Spinal Nerves – Anatomy, Types, Structure, Functions

Overview of the Spinal Nerves

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 summarized 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.

Spinal nerves, a part of the peripheral nervous system (PNS), are mixed nerves that send motor, sensory, and autonomic signals between the CNS and the body.

Spinal Nerve Anatomy

The term spinal nerve generally refers to a mixed spinal nerve that 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).

Location

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 summarized 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; it carries 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); they carry the 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, such as carrying visceral motor and sensory information to and from the visceral organs.

Cervical Nerves

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 also part of the cervical plexus.

Thoracic Nerves

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 abdomen.

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 thoracic nerves 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.

Lumbar Nerves

The lumbar nerves 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 lateral branches supply the erector spinae muscles.

Anterior Divisions

The anterior divisions of the lumbar nerves (rami anteriores) consist of long, slender branches that accompany the lumbar arteries around the sides of the vertebral bodies, beneath the psoas major.

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 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.

Sacral Nerves

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, urinary bladder, and genital organs. These pathways have both afferent and efferent fibers.

Coccygeal Nerve

The coccygeal nerve is the 31st pair of spinal nerves and arises from the conus medullaris. Its anterior root helps form the coccygeal plexus.

Function

Spinal nerve motor functions are summarized in the table below.

Actions of the spinal nerves
Level	Motor Function
C1–C6	Neck flexors
C1–T1	Neck extensors
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 the 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
T7–L1	Abdominal muscles
L1–L4	Flex thigh
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); plantarflex foot and flex toes

Branches of Spinal Nerves

The spinal nerves branch into the dorsal ramus, ventral ramus, the meningeal branches, and the rami communicantes.

Outside the vertebral column, the spinal nerves divide into branches.

• The dorsal ramus: Contains nerves that serve the dorsal portions of the trunk carrying visceral motor, somatic motor, and sensory information to and from the skin and muscles of the back.
• The ventral ramus: Contains nerves that serve the remaining ventral parts of the trunk and the upper and lower limbs carrying visceral motor, somatic motor, and sensory information to and from the ventrolateral body surface, structures in the body wall, and the limbs. Some ventral rami merge with adjacent ventral 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 meningeal branches (recurrent meningeal or sinuvertebral nerves): These 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 carry visceral motor and sensory information to and from the visceral organs.

Types of Spinal Nerve fibers

• Somatic efferent fibers originate in the anterior/ventral column of central grey matter in the spinal cord. They pass through the anterior root of the spinal nerve. They are responsible for the motor innervation of the skeletal muscles.
• Somatic afferent fibers carry sensory information from the skin, joints, and muscle to the posterior/dorsal column of grey matter in the spinal cord. These fibers pass through the dorsal root ganglion.
• Visceral efferent fibers are autonomic fibers that supply the organs. They are divided into sympathetic and parasympathetic fibers. Sympathetic fibers originate from the thoracic spinal nerves as well as L1 and L2. Parasympathetic nerves come from the S2, S3, and S4 spinal nerves only to supply the pelvic and lower abdominal viscera. The remainder of the parasympathetic nerves come from extensions of the cranial nerves into the thoracic and abdominal cavities.
• Visceral afferent fibers carry sensory information through the dorsal root ganglion and to the dorsal column of grey matter in the spinal cord.

Plexuses

A nerve plexus is a network of intersecting nerves that serve the same part of the body.

A nerve plexus is a network of intersecting nerves; multiple nerve plexuses exist in the body. Nerve plexuses are composed of afferent and efferent fibers that arise from the merging of the anterior rami of spinal nerves and blood vessels.

There are five spinal nerve plexuses—except in the thoracic region—as well as other forms of autonomic plexuses, many of which are a part of the enteric nervous system.

Spinal Plexuses

Cervical Plexus—Serves the Head, Neck, and Shoulders

• The cervical plexus is formed by the ventral rami of the upper four cervical nerves and the upper part of the fifth cervical ventral ramus. The network of rami is located deep within the neck.

Brachial Plexus—Serves the Chest, Shoulders, Arms, and Hands

• The brachial plexus is formed by the ventral rami of C5–C8 and the T1 spinal nerves, and lower and upper halves of the C4 and T2 spinal nerves. The plexus extends toward the armpit (axilla).

Lumbar Plexus—Serves the Back, Abdomen, Groin, Thighs, Knees, and Calves

• The lumbar plexus is formed by the ventral rami of L1–L5 spinal nerves with a contribution of T12 form the lumbar plexus. This plexus lies within the psoas major muscle.

Sacral Plexus—Serves the Pelvis, Buttocks, Genitals, Thighs, Calves, and Feet

• The sacral plexus is formed by the ventral rami of L4-S3, with parts of the L4 and S4 spinal nerves. It is located on the posterior wall of the pelvic cavity.

Coccygeal Plexus—Serves a Small Region over the Coccyx

• The coccygeal plexus serves a small region over the coccyx and originates from S4, S5, and Co1 spinal nerves. It is interconnected with the lower part of the sacral plexus.

In addition, the celiac plexus serves the internal organs, and Auerbach’s plexus serves the gastrointestinal tract.

Autonomic Plexuses

• Celiac plexus (solar plexus)—Serves internal organs.
• Auerbach’s plexus—Serves the gastrointestinal tract.
• Meissner’s plexus (submucosal plexus)—Serves the gastrointestinal tract.

Intercostal Nerves

The anterior divisions of the thoracic spinal nerves (T1–T11) are called the intercostal nerves.

The intercostal nerves are part of the somatic nervous system and arise from anterior divisions (rami anterior, ventral divisions) of the thoracic spinal nerves T1 to T11. The intercostal nerves are distributed chiefly to the thoracic pleura and abdominal peritoneum.

They differ from the anterior divisions of the other spinal nerves in that each pursues an independent course without plexus formation.

First Thoracic Nerve

The anterior division of the first thoracic nerve divides into two branches:

• The larger branch leaves the thorax in front of the neck of the first rib and enters the brachial plexus.
• The other smaller branch, the first intercostal nerve, runs along with the first intercostal space and ends on the front of the chest as the first anterior cutaneous branch of the thorax.

The Upper Thoracic Nerves (2nd–6th)

These are limited in their distribution to the parietes (wall) of the thorax. The anterior divisions of the second, third, fourth, fifth, and sixth thoracic nerves, and the small branch from the first thoracic are confined to the walls of the thorax and are named thoracic intercostal nerves.

Near the sternum, they cross in front of the internal mammary artery and transversus thoracic muscle, pierce the intercostales interni, the anterior intercostal membranes, and pectoralis major, and supply the integument of the front of the thorax and over the mamma, forming the anterior cutaneous branches of the thorax.

The branch from the second nerve unites with the anterior supraclavicular nerves of the cervical plexus.

The Lower Thoracic Nerves (7th–12th)

The seventh intercostal nerve terminates at the xiphoid process, at the lower end of the sternum.

The anterior divisions of the seventh, eighth, ninth, tenth, and eleventh thoracic intercostal nerves are continued anteriorly from the intercostal spaces into the abdominal wall; hence they are named thoracoabdominal nerves or thoracoabdominal intercostal nerves.

The tenth intercostal nerve terminates at the umbilicus.

The twelfth (subcostal) thoracic nerve is distributed to the abdominal wall and groin.

Unlike the nerves from the autonomic nervous system that innervate the visceral pleura of the thoracic cavity, the intercostal nerves arise from the somatic nervous system. This enables them to control the contraction of muscles, as well as provide specific sensory information regarding the skin and parietal pleura.

This explains why damage to the internal wall of the thoracic cavity can be felt as a sharp pain localized in the injured region. Damage to the visceral pleura is experienced as an unlocalized ache.

Dermatomes

A dermatome is an area of skin that is supplied by a single spinal nerve, and a myotome is a group of muscles that a single spinal nerve root innervates.

A dermatome is an area of skin that is supplied by a single spinal nerve. There are eight cervical nerves, twelve thoracic nerves, five lumbar nerves and five sacral nerves. Each of these nerves relays sensation, including pain, from a particular region of the skin to the brain.

Along the thorax and abdomen, the dermatomes are like a stack of discs, with each section supplied by a different spinal nerve. Along the arms and the legs, the pattern is different. The dermatomes run longitudinally along the limbs, so that each half of the limb has a different dermatome.

Although the general pattern is similar in all people, the precise areas of innervation are as unique to an individual as fingerprints.

Dermatomes have clinical significance, especially in the diagnosis of certain diseases. Symptoms that follow a dermatome, such as pain or a rash, may indicate a pathology that involves the related nerve root. Examples include dysfunction of the spine or a viral infection.

Viruses that remain dormant in nerve ganglia, such as the varicella zoster virus that causes both chickenpox and shingles, often cause either pain, rash, or both in a pattern defined by a dermatome.

Shingles is one of the only diseases that cause a rash in a dermatomal pattern, and as such, this is its defining symptom. The rash of shingles is almost always restricted to a specific dermatome, such as on the chest, leg, or arm caused by the residual varicella-zoster virus infection of the nerve that supplies that area of skin. Shingles typically appear years or decades after recovery from chickenpox.

Myotome

A myotome is the group of muscles that a single spinal nerve root innervates. The myotome is the motor equivalent of a dermatome.

The myotome distributions of the upper and lower extremities are listed below:

• C1/C2: Neck flexion / extension
• C3: Neck lateral flexion
• C4: Shoulder elevation
• C5: Shoulder abduction
• C6: Elbow flexion/wrist extension
• C7: Elbow extension/wrist flexion
• C8: Finger flexion
• T1: Finger abduction
• L2: Hip flexion
• L3: Knee extension
• L4: Ankle dorsiflexion
• L5: Great toe extension
• S1: Ankle plantar flexion/ankle eversion/hip extension
• S2: Knee flexion
• S3–S4: Anal reflex

The testing of myotomes provides the clinician with information about the level in the spine where a lesion may be present. During testing, the clinician looks for muscle weakness of a particular group of muscles. Results may indicate lesions to the spinal cord nerve root, or intervertebral disc herniation that presses on the spinal nerve roots.

Function and Physiology of the Spinal Nerves

Spinal nerves connect the brain and spinal cord to the limbs and organs of the body.

Review of Peripheral Nervous System Structure

The peripheral nervous system (PNS) consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system (CNS) to the limbs and organs.

Unlike the CNS, the PNS is not protected by the bones of the spine and skull, or by the blood –brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.

The peripheral nervous system includes 12 cranial nerves and 31 pairs of spinal nerves that provide communication from the CNS to the rest of the body by nerve impulses to regulate the functions of the human body. 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.

Spinal Nerve Correspondences

Each pair of spinal nerves roughly correspond to a segment of the vertebral column: 8 cervical spinal nerve pairs (C1–C8), 12 thoracic pairs (T1–T12), 5 lumbar pairs (L1–L5), 5 sacral pairs (S1–S5), and 1 coccygeal pair.

• The first 4 cervical spinal nerves, C1 through C4, split and recombine to produce a variety of nerves that subserve the neck and back of the head.
• The spinal nerve C1 (suboccipital nerve) provides motor innervation to muscles at the base of the skull.
• C2 and C3 form many of the nerves of the neck, and provides both sensory and motor control. These include the greater occipital nerve that provides sensation to the back of the head, the lesser occipital nerve that provides sensation to the area behind the ears, the greater auricular nerve, and the lesser auricular nerve.
• The phrenic nerve arises from nerve roots C3, C4, and C5. It innervates the diaphragm to enable breathing. If the spinal cord is transected above C3, then spontaneous breathing is not possible.
• The last four cervical spinal nerves, C5 through C8, and the first thoracic spinal nerve, T1, combine to form the brachial plexus, or plexus brachialis, a tangled array of nerves, splitting, combining and recombining to form the nerves that subserve the upper limb region and upper back. Although the brachial plexus may appear tangled, it is highly organized and predictable with little variation among people.

Lumbosacral Plexus

The anterior divisions of the lumbar, sacral, and coccygeal nerves form the lumbosacral plexus, the first lumbar nerve being frequently joined by a branch from the twelfth thoracic. For descriptive purposes, this plexus is usually divided into three parts: lumbar plexus, sacral plexus, and pudendal plexus.

Autonomic Nervous System Function (ANS)

The sympathetic division typically functions in actions that need quick responses. The parasympathetic division functions with actions that do not require immediate reaction.

The sympathetic system is often considered the fight or flight system, while the parasympathetic system is often considered the rest and digest or feed and breed system.

Some typical actions of the sympathetic and parasympathetic systems are listed below.

Sympathetic Nervous System

• Diverts blood flow away from the gastrointestinal (GI) tract and skin via vasoconstriction.
• Enhances blood flow to skeletal muscles and the lungs.
• Dilates bronchioles of the lung by circulating epinephrine to allow for greater alveolar oxygen exchange.
• Increases the heart rate and contractility of cardiac muscle for enhanced blood flow to skeletal muscles.
• Dilates pupils and relaxes the ciliary muscle to the lens for far vision.
• Provides vasodilation for the coronary vessels of the heart.
• Constricts all the intestinal sphincters and the urinary sphincter.
• Inhibits peristalsis.
• Stimulates orgasm.

Parasympathetic Nervous System

• Dilates blood vessels that lead to the GI tract to increase blood flow; this is important following food consumption due to the greater metabolic demands placed on the body by the gut.
• Constricts the bronchiolar diameter when the need for oxygen has diminished.
• Manages heart control via dedicated cardiac branches of the vagus and thoracic spinal accessory nerves.
• Constricts the pupil and contracts the ciliary muscles to facilitate accommodation for closer vision.
• Stimulates salivary gland secretion and accelerates peristalsis to mediate the digestion of food.
• PNS nerves are involved in the erection of genital tissues via the pelvic splanchnic nerves 2–4. They are also responsible for stimulating sexual arousal.

Neurotransmitters

• Acetylcholine is the preganglionic neurotransmitter for both divisions of the ANS, as well as the postganglionic neurotransmitter of parasympathetic neurons.
• Nerves that release acetylcholine are said to be cholinergic. In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter to stimulate muscarinic receptors.
• At the adrenal medulla, there is no postsynaptic neuron. Instead, the presynaptic neuron releases acetylcholine to act on nicotinic receptors.
• Stimulation of the adrenal medulla releases adrenaline (epinephrine) into the bloodstream, which acts on adrenoceptors, producing a widespread increase in sympathetic activity.

Somatic Nervous System Function (SoNS)

The somatic nervous system consists of afferent and efferent nerves and is associated with the voluntary control of skeletal muscle movements. The afferent nerves are responsible for relaying sensations from the body to the central nervous system (CNS), while the efferent nerves are responsible for sending out commands from the CNS to the body to stimulate muscle contraction.

Upper motor neurons release acetylcholine. Acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors (nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers.

Blood Supply and Lymphatics

The spinal cord receives both longitudinal and segmental blood supply.  The vasculature is highly complex and anastomotic which ensures adequate delivery to the entire structure.  Longitudinally, the anterior spinal artery (ASA) and a pair of posterior spinal arteries (PSAs) are the predominant supply of the spinal cord.  The three arteries branch from the distal vertebral arteries at the base of the skull.  The three vessels connect circumferentially around the cord as the vaso-corona to supply the entire periphery of the cord.[rx] Numerous radicular arteries are also arranged segmentally which provide additional blood flow to the entire vertebral column.  Many of these are branches of segmental intercostal and lumbar arteries.[rx][rx]

or

Three lumbar vertebral arteries surround each lumbar vertebral body. Lumbar vertebral arteries are direct branches off of the aorta. Spinal arterial branches differentiate into radicular and segmental arteries. Segmental branches supply vertebral bodies, and posterior arterial branches supply vertebral arches. Spinal branches enter the vertebral canal through intervertebral foramina to supply the bones, periosteum, ligaments, and meninges. Radicular and segmental arteries supply spinal nerve roots, spinal nerves, and the spinal cord. Venous drainage of the vertebral column parallels the arterial system. Additionally, there is an internal anterior lumbar venous plexus; and a posterior external venous plexus.[rx]

References

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