Rat Brain and Structure

Of all physiological facts, those concerning the brain seem most likely to help to explain behavior; and, although this statement is a truism, we may now ask why it is so. The answer given in this chapter inevitably refers to kinds of behavior and experimental methods, that are more fully described later. The central nervous system is the rat organ through which the sense organs act on the effectors; although all other organs may influence behavior, the CNS and the nerves carry especially large amounts of information to the rat muscles and rat glands. While the blood enables the muscles to remain operational, by carrying substances to and from them, the nervous system determines the moment, durations to the contractions of other muscles.

Neutral action, then, imposed a pattern of activity on dozen of muscles and many millions of muscle fibers. Patterning exists even in a simple reflex. C. S. Sherrington (1859-1952) wrote: "A great principle in the plan of the nervous system is that an effector shall be at the behest of many receptors, and that one receptor shall be able to employ many effectors". This arrangement makes possible the interaction of many afferent inputs to produce the animals pattern of activity.

The brain stem of rat (medulla, pons and midbrain) is, like the spinal cord, largely an assembly of reflex centers; it receives the inputs of the cranial nerves, except the olfactory and most of the optic, and contains the origins of the motor fibers in them. Particular regions are associated with specific reflex acts such as lachrymation, with more complex patterns such as breathing and (in the midbrain) with postural adjustments. There is also a large cerebellum of which the function is evidently the control of the details of movement; it is the cerebellum which (as far as we know) makes the proprioceptive feedback effective. In all this the mammalian brain resembles that of other classes of vertebrates.