The functional units of the nervous system as contrasted with the structural units are called reflex arcs, which involve two or more neurons, and in addition a non-nervous element in the form of a muscle cell, for example, or a gland cell that acts in consequence of the stimulus received.
A reflex arc consists in each case of at least five fundamental links, each with a different function, namely: (1) a receptor; (2) a sensory transmitter; (3) a reflex center, including the synapse; (4) a motor transmitter; and (5) an effector (Fig. 596). Together these five links form a living chain or bridge extending from the point of stimulation to the organ which functions as an effector. The receptor and the sensory transmitter together consist of a single neuron, so placed as to receive stimuli at the dendritic pole of the cell body, either from the outside world or from within the body itself, while the neurite, or axon, becomes the sensory transmitter to another neuron.
Centers, in which synapses determine the courses to be taken by impulses and consequently what is to be done, are usually withdrawn from the surface into a region protected from the direct bombardment of stimuli. A neurite extending from a cell body in the center forms the motor transmitter, or the fourth link in the chain that carries the decision of the center to the effector (Fig. 597).
The most numerous effectors in reflex arcs are no doubt muscle cells, although glands, luminescent organs, and chromatophores, which are modifiable pigment-bearing cells, are also effectors that respond to stimuli by action of some sort.
Of the two fundamental neurons in a reflex arc the sensory cells usually have long dendrites and short neurites, while the reverse is true of motor cells. Such simple reflexes, involving only two neurons, are extremely rare. Possibly the knee-jerk is one. In most reflex arcs one or more additional, intermediate links, the so-called association neurons, or relayers, are inserted between the sensory and the motor neurons. A large part of both the spinal cord and the brain is made up of such intermediate neurons which increase the distance in the chain between receptors and effectors, making complex relationships not only possible but inevitable.
An association neuron not only lengthens its special line of communication, but also makes possible a hook-up with other lines. Thus single effectors may receive impulses from several receptors at the surface or impulses starting at a single receptor may go to several different effectors at once. In either case the receptor, relayer, and effector make a trinity that acts as a unit.