Dorsal rami innervate which parts of the body

The dura is the tough outer sheath, the arachnoid lies beneath it, and the pia closely adheres to the surface of the cord Figure 3. The spinal cord is attached to the dura by a series of lateral denticulate ligaments emanating from the pial folds. The denticulate ligament, the dorsal root ganglion A , and an enlarged drawing of the meninges B. During the initial third month of embryonic development, the spinal cord extends the entire length of the vertebral canal and both grow at about the same rate.

As development continues, the body and the vertebral column continue to grow at a much greater rate than the spinal cord proper. This results in displacement of the lower parts of the spinal cord with relation to the vertebrae column. The outcome of this uneven growth is that the adult spinal cord extends to the level of the first or second lumbar vertebrae, and the nerves grow to exit through the same intervertebral foramina as they did during embryonic development.

This growth of the nerve roots occurring within the vertebral canal, results in the lumbar, sacral, and coccygeal roots extending to their appropriate vertebral levels Figure 3. All spinal nerves, except the first, exit below their corresponding vertebrae.

In the cervical segments, there are 7 cervical vertebrae and 8 cervical nerves Figure 3. C1-C7 nerves exit above their vertebrae whereas the C8 nerve exits below the C7 vertebra. It leaves between the C7 vertebra and the first thoracic vertebra. Therefore, each subsequent nerve leaves the cord below the corresponding vertebra. In the thoracic and upper lumbar regions, the difference between the vertebrae and cord level is three segments.

Therefore, the root filaments of spinal cord segments have to travel longer distances to reach the corresponding intervertebral foramen from which the spinal nerves emerge. The lumbosacral roots are known as the cauda equina Figure 3. Each spinal nerve is composed of nerve fibers that are related to the region of the muscles and skin that develops from one body somite segment. A spinal segment is defined by dorsal roots entering and ventral roots exiting the cord, i.

B Drawing of the spinal vertebrate. C Section of the spinal cord, its meninges and the dorsal and ventral roots of three segments. A dermatome is an area of skin supplied by peripheral nerve fibers originating from a single dorsal root ganglion. If a nerve is cut, one loses sensation from that dermatome. Because each segment of the cord innervates a different region of the body, dermatomes can be precisely mapped on the body surface, and loss of sensation in a dermatome can indicate the exact level of spinal cord damage in clinical assessment of injury Figure 3.

It is important to consider that there is some overlap between neighboring dermatomes. Most axons in the ventral roots arise from motor neurons in the ventral horn of the spinal cord and innervate skeletal muscle. Others arise from the lateral horn and synapse on autonomic ganglia that innervate visceral organs.

The ventral root axons join with the peripheral processes of the dorsal root ganglion cells to form mixed afferent and efferent spinal nerves, which merge to form peripheral nerves. Knowledge of the segmental innervation of the cutaneous area and the muscles is essential to diagnose the site of an injury. A transverse section of the adult spinal cord shows white matter in the periphery, gray matter inside, and a tiny central canal filled with CSF at its center.

Surrounding the canal is a single layer of cells, the ependymal layer. The shape and size of the gray matter varies according to spinal cord level. At the lower levels, the ratio between gray matter and white matter is greater than in higher levels, mainly because lower levels contain less ascending and descending nerve fibers. The gray matter mainly contains the cell bodies of neurons and glia and is divided into four main columns: dorsal horn, intermediate column, lateral horn and ventral horn column.

The dorsal horn is found at all spinal cord levels and is comprised of sensory nuclei that receive and process incoming somatosensory information. From there, ascending projections emerge to transmit the sensory information to the midbrain and diencephalon.

The intermediate column and the lateral horn comprise autonomic neurons innervating visceral and pelvic organs. The ventral horn comprises motor neurons that innervate skeletal muscle. At all the levels of the spinal cord, nerve cells in the gray substance are multipolar, varying much in their morphology. The axons of Golgi type I are long and pass out of the gray matter into the ventral spinal roots or the fiber tracts of the white matter.

The axons and dendrites of the Golgi type II cells are largely confined to the neighboring neurons in the gray matter.

A more recent classification of neurons within the gray matter is based on function. These cells are located at all levels of the spinal cord and are grouped into three main categories: root cells, column or tract cells and propriospinal cells.

The root cells are situated in the ventral and lateral gray horns and vary greatly in size. The root cells contribute their axons to the ventral roots of the spinal nerves and are grouped into two major divisions: 1 somatic efferent root neurons, which innervate the skeletal musculature; and 2 the visceral efferent root neurons, also called preganglionic autonomic axons, which send their axons to various autonomic ganglia. The column or tract cells and their processes are located mainly in the dorsal gray horn and are confined entirely within the CNS.

The axons of the column cells form longitudinal ascending tracts that ascend in the white columns and terminate upon neurons located rostrally in the brain stem, cerebellum or diencephalon. Some column cells send their axons up and down the cord to terminate in gray matter close to their origin and are known as intersegmental association column cells.

Other column cell axons terminate within the segment in which they originate and are called intrasegmental association column cells. Still other column cells send their axons across the midline to terminate in gray matter close to their origin and are called commissure association column cells.

The propriospinal cells are spinal interneurons whose axons do not leave the spinal cord proper. Some of these fibers also are found around the margin of the gray matter of the cord and are collectively called the fasciculus proprius or the propriospinal or the archispinothalamic tract.

The prominent nuclear groups of cell columns within the spinal cord from dorsal to ventral are the marginal zone, substantia gelatinosa, nucleus proprius, dorsal nucleus of Clarke, intermediolateral nucleus and the lower motor neuron nuclei. The axons of its neurons contribute to the lateral spinothalamic tract which relays pain and temperature information to the diencephalon Figure 3.

Substantia gelatinosa is found at all levels of the spinal cord. Located in the dorsal cap-like portion of the head of the dorsal horn, it relays pain, temperature and mechanical light touch information and consists mainly of column cells intersegmental column cells. These column cells synapse in cell at Rexed layers IV to VII, whose axons contribute to the ventral anterior and lateral spinal thalamic tracts. The homologous substantia gelatinosa in the medulla is the spinal trigeminal nucleus.

Nucleus proprius is located below the substantia gelatinosa in the head and neck of the dorsal horn. This cell group, sometimes called the chief sensory nucleus, is associated with mechanical and temperature sensations. It is a poorly defined cell column which extends through all segments of the spinal cord and its neurons contribute to ventral and lateral spinal thalamic tracts, as well as to spinal cerebellar tracts.

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 posterior division dorsal ramus is labeled at the top right. 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.

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

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 sacral plexus.

Brachial plexus : Cervical C5—C8 and thoracic T1 nerves comprise the brachial plexus, which is a nerve plexus that provides sensory and motor function to the shoulders and upper limbs.

Lumbar plexus : The lumbar plexus is comprised of the ventral rami of the lumbar spinal nerves L1—L5 and a contribution from thoracic nerve T The posterior green and anterior yellow divisions of the lumbar plexus are shown in the diagram. The intercostal nerves are part of the somatic nervous system and arise from anterior divisions rami anteriores, ventral divisions of the thoracic spinal nerves T1 to T The intercostal nerves are distributed chiefly to the thoracic pleura and abdominal peritoneum.

Intercostal nerves : An image of the intercostal brachial nerves. They differ from the anterior divisions of the other spinal nerves in that each pursues an independent course without plexus formation. 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 thoracis 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 seventh intercostal nerve terminates at the xyphoid 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 thoraco-abdominal nerves or thoracicoabdominal intercostal nerves.

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.

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. Dermatomes : Dermatomes are areas of skin supplied by sensory neurons that arise from a spinal nerve ganglion.

Dermatomes and the associated major cutaneous nerves are shown here in a ventral view. 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 rash : The shingles rash appears across a dermatome. In this patient, one of the dermatomes in the arm is affected, restricting the rash to the length of the back of the arm. From T7 to L1 or L2 , this pattern is reversed:. The lumbar dorsal rami originate perpendicularly from the spinal nerves and divide within medial as well as lateral branches immediately after their departure from the intervertebral foramina.

The multifidi and erector spinae muscles are supplied by the medial and lateral branches, respectively. The L1-L4 dorsal rami create three branches — medial, lateral, and intermediate, which are, respectively, distributed towards the multifidi, iliocostalis and longissimus muscles.

At L1-L3 the lateral and intermediate divisions may emerge in an alternative manner from a short common stem, while the L4 dorsal ramus regularly creates three divisions.