Human spine
The spinal cord is a thin, tubular structure that is an extension of the central nervous system and is enclosed in and protected by the bony vertebral column.
Structure
The spinal cord extends from the medulla oblongata in the brainstem to the conus medullaris near the lumbar level at L1-2, ending in a fibrous extension known as the filum terminale. In an adult, the spinal cord is approximately 18 inches long. The peripheral regions of the cord contain neuronal white matter tracts that contain sensory and motor neurons. In the centre of the cord is a fluid-filled channel called the 'central canal'; this is filled with cerebrospinal fluid, and is continuous with the fourth cerebral ventricle; this is the fluid that is sampled when a lumbar puncture is taken. The three meninges that cover the spinal cord -- the outer dura mater, the arachnoid membrane, and the innermost pia mater; these are continuous with those in the brainstem and cerebral hemispheres, with cerebrospinal fluid found in the subarachnoid space. The cord is stabilized by the connecting denticulate ligaments which extend from the pia mater laterally between the dorsal and ventral roots.
Spinal cord segments
The spinal cord has 31 segments. From each segment, motor nerve roots leave from the ventral side, and sensory nerve roots enter from the dorsal side. The ventral and dorsal roots join to form paired 'spinal nerves', one on each side of the spinal cord.
There are 31 spinal cord segments:
- 8 cervical segments
- 12 thoracic segments
- 5 lumbar segments
- 5 sacral segments
- 1 coccygeal segment
Because the vertebral column grows longer than the spinal cord, spinal cord segments become higher than the corresponding vertebra, especially in the lower spinal cord segments in adults. In a fetus, the vertebral levels originally correspond with the spinal cord segments. In the adult, the cord ends around the L1/L2 vertebral level at the conus medullaris, with all of the spinal cord segments located superiorly to this. For example, the segments for the lumbar and sacral regions are found between the vertebral levels of T9 and L2. The S4 spinal nerve roots arise from the cord around the upper lumbar/lower thoracic vertebral region, and descend downward in the vertebral canal. After they pass the end of the spinal cord, they are considered to be part of the cauda equina. The roots for S4 finally leave the vertebral canal in the sacrum.
There are two regions where the spinal cord enlarges:
- Cervical enlargement - corresponds roughly to the brachial plexus nerves, which innervate the upper limbs. It includes spinal cord segments from about C4 to T1. The vertebral levels of the enlargement are roughly the same (C4 to T1).
- Lumbosacral enlargement - corresponds to the lumbosacral plexus nerves, which innervate the lower limbs. It comprises the spinal cord segments from L2 to S3, and is found about the vertebral levels of T9 to T12.
Embryology
In the human fetus, the spinal cord extends all the way down to the sacral vertebrae. As a person matures, the rest of the body grows faster than the spinal cord, so that by adulthood, the spinal cord reaches only to the level of the first or second lumbar vertebrae. There is no spinal cord below this level, only the individual spinal nerves that form the cauda equina.
Injury
Spinal cord injuries can be caused by falling on the neck or back, or having the spinal cord moved or disrupted in another way. The vertebral bones or intervertebral disks can shatter, causing the spinal cord to be punctured by a sharp fragment of bone. Usually, victims of spinal cord injuries will suffer loss of feeling in certain parts of their body. In milder cases a victim might only suffer loss of hand or foot function. More severe injury may result in paraplegia, tetraplegia, or full body paralysis below the site of injury to the spinal cord.
The two areas of the spinal cord most often injured are the cervical spine (C1-C7) and the lumbar spine (L1-L5). (The prefixes C,T and L refer to the location of a vertebra in the cervical, thoracic, or lumbar region of the spine.)