SPH405

GOAL: TO PRESENT THE SPECIAL CHARACTERISTICS OF NEURAL TISSUE

Objectives:
Distinguish neural tissue from other tissue types

Describe the roles of neurotransmitters in the propagation of action potentials.

Describe the general anatomy of the neuron and relate it to nervous system function.

Differentiate the functions, morphology and locations of the three types of neurons.

REFERENCES:

TEXT: Chapter 5

Hamilton, W. (1976). Textbook of Human Anatomy. St. Louis, MO: C.V. Mosby.

Leeson, C., and Leeson, T (1976). Histology (3rd, Ed.)). Philadelphia: W. B. Saunders.

Zemlin, W. (1988). Speech and Hearing Science: Anatomy and Physiology (3rd. Ed.). Englewood Cliffs, NJ: Prentice-Hall.

Neural Tissue is composed of special types of cells.
The primary characteristic that distinguish neural tissue from other tissue types are the properties of EXCITABILITY (IRRITABILITY) AND CONDUCTIVITY
Excitability is the capacity for response to physical or chemical agents.

1. To some extent, excitability and conductivity are properties common to all living protoplasm.



2. If you probe an amoeba with a point, it will withdraw, as a whole unit.



3. The cell of the unicellular animal is excitable, and the excitement spreads (is conducted) throughout the entire organism.

In higher animals, there are specialized cells in which these properties are more highly developed: NEURONS.
In neurons, excitability has developed to the point that has conductivity. Nervous tissue is distinguished from other tissue types by two properties:

1. Conductivity is the ability to transmit an impulse from one location to another.



2. Neurons show polarity or directionality: The impulses are conducted in one direction only.

Another distinguishing feature of nervous tissue is its variety of cell sizes and shapes (Leeson and Leeson, 1976).
The neuron is the basic functional and anatomic unit of the nervous system.

1. Other cells in the nervous system do not transmit action potentials.



2. They are support cells called Glial Cells.

The neuron focuses its excitability through uniform changes in the chemical polarity of its cell membrane.

1. The depolarization of the cell membrane is called the ACTION POTENTIAL.



1. Action Potentials are measured in terms of millivolts (usually +/- 70mv.).



2. These voltages are believed to be the result of chemical (ionic) imbalance across the membrane of the neuron.



2. In order for a nerve to become excited and begin transmitting (propagating) an action potential impulse, there must be a stimulus of sufficient intensity.



1. The magnitude of depolarization required to generate an action potential is called its THRESHOLD.



1. Usually, it takes the summation of several AP's at the ends of several neurons to reach the threshold.



2. Threshold has different meanings to the neurologist and to the psychologist. (Question: What is the difference?)



3. The POLARITY or electrical balance of the outer membrane of each neuron is a chemical phenomenon.



1. The inner charge of the membrane is negative, while the outer charge is positive.



2. The magnitude of these charges is equal (approx. 70 mV.) when the neuron is resting.



3. When the neuron is stimulated by the application of a chemical agent, the balance is upset.



1. The inner potential is raised by approx. 12 mV to -58 mV).



2. Then a chain reaction starts.



3. When this condition exists, the membrane is said to be DEPOLARIZED.



4. This depolarization moves along the structure of the neuron in one direction.



1. This means the nerve shouldn't "backfire."



2. The membrane returns to its resting state slowly, inhibiting the reoccurrence of the depolarization.



3. This stage is called a "refractory" state": a period when the neuron must rest.

General Anatomy of the Neuron
Each neuron is composed of a nerve cell ("cell body"; "soma" "perikaryon") and one to a dozen protoplasmic projections.

1. The CELL BODY consists of a cytoplasm filled cell wall and a nucleus.



1. Intranuclear structures, such as ribosomes, etc, also occupy the nucleus.



2. The nucleus of the neuron produces special chemicals to propagate excitability



2. There are two types of PROTOPLASMIC PROJECTIONS:

Axons conduct impulses away from the cell body and form THE BULK OF THE NERVOUS SYSTEM.


1. Axons are protoplasmic processes or filaments with (we think) a neurofibrillary structure.



2. These vary in thickness from 1 micrometer to 30 micrometers.



3. The fastest conducting fibers are the thickest.



4. The end of an axon forms an end brush of telodendria



5. Each telodendria is tipped by synaptic knob or bouton.



6. These boutons contain a substances called neurotransmitters.



7. Neurotransmitters are conveyed from the cell body via the neurotubules in the plasma of the axon.



8. Axons' terminal ends are arborized ( that is, spread out like the branches of a tree.) and contact, but don't connect with one of the dendritic ends or with the cell bodies of adjacent neurons. Neurons in contact with other neurons can transmit their excitement to them. Special connections between neurons are called synapses.



9. Schwann cells form a sheath around the axons of PNS neurons

1. They are there even if there is no myelin sheath.



2. They seem to function to augment the regeneration of severed axons.



1. Little axonal regeneration occurs in the CNS.



2. The Schwann sheath has also been called "neurilemma," but this term is also applied to a delicate connective tissue sheath also called "endoneurium."

Dendrites conduct impulses toward the cell body. There maybe several dendrites projecting from a cell body.

Myelination
A MYELIN SHEATH encloses all but the finest axons in either the CNS or PNS.

1. Myelination, the development of the myelin sheath, increases the velocity of action potential conduction.



1. Fibers do not assume full function until myelination is completed.



2. Some myelinated fibers do not become fully enclosed until late in embryological development, even, in some cases, until after birth.



2. Myelin is a lipid (fatty) material which is whitish in color.



1. In the CNS this color gives rise to the term "white matter."



2. Unmyelinated fibers in the CNS are called "gray matter."

Periodic interruptions in the myelin sheath are called "Nodes of Ranvier."

The SYNAPSE is the means by which excitability travels from one part of the body, or from one neuron, to another.

1. The transmission of the action potential across the synapse is mediated by neurotransmitters.



1. These chemical catalysts facilitate or inhibit the passage of impulses across the synapse.



2. Some of these neurotransmitters are excitatory and some are inhibitory.



2. Neurotransmitters act by modifying the flow of Na+ (Sodium) and K+ (Potassium) ions across the synaptic cleft.



3. So far, about 30 types have been discovered, and some of these have been synthesized.



1. The most common facilitating neurotransmitter is acetylcholine (ACTH).



2. Some others are: norepinephrine; serotonin; dopamine; gamma-amino-butyric acid (GABA); glycerine and glutamic acid.



3. The facilitators are effective for only a short time.



1. This period is the EXCITATORY POSTSYNAPTIC POTENTIAL (EPSP).



2. ACTH is deactivated by two means.



1. It is destroyed by acetylcholinesterase.



2. It diffuses.



4. There is also an Inhibitory Postsynaptic Potential: IPSP.



5. Neurotransmitters have been used as drugs and can alter nervous system behavior.

1. Neurotransmitters are secreted by various types of neural tissue.



1. Different neurotransmitters are found at different sites.



2. They are conveyed from the cell body via the neurotubules in the plasma of the axon.



3. They end up in the boutons of the end brush, the very end of the axon. .



2. There is an actual space (synaptic cleft) between two neurons that must be bridged by the neural impulse for successful transmission.



1. Neurotransmitter molecules cross the gap.



2. They carry with them their electrical charges.



3. Entering the membrane of the adjacent dendrite.



4. And depolarizing its membrane.

Types of Neurons: There are THREE BASIC TYPES of neurons: Unipolar; bipolar; multipolar. All evolve from embryonic NEUROBLASTS.

1. Unipolar Neurons possess only a single axon.



1. One limb is afferent, and functions as a dendrite.



2. The other limb is efferent, and functions as an axon.



3. There is a single extension attached to the cell body. This bifurcates a very short distance from the body.



4. True Unipolar Neurons are actually pretty rare, being most commonly found in the developing nervous system.



2. Bipolar Neurons have extensions on either side of the cell body.



1. One is afferent (dendrite), one is efferent (axon).



2. Bipolar neurons are usually associated with the special senses: vision, hearing, smell, balance, taste).



3. Pseudounipolar Neurons : Some bipolar neurons develop a structure that makes them appear like unipolar neurons. They have an axon and a dendrite that spilt from a single projection. Both are myelinated.



4. Pseudounipolar neurons are found in the cerebral and cerebellar cortexes; in the brainstem (cranial nerve ganglia); and in the spinal cord (spinal nerve ganglia).



3. Multipolar Neurons have one large single axon and a large number of short dendrites.

There are several types of multipolar neurons.


1. Motoneurons have a long axon which courses from the ventral gray horn of the spinal cord to the skeletal muscles.



2. Purkinje neurons have greatly arborized dendrites, but the arborization occurs only in one plane. They are found only in the cerebellar cortex.



3. Pyramidal neurons have a cell body shaped like a pyramid and a long axon.



1. "Great cells of Betz" are pyramidal cells and part of the corticospinal tracts



2. Pyramidal cells are found only in the cerebral cortex.



Golgi Neurons, Type I have very large cell bodies and axons which may be a meter or more in length.


1. Their cell bodies are only found in the cerebral cortex.



2. They may convey impulses over great distances.



3. They may even contribute fibers out of the CNS to become contributors to PNS nerves.



Golgi Neurons, Type II, are also known as microneurons.


1. They have very small cell bodies and greatly arborized dendrites of very short length running in all planes. Since they look like stars, they are often described as "Stellate."



2. Golgi II cells are only found in the cerebral cortex.

Go on to Group Assignment 1
or
Go back to Microananatomy of the Central Nervous System

E-mail Bill Culbertson at bill.culbertson@nau.edu
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