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StatPearls . Sweetheart Island (FL): StatPearls Publishing; 2021 Jan-.


Anatomy, Back, Spinal Nerve-Muscle Innervation

Hajira Basit; Vamsi Reddy; Matthew Varacallo.

Author Information


The appropriate anatomy of the spinal nerve-muscular innervation the the earlier is centered roughly the lumbar spinal nerves, peripheral nerves of the lumbar plexus, spinal cord, and lumbar vertebral column. Within the lumbar region, the vertebral body are bigger than in the thoracic and also cervical regions due to the lumbar spine gift designed because that weight-bearing purposes. In general, the spinal cord is composed of gray and also white matter. As in the brain, the gray issue of the spinal cord includes the cabinet bodies; and the white issue of the spinal cord includes myelinated tracts. The gray matter of the spinal cord is found the main aspect of the spinal cord in the form of the letter H. Instantly surrounding the spinal cord is the pia mater, through the subarachnoid an are overlying the pia mater, the arachnoid mater overlying the subarachnoid space, and also dura mater at the outermost layer, adherent come the spinal column.<1><2>

Cerebral spinal liquid (CSF) is present in the main canal that the spinal cord in the center of the gray matter. Cerebral spinal liquid is additionally present surrounding the spinal cord, in the subarachnoid an are and bordering the spinal nerves. Over there are five lumbar vertebral bodies, 5 lumbar spinal nerves, and five lumbar spinal segments. The adult spinal cord terminates at the L1 or L2 vertebral level. The terminal element of the spinal cord is the conus medullaris, and also immediately inferior to the spinal cord is the cauda equina. The cauda equina is a cordlike structure composed the thickened and elongated nerve roots the occupy the spinal canal. The cauda equina attaches come the mid-sacral canal at roughly the level of S2. Spinal nerves departure the spinal cord via the intervertebral foramen bilaterally at the lateral aspects of the vertebral column. Spinal nerves secure in location by thickenings in the pia mater, creating thin ligaments called denticulate ligaments. Denticulate ligaments connect to the arachnoid and also dura mater stabilizing the position of every spinal nerve root within the vertebral column.<1><2>

Structure and also Function

Each segment that the spinal cord generates a bilateral pair of corresponding spinal nerve roots. Each spinal cord segment coincides to the very same numbered vertebral level in the thoracic, lumbar, and also sacral spinal regions. In the cervical spine, there space eight cervical spinal segments and also seven vertebral bodies. The primary materials of the lumbar nervous device are the: lumbar spinal segments, spinal rootlets, spinal roots, and also spinal nerves.

The main canal the the spinal cord provides the spinal cord and spinal nerves v CSF and also is contiguous v the ventricular device of the mind where the CSF is produced. In ~ the posterior facet of the gray issue of the spinal cord is the posterior horn. Materials of the peripherally located white issue of the spinal cord room the posterior funiculus, lateral funiculus, anterior funiculus, and also anterior commissure (which crosses the midline anteriorly). At the anterior element of the gray issue of the spinal cord is the anterior horn.  in ~ the main aspect that the gray matter is the gray commissure (which the cross the midline).  The transition from spinal roots to spinal nerves is in ~ the level of the pedicle of the vertebral column. The posterior (dorsal) spinal roots enter the spinal tower and kind posterior root ganglions ~ separating native the engine fibers of the mixed spinal nerve. Indigenous distal come proximal: after the posterior root ganglion is the posterior spinal root and then the posterior rootlets which get in the spinal cord at the posterior horn. The anterior (ventral) spinal rootlets exit the spinal cord native the anterior horn and then type the anterior spinal nerves. The anterior spinal nerves then incorporate with the posterior spinal root (distal to the posterior source ganglion) to kind mixed spinal nerves. The posterior spinal roots carry afferent (towards the spinal cord) sensory fibers, and also the anterior spinal roots carry efferent (away native the spinal cord) motor fibers. Blended spinal nerves are constantly a mix of sensory and motor nerves.<2>

Anterior spinal nerves carry motor yarn that impact motor nerves in ~ the motor endplates of muscles. The lumbosacral enlargement is indigenous T11 through S1 of the spinal cord. The lumbosacral enlargement provides off anterior rami the the spinal nerves which comprise the nerves the the lumbar and sacral plexus.<2>


During embryologic advancement the spinal cord, the fetal spinal cord is the same size as the vertebral column. However, together the fetus grows, and also the vertebral canal elongates in ~ a higher relative rate compared to the spinal cord itself, the eventual result. In adulthood, the spinal cord is shorter than the vertebral column. In adults, the spinal cord terminates at the conus medullaris, in ~ the level that L1 or L2.<3>

Blood Supply and also Lymphatics

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 also segmental arteries. Segmental branches it is provided vertebral bodies, and posterior arterial branches supply vertebral arches. Spinal branches go into the vertebral canal with intervertebral foramina to supply the bones, periosteum, ligaments, and also meninges. Radicular and segmental arteries it is provided spinal nerve roots, spinal nerves, and also the spinal cord. Venous drainage that the vertebral column parallels the arterial system. Additionally, over there is an inner anterior lumbar venous plexus; and a posterior exterior venous plexus.<2>


Upon exiting the spinal column, the posterior and anterior spinal nerve roots combine roughly the intervertebral foramen to form mixed spinal nerves. The mixed spinal nerves save on computer both motor and also sensory nerve fibers. Combined spinal nerves automatically divide right into two main rami – a posterior ramus and an anterior ramus. The posterior and also anterior rami save on computer both sensory and also motor nerve fibers. Peripheral motor nerves usually contain approximately 40% sensory nerve fibers. Likewise, peripheral sensory nerves contain engine nerve fibers that innervate the smooth muscle the blood vessels, hair follicles, and also sweat glands.<2>

Lumbar spinal nerves exit below their matching vertebral level. For example, an L1 spinal nerve will exit listed below the pedicle the the equivalent L1 vertebral body. Thoracic spinal nerves likewise exit below their matching vertebral level. However, in the cervical spine, the cervical nerves exit over their matching cervical vertebral level. For example, a C3 spinal nerve will certainly exit over the pedicle that the C3 vertebral body.<2>


A limited description that the details lumbar spinal nerves includes: L1 innervates the ab internal obliques via the ilioinguinal nerve; L2-4 innervates iliopsoas, a hip flexor, and other muscles via the femoral nerve; L2-4 innervates adductor longus, a hip adductor, and other muscle via the obturator nerve; L5 innervates fishing eye dorsiflexion muscles.

The patellar reflex requires the L3/4 spinal nerves. The Achilles reflex entails the S1 spinal nerves.<2>

Physiologic Variants

A not unusual physiologic different of the lumbar spinal nerve root is a conjoined lumbosacral nerve root. In conjoined lumbosacral nerve roots, there space two nerve roots combined where the upper and also lower spinal nerve roots are expected to be. Conjoined lumbosacral nerve roots an outcome in one enlargement that the nerve root. Conjoined roots eventually divide and exit via separate foramina. Conjoined lumbosacral nerve roots can cause sciatica. Redundant and anastomotic nerve roots space less usual than conjoined nerve roots in the lumbosacral spine. Redundant or “twinned” nerve roots have two nerve roots exiting with a solitary intervertebral foramen. Anastomotic nerve root anomalies involve otherwise typical nerve root that have an anastomosis that bridges the connection between two surrounding roots.<4><5>

Surgical Considerations

Surgical treatment for low back pain have to be booked for cases refractory to much more conservative treatment such as physical therapy and also epidural injection.<6>

Clinical Significance

Back ache is a common complaint in medicine and also is the number one cause of occupationally associated injury. 

Occupational workers older 보다 the period of 40 room at a better risk the developing earlier pain as contrasted to younger workers. A detailed, occupationally relevant, medical history and physical examination is crucial in making precise diagnosis and also developing an suitable treatment plan for back pain in the work setting.  

Often to do a definitive diagnosis in situations of persistent earlier pain, an MRI the the lumbar spine is needed. It is necessary to try to correlate the patient’s symptoms with the imaging findings. The different types of back pain space local, referred, radicular, and spasmodic. The many common cause of earlier pain is a lumbar muscle spasm. In instances of serious persistent ago pain, lumbar disc condition with bowl herniation/protrusion is much more likely if there is an connected motor deficit. Nerve source entrapment indigenous disc herniation/protrusion causes pain, maybe muscle spasm; and also in numerous cases, one accompanying sensory and motor deficit is also seen.<7><8>

Imaging findings may not correlate through the patient’s symptoms. For example, a far-ranging imaging detect of a ideal disc bulge in ~ L5/S1 in a patient through symptoms the left L4/L5 nerve root circulation is a discordant finding. Anatomic correlation of bowl protrusion through the influenced spinal root and/or spinal nerve relies on laterally the the bowl protrusion. For example, a lateral key protrusion at the L1/L2 vertebral level will most likely influence the L1 spinal nerve; conversely, a central disc protrusion at the exact same level will many likely influence the L2 nerve root. This is due to the place of the centrally traversing spinal nerve roots in the vertebral column and the lateral place of the exiting spinal nerves.

The likelihood of a bowl protrusion or extrusions bring about radicular symptoms or engine nerve deficit rise when there is preexisting main canal stenosis or intervertebral foramen narrowing. One example of radicular low back pain is sciatica (pain radiating indigenous the low earlier down the posterior element of one or both reduced extremities) which outcomes from compression of the L5 and/or S1 ingredient of the sciatic nerve (L5/S1).

The cause of main canal stenosis is multifactorial. Vertebral canal stenosis and also can an outcome from facet hypertrophy, ligamentum flavum hypertrophy, disc bulge, and congenital canal stenosis. Approximately 95% of bowl protrusions and extrusions occur at L4-L5 and/or L5-S1.

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The differential diagnoses for ago pain include yet are not minimal to: degenerative key disease, spinal TB or other infection, tumor, and abdominal muscle aortic aneurysm. Although compression of the L4 spinal nerve is most likely second to key herniation, the cause of nerve compression could additionally be native a different etiology such as a spinal cord meningioma. A differentiating imaging characteristic the can assist to differentiate in between disc bulge matches tumor or infection is leptomeningeal enhancement. Bowl bulges would certainly not present leptomeningeal enhancement; whereas, a spinal tumor or discitis may display leptomeningeal enhancement.<10>