The remarkable discovery of the dual nature of the nervous system, of its duplex development as a lower and upper system of "neurons," has shed much light upon the problems of practical medicine, but this construction is described under Brain; Neuropathology; Muscle And Nerve, &C.
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.
The axons of the motor neurons are, inasmuch as they are nerve fibres in nerve trunks, easily accessible to artificial stimuli.
On the other hand, in some neurons, after severance of the axon from the rest of the cell (spinal motor cell), the whole nerve cell as well as the severed axon degenerates, and may eventually die and be removed.
It seems, especially in the case of the nexus between certain neurons, that the influence, loss of which endangers nutrition, is associated with the occurrence of something more than merely the nervous impulses awakened from time to time in the leading nerve cell.
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.
In tracing the tonus of neurons to a source, one is always led link by link against the current of nerve force - so to say, "up stream" - to the first beginnings of the chain of neurons in the sensifacient surfaces of the body.
Hence, when cut off from these sources, the nutrition of the neurons of various central mechanisms suffers.
Thus the tonus of the motor neurons of the spinal cord is much lessened by rupture of the great afferent root cells which normally play upon them.
It had been held that the branches of the g3 Activity end is played upon by other neurons - spinal, cerebral and cerebellar.
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.
Bethe has succeeded in isolating their perikarya from certain of the afferent neurons of the antennule of Carcinus.
It has been urged that the neurons retract during sleep, and that thus at the synapses the gap between nerve cell and nerve cell becomes wider, or that the supporting cells expand between the nerve cells and tend to isolate the latter one from the other.