The General Anatomy of the Neuron
Of all the data with which psychologists work, the physiological influences on behavior have seemed to be among the most reliable and best understood. After all, physiological phenomena can often be observed directly and researchers in this area are frequently able to design experiments in which they are confident that the relevant variables are known and controlled.
This unit describes physiological phenomena as they are known and understood today But even in this relatively stable field, new events are threatening to upset some established beliefs. For example, the practice of acupuncture is challenging the established specificity theory of pain. Ronald Meizack (1973) observed: ". . . Chinese physicians have used the technique extensively, freeing surgeons to perform major operations on the abdomen, chest or head of a patient while the patient stays awake and alert. We don't know how acupuncture analgesia works. According to the theory of pain traditionally taught in our medical schools, it shouldn't."
This module introduces the neuron, the basic unit of the nervous system. We know what it looks like, how it functions, and what influence it has in the functioning of the nervous system, the muscles and glands.
Or do we?
Interest in neuropsychology has been has been mushrooming in recent years. There are a number of interesting sites in psychophysiology you might wish to visit.
The neuron performs all life functions except reproduction
Like all other cells in the body the neuron is able to carry on such life functions as assimilation of nutrients and excretion of waste products. Over many millions of years, however, the neuron has evolved into a specialist in the field of communication. The task it performs is the transmission of electrochemical nerve impulses to other neurons and to the muscles and glands.
The neuron has developed a high degree of specialization in conducting nerve impulses. It generally has been assumed that a neuron gradually lost the ability to reproduce itself. While the other systems of the body are continually renewing themselves by cell reproduction, the nervous system cannot replace lost neurons. However, it has been reported (Kolata, 2000) that a study by Ira Black reported in the Journal of Neuroscience Research found otherwise. He tinkered with chemical signals stimulating bone marrow cells and then transplanted them into the brain and spinal cords of rats. He found that the cells survived for the several months up to the time or writing his report.
The structure of the neuron reflects its function of transmitting neural impulses. One part receives a stimulus, another part carries the impulse, and a third part is responsible for the nutrition and maintenance of the entire cell.
A typical neuron has three structures:
Figure 1 is a diagram of a typical neuron with the basic structures labeled.
Direction of neural impulse
DENDRITES --------> CELL BODY (with nucleus) -------------> AXON
A neuron embodies structural specialization
The direction of a neural impulse is always from dendrites to cell body to axon. All of the life functions of the neuron take place in the cell body. Here nutrients are assimilated, broken down and used to maintain the existence of the entire cell. Part of the material in the cell body makes up the nucleus, which directs the life functions of the neuron.
The dendrites are sensitive to external stimuli. They receive the stimulus, and then transfer it as a neural impulse through the cell body to the axon.
The function of the axon is to propagate the neural impulse along its entire length. The destination may be a muscle, a gland, or, more commonly, the dendrites of another neuron. The neural impulses sometimes travel as fast as 390 feet per second and are seldom as slow as 10 feet per second as they travel to the hairlike end of the axon.
Now test yourself without looking back.
1. The basic parts of the neuron are indicated in the following diagram. .
Write the name and function of each structure indicated.
2. The neural impulse is always propagated from__________________________ through________________________________ to _______________________________.
3. What are three possible destinations of a neural impulse?
ANSWER KEY PAGE 60
Shown above is a diagram of a typical neuron. In the appropriate
blanks, write the names of the structures indicated by the arrows.
3. Cell body______
| a) The site of life functions of
b) Receives the neural impulse from other cells
c) Carries the neural impulse to its destination
d) Amplifies the neural impulse
The direction of the neural impulse is from the _______________ to the cell body to the _____________________.
Here are diagrams of three neurons transmitting impulses to three different destinations.
The axon transmits a neural impulse to a(n) ____________________
or a(n) _________________________1
1 muscle, gland, another neuron
2 dendrite, axon
3 a. axon
c cell body
a. Name and describe the function of number 1. _________________________________________
b. Name and describe the function of number 2. ______________________________________
c. Name and describe the function of number 3. ___________________________________
2. The neuron always conducts an impulse away from what part of the neuron? __________________
3. Which of the following might be a final destination of a neural impulse?
a. A muscle in your thumb
b. The pituitary gland
c. An intestinal muscle
d. The nucleus of a neuron
e. The dendrite of another neuron
f. The Pancreas (a gland)