(Par. 4) The fundamental concept of the nervous system, like all fundamental concepts, is remarkable in its simplicity: A pattern of receptor stimulation is conveyed to the brain and spinal cord by afferent neurons. Within the brain and spinal cord the excitatory state is modulated resulting in a particular pattern of efferent discharge to the body effectors, muscle and gland. The animal responds to its environment by the contraction of muscle and the secretion of its glands.
(Par. 6) The law of the nervous system is this: Input equals output. And a corollary is this: Varying inputs yield varying outputs. We want to appreciate that the output, the contraction of muscle and the secretion of glands, at any particular time is due not only to afferent input that is then arriving at the CNS but also to input that may have arrived minutes and possibly many minutes ago. The time difference is strictly a function of the time that the "circuitry" spends within the CNS before ultimately leading to efferent neurons that convey the excitatory state to effectors.
(Par. 3) The peripheral contact of the efferent neuron pathway is at a neuroeffector synapse; that is, at the place of contact of the axon of an efferent neuron with an effector of the body. The term "effector" refers to the organs that ultimately carry our the actions of the body: muscle and gland. Whereas there are many different kinds of receptors, each sensitive to a particular kind of stimulus, there are only two kinds of effectors, muscle and gland.
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