Lecture #17

References: Ross et al. Pages 256-267; 273-276

Wheater’s Functional Anatomy (4^th ed.) Pages 116-133; 140-142

Nerves

Since you will study the structure and function of neurons extensively next semester, this lecture, therefore, designed to superficially review some material covered previously in other classes. The main purpose of this lecture is to be able to identify components of the nervous system in the periphery. This includes identifying nerves (primarily axons), ganglion, and their sensory receptors in tissue sections.

I. The Neuron

A. Neurons are similar to all other cells in the body, except that they are specialized for receiving and conducting information; these functions are reflected in their polarized nature (as shown in this nerve in Figure 1).

1. Cell body (also called the soma) – consists of the nucleus and the perikaryon, the cytoplasm around the nucleus. Constitutes 10% of receptive area of the neuron.

a. prominent structures in the perikaryon can be visualized with “Nissl” stains, basic dyes (developed by Franz Nissl).

b. Nissl stains RNA and DNA; neurons stain prominently because they contain more RNA in their cytoplasm than any other cell type.

c. At the EM level, Nissl bodies can be shown to be stacks of rough endoplasmic reticulum. Neuron cell bodies are essentially “protein” factories

d. The nucleolus of neurons and stains densely with Nissl stains

2. The Axon – single process extending from the cell body; the axon is not just an extension of the cytoplasm of a neuron, but it is a specialized structure

a. Though it stains throughout the perikaryon, there is NO staining of the axon hillocks or axon with Nissl stains; axons have no ribosomes and no protein synthesizing capabilities. All proteins necessary in axons or at presynaptic terminals must be synthesized in the neuron cell body and transported.

b. A highly organized microtubule and microfilament network transfers proteins to the axon terminals and provides a highly organized cytoskeleton that provides internal support.

c. The axon is the conducting portion of the neuron; it conducts in an all-or-none fashion.

d. In man, most axons are myelinated by concentric layers of the Schwann cell plasma membrane. This Schwann cell insulation of the nerve and unmyelinated areas between the myelin called “Nodes of Ranvier” help increase the speed of the nerve impulse.

e. The terminal endings of axons, or telodendria, are unmyelinated. At their endings, they have specialized pre-synaptic dilations called boutons. This is where synaptic vesicles containing chemical neurotransmitters are released into the synapse. In the periphery, the neurotransmitter is almost always acetylcholine (Ach). Postganglionic sympathetic nerve fibers do utilized norepinephrine (NE).

3. Dendrites – a neuron has many dendrites that receive information.

a. processes are shorter and thicker than an axon

b. unmyelinated

c. form extensive arborization called the dendritic tree

d. no sharp delineation between cell body of neurons, unlike axon

e. has similar organelles as the cell body, except Golgi bodies which are located in proximity to the prominent nucleolus

II. Nerve fibers are organized similar to muscle fibers.

A. Endoneurium – delicate packing of loose vascular supporting tissue surround the plasma membrane of the Schwann cell

B. Perineurium – condensed layer of robust collagenous tissue invested a layer of flat epithelial cells. The perineurium invests nerves fascicles.

C. Epineurium – loose collagenous tissue that binds the fascicles together to form the strong cylindrical sheath of the peripheral nerve

III. Neurons can be divided into a number of structural and functional types, including the number of processes arising from the cell body. As shown in Figure 2, neurons can be pseudounipolar, bipolar, or multipolar.

A. Multipolar neurons are the most common type as numerous dendrites project from the cell body.

B. Bipolar neurons are rare and have only a single dendrite that arises from the cell body opposite the origin of the axon. Seen in neurons for the senses of smell, sight, and balance.

C. Pseudounipolar neurons have a single dendrite and the axon arising from a common stem of the cell body. Seen in primary sensory neurons (think dorsal root of the spinal nerve).

IV. Schwann Cells and the Myelin Sheath

A. Myelinated axons are surrounded by a lipid-rich layer called the myelin sheath, which is produced by Schwann cells that wrap in a spiral around a short segment of myelin on an axon. The lipid component of the myelin sheath gives nerves a vacuolated appearance in standard histological stains.

1. The myelin sheath is external and contiguous with the neurilemma, or sheath of Schwann, which is a layer of Schwann cell cytoplasm that contains the nucleus and most of the organelles of the Schwann cell.

2. Only the axon hillock and terminal arborizations where the axon synapses with its target cells are completely devoid of the myelin sheath.

3. The myelin sheath is segmented because it is formed by numerous Schwann cells arrayed sequentially along the axon. At the junction between two Schwann cells, the axon is unmyelinated and called the node of Ranvier.

4. The myelin sheath is formed once the axon is initially surrounded by a Schwann cell. The Schwann cell begins to wrap around the axon forming multiple Schwann cell layers. Oligodendrocytes are responsible for the CNS myelin.

V. What you can identify in peripheral tissue in this course!!!

A. Peripheral Nerves – look for the zigzag pattern of Schwann cells elongated along the longitudinal axis.

1. Found often near arteries and veins in the neurovascular bundles.

B. Sympathetic Ganglia – many cell bodies surrounded by irregularly placed satellite cells. These satellite cells are simple cuboidal cells that surround the cell bodies and provide metabolic and structural support. They are much more distinct in spinal ganglia.

C. Free Nerve Endings – the simplest form of sensory receptor (dendrite).

1. Best visualized with silver impregnation where you see the terminal branches of these unmyelinated, afferent nerve fibers.

2. Found along the junction of the dermis and epidermis in skin.

D. Meissner’s Corpuscles –small, encapsulated sensory receptors found in the dermis of skin

1. Involved in perceiving light discriminatory touch.

2. Concentrated in the papillary layer of the skin’s dermis, particularly in the fingertips, the soles of the feet, eyelids, lips, and genitalia

3. A mass of plump, oval cells surrounded by a delicate collagenous tissue

E. Pacinian Corpuscles - large, encapsulated sensory receptors found in the deeper layers of the skin, ligaments, and in joint capsules.

1. Looks like an onion; concentric lamellae of flattened cells

2. Responsive to pressure, vibrations, and tension.

F. Remember neuromuscular spindles and neuromuscular junctions have already been covered in this class.