INSTINCT AND HABIT
Whenever receptors are stimulated, reaction occurs. This is the general law to which there are no known exceptions in the normal waking animal. The reaction is not to a single, isolated stimulus, but integratively to the total stimulus pattern of the moment, and is modified by preceding stimulations. Reactions at any moment may include the inhibition or checking of activities already in progress.
The specific results of stimulation are produced through neural transits, which originate in the receptors, enter the spinal cord or brain stem, may or may not ascend to the cerebrum, emerge from the cord or brain stem, and flow through efferent fibers to muscle cells or gland cells, modifying the activities of these cells. In the case of a reflex, the current entering over a certain route has a fixed route of emergence, so that the same stimulus produces always the same, or nearly the same result, regardless of what other stimulations may be occurring simultaneously with the stimulus of the reflex. In the reflex, the afferent and efferent neurons are apparently in permanent synaptic connection, or else the afferent neuron is in permanent connection with a central neuron, which, in turn, is in permanent connection with the efferent neuron, so that current sent in over the afferent neuron will be discharged outward over the particular efferent neuron, regardless of what other currents are flowing through the nervous system. In these cases, therefore, a specific stimulus will always produce a specific action. The knee jerk, or patellar reflex, is a typical simple reaction : a blow on the patellar tendon, in the normal animal, will produce a contraction of the extensor muscle of the leg, no matter what other stimulations may be acting on the animal, although these other stimulations may influence the degree of the contraction.