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Online Lesson
Neural Tissue
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.
- To some extent, excitability and conductivity are properties
common to all living protoplasm.
- If you probe an amoeba with a point, it will withdraw, as a
whole unit.
- 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:
- Conductivity is the ability to transmit an impulse from
one location to another.
- 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.
- Other cells in the nervous system do not transmit action potentials.
- They are support cells called Glial Cells.
The neuron focuses its excitability through uniform changes in the chemical
polarity of its cell membrane.
- The depolarization of the cell membrane is called the ACTION
POTENTIAL.
- Action Potentials are measured in terms of millivolts (usually +/-
70mv.).
- These voltages are believed to be the result of chemical (ionic)
imbalance across the membrane of the neuron.
- In order for a nerve to become excited and begin transmitting
(propagating) an action potential impulse, there must be a stimulus of
sufficient intensity.
- The magnitude of depolarization required to generate an action
potential is called its THRESHOLD.
- Usually, it takes the summation of several AP's at the ends
of several neurons to reach the threshold.
- Threshold has different meanings to the neurologist and to
the psychologist. (Question: What is the difference?)
- The POLARITY or electrical balance of the outer membrane of each
neuron is a chemical phenomenon.
- The inner charge of the membrane is negative, while the outer
charge is positive.
- The magnitude of these charges is equal (approx. 70 mV.) when
the neuron is resting.
- When the neuron is stimulated by the application of a chemical
agent, the balance is upset.
- The inner potential is raised by approx. 12 mV to -58 mV).
- Then a chain reaction starts.
- When this condition exists, the membrane is said to be
DEPOLARIZED.
- This depolarization moves along the structure of the neuron in one
direction.
- This means the nerve shouldn't "backfire."
- The membrane returns to its resting state slowly, inhibiting
the reoccurrence of the depolarization.
- 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.
- The CELL BODY consists of a cytoplasm filled cell wall and a
nucleus.
- Intranuclear structures, such as ribosomes, etc, also
occupy the nucleus.
- The nucleus of the neuron produces special chemicals to
propagate excitability
- There are two types of PROTOPLASMIC PROJECTIONS:
Axons conduct impulses away from the cell body and form
THE BULK OF THE NERVOUS SYSTEM.
- Axons are protoplasmic processes or filaments with
(we think) a neurofibrillary structure.
- These vary in thickness from 1 micrometer to 30
micrometers.
- The fastest conducting fibers are the thickest.
- The end of an axon forms an end brush of telodendria
- Each telodendria is tipped by synaptic knob or
bouton.
- These boutons contain a substances called
neurotransmitters.
- Neurotransmitters are conveyed from the cell body
via the neurotubules in the plasma of the axon.
- 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.
- Schwann cells form a sheath around the axons of
PNS neurons
- They are there even if there is no myelin
sheath.
- They seem to function to augment the
regeneration of severed axons.
- Little axonal regeneration occurs in the
CNS.
- 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.
- Myelination, the development of the myelin sheath, increases the
velocity of action potential conduction.
- Fibers do not assume full function until myelination is
completed.
- Some myelinated fibers do not become fully enclosed until
late in embryological development, even, in some cases,
until after birth.
- Myelin is a lipid (fatty) material which is whitish in color.
- In the CNS this color gives rise to the term "white matter."
- 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.
- The transmission of the action potential across the synapse is mediated
by neurotransmitters.
- These chemical catalysts facilitate or inhibit the passage of
impulses across the synapse.
- Some of these neurotransmitters are excitatory and some are
inhibitory.
- Neurotransmitters act by modifying the flow of Na+ (Sodium) and K+
(Potassium) ions across the synaptic cleft.
- So far, about 30 types have been discovered, and some of these have
been synthesized.
- The most common facilitating neurotransmitter is acetylcholine
(ACTH).
- Some others are: norepinephrine; serotonin; dopamine; gamma-amino-butyric acid (GABA); glycerine and glutamic acid.
- The facilitators are effective for only a short time.
- This period is the EXCITATORY POSTSYNAPTIC
POTENTIAL (EPSP).
- ACTH is deactivated by two means.
- It is destroyed by acetylcholinesterase.
- It diffuses.
- There is also an Inhibitory Postsynaptic Potential: IPSP.
- Neurotransmitters have been used as drugs and can alter nervous
system behavior.
- Neurotransmitters are secreted by various types of neural tissue.
- Different neurotransmitters are found at different sites.
- They are conveyed from the cell body via the neurotubules
in the plasma of the axon.
- They end up in the boutons of the end brush, the very end of
the axon. .
- There is an actual space (synaptic cleft) between two neurons that
must be bridged by the neural impulse for successful transmission.
- Neurotransmitter molecules cross the gap.
- They carry with them their electrical charges.
- Entering the membrane of the adjacent dendrite.
- And depolarizing its membrane.
Types of Neurons: There are THREE BASIC TYPES of neurons: Unipolar;
bipolar; multipolar. All evolve from embryonic NEUROBLASTS.
- Unipolar Neurons possess only a single axon.
- One limb is afferent, and functions as a dendrite.
- The other limb is efferent, and functions as an axon.
- There is a single extension attached to the cell body. This
bifurcates a very short distance from the body.
- True Unipolar Neurons are actually pretty rare, being most
commonly found in the developing nervous system.
- Bipolar Neurons have extensions on either side of the cell body.
- One is afferent (dendrite), one is efferent (axon).
- Bipolar neurons are usually associated with the special senses:
vision, hearing, smell, balance, taste).
- 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.
- Pseudounipolar neurons are found in the cerebral and cerebellar
cortexes; in the brainstem (cranial nerve ganglia); and in the spinal
cord (spinal nerve ganglia).
- Multipolar Neurons have one large single axon and a large number of
short dendrites.
There are several types of multipolar neurons.
- Motoneurons have a long axon which courses from the
ventral gray horn of the spinal cord to the skeletal muscles.
- Purkinje neurons have greatly arborized dendrites, but the
arborization occurs only in one plane. They are found only
in the cerebellar cortex.
- Pyramidal neurons have a cell body shaped like a pyramid
and a long axon.
- "Great cells of Betz" are pyramidal cells and part of
the corticospinal tracts
- 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.
- Their cell bodies are only found in the cerebral
cortex.
- They may convey impulses over great distances.
- They may even contribute fibers out of the CNS to
become contributors to PNS nerves.
Golgi Neurons, Type II, are also known as microneurons.
- 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."
- Golgi II cells are only found in the cerebral cortex.
Once you have finished you should:
Go on to Group
Assignment 1
or
Go back to Microananatomy of the Central Nervous System
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