The Spinal Cord

Anatomy.

    The spinal cord is an extension of the brain, reaching out to the rest of the body. This is an important part of the central nervous system. It is located in, and protected by, the vertebral column. Then, emerging from this spinal cord, the peripheral nerves constitute the peripheral nervous system. In addition, the twelve pairs of cranial nerves emerging from the brain are also part of the peripheral nervous system.

    As a general organization of the spinal cord, the nerve fibers relaying sensory information enter the the spinal cord via the dorsal roots before reaching out to the brain. Whereas, the nerve fibers that control motor activity exit the spinal cord via the ventral roots. This organization allows the spinal cord to be at the center of our peripheral reflexes; like when we remove our hand from a heat source, even before our brain perceives the pain.

    In total there are 31 pairs of spinal nerves emerging from the spinal cord. Between the skull and the first cervical vertebra, we find the first pair of cervical spinal nerves. Then, under each of the seven cervical vertebrae (C1 C7), there are other pairs of cervical spinal nerves, for a total of eight pairs of cervical spinal nerves. Under the twelve thoracic vertebrae (T1-T12), there are twelve pairs of thoracic spinal nerves. Under the five lumbar vertebrae (L1-L5) there are five pairs of lumbar spinal nerves. Under the five sacral vertebrae (S1-S5), there are the five pairs of sacral spinal nerves. Finally, at the level of the coccyx emerges a pair of coccygeal spinal nerves.

    Like for the brain, the spinal cord is covered with the meninges. Between the vertebrae and the dura mater, the epidural space is filled with fatty material which form a cushion protecting the spinal cord. The space between the arachnoid and the pia mater is the subarachnoid space, or intrathecal space, where the cerebrospinal fluid (CSF) flows. The spinal cord proper ends at the beginning of the lumbar segments (L1-L2). At this level, it divides into several nerve bundles called the cauda equina. This ponytail-like stucture is still surrounded by the meninges and bathed in lots of cerebrospinal fluid. It is into this lumbar area (L3-L5) that intrathecal samples of CSF are usually taken. Similarly, it is at this same level that epidural injections of anesthetic agents are performed to anesthetize the lower part of our body.

    Along the spinal cord, we noted two bulges: a cervical bulge where the nerves innervating the upper limbs emerge, and a lumbar bulge where the nerves innervating the lower members emerge.

    The figures below show specific branches that originate from different levels of the spinal cord. These include the cervical and brachial plexus on the left, and the lumbar and sacred plexux on the right.

    These figures do not show the autonomic nervous system, the nerves that control the vegetative functions such as the blood pressure and the digestion. The autonomous system is divided into two part: the sympathetic system which is primarily activated under stress conditions and, upon excitation, increases blood pressure and heart rate, and the parasympathetic system which is mostly involved with the digestive functions.

Sections of the spinal cord.

    Looking at sections of the spinal cord, we could note a similar organization throughout. In the center, shaped like a butterfly, there is the gray matter. Pointing toward the back are the dorsal horns that mostly contain interneurons which receive and transmit visceral and somatic sensory information.

    Although they also contain interneurons, ventral horns, on the other hand, mostly contain the cell bodies of motor neurons which innervate the squeletal muscles. At the level of cervical and lumbar enlargements, there is more grey matter, more motor neurons innervating the upper and lower limbs, respectively. At the upper thoracic and lumbar level, the lateral horns contain the motor neurons controlling the visceral muscles.

    Exiting on each side of the spinal cord, at the level of all the 31 segments, the neuronal cell bodies carrying sensory information are located in the dorsal root ganglion and transmit to the dorsal horns. Ventrally, the motor neurons leave the spinal cord form the ventral roots. The dorsal and ventral roots fuse together to form the spinal nerves.

Gray matter in a section of the spinal cord.

    Around the gray matter, there is the white matter that consists of long bundle of nerve fibers running up and down, from and to the brain. Then, there is also some other fibers, named commissural fibers, which pass through the spinal cord from one side to the other side. Detailed description of these fiber bundles is quite complex, but one can draw some general characteristics. Please, refer to the figure for some details.

White matter in a section of the spinal cord.