#2 GROSS BRAIN AND SPINAL CORD
A.
Medial Surface of Brain. Sagittally section the whole brain with a brain knife as close to
the midline as possible.
1.
The structures described in the following sections can be seen on the medial
surface of the sagittally sectioned brain (H2-29). The photographs of the
medial surface of the brain stem (H2-30) will be helpful in locating these
structures. They also are depicted in the brain stem diagram (ND2) and on the
next page. Begin at the caudal end and observe the relationship of the fourth
ventricle (H2-30) to the medulla, pons and cerebellum. For the floor of the fourth
ventricle refer to the brain stem.
2.
The dorsal surface of the pons (H2-34) forms the rostral portion of the floor of
the fourth ventricle. The lateral boundaries of the rostral portion of the fourth
ventricle are the superior cerebellar peduncles (H2-34), comprised mainly of
efferent fibers connecting the cerebellum with the mesencephalon and higher
centers. The thin roof of the fourth ventricle is the superior medullary velum
(H2-34, labeled anterior) which lies between the superior cerebellar peduncles.
The floor of the fourth ventricle (ND99) is depicted diagrammatically in this
guide.
3.
The roof of the fourth ventricle is formed by the cerebellum and superior and
inferior medullary vela (H2-30), but only the anterior (superior) is labeled. The
inferior medullary velum is continuous with the posterior choroid plexus (H230). The inferior velum is perforated by three apertures (the two lateral foramina
of Luschka and the medial foramen of Magendie) communicating with the
subarachnoid space. These apertures are usually not demonstrable. In the floor
of the fourth ventricle (rhomboid fossa) (H2-34) note the median sulcus (H234) and the sulcus limitans (H2-34). Look for the trochlear nerve (H2-24)
emerging from the superior medullary velum. It is the only cranial nerve which
exits dorsally from the brain. Because of its fragility, it is sometimes missing.
4.
Next, on the half brain identify the cerebral aqueduct (H2-30) in the
mesencephalon. It is a small channel connecting the fourth ventricle with the
third ventricle; the latter is rostrally situated in the diencephalon. The part of the
mesencephalon lying dorsal to the cerebral aqueduct is the tectal plate (H2-30,
which means roof), which is the combination of the superior and inferior
colliculi (H2-30), whereas the portion anterior to the aqueduct is the tegmentum
(H2-30).
5.
Observe the medial surface of the diencephalon which can be seen because it
provides the boundaries for the slit-like third ventricle. Identify the fornix (H2-30)
on the medial surface of your specimen. Look for a small opening beneath the
rostral end of the fornix. This is the interventricular foramen (of Monro) (H230). Insert a probe in this foramen. This is the communication between the third
ventricle and the lateral ventricles located in the cerebral hemispheres.
B.
6.
The Thalamus. The thalami (dorsal thalami) (H2-30) are oval masses, one on
each side which form the lateral walls of much of the third ventricle. An
interthalamic adhesion, massa intermedia (H2-30) may or may not be present.
The thalami contain nuclei in the sensory pathways to the cerebral cortex.
7.
The Epithalamus (Caudal, dorsal midline structures). The only feature of the
epithalamus we will study is the pineal gland (H2-30). The pineal can also be
found on the brain stems.
8.
The Hypothalamus. The ventral portion of the diencephalon, hypothalamus (H230), includes, (a) the mammillary bodies (H2-30), which have important limbic
system connections; (b) the tuber cinereum, to which the infundibulum (H2-30)
of the pituitary gland is attached; and (c) the optic chiasm (H2-30).
Spinal Cord
Much of the basic anatomy of the spinal cord has been studied previously in Core I. This
review is desirable because certain structural features have special functions. A
videotape of a demonstration of the spinal cord is available for study. Several models
and specimens of spinal cord are also in the laboratory.
1.
2.
The Meninges
a.
The dura of the spinal cord is not fused to the surrounding bone (as in the
brain) but is separated from bone by the loose connective tissue of the
epidural space. For this reason the dura still surrounds the spinal cord
which is demonstrated. Recall the anatomy of the meninges and the cord
within the veterbral canal. The cord extends in adults to the lower border
of L1 whereas the dural sac and arachnoid extend to S2. In newborn
children the spinal cord occupies more of the vertebral canal, but the
vertebral canal elongates more than the cord during maturation, producing
the adult condition. Lumbar punctures are commonly performed for a
variety of diagnostic purposes. Use is made of the presence of a lumbar
CSF cistern and the protection afforded dorsal and ventral roots of the
cauda equina by CSF. Could a subdural vs. subarachnoid hemorrhage
be distinguished by a lumbar puncture? The particular relationships
between the meninges and peripheral nerves are important in a variety of
approaches for administration of anesthetic agents.
b.
Denticulate ligaments (H2-1), consisting of pia, are lateral stabilizers of
the cord. They can be used as important landmarks for identifying the
position of certain tracts within the spinal cord.
Gross topography of the spinal cord (H2-1, H2-2, H2-4)
a.
On a spinal cord find the anterior median fissure (H2-2). This is a deep
fissure on the ventral surface of the cord. Find the cervical enlargement
and the lumbar enlargement. What is the reason for these
enlargements? Locate the conus medullaris (H2-4) and the filum
terminale (H2-4). Find a shallow groove, the posterior median sulcus
(H2-2), on the dorsal side of the cord. On each side find the shallow
anterolateral sulcus from which emerge the filaments of the ventral
(motor) roots (H2-2) and posterolateral sulci for emerging dorsal
(sensory) roots (H2-2). Note that the anterior median fissure, the
posterior median sulcus, the two anterolateral sulci and the two
posterolateral sulci extend the length of the cord. Root filaments that form
the accessory nerve emerge from the lateral aspect of the upper cervical
cord. Note that the root filaments (dorsal and ventral) form a dorsal root
and a ventral root of the spinal nerve. A segment of the spinal cord is
defined as the area which gives rise to the rootlets that combine to form a
numbered spinal nerve. Review the approximate position of the cord
segments in reference to vertebral levels. It should be emphasized that in
neuroanatomy when reference is made to segments such as C5, T10, L1,
spinal cord segments are being referred to, not the vertebral level at which
the given peripheral nerve emerges. Transverse sections of the spinal
cord (H5-1, H5-2, H5-3, H5-4 & H5-5) demonstrate the centrally located
AH@ shaped gray matter (cell bodies) consisting of the posterior (dorsal)
horns, an area of intermediate gray matter, and two anterior (ventral)
horns. The posterior funiculus is the white matter (axons) between the
two posterior horns whereas the anterior funiculus occurs between the
two anterior horns. The two lateral funiculi occur between the anterior
and posterior horns. These features are shown on the cut upper end of
the specimen in the demonstration.
3.
The blood supply (H2-3)
The blood vessels on the lab specimens are difficult to see. Usually the veins
can be visualized.
a.
As has been pointed out, the single anterior spinal artery (H2-21) and
paired posterior spinal arteries (H2-21) arise from the vertebral arteries
and proceed down the spinal cord. The posterior spinal arteries may arise
from the vertebral or the posterior inferior cerebellar a. The anterior spinal
artery lies along the anterior median fissure and supplies the ventral and
much of the lateral funiculus. Additionally, the anterior portion of the gray
matter of the cord is supplied by this artery. The anterior spinal artery
supplies nervous tissue via sulcal arteries which alternate in supplying
right and left sides of the spinal cord. The dorsal funiculus, dorsal portion
of the lateral funiculi, and posterior part of the posterior horn receive blood
from the posterior spinal arteries. The posterior spinal arteries (H2-3)
lie parallel to the posterolateral sulci. Both the anterior and posterior
spinal arteries are reinforced along their routes by some radicular
branches (H2-3) from vertebrals, ascending cervicals, intercostals, and
lumbar arteries, but they don=t exist at all levels as the spinal arteries
diagram (H5-6) implies.
b.
The blood supply to certain levels of the cord may be easily compromised
especially in areas of transition where blood is derived from two sources.
For example, levels T1 through T4 cannot be maintained without the
integrity of the intercostal vessels and their radicular branches. Another
vulnerable level is L1. In the absence of adequate blood supply, necrosis
will occur and the resulting lesion may resemble a transection of the cord
at that level. These areas are of great concern to both neurologists and
physicians in several surgical specialties.
c.
4.
The venous drainage of the spinal cord parallels the arterial supply with
the addition of a midline posterior spinal vein. The two veins paralleling
the posterior spinal arteries are the posterolateral spinal veins. The
single anterior spinal vein courses with the anterior spinal artery. The
spinal veins are drained by many anterior and posterior radicular veins
which then empty into a venous plexus in the epidural space. Blood from
this plexus passes through channels in the intervertebral foramina to the
external vertebral plexus which is drained by vertebral, intercostal, and
lumbar veins.
The microscopic anatomy of the spinal cord
a.
Study spinal cord slides which show sacral (T1A), lumbar (T1B),
thoracic (T1C) and cervical (T1D) levels of the spinal cord. Compare
with sections from Haines of sacral (H5-1), lumbar (H5-2), thoracic (H53), lower cervical (H5-4) and upper cervical (H5-5).
b.
Re-identify in sections the topographical landmarks of sulci and fissures
referred to in B on the previous page.
c.
Distinguish between the gray and white matter. Compare the relative
amounts of each in the three cord levels presented by the sections. Why
is there relatively little gray matter in the thoracic level compared with the
other two? Why is there progressively more white matter as one ascends
the cord as noted in comparing the volume of lumbar to cervical, for
example?
d.
Is there any evidence of white matter (myelinated fibers) within the gray?
Ex., anterior white commissure (H5-4). This will be significant later in
studying the mode of cross-over of some of the sensory fibers.
e.
The nerve cells within the spinal cord should next be studied. The student
should distinguish between anterior (ventral), intermediate, and dorsal
horn cells. Each of these groups B sometimes called columns because
they form a more-or-less continuous column longitudinally throughout the
cord B have discrete functions.
f.
Thus, the intermediolateral cell column, which in this case is limited to
thoracic and upper lumbar levels of the cord, represents the cells of origin
of the preganglionic axonal fibers of the sympathetic nervous system. It
has its counterpart in the sacral region but the fibers have a different
pattern of termination and make up a component of the parasympathetic
system.
g.
Nerve cells of like structure (largely a similar Nissl pattern) and like
function (i.e., receive fibers from a common source) distribute their axons
to a common destination. Those which are important will be specifically
identified as the fiber systems are studied. Still, the student should
appreciate the fact that it is possible to identify within the cord, on the
basis of structure, discrete groups of neurons (cell bodies) which for
example innervate the diaphragm (phrenic), are dorsal spino-cerebellar
fibers (nucleus dorsalis), or are neurons projecting to specific muscles,
such as the gastrocnemius, biceps. This will become far more significant
later in studying the brain stem where very discrete nuclei have specific
functions (e.g., oculomotor, facial, vagus). In summary, for the cord,
dorsal horn nuclei are sensory, receptory and association;
intermediolateral are autonomic (smooth muscle, cardiac muscle and
sweat glands); and ventral are somatic motor (skeletal muscle).
5.
6.
Study slides NH1, NH2, NH3, NH4, & NH5 to identify cells of the spinal cord as
well as ganglia of the peripheral nervous system.
a.
Spinal Cord Cross Section (NH1). The anterior (or ventral) surface of
this spinal cord is toward the bottom of this slide. Identify the gray and
white matter. Locate the dorsal, ventral, and lateral funiculi. Note the
large spinal ganglion adjacent to the cord.
b.
Anterior Horn Cell (NH2). This is a high power view of a large, motor
neuron found in the anterior (or ventral) horn of the gray matter of the
spinal cord. Identify the pale nucleus and deeply stained nucleolus. Nissl
substance can be seen in the cytoplasm of this cell. Note the numerous
processes of this multipolar cell. It does not appear that this section
includes an axon; there is no obvious axon hillock. Surrounding the motor
cell are many darkly stained nuclei which are nuclei of neuroglial cells.
The cytoplasm of these cells does not stain unless special preparations
are made.
c.
Spinal Ganglion Cells (NH3). This photograph is of a section through a
dorsal root ganglion. Note the large, unipolar cell bodies. What may
appear to be axon hillocks are probably shrinkage artifacts. How does
the Nissl substance seen here differ from that of the motor cell? Identify
the satellite cells surrounding the cell bodies. Many nuclei of connective
cells can be seen between the cell bodies. Notice the bundle of
myelinated fibers cut in cross section at the right of this slide.
d.
Autonomic Ganglion Cells (NH4). This is a low power photograph of a
section through a sympathetic chain ganglion illustrating the morphology
of the cell bodies that give rise to postganglionic fibers. Note the
eccentrically placed nuclei of these multipolar cells. Satellite cells,
although present, are not nearly as prominent around the cell bodies as
those seen in the previous slide. Large bundles of myelinated fibers are
not evident in autonomic ganglia.
e.
Autonomic Ganglion Cells (NH5). This is a higher power view of
autonomic cell bodies.
Histology of Peripheral Nerves
Now turn your attention to the Awhite matter@ of the cord on the spinal cord
slides. These areas are made up of (for the most part) axons of nerve cells from
the gray matter or spinal ganglia which extend some distance in the CNS. Fibers
from similar origin that will terminate on a discrete target are bundled together in
what are referred to as tracts or fasciculi. However, it is not necessarily the case
that all fibers within a tract have the same function.
a.
Cross Section of Myelinated Nerve (NH12). This slide shows many
myelinated nerve processes bound together by connective tissues. In
cross section the myelin resembles a doughnut and the nerve process, or
axon, occupies the hole in the doughnut. The connective tissue between
adjacent myelinated processes is the endoneurium. Nuclei seen within
this fascicle are the nuclei of Schwann cells.
b.
Longitudinal Section of Myelinated Nerve (NH13). Identify the pink
stained myelinated fibers on this slide. What are the elongated purple
nuclei?
c.
Nerve Cross Section (NH14). This is a cross section of an entire nerve
such as you have dissected out in the Gross Anatomy Laboratory.
Surrounding the entire nerve is the epineurium a connective tissue
sheath that will hold sutures.
d.
Vein, Artery and Nerve (NH15). Arteries, veins and nerves are frequently
found together as indicated on this slide. The vein is a flattened tubular
structure on the right whereas the artery has a fairly thick muscular wall.
e.
Nerve, Cross Section (NH16). This tissue is stained with osmic acid which
stains the myelin black. Axons can be seen in some of the myelin
sheaths. Identify the perineurium.
f.
Teased Myelinated Fibers (NH17). This slide is also stained with osmic
acid to demonstrate myelin. Note the node of Ranvier at the arrow. To
the left of the node a number of funnel-shaped Schmidt-Lanterman clefts
can be seen extending in toward the axon.
g.
Oculomotor Nerve (NH18). Note the heavily myelinated fibers in this
nerve. Axons can be seen in the center of the myelin sheaths.
Surrounding the myelin is a very thin membrane composed of Schwann
cell membranes. Can you identify Schwann cell nuclei?
h.
Choroid Plexus (NH37). This is a low power photomicrograph of choroid
plexus taken from the roof of the fourth ventricle. Notice the proximity of a
large artery to the villous like processes of the choroid plexus projecting
into the ventricle.
i.
Choroid Plexus (NH38). This is a high power photomicrograph of choroid
plexus. Notice the epithelial covering. These cells are joined by tight
junctions. There is a capillary in the core of the process.
Most of the real study of neuroanatomy concerns the location of nuclei, origin of
nerve fibers, or fasciculi and their course to the next level of synapse. These will
be studied through the course as the sensory and motor systems are covered.
The material on the next three pages concerning fiber tracts in the central
nervous system should be read before the next laboratory session.
C.
Fiber Tracts of the Central Nervous System
1.
A study of fiber tracts and pathways within the central nervous system is best
accomplished by use of selected sections taken at various levels through this
system. One definition of a fasciculus is a nerve fiber tract having a common
origin and termination and frequently a common function. The names of many of
the fasciculi usually indicate the origin and termination of the nerve processes
composing the fasciculi. A good example of this system of nomenclature is the
spinothalamic tract. The cell bodies of the axons making up this tract are found in
the spinal cord and these axons terminate in the thalamus. The cell bodies of
axons in the corticospinal tract are located in the cerebral cortex and terminate in
the spinal cord. Generally speaking, ascending tracts (spinothalamic) are
sensory pathways whereas motor tracts (corticospinal) descend.
2.
You have probably noticed that in studying the white matter of the spinal cord
grossly and microscopically, it is impossible to identify any specific tract. In the
usual preparations of this material, the white matter looks fairly homogeneous
throughout. If you examine figures in your text and the photographs in your neuro
atlas you will observe that most of the tracts and pathways that are an integral
part of neuroanatomy have been precisely located. A knowledge of these
localities is fundamental to clinical neurology. Several methods have been used
in localizing these various fasciculi.
3.
You will learn in a later lecture that, if a nerve cell body is destroyed or an axon is
cut, the fiber distal to the site of the lesion will degenerate. This is anterograde
degeneration. The myelin sheath surrounding a degenerating nerve fiber can be
selectively stained with osmic acid (the Marchi method) while the surrounding
normal fibers are unstained. The pathways of large groups of axons, such as the
corticospinal tract, were originally described by observing patterns of
degeneration in autopsy material from patients with localized lesions. Although
much information was obtained with this approach, it was not sufficient to
delineate the smaller and shorter projections which constitute most of the
pathways within the CNS. Most our knowledge of neuroanatomy is derived from
animal studies in which very small amounts of tracer substances or viruses are
injected into precise regions of the CNS using stereotaxic instruments. After
sacrificing the animal, various methods are used to determine both projections
away from the site of injection as well as the sources of input to the region of
injections.
4.
During embryonic development all nerve tracts do not myelinate at the same time.
The rate of myelination of different tracts exhibits much variation. In general, the
phylogenetically older tracts myelinate first and are usually more centrally located,
such as the vestibulospinal tract which becomes myelinated during the sixth fetal
month. Phylogenetically newer tracts myelinate later on in development and
occupy a more peripheral position in the central nervous system. An extreme
example of this is seen in the corticospinal tract which does not begin to
myelinate (or function) to any great extent until one or two years after birth.
Selective staining of myelin is also a help in localizing specific tracts during
development.
D.
5.
Much of the remainder of the laboratory portion of this course will be devoted to a
study of some of the major fiber pathways of the CNS. Many of these pathways
involve several neurons functioning in a network and you will be expected to
know not only the pathway for a particular functional modality, but also, the nuclei
associated with these tracts. Rarely will a pathway remain ipsilateral, or on the
same side of the body. Most tracts will cross to the contralateral side and
knowing the exact area of decussation of these fibers is important in working out
neurological problems. Structures associated with, and adjacent to, specific
tracts are significant as a Apure@ lesion of one specific pathway is seldom, if ever,
seen. Tracts frequently change their position as they course through the CNS.
By means of a study of cross sections of the brain stem coupled with your
knowledge of gross topography you will develop a three dimensional concept of
the CNS.
6.
A convenient method of sorting out fiber pathways is to construct diagrams of
them. For this reason, you will find a set of unlabeled outline drawings of
selected sections from your slide boxes just before the next laboratory exercise.
You are encouraged to utilize these diagrams for reconstructions of fiber
pathways. You may want to use different colors to represent pathways which can
be added in subsequent sessions.
7.
The first pathways to be studied are the sensory pathways which arise from
afferent peripheral nerve fibers bringing somatosensory information into the CNS.
8.
For these pathways it will be necessary to know not only the pathway to the
conscious level, but also, the reflex pathways that are involved. As a clinician,
much of your neurological testing will be for the proper functioning of these arcs.
You will be testing the sensory neuron that brings information to the CNS, the
connections made by association neurons (if any), and the effector unit or muscle
or gland that is stimulated.
Brain Stem Slides
1.
In the succeeding exercises, continual reference will be made to the set of
brainstem slides that has been issued to your team. The collection of transverse
sections was provided by Dr. Howard Meineke, Professor of Anatomy, University
of Cincinnati, who received them from Dr. Alphonse Vonderahe, neurologist and
neuroanatomist at the University of Cincinnati. The sagittal sections date back to
the 1920's and were probably made here at UNMC by Dr. W.A. Willard. Only
representative sections have been chosen for use in the laboratory exercises and
these should be adequate to give you a good concept of the basic internal
anatomy of the CNS. The sections are stained by the Pal-Weigert technique,
which stains myelinated fibers blue to black. Unmyelinated areas do not stain
with this technique. Thus, relatively unstained areas represent nuclei within the
central nervous system. Occasionally, it will be possible to observe the bluishblack myelinated fibers coursing through unstained nuclear areas. As a result of
this staining procedure, it is possible to distinguish fiber tracts from nuclei.
Neuropathologists make use of this technique to determine pathological absence
of myelin for affected fiber tracts.
2.
Neuroanatomists have for centuries viewed transverse sections of the brain stem
with the ventral aspect at the bottom of the page and the dorsal aspect at the top.
All atlases and texts have used this convention. Unfortunately that is exactly
opposite to the convention adopted when CT scans and MRI came into vogue.
As you recall, for scans the body is to be viewed as if you are standing at the feet
of a supine individual looking toward the head. Neuro texts and atlases are not
likely to change soon, although Dr. Haines has made such a proposal. Just keep
in mind that the scans you will see will look upside down to you.
3.
You will remember from your examination of the gross features of the brain stem
that it is curved as it is followed from the medulla through the thalamus. It is
therefore very difficult to obtain true transverse sections of all parts of the brain
stem. Some sections will show parts of two different aspects of the brain stem.
The sections shown in the Haines atlas will be similar to those in your set but not
identical because the planes of the sections in two sets are seldom the same. By
using the diagrams on the first page of the atlas that you were loaned, you can
determine approximately where the transverse sections and sagittal sections in
your collection were made. Comparison of transverse and sagittal sections may
also be helpful in determining more precisely what was cut in the transverse
section.
4.
The labeled photographs of the slides in the neuro atlas will be very helpful as
you attempt to identify structures on the slides in the laboratory. Most important
structures have been identified on one or several sections, but not every structure
has been labeled in every section. These slides may also be viewed on the
intranet.
Legends for the thalamic nuclei:
A = anterior
Int. Med. Lam. = internal medullary lamina
Int. Lam. = intralaminar nuclei
MD = dorsomedial
CD = centromedian
P = pulvinar
MG = medial geniculate
LG = lateral geniculate
VPM = ventral posterior medial
VPL = ventral posterior lateral
VL = ventral lateral
VA = ventral anterior
LP = lateral posterior
LD = lateral dorsal