Like the brain the spinal cord consists of both grey and white matter – the grey matter being mostly neuron (nerve cell) bodies and supporting, and the white matter being largely axons, or nerve fibres, each surrounded by its insulating sheath, which transmits the impulses to and from the cell bodies. Unlike the brain, the grey matter of the spinal cord is largely confined to the interior where it forms an H-shape. The ‘horns’ of the grey matter reach the edge of the cord and give rise to the nerve roots. The anterior or ventral horns house the motor neurons, concerned with voluntary muscle action. The posterior or dorsal horns contain sensory nerve that receive information from the periphery. Some of their processes lead to higher levels of the central nervous system, others link directly at the same level with motor neurons. In the thoracic region there are also small lateral or side horns, giving rise to motor neurons of the sympathetic nervous system.
They innervate the involuntary muscles. The white matter is divided into separate bundles of axons that run up or down the spinal cord. These are called tracts and they carry particular information. The tracts have Latin names that tell us exactly which parts of the nervous system they connect. In the back part of the white matter there are tracts running up that convey special sensory information from muscles and joints to higher parts of the central nervous system, including vibration and the sense of position. In the cervical region these tracts divide in the funiculus cuneatus that conveys this information from pelvis and legs, and the funiculus gracilis that serves the upper parts of the body. In the side part of the white matter there are the tractus spinocerebellaris, the tractus spinotectalis and the tractus corticospinalis lateralis.
The spinocerebellar tract connects spinal neurons with the cerebellum, which is concerned with movement control. The spinotectalis tract conveys information from the spinal cord about pain and temperature to the thalamus. The corticospinal lateral tract runs from the cerebral cortex down to the spinal cord. It carries 85 per cent of the impulses from the motor cortex to the motor neurons in the spinal cord. The front part of the white matter contains the vestibulospinal tract that runs down from the vestibular apparatus to the spinal cord, giving information about balance to the motor neurons. Also located there, is the anterior corticospinal tract that carries 15 per cent of the motor axons to the motor neurons.
The cerebrospinal fluid surrounding the spinal cord is continuous with the fluid around the brain. It lies in the subarachnoid space, between the dura and arachnoid (the two outer protective meninges) and the pia (the third meningeal layer). In some diseases such as meningitis, it is important to examine the cerebrospinal fluid after extracting it surgically from the spine. Any bacteria or viruses causing the problem can then be identified and appropriate treatment given.