Histology Mentor

Nervous System: Questions 123-136

123. Question - Make a cross sectional drawing of the spinal cord in the thoracic region labeling the major gray and white matter components. Show the locations of sympathetic and somatic motor neurons.

Gray matter is surrounded by white matter, the ascending and descending fiber tracts found in the funiculi. The somatic nerve cells which innervate the skeletal muscles are also called ventral horn cells, final common path cells and lower motor neuron cells.

124. Question - Diagram three morphological types of neurons naming their component parts. What is the initial segment? What is axon transport?

Nissl granules are clumps of RER and free polysomes. They are not found in the axon or axon hillock. The initial segment is a constricted portion of the axon just beyond the axon hillock. Action potentials usually originate here. The nerve cell body is called soma, perikaryon, or cyton. Axon transport is mediated by axon microtubules. It carries vesicles and organelles from the soma toward the axon terminal (anterograde transport) or vesicles containing sampled extracellular material (neurotrophic fators, viruses, toxins etc.) from the terminal to the soma (retrograde transport).
Dendrites are cytoplasmic processes containing the same organelles as the perikaryon. They with their "dendritic spines" that project from them increase the receptive field of the neuron.

125. Question - Using diagrams compare and contrast the structure of autonomic and sensory ganglia.

These ganglia are similar in that they are important locations of nerve cells in the peripheral nervous system. They are different in that the nerve cells of autonomic ganglia are multipolar (They receive synapses from myelinated preganglionic nerve fibers) and their unmyelinated axons innervate the heart, smooth muscle and glands while the sensory ganglion cells are monopolar (no synapses). The sympathetic ganglion cells are small and scattered but the sensory cells are large and found in clusters. In contrast to the autonomic ganglion cells the sensory cells having only one process, can be very close together and can have many capsule cells surrounding them.

126. Question - Illustrate the structure of a myelinated and an unmyelinated peripheral nerve fiber. How is action potential conduction influenced by fiber diameter?

A peripheral nerve fiber consists of an axon and its ensheathing Schwann cells. The membrane of the ensheathing cells may wrap around the axon multiple times forming myelin or it may simply form grooves for the unmyelinated axons to pass through. Multiple Schwann cells ensheath a single axon and in myelinated fibers the gaps between Schwann cells are the nodes of Ranvier. In H&E sections the remnant of myelin in Schwann cells is called neurokeratin. Action potential conduction is slow in unmyelinated fibers and rapid in large thickly myelinated fibers.

127. Question - Show with a diagram how a peripheral nerve is organized.

The epineurium is irregular connective tissue that may contain fat cells and surrounds the nerve and fills in between fascicles. The perineurium is a special sheath of flat epithelial cells that surround each fascicle while the endoneurium is the connective tissue inside the fascicles surrounding individual nerve fibers. This diagram shows 4 nerve fascicles.

128. Question - Diagram the structure of the cerebellar cortex.

The cerebellar cortex is gray matter organized in three layers. The underlying white matter consists of nerve fibers coming to the cortex and fibers of Purkinje cells leaving the cortex.

129. Question - Illustrate the histology of the eye showing the relationships of features associated with the lens and iris and the flow of aqueous humor. Explain the process of accommodation.

The eye is described as having three layers. The outer consists of the cornea and sclera, the middle of the choroid that extends into the ciliary body and the inner consisting of the retina and the epithelium that extends forward to cover the ciliary processes and the back of the iris. Aqueous humor arises from the unique epithelium (stratified cuboidal with a basal lamina on both surfaces) of the ciliary processes and is absorbed in the canal of Schlemm. The vitreous body fills the space between the lens and the retina.

Accommodation (focus for near vision) occurs when the ciliary muscle that is anchored to the sclera anteriorly pulls on the elastic choroid. This relieves the tension on the oxytalan fibers that attach to the edge of the lens to allow the lens to become rounder.

130. Question - Diagram the layers of the retina illustrating the meaning of convergence. Describe and explain the significance of the fovea and the optic cup.

The fovea is the site of acute vision. All of the cell bodies are pushed off to the side allowing the light to reach the rods and cones with minimal scattering. The optic disk is the blind spot where the axons of ganglion cells leave the eye to enter the optic nerve.

Müller cells are supporting cells that extend through the entire retina and form the outer and inner limiting membranes.

131.Question - Explain the relationship of the perilymph to the endolymph. Diagram the components of the inner ear.

The inner ear consists of membranous tubules that are suspended in complementary spaces within the temporal bone. Perilymph (similar to cerebrospinal fluid) surrounds the membranous labyrinth. Endolymph (produced by the stria vascularis) is found within the tubules and is more like intracellular fluid having more potassium than sodium. Hair cells in the maculae and the cristae comprise the vestibular apparatus detecting orientation with respect to gravity and both linear and angular acceleration.

132. Question - Illustrate the components of the Organ of Corti and its relationship to the spiral ganglion.

133. Question - Diagram the structure of a neuromuscular spindle and briefly explain its function. Explain the basis of the stretch reflex.

In the stretch reflex the gamma motor neuron sets the sensitivity of the spindle to stretch. When stretch activates the afferent endings an action potential monosynaptically activates the alpha motor neuron which causes contraction of the extrafusal fibers.

134. Question - Show in a diagram the relationships of the meninges to the cerebrospinal fluid, the central nervous system and the blood.

The cerebrospinal fluid is mostly formed in the ventricles by the choroid plexuses. It flows into the subarachnoid space through special apertures and is removed from the subarachnoid space through the arachnoid granulations that project into the venous sinuses.

135. Question - Explain how the different types of synapses found in the central nervous system are named. Diagram in detail the features associated with a synapse.

Synapses are named for the direction of information flow. The source of neurotransmitter is named first. The classic synapse is an axodendritic synapse but axosomatic synapses are also numerous. Most possible combinations have been observed.

An action potential triggers the influx of calcium ions in the axon terminal and this promotes the exocytosis of the contents of synaptic vesicles. Endocytosis conserves the plasma membrane. When a neurotransmitter binds to its receptor on the postsynaptic membrane ion flux is modified to either hyperpolarize (inhibit) or depolarize (excite) the membrane. Inhibition occurs when the membrane potential becomes more negative and vice versa.

136. Question - Compare and contrast gray and white matter of the central nervous system. What are glial cells? Explain their functions.

In the central nervous system gray matter is the location of nerve cell bodies and their processes. It is in the gray matter that communication between neurons via synapses occurs. White matter consists of mostly myelinated nerve fiber bundles.

There are three types of glial cells, astrocytes, oligodendrocytes and microglia and they can be found in both gray and white matter. Since there is no collagenic connective tissue in the central nervous system these glial cells provide the supporting stroma.

Astrocytes have many processes some of them end by wrapping around capillaries while others fill in between nerve cells and their processes. Astrocytes contain bundles of intermediate filaments (glial fibrillary acidic protein) that provide physical support for nerve tissue. These cells also function to buffer changes in the ionic environment.

Oligodendrocytes are often found close to nerve cells in the gray matter where they are called satellite cells. They are also responsible for making myelin in the central nervous system. Each oligodendrocyte has several processes and each of them forms a segment of myelin much like a Schwann cell does in the peripheral nervous system. Thus an oligodendrocyte produces myelin segments for several axons.

In gray matter the feltwork of glial and nerve cell processes is called neuropil. Microglia are part of the mononuclear phagocytic system coming from blood monocytes and capable of proliferating and engulfing cellular debris in the event of injury.

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