A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into the corresponding cervical, thoracic, lumbar, sacral and coccygeal regions of the spine. There are eight pairs of cervical nerves, twelve pairs of thoracic nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one pair of coccygeal nerves. The spinal nerves are part of the peripheral nervous system.
The formation of the spinal nerve from the posterior and anterior roots
|Anatomical terms of neuroanatomy|
Each spinal nerve is a mixed nerve, formed from the combination of nerve fibers from its dorsal and ventral roots. The dorsal root is the afferent sensory root and carries sensory information to the brain. The ventral root is the efferent motor root and carries motor information from the brain. 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 (C1), which emerges between the occipital bone and the atlas (the first vertebra). Thus the cervical nerves are numbered by the vertebra below, except spinal nerve C8, which exists below vertebra C7 and above vertebra T1. The thoracic, lumbar, and sacral nerves are then numbered by the vertebra above. In the case of a lumbarized S1 vertebra (aka L6) or a sacralized L5 vertebra, the nerves are typically still counted to L5 and the next nerve is S1.
Outside the vertebral column, the nerve divides into branches. The dorsal ramus contains nerves that serve the posterior 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 anterior 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.
Some anterior rami merge with adjacent anterior rami to form a nerve plexus, a network of interconnecting nerves. Nerves emerging from a plexus contain fibers from various spinal nerves, which are now carried together to some target location. Major plexuses include the cervical, brachial, lumbar, and sacral plexuses.
The cervical nerves are the spinal nerves from the cervical vertebrae in the cervical segment of the spinal cord. Although there are seven cervical vertebrae (C1-C7), there are eight cervical nerves C1–C8. All cervical nerves except C8 emerge above their corresponding vertebrae, while the C8 nerve emerges below the C7 vertebra. Elsewhere in the spine, the nerve emerges below the vertebra with the same name.
The posterior distribution 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).
A loop of nerves called ansa cervicalis is part of the cervical plexus.
The thoracic nerves are the twelve spinal nerves emerging from the thoracic vertebrae. Each thoracic nerve T1 -T12 originates from below each corresponding thoracic vertebra. Branches also 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 abdomen.
Anterior divisions: The intercostal nerves come from thoracic nerves T1-T11, and run between the ribs. At T2 and T3, further branches form the intercostobrachial nerve. 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 sensitive 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 the five spinal nerves emerging from the lumbar vertebrae. They are divided into posterior and anterior divisions.
Posterior divisions: The 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 laterals supply the erector spinae muscles.
The upper three give off cutaneous nerves which pierce the aponeurosis of the latissimus dorsi at the lateral border of the erector spinae muscles, and descend across the posterior part of the iliac crest to the skin of the buttock, some of their twigs running as far as the level of the greater trochanter.
Anterior divisions: The anterior divisions of the lumbar nerves (rami anteriores) increase in size from above downward. They are joined, near their origins, by gray rami communicantes from the lumbar ganglia of the sympathetic trunk. These rami consist of long, slender branches which accompany the lumbar arteries around the sides of the vertebral bodies, beneath the psoas major. Their arrangement is somewhat irregular: one ganglion may give rami to two lumbar nerves, or one lumbar nerve may receive rami from two ganglia.
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 first three and the greater part of the fourth are connected together in this situation by anastomotic loops, and form the lumbar plexus.
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.
The sacral nerves are the five pairs of spinal nerves which exit the sacrum at the lower end of the vertebral column. The roots of these nerves begin inside the vertebral column at the level of the L1 vertebra, where the cauda equina begins, and then descend into the sacrum.
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 nerves divide into branches and the branches from different nerves join with one another, some of them also joining with lumbar or coccygeal nerve branches. These anastomoses of nerves form the sacral plexus and the lumbosacral plexus. The branches of these plexus give rise to nerves that supply much of the hip, thigh, leg and foot.
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. From the S2, S3 and S4 arise the pudendal nerve and parasympathetic fibers whose electrical potential supply the descending colon and rectum, urinary bladder and genital organs. These pathways have both afferent and efferent fibers and, this way, they are responsible for conduction of sensory information from these pelvic organs to the central nervous system (CNS) and motor impulses from the CNS to the pelvis that control the movements of these pelvic organs.
The coccygeal nerve is the 31st pair of spinal nerves. It arises from the conus medullaris, and its anterior root helps form the coccygeal plexus. It does not divide into a medial and lateral branch. It is distributed to the skin over the back of the coccyx.
|C3, C4, C5||Supply diaphragm (mostly C4)|
|C5, C6||Move shoulder, raise arm (deltoid); flex elbow (biceps)|
|C6||externally rotate (supinate) the arm|
|C6, C7||Extend elbow and wrist (triceps and wrist extensors); pronate wrist|
|C7, C8||Flex wrist; supply small muscles of the hand|
|T1–T6||Intercostals and trunk above the waist|
|L1–L4||Flex hip joint|
|L2, L3, L4||Adduct thigh; Extend leg at the knee (quadriceps femoris)|
|L4, L5, S1||abduct thigh; Flex leg at the knee (hamstrings); Dorsiflex foot (tibialis anterior); Extend toes|
|L5, S1, S2||Extend leg at the hip (gluteus maximus); flex foot and flex toes|
The muscles that one particular spinal root supplies are that nerve's myotome, and the dermatomes are the areas of sensory innervation on the skin for each spinal nerve. Lesions of one or more nerve roots result in typical patterns of neurologic defects (muscle weakness, abnormal sensation, changes in reflexes) that allow localization of the responsible lesion.
There are several procedures used in sacral nerve stimulation for the treatment of various related disorders.
Sciatica is generally caused by the compression of lumbar nerves L4, or L5 or sacral nerves S1, S2, or S3, or by compression of the sciatic nerve itself.
The cervical spinal nerve 3 (C3) is a spinal nerve of the cervical segment.It originates from the spinal column from above the cervical vertebra 3 (C3).Cervical spinal nerve 4
Cervical spinal nerve 4, also called C4, is a spinal nerve of the cervical segment. It originates from the spinal cord above the 4th cervical vertebra (C4). It contributes nerve fibers to the phrenic nerve, the motor nerve to the thoracoabdominal diaphragm. It also provides motor nerves for the longus capitis, longus colli, anterior scalene, middle scalene, and levator scapulae muscles. C4 contributes some sensory fibers to the supraclavicular nerves, responsible for sensation from the skin above the clavicle.Cervical spinal nerve 5
The cervical spinal nerve 5 (C5) is a spinal nerve of the cervical segment.It originates from the spinal column from above the cervical vertebra 5 (C5). It contributes to the phrenic nerve, long thoracic nerve, and dorsal scapular nerve before joining cervical spinal nerve 6 to form the upper trunk, a trunk of the brachial plexus, which then forms the lateral cord, and finally the musculocutaneous nerve.Cervical spinal nerve 6
The cervical spinal nerve 6 (C6) is a spinal nerve of the cervical segment.It originates from the spinal column from above the cervical vertebra 6 (C6).
The C6 nerve root shares a common branch from C5, and has a role in innervating many muscles of the rotator cuff and distal arm, including:
Extensor Carpi Radialis Brevis
Extensor Carpi Radialis Longus
Latissimus DorsiDamage to the C6 motor neuron, by way of impingement, ischemia, trauma, or degeneration of nerve tissue, can cause denervation of one or more of the associated muscles. Muscle atrophy and other secondary complications can occur.Cervical spinal nerve 7
The cervical spinal nerve 7 (C7) is a spinal nerve of the cervical segment.It originates from the spinal column from above the cervical vertebra 7 (C7).
It runs through the interspace between the C6 and C7 vertebrae.Cervical spinal nerve 8
The cervical spinal nerve 8 (C8) is a spinal nerve of the cervical segment.It originates from the spinal column from below the cervical vertebra 7 (C7).Dorsal ramus of spinal nerve
The dorsal ramus of spinal nerve (or posterior ramus of spinal nerve, or posterior primary division) is the posterior division of a spinal nerve. The dorsal ramus (Latin for branch, plural rami ) is the dorsal branch of a spinal nerve that forms from the dorsal root of the nerve after it emerges from the spinal cord. The spinal nerve is formed from the dorsal and ventral rami. The dorsal ramus carries information that supplies muscles and sensation to the human back.Dorsal root of spinal nerve
The dorsal root of spinal nerve (or posterior root of spinal nerve) is one of two "roots" which emerge from the spinal cord. It emerges directly from the spinal cord, and travels to the dorsal root ganglion. Nerve fibres with the ventral root then combine to form a spinal nerve. The dorsal root transmits sensory information, forming the afferent sensory root of a spinal nerve.Lumbar nerves
The lumbar nerves are the five pairs of spinal nerves emerging from the lumbar vertebrae. They are divided into posterior and anterior divisions.Sacral spinal nerve 1
The sacral spinal nerve 1 (S1) is a spinal nerve of the sacral segment.
It originates from the spinal column from below the 1st body of the sacrumSacral spinal nerve 2
The sacral spinal nerve 2 (S2) is a spinal nerve of the sacral segment.
It originates from the spinal column from below the 2nd body of the sacrumSacral spinal nerve 3
The sacral spinal nerve 3 (S3) is a spinal nerve of the sacral segment.
It originates from the spinal column from below the 3rd body of the sacrumSacral spinal nerve 4
The sacral spinal nerve 4 (S4) is a spinal nerve of the sacral segment.
It originates from the spinal column from below the 4th body of the sacrumSacral spinal nerve 5
The sacral spinal nerve 5 (S5) is a spinal nerve of the sacral segment.
It originates from the spinal column from below the 5th body of the sacrum
S5 supplies the Coccygeus muscleThoracic spinal nerve 11
The thoracic spinal nerve 11 (T11) is a spinal nerve of the thoracic segment.It originates from the spinal column from below the thoracic vertebra 11 (T11).Thoracic spinal nerve 12
The thoracic spinal nerve 12 (T12) is a spinal nerve of the thoracic segment.It originates from the spinal column from below the thoracic vertebra 12 (T12).
It may also be known as the subcostal nerve.Thoracic spinal nerve 4
The thoracic spinal nerve 4 (T4) is a spinal nerve of the thoracic segment.It originates from the spinal column from below the thoracic vertebra 4 (T4).Ventral ramus of spinal nerve
The ventral ramus (pl. rami) (Latin for branch) is the anterior division of a spinal nerve. The ventral rami supply the antero-lateral parts of the trunk and the limbs. They are mainly larger than the dorsal rami.
Shortly after a spinal nerve exits the intervertebral foramen, it branches into the dorsal ramus, ventral ramus, and ramus communicans. Each of these three structures carries both sensory and motor information. Because each spinal nerve carries both sensory and motor information, spinal nerves are referred to as “mixed nerves.”
In the thoracic region they remain distinct from each other and each innervates a narrow
strip of muscle and skin along the sides, chest, ribs, and abdominal wall. These rami are called the
intercostal nerves. In regions other than the thoracic, ventral rami converge with each other to form networks of nerves
called nerve plexuses. Within each plexus, fibers from the various ventral rami branch and
become redistributed so that each nerve exiting the plexus has fibers from several different spinal
nerves. One advantage to having plexuses is that damage to a single spinal nerve will not completely
paralyze a limb.
There are four main plexuses formed by the ventral rami:
the cervical plexus contains ventral rami from spinal nerves C1-C4. Branches of the
cervical plexus, which include the phrenic nerve, innervate muscles of the neck, the diaphragm, and the
skin of the neck and upper chest.
The brachial plexus contains ventral rami from spinal nerves C5-T1. This plexus innervates
the pectoral girdle and upper limb.
The lumbar plexus contains ventral rami from spinal nerves L1-L4. The sacral plexus contains ventral
rami from spinal nerves L4-S4. The lumbar and sacral plexuses innervate the pelvic girdle and lower
Ventral rami, including the sinuvertebral nerve branches, also supply structures anterior to the facet joint, including the vertebral bodies, the discs and their ligaments, and joins other spinal nerves to form the lumbo-sacral plexus.Ventral root of spinal nerve
In anatomy and neurology, the ventral root or anterior root is the efferent motor root of a spinal nerve.
At its distal end, the ventral root joins with the dorsal root to form a mixed spinal nerve.
|Central nervous system|
|Peripheral nervous system|