Lest We Forget - Wireless Anatomy Commentary

The Cord as a Conductor of Impulses

Messages sent through the cord pass either up or down in the white matter; but do impulses going up the cord follow the same paths as those going down? This question has been answered by experiments on some of the lower animals which are constructed essentially like man, and also by observation of the results in human beings in which the cord is diseased or has been injured by accident. These observations have shown that if a part of the cord is disabled, sometimes sensation and sometimes the power of motion is lost below the injured point.

If the power of motion is lost; e.g., the motion of the leg, then the injury must be in a descending tract, where the messages that pass from the brain down to the leg have been interfered with. If the person loses the sensation of feeling so that he has no consciousness of anything that may touch a given part of the body below the injured spot, then it is assumed that an ascending tract has been severed. Any injury on one side of the cord is, as a rule, accompanied by loss of sensation on the other side of the body but not on the same side as the injury. The conclusion is that messages brought into the spinal cord pass immediately to the other side and ascend there.

Curiously enough, however, an injury destroys the power of voluntary motion on the same side as the injury, but not on the other. Hence messages from the brain pass down the cord on the same side as that to which they finally go. These messages from the brain going down the cord cross over from left to right and vice versa, higher up, mainly in the medulla, so that all messages going to either side of the body start from the other side of the brain. Thus, the sensations and motions of each side of the body are connected with and controlled by the cells in the cerebral hemisphere of the other side. Although there are a few exceptions to this arrangement, this is, in the main, the relation of the spinal cord to the rest of the body. There are thirty-one of these spinal nerves on each side of the cord. Five of them unite to make up the brachial plexus.

(A) Sense Organ (B) Muscle Diagram illustrating a simple reflex action. The impulse from the sense organ passes in over the nerve fibre [fiber], d, and may pass up to the brain over a-c. But a part may be switched off through c down the fibre [fiber] f to the muscle, casing it to contract without the aid of the brain.

The Neurons as Transmitters of Impulses

The Nerve Impulse

We have mentioned the fact that nerve impulses pass over the nerve fibres [fibers], and considering the differences in the results they produce, one would naturally suppose that there would be corresponding differences in the impulses; e.g., that the impulse which produces a sensation of light must be very different from the one producing a sensation of sound. This is not the case, however. An electric current through a wire may produce different results according to the different kinds of apparatus employed at the end of the wire; it may ring a bell, or produce light in a bulb, or sound in a telephone; but though the results are various, the electric current is essentially the same in all cases. So in the body, the impulse which travels over a nerve is always essentially the same.