Physiology of the Nervous System : Part 4

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 21 of 27)

Work of the Sympathetic Ganglia and Nerves. - The neurons which form these ganglia aid in controlling the vital processes, especially digestion and circulation. These neurons are controlled for the most part by fibers from the bulb and spinal cord, and cannot for this reason be looked upon as forming an independent system. Their chief purpose seems to be that of spreading the influence of neurons from the central system over a wider area than they would otherwise reach. For example, a single neuron passing out from the spinal cord may, by terminating in a sympathetic ganglion, stimulate a large number of neurons, each of which will in turn stimulate the cells of muscles or of glands. Because of this function, the sympathetic neurons are sometimes called distributing neurons.

Functions of the Cerebellum. - Efforts to discover some special function of the cerebellum have been in the main unsuccessful. Its removal from animals, instead of producing definite results, usually interferes in a mild way with a number of activities. The most noticeable results are a general weakness of the muscles and an inability on the part of the animal to balance itself. This and other facts, including the manner of its connection with other parts of the nervous system, have led to the belief that the cerebellum is the chief organ for the reflex coordination of muscular movements, especially those having to do with the balancing of the body. In this connection it is subordinate to and under the control of the cerebrum. Of the relations which the cerebellum sustains to the cerebrum and to the different parts of the body, the following view is quite generally held:

In the development of secondary reflexes, as already described, conditions are established in the cerebellum, such that given stimuli may act reflexively through it and produce definite results in the way of muscular contraction. After the establishment of these conditions, afferent impulses from the eyes, ears, skin, and other places, under the general direction of the cerebrum, may cause such actions as the balancing of the body, walking, etc., as well as the delicate and varied movements of the hand. This view of its functions makes of the cerebellum the great center of secondary reflex action.

Functions of the Cerebrum. - While the work of the cerebrum is closely related to that of the general nervous system, it, more than any other part, exercises functions peculiar to itself. The cerebrum is the part of the nervous system upon which our varied experiences leave their impressions and through which these impressions are made to influence the movements of the body. But the power to alter, postpone, or entirely inhibit, nervous movements is but a part of the general work ascribed to the cerebrum as the organ of the mind. Numerous experiments performed upon the lower animals, together with observations on man, show the cerebrum to be the seat of the mental activities, and to make possible, in some way, the processes of consciousness, memory, volition, imagination, emotion, thought, and sensation.

Localization of Cerebral Functions. - Many experiments have been performed with a view to determining whether the entire cerebrum is concerned in each of its several activities or whether special functions belong to its different parts. These experiments have been made upon the lower animals and the results therefore obtained compared with observations made upon injured and imperfectly developed brains in man. The results have led to the conclusion that certain forms of the work of the cerebrum are localized and that some of its parts are concerned in processes different from those of others.

The work of locating the functions of different parts of the cerebrum forms one of the most interesting chapters in the history of brain physiology. The portions having to do with sight, voluntary motion, speech, and hearing have been rather accurately determined, while considerable evidence as to the location of other functions has been secured. Much of the cerebral surface, however, is still undetermined.

Nervous Control of Important Processes

Circulation of the Blood. - 1. Control of the Heart. - The ability to contract at regular intervals has been shown to reside in the heart muscle. Among other proofs is that furnished by cold-blooded animals, like the frog, whose heart remains active for quite a while after its removal from the body. These automatic contractions, however, are not sufficient to meet all the demands made upon the circulation. The needs of the tissues for the constituents of the blood vary with their activity, and it is therefore necessary to vary frequently the force and rapidity of the heart's contractions. Such changes the heart itself is unable to bring about.

For the purpose of controlling the rate and force of its contractions, the heart is connected with the central nervous system by two kinds of fibers:

a. Fibers that convey excitant impulses to the heart to quicken its movements.

b. Fibers that convey inhibitory impulses to the heart to retard its movements.

The cell-bodies of the excitant fibers are found in the sympathetic ganglia, but fibers from the bulb connect with and control them. The cell-bodies of the inhibitory fibers are located in the bulb, from where their fibers pass to the heart as a part of the vagus nerve.

In addition to the fibers above mentioned, are those that convey impulses from the heart to the bulb. These connect with neurons that in turn connect with blood vessels and with them act reflexively, when the heart is likely to be overstrained, to cause a dilation of the blood vessels. This lessens the pressure which the heart must exert to empty itself of blood. These fibers serve, in this way, as a kind of safety valve for the heart.

2. Control of Arteries. - Changes in the rate and force of the heart's contractions can be made to correspond only to the general needs of the body. When the blood supply to a particulary organ is to be increased or diminished, this is accomplished through the muscular coat in the arteries. The connection of the arterial muscle with the sympathetic ganglia and the method by which they vary the flow of blood to different organs has already been explained, so that only the location of the controlling neurons need be noted here. These, like the controlling neurons of the heart, have their cell-bodies in the bulb. It therefore appears that the entire control of the circulation is effected in a reflex manner through the nerve centers in the bulb. These centers are stimulated by conditions that relate to the movement of the blood through the body.

D.C. Heath and Co. - Publishers
Original copyright 1909