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Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less. In this installment I will discuss the exterior of the spinal
cord.
The spinal cord is one of the two main components of the central nervous system (the other being
the brain). It receives all of the sensory information from the periphery of the body
and carries it to the brain, and also contains motor neurons that supply the muscles we use
to move around. Neurons carrying sensory information enter the back of the spinal cord as dorsal
rootlets, which all emerge from a single dorsal root. The cell bodies of these sensory neurons
are contained in a cluster of cell bodies called a dorsal root ganglion. Motor nerves
leave the front of the cord as ventral rootlets and come together to form a ventral root;
the dorsal and ventral roots merge together to form the spinal nerves.
The spinal cord is divided into segments, each of which gives rise to a pair of spinal
nerves. There are 31 segments, named for their location: there are 8 cervical segments, 12
thoracic segments, 5 lumbar segments, 5 sacral segments, and 1 coccygeal segment.
The spinal cord leaves the skull through an opening called the foramen magnum and travels
down the vertebral column, ending in a pointed structure called the conus medullaris. The
cord is surrounded by a membrane called the dural tube, but the cord ends before the dural
tube and the spinal nerves that leave the cord from some lumbar and sacral segments
have to travel down through the dural tube to reach their site of exit from the vertebral
canal. This leaves a portion of the dural tube that does not contain the spinal cord
but is filled with spinal nerves. It is from this area--known as the lumbar cistern--that
cerebrospinal fluid is sampled as part of a lumbar puncture or spinal tap; it is done
here because there is less chance the cord could be damaged by the insertion of a needle.
The nerves in the lumbar cistern fan out like a horses tail and are resultantly called the
cauda equina (which means horse’s tail in Latin). There are two locations along the
cord that are slightly enlarged - the cervical and lumbar enlargements - because these parts
of the cord contain more neurons in order to supply the limbs. An extension of the meninges
called the filum terminale extends from the end of the cord to the tailbone to anchor
the cord in place.