1. Spinal cord. Meninges of the spinal cord. Reflectory arch

2. Spinal cord (internal structure)

3. Base of the brain. Topography of 12 cranial nerves

 

Lesson # 19

 

Theme 1. Spinal cord. Meninges of the spinal cord. Reflectory arch

     Nervous system may be divided into two parts, central and peripheral, also somatic and autonomic. The central nervous system consists of the brain [encephalon], contained within the cranium, and the medulla spinalis or spinal cord, lodged in the vertebral canal. The peripheral nervous system consists of a series of cranial and spinal nerves. Autonomic nervous system has a parasympathetic and sympathetic parts.

THE NERVOUS SYSTEM is the most complicated and highly organized of the various systems which make up the human body. It is the mechanism concerned with the correlation and integration of various bodily processes and the reactions and adjustments of the organism to its environment. In addition the cerebral cortex is concerned with conscious life. It may be divided into two parts, central and peripheral.

The central nervous system consists of the encephalon or brain, contained within the cranium, and the medulla spinalis or spinal cord, lodged in the vertebral canal; the two portions are continuous with one another at the level of the upper border of the atlas vertebra.

 

 

Segments of spinal cord and spinal nerves

 

 

Spinal cord is located in vertebral canal and extends inferiorly from the position of the foramen magnum of the occipital bone to the level of the first-second lumbar vertebra. The terminal portion of the spinal cord is called the conus medullaris and filum terminale extends inferiorly from it to the level second coccyx vertebrae. The cervical enlargement is located at the level of 6^th cervical vertebrae, and the lumbosacral enlargement at the level of 12^th thoracic vertebrae. The Central Canal (canalis centralis) runs throughout the entire length of the medulla spinalis. It is continued upward and opens into the fourth ventricle of the brain. In the lower part of the conus medullaris it exhibits a fusiform dilatation, the terminal ventricle.

 

         The medulla spinalis or spinal cord forms the elongated, nearly cylindrical, part of the central nervous system which occupies the upper two-thirds of the vertebral canal. Its average length in the male is about 45 cm., in the female from 42 to 43 cm., while its weight amounts to about 30 gms. It extends from the level of the upper border of the atlas to that of the lower border of the first, or upper border of the second, lumbar vertebra. Above, it is continuous with the brain; below, it ends in a conical extremity, the conus medullaris, from the apex of which a delicate filament, the filum terminale, descends as far as the first segment of the coccyx.

 

Diagrams of the medulla spinalis.

 

The position of the medulla spinalis varies with the movements of the vertebral column, its lower extremity being drawn slightly upward when the column is flexed. It also varies at different periods of life; up to the third month of fetal life the medulla spinalis is as long as the vertebral canal, but from this stage onward the vertebral column elongates more rapidly than the medulla spinalis, so that by the end of the fifth month the medulla spinalis terminates at the base of the sacrum, and at birth about the third lumbar vertebra.

Cauda equina and filum terminale seen from behind. The dura mater has been opened and spread out, and the arachnoid has been removed.

 

The medulla spinalis does not fill the part of the vertebral canal in which it lies; it is ensheathed by three protective membranes, separated from each other by two concentric spaces. The three membranes are named from without inward, the dura mater, the arachnoid, and the pia mater. The dura mater is a strong, fibrous membrane which forms a wide, tubular sheath; this sheath extends below the termination of the medulla spinalis and ends in a pointed cul-de-sac at the level of the lower border of the second sacral vertebra. The dura mater is separated from the wall of the vertebral canal by the epidural cavity, which contains a quantity of loose areolar tissue and a plexus of veins; between the dura mater and the subjacent arachnoid is a capillary interval, the subdural cavity, which contains a small quantity of fluid, probably of the nature of lymph. The arachnoid is a thin, transparent sheath, separated from the pia mater by a comparatively wide interval, the subarachnoid cavity, which is filled with cerebrospinal fluid. The pia mater closely invests the medulla spinalis and sends delicate septa into its substance; a narrow band, the ligamentum denticulatum, extends along each of its lateral surfaces and is attached by a series of pointed processes to the inner surface of the dura mater.

Sagittal section of vertebral canal to show the lower end of the medulla spinalis and the filum terminale.

Li, Lv. First and fifth lumbar vertebræ. Sii. Second sacral vertebra.

1. Dura mater.

 2. Lower part of tube of dura mater.

3. Lower extremity of medulla spinalis.

4. Intradural, and

5, Extradural portions of filum terminale.

6. Attachment of filum terminale to first segment of coccyx.

 

Thirty-one pairs of spinal nerves spring from the medulla spinalis, each nerve having an anterior or ventral, and a posterior or dorsal root, the latter being distinguished by the presence of an oval swelling, the spinal ganglion, which contains numerous nerve cells. Each root consists of several bundles of nerve fibers, and at its attachment extends for some distance along the side of the medulla spinalis. The pairs of spinal nerves are grouped as follows: cervical 8, thoracic 12, lumbar 5, sacral 5, coccygeal 1, and, for convenience of description, the medulla spinalis is divided into cervical, thoracic, lumbar and sacral regions, corresponding with the attachments of the different groups of nerves.

Enlargements.—The medulla spinalis is not quite cylindrical, being slightly flattened from before backward; it also presents two swellings or enlargements, an upper or cervical, and a lower or lumbar.

The cervical enlargement is the more pronounced, and corresponds with the attachments of the large nerves which supply the upper limbs. It extends from about the third cervical to the second thoracic vertebra, its maximum circumference (about 38 mm.) being on a level with the attachment of the sixth pair of cervical nerves.

The lumbar enlargement gives attachment to the nerves which supply the lower limbs. It commences about the level of the ninth thoracic vertebra, and reaches its maximum circumference, of about 33 mm., opposite the last thoracic vertebra, below which it tapers rapidly into the conus medullaris.

The Anterior Median Fissure (fissura mediana anterior) has an average depth of about 3 mm., but this is increased in the lower part of the medulla spinalis. It contains a double fold of pia mater, and its floor is formed by a transverse band of white substance, the anterior white commissure, which is perforated by bloodvessels on their way to or from the central part of the medulla spinalis.

The Posterior Median Sulcus (sulcus medianus posterior) is very shallow; from it a septum of neuroglia reaches rather more than half-way into the substance of the medulla spinalis; this septum varies in depth from 4 to 6 mm., but diminishes considerably in the lower part of the medulla spinalis.

Posterior view of spinal cord

 

On either side of the posterior median sulcus, and at a short distance from it, the posterior nerve roots are attached along a vertical furrow named the posterolateral sulcus. The portion of the medulla spinalis which lies between this and the posterior median sulcus is named the posterior funiculus. In the cervical and upper thoracic regions this funiculus presents a longitudinal furrow, the postero-intermediate sulcus; this marks the position of a septum which extends into the posterior funiculus and subdivides it into two fasciculi—a medial, named the fasciculus gracilis (tract of Goll); and a lateral, the fasciculus cuneatus (tract of Burdach). The portion of the medulla spinalis which lies in front of the posterolateral sulcus is termed the antero-lateral region. The anterior nerve roots, unlike the posterior, are not attached in linear series, and their position of exit is not marked by a sulcus. They arise by separate bundles which spring from the anterior column of gray substance and, passing forward through the white substance, emerge over an area of some slight width. The most lateral of these bundles is generally taken as a dividing line which separates the antero-lateral region into two parts, viz., an anterior funiculus, between the anterior median fissure and the most lateral of the anterior nerve roots; and a lateral funiculus, between the exit of these roots and the postero-lateral sulcus. In the upper part of the cervical region a series of nerve roots passes outward through the lateral funiculus of the medulla spinalis; these unite to form the spinal portion of the accessory nerve, which runs upward and enters the cranial cavity through the foramen magnum.

Position of spinal segments

Gray Substance (substantia grisea centralis).—The gray substance consists of two symmetrical portions, one in each half of the medulla spinalis: these are joined across the middle line by a transverse commissure of gray substance, through which runs a minute canal, the central canal, just visible to the naked eye. In a transverse section each half of the gray substance is shaped like a comma or crescent, the concavity of which is directed laterally; and these, together with the intervening gray commissure, present the appearance of the letter H. An imaginary coronal plane through the central canal serves to divide each crescent into an anterior or ventral, and a posterior or dorsal column.

 

 

 

Gray substance

 

The Anterior Column (columna anterior; anterior cornu), directed forward, is broad and of a rounded or quadrangular shape. Its posterior part is termed the base, and its anterior part the head, but these are not differentiated from each other by any well-defined constriction. It is separated from the surface of the medulla spinalis by a layer of white substance which is traversed by the bundles of the anterior nerve roots. In the thoracic region, the postero-lateral part of the anterior column projects lateralward as a triangular field, which is named the lateral column (columna lateralis; lateral cornu).

The Posterior Column (columna posterior; posterior cornu) is long and slender, and is directed backward and lateralward: it reaches almost as far as the posterolateral sulcus, from which it is separated by a thin layer of white substance, the tract of Lissauer. It consists of a base, directly continuous with the base of the anterior horn, and a neck or slightly constricted portion, which is succeeded by an oval or fusiform area, termed the head, of which the apex approaches the posterolateral sulcus. The apex is capped by a V-shaped or crescentic mass of translucent, gelatinous neuroglia, termed the substantia gelatinosa of Rolando, which contains both neuroglia cells, and small nerve cells. Between the anterior and posterior columns the gray substance extends as a series of processes into the lateral funiculus, to form a net-work called the formatio reticularis.

The anterior median fissure (fissura mediana anterior) and right and left anterolateral sulcuses located on anterior surface of the spinal cord. Posterior median sulcus (sulcus medianus posterior) and also right

and left posterolateral sulcus located on back surface of the spinal cord.

Each spinal nerve is attached to the medulla spinalis by two roots, an anterior or ventral (motor and sympathetic fibbers), and a posterior or dorsal, the being characterized by the presence of a ganglion, the spinal ganglion (with sensory pseudounipolar cells).

A spinal nerve with its anterior and posterior roots.

 

After emerging from the intervertebral foramen, each spinal nerve gives off a small meningeal branch which supplies the dura mater, and an anterior or ventral, posterior or dorsal divisions, also white and gray communicating branches for nearest sympathetic ganglion (in thoracic-lumbar part).

The peripheral nervous system consists of a series of nerves by which the central nervous system is connected with the various tissues of the body. For descriptive purposes these nerves may be arranged in two groups, cerebrospinal and sympathetic, the arrangement, however, being an arbitrary one, since the two groups are intimately connected and closely intermingled. Both the cerebrospinal and sympathetic nerves have nuclei of origin (the somatic efferent and sympathetic efferent) as well as nuclei of termination (somatic afferent and sympathetic afferent) in the central nervous system. The cerebrospinal nerves are forty-three in number on either side—twelve cranial, attached to the brain, and thirty-one spinal, to the medulla spinalis. They are associated with the functions of the special and general senses and with the voluntary movements of the body. The sympathetic nerves transmit the impulses which regulate the movements of the viscera, determine the caliber of the bloodvessels, and control the phenomena of secretion. In relation with them are two rows of central ganglia, situated one on either side of the middle line in front of the vertebral column; these ganglia are intimately connected with the medulla spinalis and the spinal nerves, and are also joined to each other by vertical strands of nerve fibers so as to constitute a pair of knotted cords, the sympathetic trunks, which reach from the base of the skull to the coccyx. The sympathetic nerves issuing from the ganglia form three great prevertebral plexuses which supply the thoracic, abdominal, and pelvic viscera; in relation to the walls of these viscera intricate nerve plexuses and numerous peripheral ganglia are found.

 

 

 

The medulla spinalis is ensheathed by three protective membranes, separated from each other by two concentric spaces:

1. The Dura mater

2. The Arachnoid

3. The Pia mater

         

The Spinal Dura Mater (dura mater spinalis; spinal dura) forms a loose sheath around the medulla spinalis, and represents only the inner or meningeal layer of the cranial dura mater; the outer or endosteal layer ceases at the foramen magnum, its place being taken by the periosteum lining the vertebral canal. The spinal dura mater is separated from the arachnoid by a potential cavity, the subdural cavity; the two membranes are, in fact, in contact with each other, except where they are separated by a minute quantity of fluid, which serves to moisten the apposed surfaces. It is separated from the wall of the vertebral canal by a space, the epidural space, which contains a quantity of loose areolar tissue and a plexus of veins; the situation of these veins between the dura mater and the periosteum of the vertebræ corresponds therefore to that of the cranial sinuses between the meningeal and endosteal layers of the cranial dura mater. The spinal dura mater is attached to the circumference of the foramen magnum, and to the second and third cervical vertebræ; it is also connected to the posterior longitudinal ligament, especially near the lower end of the vertebral canal, by fibrous slips. The subdural cavity ends at the lower border of the second sacral vertebra; below this level the dura mater closely invests the filum terminale and descends to the back of the coccyx, where it blends with the periosteum. The sheath of dura mater is much larger than is necessary for the accommodation of its contents, and its size is greater in the cervical and lumbar regions than in the thoracic. On each side may be seen the double openings which transmit the two roots of the corresponding spinal nerve, the dura mater being continued in the form of tubular prolongations on them as they pass through the intervertebral foramina. These prolongations are short in the upper part of the vertebral column, but gradually become longer below, forming a number of tubes of fibrous membrane, which enclose the lower spinal nerves and are contained in the vertebral canal.

Structure.—The spinal dura mater resembles in structure the meningeal or supporting layer of the cranial dura mater, consisting of white fibrous and elastic tissue arranged in bands or lamellæ which, for the most part, are parallel with one another and have a longitudinal arrangement. Its internal surface is smooth and covered by a layer of mesothelium. It is sparingly supplied with bloodvessels, and a few nerves have been traced into it.

 

The Arachnoid—The arachnoid is a delicate membrane enveloping the brain and medulla spinalis and lying between the pia mater internally and the dura mater externally; it is separated from the pia mater by the subarachnoid cavity, which is filled with cerebrospinal fluid. It invests the brain loosely, and does not dip into the sulci between the gyri, nor into the fissures, with the exception of the longitudinal. On the upper surface of the brain the arachnoid is thin and transparent; at the base it is thicker, and slightly opaque toward the central part, where it extends across between the two temporal lobes in front of the pons, so as to leave a considerable interval between it and the brain.

 

The Spinal Part (arachnoidea spinalis) of the arachnoid is a thin, delicate, tubular membrane loosely investing the medulla spinalis. Above, it is continuous with the cranial arachnoid; below, it widens out and invests the cauda equina and the nerves proceeding from it. It is separated from the dura mater by the subdural space, but here and there this space is traversed by isolated connective-tissue trabeculæ, which are most numerous on the posterior surface of the medulla spinalis.

The arachnoid surrounds the cranial and spinal nerves, and encloses them in loose sheaths as far as their points of exit from the skull and vertebral canal.

Structure.—The arachnoid consists of bundles of white fibrous and elastic tissue intimately blended together. Its outer surface is covered with a layer of low cuboidal mesothelium. The inner surface and the trabeculæ are likewise covered by a somewhat low type of cuboidal mesothelium which in places are flattened to a pavement type. Vessels of considerable size, but few in number, and, according to Bochdalek, a rich plexus of nerves derived from the motor root of the trigeminal, the facial, and the accessory nerves, are found in the arachnoid.

The Subarachnoid Cavity (cavum subarachnoideale; subarachnoid space) is the interval between the arachnoid and pia mater. It is occupied by a spongy tissue consisting of trabeculæ of delicate connective tissue, and intercommunicating channels in which the subarachnoid fluid is contained. This cavity is small on the surface of the hemispheres of the brain; on the summit of each gyrus the pia mater and the arachnoid are in close contact; but in the sulci between the gyri, triangular spaces are left, in which the subarachnoid trabecular tissue is found, for the pia mater dips into the sulci, whereas the arachnoid bridges across them from gyrus to gyrus. At certain parts of the base of the brain, the arachnoid is separated from the pia mater by wide intervals, which communicate freely with each other and are named subarachnoid cisternæ; in these the subarachnoid tissue is less abundant.

The dura mater is separated from the wall of the vertebral canal by the epidural cavity, which contains a quantity of fat tissue and a plexus of veins. Between the dura mater and the subjacent arachnoid is a capillary interval, the subdural cavity, which contains a small quantity of fluid, probably of the nature of lymph. The arachnoid is a thin, transparent sheath, separated from the pia mater by a comparatively wide interval, the subarachnoid cavity, which is filled with cerebrospinal fluid. The pia mater closely invests the medulla spinalis and sends delicate septa into its substance; a narrow band, the ligamentum denticulatum, extends along each of its lateral surfaces and is attached by a series of pointed processes to the inner surface of the dura mater.

 

 

The Spinal Pia Mater (pia mater spinalis; pia of the cord) is thicker, firmer, and less vascular than the cranial pia mater: this is due to the fact that it consists of two layers, the outer or additional one being composed of bundles of connective-tissue fibers, arranged for the most part longitudinally. Between the layers are cleft-like spaces which communicate with the subarachnoid cavity, and a number of bloodvessels (spinal arteries) which are enclosed in perivascular lymphatic sheaths.

 

 

The spinal pia mater covers the entire surface of the medulla spinalis, and is very intimately adherent to it; in front it sends a process backward into the anterior fissure. A longitudinal fibrous band, called the linea splendens, extends along the middle line of the anterior surface; and a somewhat similar band, the ligamentum denticulatum, is situated on either side. Below the conus medullaris, the pia mater is continued as a long, slender filament (filum terminale), which descends through the center of the mass of nerves forming the cauda equina. It blends with the dura mater at the level of the lower border of the second sacral vertebra, and extends downward as far as the base of the coccyx, where it fuses with the periosteum. It assists in maintaining the medulla spinalis in its position during the movements of the trunk, and is, from this circumstance, called the central ligament of the medulla spinalis.

The pia mater forms sheaths for the cranial and spinal nerves; these sheaths are closely connected with the nerves, and blend with their common membranous investments.

The ligamentum denticulatum (dentate ligament) is a narrow fibrous band situated on either side of the medulla spinalis throughout its entire length, and separating the anterior from the posterior nerve roots. Its medial border is continuous with the pia mater at the side of the medulla spinalis. Its lateral border presents a series of triangular tooth-like processes, the points of which are fixed at intervals to the dura mater. These processes are twenty-one in number, on either side, the first being attached to the dura mater, opposite the margin of the foramen magnum, between the vertebral artery and the hypoglossal nerve; and the last near the lower end of the medulla spinalis.

 

The cerebrospinal fluid, for the most part elaborated by the choroid plexuses, is poured into the cerebral ventricles which are lined by smooth ependyma. That portion of the fluid formed in the lateral ventricles escapes by the foramen of Monro into the third ventricle and thence by the aqueduct into the fourth ventricle. Likewise an ascending current of fluid apparently occurs in the central canal of the spinal cord; this, representing a possible product of the ependyma, may be added to the intraventricular supply. From the fourth ventricle the fluid is poured into the subarachnoid spaces through the medial foramen of Majendie and the two lateral foramina of Luschka. There is no evidence that functional communications between the cerebral ventricles and the subarachnoid spaces exist in any region except from the fourth ventricle.

 

 

In addition to the elaboration of the cerebrospinal fluid by the choroid plexuses, there seems fairly well established a second source of the fluid from the nervous system itself. The bloodvessels that enter and leave the brain are surrounded by perivascular channels. It seems most likely that the outer wall of these channels is lined by a continuation inward of the pial mesothelium while the inner wall is probably derived from the mesothelial covering of the vessels, which are thus protected throughout the subarachnoid spaces. These mesothelial cells continue inward only a short distance, neuroglia cells probably replacing on the outer surface the mesothelial elements. Through these perivascular channels there is probably a small amount of fluid flowing from nerve-cell to subarachnoid space. The chemical differences between the subarachnoid fluid (product of choroid plexuses and perivascular system) and the ventricular fluid (product of choroid plexuses alone) indicate that the products of nerve-metabolism are poured into the subarachnoid s

 

The absorption of the cerebrospinal fluid is a dual process, being chiefly a rapid drainage through the arachnoid villi into the great dural sinuses, and, in small part, a slow escape into the true lymphatic vessels, by way of an abundant but indirect perineural course.

In general the arachnoid channels are equipped as fluid retainers with unquestionable powers of diffusion or absorption in regard to certain elements in the normal cerebrospinal fluid, deriving in this way a cellular nutrition.

The subdural space (between arachnoid and dura) is usually considered to be a part of the cerebrospinal channels. It is a very small space, the two limiting surfaces being separated by merely a capillary layer of fluid. Whether this fluid is exactly similar to the cerebrospinal fluid is very difficult to ascertain. Likewise our knowledge of the connections between the subdural and subarachnoid spaces is hardly definite. In some ways the subdural space may be likened to a serous cavity. The inner surface of the dura is covered by flattened polygonal mesothelial cells but the outer surface of the arachnoid is covered by somewhat cuboidal mesothelium. The fluid of the subdural space has probably a local origin from the cells lining it.

 

The nervous tissues are composed of nerve cells and their various processes, together with a supporting tissue called neuroglia, which, however, is found only in the brain and medulla spinalis. Certain long processes of the nerve cells are of special importance, and it is convenient to consider them apart from the cells; they are known as nerve fibers.

To the naked eye a difference is obvious between certain portions of the brain and medulla spinalis, viz., the gray substance and the white substance. The gray substance is largely composed of nerve cells, while the white substance contains only their long processes, the nerve fibers. It is in the former that nervous impressions are received, stored, and transformed into efferent impulses, and by the latter that they are conducted. Hence the gray substance forms the essential constituent of all the ganglionic centers, both those in the isolated ganglia and those aggregated in the brain and medulla spinalis; while the white substance forms the bulk of the commissural portions of the nerve centers and the peripheral nerves.

 

Neuroglia.—Neuroglia, the peculiar ground substance in which are imbedded the true nervous constituents of the brain and medulla spinalis, consists of cells and fibers. Some of the cells are stellate in shape, with ill-defined cell body, and their fine processes become neuroglia fibers, which extend radially and unbranched among the nerve cells and fibers which they aid in supporting. Other cells give off fibers which branch repeatedly. Some of the fibers start from the epithelial cells lining the ventricles of the brain and central canal of the medulla spinalis, and pass through the nervous tissue, branching repeatedly to end in slight enlargements on the pia mater. Thus, neuroglia is evidently a connective tissue in function but is not so in development; it is ectodermal in origin, whereas all connective tissues are mesodermal.

 

 

Neuroglia cells of brain shown by Golgi’s method. A. Cell with branched processes. B. Spider cell with unbranched processes. (After Andriezen.)

Nerve Cells—Nerve cells are largely aggregated in the gray substance of the brain and medulla spinalis, but smaller collections of these cells also form the swellings, called ganglia, seen on many nerves. These latter are found chiefly upon the spinal and cranial nerve roots and in connection with the sympathetic nerves.

The nerve cells vary in shape and size, and have one or more processes. They may be divided for purposes of description into three groups, according to the number of processes which they possess: (1) Unipolar cells, which are found in the spinal ganglia; the single process, after a short course, divides in a T-shaped manner  Bipolar cells, also found in the spinal ganglia when the cells are in an embryonic condition. They are best demonstrated in the spinal ganglia of fish. Sometimes the processes come off from opposite poles of the cell, and the cell then assumes a spindle shape; in other cells both processes emerge at the same point. In some cases where two fibers are apparently connected with a cell, one of the fibers is really derived from an adjoining nerve cell and is passing to end in a ramification around the ganglion cell, or, again, it may be coiled spirally around the nerve process which is issuing from the cell. (3) Multipolar cells, which are pyramidal or stellate in shape, and characterized by their large size and by the numerous processes which issue from them. The processes are of two kinds: one of them is termed the axis-cylinder process or axon because it becomes the axis-cylinder of a nerve fiber. The others are termed the protoplasmic processes or dendrons; they begin to divide and subdivide soon after they emerge from the cell, and finally end in minute twigs and become lost among the other elements of the nervous tissue.

 

 

Various forms of nerve cells. A. Pyramidal cell. B. Small multipolar cell, in which the axon quickly divides into numerous branches. C. Small fusiform cell. D and E. Ganglion cells (E shows T-shaped division of axon). ax. Axon. c. Capsule.

 

 

 

Bipolar nerve cell from the spinal ganglion of the pike. (After Kölliker.)

 

 

Motor nerve cell from ventral horn of medulla spinalis of rabbit. The angular and spindle-shaped Nissl bodies are well shown. (After Nissl.)

 

The body of the nerve cell, known as the cyton, consists of a finely fibrillated protoplasmic material, of a reddish or yellowishbrown color, which occasionally presents patches of a deeper tint, caused by the aggregation of pigment granules at one side of the nucleus, as in the substantia nigra and locus cæruleus of the brain. The protoplasm also contains peculiar angular granules, which stain deeply with basic dyes, such as methylene blue; these are known as Nissl’s granules. They extend into the dendritic processes but not into the axis-cylinder; the small clear area at the point of exit of the axon in some cell types is termed the cone of origin. These granules disappear (chromatolysis) during fatigue or after prolonged stimulation of the nerve fibers connected with the cells. They are supposed to represent a store of nervous energy, and in various mental diseases are deficient or absent. The nucleus is, as a rule, a large, well-defined, spherical body, often presenting an intranuclear network, and containing a well-marked nucleolus.

 

 

Pyramidal cell from the cerebral cortex of a mouse. (After Ramón y Cajal.)

 

 

 

Cell of Purkinje from the cerebellum. Golgi method. (Cajal.) a. Axon. b. Collateral. c and d. Dendrons.

 

In addition to the protoplasmic network described above, each nerve cell may be shown to have delicate neurofibrils running through its substance; these fibrils are continuous with the fibrils of the axon, and are believed to convey nerve impulses. Golgi has also described an extracellular network, which is probably a supporting structure.

Nerve Fibers.—Nerve fibers are found universally in the peripheral nerves and in the white substance of the brain and medulla spinalis. They are of two kinds—viz., medullated or white fibers, and non-medullated or gray fibers.

The medullated fibers form the white part of the brain and medulla spinalis, and also the greater part of every cranial and spinal nerve, and give to these structures their opaque, white aspect. When perfectly fresh they appear to be homogeneous; but soon after removal from the body each fiber presents, when examined by transmitted light, a double outline or contour, as if consisting of two parts. The central portion is named the axis-cylinder; around this is a sheath of fatty material, staining black with osmic acid, named the white substance of Schwann or medullary sheath, which gives to the fiber its double contour, and the whole is enclosed in a delicate membrane, the neurolemma, primitive sheath, or nucleated sheath of Schwann

 

 

 

Theme 2. Spinal cord (internal structure)

 

Spinal cord consists of the gray substance that is surrounded by the white substance. Posterior median sulcus passes to the gray substance and divides by the white substance into two parts. Anterior median fissure does not pass to the gray substance so it is white commisura that connect anterior symmetrical regions of the white matter.

The gray substance is largely composed of nervous cell bodies. The gray substance consists of anterior, posterior and lateral columns (only in thoracic-lumbar portion) also central intermediate

zone round central canal. In transverse section columns are looking as horns, so they differ anterior, posterior and lateral horns.

 

Transverse section of the medulla spinalis in the mid-thoracic region.

 

Anterior horns contain motor cells that arranged in 5 motor nuclei (nucleus anterior [medial and lateral], central nucleus and posterior [medial and lateral] nuclei). Their axons form anterior roots that pass with spinal nerves to the skeletal muscles of the trunk and limbs. Posterior horns contain intermediate cells. They receive impulse from sensory cells and carry them to the another cell. Intermediate cells form the spongious zone, gelatinous substance, proper nucleus and thoracic nucleus (Clarc-Steiling column).

Lateral columns in thoracic-lumbar portion contain autonomic cells that form lateral intermediate nucleus. In intermediate central part medial intermediate nucleus carry intermediate cells.

White substance contains only neuron long processes (axons), the nerve fibbers which form ascending (sensory, afferent) and descending (motor, efferent) pathways. White substance divided by sulcuses and is arranged in three funiculi: anterior, lateral, and posterior. Both anterior funiculi are communicated each other by white commisura. 

Fasciculi in the Posterior Funiculus comprises the fasciculus gracilis [tract of Goll] (lies next the posterior median septum) and the fasciculus cuneatus [tract of Burdach] (laterally). They conduct impulses of conscious muscle sense.

Pathways in the Anterior Funiculus (descending): anterior corticospinal [pyramidal] tract, tectospinal tract, reticulospinal tract, olivospinal tract, and vestibulospinal tract. Ascending pathway  - anterior spinothalamic tract.

 

Diagram of the principal fasciculi of the spinal cord.

 

Ascending pathways in the Lateral Funiculus:

·        dorsal spinocerebellar tract (tract of Flechsig)

·        ventral spinocerebellar tract (tract of Gowers)

·        lateral spinothalamic

·        tract

Medially they can find descending pathways:

·        corticospinal [pyramidal] tract

·        rubrospinal tract (of Monakow)

 

 

Portion of the spinal cord that carries two pairs of ventral and dorsal roots or one pair of spinal nerves called segment. There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal segments. Roots which exit from the spinal cord lower then second lumbar segment form together with the filum termanale cauda equina.

Gray Substance (substantia grisea centralis).—The gray substance consists of two symmetrical portions, one in each half of the medulla spinalis: these are joined across the middle line by a transverse commissure of gray substance, through which runs a minute canal, the central canal, just visible to the naked eye. In a transverse section each half of the gray substance is shaped like a comma or crescent, the concavity of which is directed laterally; and these, together with the intervening gray commissure, present the appearance of the letter H. An imaginary coronal plane through the central canal serves to divide each crescent into an anterior or ventral, and a posterior or dorsal column.

  The Anterior Column (columna anterior; anterior cornu), directed forward, is broad and of a rounded or quadrangular shape. Its posterior part is termed the base, and its anterior part the head, but these are not differentiated from each other by any well-defined constriction. It is separated from the surface of the medulla spinalis by a layer of white substance which is traversed by the bundles of the anterior nerve roots. In the thoracic region, the postero-lateral part of the anterior column projects lateralward as a triangular field, which is named the lateral column (columna lateralis; lateral cornu).

  The Posterior Column (columna posterior; posterior cornu) is long and slender, and is directed backward and lateralward: it reaches almost as far as the posterolateral sulcus, from which it is separated by a thin layer of white substance, the tract of Lissauer. It consists of a base, directly continuous with the base of the anterior horn, and a neck or slightly constricted portion, which is succeeded by an oval or fusiform area, termed the head, of which the apex approaches the posterolateral sulcus. The apex is capped by a V-shaped or crescentic mass of translucent, gelatinous neuroglia, termed the substantia gelatinosa of Rolando, which contains both neuroglia cells, and small nerve cells. Between the anterior and posterior columns the gray substance extends as a series of processes into the lateral funiculus, to form a net-work called the formatio reticularis.

 

Pathways in white matter and gray matter’s nuclei of the spinal cord

  The quantity of gray substance, as well as the form which it presents on transverse section, varies markedly at different levels. In the thoracic region it is small, not only in amount but relatively to the surrounding white substance. In the cervical and lumbar enlargements it is greatly increased: in the latter, and especially in the conus medullaris, its proportion to the white substance is greatest.  In the cervical region its posterior column is comparatively narrow, while its anterior is broad and expanded; in the thoracic region, both columns are attenuated, and the lateral column is evident; in the lumbar enlargement, both are expanded; while in the conus medullaris the gray substance assumes the form of two oval masses, one in each half of the cord, connected together by a broad gray commissure.

  The Central Canal (canalis centralis) runs throughout the entire length of the medulla spinalis. The portion of gray substance in front of the canal is named the anterior gray commissure; that behind it, the posterior gray commissure. The former is thin, and is in contact anteriorly with the anterior white commissure: it contains a couple of longitudinal veins, one on either side of the middle line. The posterior gray commissure reaches from the central canal to the posterior median septum, and is thinnest in the thoracic region, and thickest in the conus medullaris. The central canal is continued upward through the lower part of the medulla oblongata, and opens into the fourth ventricle of the brain; below, it reaches for a short distance into the filum terminale. In the lower part of the conus medullaris it exhibits a fusiform dilatation, the terminal ventricle; this has a vertical measurement of from 8 to 10 mm., is triangular on cross-section with its base directed forward, and tends to undergo obliteration after the age of forty years.

Throughout the cervical and thoracic regions the central canal is situated in the anterior third of the medulla spinalis; in the lumbar enlargement it is near the middle, and in the conus medullaris it approaches the posterior surface. It is filled with cerebrospinal fluid, and lined by ciliated, columnar epithelium, outside of which is an encircling band of gelatinous substance, the substantia gelatinosa centralis. This gelatinous substance consists mainly of neuroglia, but contains a few nerve cells and fibers; it is traversed by processes from the deep ends of the columnar ciliated cells which line the central canal.

Structure of the Gray Substance.—The gray substance consists of numerous nerve cells and nerve fibers held together by neuroglia. Throughout the greater part of the gray substance the neuroglia presents the appearance of a sponge-like network, but around the central canal and on the apices of the posterior columns it consists of the gelatinous substance already referred to. The nerve cells are multipolar, and vary greatly in size and shape. They consist of (1) motor cells of large size, which are situated in the anterior horn, and are especially numerous in the cervical and lumbar enlargements; the axons of most of these cells pass out to form the anterior nerve roots, but before leaving the white substance they frequently give off collaterals, which reënter and ramify in the gray substance. Cells of small or medium size, whose axons pass into the white matter, where some pursue an ascending, and others a descending course, but most of them divide in a T-shape manner into descending and ascending processes. They give off collaterals which enter and ramify in the gray substance, and the terminations of the axons behave in a similar manner. The lengths of these axons vary greatly: some are short and pass only between adjoining spinal segments, while others are longer and connect more distant segments. These cells and their processes constitute a series of association or intersegmental neurons which link together the different parts of the medulla spinalis. The axons of most of these cells are confined to that side of the medulla spinalis in which the nerve cells are situated, but some cross to the opposite side through the anterior commissure, and are termed crossed commissural fibers. Some of these latter end directly in the gray substance, while others enter the white substance, and ascend or descend in it for varying distances, before finally terminating in the gray substance. (3) Cells of the type II of Golgi, limited for the most part to the posterior column, are found also in the substantia gelatinosa of Rolando; their axons are short and entirely confined to the gray substance, in which they break up into numerous fine filaments. Most of the nerve cells are arranged in longitudinal columns, and appear as groups on transverse section.

Nerve Cells in the Anterior Column.—The nerve cells in the anterior column are arranged in columns of varying length. The longest occupies the medial part of the anterior column, and is named the antero-medial column: it is well marked in C4, C5, again from C8 to L4, it disappears in L5 and S1 but is well marked in S2, S3 and S4 (Bruce). Behind it is a dorso-medial column of small cells, which is not represented in L5, S1, S2 nor below S4. Its axons probably pass into the dorsal rami of the spinal nerves to supply the dorsal musculature of the spinal column. In the cervical and lumbar enlargements, where the anterior column is expanded in a lateral direction, the following additional columns are present, viz.: (a) antero-lateral, which consists of two groups, one in C4, C5, C6 the other in C6, C7, C8 in the cervical enlargement and of a group from L2 to S2 in the lumbo-sacral enlargement; (b) postero-lateral, in the lower five cervical, lower four lumbar, and upper three sacral segments; (c) post-postero-lateral, in the last cervical, first thoracic, and upper three sacral segments; and (d) a central, in the lower four lumbar and upper two sacral segments. These cell groups are evidently related to the nerve roots of the brachial and sacral plexuses and supply fibers to the muscles of the arm and leg. Throughout the base of the anterior column are scattered solitary cells, the axons of some of which form crossed commissural fibers, while others constitute the motor fibers of the posterior nerve roots.

 

Transverse sections of the medulla spinalis at different

Nerve Cells in the Lateral Column.—These form a column which is best marked where the lateral gray column is differentiated, viz., in the thoracic region; but it can be traced throughout the entire length of the medulla spinalis in the form of groups of small cells which are situated in the anterior part of the formatio reticularis. In the upper part of the cervical region and lower part of the medulla oblongata as well as in the third and fourth sacral segments this column is again differentiated. In the medulla it is known as the lateral nucleus. The cells of this column are fusiform or star-shaped, and of a medium size: the axons of some of them pass into the anterior nerve roots, by which they are carried to the sympathetic nerves: they constitute the white rami and are sympathetic or visceral efferent fibers; they are also known as preganglionic fibers of the sympathetic system; the axons of others pass into the anterior and lateral funiculi, where they become longitudinal.

 Nerve Cells in the Posterior Column.—1. The dorsal nucleus (nucleus dorsalis; column of Clarke) occupies the medial part of the base of the posterior column, and appears on the transverse section as a well-defined oval area. It begins below at the level of the second or third lumbar nerve, and reaches its maximum size opposite the twelfth thoracic nerve. Above the level of the ninth thoracic nerve its size diminishes, and the column ends opposite the last cervical or first thoracic nerve. It is represented, however, in the other regions by scattered cells, which become aggregated to form a cervical nucleus opposite the third cervical nerve, and a sacral nucleus in the middle and lower part of the sacral region. Its cells are of medium size, and of an oval or pyriform shape; their axons pass into the peripheral part of the lateral funiculus of the same side, and there ascend, probably in dorsal spinocerebellar (direct cerebellar) fasciculus. 2. The nerve cells in the substantia gelatinosa of Rolando are arranged in three zones: a posterior or marginal, of large angular or fusiform cells; an intermediate, of small fusiform cells; and an anterior, of star-shaped cells. The axons of these cells pass into the lateral and posterior funiculi, and there assume a vertical course. In the anterior zone some Golgi cells are found whose short axons ramify in the gray substance. 3. Solitary cells of varying form and size are scattered throughout the posterior column. Some of these are grouped to form the posterior basal column in the base of the posterior column, lateral to the dorsal nucleus; the posterior basal column is well-marked in the gorilla (Waldeyer), but is ill-defined in man. The axons of its cells pass partly to the posterior and lateral funiculi of the same side, and partly through the anterior white commissure to the lateral funiculus of the opposite side. Golgi cells, type II, located in this region send axons to the lateral and ventral columns.

  A few star-shaped or fusiform nerve cells of varying size are found in the substantia gelatinosa centralis. Their axons pass into the lateral funiculus of the same, or of the opposite side.

  The nerve fibers in the gray substance form a dense interlacement of minute fibrils among the nerve cells. This interlacement is formed partly of axons which pass from the cells in the gray substance to enter the white funiculi or nerve roots; partly of the axons of Golgi’s cells which ramify only in the gray substance; and partly of collaterals from the nerve fibers in the white funiculi which, as already stated, enter the gray substance and ramify within it.

 

             White Substance (substantia alba).—The white substance of the medulla spinalis consists of medullated nerve fibers imbedded in a spongelike net-work of neuroglia, and is arranged in three funiculi: anterior, lateral, and posterior. The anterior funiculus lies between the anterior median fissure and the most lateral of the anterior nerve roots: the lateral funiculus between these nerve roots and the postero-lateral sulcus; and the posterior funiculus between the postero-lateral and the posterior median sulci. The fibers vary greatly in thickness, the smallest being found in the fasciculus gracilis, the tract of Lissauer, and inner part of the lateral funiculus; while the largest are situated in the anterior funiculus, and in the peripheral part of the lateral funiculus. Some of the nerve fibers assume a more or less transverse direction, as for example those which cross from side to side in the anterior white commissure, but the majority pursue a longitudinal course and are divisible into (1) those connecting the medulla spinalis with the brain and conveying impulses to or from the latter, and (2) those which are confined to the medulla spinalis and link together its different segments, i. e., intersegmental or association fibers.

 

Nerve Fasciculi.—The longitudinal fibers are grouped into more or less definite bundles or fasciculi. These are not recognizable from each other in the normal state, and their existence has been determined by the following methods: (1) A. Waller discovered that if a bundle of nerve fibers be cut, the portions of the fibers which are separated from their cells rapidly degenerate and become atrophied, while the cells and the parts of the fibers connected with them undergo little alteration. This is known as Wallerian degeneration. Similarly, if a group of nerve cells be destroyed, the fibers arising from them undergo degeneration. Thus, if the motor cells of the cerebral cortex be destroyed, or if the fibers arising from these cells be severed, a descending degeneration from the seat of injury takes place in the fibers. In the same manner, if a spinal ganglion be destroyed, or the fibers which pass from it into the medulla spinalis be cut, an ascending degeneration will extend along these fibers. (2) Pathological changes, especially in man, have given important information by causing ascending and descending degenerations. (3) By tracing the development of the nervous system, it has been observed that at first the nerve fibers are merely naked axis-cylinders, and that they do not all acquire their medullary sheaths at the same time; hence the fibers can be grouped into different bundles according to the dates at which they receive their medullary sheaths. (4) Various methods of staining nervous tissue are of great value in tracing the course and mode of termination of the axis-cylinder processes.

Fasciculi in the Anterior Funiculus.—Descending Fasciculi.—The anterior cerebrospinal (fasciculus cerebrospinalis anterior; direct pyramidal tract), which is usually small, but varies inversely in size with the lateral cerebrospinal fasciculus. It lies close to the anterior median fissure, and is present only in the upper part of the medulla spinalis; gradually diminishing in size as it descends, it ends about the middle of the thoracic region. It consists of descending fibers which arise from cells in the motor area of the cerebral hemisphere of the same side, and which, as they run downward in the medulla spinalis, cross in succession through the anterior white commissure to the opposite side, where they end, either directly or indirectly, by arborizing around the motor cells in the anterior column. A few of its fibers are said to pass to the lateral column of the same side and to the gray matter at the base of the posterior column. They conduct voluntary motor impulses from the precentral gyrus to the motor centers of the cord.

  The vestibulospinal fasciculus, situated chiefly in the marginal part of the funiculus and mainly derived from the cells of Deiters’ nucleus, of the same and the opposite side, i. e., the chief terminal nucleus of the vestibular nerve. Fibers are also contributed to this fasciculus from scattered cells of the articular formation of the medulla oblongata, the pons and the mid-brain (tegmentum). The other terminal nuclei of the vestibular nerve also contribute fibers. In the brain stem these fibers form part of the median longitudinal bundle. The fasciculus can be traced to the sacral region. Its terminals and collaterals end either directly or indirectly among the motor cells of the anterior column. This fasciculus is probably concerned with equilibratory reflexes.

  The tectospinal fasciculus, situated partly in the anterior and partly in the lateral funiculus, is mainly derived from the opposite superior colliculus of the mid-brain. The fibers from the superior colliculus cross the median raphé in the fountain decussation of Meynert and descend as the ventral longitudinal bundle in the reticular formation of the brain-stem. Its collaterals and terminals end either directly or indirectly among the motor cells of the anterior column of the same side. Since the superior colliculus is an important visual reflex center, the tectospinal fasciculus is probably concerned with visual reflexes.

  Ascending Fasciculi.—The ventral spinothalamic fasciculus, situated in the marginal part of the funiculus and intermingled more or less with the vestibulo-spinal fasciculus, is derived from cells in the posterior column or intermediate gray matter of the opposite side. Their axons cross in the anterior commissure. This is a somewhat doubtful fasciculus and its fibers are supposed to end in the thalamus and to conduct certain of the touch impulses.

  The remaining fibers of the anterior funiculus constitute what is termed the anterior proper fasciculus (fasciculus anterior proprius; anterior basis bundle). It consists of (a) longitudinal intersegmental fibers which arise from cells in the gray substance, more especially from those of the medial group of the anterior column, and, after a longer or shorter course, reënter the gray substance; (b) fibers which cross in the anterior white commissure from the gray substance of the opposite side.

 

Fasciculi in the Lateral Funiculus.—1. Descending Fasciculi.—(a) The lateral cerebrospinal fasciculus (fasciculus cerebrospinalis lateralis; crossed pyramidal tract) extends throughout the entire length of the medulla spinalis, and on transverse section appears as an oval area in front of the posterior column and medial to the cerebellospinal. Its fibers arise from cells in the motor area of the cerebral hemisphere of the opposite side. They pass downward in company with those of the anterior cerebrospinal fasciculus through the same side of the brain as that from which they originate, but they cross to the opposite side in the medulla oblongata and descend in the lateral funiculus of the medulla spinalis.

  It is probable that the fibers of the anterior and lateral cerebrospinal fasciculi are not related in this direct manner with the cells of the anterior column, but terminate by arborizing around the cells at the base of the posterior column and the cells of Clarke’s column, which in turn link them to the motor cells in the anterior column, usually of several segments of the cord. In consequence of these interposed neurons the fibers of the cerebrospinal fasciculi correspond not to individual muscles, but to associated groups of muscles.

  The anterior and lateral cerebrospinal fasciculi constitute the motor fasciculi of the medulla spinalis and have their origins in the motor cells of the cerebral cortex. They descend through the internal capsule of the cerebrum, traverse the cerebral peduncles and pons and enter the pyramid of the medulla oblongata. In the lower part of the latter about two-thirds of them cross the middle line and run downward in the lateral funiculus as the lateral cerebrospinal fasciculus, while the remaining fibers do not cross the middle line, but are continued into the same side of the medulla spinalis, where they form the anterior cerebrospinal fasciculus. The fibers of the latter, however, cross the middle line in the anterior white commissure, and thus all the motor fibers from one side of the brain ultimately reach the opposite side of the medulla spinalis. The proportion of fibers which cross in the medulla oblongata is not a constant one, and thus the anterior and lateral cerebrospinal fasciculi vary inversely in size. Sometimes the former is absent, and in such cases it may be presumed that the decussation of the motor fibers in the medulla oblongata has been complete. The fibers of these two fasciculi do not acquire their medullary sheaths until after birth. In some animals the motor fibers are situated in the posterior funiculus.

 

Cortico-spinal and rubrospinal tracts

 

       (b) The rubrospinal fasciculus (Monakow) (prepyramidal tract), lies on the ventral aspect of the lateral cerebrospinal fasciculus and on transverse section appears as a somewhat triangular area. Its fibers descend from the mid-brain, where they have their origin in the red nucleus of the tegmentum of the opposite side. Its terminals and collaterals end either directly or indirectly in relation with the motor cells of the anterior column. The rubrospinal fasciculus is supposed to be concerned with cerebellar reflexes since fibers which pass from the cerebellum through the superior peduncle send many collaterals and terminals to the red nucleus.

(c) The olivospinal fasciculus (Helweg) arises in the vicinity of the inferior olivary nucleus in the medulla oblongata, and is seen only in the cervical region of the medulla spinalis, where it forms a small triangular area at the periphery, close to the most lateral of the anterior nerve roots. Its exact origin and its mode of ending have not yet been definitely made out.

  2. Ascending Fasciculi.—(a) The dorsal spinocerebellar fasciculus (fasciculus cerebellospinalis; direct cerebellar tract of Flechsig) is situated at the periphery of the posterior part of the lateral funiculus, and on transverse section appears as a flattened band reaching as far forward as a line drawn transversely through the central canal. Medially, it is in contact with the lateral cerebrospinal fasciculus, behind, with the fasciculus of Lissauer. It begins about the level of the second or third lumbar nerve, and increasing in size as it ascends, passes to the vermis of the cerebellum through the inferior peduncle. Its fibers are generally regarded as being formed by the axons of the cells of the dorsal nucleus (Clarke’s column); they receive their medullary sheaths about the sixth or seventh month of fetal life. Its fibers are supposed to conduct impulses of unconscious muscle sense.

  The superficial antero-lateral fasciculus (tract of Gowers) consists of four fasciculi, the ventral spinocerebellar, the lateral spinothalamic, the spinotectal and the ventral spinothalamic.

  (b) The ventral spinocerebellar fasciculus (Gowers) skirts the periphery of the lateral funiculus in front of the dorsal spinocerebellar fasciculus. In transverse section it is shaped somewhat like a comma, the expanded end of which lies in front of the dorsal spinocerebellar fasciculus while the tail reaches forward into the anterior funiculus. Its fibers come from the same but mostly from the opposite side of the medulla spinalis and cross both in the anterior white commissure and in the gray commissure; they are probably derived from the cells of the dorsal nucleus and from other cells of the posterior column and the intermediate portion of the gray matter. The ventral spinocerebellar fasciculus begins about the level of the third pair of lumbar nerves, and can be followed into the medulla oblongata and pons almost to the level of the inferior colliculus where it crosses over the superior peduncle and then passes backward along its medial border to reach the vermis of the cerebellum. In the pons it lies along the lateral edge of the lateral lemniscus. Some of its fibers join the dorsal spinocerebellar fasciculus at the level of the inferior peduncle and pass with them into the cerebellum. Other fibers are said to continue upward in the dorso-lateral part of the tegmentum of the mid-brain probably as far as the thalamus.

  (c) The lateral spinothalamic fasciculus is supposed to come from cells in the dorsal column and the intermediate gray matter whose axons cross in the anterior commissure to the opposite lateral funiculus where they pass upward on the medial side of the ventral spinocerebellar fasciculus; on reaching the medulla oblongata they continue in the formatio reticularis near the median fillet and probably terminate in the ventro-lateral region of the thalamus. It is supposed to conduct impulses of pain and temperature. The lateral and ventral spinothalamic fasciculi are sometimes termed the secondary sensory fasciculus or spinal lemniscus.

  (d) The spinotectal fasciculus is supposed to arise in the dorsal column and terminate in the (inferior ?) and superior colliculi. It is situated ventral to the lateral spinothalamic fasciculus, but its fibers are more or less intermingled with it. It is also known as the spino-quadrigeminal system of Mott. In the brain-stem the fibers run lateral from the inferior olive, ventro-lateral from the superior olive, then ventro-medial from the spinal tract of the trigeminal; the fibers come to lie in the medial portion of the lateral lemniscus.

  (e) The fasciculus of Lissauer is a small strand situated in relation to the tip of the posterior column close to the entrance of the posterior nerve roots. It consists of fine fibers which do not receive their medullary sheaths until toward the close of fetal life. It is usually regarded as being formed by some of the fibers of the posterior nerve roots, which ascend for a short distance in the tract and then enter the posterior column, but since its fibers are myelinated later than those of the posterior nerve roots, and do not undergo degeneration in locomotor ataxia, they are probably intersegmental in character.

  In addition the fasciculus or tract of Lissauer contains great numbers of fine non-medullated fibers derived mostly from the dorsal roots but partly endogenous in origin. These fibers are intimately related to the substantia gelatinosa which is probably the terminal nucleus. The non-medullated fibers ascend or descend for short distances not exceeding one or two segments, but most of them enter the substantia gelatinosa at or near the level of their origin. Ransom 118 suggests that these non-medullated fibers and the substantia gelatinosa are concerned with the reflexes associated with pain impulses.

  (f) The lateral proper fasciculus (fasciculus lateralis proprius; lateral basis bundle) constitutes the remainder of the lateral column, and is continuous in front with the anterior proper fasciculus. It consists chiefly of intersegmental fibers which arise from cells in the gray substance, and, after a longer or shorter course, reënter the gray substance and ramify in it. Some of its fibers are, however, continued upward into the brain under the name of the medial longitudinal fasciculus.

 

Fasciculi in the Posterior Funiculus.—This funiculus comprises two main fasciculi, viz., the fasciculus gracilis, and the fasciculus cuneatus. These are separated from each other in the cervical and upper thoracic regions by the postero-intermediate septum, and consist mainly of ascending fibers derived from the posterior nerve roots.

  The fasciculus gracilis (tract of Goll) is wedge-shaped on transverse section, and lies next the posterior median septum, its base being at the surface of the medulla spinalis, and its apex directed toward the posterior gray commissure. It increases in size from below upward, and consists of long thin fibers which are derived from the posterior nerve roots, and ascend as far as the medulla oblongata, where they end in the nucleus gracilis.

  The fasciculus cuneatus (tract of Burdach) is triangular on transverse section, and lies between the fasciculus gracilis and the posterior column, its base corresponding with the surface of the medulla spinalis. Its fibers, larger than those of the fasciculus gracilis, are mostly derived from the same source, viz., the posterior nerve roots. Some ascend for only a short distance in the tract, and, entering the gray matter, come into close relationship with the cells of the dorsal nucleus; while others can be traced as far as the medulla oblongata, where they end in the gracile and cuneate nuclei.

 

 

Fasciculus gracilis and fasciculus cuneatus

The fasciculus gracilis and fasciculus cuneatus conduct (1) impulses of conscious muscle sense, neurons of the second order from the nucleus gracilis and nucleus cuneatus, pass in the median lemniscus to the thalamus and neurons of the third order from the thalamus to the cerebral cortex; (2) impulses of unconscious muscle sense, via neurons of the second order from the nucleus gracilis and nucleus cuneatus pass in the internal and external arcuate fibers of the medulla oblongata to the inferior peduncle and through it to the cerebellum; (3) impulses of tactile discrimination, via neurons of the second order from the nucleus cuneatus and nucleus gracilis pass in the median lemniscus to the thalamus, neurons of the third order pass from the thalamus to the cortex.

  The Posterior Proper Fasciculus (posterior ground bundle; posterior basis bundle) arises from cells in the posterior column; their axons bifurcate into ascending and descending branches which occupy the ventral part of the funiculus close to the gray column. They are intersegmental and run for varying distances sending off collaterals and terminals to the gray matter.

  Some descending fibers occupy different parts at different levels. In the cervical and upper thoracic regions they appear as a comma-shaped fasciculus in the lateral part of the fasciculus cuneatus, the blunt end of the comma being directed toward the posterior gray commissure; in the lower thoracic region they form a dorsal peripheral band on the posterior surface of the funiculus; in the lumbar region, they are situated by the side of the posterior median septum, and appear on section as a semi-elliptical bundle, which, together with the corresponding bundle of the opposite side, forms the oval area of Flechsig; while in the conus medullaris they assume the form of a triangular strand in the postero-medial part of the fasciculus gracilis. These descending fibers are mainly intersegmental in character and derived from cells in the posterior column, but some consist of the descending branches of the posterior nerve roots. The comma-shaped fasciculus was supposed to belong to the second category, but against this view is the fact that it does not undergo descending degeneration when the posterior nerve roots are destroyed.

 

Roots of the Spinal Nerves.—As already stated, each spinal nerve possesses two roots, an anterior and a posterior, which are attached to the surface of the medulla spinalis opposite the corresponding column of gray substance, their fibers become medullated about the fifth month of fetal life.

  The Anterior Nerve Root (radix anterior) consists of efferent fibers, which are the axons of the nerve cells in the ventral part of the anterior and lateral columns. A short distance from their origins, these axons are invested by medullary sheaths and, passing forward, emerge in two or three irregular rows over an area which measures about 3 mm. in width.

  The Posterior Root (radix posterior) comprises some six or eight fasciculi, attached in linear series along the postero-lateral sulcus. It consists of afferent fibers which arise from the nerve cells in a spinal ganglion. Each ganglion cell gives off a single fiber which divides in a T-shaped manner into two processes, medial and lateral. The lateral processes extend to the sensory end-organs of the skin, muscles, tendons, joints, etc. (somatic receptors), and to the sensory end-organs of the viscera (visceral receptors). The medial processes of the ganglion cells grow into the medulla spinalis as the posterior roots of the spinal nerves.

  The posterior nerve root enters the medulla spinalis in three chief bundles, medial, intermediate, and lateral. The medial strand passes directly into the fasciculus cuneatus: it consists of coarse fibers, which acquire their medullary sheaths about the fifth month of intrauterine life; the intermediate strand consists of coarse fibers, which enter the gelatinous substance of Rolando; the lateral is composed of fine fibers, which assume a longitudinal direction in the tract of Lissauer, and do not acquire their medullary sheaths until after birth. In addition to these medullated fibers there are great numbers of non-medullated fibers which enter with the lateral bundle. They are more numerous than the myelinated fibers. They arise from the small cells of the spinal ganglia by T-shaped axons similar to the myelinated. They are distributed with the peripheral nerves chiefly to the skin, only a few are found in the nerves to the muscles. 

  Having entered the medulla spinalis, all the fibers of the posterior nerve roots divide into ascending and descending branches, and these in their turn give off collaterals which enter the gray substance. The descending fibers are short, and soon enter the gray substance. The ascending fibers are grouped into long, short, and intermediate: the long fibers ascend in the fasciculus cuneatus and fasciculus gracilis as far as the medulla oblongata, where they end by arborizing around the cells of the cuneate and gracile nuclei; the short fibers run upward for a distance of only 5 or 6 mm. and enter the gray substance; while the intermediate fibers, after a somewhat longer course, have a similar destination. All fibers entering the gray substance end by arborizing around its nerve cells or the dendrites of cells, those of intermediate length being especially associated with the cells of the dorsal nucleus.

 

Autonomic nuclei and nerves of brain and spinal cord

    

 The long fibers of the posterior nerve roots pursue an oblique course upward, being situated at first in the lateral part of the fasciculus cuneatus: higher up, they occupy the middle of this fasciculus, having been displaced by the accession of other entering fibers; while still higher, they ascend in the fasciculus gracilis. The upper cervical fibers do not reach this fasciculus, but are entirely confined to the fasciculus cuneatus. The localization of these fibers is very precise: the sacral nerves lie in the medial part of the fasciculus gracilis and near its periphery, the lumbar nerves lateral to them, the thoracic nerves still more laterally; while the cervical nerves are confined to the fasciculus cuneatus.

 

 

 

 

 

Theme 3. Base of the brain. Topography of 12 cranial nerves

 

The Brain [encephalon, cerebrum] during the fourth week develops from three primary cerebral vesicles: prosencephalon, mesencephalon and rhomboencephalon. Prosencephalon forms two secondary cerebral vesicles: telencephalon and diencephalon. Rhomboencephalon gives origin for medulla oblongata and metencephalon. Mesencephalon separates from rhomboencephalon by isthmus.

Metencephalon developes into pons and cerebellum. Midbrain comprises tectum and pedunculi cerebri. Diencephalon contains thalamus and hypothalamus. Telencephalon (forebrain) gives origin for rhinencephalon, basal nuclei of gray matter, pallium of hemispheres, corpus callosum and fornix.

 

Scheme showing the connections of the several parts of the brain.

 

The brain can be divided into two cerebral hemispheres, brain stem and cerebellum. A hemisphere has a base, dorsolateral and medial surfaces. 12 pairs of the cranial nerves exit from the encephalon base (see table below).

 

On the brain base student should find:

·        medulla oblongata

·        pons

·        medii cerebellar pedunculi

·        cerebral pedunculi, interpeduncular fos­sa lies between the two peduncles. Its floor is perforated by large numbers of blood vessels, the posterior perforated sub­stance

·        2 mammillary bodies are located forward from interpeduncular fos­sa

·        tuber cinereum with infundibulum that carries the hypophysis

·        optic tracts which join each other and form optic chiasm

·        optic nerve

·        olfactory triangles with anterior perforated sub­stance

·        olfactory tracts

·        olfactory bulbs

 

Cranial Nerves:

Cranial nerves on brain base

 

 

 

 

             Brain or Encephalon. General Considerations and Divisions.—The brain, is contained within the cranium, and constitutes the upper, greatly expanded part of the central nervous system. In its early embryonic condition it consists of three hollow vesicles, termed the hind-brain or rhombencephalon, the mid-brain or mesencephalon, and the fore-brain or prosencephalon; and the parts derived from each of these can be recognized in the adult.

Base of brain.

 

 

 Thus in the process of development the wall of the hind-brain undergoes modification to form the medulla oblongata, the pons, and cerebellum, while its cavity is expanded to form the fourth ventricle. The mid-brain forms only a small part of the adult brain; its cavity becomes the cerebral aqueduct (aqueduct of Sylvius), which serves as a tubular communication between the third and fourth ventricles; while its walls are thickened to form the corpora quadrigemina and cerebral peduncles. The fore-brain undergoes great modification: its anterior part or telencephalon expands laterally in the form of two hollow vesicles, the cavities of which become the lateral ventricles, while the surrounding walls form the cerebral hemispheres and their commissures; the cavity of the posterior part or diencephalon forms the greater part of the third ventricle, and from its walls are developed most of the structures which bound that cavity.

Schematic representation of the chief ganglionic categories (I to V).

 

 

The telencephalon includes: (1) the cerebral hemispheres with their cavities, the lateral ventricles; and (2) the pars optica hypothalami and the anterior portion of the third ventricle (already described under the diencephalon). As previously stated (see page 744), each cerebral hemisphere may be divided into three fundamental parts, viz., the rhinencephalon, the corpus striatum, and the neopallium. The rhinencephalon, associated with the sense of smell, is the oldest part of the telencephalon, and forms almost the whole of the hemisphere in some of the lower animals, e. g., fishes, amphibians, and reptiles. In man it is rudimentary, whereas the neopallium undergoes great development and forms the chief part of the hemisphere.

 

 

Sagittal dissection of the brain

The Cerebral Hemispheres.—The cerebral hemispheres constitute the largest part of the brain, and, when viewed together from above, assume the form of an ovoid mass broader behind than in front, the greatest transverse diameter corresponding with a line connecting the two parietal eminences. The hemispheres are separated medially by a deep cleft, named the longitudinal cerebral fissure, and each possesses a central cavity, the lateral ventricle.

  The Longitudinal Cerebral Fissure (fissura cerebri longitudinalis; great longitudinal fissure) contains a sickle-shaped process of dura mater, the falx cerebri. It front and behind, the fissure extends from the upper to the under surfaces of the hemispheres and completely separates them, but its middle portion separates them for only about one-half of their vertical extent; for at this part they are connected across the middle line by a great central white commissure, the corpus callosum.

  In a median sagittal section the cut corpus callosum presents the appearance of a broad, arched band. Its thick posterior end, termed the splenium, overlaps the mid-brain, but is separated from it by the tela chorioidea of the third ventricle and the pineal body. Its anterior curved end, termed the genu, gradually tapers into a thinner portion, the rostrum, which is continued downward and backward in front of the anterior commissure to join the lamina terminalis. Arching backward from immediately behind the anterior commissure to the under surface of the splenium is a second white band named the fornix: between this and the corpus callosum are the laminæ and cavity of the septum pellucidum.

 

Surfaces of the Cerebral Hemispheres.—Each hemisphere presents three surfaces: lateral, medial, and inferior.

  The lateral surface is convex in adaptation to the concavity of the corresponding half of the vault of the cranium. The medial surface is flat and vertical, and is separated from that of the opposite hemisphere by the great longitudinal fissure and the falx cerebri. The inferior surface is of an irregular form, and may be divided into three areas: anterior, middle, and posterior. The anterior area, formed by the orbital surface of the frontal lobe, is concave, and rests on the roof of the orbit and nose; the middle area is convex, and consists of the under surface of the temporal lobe: it is adapted to the corresponding half of the middle cranial fossa. The posterior area is concave, directed medialward as well as downward, and is named the tentorial surface, since it rests upon the tentorium cerebelli, which intervenes between it and the upper surface of the cerebellum.

Lateral surface of left cerebral hemisphere, viewed from above.

 

  These three surfaces are separated from each other by the following borders: (a) supero-medial, between the lateral and medial surfaces; (b) infero-lateral, between the lateral and inferior surfaces; the anterior part of this border separating the lateral from the orbital surface, is known as the superciliary border; (c) medial occipital, separating the medial and tentorial surfaces; and (d) medial orbital, separating the orbital from the medial surface. The anterior end of the hemisphere is named the frontal pole; the posterior, the occipital pole; and the anterior end of the temporal lobe, the temporal pole. About 5 cm. in front of the occipital pole on the infero-lateral border is an indentation or notch, named the preoccipital notch.

 

Cranial nerves on brain base

       

The surfaces of the hemispheres are moulded into a number of irregular eminences, named gyri or convolutions, and separated by furrows termed fissures and sulci. The furrows are of two kinds, complete and incomplete. The former appear early in fetal life, are few in number, and are produced by infoldings of the entire thickness of the brain wall, and give rise to corresponding elevations in the interior of the ventricle. They comprise the hippocampal fissure, and parts of the calcarine and collateral fissures. The incomplete furrows are very numerous, and only indent the subjacent white substance, without producing any corresponding elevations in the ventricular cavity.

 

 

 

Prepared by

Reminetskyy B.Y.