Spinal nerves
There are 31 pairs of
spinal nerves that leave the vertebral canal by passing through the
intervertebral foramina formed by adjacent vertebrae.
They are named and grouped according to the vertebrae with which they are associated:
They are named and grouped according to the vertebrae with which they are associated:
. 8 cervical
. 12 thoracic
. 5 lumbar
. 5 sacral
. 1 coccygeal.
Although there are
only seven cervical vertebrae, there are eight nerves because the first pair
leaves the vertebral canal between the occipital bone and the atlas and the
eighth pair leave below the last cervical vertebra.
Thereafter the nerves are given the name and number of the vertebra immediately above.
Thereafter the nerves are given the name and number of the vertebra immediately above.
The lumbar, sacral
and coccygeal nerves leave the spinal cord near its termination at the level of
the first lumbar vertebra, and extend downwards inside the vertebral canal in
the subarachnoid space, forming a sheaf of nerves which resembles a horse’s
tail, the cauda equine.
These nerves leave the vertebral canal at the appropriate lumbar, sacral or coccygeal level, depending on their destination.
These nerves leave the vertebral canal at the appropriate lumbar, sacral or coccygeal level, depending on their destination.
Nerve roots
The spinal nerves
arise from both sides of the spinal cord and emerge through the intervertebral
foramina.
Each nerve is formed by the union of a motor (anterior) and a sensory (posterior) nerve root and is, therefore, a mixed nerve.
Thoracic and upper lumb ar (L1 and L2) spinal nerves have a contribution from the sympathetic part of the autonomic nervous system in the form of a preganglionic fibre.
Bones and joints are supplied by adjacent nerves.
Each nerve is formed by the union of a motor (anterior) and a sensory (posterior) nerve root and is, therefore, a mixed nerve.
Thoracic and upper lumb ar (L1 and L2) spinal nerves have a contribution from the sympathetic part of the autonomic nervous system in the form of a preganglionic fibre.
Bones and joints are supplied by adjacent nerves.
The anterior nerve
root consists of motor nerve fibres, which are the axons of the lower motor
neurons from the anterior column of grey matter in the spinal cord and, in the thoracic
and lumbar regions, sympathetic nerve fibres, which are the axons of cells in
the lateral columns of grey matter.
The posterior nerve
root consists of sensory nerve fibres. Just outside the spinal cord there is a
spinal ganglion (posterior root ganglion), consisting of a little cluster of
cell bodies.
Sensory nerve fibres pass through these ganglia before entering the spinal cord.
The area of skin whose sensory receptors contribute to each nerve is called a dermatome.
Sensory nerve fibres pass through these ganglia before entering the spinal cord.
The area of skin whose sensory receptors contribute to each nerve is called a dermatome.
For a very short
distance after leaving the spinal cord the nerve roots have a covering of dura
and arachnoid maters.
These terminate before the two roots join to form the mixed spinal nerve. The nerve roots have no covering of pia matter.
These terminate before the two roots join to form the mixed spinal nerve. The nerve roots have no covering of pia matter.
Branches
Immediately after
emerging from the intervertebral foraminen, spinal nerves divide into brances,
or rami: a ramus communicans, a posterior ramus and an anterior ramus.
The rami communicante
are part of preganglionic sympathetic neurons of the autonomic nervous system.
The posterior rami
pass backwards and divide into medial and lateral branches to supply skin and
muscles of relatively small area of the posterior aspect of the head, neck and
trunk.
The anterior rami
supply the anterior and lateral aspects of the neck, trunk, and upper and lower
limbs.
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