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Unit 3 - Microscopic Anatomy of
the Central Nervous System

Unit 3 Learning Objectives

A. Neurons
B. Neuroglia
C.Processing of Information

The C.N.S. is comprised of two basic neural elements, the neurons and the neuroglia, in addition to connective tissue elements, blood vessels, and extracellular fluids similar to those present in other regions of the body.

Links to related animations:

25. Cerebral Arteries

A. Neurons

Nerve cells, or neurons, contain the usual organelles and inclusion bodies and are the functional units of the nervous system.   Structurally, neurons are distinguished from other cell types by their unusually long protoplasmic processes and by the presence of nissl substance in the cell body. Functionally, they differ from other cell types by the properties of irritability and conductivity.

Although most cells within the body can regenerate following injury, neurons cannot due to various physiological processes which occur at the time of injury or disease.Neurons have a great deal of variability in size and shape.   They can run between 5u (e.g. the granular cells of the cerebellum) and 80u (e.g. anterior horn cells in the lumbosacral region) in size, and vary from the pyramidal shaped efferent nerve cells in the cerebral cortex to the spherical dorsal root ganglia and the oval shaped sensory nuclei in the spinal cord.

1. Structural Characteristics of Neurons

a. Nissl Substance

Nissl Substance consists of a special organization of endoplasmic reticulum and ribosomes which contain RNA. The amount, shape, and distribution of the substance varies from cell to cell, apparently being directly related to cell function.
The main function of nissl substance is to replace protein consumed during normal cellular activity to allow for continuous conductivity.

b. Dendrites

Dendrites are protoplasmic processes, usually shorter in length than axons, which represent an extension of the cell body.   Usually more than one per neuron and branching profusely via dendritic spines, dendrites serve to increase the surface area of the neuron and enable it to receive impulses from more numerous and more widely separated sources. Dendrites are considered afferent and carry information toward the cell body.

c. Axons

The protoplasmic axon, in contrast to dendrites, is longer, of more uniform diameter, and does not generally branch until near its termination. Although there is usually one axon per neuron, collateral axons may occur along its course. The terminal portion of the axon is specialized for the transmission of impulses and divides into several telodendria (terminal feet). Axons are considered efferent and carry information away from the cell body. An axon may be wrapped in a myelin sheath or it may be naked (non-myelinated). If myelinated, the myelin is interrupted at various points called Nodes of Ranvier along the length of the axon. These “nodes” enable the nerve impulse to literally jump along the axon by way of saltatory conduction, thus giving myelinated axons a faster conduction velocity than non-myelinated fibers.

d. Synapse

Synapses are the functional connections between separate neurons. A synapse consists of the specialized terminal ending of the axon called a bouton terminal, or terminal button, a synaptic cleft, and either the axon, the body, or the dendrite of a second neuron. Bouton terminals contain many mitochondria and small pre-synaptic vesicles that hold a transmitting agent, which may be either facilitory or inhibitory. The receiving area of the synapse houses specialized post-synaptic vesicles, which contain a deactivating chemical.

Fig. 1 –

Neuron Structure, The compound action potential, biomedia.bio.purdue.edu/GenBioLM/ GBActPotl/html/neuron_structure.html

Click for Printable PDF Figure 1.

Fig. 2 –

Neuron,The Brain, http://www.enchantedlearning.com/subjects/anatomy/brain/Neuron.shtml

2. Neuron Types

Three basic types of neurons have been named on the basis of the number of cell processes. Each type differs in its functional capacity, i.e. its ability to process information.

a. Unipolar

Unipolar neurons have only one cell process which divides into two parts a short distance from the cell body, and thus are referred to as “pseudounipolar The only example of this type in humans is the cells of the dorsal root ganglia.

b. Bipolar

Bipolar neurons possess two cell processes (central and distal) (axon and dendrite). Although common during development, bipolar neurons are only found in the vestibular ganglia, the auditory ganglia, the olfactory ganglia, and the retina ganglia of the mature adult nervous system.

c. Multipolar

Multipolar neurons are most commonly found in the adult. They consist of one axon and many dendrites (e.g. Type I and Type II).

Fig. 3 –

Unipolar neuron http://www.psyweb.com/ Physiological/Neurons/unineuron.html

Fig. 4 –

Bipolar neuron http://www.psyweb.com/ Physiological/Neurons/unineuron.html

Fig. 5 –

Multipolar neuron http://www.psyweb.com/Physiological/Neurons/unineuron.html

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3. Classification of Nerve Fibers

Many studies have been conducted to determine the divisions and classifications of the peripheral nerve fibers. The three variables used in this classification are conduction rates, which depend upon diameter of the fiber and the amount of myelinization present on the fiber. (see following chart)

Erlanger and Gasser	Lloyd	Function	Myelin	C.R. (m/sec)	Diameter (v)
A-Alpha	I. A	1. Efferent to skeletal muscle 2. Afferent from primary ending of the muscle spindle (nuclear bag and chain)	Very heavily myelinated	Very fast	12-200
A- Alpha	I. B	1. Afferent from Golgi Tendon Organs	Very heavily myelinated	Very fast	12-20
A-Beta	II.	1. Afferent from secondary endings of muscle spindle (nuclear chain only) 2. Touch (specific)	Heavy	Fast	5-12
A-Gamma		1. Efferent to muscle spindle	Heavily or Moderately myelinated	Fairly fast	5-12
A-Delta	III.	1. Non-specific touch and pressure receptors 2. Pain, temperature, and pressure receptors (specific)	Moderately myelinated	Mod. fast	3-5
B	III.	1. All pre-ganglionic fibers (ANS) 2. Some post-ganglionic fibers (ANS)	Finely myelinated	Slow	3-5
C	IV.	1. ANS-efferent post-ganglionic fibers 2. Poor localized pain & touch 3. Temperature	Unmyelinated	Very slow	0.5-1.0

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B. Neuroglia

Neuroglia, or Glial Cells, have long been known as the supportive framework of the nervous system representing nearly one half of its total volume and outnumbering neurons by five to ten times. Yet their function does not end with a mere supportive one. Like neurons, glial cells possess long protoplasmic processes, but unlike neurons, mature neuroglia are not specialized for excitation and conduction. One other important difference is that mature glial cells are capable of undergoing mitosis and are known to proliferate in response to infection and injury. In fact, it is the neuroglial cells, and not the neurons, which are most frequently involved in tumors in the nervous system. Neuroglia can be divided into three main classes based on their relative size and primitive cell origin.

1.  Types of Neuroglia

a.   Macroglia

1. Astrocytes

Astrocytes, or Astroglia, are relatively large glial cells that possess star-like radiating processes.   Some of these processes end on and actually envelop small blood vessels. Others join with the pia mater to invest the brain and spinal cord.   Since they are confined to the C.N.S., astrocytes are truly the “skeleton” of the nervous system. They also serve an important function as “insulators” between adjacent neuronal processes. They are also part of a system that transports the chemicals necessary for cellular metabolism to the neurons, which allows the neurons to produce the energy needed for function. Astrocytes proliferate in response to injury, infections and degeneration of the C.N.S in an attempt to re-wire the damaged system. However, this proliferation can be so extensive as to form a glial scar, or tumors, which may in itself be the cause of severed ends of axons being incapable of regeneration.

2. Oligodendrocyte

Also large and confined to the C.N.S. are the glial cells known as oligodendrocytes, or oligodendroglia. They have fewer processes than the astrocytes, and they are often found adjacent to the nerve cell bodies where they are known as satellite cells. These cells are responsible for the formation of myelin sheaths around axis cylinders. Unmyelinated fibers, as well, lie in indentations of the cell membrane of the oligodendrocytes.

3. Ependymal Cells

This group of macroglia was previously described, their function being to line the ventricular system of the C.N.S

b. Microglia

Long thought to be of mesodermal origin, it has now been suggested by some that microglia are actually undifferentiated glial cells. Smaller than other types, their chief function appears to be to differentiate into phagocytes and assist in removing the debris of degeneration and infection.

c. Schwann Cells

Schwann cells are the glial cells of the peripheral nervous system.  Like the oligodendrocytes of the C.N.S., they form myelin sheaths around cranial and spinal nerves.

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C. Processing of Information

The processing of information, including transmission, correlation, and storage is the basic function of the nervous system.   This is dependent on a vast variety of neural circuitry.   Those circuits common to sensory and motor systems are as follows:

1. Patterns of Neural Circuitry

a. Convergence

Convergence means that many input fibers simultaneously supply and influence the activity of a single neuron and that neuron’s output fibers. The input fibers can be from the same source, or from two different sources.

b. Divergence

Divergence is when one neuron supplies and influences the activity of numerous neurons and their output fibers.

Fig. 6 –

REACTION TIME AND NEURAL CIRCUITRY, Directions for Teachers. http://www.nabt.org/sup/publications/
nlca/nlcapdf/13NLCAchp11.pdf

2. Feedback Circuits Used for Motor Control

a. Closed Circuits

Fig. 7 –

In a closed circuit feedback, recurrent collaterals branching from the output fiber (Alpha fiber) connect with interneurons, or Renshaw cells, which supply and influence (inhibits) the activity of the parent neuron. This circuitry allows for non-repetitive actions.

b. Open Circuits

Fig. 8 –

In an open circuit feedback, circuit collaterals from the output fiber connect with many interneurons which in turn facilitates (disynaptic or multisynaptic) connections with the parent neuron.   This permits a “reverberating” influence on the parent neuron. This circuitry allows for prolonged repetitive actions.

c. Parallel Circuits

Fig. 9 –

In a parallel circuit feedback, there are a number of parent neurons, each of which has a collateral that is facilitory to the terminal neuron.   This circuitry allows for repetitive actions that are not prolonged in nature such as walking.

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