The Peripheral Nervous System, Neuromuscular Junction, and Muscle

Neurological problems frequently result from lesions that spare the central nervous system, but involve nerve roots, plexuses, peripheral nerves, neuromuscular junction or muscle. We cannot hope to review the anatomy of all the nerves and muscles in the body, but we will summarize the deficits resulting from pathology in various peripheral sites, and will list a few nerves that you should be familiar with.

Roots, plexuses and nerves:

Motor deficits

These are the features of lower motor neuron dysfunction, and include weakness, atrophy, and, at times, fasciculations (spontaneous firing of one nerve fiber, resulting in contraction of the muscles fibers that it innervates). The distribution depends upon the nerve(s) affected.

Sensory deficits

These may reflect damage to large sensory fibers (loss of vibration and position) or to small fibers (loss of pain and temperature) or both.

Autonomic deficits

Changes in skin temperature and sweating often accompany peripheral nerve lesions, but are difficult to assess reliably.

Tendon reflexes

mediated by affected nerves are diminished or absent, unless the lesion spares large sensory fibers that carry the afferent impulse from muscle spindles to the spinal cord.

Neuromuscular junction

Diseases restricted to the neuromuscular junction obviously produce no sensory symptoms. The distribution of weakness depends upon the condition: myasthenia gravis has a predilection for eyelid, extraocular, bulbar and proximal limb muscles, and thus presents most commonly with ptosis, diplopia, dysarthria, dysphagia, and proximal limb weakness.

Muscle

For reasons that are not clear, most primary diseases of muscle (myopathies) present with weakness that is greater proximally than distally. Myopathy should be strongly considered in patients with proximal weakness, normal sensation, and normal or depressed reflexes (severe muscle weakness or damage to the muscle spindle may depress reflexes in myopathy).

A more detailed consideration of the distribution of findings in disorders affecting roots, plexus and peripheral nerves

The diagnosis of disorders affecting the peripheral nervous system affecting the peripheral nervous system begins with localization. Although it is not possible to teach you a single set of algorithms that cover every condition, the following are basic and clinically very useful distinctions:

Median nerve

Focal neuropathies (mononeuropathy, multiple mononeuropathy)

These are isolated disturbances of single, named, peripheral nerves, such as the median or peroneal nerves. Pain can be a prominent complaint.

Mononeuropathies

Involvement of a single nerve. Trauma and nerve entrapments are the usual causes. You should be familiar with the following commonly affected nerves:

Median nerve

Often compressed as it runs through the carpal tunnel, the median nerve supplies sensation to the thumb, index, middle and half of the ring finger, and the lateral aspect of the palm. Atrophy and weakness is most obvious in the thenar muscles (abductor and opponens pollicis muscles). Carpal tunnel syndrome usually presents with pain associated with wrist flexion or extension (which narrow the carpal tunnel), and intermittent numbness. The pain may be referred to the forearm, arm, shoulder or even neck; the numbness is in the distribution of the median nerve.

Ulnar nerve

Often damaged as it wraps around the elbow, the ulnar nerve supplies sensation to the little and half of the ring finger, and the adjacent portions of the hand. Atrophy and weakness affect the interossei (most visible in the first dorsal interosseous muscle), and thumb adductor. Finger spreading and thumb adduction are weak.

Radial nerve

Usually compressed in the arm as it winds around the humerus, the radial nerve supplies sensation to the portions of the hand not innervated by median or ulnar nerves, but motor deficits predominate. There is wrist drop with weakness of finger and wrist extensors. The brachioradialis muscle is also affected. Usually the nerve is compressed after it has given off its branches to the triceps, so this muscle (and the triceps reflex) is spared.

Peroneal nerve

The peroneal nerve is often damaged as it runs behind the head of the fibula just below the knee. Deficits include a foot drop with weakness of ankle eversion and dorsiflexion, and sensory loss along the anterolateral aspect of the leg and the dorsum of the foot.

Lateral femoral cutaneous nerve

This purely sensory nerve supplies sensation to the anterolateral aspect of the thigh. Patients complain of tingling or burning sensation in this region. The syndrome is called meralgia paresthetica. It is thought to result from entrapment or compression of the nerve as it crosses the inguinal ligament.

Multifocal neuropathy

(AKA multiple mononeuropathy, or mononeuropathy multiplex): Successive involvement of individual nerves. When the onset of symptoms in each nerve is acute and associated with pain, suspect a vasculitis (inflammation affecting the vasa nervorum).

Polyneuropathy

Polyneuropathy is the commonest generalized disorder affecting the peripheral nervous system. In polyneuropathy, usually the longest nerves are affected most severely. The initial symptoms are therefore in the feet (tingling, burning, numbness), and the earliest signs are atrophy of intrinsic foot muscles, a graded stocking distribution of sensory loss, and decreased or absent ankle stretch reflexes. As the neuropathy becomes more severe, the sensory loss ascends the legs, and then begins in the fingers. Polyneuropathies are most often caused by drugs, toxins, and metabolic diseases.

Polyradiculoneuropathies

This term is used to describe processes that can affect many portions of many nerves, resulting in both proximal and distal motor and/or sensory changes, usually with diffusely hypoactive or absent tendon reflexes. Acute inflammatory demyelinating polyradiculoneuropathy (AIDP, commonly known as the Guillain-Barr syndrome), an autoimmune process, is the commonest example of such a neuropathy.

Plexopathy

This localization should be suspected when there are motor and reflex deficits restricted to one limb, but involving more than the territory of a single nerve or nerve root. Pain is common. Trauma, tumors, and autoimmune neuropathies may affect the brachial or lumbosacral plexus. Diabetes is a common cause of lumbosacral plexopathy.

Radiculopathy

Radiculopathy is commonly associated with pain radiating from the neck or back to the limb. Motor, sensory and reflex deficits may be absent or slight. When present, they should be confined to the distribution of a single nerve root. Herniated disks, spondylosis, and Herpes zoster (shingles) are common causes of radiculopathy. Tumors must be considered; Lyme disease may present with polyradiculopathy. Disks, spondylosis, and tumors may also compress the spinal cord, producing combined radiculopathy and myelopathy (spinal cord dysfunction).

A note about diabetes

A note about diabetes: This is far from neuroanatomy, but this is a convenient place to mention that diabetes can cause almost any kind of neuropathy. Polyneuropathy, plexopathy, radiculopathy, mononeuropathy and multifocal neuropathy have all been ascribed to diabetes.

This creates an interesting finding on the reflex examination: when you tap the biceps tendon, the elbow extends! This occurs because (1)šthe biceps reflex is reduced, and (2)šthe triceps reflex is so brisk that the movement induced by tapping the biceps tendon triggers a triceps reflex. This has been called Ainversion of the biceps reflex. In addition, the finger flexor reflex can also be triggered.

It is now known that there are multiple representations of the homunculus in primary motor and sensory cortices, and there are multiple visual association areas, ecah subserving a different function (e.g., perception of color, form, or motion). This suggests that there is parallel processing of motor and sensory information. We know that spatial localization of visual stimuli is mediated by projections from occipital to parietal cortex, whereas the identification of objects by vision is mediated by projections from occipital to inferior temporal cortex. Bilateral parietal lesions can impair the patient=s ability to understand the spatial relations of objects, each of which can be identified; whereas bilateral inferotemporal lesions can impair object identification, without impairing spatial localization.