The sensory nerve fibres stream towards the spinal cord, gather together in the dorsal root, and enter the spinal cord. The special nerve for the head and face, called the trigeminal nerve, has exactly the same organization but its fibres enter the hindbrain, which is the forward extension of the spinal cord.
The spinal cord is a cylinder of nerve tissue running the entire length of the body within the vertebrae. It enters the head through a large hole in the base of the skull and continues as the brainstem. In cross-section, the cord is round with its outer parts, shown in black in the photograph, made up of a mass of central nerve fibres, which connect the cord to the brain, and of nerve fibres running in the opposite direction, which connect the brain to the spinal cord. Within this nerve-fibre highway, there is a clear area containing the nerve cells. The upper area, called the dorsal horn because it is near the back (dorsum), contains the cells which receive the sensory messages, process them and send on the analyzed messages. The lower area, called the ventral horn because it is near the belly (ventrum), contains the cells which generate the motor orders and send their messages back to the muscles and tissues.
We are concerned with the upper sensory part. The cells are arranged in laminae which run the entire length of cord. The upper three laminae make up a clear area. Here all the incoming small C fibres end on cells which also receive inputs from the other sensory fibres. They are mainly concerned with local, important, busy work, which we will describe. Only a few of them send messages to the brain. The lower laminae contain larger cells which receive the larger sensory input fibres (A beta and delta) and send their messages on to the motor ventral horn and to the brain. All parts of the cord receive sensory messages and send on processed messages to the motor part of the cord and to the brain, but it is essential to realize that none of this mechanism works properly if it does not receive messages from the brain. These modulating descending controls reach each area of the cord by way of fibres running in the fibre tracts in the outer rim of the cord.
Let us step aside for a moment to examine two cases that illustrate the relation between the brain and the spinal cord.
Case 1
A young Irish soldier was shot in the middle of his back while he was in a United Nations peace-keeping unit. A single bullet had cut completely across his spinal cord and he was immediately paraplegic. When he was examined some months later, his legs lay floppy on the bed and he could will no movement at all in his legs. He could feel no sign of any stimulus applied to his body below his navel. Yet the part of his spinal cord below the lesion was working in its fashion, even though it was isolated from the brain, unable to send or receive messages to or from the brain. Tapping his knee produced a lively knee jerk. Running a thumb nail along the bottom of his foot produced a powerful, long-lasting withdrawal of the leg, which relaxed after many seconds with a long series of jerks. Even his bladder emptied automatically without his knowing when it filled up. Yet he sensed a clear phantom image of his body with his legs in a knees-up position, even though his legs were in fact straight. A grim, deep pain in his completely numb body was developing.
Case 2
A lively, intelligent young woman had suffered localized brain damage during her birth. Her right arm was held, unmoving and flexed with her fingers curled. She could move it a little in a weak, clumsy fashion. She could not send motor orders to the right-arm area of her spinal cord. She could sense touch and pain in the arm but in a peculiar way. It ached all the time. She could not tell if a touch was moving or stationary. If a number was drawn on her arm, she had no idea what was happening. She was unable to send modulating orders down to the cord which would have permitted her to unravel subtle aspects of the sensory messages.