THE ELEMENTARY FACTS ABOUT THE NEURO-PHYSIOLOGICAL BASIS OF ACTION
Introduction. --Having studied the receptors and found that their activity involves the initiation of neural impulses, our next problem is to learn something about neural conduction and the arrangement of pathways over which such impulses must pass in order to reach the effectors--the muscles and glands. We should say in the beginning that all neural impulses initiated in a sense organ have to pass either through the spinal cord or the brain or both of these organs before reaching the muscles and glands. Hence it is necessary for us to take up the elementary facts about their structure and functions. If we attempted to make a study of the whole nervous system, even in outline, we should find that our task could not be accomplished without going into a laboratory and actually working with neurological material. We can, though, apart from such laboratories, obtain a fairly good working notion of (1) many of the things which the nervous system as a whole has to do, (2) of the elementary neural structures and (3) the way the latter are chained together to form the reflex arcs that make possible our acts in daily life.
The Unit of the Nervous System. --The unit of the nervous system is the neurone. A complete neurone is shown in Fig. 21. It consists (1) of a cell body with (2) its axone and (3) its dendrites. The cell body is a somewhat complicated and not thoroughly understood structure. It contains a nucleus which does not differ greatly from the nucleus of any other cell. The most characteristic part of the cell is its cytoplasm, which is made up of neurofibrils, fine fibrils that are continuous throughout the axone, cell body and dendrites. The perifibrillar substance is a fluidlike substance that surrounds the neurofibrils. The cell contains in addition chromophilic substance, flake-like masses scattered