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In this respect, the neuron does not differ essentially from the cell of a muscle. Just as the muscle cell requires a stimulus to make it contract, so does the neuron require a stimulus to start the impulse. Hence, in accounting for the activities of the body, it is not sufficient to say they are caused by nervous impulses.
#Anatomy.# A nerve-trunk is made up of a variable number of bundles of nerve fibres surrounded and supported by a framework of connective tissue. The nerve fibres are chiefly of the medullated type, and they run without interruption from a nerve cell or neuron in the brain or spinal medulla to their peripheral terminations in muscle, skin, and secretory glands.
It was a common saying of Myson that men ought not to investigate things from words, but words from things; for that things are not made for the sake of words, but words for things. Diogenes Laertius. The word is derived from the Greek neuron nerve, and astheneia weakness.
On account of this property, an excitation, or disturbance, in any part of a neuron is conducted or carried to all the other parts. Thus a disturbance at the distant ends of the dendrites causes a movement toward the cell-body and, reaching the cell-body, the disturbance is passed through it into the axon. This movement through the neuron is called the nervous impulse.
Certain of the axons have no primitive sheath and others are without a medullary sheath. *Form and Length of Axons.*—Where the axons terminate they usually separate into a number of small divisions, thereby increasing the number of their connections. These collateral branches, by distributing themselves in a manner similar to the main fiber, greatly extend the influence of a single neuron.
The axis cylinder depends for its nutrition on the central neuron with which it is connected, and from which it originally developed, and it degenerates if it is separated from its neuron.
The dendrites serve two purposes: first, they extend the surface of the cell-body, thereby enabling it to absorb a greater amount of nourishment from the surrounding lymph; second, they act as receivers of stimuli from other neurons. The same impulse does not pass from one neuron to another. The special function of the axon is to transmit the impulse.
The cerebrum, in addition to controlling the voluntary movements, is able to establish the necessary conditions for secondary reflex actions, such as walking, writing, etc. Although certain of the divisions of the nervous system exercise special functions, all parts of it are closely related. Exercises.—1. Give the function of each of the parts of a neuron.
The mechanism of these processes is entirely unknown to me; any one who wishes to follow up these ideas must try to find the physical analogies and prepare the way for a demonstration of the process of motion in the stimulation of the neuron.
But it serves as a stimulus for the neuron with which the first connects and starts an impulse in this connecting neuron, the point of stimulation being where the fiber terminations of the first neuron make connection with the dendrites of the second. This impulse in turn stimulates the next neuron, and so on, producing a series of impulses along a given nerve path.
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