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.
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.