1. Limbic system. Structure of
white matter. Corpus callosum, fornix. Lateral
ventricles. Basal nuclei
2. Pathways of the brain and spinal cord
(associative, commissural, projective). Ascending pathways
3. Descending pathways: pyramidal, exrapyramidal.
Topography of pathways on section of spinal cord
Lesson No 23
Theme 1.
Limbic system. Structure of white
matter. Corpus callosum, fornix. Lateral ventricles.
Basal nuclei
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. It divides into
central and peripheral parts. The central
part includes the hippocampus, gyrus fornicatus (gyrus cinguli + gyrus
hyppocampi), gyrus dentatus, septum pellucidum and uncus. The peripheral part includes the olfactory
bulb, olfactory tract, olfactory trigone and anterior perforated substance.
Cortical smell analyzer located in the uncus. Rhinencephalon is
a center of emotional colouring of sensible perception of external environment
(Limbic system). Together from all
subcortical centers it is by energy source for cortex and answers for vitally
important man reactions regulates activity of internal organs: hunger feeling
and thirst, sounds perceptions and smells. Here are the memory mechanisms.
The rhinencephalon comprises the olfactory lobe, the uncus,
the subcallosal and supracallosal gyri, the fascia dentata
hippocampi, the septum pellucidum, the fornix, and the hippocampus.
1. The Olfactory Lobe (lobus olfactorius) is
situated under the inferior or orbital surface of the frontal lobe. In many vertebrates
it constitutes a well-marked portion of the hemisphere and contains an
extension of the lateral ventricle; but in man and some other mammals it is
rudimentary. It consists of the olfactory bulb and tract, the olfactory
trigone, the parolfactory area of Broca, and the anterior
perforated substance.
(a) The olfactory bulb (bulbus olfactorius) is an oval,
reddish-gray mass which rests on the cribriform plate of the ethmoid and forms
the anterior expanded extremity of the olfactory tract. Its
under surface receives the olfactory nerves, which pass upward through the
cribriform plate from the olfactory region of the nasal cavity.
Scheme of rhinencephalon.
(b) The olfactory tract (tractus
olfactorius) is a narrow white band, triangular on coronal section, the
apex being directed upward. It lies in the olfactory sulcus on the inferior
surface of the frontal lobe, and divides posteriorly into two striæ, a
medial and a lateral. The lateral stria is directed across the lateral
part of the anterior perforated substance and then bends abruptly medialward
toward the uncus of the hippocampal gyrus. The medial stria turns
medialward behind the parolfactory area and ends in the subcallosal gyrus; in
some cases a small intermediate stria is seen running backward to the
anterior perforated substance.
(c) The olfactory trigone (trigonum
olfactorium) is a small triangular area in front of the anterior perforated
substance. Its apex, directed forward, occupies the posterior part of the olfactory
sulcus, and is brought into view by throwing back the olfactory tract.
(d) The parolfactory area of Broca (area
parolfactoria) is a small triangular field on the medial surface of the
hemisphere in front of the subcallosal gyrus, from which it is separated by the
posterior parolfactory sulcus; it is continuous below with the olfactory
trigone, and above and in front with the cingulate gyrus; it is limited
anteriorly by the anterior parolfactory sulcus.
(e) The anterior perforated substance (substantia
perforata anterior) is an irregularly quadrilateral area in front of the
optic tract and behind the olfactory trigone, from which it is separated by the
fissure prima; medially and in front it is continuous with the
subcallosal gyrus; laterally it is bounded by the lateral stria of the
olfactory tract and is continued into the uncus. Its gray substance is
confluent above with that of the corpus striatum, and is perforated anteriorly
by numerous small bloodvessels.
2. The Uncus has already been described (page 826) as
the recurved, hook-like portion of the hippocampal gyrus.
3. The Subcallosal, Supracallosal, and Dentate
Gyri form a rudimentary arch-shaped lamina of gray substance extending over
the corpus callosum and above the hippocampal gyrus from the anterior
perforated substance to the uncus.
(a) The subcallosal gyrus (gyrus
subcallosus; peduncle of the corpus callosum) is a narrow lamina on the
medial surface of the hemisphere in front of the lamina terminalis, behind the
parolfactory area, and below the rostrum of the corpus callosum. It is
continuous around the genu of the corpus callosum with the supracallosal gyrus.
(b) The supracallosal gyrus (indusium
griseum; gyrus epicallosus) consists of a thin layer of gray substance in
contact with the upper surface of the corpus callosum and continuous laterally
with the gray substance of the cingulate gyrus. It contains two longitudinally
directed strands of fibers termed respectively the medial and lateral
longitudinal striæ. The supracallosal gyrus is prolonged around the
splenium of the corpus callosum as a delicate lamina, the fasciola cinerea,
which is continuous below with the fascia dentata hippocampi.
(c) The fascia dentata hippocampi (gyrus
dentatus) is a narrow band extending downward and forward above the
hippocampal gyrus but separated from it by the hippocampal fissure; its free
margin is notched and overlapped by the fimbria—the fimbriodentate fissure
intervening. Anteriorly it is continued into the notch of the uncus, where it forms
a sharp bend and is then prolonged as a delicate band, the band of
Giacomini, over the uncus, on the lateral surface of which it is lost.
The remaining parts of the rhinencephalon, viz., the septum
pellucidum, fornix, and hippocampus, will be described in connection with the
lateral ventricle.
White Matter of the Cerebrum. The
external
capsule located between putamen and claustrum. The extrema capsule separates
the claustrum and cortex of the insula.
The
corpus callosum connect right and left cerebral hemispheres.
Corpus callosum anteriorly carries a genu
that passes into rostrum. Last
continue as a lamina rostralis and lamina terminalis. Back part of the
corpus callosum called splemium.
Anterior fibers of the corpus callosum form the frontal forceps, posterior fibers of the corpus callosum - the occipital forceps.
The Corpus Callosum is the great transverse commissure which
unites the cerebral hemispheres and roofs in the lateral ventricles. A good
conception of its position and size is obtained by examining a median sagittal
section of the brain,
when it is seen to form an arched structure about
Corpus callosum from
above.
The anterior end is named the genu, and is
bent downward and backward in front of the septum pellucidum; diminishing
rapidly in thickness, it is prolonged backward under the name of the rostrum,
which is connected below with the lamina terminalis. The anterior cerebral
arteries are in contact with the under surface of the rostrum; they then arch
over the front of the genu, and are carried backward above the body of the
corpus callosum.
The posterior end is termed the splenium and
constitutes the thickest part of the corpus callosum. It overlaps the tela
chorioidea of the third ventricle and the mid-brain, and ends in a thick, convex,
free border. A sagittal section of the splenium shows that the posterior end of
the corpus callosum is acutely bent forward, the upper and lower parts being
applied to each other.
The superior surface is convex from before backward,
and is about
The inferior surface is concave, and forms on either
side of the middle line the roof of the lateral ventricle. Medially, this
surface is attached in front to the septum pellucidum; behind this it is fused
with the upper surface of the body of the fornix, while the splenium is in
contact with the tela chorioidea.
On either side, the fibers of the corpus callosum radiate in
the white substance and pass to the various parts of the cerebral cortex; those
curving forward from the genu into the frontal lobe constitute the forceps
anterior, and those curving backward into the occipital lobe, the forceps
posterior. Between these two parts is the main body of the fibers which
constitute the tapetum and extend laterally on either side into the
temporal lobe, and cover in the central part of the lateral ventricle.
The
fornix cerebri located under corpus callosum and has a body, columna fornicis (anteriorly) and crura fornicis (posteriorly). Crus fused
with the hippocampus and form the fimbria
hippocampi. Anterior commissura
positioned closly to the
columna fornicis.
The Fornix is a
longitudinal, arch-shaped lamella of white substance, situated below the corpus
callosum, and continuous with it behind, but separated from it in front by the
septum pellucidum. It may be described as consisting of two symmetrical bands,
one for either hemisphere. The two portions are not united to each other in
front and behind, but their central parts are joined together in the middle
line. The anterior parts are called the columns of the fornix; the
intermediate united portions, the body; and the posterior parts, the crura.
Diagram of the tracts
in the internal capsule. Motor tract red. The
sensory tract (blue) is not direct, but formed of neurons receiving impulses
from below in the thalamus and transmitting them to the cortex. The optic
radiation (occipitothalamic) is shown in violet.
The body (corpus fornicis) of the fornix is
triangular, narrow in front, and broad behind. The medial part of its upper
surface is connected to the septum pellucidum in front and to the corpus
callosum behind. The lateral portion of this surface forms part of the floor of
the lateral ventricle, and is covered by the ventricular epithelium. Its
lateral edge overlaps the choroid plexus, and is continuous with the epithelial
covering of this structure. The under surface rests upon the tela chorioidea of
the third ventricle, which separates it from the epithelial roof of that
cavity, and from the medial portions of the upper surfaces of the thalami.
Below, the lateral portions of the body of the fornix are joined by a thin
triangular lamina, named the psalterium (lyra).
This lamina contains some transverse fibers which connect the two hippocampi
across the middle line and constitute the hippocampal commissure.
Between the psalterium and the corpus callosum a horizontal cleft, the
so-called ventricle of the fornix (ventricle of Verga), is
sometimes found.
The columns (columna fornicis; anterior pillars;
fornicolumns) of the fornix arch downward in front of the interventricular
foramen and behind the anterior commissure, and each descends through the gray
substance in the lateral wall of the third ventricle to the base of the brain,
where it ends in the corpus mammillare. From the cells of the corpus mammillare
the thalamomammillary fasciculus (bundle of Vicq d’Azyr) takes
origin and is prolonged into the anterior nucleus of the thalamus. The column
of the fornix and the thalamomammillary fasciculus together form a loop
resembling the figure 8, but the continuity of the loop is broken in the corpus
mammillare. The column of the fornix is joined by the stria medullaris of the
pineal body and by the superficial fibers of the stria terminalis, and is said
to receive also fibers from the septum pellucidum. Zuckerkandl describes an olfactory
fasciculus which becomes detached from the main portion of the column of
the fornix, and passes downward in front of the anterior commissure to the base
of the brain, where it divides into two bundles, one joining the medial stria
of the olfactory tract; the other joins the subcallosal gyrus, and through it
reaches the hippocampal gyrus.
Diagram
of the fornix.
The fornix and corpus
callosum from below.
The crura (crus fornicis; posterior pillars)
of the fornix are prolonged backward from the body. They are flattened bands,
and at their commencement are intimately connected with the under surface of
the corpus callosum. Diverging from one another, each curves around the
posterior end of the thalamus, and passes downward and forward into the
inferior cornu of the lateral ventricle (750). Here it lies along the concavity of the hippocampus, on the surface of
which some of its fibers are spread out to form the alveus, while the
remainder are continued as a narrow white band, the fimbria hippocampi,
which is prolonged into the uncus of the hippocampal gyrus. The inner edge of
the fimbria overlaps the fascia dentata hippocampi (dentate gyrus)
(page 827), from which it is separated by the fimbriodentate fissure;
from its lateral margin, which is thin and ragged, the ventricular epithelium
is reflected over the choroid plexus as the latter projects into the chorioidal
fissure.
Interventricular Foramen (foramen of Monro).—Between the columns of the fornix and the
anterior ends of the thalami, an oval aperture is present on either side: this
is the interventricular foramen, and through it the lateral ventricles
communicate with the third ventricle. Behind the epithelial lining of the
foramen the choroid plexuses of the lateral ventricles are joined across the
middle line.
The Anterior Commissure (precommissure) is a
bundle of white fibers, connecting the two cerebral hemispheres across the
middle line, and placed in front of the columns of the fornix. On sagittal
section it is oval in shape, its long diameter being vertical and measuring
about
The Septum Pellucidum (septum lucidum) (720) is a thin, vertically placed partition consisting of two laminæ,
separated in the greater part of their extent by a narrow chink or interval,
the cavity of the septum pellucidum. It is attached, above, to the under
surface of the corpus callosum; below, to the anterior part of the fornix
behind, and the reflected portion of the corpus callosum in front. It is
triangular in form, broad in front and narrow behind;
its inferior angle corresponds with the upper part of the anterior commissure.
The lateral surface of each lamina is directed toward the body and anterior
cornu of the lateral ventricle, and is covered by the ependyma of that cavity.
The cavity of the septum pellucidum (cavum septi
pellucidi; pseudocele; fifth ventricle) is generally regarded as part of
the longitudinal cerebral fissure, which has become shut off by the union of
the hemispheres in the formation of the corpus callosum above and the fornix
below. Each half of the septum therefore forms part of the medial wall of the
hemisphere, and consists of a medial layer of gray substance, derived from that
of the cortex, and a lateral layer of white substance continuous with that of the
cerebral hemispheres. This cavity is not developed from the cavity of the
cerebral vesicles, and never communicates with the ventricles of the brain.
The septum
pellucidum is tightened between corpus callosum and fornix. It consists of
the 2 laminae and cavity between them.
Lateral ventricles
The two lateral ventricles are cavities situated in the lower and medial
parts of the cerebral hemispheres. They are separated from each other by a
median vertical partition, the septum
pellucidum, but communicate with the third ventricle and indirectly with
each other through the interventricular
foramen. Each lateral ventricle consists of a central part, and three
prolongations from it, termed anterior, posterior and inferior cornua.
The anterior horns passes forward into the frontal lobe. It
bordered:
*
medially - by lamina of septum pellucidum
*
laterally - by head of caudate nucleus
*
anteriorly and roof - by corpus callosum
The Lateral Ventricles (ventriculus
lateralis) are irregular cavities situated in the
lower and medial parts of the cerebral hemispheres, one on either side of the
middle line. They are separated from each other by a median vertical partition,
the septum pellucidum, but communicate with the third ventricle and
indirectly with each other through the interventricular foramen. They
are lined by a thin, diaphanous membrane, the ependyma, covered by
ciliated epithelium, and contain cerebrospinal fluid, which, even in health,
may be secreted in considerable amount. Each lateral ventricle consists of a central
part or body, and three prolongations from it, termed cornua.
The central part (pars centralis ventriculi
lateralis; cella) of the lateral ventricle extends from the
interventricular foramen to the splenium of the corpus callosum. It is an
irregularly curved cavity, triangular on transverse section, with a roof, a
floor, and a medial wall. The roof is formed by the under surface of the corpus
callosum; the floor by the following parts, enumerated in their order of
position, from before backward: the caudate nucleus of the corpus striatum, the
stria terminalis and the terminal vein, the lateral portion of the upper
surface of the thalamus, the choroid plexus, and the lateral part of the
fornix; the medial wall is the posterior part of the septum pellucidum, which
separates it from the opposite ventricle.
Central
part is found in parietal lobe. It is limited:
·
below - by body of caudate nucleus
and dorsal surface of the thalamus;
·
upwards
and laterally - by fibers of corpus callosum, which form a roof.
The anterior cornu (cornu anterius; anterior
horn; precornu) (Fig.
736) passes forward and lateralward, with a slight inclination downward,
from the interventricular foramen into the frontal lobe, curving around the
anterior end of the caudate nucleus. Its floor is formed by the upper surface
of the reflected portion of the corpus callosum, the rostrum. It is
bounded medially by the anterior portion of the septum pellucidum, and
laterally by the head of the caudate nucleus. Its apex reaches the posterior
surface of the genu of the corpus callosum.
Drawing of a cast of the ventricular
cavities, viewed from the side. (Retzius.)
The posterior cornu (cornu posterius; postcornu)
passes into the occipital lobe, its direction being backward and lateralward,
and then medialward. Its roof is formed by the fibers of the corpus callosum
passing to the temporal and occipital lobes. On its medial wall is a
longitudinal eminence, the calcar avis (hippocampus minor), which
is an involution of the ventricular wall produced by the calcarine fissure.
Above this the forceps posterior of the corpus callosum, sweeping around to
enter the occipital lobe, causes another projection, termed the bulb of the
posterior cornu. The calcar avis and bulb of the posterior cornu are
extremely variable in their degree of development; in some cases they are
ill-defined, in others prominent.
Central part and anterior and posterior
cornua of lateral ventricles exposed from above
The inferior cornu (cornu inferior; descending
horn; middle horn; medicornu) the largest of the three, traverses the
temporal lobe of the brain, forming in its course a curve around the posterior
end of the thalamus. It passes at first backward, lateralward, and downward,
and then curves forward to within 2.5 cm. of the apex of the temporal lobe, its
direction being fairly well indicated on the surface of the brain by that of
the superior temporal sulcus. Its roof is formed chiefly by the inferior
surface of the tapetum of the corpus callosum, but the tail of the caudate
nucleus and the stria terminalis also extend forward in the roof of the
inferior cornu to its extremity; the tail of the caudate nucleus joins the
putamen. Its floor presents the following parts: the hippocampus, the fimbria
hippocampi, the collateral eminence, and the choroid plexus. When the choroid
plexus is removed, a cleft-like opening is left along the medial wall of the
inferior cornu; this cleft constitutes the lower part of the choroidal fissure.
Coronal section through posterior cornua of lateral
ventricle
Posterior and inferior cornua of left lateral ventricle
exposed from the side
The
posterior horns localised in occipital lobes and cover by white
matter ‘tapetum’. They carry the bulb and the calcar avis on medial wall, and a
collateral triangle on the floor.
The inferior horns
are found in temporal lobe. They are boundered:
·
medially - by hippocampus;
·
below - by white matter, which forms
collateral eminence;
·
superolaterally - by white matter;
·
superomedially -
by a tail of caudate nucleus.
The
central part and temporal horn of lateral ventricle contain choroid plexus of lateral ventricle
generated of penetration pia mater by vessels. Choroid plexus passes to the III
ventricle through interventricular foramen. Choroid plexus takes part in
formation of larger half of cerebrospinal fluid.
A cerebrospinal fluid passes
from lateral ventricles through the interventricular foramen into third
ventricle, where its amount increases. Then it flows from third ventricle
through the cerebral aqueduct into fourth ventricle. In the fourth ventricle a
cerebrospinal fluid passes the subarachnoid space through the median (of
Magendie) and lateral (of Luschka) aperture and also to the central canal of
the spinal cord. Cerebrospinal fluid of the subarachnoid space returns to the
venous blood in the venous sinuses through the Pachioni’s arachnoid
granulation.
Basal nuclei
are the paired masses of gray matter located deep within the white matter in
base of the forebrain. Basal nuclei include 1) corpus striatum, 2) claustrum
and 3) nucleus amygdaloideus.
Corpus striatum
is composed of caudate nucleus (it
has a head, body and tail) and lentiform nucleus (it consists of medial and lateral globus
pallidus and putamen). There is a thick lamina of white substance,
the internal
capsule between caudate nucleus and globus pallidus. It has the
prominence of the curve is called the genu,
the frontal crus and the occipital crus. The occipital crus separates the lentiform nucleus from the thalamus and
carries optic and acoustic radiation.
The nucleus amygdaloideus is
an ovoid gray mass situated at the lower end of the inferior cornu of the
lateral ventricle (in temporal lobe).
The corpus striatum has
received its name from the striped appearance which a section of its anterior
part presents, in consequence of diverging white fibers being mixed with the gray
substance which forms its chief mass. A part of the corpus striatum is imbedded
in the white substance of the hemisphere, and is therefore external to the
ventricle; it is termed the extraventricular portion, or the lentiform
nucleus; the remainder, however, projects into the ventricle, and is named
the intraventricular portion, or the caudate nucleus.
The caudate nucleus (nucleus caudatus; caudatum)
is a pear-shaped, highly arched gray mass; its broad extremity, or head,
is directed forward into the anterior cornu of the lateral ventricle, and is
continuous with the anterior perforated substance and with the anterior end of
the lentiform nucleus; its narrow end, or tail, is directed backward on
the lateral side of the thalamus, from which it is separated by the stria
terminalis and the terminal vein. It is then continued downward into the roof
of the inferior cornu, and ends in the putamen near the apex of the temporal
lobe. It is covered by the lining of the ventricle, and crossed by some veins
of considerable size. It is separated from the lentiform nucleus, in the
greater part of its extent, by a thick lamina of white substance, called the
internal capsule, but the two portions of the corpus
striatum are united in front.
Horizontal section of
right cerebral hemisphere.
The lentiform nucleus
(nucleus lentiformis; lenticular nucleus; lenticula) is lateral to the caudate nucleus and thalamus, and is seen
only in sections of the hemisphere. When divided horizontally, it exhibits, to
some extent, the appearance of a biconvex lens.
while a coronal section of its central part presents a
somewhat triangular outline. It is shorter than the caudate nucleus and does
not extend as far forward. It is bounded laterally by a lamina of white substance
called the external capsule, and lateral to this is a thin layer of gray
substance termed the claustrum. Its anterior end is continuous with the
lower part of the head of the caudate nucleus and with the anterior perforated
substance.
In a coronal section through the middle of the lentiform
nucleus, two medullary laminæ are seen dividing it into three
parts. The lateral and largest part is of a reddish color, and is known as the putamen,
while the medial and intermediate are of a yellowish tint, and together
constitute the globus pallidus; all three are marked by fine radiating
white fibers, which are most distinct in the putamen.
The gray substance of the corpus striatum is traversed by
nerve fibers, some of which originate in it. The cells are multipolar, both
large and small; those of the lentiform nucleus contain yellow pigment. The
caudate and lentiform nuclei are not only directly continuous with each other
anteriorly, but are connected to each other by numerous fibers. The corpus
striatum is also connected: (1) to the cerebral cortex, by what are termed the corticostriate
fibers; (2) to the thalamus, by fibers which pass through the internal
capsule, and by a strand named the ansa lentiformis; (3) to the cerebral
peduncle, by fibers which leave the lower aspect of the caudate and lentiform
nuclei.
Coronal section through anterior cornua
of lateral ventricles.
The claustrum
is a thin layer of gray substance, situated on the lateral surface of the
external capsule. Its transverse section is triangular, with the apex directed
upward. Its medial surface, contiguous to the external capsule, is smooth, but
its lateral surface presents ridges and furrows corresponding with the gyri and
sulci of the insula, with which it is in close relationship. The claustrum is
regarded as a detached portion of the gray substance of the insula, from which
it is separated by a layer of white fibers, the capsula extrema (band
of Baillarger). Its cells are small and spindle-shaped, and contain yellow
pigment; they are similar to those of the deepest layer of the cortex.
The nucleus amygdalæ (amygdala) is an ovoid gray mass, situated at the lower end of the roof
of the inferior cornu. It is merely a localized thickening of the gray cortex,
continuous with that of the uncus; in front it is continuous with the putamen,
behind with the stria terminalis and the tail of the caudate nucleus.
The internal capsule (capsula interna) is a
flattened band of white fibers, between the lentiform nucleus on the lateral
side and the caudate nucleus and thalamus on the medial side. In horizontal
section it is seen to be somewhat abruptly curved, with its convexity inward;
the prominence of the curve is called the genu, and projects between the
caudate nucleus and the thalamus. The portion in front of the genu is termed
the frontal part, and separates the lentiform from the caudate nucleus; the
portion behind the genu is the occipital part, and separates the lentiform
nucleus from the thalamus.
Coronal section of brain through
anterior commissure.
The frontal part
of the internal capsule contains: (1) fibers running from the thalamus to the
frontal lobe; (2) fibers connecting the lentiform and caudate nuclei; (3)
fibers connecting the cortex with the corpus striatum; and (4) fibers passing
from the frontal lobe through the medial fifth of the base of the cerebral
peduncle to the nuclei pontis. The fibers in the region of the genu are named
the geniculate fibers; they originate in the motor part of the cerebral
cortex, and, after passing downward through the base of the cerebral peduncle
with the cerebrospinal fibers, undergo decussation and end in the motor nuclei
of the cranial nerves of the opposite side. The anterior two-thirds of the
occipital part of the internal capsule contains the cerebrospinal fibers,
which arise in the motor area of the cerebral cortex and, passing downward
through the middle three-fifths of the base of the cerebral peduncle, are
continued into the pyramids of the medulla oblongata. The posterior third of
the occipital part contains: (1) sensory fibers, largely derived from the
thalamus, though some may be continued upward from the medial lemniscus; (2)
the fibers of optic radiation, from the lower visual centers to the cortex of
the occipital lobe; (3) acoustic fibers, from the lateral lemniscus to the
temporal lobe; and (4) fibers which pass from the occipital and temporal lobes
to the nuclei pontis.
The fibers of the internal capsule radiate widely as they
pass to and from the various parts of the cerebral cortex, forming the corona
radiata (745) and intermingling with the fibers of the corpus callosum.
The external capsule (capsula externa) is a
lamina of white substance, situated lateral to the lentiform nucleus, between
it and the claustrum, and continuous with the internal capsule below and behind
the lentiform nucleus. It probably contains fibers derived from the thalamus,
the anterior commissure, and the subthalamic region.
The substantia
innominata of Meynert is a stratum consisting partly of gray and partly of
white substance, which lies below the anter
ior
part of the thalamus and lentiform nucleus. It consists of three layers,
superior, middle, and inferior. The superior layer is named the ansa
lentiformis, and its fibers, derived from the medullary lamina of the
lentiform nucleus, pass medially to end in the thalamus and subthalamic region,
while others are said to end in the tegmentum and red nucleus.
Dissection showing the course of the
cerebrospinal fibers.
The middle layer consists of nerve cells and nerve fibers; fibers
enter it from the parietal lobe through the external capsule, while others are
said to connect it with the medial longitudinal fasciculus. The inferior
layer forms the main part of the inferior stalk of the thalamus, and connects
this body with the temporal lobe and the insula.
The stria terminalis (tænia semicircularis)
is a narrow band of white substance situated in the depression between the
caudate nucleus and the thalamus. Anteriorly, its fibers are partly continued
into the column of the fornix; some, however, pass over the anterior commissure
to the gray substance between the caudate nucleus and septum pellucidum, while
others are said to enter the caudate nucleus. Posteriorly, it is continued into
the roof of the inferior cornu of the lateral ventricle, at the extremity of
which it enters the nucleus amygdalæ. Superficial to it is a large vein,
the terminal vein (vein of the corpus striatum), which receives
numerous tributaries from the corpus striatum and thalamus; it runs forward to
the interventricular foramen and there joins with the vein of the choroid
plexus to form the corresponding internal cerebral vein.
Theme 2.
Pathways of the brain and spinal cord (associative,
commissural, projective). Ascending pathways.
Pathways of the brain and spinal
cord subdivide
into: associative, commissural and projection fibers.
Associative pathways
are the tracts communicating functional areas of one hemisphere. They can be
divided into long associative fibres and short associative fibres. The long
associative pathways include:
1.
Superior longitudinal fascicle
communicates frontal, parietal and occipital lobes;
2.
Inferior longitudinal fascicle
communicates parietal, occipital, temporal lobes;
3.
Uncinate fascicle - communicates frontal,
parietal and temporal lobes;
4.
Fornicate fascicle - communicates
central areas of rhinencephalon;
To short associative
tracts are arcuate fibres of the
cerebrum, which communicate neighbouring convolutions within hemisphere.
associationis breves).
The association fibers unite different parts of
the same hemisphere, and are of two kinds: (1) those connecting adjacent gyri, short
association fibers; (2) those passing between more distant parts, long
association fibers.
The short association fibers lie immediately
beneath the gray substance of the cortex of the hemispheres, and connect
together adjacent gyri.
The long association fibers include the following:
(a) the uncinate fasciculus; (b) the cingulum; (c) the
superior longitudinal fasciculus; (d) the inferior longitudinal
fasciculus; (e) the perpendicular fasciculus; (f) the
occipitofrontal fasciculus; and (g) the fornix.
(a) The uncinate fasciculus passes across
the bottom of the lateral fissure, and unites the gyri of the frontal lobe with
the anterior end of the temporal lobe.
(b) The cingulum is a band of white matter
contained within the cingulate gyrus. Beginning in front at the anterior
perforated substance, it passes forward and upward parallel with the rostrum,
winds around the genu, runs backward above the corpus callosum, turns around
the splenium, and ends in the hippocampal gyrus.
(c) The superior longitudinal fasciculus
passes backward from the frontal lobe above the lentiform nucleus and insula;
some of its fibers end in the occipital lobe, and others curve downward and
forward into the temporal lobe.
Dissection of cortex and brain-stem showing
association fibers and island of Reil after removal of its superficial gray
substance
(d) The inferior longitudinal fasciculus
connects the temporal and occipital lobes, running along the lateral walls of
the inferior and posterior cornua of the lateral ventricle.
Deep dissection of cortex and brain-stem
(e) The perpendicular fasciculus runs
vertically through the front part of the occipital lobe, and connects the
inferior parietal lobule with the fusiform gyrus.
(f) The occipitofrontal fasciculus passes
backward from the frontal lobe, along the lateral border of the caudate
nucleus, and on the mesial aspect of the corona radiata; its fibers radiate in
a fan-like manner and pass into the occipital and temporal lobes lateral to the
posterior and inferior cornua. Déjerine regards the fibers of the
tapetum as being derived from this fasciculus, and not from the corpus
callosum.
(g) The fornix connects the hippocampal
gyrus with the corpus mammillare and, by means of the thalamomammillary
fasciculus, with the thalamus (see page 839). Through the fibers of the
hippocampal commissure it probably also unites the opposite hippocampal gyri.
The
Commissural pathways communicate
symmetric areas of both hemispheres of cerebrum and both halves of spinal cord
for co-ordination of their activity. There are:
1.
corpus callosum;
2.
anterior cerebral commissura;
3.
posterior cerebral commissura;
4.
habenular commissura ;
5.
fornicate commissura ;
6.
interthalamic adhesion;
7.
posterior
white commissura of spinal cord.
The
corpus callosum connects right and
left cerebral hemispheres. Corpus callosum anteriorly carries a genu that passes into rostrum. Last continue as a lamina rostralis and lamina terminalis. Back part of the
corpus callosum called splemium.
Anterior fibers of the corpus callosum form the frontal forceps, posterior fibers of the corpus callosum - the occipital forceps.
The Corpus Callosum is the great transverse commissure which
unites the cerebral hemispheres and roofs in the lateral ventricles. A good
conception of its position and size is obtained by examining a median sagittal
section of the brain,
when it is seen to form an arched structure about
Corpus callosum from above
The anterior end is named the genu, and is
bent downward and backward in front of the septum pellucidum; diminishing
rapidly in thickness, it is prolonged backward under the name of the rostrum,
which is connected below with the lamina terminalis. The anterior cerebral
arteries are in contact with the under surface of the rostrum; they then arch
over the front of the genu, and are carried backward above the body of the
corpus callosum.
The posterior end is termed the splenium and
constitutes the thickest part of the corpus callosum. It overlaps the tela
chorioidea of the third ventricle and the mid-brain, and ends in a thick,
convex, free border. A sagittal section of the splenium shows that the
posterior end of the corpus callosum is acutely bent forward, the upper and
lower parts being applied to each other.
The superior surface is convex from before backward,
and is about
The inferior surface is concave, and forms on either
side of the middle line the roof of the lateral ventricle. Medially, this
surface is attached in front to the septum pellucidum; behind this it is fused
with the upper surface of the body of the fornix, while the splenium is in
contact with the tela chorioidea.
On either side, the fibers of the corpus callosum radiate in
the white substance and pass to the various parts of the cerebral cortex; those
curving forward from the genu into the frontal lobe constitute the forceps
anterior, and those curving backward into the occipital lobe, the forceps
posterior. Between these two parts is the main body of the fibers which
constitute the tapetum and extend laterally on either side into the
temporal lobe, and cover in the central part of the lateral ventricle.
The Projecting pathways are the tracts, which communicate the cerebrum and
spinal cord with working organs. They subdivided into ascending (sensible) and
descending (motor). The sensible projecting tracts into its turn
subdivide into exteroreceptive, interoceptive and proprioreceptive. The motor
projecting fibers tracts are pyramidalі and extrapyramidal.
Sensory Pathways from the Spinal Cord to the Brain.—The posterior root fibers conducting the impulses of conscious muscle
sense, tendon sense and joint sense, those impulses which have to do with
the coördination and adjustment of muscular movements, ascend in the
fasciculus gracilis and fasciculus cuneatus to the nucleus gracilis and nucleus
cuneatus in the medulla oblongata.
In the nucleus gracilis and nucleus cuneatus synaptic
relations are found with neurons whose cell bodies are located in these nuclei
and whose axons pass by way of the internal arcuate fibers, cross in the
raphé to the opposite side in the region between the olives and turn
abruptly upward to form the medial lemniscus or medial fillet. The medial
fillet passes upward in the ventral part of the formatio reticularis through
the medulla oblongata, pons and mid-brain to the principal sensory nucleus of
the ventro-lateral region of the thalamus. Here the terminals form synapses
with neurons of the third order whose axons pass through the internal capsule
and corona radiata to the somatic sensory area of the cortex in the
post-central gyrus.
Fibers conducting the impulses of unconscious muscle
sense pass to the cerebellum partly by way of the fasciculus gracilis and
fasciculus cuneatus to the nucleus gracilis and nucleus cuneatus, thence
neurons of the second order convey the impulses either via the dorsal external
arcuate fibers directly into the inferior peduncle of the cerebellum or via the
ventral external arcuate fibers which are continued from the internal arcuate
fibers through the ventral part of the raphé and after crossing the
midline emerge on the surface of the medulla in the ventral sulcus between the
pyramids or in the groove between the pyramid and the olive. They pass over the
lateral surface of the medulla and olive to reach the inferior peduncle through
which they pass to the cerebellum.
Other fibers conducting impulses of unconscious muscle
sense pass upward in the dorsal spinocerebellar fasciculus, which arises from
cells in the nucleus dorsalis. The posterior root fibers conducting these
impulses pass into the fasciculus cuneatus and the collaterals from them to the
nucleus dorsalis are said to come almost exclusively from the middle area of
the fasciculus cuneatus. They form by their multiple division baskets about the
individual cells of the nucleus dorsalis, each fiber coming in relation with
the bodies and dendrites of several cells. The axons of the second order pass
into the dorsal spinocerebellar fasciculus of the same side and ascend along
the lateral surface of the spinal cord and medulla oblongata until they arrive
at the level of the olive, they then curve backward beneath the external
arcuate fibers into the inferior peduncle and pass into the cerebellum. Here
they give off collaterals to the dentate nucleus and finally terminate in the
cortex of the dorsal and superior portion of the vermis, partly on the same
side, but to a great extent by way of a large commissure to the opposite side.
The fibers lose their myelin sheaths as they enter the gray substance and
terminate by end ramifications among the nerve cells and their processes. Some
of the fibers are said to end in the nucleus dentatus and the roof nuclei of
the cerebellum (the nucleus globosus, nucleus emboliformis and nucleus
fastigius) and others pass through them to terminate in the inferior vermis. A
few fibers of the dorsal spinocerebellar fasciculus are said not to enter the
inferior peduncle but to pass with the ventral spinocerebellar fasciculus. The
cerebellar reflex are is supposed to be completed by the fibers of the superior
peduncle which pass from the cerebellum to the red nucleus of the mid-brain
where some of their terminals and collaterals form synapses with neurons whose
axons descend to the spinal cord in the rubrospinal fasciculus. The terminal
and collaterals of this fasciculus end either directly or indirectly about the
motor cells in the anterior column.
The ventral spinocerebellar fasciculus, since most of its
fibers pass to the cerebellum, is also supposed to be concerned in the
conduction of unconscious muscle sense. The location of its cells of origin is
uncertain. They are probably in or near the dorsal nucleus of the same and the
opposite side; various other locations are given, the dorsal column, the
intermediate zone of the gray matter and the central portion of the anterior
column. The neurons of the first order whose central fibers enter the
fasciculus cuneatus from the dorsal roots send collaterals and terminals to
form synapses with these cells. The fibers which come from the opposite gray
columns cross some in the white and some in the gray commissure and pass with
fibers from the same side through the lateral funiculus to the marginal region
ventral to the dorsal spinocerebellar fasciculus. The fasciculus begins about
the level of the third lumbar nerve and continues upward on the lateral surface
of the spinal cord and medulla oblongata until it passes under cover of the
external arcuate fibers. It passes just dorsal to the olive and above this
joins the lateral edge of the lateral lemniscus along which it runs, ventral to
the roots of the trigeminal nerve, almost to the level of the superior
colliculus, it then crosses over the superior peduncle, turns abruptly backward
along its medial border, enters the cerebellum with it and ends in the vermis
of the same and the opposite side. Some of its fibers are said to join the
dorsal spinocerebellar fasciculus in the medulla oblongata and enter the
cerebellum through the inferior peduncle. A number of fibers are said to
continue upward in the dorsolateral part of the tegmentum as far as the
superior colliculus and a few pass to the thalamus. They probably form part of
the sensory or higher reflex path.
The posterior root fibers conducting impulses of pain
and temperature probably terminate in the posterior column or the
intermediate region of the gray matter soon after they enter the spinal cord.
The neurons of the second order are supposed to pass through the anterior
commissure to the superficial antero-lateral fasciculus (tract of Gowers) and
pass upward in that portion of it known as the lateral spinothalamic
fasciculus. This fasciculus lies along the medial side of the ventral
spinocerebellar fasciculus. It is stated by some authors that the pain fibers
pass upward in the antero-lateral ground bundles. In some of the lower mammals
this pathway carries the pain fibers upward by a series of neurons some of
which cross to the opposite side, so that in part there is a double path. In
man, however, the lateral spinothalamic fasciculus is probably the most
important pathway. On reaching the medulla these fibers continue upward through
the formatio reticularis in the neighborhood of the median fillet to the
thalamus, probably its ventro-lateral region. Whether higher neurons convey the
pain impulses to the cortex through the internal capsule is uncertain. The
pathway is probably more complex and Head is of the opinion that our sensations
of pain are essentially thalamic. The pain and temperature pathways in the
lateral spinothalamic fasciculus are not so closely intermingled but that one
can be destroyed without injury to the other.
Ransom suggests that the non-medullated fibers of the posterior
roots, which turn into Lissauer’s tract and ascend or descend for short
distances not exceeding one or two segments and finally end in the substantia
gelatinosa, are in part at least pain fibers and that the fasciculus of
Lissauer and the substantia gelatinosa represent part of the mechanism for
reflexes associated with pain conduction and reception while the fibers to the
higher centers pass up in the spinothalamic tract.
The fibers of tactile discrimination, according to
Head and Thompson, pass up in the fasciculus cuneatus and fasciculus gracilis
of the same side and follow the path of the muscle-sense fibers. The axons of
the second order arising in the nucleus cuneatus and gracilis cross with the
internal arcuate fibers and ascend to the thalamus with the medial lemniscus,
thence by neurons of higher order the impulses are carried to the somatic
sensory area of the cortex through the internal capsule. The other touch
fibers, shortly after entering the spinal cord, terminate in the dorsal
column or intermediate gray matter. Neurons of the second order send their
axons through the anterior commissure to pass upward in the antero-lateral
funiculus probably in the ventral spinothalamic fasciculus. In the
medulla they join or pass upward in the neighborhood of the medial lemniscus to
the thalamus and thence by neurons of higher order to the somatic sensory area
of the cortex.
The remaining ascending fasciculi form a part of the
complex known as the superficial antero-lateral fasciculus (tract of
Gowers). The spinotectal fasciculus, as its name indicates, is
supposed to have its origin in the gray matter of the cord and terminations in
the superior and inferior (?) colliculi of the mid-brain serving for reflexes
between the cord and the visceral and auditory centers of the mid-brain.
The spino-olivary fasciculus (olivospinal;
bulbospinal, Helweg’s bundle) is likewise of unknown constitution and
function; there is uncertainty even in regard to the direction of its fibers.
Sympathetic afferent fibers (visceral
afferent; viscero-sensory; splanchnic afferent) enter the spinal cord by
the posterior roots of the thoracic and first two or three lumbar nerves and
the second to the fourth sacral nerves. The fibers pass to these nerves from
the peripheral sympathetic system through the white rami communicantes. Some of
the cell bodies of these afferent fibers are located in the spinal ganglia and
others are in the sympathetic ganglia. Some of the afferent sympathetic fibers
end about the cell bodies of somatic sensory neurons and visceral impulses are
thus transmitted to these neurons which conduct them as well as their own
special impulses to the spinal cord. Other sympathetic afferent neurons whose
cell bodies are located in the spinal ganglia send collaterals to neighboring
cells of somatic sensory neurons and thus have a double path of transmission to
the spinal cord. Such an arrangement provides a mechanism for some of the
referred pains.
To
sernsory
exteroreceptive (E)
tracts belong the tracts temperatural, palpable, pain sensivity, also vision,
hearing, olfaction. The proprioreceptive (P) tracts subdivided into
tracts of cortical direction (Goll tracts and Burdach) and cerebellar
directions (Flechsig and Gowers tracts). Anterior and lateral corticospinal
tracts and also corticonuclear tract belong to pyramidal pathways. The
extrapyramidal tracts include rubrospinal, vestibulospinal, reticulospinal,
olivospinal pathways.
(E)
Pain and temperature sensation pathway (Lateral
spinothalamic tract).
1.
Body of first neuron is found in spinal ganglion. The peripheral process
of this neuron terminates by exteroreceptor in skin, and central - on the
strength of posterior rootlets into posterior horns of spinal cord, where
contacts with second neuron.
2.
The second neuron localised in proper
nucleus of posterior horns of spinal cord. Its axons make a crossing in grey
commissura of spinal cord and pass in lateral funiculus (lateral spinothalamic
tract). Axons within the medial of lemniscus reach the thalamus, where
terminate by synapse with body of third neuron.
3.
The third neuron is in lateral
nucleus of thalamus, and its axons
‘thalamocortical tract’ pass to postcentral gyrus (cortical analyser of skin
sensation) through the back third of internal capsule.
(E) Pressure and touch
sensation pathway (Anterior
spinothalamic tract).
1.
Body of first neuron is found in spinal ganglion. The peripheral process of
this neuron terminates by exteroreceptor in skin, and central – in the
gelatinose substance of spinal cord, where contacts with second neuron.
2.
The second neuron axons make a
crossing in grey commissura of spinal cord and pass in lateral funiculus (anterior
spinothalamic tract). Axons reach the thalamus, where terminate by synapse with
body of third neuron.
3.
The third neuron is in thalamus, it passes to postcentral gyrus
(cortical analyser of skin sensation) through the back third of internal
capsule:
Proprioreceptive
pathway (cortical direction) - Bulbothalamic tract.
1.
The first neuron of this tract lies
in spinal ganglion. Its peripheral process terminates by proprioreceptor in
muscles, tendoms, jonts capsules and ligaments. The central process passeswith
posterior rootlets into spinal cord and form in white matter Goll tract -
fascicle (upper 11 segments). The process passes to medulla oblongata, where terminate by synapse with second neuron:
2.
The second neurons are in gracilis
and cuneate nuclei of medulla oblongata.
Axons this neuron form bulbothalamic tract, which composes a base of medial lemniscus.
Axons of second neuron cross (deccusation of lemniscus) and reach the thalamus, where terminate by synapse
with third neuron.
3.
The third neurons pass through the
internal capsule and form the thalamocortical tract:
(P)
Proprioreceptive pathways with cerebellar direction (Spinocerebellar tracts).
Posterior spinocerebellar tract
(Flechsig tract)
1.
The first neuron is in spinal ganglion. Its peripheral process terminates
by proprioreceptor and central with the posterior rootlets of spinal cord
passes to the grey matter.
2.
The second neuron lies in thoracic
nucleus of posterior horns (Clarc-Steiling). Its axons pass in lateral
funiculus. Then it reaches the cortex of the cerebellar vermis through the inferior cerebellar pedunculi and dentate nucleus. Nervous impulse passes
to the ruber nucleus:
Anterior spinocerebellar tract (Gowers tract). Two-neurons way.
This tract differs from previous by its second neuron, a body of which is found
in medial intermediate nucleus of spinal cord. Axons of the second neuron get
across and pass into lateral funiculus, reaching the superior cerebral velum.
They cross here and pass to the vermis and dentate nucleus. Nervous impulse
also passes to the ruber nucleus.
Theme 3.
Descending pathways: pyramidal, exrapyramidal. Topography of pathways on
section of spinal cord
Lateral
corticospinal (pyramidal) tract
1.
A Body of first neuron is found in gigantic
pyramidal cells (Bets) of top and middle third of precentral gyrus of the cortex. Axons pass through anterior
third part of internal capsule, pons, medulla oblongata; part of fibres gets
across forming the pyramids. The crossed fibres of first neuron pass in lateral
funiculus of the spinal cord ‘lateral corticospinal (pyramidal) tract’ and
terminate in anterior horns of spinal cord.
2.
The second neuron lies in motor
nucleus of anterior horns, and its axons pass with the anterior rootlets and spinal nerves as far as skeletal muscles
of the trunk and extremities:
The descending fasciculi which convey impulses from the
higher centers to the spinal cord and located in the lateral and ventral
funiculi.
The Motor Tract conveying voluntary impulses,
arises from the pyramid cells situated in the motor area of the cortex, the
anterior central and the posterior portions of the frontal gyri and the
paracentral lobule. The fibers are at first somewhat widely diffused, but as
they descend through the corona radiata they gradually approach each other, and
pass between the lentiform nucleus and thalamus, in the genu and anterior
two-thirds of the occipital part of the internal capsule; those in the genu are
named the geniculate fibers, while the remainder constitute the cerebrospinal
fibers; proceeding downward they enter the middle three-fifths of the base
of the cerebral peduncle. The geniculate fibers cross the middle line, and end
by arborizing around the cells of the motor nuclei of the cranial nerves. The
cerebrospinal fibers are continued downward into the pyramids of the medulla
oblongata, and the transit of the fibers from the medulla oblongata is effected
by two paths. The fibers nearest to the anterior median fissure cross the
middle line, forming the decussation of the pyramids, and descend in the
opposite side of the medulla spinalis, as the lateral cerebrospinal
fasciculus (crossed pyramidal tract). Throughout the length of the
medulla spinalis fibers from this column pass into the gray substance, to
terminate either directly or indirectly around the motor cells of the anterior
column. The more laterally placed portion of the tract does not decussate in
the medulla oblongata, but descends as the anterior cerebrospinal fasciculus
(direct pyramidal tract); these fibers, however, end in the anterior
gray column of the opposite side of the medulla spinalis by passing across in
the anterior white commissure. There is considerable variation in the extent to
which decussation takes place in the medulla oblongata; about two-thirds or
three-fourths of the fibers usually decussate in the medulla oblongata and the
remainder in the medulla spinalis.
The axons of the motor cells in the anterior column pass
out as the fibers of the anterior roots of the spinal nerves, along which the
impulses are conducted to the muscles of the trunk and limbs.
From this it will be seen that all the fibers of the
motor tract pass to the nuclei of the motor nerves on the opposite side of the
brain or medulla spinalis, a fact which explains why a lesion involving the
motor area of one side causes paralysis of the muscles of the opposite side of
the body. Further, it will be seen that there is a break in the continuity of
the motor chain; in the case of the cranial nerves this break occurs in the
nuclei of these nerves; and in the case of the spinal nerves, in the anterior
gray column of the medulla spinalis. For clinical purposes it is convenient to
emphasize this break and divide the motor tract into two portions: (1) a series
of upper motor neurons which comprises the motor cells in the cortex and
their descending fibers down to the nuclei of the motor nerves; (2) a series of
lower motor neurons which includes the cells of the nuclei of the motor
cerebral nerves or the cells of the anterior columns of the medulla spinalis
and their axiscylinder processes to the periphery.
The rubrospinal fasciculus arises from the large
cells of the red nucleus. The fibers cross the raphé of the mid-brain in
the decussation of Forel and descend in the formatio reticularis of the pons
and medulla dorsal to the medial lemniscus and as they pass into the spinal
cord come to lie in a position ventral to the crossed pyramidal tracts in the
lateral funiculus. The rubrospinal fibers end either directly or indirectly by
terminals and collaterals about the motor cells in the anterior column on the
side opposite from their origin in the red nucleus. A few are said to pass down
on the same side. Since the red nucleus is intimately related to the cerebellum
by terminals and collaterals of the superior peduncle which arises in the
dentate nucleus of the cerebellum, the rubrospinal fasciculus is supposed to be
concerned with cerebellar reflexes, complex motor coördinations necessary
in locomotion and equilibrium. The afferent paths concerned in these reflexes
have already been partly considered, namely, the dorsal and ventral
spinocerebellar fasciculi, and probably some of the fibers of the posterior
funiculi which reach the cerebellum by the inferior peduncle.
The tectospinal fasciculus arises from the
superior colliculus of the roof (tectum) of the mid-brain. The axons come from
large cells in the stratum opticum and stratum lemnisci and sweep ventrally
around the central gray matter of the aqueduct, cross the raphé in the
fountain decussation of Meynert and turn downward in the tegmentum in the
ventral longitudinal bundle. Some of the fibers do not cross in the
raphé but pass down on the same side; it is uncertain whether they come
from the superior colliculus of the same side or arch over the aqueduct from
the colliculus of the opposite side. The tectospinal fasciculus which comprises
the major part of the ventral longitudinal bundle passes down through the
tegmentum and reticular formation of the pons and medulla oblongata ventral to the
medial longitudinal bundle. In the medulla the two bundles are more or less
intermingled and the tectospinal portion is continued into the antero-lateral
funiculus of the spinal cord ventral to the rubrospinal fasciculus with which
some of its fibers are intermingled. Some of the fibers of the tectospinal
fasciculus pass through the red nucleus giving off collaterals to it, others
are given off to the motor nuclei of the cranial nerves and in the spinal cord
they terminate either directly or indirectly by terminals and collaterals among
the nuclei of the anterior column. Since the superior colliculus is an
important optic reflex center, this tract is probably concerned in optic
reflexes; and possibly also with auditory reflexes since some of the fibers of the
central auditory path, the lateral lemniscus, terminate in the superior
colliculus.
The vestibulospinal fasciculus (part of the
anterior marginal fasciculus or Loewenthal’s tract) situated chiefly
in the marginal part of the anterior funiculus is mainly derived from the cells
of the terminal nuclei of the vestibular nerve, probably Deiters’s and
Bechterew’s, and some of its fibers are supposed to come from the nucleus
fastigius (roof nucleus of the cerebellum). The latter nucleus is intimately
connected with Dieters’s and Bechterew’s nuclei. The vestibulospinal fasciculus
is concerned with equilibratory reflexes. Its terminals and collaterals end
about the motor cells in the anterior column. It extends to the sacral region
of the cord. Its fibers are intermingled with the ascending spinothalamic
fasciculus, with the anterior proper fasciculus and laterally with the
tectospinal fasciculus. Its fibers are supposed to be both crossed and
uncrossed. In the brain-stem it is associated with the dorsal longitudinal
bundle.
The pontospinal fasciculus (Bechterew)
arises from the cells in the reticular formation of the pons from the same and
the opposite side and is associated in the brain-stem with the ventral
longitudinal bundle. In the cord it is intermingled with the fibers of the
vestibulospinal fasciculus in the anterior funiculus. Not much is known about
this tract.
There are probably other descending fasciculi such as the
thalamospinal but not much is known about them.
Anterior corticospinal
(pyramidal) tract
1.
Major part of fibres of first neurons
of anterior corticospinal tract does not cross in pyramids of medulla
oblongata, and passes in anterior funiculus of the spinal cord. Axons cross in
grey commissura terminate by synapse with bodies of second neurons.first
2.
The second neuron lies in motor
nucleus of anterior horns, and its axons pass within the anterior rootlets and
spinal nerves as far as skeletal muscles of the trunk and extremities.
Corticonuclear tract
1.
The first neuron of this tract is
found in gigantic pyramidal cells of cortex in lower third of precentral gyrus.
Axons pass through the genu of internal capsule, base of cerebral peduncles and
terminate in motor nucleus of rhomboid fossa and midbrain, previously passing
across partially on opposite side.
2.
The second neuron lies in motor
nucleus of rhomboid fossa and midbrain of opposite side, and its axons with
cranial nerves pass as far as stripped muscles of the head and superficial neck
muscles:
Extrapyramidal
pathways transmit impulses providing muscles tone and
reflexes of equilibrium and execution of automated motions. They include
rubrospinal, tectospinal, vestibulospinal, reticulospinal and olivospinal
tracts.
Rubrospinal
tract
1.
The first neuron is found in red
nucleus (n. ruber). Its axons make tegmental deccusation and pass through the
base of cerebral peduncles, pons, and medulla oblongata. They form tract in
lateral funiculus and reach motor nuclei in anterior horns of spinal cord.
2.
The second neuron lies in motor
nucleus of anterior horns of spinal cord, and its axons on the strength of
anterior rootlets and spinal nerves reach trunk and extremities muscles.
3.
This
2-neuron tract is descending link for reflex link of unconscious motion
coordination. Spinocerebellar tracts (Flechsig and Gowers tracts) are the ascending
links for this regulation. Spinocerebellar tracts send impulses for rubrospinal
tracts through the intermediate link (from cerebellar vermis and dentate nuclei
to the ruber nuclei).
Corticopontocerebellar tract
(way of cortical correction of the cerebellum)
This
2-neuron tract starts from all lobes of cerebral hemispheres. Axons run through
the internal capsula to the proper pontini nuclei. Second neurons cross and
pass to the cerebellar vermis through the middle cerebellar pedunculi. Some
links directed also to dentate and ruber nuclei. This pathway materializes
cortical correction of unconscious motion co-ordination.
,
Prepared
by
Reminetskyy
B.Y.