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Spinal nerves, a part of the PNS, generally refers to mixed nerves, with motor, sensory, and autonomic signals between the CNS and the body.
Describe spinal nerves of the peripheral nervous system
Afferent sensory axons, bringing sensory information from the body to the spinal cord and brain, travel through the dorsal roots of the spinal cord, and efferent motor axons, bringing motor information from the brain to the body, travel through the ventral roots of the spinal cord.
The foramen allows for the passage of the spinal nerve root, dorsal root ganglion, the spinal artery of the segmental artery, communicating veins between the internal and external plexuses, recurrent meningeal (sinu-vertebral) nerves, and transforaminal ligaments.
The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body.
Humans have 31 left-right pairs of spinal nerves, each roughly corresponding to a segment of the vertebral column: eight cervical spinal nerve pairs (C1-C8), 12 thoracic pairs (T1-T12), five lumbar pairs (L1-L5), five sacral pairs (S1-S5), and one coccygeal pair. The spinal nerves are part of the peripheral nervous system (PNS).
Each spinal nerve is formed by the combination of nerve fibers from the dorsal and ventral roots of the spinal cord. The dorsal roots carry afferent sensory axons, while the ventral roots carry efferent motor axons. The spinal nerve emerges from the spinal column through an opening (intervertebral foramen) between adjacent vertebrae.
This is true for all spinal nerves except for the first spinal nerve pair, which emerges between the occipital bone and the atlas (the first vertebra). Thus the cervical nerves are numbered by the vertebra below, except C8, which exists below C7 and above T1. The thoracic, lumbar, and sacral nerves are then numbered by the vertebra above. In the case of a lumbarized S1 vertebra (i.e., L6) or a sacralized L5 vertebra, the nerves are typically still counted to L5 and the next nerve is S1.
Spinal Nerve Innervation
Outside the vertebral column, the nerve divides into branches. The dorsal ramus contains nerves that serve the dorsal portions of the trunk, carrying visceral motor, somatic motor, and somatic sensory information to and from the skin and muscles of the back (epaxial muscles).
The ventral ramus contains nerves that serve the remaining ventral parts of the trunk and the upper and lower limbs (hypaxial muscles), carrying visceral motor, somatic motor, and sensory information to and from the ventrolateral body surface, structures in the body wall, and the limbs. The meningeal branches (recurrent meningeal or sinuvertebral nerves) branch from the spinal nerve and re-enter the intervertebral foramen to serve the ligaments, dura, blood vessels, intervertebral discs, facet joints, and periosteum of the vertebrae.
The rami communicantes contain autonomic nerves that serve visceral functions carrying visceral motor and sensory information to and from the visceral organs.
The posterior distribution of the cervical nerves includes the suboccipital nerve (C1), the greater occipital nerve (C2) and the third occipital nerve (C3). The anterior
distribution includes the cervical plexus (C1-C4) and brachial plexus (C5-T1).
The muscles innervated by the cervical nerves are the sternohyoid, sternothyroid and omohyoid
muscles. A loop of nerves called ansa
cervicalis is part of the cervical plexus.
Thoracic nerve branches exit
the spine and go directly to the paravertebral ganglia of the autonomic nervous system where they are
involved in the functions of organs and glands in the head, neck, thorax and
Anterior divisions: The intercostal nerves come from thoracic nerves
T1-T11, and run between the ribs. The subcostal
nerve comes from nerve T12, and runs below the twelfth rib.
Posterior divisions: The
medial branches (ramus medialis) of the posterior branches of the upper six
thoracic nerves run between the semispinalis dorsi and multifidus,
which they supply; they then pierce the rhomboid
and trapezius muscles, and reach the skin by the sides
of the spinous processes. This branch is called the medial cutaneous ramus.
The medial branches of the
lower six are distributed chiefly to the multifidus and longissimus
dorsi, occasionally they give off filaments to the skin near the middle
line. This sensitive branch is called the posterior cutaneous ramus.
The lumbar nerves are divided
into posterior and anterior divisions.
medial branches of the posterior divisions of the lumbar nerves run close to
the articular processes of the vertebrae and end in the multifidus
muscle. The lateral branches supply the erector spinae muscles.
anterior divisions of the lumbar nerves (rami anteriores) consist of long,
slender branches which accompany the lumbar
arteries around the sides of the vertebral bodies, beneath the psoas
The first and second, and
sometimes the third and fourth lumbar nerves are each connected with the lumbar
part of the sympathetic trunk by a white ramus communicans.
The nerves pass obliquely
outward behind the psoas major, or between its fasciculi,
distributing filaments to it and the quadratus lumborum.
The first three and the
greater part of the fourth are connected by anastomotic loops and form the lumbar
The smaller part of the
fourth joins with the fifth to form the lumbosacral
trunk, which assists in the formation of the sacral
plexus. The fourth nerve is named the furcal nerve, from the fact that
it is subdivided between the two plexuses.
There are five paired
sacral nerves, half of them arising through the sacrum on the left side and the
other half on the right side. Each nerve emerges in two divisions: one division
through the anterior sacral foramina and the other
division through the posterior sacral foramina.
The sacral nerves have
both afferent and efferent
fibers, thus they are responsible for part of the sensory perception and the movements of the
lower extremities of the human body. The pudendal nerve and parasympathetic
fibers arise from S2, S3, and S4. They supply the descending
colon and rectum,
bladder, and genital organs. These pathways have both afferent
and efferent fibers.
coccygeal nerve is the 31st pair of spinal nerves and arises from the conus
medullaris. Its anterior root helps form the coccygeal
Spinal nerve motor
functions are summarized in the table below.
Source: Boundless. “Overview of the Spinal Nerves.” Boundless Anatomy and Physiology. Boundless, 29 Jul. 2016. Retrieved 31 Aug. 2016 from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/peripheral-nervous-system-13/spinal-nerves-132/overview-of-the-spinal-nerves-710-5078/