Any mass of living protoplasm, such as an amoeba, can receive extrinsic stimuli, which affect directly its conditions, and though the impression may be very localized in its application, yet all the parts of the cell participate in the sensation, and probably take part in the resulting movements.
Besides those acts of which we can recognize the cause, many others occur in amoebae which we are not able to trace to any definite cause other than the energies derived from its special powers of assimilation. We say that not only can an amoeba feel local stimulation, transmit the impulse to remoter parts of its body, and respond by movement to the stimulus, but it can also initiate impulses which appear as motions, etc., as the result of intrinsic processes of a chemical nature. We may conclude from this fact alone that automatic action is one of the properties of protoplasm derived from its proper chemical activities.
In the nerve centres of all the more complex animals we find that each of these kinds of action is distributed to different varieties of cells, and thus an important division of labor takes place. The first act is performed by a wonderfully elaborate set of special organs adapted to the reception of the various extrinsic impulses or sensations from without. The excitation is then sent by nerve fibres to another group of central nerve cells, which are apparently employed solely in receiving the stimuli from the peripheral organs, and then distributing the impulses to their neighbors, which can direct, modify, analyze, classify, redistribute, or check the impulses, so that the higher nerve cells may have less work, and at the same time lose none of the advantage that is to be gained from the income derived from stimulus coming from without. Connected with the last group is another, the nerve cells which lie out of the reach of the ordinary peripheral impulses, but are capable of developing within themselves energies, and can initiate impulses with no other aid than that of their nutrition and the chemical changes resulting from their assimilation.
These impulses are distributed to the peripheral active tissues, muscles, glands, etc., probably through the medium of other sets of cells analogous to the last group situated in the nerve centres as well as to the local distributors which act as unions between the other textures and the nerve fibres.
The functions of nerve cells which form centres of action may be classified thus: -
Many cells are capable of reflecting impulses received from an afferent nerve; that is to say, they send it by an efferent nerve to some active tissue, such as a muscle or gland. This kind of direction is spoken of as a simple reflex action. For instance, if a grain of red pepper be placed on the tongue, an impulse soon travels from the peripheral receiving terminal, along an afferent nerve to its centra] terminal, which reflects the impulse to the efferent nerve, going to the salivary gland, and the result is an increased secretion of saliva.
There are but few reflex acts that do not require the co-operation of several cells, and these work together in an orderly manner, the resulting activity being well arranged and usually adapted to some purpose. The first act of the receiving cells of a reflex centre must then be to distribute and direct the impulse into those channels which lead to groups of cells capable of sending impulses in an orderly and definite direction. This directing and arranging power is spoken of as co-ordination, and probably is an attribute common to all nerve cells.
The force of the reflected efferent impulse bears a direct relation to the afferent impulse as determined by the strength of the stimulus. Thus, if the amount of pepper on the tongue be much increased, not only is the flow of saliva greater, but the excitation spreads from one central cell to another until the neighboring centres are affected. Thus, we often find the lachrymal glands are influenced by very strong stimulation of the tongue, and pour out their secretion, as is said, "in sympathy" with the mouth glands. But the amount of the afferent impulse is not the only factor in determining the energy of response to be reflected along the efferent channels. Some nerve cells have a distinct power of increasing the amount of response to a given stimulus. When an irritant falls near the mucous membrane in the neighborhood of the laryngeal opening, a very different result is produced. The greater response to an equal stimulus in such cases probably depends rather on a peculiar augmenting power of some central cells than upon any special local mechanisms.
Under certain conditions, which will be more fully explained presently, nerve cells appear to have the power of restraining the activity of other cells or tissues, of checking their receptive or executive power, or lessening the impulse reflected so as to produce less effect; this is called inhibition.
Nerve cells are supposed to have the power of originating activity, i. e., discharging impulses without receiving any exciting impulses from other nervous agencies that we can find out. Examples may be found among those carrying on operations which require to be of a more or less permanent kind, such as the partial contraction of the muscle cells of the arteries. Automatic actions are sometimes classified as those acting continuously and those that undergo rhythmical changes. If carefully examined, most of the so-called constant automatic actions will be found to show traces of rhythmic relaxation. The centre governing respiratory movement is an example of an automatic group of cells. Impulses are discharged from it even when the connections with all the afferent nerves which influence it normally are cut off, and it has no other excitant than the warm blood supplying it with nutriment. Respirations are, however, normally regulated by a reflex mechanism, the channels of which reside in the vagus nerve.
In the nerve cells we must also seek mental activity, under which term maybe considered perception, volition, thought and memory. It is very difficult to allocate the due proportions of reflexion, co-ordination, augmentation, inhibition, automatism, etc., requisite for the development of mental faculties. In all probability, what we call mental operations are related to activities called forth as the resultant of a long series of external and internal excitations, modified by intrinsic nutritive influences, acting upon innumerable groups and complex associations of nerve cells, the general outline of whose function and tendency of action, character, has been rough hewn by hereditary transmission.