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Case 1:

Dr. Staugaitis will demonstrate patellar reflex on several class volunteers. Everyone will be amazed at how rapid the response is. This will introduce discussion of the cellular anatomy of the cell types that participate in the monosynaptic reflex arc.

Slides: [Diagram of monosynaptic reflex arc]

This image will help you follow the pathway of the monosynaptic reflex arc. Each letter is keyed to images illustrating that part of the pathway.

[A- Sensory receptors]

Most sensory information is provided to sensory neurons via specialized sensory receptors. These are usually composed of one or more cells that relay information about the quantity and quality of the stimulus. This image shows a pacinian corpuscle, a cell complex in the deep dermis that is sensitive to pressure. For supplementary reading on sensory receptors, see Kierszenbaum, Histology and Cell Biology, pages 308-311.

B- Axons (Sensory and Motor)

[B1- Electron micrograph of a myelinated peripheral nerve axon]

1. Identify Schwann cell nucleus, myelin, axon, basal lamina, collagen.
2. What structures can you identify within the axon cytoplasm?

[B2- High magnification image of teased fiber preparation]

1. Identify the Node of Ranvier.

C- Nerve

[C1- Low magnification image of peripheral nerve]

1. Identify epineurium, epineurial blood vessels, nerve fascicle, perineurium.

[C2- Masson trichrome stain of cross section of peripheral nerve fascicle]

1. Can you distinguish the myelin from the endoneurial collagen?

[C3- Masson trichrome stain of longitudinal section of peripheral nerve fascicle]

1. Can you identify the node of Ranvier in this image?

[C4- Low magnification electron micrograph of portion of peripheral nerve fascicle]

1. Identify large myelinated fibers, small myelinated fibers, non-myelinated fibers.
2. What is the relationship between axon diameter and thickness of myelin?
3. What is the difference (to the Schwann cell) between myelinated fibers and non-myelinated fibers? Do oligodendrocytes have the same relationship with CNS axons?

D- Sensory ganglion (Dorsal root ganglion)

[Low magnification image of dorsal root ganglion]

The cell bodies of sensory neurons are organized into clusters called ganglia and neuronal cell bodies are often called ganglion cells. The dorsal root ganglia are located in the dorsal or posterior spinal nerve root. What cranial nerves have sensory ganglia? Where are the ganglia of the autonomic and enteric nervous systems and what are the different ganglia called? (You will learn about these in other organ blocks.)

1. What does pseudounipolar mean? Can you distinguish between the “axon” and “dendrite” of a pseudounipolar neuron?

E- Spinal cord

[E1- Virtual slide of spinal cord]

1. Identify dorsal root, dorsal horn, ventral horn, ventral root.

[E2- Medium magnification of ventral horn]

1. Identify motor neurons.
2. Identify nucleus, nucleolus, Nissl substance.
3. What is Nissl substance? What does the abundance of Nissl in neurons imply about neuronal protein synthesis?
4. How are these proteins transported to the synapse?

F- Skeletal muscle

[F1- Low magnification cross section of skeletal muscle]

1. Epimysium is not shown in this image. Identify perimysium, muscle fascicle,endomysium, muscle fiber/cell, perimysial/interfascicular blood vessel.
2. Describe two major features of the skeletal muscle cell (hint- number and location of nuclei).

[F2- Esterase histochemistry of skeletal muscle]

1. Can you see the endomysial nerve twigs?
2. The endomysial nerves end in foci of strong esterase staining. What are these structures? (Hint: What esterase is important in neuromuscular function? Where is this esterase located and what does it do?)

[F3- ATPase histochemistry (pH 9.8) of skeletal muscle]

1. This preparation demonstrates three fiber types: 1, 2a and 2b. How do type 1 and type 2 fibers differ physiologically?

[F4- High magnification electron micrograph of longitudinal section of skeletal muscle]

1. What is a sarcomere? Identify the limits of the sarcomere.
2. Where are the thick and thin filaments located? What is the major protein composition of the filaments?
3. The molecular details of muscle cytoskeleton and contraction will be presented in Week 6.

Case 2:

A 68-year-old female presents with a 6-month history of weight loss (20 lbs), paresthesias (tingling and other abnormal sensations) and intermittent pain in the lower extremities. Reflexes are diminished. Erythrocyte sedimentation rate was elevated. Muscle and nerve biopsies were performed. The muscle biopsy was abnormal but indicated that abnormality was due to nerve injury. The nerve showed a necrotizing vasculitis involving the epineurial arteries.

Slides: [Low magnification image of normal epineurial blood vessel] [This patient's epineurial blood vessel]

1. What is the difference between the normal vessel and the one involved by vasculitis?
2. What effect could vascular injury have on the nerves supplied by this vessel?

[Normal teased fiber preparation] [Teased fiber preparation of this patient]

1. What is the difference between the normal teased nerve fiber and the affected one from our patient with vasculitis?
2. What is the eponym given to this degenerative process? Was this sample taken proximal or distal to the transection? What class of inflammatory cells is involved in clearing the debris?

[Normal anterior horn cells] [This patient's anterior horn cells]

1. What is the difference between the normal anterior horn cells and the affected ones in our patient? (Note: the original magnification for the abnormal is 2x that of the normal image.)
2. Central chromatolysis is a descriptive term applied to the reparative phenomenon seen in our patient. What characteristic feature of motor neurons is no longer visible? What does this imply about protein synthesis in the motor neuron? (Hint: What are the major proteins required for axonal regeneration?)

[Normal skeletal muscle] [This patient's skeletal muscle]

1. What is the difference between the normal skeletal and the affected muscle in our patient?
2. What concept learned in Week 3 does the histology of our patient's affected muscle illustrate?
3. Do you think the pathology in this image resulted from interruption of one or more than one motor unit?

Case 3:

The patient above is treated and over time her symptoms are beginning to resolve. Unfortunately, she develops peptic ulcer disease and is hospitalized after an upper GI bleed. She dies of hypotensive shock. An autopsy is performed. Permission is given to evaluate peripheral nerves and skeletal muscle.

Slides: [High magnification electron micrograph of normal large myelinated axon] [High magnification electron micrograph of large remyelinated axon from this patient]

1. What is the difference between native and regenerated myelin? What is the difference in the length of the internode (not shown)? What is the functional implication of these differences?

[ATPase histochemistry (pH 9.8) of normal skeletal muscle] [ATPase histochemistry (pH 9.8) of affected skeletal muscle from this patient]

1. The patient's muscle shows "fiber type grouping".
2. What is a motor unit? What does fiber type grouping tell you about the size of a motor unit in reinnervated muscle?
3. What is the functional implication if the motor neuron supplying a large motor unit dies?