Although details of the structural organization of the brain and spinal cord will come in the Neuroscience course, it is important from the beginning to place primary sensory and motor neurons in their proper relation to the spinal cord. This slide is an overview of one half of a transverse section of the spinal cord, along with its ventral and dorsal roots and a spinal ganglion. At the extreme left (which is close to the midline of the cord) notice a small central canal lined by a dark layer of ependymal this contains cerebrospinal fluid in life. Above the canal lies the narrow slit of the posterior median sulcus, and below the canal is a wider, bulging separation called the anterior median fissure. Lateral to all these spaces lies the gray matter of the cord (quite pink here), where neuronal cell bodies lie. Surrounding the gray matter is a layer of white matter, consisting of nerve cell processes, all of them axons, running up or down the length of the cord and therefore cut in cross-section here. Outside the cord, to the right, lies a mass of nerve cell bodies, the spinal ganglion, interrupting the course of the dorsal root. Below the ganglion lies the ventral root. Surrounding the entire complex is a well-defined, pink band of dura mater which consists of dense collagenous connective tissue. The wedge of delicate areolar c.t. at the bottom of the anterior median fissure is the arachnoid; note the round cross-cut of a blood vessel lying in it. The pia mater, invisible here, is an extremely thin connective tissue layer immediately investing the spinal cord.
In terms of a simple reflex arc (sensory information comes to the cord and motor information is sent from the cord) picture some basic nerve cell bodies and processes as follows:
- A pseudounipolar, sensory cell body lies within the spinal ganglion. It has one long dendrite coming in from the extreme right in this picture, from the body periphery (either from muscle or skin). This dendrite is continuous with the cell body (no synapses are involved here). The cell's axon leaves along the same "stalk" with the dendrite and then turns to course through the dorsal root, into the spinal cord. There its axonal endings synapse upon the dendrites of a small, intermediate multipolar neuron lying in the dorsal horn of the gray matter. This intermediary cell sends its axon to the ventral horn of the, gray matter and synapses upon the dendrites of a large, multipolar, motor neuron lying there. The axon of the motor neuron courses out of the cord via the ventral root and proceeds out of this picture, to the right, until it ends Upon voluntary muscle
A group of large multipolar neurons, as found in the gray matter of the anterior horn. Cell nuclei are pale (or vesicular) and round and contain a large amount of Nissl substance (RER). The smallest nuclei in the field belong to glial cells. In an area like this, glia play a supportive and nutritive role. They take the place of connective tissue within the central nervous system (i.e., the brain and spinal cord).
Higher power of multipolar neuron in gray matter stained with silver. Notice the meshwork of processes comprising the neuropil around the cell Processes may be dendrites of local neurons, or axons of distant neurons either passing through the field or ending upon local neurons.
A large, multipolar, motor neuron of the anterior horn, seen whole, with all its processes stretched out in a spinal cord smear. Notice the dark clumps of Nissl substance in the cytoplasm. The axon cannot be identified with certainty in this particular view. Neuroglial nuclei surround the neuron. Of these small nuclei, the lightest ones, showing small clumps of chromatin, belong to astrocytes; any dark, round ones (such as the one in the upper right corner) belong to oligodendroglia; and any dark, thin, cigar-shaped ones to microglia (see possible one just to right of the neuron).
Glial nuclei seen in white matter of the cord, cut so that nerve processes are seen running longitudinally. Most of these are round, dark oligodendroglial nuclei; these are the cells responsible for the myelin wrapping of axons of the central nervous system.
Silvered preparation of astrocytes, showing their many fine cytoplasmic processes. Note their close relationship to capillaries, the heavy black structures. Since astrocytes touch both capillaries and neurons, they are thought to play an important intermediary role in the nutrition and metabolism of neurons.
Spinal ganglion in Mallory connective tissue stain. The pseudounipolar cells are in characteristic groups or clumps, separated by bands of nerve processes. The processes might be either dendrites arriving from the body periphery or axons proceeding on to the spinal cord. Either way, the cell bodies or origin for the processes lie within the spinal ganglion and are sensory neurons. The dark blue sheath outside the ganglion is the dense collagenous connective tissue dura mater.
Detail of pseudounipolar spinal ganglion each one encapsulated by a layer of small satellite cells. Bright blue material is the supportive connective tissue, which is directly continuous with the endoneurium surrounding the individual nerve processes entering and leaving the ganglion. Remember that connective tissue is the supportive tissue of the peripheral nervous system.
Higher power of spinal ganglion stained with H&E. Satellite cell capsules are clear. The large neuron in the center of the field has a pale axon hillock where the seemingly single process enters and leaves. In such a pseudounipolar cell, the incoming dendrite and outgoing axon seem to be related to the cell body by means of a single "stalk". The paleness of the hillock is due to the absence of RER (Nissl substance) in this area.
Cells of autonomic (sympathetic) ganglion, at same magnification as previous slide. These motor neurons are actually multipolar in shape and are generally smaller than spinal ganglion neurons; they are also scattered more randomly and individually in their ganglion, and have less well defined capsules of satellite cells. Some of the cells in this picture contain yellow lipofuscin granules, a sign of age. (Lipofuscin is sometimes spelled lipofuchsin; these granules represent the undigested residual material of lysosomal activity.) Autonomic ganglion neurons are the second order neurons in the two cell autonomic chain; the first order neurons lie in the central nervous system and send out axons to synapse upon the dendrites of the ganglion neurons.
Autonomic parasympathetic neurons lying between muscle layers in the intestinal wall. Note their large size in comparison with surrounding satellite cells. The neuronal nuclei here are often eccentric. Remember that although autonomic neurons look generally rounded in outline, they are actually multipolar neurons with very fine dendritic processes, and they are visceral motor neurons, responsible for involuntary control of smooth and cardiac muscle.
Cross-cut of a peripheral nerve showing characteristically round bundles of nerve processes surrounded by pale gray-blue connective tissue sheaths. The outer connective tissue sheath surrounding the entire nerve is the epineurium. The connective tissue sheath surrounding each round bundle is the perineurium. Surrounding each individual nerve process within a bundle is the delicate connective tissue endoneurium (not visible at this magnification).
Scanning electron micrograph of a cross-section of a peripheral nerve showing individual axons surrounded by myelin sheaths. The axons have undergone some shrinkage with specimen preparation and have receded from the surface of the section. Myelinated axons are visible beneath the translucent perineurium.
A higher magnification of one bundle of peripheral nerve, showing cross-cuts of individual processes. The ones in the center are the truest cut; those on either side are tangentially cut. The best ones show a darker axon in the center of the fiber, surrounded by a paler myelin sheath. Remember that some of these fibers are axons of motor neurons, whose cell bodies are in the anterior horn of the spinal cord, while other fibers are dendrites of the pseudounipolar sensory cells of the spinal ganglion. This is the one instance where functional dendrites (i.e., processes coming into the cell body) are structurallv like axons with myelin sheaths. The dense sheath at the outer edge of the bundle here is perineurium. The lines of pink surrounding each process represent endoneurium.
Low power view of longitudinal section of peripheral nerve, again showing distinct division into bundles of processes. The "'waviness" of the processes themselves is often typical of nerve.
Higher magnification of longitudinally cut nerve, showing a clear node of Ranvier in the center of the field. Note that the axon is continuous through the node. Notice also the "foamy", grainy appearance of the myelin sheaths; this represents the proteinaceous material of the cell membrane wrappings of the sheath, often called "neurokeratin" although this is a misnomer. The lipid portion of the membranes has been dissolved out during tissue fixation.
Detail of node of Ranvier, with axon continuing through it. Axons stain deep pink. Myelin is pale because the lipid material disolves out. The dark strands of protein neurokeratin give the "foamy" look to the myelin in light microscopy. Nuclei, seen here near the bottom of the picture, lie between nerve processes and belong to either Schwann cells or endoneurial connective tissue cells (such as fibroblasts).
Drawing of relation of an oligodendrocyte to a neuronal axon in the CNS, as seen in E.M. An extension of cell cytoplasm wraps around the axon, making a multi-layered myelin sheath. Ordinarily there is one oligodendrocyte between two successive nodes of Ranvier. Notice that the cell has other cytoplasmic extensions up above, which are free to as sociate with other axons. This same principle of lamellated (layered) myelin sheath formation holds true also for Schwann cells and peripheral nerves. One difference, however, is that a Schwann cell is believed to wrap only one axon instead of several. Notice that the plasma membrane of the axon is bare at the point of the node; this allows for rapid saltatory conduction as the impulse jumps from node to node to node.
EM of myelinated axons of peripheral nerve. The dark, many-layered myelin sheaths surround pale axons. At the upper edge of the picture is a nucleus of a Schwann cell, with its outer rim of cytoplasm continuous with the outer rim of the myelin sheath of the axon in the left corner. (Remember that non-myelinated axons are also closely related to Schwann cells, but the Schwann cells form no layered wrappings around them. Note, too, that one Schwann cell can be related to several axons when these are non-myelinated.)
Cross-cuts of small peripheral nerve bundles as seen in ordinary tissue sections. The processes have a typically wavy appearance.
Detail of a motor nerve ending upon a skeletal muscle cell (voluntary muscle). The axon terminal is highly branched to form an oval motor end plate. The cell body which sends out this axon is a multipolar motor neuron, such as those in the anterior horn of the spinal cord.
Diagram of motor end plate (myoneural junction) as seen with electron microscopy. This drawing shows a detail of one knob of an end plate as it rests in a trough on the surface of a muscle cell. The "subneural clefts" labelled here are also called "gutters" in the sarcolemmal membrane. The label "glycoprotein" indicates the position of the basal lamina of the muscle cell.
EM detail of neuro-neural synapse in the brain or spinal cord. The axon terminal contains many seed-like synpatic vesicles containing transmitter substances. The intercellular cleft between the axon and the contacted dendrite can be seen. Just below the dendritic cell membrane is a dark, filamentous post-synaptic density. Other profiles in this field, most of them very irregular in outline, belong to both neuronal processes and glial processes. There is one large and one small mitochondrion just left of the synaptic vesicles.
Muscle spindle -- a specialized sensory receptor for muscle stretch and position sense, as related particularly to unconscious maintenance of skeletal muscle tone and proper balance of postural muscle activity. The spindle is the encapsulated group of muscle fibers lying in the center of the field of regular skeletal muscle fibers, all cut in cross-section. The sensory nerve endings themselves (not visible here) wrap around the muscle fibers within the spindle. Such endings relay sensory information along dendrites within peripheral nerves, back to pseudounipolar cell bodies in a spinal ganglion, and thence to the spinal cord.
Pacinian corpuscle -- another specialized sensory ending, this time for deep pressure. This particular view is from a whole mount of mesentery, so you are seeing the corpuscle three-dimensionally. They are also found in subcutaneous tissue, deep to skin. Notice the onion-like layers of specialized connective tissue surrounding a dark pink dendritic terminal. Again, the cell body for this dendrite lies in a spinal ganglion, and the axon of that same cell then proceeds into the spinal cord.
The following five slides show some specializations of the brain. First is an overview mid-sagittal cut of the brain, showing the many folds (or gyri) of the external cerebral cortex, and the much smaller, more delicate folds (or folia) of the cerebellar cortex seen to the left. As seen in this kind of cut, the cerebellar folia have a branching, tree-like appearance. (The brain stem is the solid-looking structure along the base of the brain, and continuous with the spinal cord at lower left.)
Section of cerebral cortex, showing cuts of two gyri. The pale cortex follows along the contours of the gyri. White matter (composed of nerve processes) lies below and stains a darker pink. Very little cytoarchitecture is seen with H&E stain.
Cerebral cortex stained with silver to show silhouettes of pyramidal cells. Now each triangular cell body can be seen, as well as the ascending apical dendrite, several basal dendrites, and a very fine descending axon. These are specialized multipolar neurons with such a definite shape that they can be recognized as such. You will learn more about them in Neuroscience.
Section of cerebellar cortex, showing several folia. Each folium has a central core of bright blue white matter, consisting of nerve processes entering and leaving the superficial cortex. The cortex has an external pale layer and a darker staining granular layer beneath it. Large Purkinje cells lie in a row between these two layers but are not visible at this magnification.
Higher magnification of cerebellar cortex, showing the row of large Purkinje cells lying between the outer and inner cortical layers. The stubs of the dendritic trees of the Purkinje cells look rather like "antlers" arising from the cell bodies. Very complex dendritic branchings actually extend throughout the molecular layer above the Purkinje c ells. A single axon leaves each Purkinje cell at its base and descends through the granular layer to deeper relay stations within the brain. Again, these are neurons with a very distinctive shape; you'll study their function and their connections next semester.