Spinal Cord

Spinal Cord Regions

The spinal cord is divided into thirty-one different segments, with pairs of spinal nerves (either motor, sensory, or mixed) forming at each segment. From these nerve pairs, six to eight motor nerve rootlets branch out of right and left ventro lateral sulci and combine to form motor nerve roots. The sensory nerve rootlets form off left and right dorsal lateral sulci and form sensory nerve roots. These nerve roots, ventral (motor), and dorsal (sensory), combine into spinal nerves which exit the spinal column one on each side. These spinal nerves form in the intervertebral foramen, with the exception of those at C1 and C2 (the Atlas and the Axis) in the neck.

Inside the spinal cord there is a peripheral region, closest to the pia mater, which contains neuronal white matter including sensory neurons and motor neurons; these are mostly myelinated which is what gives the area its whitish hue. The central region of the spinal cord is gray in color and is butterfly-shaped, consisting of interneurons and motor neurons along with neuroglia cells and unmyelinated axons. Inside the central canal is cerebrospinal fluid and ventricles which are an anatomic extension of the ventricles in the brain.

In the central nervous system (CNS) the nerve cell bodies are, for the most part, organized into clusters according t function. Axons in the CNS are collected into tracts. The spinal cord nerve segments include eight cervical segments, each with a pair of cervical nerves. These start as C1 spinal nerves leave the spinal column between the occiput and the first cervical vertebra (C1), and continue until the eight cervical nerve pair exits between C7 and T1.

Spinal Cord
A cut away view shows how signals are sent to and from the spinal cord

The Sensation of Pain

The somatosensory organization of the spinal cord is divided into the anterolateral system, which deals with pain and temperature sensation, and the dorsal column-medial leminscus tract, which senses touch, proprioception (position), and vibration. The anterolateral system (ALS) has a slightly different mechanism than the medial leminiscus tract, in that its primary neurons enter the spinal cord and then travel upward for on or two levels before synapsing in the substantia gelatinosa. The resulting secondary axons ascend in the anterior lateral portion of the spinal cord all the way to the ventro-posterolateral nucleus (VPL) where they synapse with tertiary neurons. The tertiary neuronal axons then connect to the primary sensory cortex.

However, some ‘pain fibers’ take a different route to the brain, arriving instead at the reticular formation in the midbrain where they then proceed to the hippocampus, and the centromedia nucleus. The first of these, the hippocampus, acts to create memories of the pain, whilst the second, the centriomedian nucleus, causes non-specific, diffuse pain. In order to control pain sensation to some extent, some of the ALS axons travel to the periaqueductal gray in the pons, and then project to the nucleus raphe magnus which travels to the pain signals origin and inhibits it.

It is clear that there are many ways in which our response and perception of pain can become confused, either heightened or suppressed. By incurring damage to the nerves there may be an inability to sense pain or temperature resulting in further injury such as burning of the hands whilst cooking. Patients who experience a reduced or altered sensation of pain or temperature in their peripheral limbs should consult a doctor immediately in order to rule out problems with the spinal cord and its nerves

Next Read About: Spinal Cord Injury





Last Updated: 2/02/2011