The unit of the nervous system is the neurone

The unit of the nervous system is the neurone,--a nerve cell with all of its processes. The nervous system is composed of myriads of neurones joined into a network by means of synapses.

Each muscle fiber is supplied by at least one motor nerve fiber which ends beneath the sarcolemma in the motor end organ. This constitutes the efferent nerve supply. From the muscle and tendon spindles, muscles make afferent connections with the brain and spinal cord (central nervous system), in part sensory and in part non-sensory.

The simple nervous connections between peripheral receptors to the center (afferent) and the return to the effector (efferent) is known as a reflex arc, and is composed of at least two neurones. More complex arcs are established by means of one or more connector neurones and may extend to the opposite side of the cord, to other levels, and to the cerebrum. In the last case they become sensory fibers since they arouse sensations. Voluntary discharges may issue from the motor area of the cerebrum to the lower reflex mechanisms and elicit voluntary movements.The term reflex action is used to designate involuntary action. Reflexes are dependent upon the integrity of the reflex arc. Pavlov has classified all known reflex actions into unconditioned or inborn reflexes, and conditioned or acquired reflexes.Muscle tonus is that state of partially maintained contraction of muscle by which its length and tension, especially true of antagonistic muscles, are automatically adapted to the new positions taken by parts of the body as in locomotion or of parts of the body to the whole as in posture.

The physical properties of muscle, extensibility and elasticity, are increased or decreased directly as muscle tonus is increased or decreased.Muscle tonus and postural tonus are dependent upon the integrity of the myotatic or monomuscular arc, and upon connections with the semicircular canals, cerebrum, and cerebellum. Loss of any one will alter muscle and postural tonus.The nicety of adjustment between the position of the body to its environment or of the parts of the body to the whole, of muscle tonus, action of antagonists, and in postural tonus is dependent upon the phenomena of reciprocal action and reciprocal innervation. When one group of muscles contract an antagonistic group must relax and vice versa. Movements of locomotion would be impossible without these mechanisms.