Spinal cord · Grey Matter

The spinal cord is the cable of nervous tissue in the vertebral canal that carries sensation up and commands down between the brain and the body, and that runs simple reflexes by itself.

Spinal cord. The spinal cord is the cable of nervous tissue in the vertebral canal that carries sensation up and commands down between the brain and the body, and that runs simple reflexes by itself.

It continues the brainstem from the foramen magnum down to about the first or second lumbar vertebra, about 45 cm long in men and 43 cm in women, and between roughly 6 and 13 mm across, thickest where the nerves of the arms and legs leave it. In cross-section its grey matter forms an H or butterfly in the centre (sensory cells in the back horns, motor neurons in the front horns), wrapped by white matter whose columns carry the ascending and descending tracts. Thirty-one pairs of spinal nerves leave it, one for each segment.

Reflexes need no brain. A tap below the kneecap stretches a muscle, sensory fibres excite the motor neurons of the same muscle inside the cord, and the leg kicks before the brain is informed; withdrawal from a burn works the same way through a few more synapses.

Injury cuts off everything below the level. A complete lesion removes movement and sensation below it, and the higher the level the more of the body is lost.

An injury at or above the third to fifth cervical segments affects breathing, because the phrenic nerve to the diaphragm leaves the cord there; injuries below that level leave the diaphragm working.

Questions: Why can a spinal injury high in the neck stop breathing when one lower down does not? The rhythm of breathing is generated in the medulla, and its commands travel down the spinal cord to the motor neurons of the phrenic nerve, which leave the cord at the third, fourth and fifth cervical segments and drive the diaphragm. An injury above that level cuts the medulla off from the diaphragm and breathing stops unless a ventilator takes over. An injury below it leaves the diaphragm working, even when the arms or legs are paralysed. Why is the spinal cord's grey matter inside, the reverse of the brain's cortex, and what does it do there? In the cord the grey matter forms a central H, with sensory neurons in the back horns and motor neurons in the front horns, and the white matter around it is made of the long tracts that run between the body and the brain. The layout keeps the local circuits compact, where sensory fibres enter and motor fibres leave at every segment, while the through traffic runs on the outside. Those local circuits are enough for reflexes: in the knee jerk a stretch receptor's fibre excites the motor neuron of the same muscle directly in the front horn, and the leg kicks with no part played by the brain. Why does a stroke in the left hemisphere weaken the right side of the body? The axons of the motor cortex descend as the corticospinal tract through the internal capsule and the brainstem, and in the lower medulla about 85 to 90 % of them cross to the other side at the pyramidal decussation before running down the spinal cord. The left motor cortex therefore commands the right arm and leg, and a lesion anywhere above the crossing weakens the opposite side. Below the crossing, in the spinal cord, a lesion weakens the same side.