Neuro 101: Peripheral Nervous System

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.
Peripheral nerve anatomy
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.

The brachial plexus

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’s palsy: claw hand, forearm supination, wrist/finger flexion
  • Horner’s 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

Upper extremity motor examination

  • 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
  • Musculocutaneous
  • Radial
  • Ulnar
  • Median

Upper extremity sensory examination

  • Musculocutaneous
  • Median
  • Ulnar
  • Radial

Lower extremity motor examination

  • L1–L2: hip flexion
  • 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
  • Sciatic
  • Tibial
  • Common peroneal

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 paresthesia, positive Tinel’s and Phalen’s tests

Pronator teres syndrome

  • Median nerve compression in forearm
  • Daytime symptoms, pain with pronation against resistance

Ulnar neuropathy

  • Compression at medial epicondyle or Guyon’s canal
  • Tingling in medial hand, weakness of intrinsic hand muscles

Saturday night palsy

  • Radial nerve compression → wrist drop

Posterior interosseous syndrome

  • Radial nerve branch compression
  • Weak finger extension, radial deviation of wrist

Peroneal neuropathy

  • Compression at fibular head
  • Foot drop, weakness of dorsiflexion and eversion

L5 radiculopathy

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

Meralgia paresthetica

  • Lateral femoral cutaneous nerve compression
  • Anterolateral thigh pain and paresthesia
Diseases Affecting the Neuromuscular Junction

Myasthenia gravis

  • Autoimmune acetylcholine receptor disorder
  • 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

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.

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References

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LITFL author Robert Coni DO EdS

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 |

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