Lesson 6
Spinal cord
Structure of the white and
gray matter
Brain
Meninges of the brain
The adult spinal cord measures approximately 45
cm (18 in.) in length and has a maximum width of roughly 14 mm (0.55 in.). The
posterior (dorsal) surface of the spinal cord bears a shallow longitudinal
groove, the posterior median sulcus. The anterior median fissure is a deeper groove along the anterior (ventral)
surface.
The amount of gray matter is greatest in
segments of the spinal cord that deal with the sensory and motor control of the
limbs. These areas are expanded, forming the enlargements of the spinal cord.
The cervical
enlargement
supplies nerves to the shoulder girdles and upper limbs; the lumbar enlargement provides innervation to structures
of the pelvis and lower limbs. Inferior to the lumbar enlargement, the spinal
cord becomes tapered and conical; this region is the conus medullaris. The filum terminale ("terminal thread"), a slender strand
of fibrous tissue, extends from the inferior tip of the conus medullaris. It
continues along the length of the vertebral canal as far as the second sacral
vertebra. There it provides longitudinal support to the spinal cord as a
component of the coccygeal ligament.
The amount of gray matter is greatest in
segments of the spinal cord that deal with the sensory and motor control of the
limbs. These areas are expanded, forming the enlargements of the spinal cord.
The cervical
enlargement
supplies nerves to the shoulder girdles and upper limbs; the lumbar enlargement provides innervation to structures
of the pelvis and lower limbs. Inferior to the lumbar enlargement, the spinal
cord becomes tapered and conical; this region is the conus medullaris. The filum terminale ("terminal thread"), a slender strand
of fibrous tissue, extends from the inferior tip of the conus medullaris. It
continues along the length of the vertebral canal as far as the second sacral
vertebra. There it provides longitudinal support to the spinal cord as a
component of the coccygeal ligament.
The amount of gray matter is greatest in
segments of the spinal cord that deal with the sensory and motor control of the
limbs. These areas are expanded, forming the enlargements of the spinal cord.
The cervical
enlargement
supplies nerves to the shoulder girdles and upper limbs; the lumbar enlargement provides innervation to structures
of the pelvis and lower limbs. Inferior to the lumbar enlargement, the spinal
cord becomes tapered and conical; this region is the conus medullaris. The filum terminale ("terminal thread"), a slender strand
of fibrous tissue, extends from the inferior tip of the conus medullaris. It
continues along the length of the vertebral canal as far as the second sacral
vertebra. There it provides longitudinal support to the spinal cord as a
component of the coccygeal ligament.
Figure 13-1 provides a series of sectional views that
demonstrate the variations in the relative mass of gray and white matter in the
cervical, thoracic, lumbar, and sacral regions of the spinal cord. The entire
spinal cord can be divided into 31 segments on the basis of the origins of the
spinal nerves. Each segment is identified by a letter and number designation,
as used in the identification of individual vertebrae. For example, C3,
the segment in the uppermost section of Figure 13-1, is the third cervical segment.
Every spinal segment is associated with a pair
of dorsal root
ganglia
(Figure 13-1) that contains the cell bodies of sensory
neurons. The dorsal roots, which contain the axons of these neurons, bring
sensory information into the spinal cord. A pair of ventral roots contains the axons of motor neurons that
extend into the periphery to control somatic and visceral effectors. On either
side, the dorsal and ventral roots of each segment pass between the vertebral
canal and the periphery at the intervertebral foramen between successive
vertebrae. The dorsal root ganglion lies between the pedicles of the adjacent
vertebrae. (You may wish to review the description of vertebral anatomy in
Chapter 7, Figure 7-18)
Distal to each dorsal root ganglion, the sensory
and motor roots are bound together into a single spinal nerve. Spinal nerves
are classified as mixed nerves because they contain both afferent (sensory)
and efferent (motor) fibers. There are 31 pairs of spinal nerves, each
identified by its association with adjacent vertebrae. For example, we may
speak of "cervical spinal nerves" or even "cervical nerves"
when we make a general reference to spinal nerves of the neck. When we indicate
specific spinal nerves, it is customary to give them a regional number, as
indicated in Figure 13-1. Each spinal nerve caudal to the first thoracic
vertebra takes its name from the vertebra immediately preceding it. Thus,
spinal nerve T1 emerges immediately caudal to vertebra T1, spinal nerve T2 follows vertebra T2,
and so forth.
The arrangement differs in the cervical region
because the first pair of spinal nerves, C1, passes between the
skull and the first cervical vertebra. For this reason, each cervical nerve
takes its name from the vertebra immediately following it. In other
words, cervical nerve C2 precedes vertebra C2, and
the same system is used for the rest of the cervical series. The transition
from one numbering system to another occurs between the last cervical and first
thoracic vertebrae. Because the spinal nerve lying between them has been
designated C8, there are only seven cervical vertebrae but eight cervical
nerves.
The spinal cord continues to enlarge and
elongate until an individual is approximately 4 years old. Up to that time,
enlargement of the spinal cord keeps pace with the growth of the vertebral
column. Throughout this period, the ventral and dorsal roots are very short,
and they enter the intervertebral foramina immediately adjacent to their spinal
segment. After age 4, the vertebral column continues to elongate, but the
spinal cord does not. This vertebral growth carries the dorsal roots and spinal
nerves farther and farther from their original positions relative to the spinal
cord. As a result, the dorsal and ventral roots gradually elongate, and the
correspondence between spinal segment and the vertebral segment is lost. For
example, in the adult the sacral segments of the spinal cord are at the level
of vertebrae L1-L2.
Because the adult spinal cord extends only to
the level of the first or second lumbar vertebra, the dorsal and ventral roots
of spinal segments L2 to S5 extend caudally, past the
inferior tip of the conus medullaris. When seen in gross dissection, the filum
terminale and the long ventral and dorsal roots resemble a horse's tail. As a
result, early anatomists called the complex the cauda equina (cauda, tail + equus, horse).
SPINAL CORD (MEDULLA)
The spinal cord
occupies the superior two-thirds of the vertebral canal (Fig. 18.1, Fig. 43.1).
It is continuous cranially with the medulla oblongata, and narrows caudally to
the conus medullaris, from whose apex a connective tissue filament, the filum
terminale, descends to the dorsum of the first coccygeal vertebral segment. The
cord extends from the upper border of the atlas to the junction between the
first and second lumbar vertebrae: its average length in European males is 45
cm, its weight approximately 30 g. (For dimensional data consult Barson &
Sands 1977.)
During
development, the vertebral column elongates more rapidly than the spinal cord,
so that there is an increasing discrepancy between the anatomical level of
spinal cord segments and their corresponding vertebrae. At stage 23, the
vertebral column and spinal cord are the same length, and the cord ends at the
last coccygeal vertebra: this arrangement continues until the third fetal
month. At birth, the spinal cord terminates at the lower border of the second
lumbar vertebra, and may sometimes reach the third lumbar vertebra. In the
adult, the spinal cord is said to terminate at the level of the disc between
the first and second lumbar vertebral bodies, which lies a little above the
level of the elbow joint when the arm is by the side, and also lies
approximately in the transpyloric plane (p. 1054). However, there is
considerable variation in the level at which the spinal cord ends. It may end
below this level in as many as 40% of subjects, or opposite the body of either
the first or second lumbar vertebra: very occasionally it ends as high as the
caudal third of twelfth thoracic or as low as the disc between the second and
third lumbar vertebrae. Its position rises slightly in vertebral flexion, and
there is some correlation with the length of the trunk, especially in females.
The spinal cord varies in transverse width, gradually tapering craniocaudally,
except at the levels of the enlargements. It is not cylindrical, being wider
transversely at all levels, especially in the cervical segments.
Fig. 43.1
A, Brain and spinal cord with attached spinal nerve roots and dorsal root
ganglia, photographed from the dorsal aspect. Note the fusiform cervical and
lumbar enlargements of the cord, and the changing
obliquity of the spinal nerve roots as the cord is descended. The cauda
equina is undisturbed on the right but has been spread out on the left to
show its individual components. B–D, Formation of typical spinal nerve,
ventral aspect. B, Cervical level; C, Thoracic level; D, Lumbar level. E,
Lower end of spinal cord, filum terminale and cauda equina exposed from
behind. The dura mater and the arachnoid have been opened and spread out. F,
Spinal cord segment showing mode of formation of a typical spinal nerve and
the gross relationships of the grey and white matter. (B–D, From Sobotta
2006.) |
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The cervical enlargement is the source of the large spinal nerves that supply
the upper limbs. It extends from the third cervical to the second thoracic segments, its maximum circumference (approximately 38 mm) is
in the sixth cervical segment. (A spinal cord segment provides the attachment
of the rootlets of a pair of spinal nerves.) The lumbar enlargement is the
source of the large spinal nerves that supply the lower limbs, and extends from
the first lumbar to the third sacral segments, the equivalent vertebral levels
being the ninth to twelfth thoracic vertebrae. Its greatest circumference
(approximately 35 mm) is near the lower part of the body of the twelfth thoracic
vertebra, below which it rapidly dwindles into the conus medullaris.
Fissures and sulci
extend along most of the external surface. An anterior median fissure and a
posterior median sulcus and septum almost completely separate the cord into
right and left halves, but they are joined by a commissural band of nervous
tissue which contains a central canal.
The anterior
median fissure extends along the whole ventral surface with an average depth of
3 mm, although it is deeper at caudal levels. It contains a reticulum of pia
mater. Dorsal to it is the anterior white commissure. Perforating branches of
the spinal vessels pass from the fissure to the commissure to supply the
central spinal region. The posterior median sulcus is shallower, and from it a
posterior median septum penetrates more than halfway into the cord, almost to
the central canal. The septum varies in anteroposterior extent from 4 to 6 mm,
and diminishes caudally as the canal becomes more dorsally placed and the cord
contracts.
A posterolateral
sulcus exists from 1.5 to 2.5 mm lateral to each side of the posterior median
sulcus. Dorsal roots (strictly rootlets) of spinal nerves enter the cord along
the sulcus. The white substance between the posterior median and posterolateral
sulcus on each side is the posterior funiculus. In cervical and upper thoracic
segments a longitudinal posterointermediate sulcus marks a septum dividing each
posterior funiculus into two large tracts: the fasciculus gracilis (medial) and
fasciculus cuneatus (lateral). Between the posterolateral sulcus and anterior
median fissure is the anterolateral funiculus. This is subdivided into anterior
and lateral funiculi by ventral spinal rootlets which pass through its
substance to issue from the surface of the cord. The anterior funiculus is
medial to, and includes, the emerging ventral rootlets, whilst the lateral
funiculus lies between the roots and the posterolateral sulcus. In upper
cervical segments, nerve rootlets emerge through each lateral funiculus to form
the spinal accessory nerve which ascends in the vertebral canal lateral to the
spinal cord and enters the posterior cranial fossa via the foramen magnum (Fig. 28.11).
The filum
terminale, a filament of connective tissue approximately 20 cm long, descends
from the apex of the conus medullaris. Its upper 15 cm, the filum terminale
internum, is continued within extensions of the dural and arachnoid meninges
and reaches the caudal border of the second sacral vertebra. Its final 5 cm,
the filum terminale externum, fuses with the investing dura mater, and then
descends to the dorsum of the first coccygeal vertebral segment. The filum is
continuous above with the spinal pia mater. A few strands of nerve fibres which
probably represent roots of rudimentary second and third coccygeal spinal
nerves adhere to its upper part. The central canal is continued into the filum
for 5–6 mm. A capacious part of the subarachnoid space surrounds the filum
terminale internum, and is the site of election for access to the CSF (lumbar
puncture).
The paired dorsal
and ventral roots of the spinal nerves are continuous with the spinal cord
(Fig. 43.1F; see also p. 754). They cross the subarachnoid space and traverse
the dura mater separately, uniting in or close to their intervertebral foramina
to form the (mixed) spinal nerves. Since the spinal cord is shorter than the
vertebral column, the more caudal spinal roots descend for varying distances
around and beyond the cord to reach their corresponding foramina. In so doing
they form a divergent sheaf of spinal nerve roots, the cauda equina, which is
gathered round the filum terminale in the spinal theca, mostly distal to the
apex of the cord.
Ventral spinal
roots contain efferent somatic and, at some levels, preganglionic sympathetic,
axons which extend from neuronal cell bodies in the ventral horns
and intermediolateral columns respectively. There are also afferent nerve
fibres in these roots. The rootlets comprising each ventral root emerge from
the anterolateral sulcus in groups over an elongated vertical elliptical area
(Fig. 43.1F). Dorsal spinal roots bear ovoid swellings, the spinal ganglia, one
on each root proximal to its junction with a corresponding ventral root in an
intervertebral foramen. Each root fans out into six to eight rootlets before
entering the cord in a vertical row in the posterolateral sulcus. Dorsal roots
are usually said to contain only afferent axons (both somatic and visceral)
which are the central processes of unipolar neurones in the spinal root ganglia,
but they may also contain a small number (3%) of efferent fibres and autonomic
vasodilator fibres.
Each ganglionic
neurone has a single short stem which divides into a medial (central) branch
which enters the spinal cord via a dorsal root, and a lateral (peripheral)
branch which passes peripherally to a sensory end organ. The central branch is
an axon while the peripheral one is an elongated dendrite (but when traversing
a peripheral nerve is, in general structural terms, indistinguishable from an
axon). The region of spinal cord associated with the emergence of a pair of
nerves is a spinal segment, but there is no actual surface indication of
segmentation. Moreover, the deep neural sources or destinations of radicular
fibres may lie far beyond the confines of the ‘segment’ so defined.
Spinal Meninges
The vertebral column and its surrounding
ligaments, tendons, and muscles isolate the spinal cord from the external
environment. The delicate neural tissues must also be defended against damaging
contacts with the surrounding bony walls of the vertebral canal. A series of
specialized membranes, the spinal meninges (meninx, membrane), provide
the necessary physical stability and shock absorption. Blood vessels branching
within these layers also deliver oxygen and nutrients to the spinal cord.
The
relationships among the spinal meninges are shown in Figure 13-2a. There are three meningeal layers: (1) the dura
mater, (2) the arachnoid, and (3) the pia mater. At the
foramen magnum of the skull, the spinal meninges are continuous with the
cranial meninges that surround the brain. (We shall discuss the cranial
meninges, which have the same three layers, in Chapter 14.)
Bacterial or viral infection can cause meningitis, or inflammation of the meningeal membranes.
Meningitis is dangerous because it can disrupt the normal circulatory and
cerebrospinal fluid supplies, damaging or killing neurons and glial cells in
the affected areas. Although an initial diagnosis may specify the meninges of
the spinal cord (spinal meningitis) or brain (cerebral meningitis),
in later stages the entire meningeal system is usually affected.
The Dura Mater
The tough, fibrous dura mater (dura, hard + mater, mother)
forms the outermost covering of the spinal cord (Figure 13-2a). The dense collagen fibers of the dura mater
are oriented along the longitudinal axis of the cord. Between the dura mater
and the walls of the vertebral canal lies the epidural space, which contains loose connective tissue, blood
vessels, and a protective padding of adipose tissue (Figure 13-2b).
The dura mater does not have extensive, firm
connections to the surrounding vertebrae. Longitudinal stability is provided by
localized attachment sites at either end of the vertebral canal. Cranially, the
outer layer of the dura mater fuses with the periosteum of the occipital bone
around the margins of the foramen magnum. Within the sacral canal, the dura
mater tapers from a sheath to a dense cord of collagen fibers that blend with
components of the filum terminale to form the coccygeal
ligament.
The coccygeal ligament continues along the
sacral canal, ultimately blending into the periosteum of the coccyx. Lateral
support for the dura mater is provided by loose connective tissue and adipose
tissue within the epidural space. In addition, the dura mater extends between
adjacent vertebrae at each intervertebral foramen, fusing with the connective
tissues that surround the spinal nerves.
Injecting an anesthetic into the epidural space
will affect only the spinal nerves in the immediate area of the injection. The
result is a temporary sensory and motor paralysis known as an epidural block. Epidural blocks in the lower lumbar or sacral
regions may be used to control pain during childbirth.
The Arachnoid
In most anatomical and histological
preparations, a narrow subdural space separates the dura mater from deeper
meningeal layers. It is likely, however, that in life no such space exists and
that the inner surface of the dura mater is in contact with the outer surface
of the arachnoid (arachne,
spider) (Figure 13-2b). The inner surface of the dura mater and the
outer surface of the arachnoid are covered by simple squamous epithelia. The
arachnoid includes the epithelium and the subarachnoid
space, which
contains the arachnoid trabeculae, a delicate network of collagen and
elastic fibers maintained by modified fibroblasts. The subarachnoid space is
filled with cerebrospinal fluid, which acts as a shock absorber as well as a
diffusion medium for dissolved gases, nutrients, chemical messengers, and waste
products.
The arachnoid membrane extends caudally as far
as the filum terminale, and the dorsal and ventral roots of the cauda equina
travel within the fluid-filled subarachnoid space. In adults, the withdrawal of
cerebrospinal fluid, a procedure known as a spinal tap, involves the insertion
of a needle into the subarachnoid space in the lower lumbar region (Figure
13-3a). This placement avoids the possibility of
damage to the spinal cord. Spinal taps are performed when CNS infection is
suspected or to diagnose severe back pain, headaches, disc problems, and some
types of strokes.
The Pia Mater
The subarachnoid space bridges the gap between
the arachnoid epithelium and the innermost meningeal layer, the pia mater (pia, delicate + mater, mother).
The meshwork of elastic and collagen fibers of the pia mater
are interwoven with those of the subarachnoid space. The blood vessels
servicing the spinal cord are found here. Unlike more superficial meninges, the
pia mater is firmly bound to the underlying neural tissue (Figure 13-2b).
Along the length of the spinal cord, paired denticulate
ligaments
extend from the pia mater through the arachnoid to the dura mater. Denticulate
ligaments, which originate along either side of the spinal cord (Figure 13-2b), prevent lateral (side-to-side) movement. The
dural connections at the foramen magnum and the coccygeal ligament prevent
longitudinal (superior/inferior) movement.
The spinal meninges accompany the dorsal and
ventral roots as they pass through the intervertebral foramina. As indicated in
the sectional view of Figure 13-2b, the meningeal membranes are continuous with
the connective tissues that surround the spinal nerves and their peripheral
branches.
CONCEPT CHECK QUESTIONS
1. Damage to which root of a spinal nerve would
interfere with motor function?
2. Where is the cerebrospinal fluid that surrounds
the spinal cord located?
Sectional Anatomy of the Spinal Cord
To understand the functional organization of the
spinal cord, you must become familiar with its sectional organization. The anterior
median fissure and the posterior median sulcus mark the division
between left and right sides of the spinal cord. The superficial white
matter contains large numbers of myelinated and unmyelinated axons. The gray
matter, dominated by the cell bodies of neurons, glial cells, and
unmyelinated axons, surrounds the narrow central canal and forms an H or
butterfly shape. The projections of gray matter toward the outer surface of the
spinal cord are called horns. Figure 13-4 presents a typical section through the spinal
cord.
Organization of Gray Matter
The cell bodies of neurons in the gray matter of
the spinal cord are organized into functional groups called nuclei. Sensory
nuclei receive and relay sensory information from peripheral receptors. Motor
nuclei issue motor commands to peripheral effectors. Although sensory and motor
nuclei appear rather small in transverse section, they may extend for a
considerable distance along the length of the spinal cord.
A frontal section along the length of the central
canal of the spinal cord will separate the sensory (posterior,
or dorsal) nuclei from the motor (anterior, or ventral) nuclei. The posterior
gray horns contain somatic and visceral sensory nuclei, whereas the anterior
gray horns contain somatic motor nuclei. The lateral gray horns, located only
in the thoracic and lumbar segments, contain visceral motor nuclei. The gray
commissures (commissura, a joining together) posterior to and anterior
to the central canal contain axons that cross from one side of the cord to the
other before they reach a destination within the gray matter.
Figure shows the relationship between the
function of a particular nucleus (sensory or motor) and its relative position
within the gray matter of the spinal cord. The nuclei within each gray horn are
also organized. For example, the anterior gray horns of the cervical
enlargement contain nuclei whose motor neurons control the muscles of the upper
limbs. On each side of the spinal cord, in medial to lateral sequence, are motor
nuclei that control (1) muscles that position the shoulder girdle, (2) muscles
that position the arm, (3) muscles that move the forearm and hand, and (4)
muscles that move the hand and fingers. Within each of these regions, the motor
neurons that control flexor muscles are grouped separately from those that
control extensor muscles. Because the spinal cord is so highly organized, we
can predict the muscles that will be affected by damage to a specific area of
gray matter.
Organization of White Matter
The white matter on each side of the spinal cord
can be divided into three regions called columns, or funiculi (Figure
13-4a). The posterior white columns lie between the
posterior gray horns and the posterior median sulcus. The anterior white
columns lie between the anterior gray horns and the anterior median fissure.
The anterior white columns are interconnected by the anterior white commissure.
The white matter between the anterior and posterior columns on each side makes
up the lateral white column.
Each column contains tracts whose axons share
functional and structural characteristics. A tract, or fasciculus (bundle), is
a bundle of axons in the CNS that are relatively uniform with respect to
diameter, myelination, and conduction speed. All the axons within a tract relay
the same type of information (sensory or motor) in the same direction. Short
tracts carry sensory or motor signals between segments of the spinal cord, and
longer tracts connect the spinal cord with the brain.
INTERNAL ORGANIZATION
In transverse section, the spinal cord is incompletely
divided into symmetrical halves by a dorsal (posterior) median septum and a
ventral (anterior) median sulcus (Fig. 18.1). It consists of an outer layer of
white matter and an inner core of grey matter; their relative sizes and
configuration vary according to level. The amount of grey matter reflects the
number of neurones present; it is proportionately largest in the cervical and
lumbar enlargements, which contain the neurones that innervate the limbs. The
absolute amount of white matter is greatest at cervical levels, and decreases
progressively at lower levels, because descending tracts shed fibres as they
descend and ascending tracts accumulate fibres as they ascend.
Fig. 18.1 Transverse
sections through the spinal cord at representative levels. Approximately
×5. (Figure enhanced by B Crossman.) |
|
A diminutive central canal, lined by columnar,
ciliated epithelium (ependyma) and containing cerebrospinal fluid (CSF),
extends the whole length of the spinal cord lying in the centre of the spinal
grey matter. Rostrally, the central canal extends into the caudal half of the
medulla oblongata and then opens into the fourth ventricle.
SPINAL GREY MATTER
In three dimensions, the spinal grey matter is shaped
like a fluted column (Fig.
43.1F). In transverse section the column is often described as being
‘butterfly-shaped’ or resembling the letter ‘H’ (Fig. 18.1). It consists of
four linked cellular masses, the right and left dorsal and ventral horns, that
project dorsolaterally and ventrolaterally towards the surface respectively.
The grey matter that immediately surrounds the central canal and unites the two
sides constitutes the dorsal and ventral grey commissures. The dorsal horn is
the site of termination of the primary afferent fibres that enter the cord via
the dorsal roots of spinal nerves. The tip of the dorsal horn is separated from
the dorsolateral surface of the cord by a thin fasciculus or tract (of
Lissauer) in which primary afferent fibres ascend and descend for a short
distance before terminating in the subjacent grey matter. The ventral horn
contains efferent neurones whose axons leave the spinal cord in ventral nerve
roots. A small intermediate, or lateral, horn is present at thoracic and upper
lumbar levels; it contains the cell bodies of preganglionic sympathetic
neurones.
Spinal grey matter (Fig. 18.2) is a complex mixture of
neuronal cell bodies, their processes and synaptic connections, neuroglia and
blood vessels. Neurones in the grey matter are multipolar. They vary in size
and features such as the length and the arrangement of their axons and
dendrites. Neurones may be intrasegmental, i.e. contained within a single
segment, or intersegmental, i.e. their ramifications spread through several
segments.
Fig. 18.2 Transverse
section of spinal cord at a midlumbar level. The larger motor neurones in the
ventral grey column are visibly grouped. Stained with cresyl fast violet. |
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Neuronal cell groups of the
spinal cord
Viewed from the perspective of its longitudinal
columnar organization, the grey matter of the spinal cord consists of a series
of discontinuous cell groupings associated with their corresponding segmentally
arranged spinal nerves. At any particular cross-sectional level these cell
groupings are often considered to correspond approximately with one or more of
ten cell layers, known as Rexed's laminae. These laminae are defined on the
basis of neuronal size, shape, cytological features and density and are
numbered in a dorsoventral sequence.
Laminae I–IV correspond to the dorsal part of the
dorsal horn, and are the main site of termination of cutaneous primary afferent
terminals and their collaterals. Many complex polysynaptic reflex paths
(ipsilateral, contralateral, intrasegmental and intersegmental) start from this
region, as also do many long ascending tract fibres which pass to higher
levels. Lamina I (lamina marginalis) is a very thin layer with an ill-defined
boundary at the dorsolateral tip of the dorsal horn. It has a reticular
appearance, reflecting its content of intermingling bundles of coarse and fine
nerve fibres. It contains small, intermediate and large neuronal somata, many
of which are fusiform in shape. The much larger lamina II consists of densely
packed small neurones, responsible for its dark appearance in Nissl-stained
sections. With myelin stains, lamina II is characteristically distinguished
from adjacent laminae by the almost total lack of myelinated fibres. Lamina II
corresponds to the substantia gelatinosa. Lamina III consists of somata which
are mostly larger, more variable and less closely packed than those in lamina
II. It also contains many myelinated fibres. Some workers consider that the
substantia gelatinosa contains part or all of lamina III as well as lamina II.
The ill-defined nucleus proprius of the dorsal horn corresponds to some of the
cell constituents of laminae III and IV. Lamina IV is a thick, loosely packed,
heterogeneous zone permeated by fibres. Its neuronal somata vary considerably
in size and shape, from small and round, through intermediate and triangular,
to very large and stellate.
Laminae V and VI lie at the base of the dorsal horn.
They receive most of the terminals of proprioceptive primary afferents, profuse
corticospinal projections from the motor and sensory cortex and input from
subcortical levels, suggesting their involvement in the regulation of movement.
Lamina V is a thick layer, divisible into a lateral third and medial
two-thirds. Both have a mixed cell population but the former contains many
prominent well-staining somata interlaced by numerous bundles of transverse,
dorsoventral and longitudinal fibres. Lamina VI is most prominent in the limb
enlargements. It has a densely staining medial third of
small, densely packed neurones and a lateral two-thirds containing larger, more
loosely packed, triangular or stellate somata.
Laminae VII–IX show a variety
of forms in the limb enlargements. Lamina VII includes much of the intermediate
(lateral) horn. It contains prominent neurones of Clarke's column (nucleus
dorsalis, nucleus thoracis, thoracic nucleus) and
intermediomedial and intermediolateral cell groupings (Fig. 18.3). The lateral
part of lamina VII has extensive ascending and descending connections with the
midbrain and cerebellum (via the spinocerebellar, spinotectal, spinoreticular,
tectospinal, reticulospinal and rubrospinal tracts) and is thus involved in the
regulation of posture and movement. Its medial part has numerous propriospinal
reflex connections with the adjacent grey matter and segments concerned both
with movement and autonomic functions. Lamina VIII spans the base of the
thoracic ventral horn but is restricted to its medial aspect in limb
enlargements. Its neurones display a heterogeneous mixture of sizes and shapes
from small to moderately large. Lamina VIII is a mass of propriospinal
interneurones. It receives terminals from the adjacent laminae, many
commissural fibres from the contralateral lamina VIII, and descending
connections from the interstitiospinal, reticulospinal and vestibulospinal
tracts and the medial longitudinal fasciculus. The axons from these
interneurones influence α motor neurone activity bilaterally, perhaps
directly but more probably by excitation of small γ motor neurones
supplying efferent fibres to muscle spindles. Lamina IX is a complex array of
cells consisting of α and γ motor neurones and many interneurones.
The large α motor neurones supply motor end-plates of extrafusal muscle
fibres in striated muscle. Recording techniques have demonstrated tonic and
phasic α motor neurones. The former have a lower rate of firing and lower
conduction velocity and tend to innervate type S muscle units. The latter have
higher conduction velocity and tend to supply fast twitch (type FR, FF) muscle
units. The smaller γ motor neurones give rise to small-diameter efferent
axons (fusimotor fibres), which innervate the intrafusal muscle fibres in
muscle spindles. There are several functionally distinct types of γ motor
neurone. The ‘static’ and ‘dynamic’ responses of muscle spindles have separate
controls mediated by static and dynamic fusimotor fibres, which are distributed
variously to nuclear chain and nuclear bag fibres.
Lamina X surrounds the central canal and consists of
the dorsal and ventral grey commissures.
Dorsal horn
The dorsal horn is a major zone of termination of
primary afferent fibres, which enter the spinal cord through the dorsal roots
of spinal nerves. Dorsal root fibres contain numerous molecules, which are
either known, or suspected, to fulfil a neurotransmitter or neuromodulator
role. These include glutamic acid, substance P, calcitonin gene-related peptide
(CGRP), bombesin, vasoactive intestinal polypeptide (VIP), cholecystokinin
(CCK), somatostatin, dynorphin and angiotensin II. Dorsal root afferents carry
exteroceptive, proprioceptive and interoceptive information. Laminae I–IV are
the main cutaneous receptive areas; lamina V receives fine afferents from the
skin, muscle and viscera; lamina VI receives proprioceptive and some cutaneous
afferents. Most, if not all, primary afferent fibres divide into ascending and
descending branches on entering the cord. These then travel for variable
distances in the tract of Lissauer, near the surface of the cord, and send
collaterals into the subjacent grey matter. The formation, topography and
division of dorsal spinal roots have all been confirmed in man.
The lamina marginalis is a thin lamina of neurones at
the dorsolateral tip of the dorsal horn, deep to the tract of Lissauer. Beneath
it lies the substantia gelatinosa (laminae II and III), which is present at all
levels, and consists mostly of small neurones, together with some larger
neurones. The substantia gelatinosa receives afferents via the dorsal roots, and its neurones give
rise to fibres that form the contralateral spinothalamic tract. The large
propriospinal neurones of the nucleus proprius lie ventral to the substantia
gelatinosa; they link segments for the mediation of intraspinal coordination
(Fig. 18.3).
Fig. 18.3 The groups
of nerve cells in the grey columns of the spinal cord. The relative positions
of these columnar groups and their extent through spinal segments are
indicated. |
|
Clarke's column lies at the base of the dorsal horn.
At most levels, it is near the dorsal white funiculus and may project into it.
In the human spinal cord, it can usually be identified from the eighth cervical
to the third or fourth lumbar segments. Neurones of Clarke's column vary in
size, but most are large, especially in the lower thoracic and lumbar segments.
Some send axons into the dorsal spinocerebellar tracts and others are
interneurones.
Lateral horn
The lateral horn is a small lateral projection of grey
matter located between the dorsal and ventral horns. It is present from the
eighth cervical or first thoracic segment to the second or third lumbar segment.
It contains the cell bodies of preganglionic sympathetic neurones. These
develop in the embryonic cord dorsolateral to the central canal and migrate
laterally, forming intermediomedial and intermediolateral cell columns. Their
axons travel via ventral spinal roots and white rami communicantes to the
sympathetic trunk. A similar cell group is found in the second to fourth sacral
segments, but unlike the thoracolumbar lateral cell column, it does not form a
visible lateral projection. It is the source of the sacral outflow of
parasympathetic preganglionic nerve fibres.
Ventral horn
Neurones in the ventral horn vary in size. The largest
cell bodies, which may exceed 25 μm in diameter, are those of α motor
neurones, the axons of which emerge in ventral roots to innervate extrafusal
fibres in striated skeletal muscles. Large numbers of smaller neurones, 15–25
μm in diameter, are also present. Some of these are γ motor neurones,
which innervate intrafusal fibres of muscle spindles, and the rest are interneurones.
Motor neurones utilize acetylcholine as their neurotransmitter.
Considered longitudinally, ventral horn neurones are
arranged in elongated groups, and form a number of separate columns, which
extend through several segments. These are seen most easily in transverse
sections. The ventral horn may be divided into medial, central and lateral cell
columns, which all exhibit subdivision at certain levels, usually into dorsal
and ventral parts (Fig. 18.3). The medial group extends throughout the cord, but
may be absent in the fifth lumbar and first sacral segments. In the thoracic
and the upper four lumbar segments, it is subdivided into ventromedial and
dorsomedial groups. In segments cranial and caudal to this region, the medial
group has only a ventromedial moiety, except in the first cervical segment,
where only the dorsomedial group exists.
The central group of cells is the least extensive, and
is found only in some cervical and lumbosacral segments. The centrally situated
phrenic nucleus, containing the motor neurones that innervate the diaphragm,
lies in the third to seventh cervical segments. An irregular accessory group of
neurones in the upper five or six cervical segments at the ventral border of
the ventral horn give rise to axons that are thought to enter the spinal
accessory nerve (Fig. 18.3).
The lateral group of cells in the ventral horn is
subdivided into ventral, dorsal and retrodorsal groups, largely confined to the
spinal segments which innervate the limbs. The nucleus of Onuf, which is thought
to innervate the perineal striated muscles, is a ventrolateral group of cells
in the first and second sacral segments.
The motor neurones of the ventral horn are
somatotopically organized. The basic arrangement is that medial cell groups
innervate the axial musculature, and lateral cell groups innervate the limbs.
The basic building block of the somatic motor neuronal populations is
represented by a longitudinally disposed group of neurones, which innervate a
given muscle, and in which the α and γ motor neurones are intermixed.
The various groups innervating different muscles are aggregated into two major
longitudinal columns, medial and lateral. In transverse section these form the
medial and lateral cell groups in the ventral horn (Fig. 18.4).
Fig. 18.4 The
approximate location of motor cell groups at C8 segmental level of the spinal
cord. |
|
The medial longitudinal motor column extends
throughout the length of the spinal cord. Its neurones innervate epaxial and
hypaxial muscle groups. Basically, epaxial muscles include the erector spinae
group (which extend the head and vertebral column), while hypaxial muscles
include prevertebral muscles of the neck, intercostal and anterior abdominal
wall muscles (which flex the neck and the trunk). The epaxial muscles are
innervated by branches of the dorsal primary rami of the spinal nerves, and the
hypaxial muscles by branches of the ventral primary rami. In the medial column,
motor neurones supplying epaxial muscles are sited ventral to those supplying
hypaxial muscles.
The lateral longitudinal motor column is found only in
the enlargements of the spinal cord. The motor neurones in this column in the
cervical and lumbar enlargements innervate muscles of the upper and lower
limbs, respectively. In the cervical enlargement, motor neurones which supply
muscles intrinsic to the upper limb are situated dorsally in the ventral grey
column, and those innervating the most distal (hand) muscles are sited further
dorsally. Motor neurones of the girdle muscles lie in the ventrolateral part of
the ventral horn. There is a further somatotopic organization in that the
proximal muscles of the limb are supplied from motor cell groups located more
rostrally in the enlargement than those supplying the distal muscles. For
example, motor neurones innervating intrinsic muscles of the hand are sited in
segments C8 and T1, while motor neurones of shoulder muscles are in segments C5
and 6. A similar overall arrangement of motor neurones innervating lower limb
muscles applies in the lumbosacral cord (Fig. 18.5).
Fig. 18.5 The
segmental arrangement of motor neurones innervating muscles of the lower
limb. |
|
The main afferent connections to motor neurones are:
direct monosynaptic connections from proprioceptive dorsal root afferents in
the same or nearby segments; connections from axonal collaterals of dorsal horn
and other interneurones; direct monosynaptic connections from the
vestibulospinal and corticospinal tracts.
The intrinsic connections of the spinal cord and brain
stem subserve a number of reflexes by which the functions of peripheral
structures are modulated in response to afferent information in a relatively
automatic or autonomous fashion. The fundamental components of such reflex
‘arcs’ are, thus, an afferent and an efferent neurone. However, in all but the
simplest of reflexes, interneurones intervene between the afferent and efferent
components, conferring increased versatility and complexity on reflex
responses. Reflexes, by their very nature, are relatively fixed and stereotyped
in form. Nevertheless, they are strongly influenced and modulated by descending
connections. In the case of spinal reflexes these descending controls come from
both the brain stem and the cerebral cortex. Pathology of descending
supraspinal pathways commonly causes abnormalities of spinal reflex activity,
which are routinely tested for in neurological examination. During development,
descending control mechanisms suppress what may be regarded as ‘primitive’
spinal reflex responses, such as the extensor plantar reflex and the grasp
reflex. When the higher control mechanisms become damaged, these reflexes are
released and reappear as a sign of CNS pathology (e.g. the Babinski reflex).
Stretch reflex
The stretch reflex is the mechanism by which stretch
applied to a muscle elicits its reflex contraction. It is essential for the
maintenance of both muscle tone and an upright stance (via the innervation of
the postural muscles of the neck, back and lower limbs). Anatomically it is the
simplest of reflexes, since it is mediated solely by an afferent and an
efferent neurone. The afferent component arises from stretch receptors
associated with intrafusal muscle fibres located within muscle spindles. The
primary or annulospiral endings of these receptive cells give rise to primary
afferent fibres which enter the spinal cord, where they make excitatory
synaptic contact directly onto α motor neurones innervating the same
muscle (Fig. 18.6). The α motor neurones of antagonistic muscles are
simultaneously inhibited via collateral connections to inhibitory
interneurones.
Fig. 18.6 The stretch
reflex. |
|
Gamma reflex
As well as α motor neurones innervating
extrafusal muscle fibres, muscles also receive γ motor neurones, which
innervate intrafusal muscle fibres. Activation of γ motor neurones
increases the sensitivity of the intrafusal fibres to stretch (Fig. 18.7).
Therefore, changes in γ activity have a profound effect upon the stretch
reflex and upon muscle tone. Like α motor neurones, γ motor neurones
are under the influence of descending pathways from the brain stem and cerebral
cortex. Changes in the activity of the stretch reflex and of muscle tone are
commonly found in disorders of the CNS as well as the PNS.
Fig. 18.7 The gamma
reflex. |
|
Painful stimulation of the limbs leads to flexion
withdrawal, that is mediated by a polysynaptic reflex (Fig. 18.8) in which
interneurones link the afferent and efferent neurones. Thus, activation of
nociceptive primary afferents indirectly causes activation of limb flexor motor
neurones. Collateralization of fibres to nearby spinal segments mediates
flexion of a limb at several joints, depending on the intensity of the stimulus.
Decussating connections to the contralateral side of the cord activate α
motor neurones innervating corresponding extensor muscles, which produces the
so-called crossed extensor reflex. In principle, virtually any cutaneous
stimulus has the potential to induce a flexor reflex, but, other than in the
case of noxious stimuli, this response is normally inhibited by descending
pathways. When descending influences are lost, even harmless cutaneous
stimulation can elicit flexion of the limbs. The Babinski (extensor plantar)
reflex, which is generally regarded as pathognomonic of damage to the
corticospinal tract, is part of a flexion withdrawal of the
lower limb in response to stimulation of the sole of the foot.
Fig. 18.8 The flexor
reflex and crossed extensor reflex. |
|
Ascending tracts carry sensory information toward the brain, and
descending tracts convey motor commands into the spinal cord.
CONCEPT CHECK QUESTIONS
1. A person with polio has lost the use of his leg
muscles. In what area of the spinal cord would you expect the virally infected
motor neurons to be in this individual?
2. What portion of the spinal cord would be
affected by a disease that damages myelin sheaths?
Brain
Meninges
of the brain
Our perceptions of the world around
us depend on thousands of interactions among neurons within the central nervous
system. We seldom realize how complex these processes are unless they go wrong
in some way. For example, a child with dyslexia has a condition characterized
by difficulties with the recognition and use of words. Although the cause of
dyslexia remains a mystery, there is general agreement that it results from
problems with the integration and processing of visual or auditory information.
There is much that we still do not understand about such activities, which
ultimately create our consciousness and our unique personalities. This chapter
introduces the brain regions involved in our conscious and subconscious thought
processes and considers complex neural functions, such as memory and learning.
The adult human brain contains almost 98 percent of
the body's neural tissue. A "typical" brain weighs
To introduce the organization of the adult brain, we
will consider its embryological origins. The development of the brain is
detailed in the Embryology Summary on pages 454-455. The CNS begins as a hollow
neural tube with a fluid-filled internal cavity called the neurocoel.
In the cephalic portion of the neural tube, three areas enlarge rapidly through
expansion of the neurocoel. This enlargement creates three prominent divisions
called primary brain vesicles. The primary brain vesicles are named for their
relative positions: the prosencephalon (proso, forward + enkephalos,
brain), or "forebrain"; the mesencephalon (mesos, middle), or
"midbrain"; and the rhombencephalon, or "hindbrain."
The fate of the three primary divisions of the brain is
summarized in Table 14-1. The prosencephalon and rhombencephalon are subdivided
further, forming secondary brain vesicles. The prosencephalon forms the telencephalon (telos, end) and the diencephalon (dia, through). The tel-encephalon will
ultimately form the cerebrum of the adult brain. The mesencephalon
thickens, and the neurocoel becomes a relatively narrow passageway comparable
to the central canal of the spinal cord. The portion of the rhombencephalon
adjacent to the mesencephalon forms the metencephalon (meta,
after). The dorsal portion of the metencephalon will become the cerebellum,
and the ventral portion will develop into the pons. The portion of the
rhombencephalon closer to the spinal cord forms the myelencephalon (myelon, spinal cord), which will become
the medulla oblongata.
A Preview of Major Regions and Landmarks
The adult brain is dominated in size by the cerebrum:
Second in size to the cerebrum is the
cerebellum:
The other major regions of the brain can best be
examined after the cerebral hemispheres have been removed (Figure 14-2).
The diencephalon is a structural and functional link
between the cerebral hemispheres and the components of the brain stem.
The brain stem includes the mesencephalon, pons, and medulla
oblongata. (Some sources consider the brain stem to include the diencephalon.
We will use the more restrictive definition here). It contains a variety of
important processing centers and nuclei that relay information headed to or
from the cerebrum or cerebellum:
The boundaries and general functions of the diencephalon
and brain stem are listed in Figure 14-2.
CONCEPT CHECK QUESTIONS
1. What are the three primary brain vesicles, and
what does each contribute to the structure of the adult brain?
2. In response to a loud noise, your head
automatically turns toward the source of the sound. What part of the brain directs this response?
During development, the neurocoel within the cerebral
hemispheres, diencephalon, metencephalon, and medulla oblongata expands to form
chambers called ventricles. The ventricles are lined
by cells of the ependyma.
Each cerebral hemisphere contains an enlarged
ventricle. A thin medial partition, the septum pellucidum, separates this pair
of lateral ventricles. There is no direct connection between the two lateral
ventricles, but each communicates with the ventricle of the diencephalon
through an interventricular
foramen
(foramen of Monro) (Figure 14-3). Because there are two lateral ventricles
(first and second), the one in the diencephalon is called the third ventricle.
The mesencephalon has a slender canal known as the mesencephalic
aqueduct
(aqueduct of Sylvius or cerebral aqueduct). This passageway
connects the third ventricle with the fourth ventricle. The superior portion of
the fourth ventricle lies between the posterior surface of the pons and the
anterior surface of the cerebellum. The fourth ventricle extends into the
superior portion of the medulla oblongata (Figure 14-3a). This ventricle then narrows and becomes
continuous with the central canal of the spinal cord (Figure 14-3a).
The ventricles are filled with cerebrospinal fluid.
There is a continuous circulation of cerebrospinalfluid (CSF) from the
ventricles and central canal into the subarachnoid space of the meninges
that surround the CNS. The CSF passes between the interior and exterior of the
CNS through foramina in the roof of the fourth ventricle.
PROTECTION AND SUPPORT OF THE BRAIN
The delicate tissues of the brain are protected from
mechanical forces by (1) the bones of the cranium,(2)
the cranial meninges, and (3) cerebrospinal fluid. In addition, the
neural tissue of the brain is biochemically isolated from the general
circulation by the blood-brain barrier.
The layers that make up the cranial meninges—the dura
mater, arachnoid, and pia mater—are continuous with those of the
spinal cord. However, the cranial meninges have distinctive
anatomical and functional characteristics.
Functions of the Cranial Meninges
The brain is cradled within the cranium. There is an
obvious correspondence between the shape of the brain and that of the cranial
cavity (Figure 14-4a). The massive cranial bones provide mechanical
protection, but they also pose a threat. The brain is like a person driving a
car. If the car hits a tree, the car protects the driver from contact with the
tree, but serious injury will occur unless a seat belt or airbag protects the
driver from contact with the car.
Cranial trauma is a head injury resulting from impact
with another object. There are roughly 8 million cases of cranial trauma each
year in the United States, but only 1 case in 8 results in serious brain
damage. The percentage is relatively low because the cranial meninges provide
effective protection for the brain. Tough, fibrous dural folds act like
safety belts that hold the brain in position. The cerebrospinal fluid contained
in the subarachnoid space acts like an airbag that cushions sudden jolts and
shocks.
Dural Folds
In several locations, the inner layer of the dura mater extends
into the cranial cavity, forming a sheet that dips inward and then returns.
These dural folds provide additional stabilization and support to the brain.
Dural sinuses may be found between the two layers of a dural fold. The three
largest dural folds are called the falx cerebri, the tentorium
cerebelli, and the falx cerebelli (Figure 14-4b):
1. The falx cerebri (falx, curving or sickle-shaped) is a
fold of dura mater that projects between the cerebral hemispheres in the longitudinal
fissure. Its inferior portions attach anteriorly to the crista galli and
posteriorly to the internal occipital crest. Two large venous sinuses,
the superior sagittal sinus and the inferior sagittal sinus, travel within this
dural fold. The posterior margin of the falx cerebri intersects the tentorium
cerebelli.
2. The tentorium
cerebelli
(tentorium, a covering) separates and protects the cerebellar hemispheres
from those of the cerebrum. It extends across the cranium at right angles to
the falx cerebri. The transverse sinus
lies within the tentorium cerebelli.
3. The falx cerebelli divides the two cerebellar
hemispheres along the midsagittal line inferior to the tentorium cerebelli.
Cerebrospinal Fluid
Cerebrospinal fluid (CSF) completely surrounds and
bathes the exposed surfaces of the CNS. The
CSF has several important functions, including the following:
Because free exchange occurs between the interstitial
fluid and CSF, changes in CNS function may produce changes in the composition
of the CSF. As we noted in Chapter
The choroid plexus (choroid, vascular coat; plexus,
network) consists of a combination of specialized ependymal cells and permeable
capillaries for the production of cerebrospinal fluid. Two extensive folds of
the choroid plexus originate in the roof of the third ventricle and extend
through the interventricular foramina. These folds cover the floors of the
lateral ventricles (Figure 14-5a). In the lower brain stem, a region of the
choroid plexus in the roof of the fourth ventricle projects between the
cerebellum and pons.
Specialized ependymal cells, interconnected by tight
junctions, surround the capillaries of the choroid plexus. The ependymal cells
secrete CSF into the ventricles; they also remove waste products from the CSF
and adjust its composition over time. The differences in composition between
CSF and blood plasma (blood with the cellular elements removed) are quite
pronounced. For example, the blood contains high concentrations of soluble
proteins, but the CSF does not. There are also differences in the
concentrations of individual ions and in the levels of amino acids, lipids, and
waste products.
The choroid plexus produces CSF at a rate of about 500
ml/day. The total volume of CSF at any given moment is approximately 150 ml;
thus, the entire volume of CSF is replaced roughly every 8 hours. Despite this
rapid turnover, the composition of CSF is closely regulated, and the rate of
removal normally keeps pace with the rate of production.
The CSF circulates from the choroid plexus through the
ventricles and the central canal of the spinal cord (Figure 14-5a). As the CSF circulates, there is unrestricted
diffusion between it and the interstitial fluid of the CNS between and across
the ependymal cells. The CSF reaches the subarachnoid space through two lateral apertures and a single median aperture in the roof of the
fourth ventricle. Cerebrospinal fluid then flows through the subarachnoid space
surrounding the brain, spinal cord, and cauda equina.
Along the axis of the superior sagittal sinus,
fingerlike extensions of the arachnoid membrane, called the arachnoid villi,
penetrate the dura mater. In adults, clusters of villi form large arachnoid
granulations
(Figure 14-5b). Cerebrospinal fluid is absorbed into the
venous circulation at the arachnoid granulations. If the normal circulation or
reabsorption of CSF is interrupted, a variety of clinical problems may appear.
For example, a problem with the reabsorption of CSF in infancy is responsible
for symptoms of hydrocephalus, or "water on the brain."
Infants with this condition have enormously expanded skulls due to the presence
of an abnormally large volume of CSF.
In an adult, failure of reabsorption or blockage of
CSF circulation can cause distortion and damage to the brain.
The Blood Supply to the Brain
As we noted in Chapter 12, neurons have a high demand
for energy, and they have neither energy reserves, in the form of carbohydrates
or lipids, nor oxygen reserves, in the form of myoglobin. Your brain, with billions of neurons, is an
extremely active organ with a continuous demand for nutrients and oxygen. These
demands are met by an extensive circulatory supply. Arterial blood reaches the
brain through the internal carotid arteries and the vertebral
arteries. Most of the venous blood from the brain leaves the cranium in the
internal jugular veins, which drain the dural sinuses. A head injury
that damages cerebral blood vessels may cause bleeding into dura mater, either
near the dural epithelium or between the outer layer of the dura mater and the
bones of the skull. These are serious conditions because the blood entering
these spaces compresses and distorts the relatively soft tissues of the brain.
Edipural snd Subdural Hemorrhages
Cerebrovascular diseases are circulatory disorders that
interfere with the normal circulatory supply to the brain. The particular
distribution of the vessel involved will determine the symptoms, and the degree
of oxygen or nutrient starvation will determine their severity. A stroke,
or cerebrovascular
accident (CVA)
, occurs when the blood supply to a portion of the brain is shut off. Affected
neurons begin to die in a matter of minutes.
Neural tissue in the CNS is isolated from the general
circulation by the blood-brain barrier. This barrier exists because the
endothelial cells that line the capillaries of the CNS are extensively
interconnected by tight junctions. These junctions prevent the diffusion of
materials between adjacent endothelial cells. In general, only lipid-soluble
compounds (including carbon dioxide, oxygen, ammonia,
lipids, such as steroids or prostaglandins, and small alcohols) can diffuse
across the lipid bilayer membranes of endothelial cells into the interstitial
fluid of the brain and spinal cord. Water and ions must pass through channels
in the inner and outer cell membranes. Larger water-soluble compounds can cross
the capillary walls only through active or passive transport. The restricted
permeability characteristics of the endothelial lining of brain capillaries are
in some way dependent on chemicals secreted by astrocytes. We described these
cells, which are in close contact with CNS capillaries, in Chapter 12. The outer surfaces of the endothelial cells are
covered by the processes of astrocytes. Because the astrocytes release
chemicals that control the permeabilities of the endothelium, these cells play
a key supporting role in the blood-brain barrier. If the astrocytes are damaged
or stop stimulating the endothelial cells, the blood-brain barrier disappears.
The choroid plexus lies outside the neural tissue of
the brain, and there are no astrocytes in contact with the endothelial cells.
As a result, capillaries there are highly permeable. Substances do not have
free access to the CNS, however, because a blood-CSF
barrier
is created by specialized ependymal cells. These cells, interconnected by tight
junctions, surround the capillaries of the choroid plexus.
Transport across the blood-brain and blood-CSF
barriers is selective and directional. Even the passage of small ions, such as
sodium, hydrogen, potassium, or chloride, is controlled. As a result, the pH
and concentrations of sodium, potassium, calcium, and magnesium ions in the
blood and CSF are different. Some organic compounds are readily transported,
and others cross only in minute amounts. Neurons have a constant need for
glucose. This need must be met regardless of the relative concentrations in the
blood and interstitial fluid. Even when circulating glucose levels are low,
endothelial cells continue to transport glucose from the blood to the
interstitial fluid of the brain. In contrast, only trace amounts of circulating
norepinephrine, epinephrine, dopamine, or serotonin pass into the interstitial
fluid or CSF of the brain. This limitation is important because these compounds
are neurotransmitters, and their entry from the circulation (where
concentrations can be relatively high) could result in the uncontrolled
stimulation of neurons throughout the brain.
The blood-brain barrier remains intact throughout the
CNS, with four noteworthy exceptions:
1. In portions of the hypothalamus, the capillary
endothelium is extremely permeable. This permeability exposes hypothalamic
nuclei to circulating hormones and permits the diffusion of hypothalamic
hormones into the circulation.
2. Capillaries in the posterior pituitary gland are
highly permeable. At this site, the hormones ADH and oxytocin,
produced by hypothalamic neurons, are released into the circulation.
3. Capillaries in the pineal gland are also
very permeable. The pineal gland, an endocrine structure, is located in the
posterior, superior surface of the diencephalon. The capillary permeability
allows pineal secretions into the general circulation.
4. Capillaries at the choroid plexus are extremely
permeable. Although the capillary characteristics of the blood-brain barrier
are lost there, the transport activities of specialized ependymal cells within
the choroid plexus maintain the blood-CSF barrier.
CONCEPT CHECK QUESTIONS
1. What would happen if an interventricular foramen
became blocked?
2. How would decreased diffusion across the
arachnoid granulations affect the volume of cerebrospinal fluid in the
ventricles?
3. Many water-soluble molecules found in the blood
in relatively large amounts occur in small amounts or not at all in the
extracellular fluid of the brain. Why?