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Peripheral Nervous System (PNS)
by Annie Burke-Doe, PT, MPT, PhD
Practicing physical therapist and associate professor at the University of St. Augustine for Health Sciences in San Diego, California

Slide 1: Peripheral Nervous System (PNS)

Hello. Welcome to neuroanatomy and physical therapy. I'm Dr. Annie Burke-Doe, a practicing physical therapist and an associate professor at the University of St. Augustine for Health Science in San Diego, California.

This lecture series has been developed for physical therapists embarking on the study of neurology. Neuroanatomy is the study of the anatomical organization of the brain, and it is also considered a branch of neuroscience, which deals with the study of gross structure of the brain and the nervous system. The peripheral nervous system (PNS) is composed of specialized clusters of neurons and peripheral nerves. The peripheral nervous system relays information to the central nervous system and executes motor commands generated in the brain and spinal cord. We will begin with the gross anatomy and discuss clinical disorders of nerves, nerve roots, and nerve plexus.

Slide 2: Spinal Cord

In our previous topics, we discussed the main motor and sensory pathways, and we will now follow these pathways to the peripheral nervous system. Remember that along the length of the spinal cord, there is a variation in size and shape. The white matter, which is made up of longitudinal tracts, pictured here, is thickest in the cervical levels, where most ascending fibers have already entered the cord and most descending fibers have not yet terminated on their targets, while the sacral cord is mostly gray matter.

The amount of gray matter is greatest in the segments of the spinal cord dedicated to the sensory and motor control of the extremities. These segments are expanded, forming enlargements of the spinal cord. The cervical enlargement supplies nerves to the shoulder and upper limbs, while the lumbosacral enlargement provides enervation to the structures of the pelvis and lower limbs. Inferior to the lumbar enlargement, the spinal cord becomes tapered and conical. This region is the conus medullaris. The filum terminale is a slender strand of fibrous tissue that extends from the inferior tip of the conus medullaris. It continues along the length of the vertebral canal as far as the second sacral vertebra, where it provides support to the spinal cord as a component of the coccygeal ligament.

Slide 3: Regions of the Spinal Cord

When looking at the regions of the spinal cord here on slide 3, there are 31 segments, each named according to the level they exit the bony vertebral bodies. In the cervical region, the first pair of spinal nerves, C1, passes between the skull and the first cervical vertebra. Each cervical nerve takes its name from the vertebra immediately inferior to it. So, in other words, cervical nerve C2 precedes vertebra C2 and the same system is used for the rest of the cervical series. In the thoracic region, a transition for the numbering system occurs between the last cervical vertebra and the first thoracic vertebra. The spinal nerve at this location is C8. Each spinal nerve inferior to the first thoracic vertebra takes its name from the vertebra immediately superior to it. The spinal nerve T1 emerges immediately inferior to vertebra T1, spinal nerve T2 follows vertebra T2, and so forth.

Note that the cord itself is not as long as the vertebra column because during development the bony vertebral canal increases in length faster than the spinal cord. The spinal cord continues to enlarge and elongate until an individual is approximately 4 years old. Up to that time, enlargement of the spinal cord keeps pace with growth of the vertebral column. Throughout this period, the ventral and dorsal roots are very short, and they enter the intervertebral foramina immediately adjacent to their spinal segments. After age 4, the vertebral column continues to elongate, but the spinal cord does not. This vertebral growth moves the intervertebral foramina, and thus the spinal nerves, farther and farther from their original position relative to the spinal cord. Because the adult spinal cord extends only to the level of the first or second lumbar vertebra, the dorsal and ventral roots of spinal segments L2 through S5 extend inferiorly, past the inferior tip of the conus medullaris. When seen in gross dissection, the filum terminale and the long ventral and dorsal roots resemble a horse's tail and are called the cauda equina.

Slide 4: Spinal Cord and Peripheral Nervous System

Every spinal segment is associated with a pair of dorsal root ganglia, situated near the spinal cord. These ganglia, pictured here, contain the cell bodies of sensory neurons. The axons of the neurons form the dorsal roots, which bring sensory information into the spinal cord. There is also a pair of ventral roots containing the axons of motor neurons that extend into the periphery to control somatic and visceral effectors. On both sides, the dorsal and the ventral roots of each segment pass between the vertebral canal and the periphery at the intervertebral foramen between successive vertebrae. Distal to the dorsal root ganglion, the sensory and motor roots are bound together into a single spinal nerve. Spinal nerves are classified as mixed nerves; that is, they contain both afferent or sensory information and efferent or motor fibers.

Slide 5: Vertebral Bones

You can see here, on slide 5, the vertebral bones function as a central mechanical support for the body and as protection for the spinal cord. Each bone has a cylindrical vertebral body located anteriorly. The bodies are separated from each other by an intervertebral disc consisting of a nucleus pulposus surrounded by a capsule called the annulus fibrosus. Posteriorly, the neural elements are surrounded by an arched bone formed by the pedicles, transverse process, laminae, and spinous process. The superior and inferior articular process, or facet joints, form additional points of mechanical contact between adjacent vertebrae. Stop here and identify each landmark.

Slide 6: Organization of Spinal Nerves

The spinal cord runs through the spinal canal and is surrounded by the spinal meninges. The spinal meninges consists of three layers: the pia, arachnoid, and dura mater. As the dura exits the skull at the foramen magnum, the inner layer continues, and the outer layer becomes indistinguishable from the periosteum. Unlike the cranium, there is a layer of epidural fat between the dura and the periosteum in the spinal canal, which is a useful landmark on MRI scans. Bacterial or viral infections can cause meningitis or inflammation of the meningial membranes. Meningitis is dangerous because it can disrupt the normal circulatory and cerebrospinal fluid supplies, damaging or killing neurons or neuroglia in affected areas. The nerve roots exit the spinal canal via the neural foramina.

Slide 7: Relationship of Cervical and Lumbar Nerve Roots to Intervertebral Discs

Pictured here once more, we can see the important relationship of the cervical and lumbar nerve roots to the intervertebral discs. We see that the cervical nerve roots exit above the corresponding vertebral bone, except for C8, which has no corresponding vertebral bone and exits between C7 and T1. Cervical nerve roots have a fairly horizontal course as they emerge from the dura or thecal sac near the intervertebral disc and exit through the intervertebral foramina. Cervical discs are usually constrained by the posterior longitudinal ligament, which is not pictured here, and will herniate laterally toward the nerve root rather than centrally toward the spinal cord. Thus, in the cervical cord, the nerve root involvement usually corresponds to the lower vertebral bone of the disc space.

Unlike cervical roots, lumbar and sacral roots must travel down several levels before they exit the spinal canal. In addition, the intervertebral foramina of the lumbosacral spine are such that the nerve roots exit some distance above the intervertebral discs. As they are about to exit, the nerve roots move into the lateral recesses of the spinal canal, and it is at this point that they are closest to the disc. Thus, posterolateral disc herniation in the lumbosacral spine usually impinges on nerve roots on their way to exit beneath the next lower vertebral bone, which corresponds to the number of the nerve root involved.

Slide 8: Herniation

Disc herniations are most common at the cervical and lumbosacral levels, which are pictured here. An understanding of the anatomy of the nerve roots and discs should make clear the following important rule: For both cervical and lumbosacral disc herniation, the nerve root involved usually corresponds to the lower of the adjacent two vertebrae. For example, a L5/S1 disc usually produces the S1 radiculopathy. The rule is different for cervical versus lumbosacral discs, as discussed earlier. A posterior lateral disc herniation in the lumbosacral spine usually impinges on the nerve roots on their way to exit beneath the next lower vertebral bone, which corresponds to the number of the nerve root involved. Far lateral lumbosacral disc herniation affects the nerve root exiting that level, and central lumbosacral disc herniation can cause cauda equina syndrome.

Slide 9: Dermatomes

The sensory region of the skin innervated by a nerve root is called a dermatome. Dermatome maps often vary from resource to resource. This variation is likely due to differences in both the methods of testing and individual patients being studied. Being familiar with locations of dermatomes will assist you as a clinician in determination of the correct neurologic level of involvement with injuries.

Sensation for the face is provided by the trigeminal nerve, while most of the remainder of the head is provided by C2. C5 is represented at the shoulder, C6 in the lateral arm and the first two digits, C7 in the middle digit, and C8 the fourth and fifth digits. The L4 representation extends over the anteromedial shin, L5 extends down the anterior lateral shin and dorsum of the foot to the big toe, and S1 is in the small toe, lateral foot, sole, and calf. S2, 3, and 4 innervate the perianal area in a saddle-like distribution.

Note that there is a considerable amount of overlap between adjacent dermatomes. So, lesions of a single nerve root ordinarily cause a decrease but not a complete loss of sensation in a given dermatome. There may be less overlap for smaller fibers, so pinprick is a more sensitive test for dermatomal sensory loss than touch.

Slide 10: Myotomes

Patients with frontal lobe dysfunction may have a particular difficulty changing from one action to the next when performing a repeated sequence of action, such as drawing a pattern. At times, the patient may perseverate or get stuck on one aspect of the task. The Luria manual sequencing task may be helpful in determining perseveration. Additional support for frontal lobe pathology comes from the presence of frontal release signs, such as the grasp reflex.

Slide 11: Myotomes

Slide 12: Myotomes

Slide 13: Myotomes

Slide 14: Myotomes

Slide 15: Disorders

A variety of disorders can affect the peripheral nervous system at multiple levels. Disorders of the peripheral nervous system can often be distinguished from central nervous system dysfunction by an anatomical pattern of sensory or motor deficits. In addition, the presence of lower motor neuron signs including atrophy, fasciculations, decreased tone, and hyperreflexia suggests peripheral nervous system dysfunction as do paresthesias in peripheral nerve distributions.

Slide 16: Disorders

Neuropathy is a general term meaning nerve disorder. The site of pathology can be in axons, myelin, or both and can affect large diameter fibers, small diameter fibers, or both. Usually, neuropathies affect both sensory and motor fibers in the nerve, although one or the other may be preferentially involved. Damage may be reversible or permanent. The location of the neuropathy can be focal, as in a mononeuropathy; multifocal in mononeuropathy multiplex; or generalized in polyneuropathy.

Remember, like neuropathies, motor neuron disorders can also cause lower motor neuron-type weakness, but motor neuron disorders do not cause sensory involvement.

Etiologies such as diabetic neuropathy due to diabetes, mechanical compression, traction, laceration, or entrapment of a peripheral nerve, or syndromes, such as acute inflammatory demyelinating polyneuropathy, can also lead to peripheral nerve damage. A variety of disorders can affect the peripheral nervous system at multiple levels. Disorders of the peripheral nervous system can often be distinguished from central nervous system dysfunction by an anatomical pattern of sensory or motor deficits. In addition, the presence of lower motor neuron signs including atrophy, fasciculations, decreased tone, and hyperreflexia suggest peripheral nervous system dysfunction, as do paresthesias in peripheral nervous distributions.

Slide 17: Disorders

Another disorder could be impaired neuromuscular transmission, which leads to motor weakness without sensory deficits. Examples of impaired neuromuscular junction disorders include myasthenia gravis, Lambert-Eaton myasthenic syndrome, and botulism poisoning.

Slide 18: Disorders

Myopathies, or muscle disorders, produce weakness that is typically more severe in proximal musculature than in distal musculature and without loss of sensation or reflexes. Myopathies can be found in thyroid disorders, polymyositis, and Duchenne muscular dystrophy.

Slide 19: Back Pain

Back pain is one of the most common reasons that people seek medical attention and can have numerous causes, as listed here on slide 19.

Slide 20: Radiculopathy

Back pain can also cause a neuropathy affecting spinal nerve roots, which is called radiculopathy and causes a sensory or motor dysfunction. Radiculopathy is often associated with burning, tingling pain that radiates or shoots down a limb in a dermatome of the effected nerve root. There may be a loss of reflexes and motor strength in a radicular distribution as well. Chronic radiculopathy can result in atrophy and fasciculations. Sensations may be diminished if a single dermatome is involved, but because of overlap from adjacent dermatomes, sensation is usually not absent.

Slide 21: Important Nerve Roots of Arm

Clinically, it is very important to be familiar with reflexes, motor and sensory functions associated with the upper extremity, specifically C5, C6, and C7. When examining patients, it is helpful to have memorized at least one muscle that gets its major innervation from each of these three nerve roots. Here pictured, you can see C5 is associated with shoulder abduction, C5/6 is associated with elbow flexion and the bicep tendon reflex, and C7 is associated with elbow extension and the triceps tendon reflex. It is also helpful to know that C8 radiculopathy accounts for about 6% of cervical radiculopathies and is caused by C7-T1 disc herniation. It's also associated with weakness of the intrinsic hand muscles and decreased sensation over the fourth and fifth digits of the medial forearm. About 20% of all cervical radiculopathies involve two or more cervical levels.

Slide 22: Important Nerve Roots of Leg

One must also be familiar with reflexes, motor and sensory functions associated with the lower extremities, specifically L4, L5, and S1. As pictured, L4 mediates leg extension at the knee and the patellar tendon reflex. L5 mediates dorsiflexion at the ankle and great toe extension, and S1 mediates plantar flexion at the ankle and the Achilles tendon reflex.

Slide 23: Case 1

The following clinical cases have been developed for your review. They contain subject matter that is clinically related and will reinforce lecture slide content. The questions for the case follow the introduction of the case slide, and the discussion for the case is in the slide notes. I recommend not looking for the answers in the discussion notes until you have attempted to answer the questions on your own, using the slide content. Good luck, and I will see you in the next topic.

Slide 24: Case 1: Questions

Slide 25: Case 2

Slide 26: Case 2: Questions

Slide 27: References