Objectives:

1. Be able to identify cells and tissues in the peripheral nervous system (nerves, neurons and glia).



2. Describe the organization of a typical neuron and the direction of information flow.



3. Describe and contrast the function and organization of sensory and motor neurons.



4. Describe the process of myelination, and the function of myelin, including Nodes of Ranvier. Explain the role of the Schwann cell, with respect to both myelinated and unmyelinated neurons.



5. Describe the organization of connective tissue in a nerve.



6. Be able to identify tissues in the nervous system (nerves, cell bodies and ganglia, and white vs. gray matter in the spinal cord).
   
   
   
7. Be able to identify and know the function of Meissner's and Pacinian corpuscles

The nervous system is divided structurally into the central nervous system (CNS) and the peripheral nervous system (PNS), although remember that these are really two components of one, integrated system.  The CNS consists of the spinal cord and the "brain" (everything in the cranial vault: cerebral hemispheres, thalamus, midbrain, pons, cerebellum, and medulla oblongata), while the PNS is composed of nerves and groups of nerve cells (neurons), called ganglia. The nerves of the PNS carry sensory (afferent) inputs to the CNS and motor (efferent) output from the CNS to the skeletal and cardiac muscles and the smooth muscles of blood vessels, organs and glands.
           
Another way to think about the nervous system is according to its two main functional subdivisions: the somatic (voluntary, conscious) and autonomic (involuntary, unconscious) components. 

Peripheral Nervous System

I. Ganglia

In the peripheral nervous system, clusters of neurons with associated nerve fibers and supporting cells are referred to as ganglia.  (In the central nervous system, clusters of neurons are referred to as nuclei, an unfortunate terminology.)

A. Dorsal Root (Spinal) Ganglia
Slide 65-1N (spinal cord, trichrome) [WinLab] [Mac] [WinHome]
Slide 65-2 (spinal cord, H&E) [WinLab] [Mac] [WinHome]
Slide 65-1 (spinal cord, trichrome) [WinLab] [Mac] [WinHome]

Dorsal root ganglia contain the cell bodies of sensory neurons.  Return to slide #65 (W pg 139, 7.20; R pg 355, Plate 23, Figs 3, 4) and locate a dorsal root ganglion near, but outside, the spinal cord.  The neuron cell bodies belong to large, pseudounipolar sensory neurons that have a single "T-shaped" process; these are the afferent fibers carrying sensory information from the periphery (sensory receptors in the skin, joints and muscles that respond to touch, temperature, pain, stretch) to the dorsal horn, where they synapse on neurons in the spinal cord.

NOTE: these sensory neurons are an exception to the typical neuron, in that they do not have separate dendrites and an axonal process, but rather one branched process that serves both functions.

Many of the clusters of sensory neuron cell bodies are peripheral in the ganglion, and others lie between bundles of nerve fibers running in parallel through the ganglion. There are no synapses in these ganglia. You will seldom see a process coming from the cell body, since cells are pseudounipolar and the process will not usually be included in the plane of section.
 
The nuclei in dorsal root ganglia are generally located centrally in the cell bodies of the neurons. Numerous satellite cells (a type of glial cell) form a prominent capsule around each cell body evident in H&E-stained slide 65-2 [example] and Masson-stained slides 65-1 [example] and 65-1N [example] . Just as in the spinal cord, many neurons may appear shrunken and surrounded by an empty space due to poor fixation. The perikarya of surrounding glial cells are typically much smaller than neurons, and their nuclei contain markedly less euchromatin.

B. Autonomic ganglia
Slide 74 (sympathetic ganglion, toluoidine blue) [WinLab] [Mac] [WinHome]
Slide 250-1 (vagina, H&E) [WinLab] [Mac] [WinHome]
Slide 250-2 (vagina, trichrome) [WinLab] [Mac] [WinHome]
Slide 75 (seminal vesicle, H&E) [WinLab] [Mac] [WinHome]

Autonomic ganglia contain cell bodies of sympathetic or parasympathetic motor neurons, which receive synaptic input from preganglionic autonomic neurons whose cell bodies are located in the CNS. The autonomic motor neurons in the ganglia send efferent fibers (postganglionic autonomic nerve fibers) to innervate cardiac muscle fibers of the heart and smooth muscle fibers of body organs and glands.

Examine a sympathetic ganglion in slide #74 (W pg 139, 7.21) [example].  Compare this autonomic ganglion with the dorsal root (spinal) ganglia studied above. Sympathetic ganglia are similar in that they are isolated collections of neurons innervating target organs that are usually quite a distance away.   However, in sympathetic ganglia, the neuron cell bodies are often more widely dispersed, with a meshwork of nerve fibers lying between them, and the nerve fibers generally are not as well organized. Unlike the dorsal root ganglia, which have no synapses and therefore no neuropil, in sympathetic ganglia many preganglionic sympathetic fibers from the spinal cord synapse on the sympathetic neurons, and others travel through the ganglia without synapsing. The cell bodies of sympathetic neurons are smaller than those of sensory neurons in the dorsal root ganglion, and often have eccentrically placed nuclei. The cell profile appears somewhat angular, since these cells are multipolar, and the roots of their processes are often included in the plane of section. The satellite cells (glial cells) are sparse and less apparent.

Parasympathetic ganglia  (W pg 139, 7.22; R pg 336, 12.17) are located in the organ that is being innervated.  Go to slide #250 (vagina) stained with H&E [example] or Masson trichrome [example] or slide #75 (seminal vesicle) [example] and see if you can identify parasympathetic ganglia amongst large blood vessels and nerves in the deep connective tissue in the vaginal wall (outer 1/3), or in the connective surrounding the seminal vesicle.

You should be able to distinguish dorsal root (spinal) ganglia from autonomic ganglia and to identify neurons and satellite cells in these ganglia. What neuronal perikarya are found outside the CNS? (NS2) For the autonomic ganglia, you should be able to differentiate sympathetic ganglia from parasympathetic ganglia (remember: parasympathetic ganglia are IN THE WALL of the organ they innervate, whereas sympathetic ganglia are isolated clusters of neurons located near the spinal chord or associated with the dorsal aorta).

II. Peripheral nerves (and more ganglia)

In the peripheral nervous system, the larger diameter axons are surrounded by a lipid-rich myelin sheath formed by the Schwann cells (W pg. 138, 7.18). The Schwann cells (in the peripheral nerves) and the satellite cells (in the ganglia) are glial cells (supporting cells) of the PNS. Using slide #68 (W pg. 136-7, 7.13, 14, 16, 17; R pg 357, Plate 24, Figs 1, 2), examine a cross section of a nerve trunk.  It is made up of several fascicles, two of which are larger than the others.  Within one of the larger fascicles, study the axons in cross section, noting the wide range in axon diameter. The axon appears as a dot in the middle of a clear space, which is the region occupied by lipid-rich myelin (extracted during tissue preparation).  Most peripheral nerves carry both afferent (sensory) and efferent (motor) fibers.

The nerve and the fascicles (bundles of nerve fibers) that comprise it in this section are invested with a thick layer of loose connective tissue or epineurium.  Each fascicle is surrounded intimately by the perineurium, which is a layer of very flattened cells lying between the epineurium and groups of axons of the fascicle.  The endoneurium is a delicate layer of reticular fibers and other connective tissue components surrounding each individual axon.  What distinguishes endoneurium, perineurium and epineurium? (NS3)

Next, examine a longitudinal section of peripheral nerve, slide #67 (W pg 130, 7.7; pg 137, 7.15; R pg 357, Plate 24, Figs 3, 4).  Note the wavy course of the myelinated axons, which is characteristic of nerves seen in histological sections.  Note that you can see the slightly darker axon running through the clear myelin space.  Look for nodes of Ranvier [example] in places where the axons are cut in particularly favorable longitudinal section.  Myelinated axons may also be found in the dorsal nerve root in slide 65-2 [example]. As before, the nuclei present among the axons belong mostly to Schwann cells or to fibroblasts, although there are also occasional capillaries.  What are the numerous nuclei observed in non-myelinated nerves? (PNS4)

Examine slides #29 and #169 to find small nerve fibers (W pg 138, Fig. 7.19) and more autonomic (parasympathetic) ganglia (W. pg 139, 7.22; pg 267, 14.4c).  Look in between the layers of smooth muscle you studied in the connective tissue lab session.  An extensive plexus of nerves and parasympathetic ganglia (myenteric plexus) is present in the connective tissue separating these muscle layers (W pg 266, 14.4c) in slide 29 [example] and slide 169 [example].  Identify both nerve fibers and neurons of the parasympathetic ganglia.  This is a good way to practice distinguishing smooth muscle from adjacent nerve fibers. Small parasympathetic ganglia and nerve fibers may also be found in the loose connective tissue of the submucosa in slide 29 [example].

III. Neuromuscular junctions and muscle spindles

Alpha (somatic) motor neurons from the ventral horn of the spinal cord innervate muscle fibers (their effector cells) at specialized synapses called neuromuscular junctions (or motor end plates of skeletal muscles) [example]. These are best visualized with special stains that use heavy metals (gold) to label the nerve fibers or histochemical methods for acetylcholinesterase (an enzyme that hydrolyzes the neurotransmitter used by somatic motor neurons, acetylcholine).  The slide #71 in even numbered boxes (digital slide #71-2A) show a similar preparation of motor end plates.   Also, review the diagrams in your atlas (W pg.134-5, 7.12; R pg 291, 11.9; pg 311, Plate 19, Fig 3).

The terminal bouton of the motor axon has numerous synaptic vesicles that contain the neurotransmitter, acetylcholine. The terminal bouton lies in a depression in the surface of the muscle fiber, and is separated from it by a gap, the synaptic cleft, of uniform width. The plasma membrane of the muscle fiber is highly folded, and a basal lamina (also called the external lamina), is interposed between the nerve fiber and muscle fiber.  Synaptic transmission of nerve impulses across the synaptic cleft is accomplished by the release of acetylcholine from the synaptic vesicles (by exocytosis) into the synaptic cleft, where it diffuses to the muscle fiber membrane and activates acetylcholine receptors, which trigger membrane depolarization and subsequent muscle contraction.
           
Neuromuscular spindles are stretch receptor organs that regulate muscle tone via the spinal stretch reflex (W pg. 150, 7.33; R pg 294-5, 11.12). Look at slide #71 in odd-numbered boxes (digital slide # 71-1B) and identify the neuromuscular spindle [example] in the belly of the muscle. In this preparation (H&E, transverse section), the sensory nerve fibers of the spindle are not visible, but the modified skeletal muscle fibers (intrafusal fibers), which are smaller than the muscle fibers proper (extrafusal fibers), are easily visualized -- 2 to 10 are contained in a fluid-filled space within a discrete, external connective tissue capsule. Note the intrafusal fibers are bundled together by a delicate internal capsule that is not so evident in these sections. The sensory receptors (nerve endings) are activated by stretching of the intrafusal fibers, which evokes a reflex contraction of the extrafusal fibers that is driven by large (alpha) somatic motor neurons (located in the ventral horn) in a two-neuron spinal reflex arc. It is worth noting that, in addition to being stretch receptors, the intrafusal fibers are functional, contractile muscle cells. They are innervated by special (gamma) motor neurons that set the tone of the intrafusal fibers thus modulating sensitivity of the stretch receptor.

IV. Peripheral mechanosensory receptors

A. Meissner's Corpuscles (Ross pg 455, fig 15.13)
Slide 106 (plantar skin, H&E) [WinLab] [Mac] [WinHome]
Slide 112 (plantar skin, H&E) [WinLab] [Mac] [WinHome]

Meissner's corpuscles [example] are touch receptors that are responsive to low-frequency stimuli and are usually associated with hairless skin of the lips and palmar and plantar surfaces, particularly those of the fingers and toes. Generally, these receptors are tapered cylinders located in the undulating connective tissue just underneath the stratified epithelium of the skin. The long axis of the cylinder is perpendicular to that of the overlying epidermis and is usually about 150 um long and is usually tucked within extensions of the underlying connective tissue dermis (called "dermal papillae") that project into the underside of the epidermis. Within these receptors, one or two nonmyelinated endings of myelinated nerve fibers follow a spiral path through the corpuscle. The fibers are accompanied by ensheathing Schwann cells, the nuclei of which are flattened and stacked on top of each other giving the corpuscle its characteristic irregular, lamellar appearance.

B. Pacinian Corpuscles (Ross pg 455, fig 15.13)
Slide 42 (mesentery, H&E) [WinLab] [Mac] [WinHome]
Slide 95 (mesentery, H&E) [WinLab] [Mac] [WinHome]
Slide 95M (mesentery, trichrome) [WinLab] [Mac] [WinHome]

Pacinian corpuscles [example] are large, ovoid structures up to 1 mm in diameter found in the dermis and hypodermis of the skin and also in the connective tissue associated with bones, joints, and internal organs. They respond primarily to pressure and vibration and are composed of a myelinated nerve ending surrounded by a capsule. The nerve enters the capsule at one pole (which might be out of the plane of section and therefore not visible) with its myelin sheath intact but then it is quickly lost. The unmyelinated portion of the axon extends toward the opposite pole from which it entered and its length is covered by flattened Schwann cell lamellae that form the inner core of the corpuscle. The remaining bulk of the capsule, or outer core, is comprised of a series of concentric, onionlike lamellae with each layer separated by an extracellular fluid similar to lymph. Each lamella is composed of flattened Schwann cells and endoneurial fibroblasts. In addition the fluid between each layer, delicate collagen fibers may be present as well as occasional capillaries. Displacement of the lamellae by pressure or vibrations effectively causes depolarization of the axon, which sends the signal to the central nervous system.

Objectives

1. Be able to identify tissues in the nervous system (nerves, cell bodies and ganglia, and white vs. gray matter in the spinal cord, cerebellum, and cerebrum).
   
2. Describe the organization and understand some of the basic functions of regions of the:
   • spinal cord (e.g. dorsal horn, ventral horn, lateral horn, and dorsal nucleus of Clarke),
   • cerebellum (e.g. molecular, Purkinje, and granule cell layers and the general interactions of the cells therein)
   • cerebral cortex (e.g. layers I through VI, particularly pyramidal cells of layers III and V)
3. Observe ependymal cells of the choroid plexus, noting that these are the cells responsible for the production of CSF.
4. Observe the 3-layered organization of the hippocampus and dentate gyrus (archicortex) as opposed to the 6-layered organization observed in other regions of the cerebral cortex (neocortex).
5. Be able to identify pyramidal cells of the hippocampus and granule cells of the dentate gyrus.

I. Spinal Cord
Slide 65-1N (lumbar spinal cord, trichrome) [WinLab] [Mac] [WinHome]
Slide 65-2 (lumbar spinal cord, H&E) [WinLab] [Mac] [WinHome]
Slide 65-1 (lumbar spinal cord, H&E) [WinLab] [Mac] [WinHome]
Slide 66a (thoracic spinal cord, luxol blue & cresyl violet) [WinLab] [Mac] [WinHome]

Review the organization of the spinal cord using your atlas (W pg 393, 20.2; R pg 363, Plate 27).  Examine the cross section of the lumbar spinal cord in slide #65-2.  At low magnification, differentiate inner gray from outer white matter and identify dorsal and ventral horns of the gray matter. You should also identify the dorsal and ventral horns in Slide 65-1N stained with Masson trichrome. In these slides, dorsal happens to be "up," but you should be able to tell dorsal and ventral horns based on morphology and the cells present rather than the orientation. The perikarya of large somatic motor neurons [example] located in the ventral horn of the cord innervate the skeletal muscles of the limbs and trunk, which are embryologically derived from somites (hence, somatic muscles). Observe that the perikarya of neurons in the dorsal horn are much smaller. Why? (NS1)

Slide 66a shows a section of thoracic spinal cord. In addition to the dorsal and ventral horns, two structures especially obvious in the thoracic cord are the dorsal nucleus of Clarke and the lateral horn. The dorsal nucleus of Clarke [example] is in the dorsal horn and contains relatively large, multipolar neurons that receive proprioceptive information from dorsal root ganglion cells innervating muscle spindles in the trunk and lower limb. The cells of Clarke's nucleus then relay this information via axonal projections that extend all the way up into the cerebellum (hence the reason why the cells are so large) where it is processed to allow for coordinated movement. The lateral horn [example] contains relatively large, multipolar motor neurons of the intermediolateral cell column that extends from levels T1 through L2 of the spinal cord. The cells are preganglionic sympathetic neurons whose axons terminate in either sympathetic chain ganglia or the "visceral" ganglia associated with the major branches of the abdominal aorta (e.g. celiac, aorticorenal, and superior/inferior mesenteric ganglia).

Many neurons in the spinal cord may appear shrunken and surrounded by an empty space due to poor fixation.  Cells that are well preserved show features characteristic of most neurons: large cell body, large pale nucleus, Nissl substance, and cell processes (most of which are dendrites). The delicate meshwork of dendritic processes and nerve fibers (axons) lying between cells in the gray matter is called the neuropil. The white matter contains nerve fibers (axons) entering and exiting the gray matter, and traveling up and down the spinal cord, linking it to the brain. Another feature commonly found in nervous tissue are starchlike granules known as "corpora amylacea" [example] (amylon = starch, Greek) which are aggregates of dead cells and/or proteinaceous secretions that may be found in either white or gray matter. These granules are of little pathological significance, but they generally increase with age.

Nervous tissue contains two basic categories of cells: neurons and support cells (glia). Both neurons and glia have fine processes projecting from the cell body, which generally cannot be resolved in the light microscope without special staining techniques. Astrocytes in the CNS provide metabolic support for neurons and play an important role in maintenance of the blood-brain barrier whereas oligodendrocytes (another type of glial cell) are responsible for myelination of CNS axons. Recall that Schwann cells are the glial cells responsible for myelination in the peripheral nervous system. Myelin is lipid-rich, and on gross inspection appears white. Thus, in the 'white matter' of the brain and spinal cord, myelinated axons are the predominant neuronal component whereas most of the the nuclei that you see in white matter are primarily of glial cells. The ‘gray matter’ contains relatively more neuronal and glial perikarya as well as non-myelinated (e.g. dendritic) processes. The other major glial cell type you should know about are microglia which are small cells derived from blood monocytes. They are considered part of the mononuclear phagocytic system and will prolifereate and become actively phagocytic in regions of injury and/or inflammation. Because of the difficulty of discerning each glial cell type by routine light microscopy, you will not be required to identify glial cells by light microscopy, but you should be aware of their functions.

II. Neurons (Slide #65)

Neurons are characterized by a large cell body or perikaryon containing a large, pale (active, euchromatic) nucleus with a prominent nucleolus.  Scattered in the cytoplasm are the characteristic clusters of ribosomes and rough ER termed Nissl bodies  or Nissl substance [example].  One or more cell processes may also be seen emerging from the neuronal perikaryon. 
           
Review diagrams illustrating the morphology of neurons in your textbooks (R pgs 320-1, 12.1-2; W pgs 123-4, 7.1-7.3). The dendrites receive neural input from other neurons via synapses (or they are specialized to receive sensory stimuli), and they transmit neural information toward the perikaryon. A single axon (often called a nerve fiber) leaves the perikaryon and transmits neural signals to other neurons or to the effector organ (e.g., skeletal muscles) via synapses.

III. Cerebellum (W pg. 396, 20.5)
Slide 77 20x (cerebellum, H&E) [WinLab] [Mac] [WinHome]
Slide 77 40x (H&E) [WinLab] [Mac] [WinHome]
Slide 77a 40x (luxol blue/cresyl violet) [WinLab] [Mac] [WinHome]
Slide 78 20x (luxol blue/cresyl violet) [WinLab] [Mac] [WinHome]

Using slide 77, determine that the cerebellar cortex is organized into an outer molecular layer [example], containing basket and stellate cells (not distinguishable by routine light microscopy) as well as axons of granule cells found in the deeper, highly cellular granule layer [example].  Still deeper is the white matter [example] (medulla in W pg. 396, 20.5a) of the cerebellum, which contains nerve fibers, neuroglial cells, small blood vessels, but no neuronal cell bodies.

Examine the boundary between molecular and granule cell layers.  Here you will see the Purkinje cell bodies [example].  In these slides you will not be able to discern the amazing dendritic tree that extends from the Purkinje cell bodies into the molecular layer, nor will you be able to see their axons, which extend down through the granular layer into deeper parts of the cerebellum.  The dendritic tree and axon or each Purkinje cell can only be seen in thicker sections stained with special silver stains.  Most of the nuclei visible in the granular layer belong to very small neurons, granule cells, which participate in the extensive intercommunication involved in the cerebellum’s role in balance and coordination.

Slide 78 is a section of cerebellum still in the process of development (evidenced by the numerous small round progenitor cells, or neuroblasts [example], seen in the outermost margin of the molecular layer --some will remain in the molecular layer and differentiate into basket and stellate cells whereas most will migrate inward and differentiate into neurons in the granule cell layer). However, the general architecture of the cerebellum has been established, so you should still be able to discern the molecular layer, Purkinje cell layer, and granule layer of the gray matter and the internal white matter.

III. Cerebrum (W pg 140, 7.23; W pg 398-9, 20.8)
Slide 76 (cerebrum, luxol blue/cresyl violet) [WinLab] [Mac] [WinHome]
Slide 76b (toluidine blue & eosin) [WinLab] [Mac] [WinHome]
  
Unlike the highly organized cerebellar cortex, the cerebral cortex appears to be less well-organized when viewed with the light microscope.  Nonetheless, it is loosely stratified into layers containing scattered nuclei of both neurons and glial cells.  Examine the layered organization of the cerebral cortex using slide 76 stained with luxol blue/cresyl violet [ORIENTATION] (which stains white matter tracts and cell bodies) or toluidine blue and eosin [ORIENTATION] (TB&E, toluidine blue stains the nuclei and RER of cells whereas eosin stains membranes and axon tracts).  Typically one or more sulci (infoldings) will extend inward from one edge of the section.  Examine the gray matter on each side of the sulcus using first low and then high power.  Neurons of the cerebral cortex are of varying shapes and sizes, but the most obvious are pyramidal cells.  As the name implies, the cell body is shaped somewhat like a pyramid, with a large, branching dendrite extending from the apex of the pyramid toward the cortical surface, and with an axon extending downward from the base of the pyramid.  In addition to pyramidal cells, other nuclei seen in these sections may belong to other neurons or to glial cells also present in the cortex.  You may be able to see subtle differences in the distribution of cell types in rather loosely demarcated layers. There are 6 classically recognized layers of the cortex:

1. Outer plexiform (molecular) layer: sparse neurons and glia
2. Outer granular layer: small pyramidal and stellate neurons
3. Outer pyramidal layer: moderate sized pyramidal neurons (should be able to see these in either luxol blue [example] or TB&E-stained [example] sections)
4. Inner granular layer: densely packed stellate neurons (usually the numerous processes aren’t visible, but there are lots of nuclei reflecting the cell density)
5. Ganglionic or inner pyramidal layer: large pyramidal neurons (should be able to see these in either luxol blue [example] or TB&E-stained [example] sections)
6. Multiform cell layer: mixture of small pyramidal and stellate neurons

Pyramidal cells in layers III and V tend to be larger because their axons contribute to efferent projections that extend to other regions of the CNS –pyramidal neurons in layer V of motor cortices send projections all they way down to motor neurons in the spinal cord!

Deep to the gray matter of the cerebral cortex is the white matter that conveys myelinated fibers between different parts of the cortex and other regions of the CNS. Be sure you identify the white matter in both luxol blue [example] and TB&E-stained [example] sections, as it will appear differently in these two stains. Review the organization of gray and white matter in cerebral cortex vs. spinal cord.

IV. Hippocampal Region (virtual slide ONLY)
Slide NP004N (hippocampal region, coronal section, luxol blue) [WinLab] [Mac] [WinHome] [ORIENTATION]

This coronal section includes the hippocampus (hippocampus = sea horse), dentate gyrus, and adjacent temporal lobe gyrus (entorhinal cortex). Above the temporal (ventral or inferior) horn of the lateral ventricle the lateral geniculate nucleus is present. Lateral to this structure is the tail of the caudate. The medial surface of the section is the posterior portion of the thalamus and a small portion of the cerebral peduncle. Look at the margins of the ventricle at higher magnification and note that it is entirely lined by ependymal cells. Just medial (to the right) of the tail of the caudate, note the choroid plexus, which consists of highly convoluted and vascularized villi covered by ependymal cells which are specialized for the production of cerebrospinal fluid, or CSF.

Direct your attention to the hippocampus and dentate gyrus and observe the presence of three distinct layers rather than the six layers found in the cerebral cortex (evolutionarily speaking, the three-layered organization is considered to be "older," so this type of cortex is also known as "archicortex" whereas the "newer" six-layered cerebral cortex is "neocortex").

In the hippocampus [ORIENTATION], observe:

• ("1" in the orientation figure) a polymorphic layer containing many nerve fibers and small cell bodies of interneurons,
• ("2" in the orientation figure) a middle pyramidal cell layer containing hippocampal pyramidal cells, and
• ("3" in the orientation figure) a molecular layer containing dendrites of the pyramidal cells.

In the dentate gyrus [ORIENTATION], observe:

• ("4" in the orientation figure) a polymorphic layer containing nerve fibers (known as "mossy fibers") and cell bodies of interneurons,
• ("5" in the orientation figure) a middle granule cell layer containing the round, neuronal cell bodies of dentate granule cells, and
• ("6" in the orientation figure) a molecular layer containing dendrites of the granule cells.

The "hilus" is the region where the head of hippocampus abuts the dentate gyrus. The multipolar neurons in this areas are known as "mossy cells" and they primarily receive input from mossy fibers of the granule cells of the dentate gyrus and then relay those signals back to other cells in the dentate. In terms of clinical significance, the pyramidal cells of the hippocampus are particuarly vulnerable to damage in severe circulatory failure and by anoxia of persistent severe seizures. You may see small calcific bodies in part of the hippocampus, which occur as a normal part of the aging process. Calcific bodies are present in the choroid plexus, another common site of accumulation as the years pass.

Electron Micrograph Wall Charts

#51 MOTOR NEURON [WinLab] [Mac] [WinHome] (ventral horn, rat)

In this electron micrograph, note some of the features you saw in ventral horn motor neurons with the light microscope, such as the large, pale nucleus, prominent nucleolus, Nissl bodies, dendrites and axon.  Adjacent to the neuron, note myelinated axons of various sizes, and also that there is no space between cell processes--all space is occupied either by the processes of neurons or glia, or by capillaries (capillaries are somewhat swollen here because the tissue was fixed by perfusion).  (see aslo W pgs 124-5, 7.3).

#55 NERVE PROCESS [WinLab] [Mac] [WinHome] (autonomic nerve, kidney, rat) (PNS)

Most of the axons seen in this electron micrograph of an autonomic nerve are non-myelinated.  All of the non-myelinated fibers are embedded in grooves in the surface of Schwann cells (in some cases there may be more than one axon per groove), with each Schwann cell thus supporting a considerable number of these small axons.  Although the axons are very close together, you will observe thin partitions of Schwann cell between them.  Also note a few myelinated fibers and a very sparse endoneurium.  A very thin perineurium surrounds the nerve.

#57 SCHWANN CELL & MYELINATED AXON [WinLab] [Mac] [WinHome] (sciatic nerve, cross section rat)

In this cross section of a myelinated nerve process, note the axon, containing microtubules and neurofilaments, and bounded by a plasma membrane ("axolemma").  Outside the plasma membrane of the axon is the myelin sheath, which you will remember is composed of tightly wrapped plasma membranes of the Schwann cell.  Also, note the nucleus and cytoplasmic organelles of the Schwann cell.  Remember that the myelin is part of the Schwann cell, not of the axon.

#58 NODE OF RANVIER [WinLab] [Mac] [WinHome] (sciatic nerve, rat)

The myelin is much better preserved in this electron micrograph than in the earlier light microscope slides, but otherwise you are viewing the same structures.  Remember that the node of Ranvier is actually a short segment of the axon that is bare at the junction between two Schwann cells, making "saltatory conduction" possible.  Note the manner in which the myelin ends in each Schwann cell at the junction, by a "peeling off" of successive myelin layers, which come to lie against the axon as small cytoplasmic swellings.  (see also W pg. 130, 7.7).

#53 NERVE PROCESS [WinLab] [Mac] [WinHome] (CNS)

In this field you see two oligodendrocytes, the cells that make myelin in the CNS, surrounded by numerous myelinated axons of various size, cut in cross section.  Compare and contrast the cellular processes of myelination in the CNS and PNS. (NS5)

Review Question Answers

NS1: Why are perikarya of dorsal horn neurons smaller than those in the ventral horn?

answer

NS2: What neuronal perikarya are found outside the CNS?

answer

NS3: What distinguishes endoneurium, perineurium and epineurium?

answer

NS4: What are the numerous nuclei observed in non-myelinated peripheral nerves?

answer

NS5: Compare and contrast the cellular process of myelination in the CNS and PNS.

answer

Practice Questions

Click here to view image.
1. The nuclei indicated by the arrows in Panel A and labeled "N" in the electron micrograph in Panel B (which is a high magnification vewi of the boxed area in panel A) belong to:

1. Schwann cells
2. smooth muscle cells
3. fibroblasts of perineurium
4. dorsal root (sensory) ganglion neurons
5. autonomic ganglion neurons

ANSWER

Click here to view image.
2. What type of cell junction plays an important role in the FUNCTION of the connective tissue layer indicated by the arrows?

1. gap junction
2. desmosome
3. hemidesmosome
4. zonlula adherens
5. tight junction

ANSWER

Select the MAC link or PC link to view image.
3. The cell indicated by the arrow is a/an:

1. dorsal root ganglion neuron
2. autonomic ganglion neuron
3. satellite cell
4. Schwann cell
5. intrafusal muscle fiber (cell)

ANSWER

click here for image
4. The arrow indicates a morphological type of neuron. Neurons such as this are found in ALL areas of the nervous system, EXCEPT in the:

1. cerebellar cortex.
2. cerebral cortex.
3. gray matter of the spinal cord.
4. dorsal root ganglia.
5. peripheral autonomic ganglia.

ANSWER

click here for image
5. Two views of the same tissue at 40X and 400X are shown. The higher magnification view is from an area similar to that outlined by a square on the left panel. The tissue is most likely from the:

1. gray matter of the spinal cord.
2. white matter of the spinal cord.
3. gray matter of the cerebral cortex.
4. white matter of the cerebral cortex.
5. gray matter of the cerebellar cortex.
6. white matter of the cerebellar cortex.

ANSWER

6. Which of the following cell types contributes to maintenance of the blood-brain barrier?

1. astrocytes.
2. oligodendrocytes.
3. microglia.
4. ependymal cells.
5. neurons.
6. NONE of the above

ANSWER

Click here to view low magnification image
Click here to view area in white box at higher magnification
7. The section shown is from which region of the nervous system?

1. peripheral autonomic ganglia
2. dorsal root (spinal) ganglia
3. spinal cord dorsal horn
4. spinal cord ventral horn
5. cerebellum
6. cerebral cortex

ANSWER