Neuro 101: Peripheral Nervous System • LITFL • Neurology library

The peripheral nervous system (PNS) connects the central nervous system (CNS) to the limbs and organs, enabling voluntary movement, sensation, and autonomic function. Disorders of the PNS can result in characteristic patterns of weakness, sensory loss, and reflex changes that assist with lesion localisation.

In this section, we will cover the anatomy of the peripheral nervous system, localisation of nerve injuries, pathology affecting peripheral nerves, the brachial and lumbosacral plexuses, peripheral nerve examination, common peripheral neuropathies, and diseases affecting the neuromuscular junction.

Function

The peripheral nervous system transmits motor, sensory, and autonomic signals between the CNS and the body. Peripheral nerves are made up of a mix of sensory and motor fibers, which are derived from several spinal roots via the plexuses.

Conducts efferent (motor) output from the CNS to muscles and glands
Conducts afferent (sensory) input from the periphery to the CNS
Mediates reflexes in conjunction with spinal circuits

Pathology of nerves can involve the destruction of axons, or the deterioration of the myelin coatings.

Pathology

Anatomy

Connection to the CNS occurs at the nerve root level
Nerve roots form plexuses where motor and sensory fibres mix before forming peripheral nerves
Peripheral nerves consist of mixed motor and sensory fibres derived from several spinal roots
Axons may be myelinated or unmyelinated; pathology may affect axons or myelin

Motor Unit

A motor unit consists of one anterior horn cell and all the muscle fibres it innervates

Neuromuscular Junction

The connection a motor horn cell axon has with each muscle fibril is called the neuromuscular junction.
This is a chemical synapse between the axon terminus and the muscle fiber. Acetylcholine is the neurotransmitter released from the axon terminal. Acetylcholine traverses the synaptic space, binding to specific receptors, which activates the muscle, resulting in contraction.

Testing

Clinical examination of the PNS requires careful assessment of motor, sensory, and reflex function to aid localisation.

Identify patterns of weakness and sensory loss
Assess for dermatomal or peripheral nerve distribution patterns
Evaluate reflexes for asymmetry or loss
Observe for signs of muscle atrophy, fasciculations, and trophic changes
Distinguish between nerve root, plexus, or peripheral nerve involvement

Localising a Peripheral Nerve Injury

Signs of lower motor neuron injury:

Atrophy
Fasciculations
Weakness and fatigue
Reflex changes

Sensory findings:

Anaesthesia
Hypoaesthesia
Hyperaesthesia

Axonal damage:

Distal > proximal impairment (glove and stocking pattern)
Atrophy in advanced cases (e.g. diabetic neuropathy)

Demyelination:

Loss of stretch reflexes
Weakness
Sensory changes

Pathology Affecting Peripheral Nerves

Motor neuron disease

Degeneration of anterior horn cells
Varying degrees of upper motor neuron loss
No sensory loss
Amyotrophic lateral sclerosis (ALS)
- upper and lower motor neuron involvement;
- Babinski response; hyperreflexia
- atrophy; fasciculations

Nerve root lesions

Herniated disc → radiculopathy
C5–C7 and L4–S1 most commonly affected
Sensory and/or motor symptoms
Cauda equina syndrome → bilateral symptoms

Dorsal root lesions

Shingles (herpes zoster)
Tabes dorsalis (syphilis)

The Brachial Plexus

Innervates upper limb motor and sensory function.

Upper plexus injuries

Mechanism: traction injury (e.g. difficult birth)
Erb’s palsy: medial rotation of arm, forearm pronation, wrist/finger flexion, winged scapula

Lower plexus injuries

Mechanism: compression (e.g. Pancoast tumour)
Klumpke palsy: claw hand, forearm supination, wrist/finger flexion
Horner syndrome: miosis, anhidrosis, ptosis

Other causes

Diabetic amyotrophy
Brachial plexitis
Neoplasia
Post-irradiation injury
Obstetric palsy
Postoperative plexopathy
Thoracic outlet syndrome

The Lumbosacral Plexus

Two plexuses (lumbar and sacral) innervate lower limb motor and sensory function.

Clinically important nerves

Femoral
Sciatic (tibial, peroneal)

Common causes of injury

Diabetes
Neoplasia
Retroperitoneal haemorrhage
Post-irradiation plexopathy

Peripheral Examination

Examination of peripheral nervous system issues requires pattern recognition. One needs to examine and assess the appearance of the muscles, muscle power, peripheral sensation, and muscle reflexes to get a good localization of peripheral nervous system impairments.

Upper extremity motor examination

The strongest muscles in the upper arms are the flexors. The most likely muscles to show subtle weakness are the extensors. Assess pronator drift to look for lateralized muscle weakness. Observe for atrophy and fasciculations.

C5: shoulder abduction, elbow flexion → biceps reflex
C6: elbow flexion (semi-pronated) → brachioradialis reflex
C7: elbow extension, finger extension → triceps reflex
C8: finger flexion → finger flexor reflex
T1: intrinsic hand muscles → finger abduction test

Important motor nerves

Axillary: innervates deltoid muscle (movements of the shoulder)
Musculocutaneous: innervates muscles important for flexion of arm at the elbow
Radial: innervates all extensor muscles of upper extremity
Ulnar: innervates all but four intrinsic hand muscles
Median: innervates most flexor and pronator muscles of forearm and several intrinsic hand muscles

Upper extremity sensory examination

Musculocutaneous
Median
Ulnar
Radial

Lower extremity motor examination

The strongest muscles in the lower extremities are the extensor muscles. Weakness is more easily shown in the flexor groups. Observe for atrophy, fasciculations, and trophic changes such as changes in hair patterns, thinning of skin, and changes in arches.

In the leg, as in the arm, all muscles are innervated by more than one nerve root so that recognition of patterns of weakness become most important in peripheral neurological assessment. This can be accomplished best by reviewing general motor control and reflexes by nerve root.

L1–L2: hip flexion → test patient laying down—have them flex hip and hold against resistance
L3–L4: knee extension → knee jerk reflex
L5: foot dorsiflexion, inversion, eversion, big toe extension
S1: hip extension, knee flexion, foot plantarflexion → ankle reflex

Important motor nerves

Femoral: supplies muscles associated with knee extension
Sciatic: supplies knee flexion; separates into tibial and common peroneal branches
Tibial: supplies foot plantar flexion and inversion as well as small muscles of foot
Common peroneal: supplies dorsiflexion of foot and eversion of ankle

Lower extremity sensory examination

Dermatomes: L4, L5, S1 commonly affected by radiculopathy
Nerves: anterior femoral cutaneous, medial femoral cutaneous, saphenous, sciatic branches

Common Peripheral Neuropathies

Carpal tunnel syndrome

Median nerve compression at wrist
Nocturnal paraesthesia, positive Tinel test and Phalen tests (elicits paresthesia)

Pronator teres syndrome

Median nerve compression passing through pronator teres muscle
Pain in forearm and hand; less pain at night
Daytime symptoms, pain with pronation against resistance
Weakness of wrist pronators and distal finger flexors; Difficulty pinching
Positive Tinel; Negative Phalen

Ulnar neuropathy

Compression at medial epicondyle or Guyon’s canal
Tingling in medial hand, weakness of intrinsic hand muscles
Difficulty with fine motor control of hand (due to weakness of intrinsic muscles)

Saturday night palsy

Compression of radial nerve in spiral groove
Radial nerve compression → wrist drop
Weak finger extension at metacarpophalangeal joints

Posterior interosseous syndrome

Compression of posterior interosseous nerve (branch of radial) between heads of supinator muscle
Radial nerve branch compression
Weak finger extension, radial deviation of wrist

Peroneal neuropathy

Compression of common peroneal nerve as it winds around fibular head
Foot drop, weakness of dorsiflexion and eversion

L5 radiculopathy

Root compression → lancinating leg pain, weakness of inversion and big toe extension

Meralgia paraesthetica

Compression of the lateral femoral cutaneous nerve beneath inguinal ligament
Anterolateral thigh pain and paresthesia

Diseases Affecting the Neuromuscular Junction

The neuromuscular junction is a specialized synapse allowing the conversion of nerve electrical action potentials to chemical impulses, which transmit the message for a muscle to contract.

There are multiple clinical syndromes affecting the neuromuscular junction, with the most common being myasthenia gravis.

Myasthenia gravis

Autoimmune acetylcholine receptor disorder
Generally occurs in younger women or in older men
Fluctuating weakness, facial and limb involvement, ptosis, diplopia, dysphagia, respiratory fatigue

Testing

Repetitive movement testing
Respiratory fatigue testing
Extended upgaze for ptosis
Ice pack test; improvement of ptosis suggests neuromuscular junction problem (not stroke or nerve injury)

This is an edited excerpt from the Medmastery course Clinical Neurology Essentials by Robert Coni, DO, EdS, FAAN. Acknowledgement and attribution to Medmastery for providing course transcripts.

Coni R. Headache Masterclass. Medmastery
Simmonds GR. Neurology Masterclass: Managing Common Diseases. Medmastery
Simmonds GR. Neurology Masterclass: Managing Emergencies. Medmastery

Clinical Neurology Essentials

Coni R. Neuro 101: Neurological Examination. LITFL
Coni R. Neuro 101: Cerebral Hemispheres. LITFL
Coni R. Neuro 101: Cerebellum and Basal Ganglia. LITFL
Coni R. Neuro 101: Brainstem. LITFL
Coni R. Neuro 101: Cranial Nerves. LITFL
Coni R. Neuro 101: Spinal Cord. LITFL
Coni R. Neuro 101: Peripheral Nervous System. LITFL

References

Robert Coni

Robert Coni, DO, EdS, FAAN. Vascular neurologist and neurohospitalist and Neurology Subspecialty Coordinator at the Grand Strand Medical Center in South Carolina. Former neuroscience curriculum coordinator at St. Luke’s / Temple Medical School and fellow of the American Academy of Neurology. In my spare time, I like to play guitar and go fly fishing. | Medmastery | Linkedin |