Spinal cord
In vertebrates, the spinal cord is the part of the
central nervous system that is enclosed in and protected by the vertebral column.
Encyclopedia
In vertebrates, the
spinal cord is the part of the
central nervous system that is enclosed in and protected by the vertebral column.
Structure
In humans, the spinal cord extends through the spinal canal, starting at the
brain and extending downward. In the adult, it ends at the vertebral level of L1 or L2. This terminal part of the spinal cord is known as the conus medullaris. Like the brain, the spinal cord is covered by three meninges: the outer dura mater, the arachnoid membrane, and the innermost pia mater, with cerebrospinal fluid found in the subarachnoid space. The cord within the pia mater is stabilized within the dura mater by the connecting denticulate ligaments which come off the pia mater laterally between the dorsal and ventral roots.
Spinal cord segments
The spinal cord can be divided up into 31 different sections known as
spinal cord segments. In a given spinal cord segment, two sets of nerve rootlets emerge from the cord on each side. The
anterior rootlets form the
ventral roots, while the
posterior rootlets form the dorsal roots. The ventral and dorsal roots will later join to form paired
spinal nerves, one on each side of the spinal cord. The segment is then named after these nerves.
There are 31 spinal cord segments:
- 8 cervical segments
- 12 thoracic segments
- 5 lumbar segments
- 5 sacral segments
- 1 coccygeal segment
It is important to note that a spinal cord segment is not always found at the same level as its corresponding
vertebra, especially in the lower spinal cord segments. In a fetus, the vertebral levels originally correspond with the spinal cord segments. The vertebral column, however, grows longer than the spinal cord. In the adult, the cord ends around the L1/L2 vertebral level, 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 limb. It includes spinal cord segments from about C4 to T1. The vertebral levels of the enlargement are roughly the same .
- Lumbosacral enlargement - corresponds to the lumbosacral plexus nerves, which innervate the lower limb. It comprises the spinal cord segments from L2 to S3, and is found about the vertebral levels of T9 to T12.
Injury
Spinal cord injuries are caused by damage to the spinal cord, such as 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 good 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 most common areas of the spinal cord most injured are the cervical spine and the lumbar spine .
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 adulthood, the spinal cord reaches only to the level of the first or second lumbar vertebrae. Below this level, there is no spinal cord, only
spinal nerves that form the cauda equina.
See also
External links
