This gentle continuous activity of the neuron is called its tonus.
Sleep is not exhaustion of the neuron in the sense that prolonged activity has reduced its excitability to zero.
The synapse, therefore, as the place of meeting of one neuron with the next is called, is said to valve the nerve circuits.
In the myelinated cell branches of the neuron, that is, in the ordinary nerve fibres, no visible change has ever been demonstrated as the result of any normal activity, however great - a striking contrast to the observations obtained on the perikarya.
In addition to the trophic influence exerted by each part of the neuron on its other parts, notably by the perikaryon on the cell branches, one neuron also in many instances influences the nutrition of other neurons.
Among the properties of the neuron is conductivity in all directions.
The wave of change (nervous impulse) induced in a neuron by advent of a stimulus is after all only a sudden augmentation of an activity continuous within the neuron - a transient accentuation of one (the disintegrative) phase of the metabolism inherent in and inseparable from its life.
Each neuron or nerve cell is a morphologically distinct and discrete unit connected functionally but not structurally with its neighbours, and leading its own life independently of the destiny of its neighbours.
The neuron is described as having a cell body or perikaryon from which the cell branches - dendrites and axon - extend, and it is this perikaryon which, as its name implies, muscle produces lactic acids during activity, it has been suggested that acids are among the "fatigue substances" with which muscle poisons itself when deprived of circulating blood.
This trophic influence which one neuron exerts upon others, or upon the cells of an extrinsic tissue, such as muscle, is exerted in that direction which is the one normally taken by the natural nerve impulses.
A continuous lesser "change" or stream of changes sets through the neuron, and is distributed by it to other neurons in the same direction and by the same synapses as are its nerve impulses.
Although the cell body or perikaryon of the neuron, with its contained nucleus, is essential for the maintenance of the life of the cell branches, it has become recognized that the actual process and function of "conduction" in many neurons can, and does, go on without the cell body being directly concerned in the conduction.
It is a muscular tonus of central source consequent on the continual glow of excitement in the spinal motor neuron, whose outgoing end plays upon the muscle cells, whose ingoing Yet when the muscular contraction is taken as index ology.
But when neurons are linked together it is found that nerve impulses will only pass from neuron A to neuron B, and not from neuron B to neuron A; that is, the transmission of the excited state or nervous impulse, although possible in each neuron both up and down its own cell branches, is possible from one nerve cell to another in one direction only.