Understanding Spinal Nerve Plexuses

M15.3 Spinal Nerve Plexuses Major Nerve Plexuses Ventral rami from adjacent (but different) spinal nerves will come together to form a network or mingling (knot, tangle, net) of interconnecting nerves called a nerve plexus. In this instance, the word plexus is used to describe networks of nerve fibers with no associated cell bodies. As the axons mingle, they reorganize and create peripheral nerves that follow different courses. Peripheral (named) nerves emerging from a plexus contain fibers from various spinal nerves, which are now carried together to innervate various body structures. The plexuses contain both sensory and motor fibers (they are mixed). This means that: * Some fibers extend from receptors and send action potentials into the CNS. Those are axons of sensory neurons in the dorsal root ganglia that enter the spinal cord through the dorsal nerve root. * Some fibers are the axons of motor neurons of the anterior horn of the spinal cord, which emerge in the ventral nerve root and send action potentials to cause skeletal muscles to contract in their target regions. * For example, the radial n. contains fibers of cutaneous sensation in the arm, as well as motor fibers that move muscles in the arm. The mingling of spinal nerves happens at four major places along the length of the vertebral column. Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level. Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the intercostal nerves found between the ribs, which articulate with the vertebrae surrounding the spinal nerve. Video Image Intercostal nerves @Diana Botnaru, 2020. Length: 18:07. Description of plexuses * The cervical plexus is composed of axons from spinal nerves C1-C5 and branches into nerves in the posterior neck and head, as well as the phrenic n., which connects to the diaphragm at the base of the thoracic cavity. This plexus is responsible for sensation from the head and neck and motor control of respiration. * The brachial plexus is composed of axons from spinal nerves C5-T1 and give rise to the nerves of the arms. A large nerve from this plexus is the radial n. from which the axillary n. branches to go to the armpit region. The radial nerve continues through the arm and is paralleled by the ulnar n. and the median n. This plexus is responsible for functions like flexion of the elbow and abduction of shoulder, as well as sensation from the shoulder and upper limb. * The lumbar plexus arises from all the lumbar spinal nerves, L1-L5, and gives rise to nerves innervating the pelvic region and the anterior leg. The femoral n. is one of the major nerves from this plexus, which gives rise to the saphenous n. as a branch that extends through the anterior lower leg. The other big nerve of this plexus is the obturator n. This plexus is responsible for functions like knee extension and sensation from teh lower abdomen. * The sacral plexus arises from the lower lumbar nerves L4-L5, sacral nerves S1-S4 and coccygeal(Co). The most significant systemic nerve to come from this plexus is the sciatic n., which is a combination of the tibial n. and the fibular n.. The sciatic nerve extends across the hip joint and is most commonly associated with the condition sciatica, which is the result of compression or irritation of the nerve or any of the spinal nerves giving rise to it. This plexus is responsible for functions like plantar flexion and sensation from the sacroiliac joint. * *Commonly the term ‘lumbosacral’ is used for the lumbar, sacral, and coccygeal plexus (S4-Co). Dermatomes and Myotomes A dermatome is an area of skin that is primarily supplied by a single spinal nerve root (except C1): communicating sensation from this skin region to the brain. Somatosensation includes feedback from mechanoreceptors, thermoreceptors, proprioceptors, pain receptors, and chemoreceptors. The body may be divided into sensory segments of the skin that collectively make up a dermatome map. Such a map is oversimplified, however, because the dermatomes overlap at their edges by as much as 50%. Dermatomes form a stack of horizontal layers on the trunk and run lengthwise in the extremities. There are eight cervical nerves (although C1 does not have its own dermatome), 12 thoracic nerves, 5 lumbar nerves, and 5 sacral nerves. The dermatomes of the head are supplied by the branches of the trigeminal n.. Lesions of one or more nerve roots result in typical patterns of neurologic defects that allow localization of the lesion. A myotome is the group of muscles that a single spinal nerve root innervates. The myotome is the motor equivalent of a dermatome. The generalized myotome distribution of the upper and lower extremities is listed below, but it is not required for the final. Video (optional) Myotome distribution @Carlee Wheeler, 2017. Length: 1:59. * 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 dorsi-flexion * L5: Great toe extension * S1: Ankle plantar flexion/ankle eversion/hip extension * S2: Knee flexion * S3–S4: Anal reflex Real Life Knowledge of dermatomes can aid in the diagnosis of disease. * Herpes zoster (shingles). Chickenpox (varicella) is caused by the varicella-zoster virus. It produces an itchy rash that usually clears up without complications. The virus, however, remains for life in the dorsal root ganglia, kept in check by the immune system. If the immune system is compromised, however, the virus can travel along the sensory nerve fibers by fast axonal transport and cause shingles, producing a rash and pain along the corresponding dermatome. * Lumbar radiculopathy, a radiating back pain extending down a dermatome is due to compression of a spinal nerve root (sciatica). * Referred pain from a visceral organ can be perceived at a location other than the site of the stimulus or origin in a nearby dermatome. 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.

M15.3 Spinal Nerve Plexuses
Major Nerve Plexuses
Ventral rami from adjacent (but different) spinal nerves will come together to form a network or mingling (knot, tangle, net) of interconnecting nerves called a nerve plexus. In this instance, the word plexus is used to describe networks of nerve fibers with no associated cell bodies. As the axons mingle, they reorganize and create peripheral nerves that follow different courses. Peripheral (named) nerves emerging from a plexus contain fibers from various spinal nerves, which are now carried together to innervate various body structures. The plexuses contain both sensory and motor fibers (they are mixed). This means that:
* Some fibers extend from receptors and send action potentials into the CNS. Those are axons of sensory neurons in the dorsal root ganglia that enter the spinal cord through the dorsal nerve root.
* Some fibers are the axons of motor neurons of the anterior horn of the spinal cord, which emerge in the ventral nerve root and send action potentials to cause skeletal muscles to contract in their target regions.
* For example, the radial n. contains fibers of cutaneous sensation in the arm, as well as motor fibers that move muscles in the arm. 
The mingling of spinal nerves happens at four major places along the length of the vertebral column. Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level. Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the intercostal nerves found between the ribs, which articulate with the vertebrae surrounding the spinal nerve.
Video
	Image
	Intercostal nerves
	@Diana Botnaru, 2020. Length: 18:07. 
	
	
	Description of plexuses
* The cervical plexus is composed of axons from spinal nerves C1-C5 and branches into nerves in the posterior neck and head, as well as the phrenic n., which connects to the diaphragm at the base of the thoracic cavity. This plexus is responsible for sensation from the head and neck and motor control of respiration.
* The brachial plexus is composed of axons from spinal nerves C5-T1 and give rise to the nerves of the arms. A large nerve from this plexus is the radial n. from which the axillary n. branches to go to the armpit region. The radial nerve continues through the arm and is paralleled by the ulnar n. and the median n. This plexus is responsible for functions like flexion of the elbow and abduction of shoulder, as well as sensation from the shoulder and upper limb.  
* The lumbar plexus arises from all the lumbar spinal nerves, L1-L5, and gives rise to nerves innervating the pelvic region and the anterior leg. The femoral n. is one of the major nerves from this plexus, which gives rise to the saphenous n. as a branch that extends through the anterior lower leg. The other big nerve of this plexus is the obturator n. This plexus is responsible for functions like knee extension and sensation from teh lower abdomen. 
* The sacral plexus arises from the lower lumbar nerves L4-L5, sacral nerves S1-S4 and coccygeal(Co). The most significant systemic nerve to come from this plexus is the sciatic n., which is a combination of the tibial n. and the fibular n.. The sciatic nerve extends across the hip joint and is most commonly associated with the condition sciatica, which is the result of compression or irritation of the nerve or any of the spinal nerves giving rise to it. This plexus is responsible for functions like plantar flexion and sensation from the sacroiliac joint. 
   * *Commonly the term ‘lumbosacral’ is used for the lumbar, sacral, and coccygeal plexus (S4-Co).
Dermatomes and Myotomes
A dermatome is an area of skin that is primarily supplied by a single spinal nerve root (except C1): communicating sensation from this skin region to the brain. Somatosensation includes feedback from mechanoreceptors, thermoreceptors, proprioceptors, pain receptors, and chemoreceptors. The body may be divided into sensory segments of the skin that collectively make up a dermatome map. Such a map is oversimplified, however, because the dermatomes overlap at their edges by as much as 50%. Dermatomes form a stack of horizontal layers on the trunk and run lengthwise in the extremities. There are eight cervical nerves (although C1 does not have its own dermatome), 12 thoracic nerves, 5 lumbar nerves, and 5 sacral nerves. The dermatomes of the head are supplied by the branches of the trigeminal n.. Lesions of one or more nerve roots result in typical patterns of neurologic defects that allow localization of the lesion.
A myotome is the group of muscles that a single spinal nerve root innervates. The myotome is the motor equivalent of a dermatome. The generalized myotome distribution of the upper and lower extremities is listed below, but it is not required for the final. 
Video (optional)
	Myotome distribution 
	@Carlee Wheeler, 2017. Length: 1:59. 
	* 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 dorsi-flexion
* L5: Great toe extension
* S1: Ankle plantar flexion/ankle eversion/hip extension
* S2: Knee flexion
* S3–S4: Anal reflex
	Real Life 
Knowledge of dermatomes can aid in the diagnosis of disease.
* Herpes zoster (shingles). Chickenpox (varicella) is caused by the varicella-zoster virus. It produces an itchy rash that usually clears up without complications. The virus, however, remains for life in the dorsal root ganglia, kept in check by the immune system. If the immune system is compromised, however, the virus can travel along the sensory nerve fibers by fast axonal transport and cause shingles, producing a rash and pain along the corresponding dermatome.
* Lumbar radiculopathy, a radiating back pain extending down a dermatome is due to compression of a spinal nerve root (sciatica).
* Referred pain from a visceral organ can be perceived at a location other than the site of the stimulus or origin in a nearby dermatome.
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.

Transcript for:Understanding Spinal Nerve Plexuses

Transcript for:
Understanding Spinal Nerve Plexuses