26. Peripheral Nervous System Framework

What are the six key localization categories in the PNS framework?

Motor neuron, nerve root, plexus, peripheral nerve, neuromuscular junction, muscle

What is the clinical value of this localization framework?

Helps narrow the differential diagnosis using history and physical exam findings

Where is the lower motor neuron cell body located?

In the anterior horn of the spinal cord

What is another name for a lower motor neuron cell body?

Anterior horn cell

What symptoms suggest anterior horn cell pathology?

Weakness without numbness, lower motor neuron signs

What are two classic LMN signs?

Atrophy and fasciculations

What causes fasciculations?

Spontaneous firing of dying motor neurons

Can motor neuron disorders involve UMN signs too?

Yes — some cause both UMN and LMN findings

What causes hyperreflexia in UMN disorders?

Loss of inhibition from corticospinal tract

What is spasticity?

Velocity-dependent increase in muscle tone (same mechanism as hyperreflexia)

What is ALS?

A neurodegenerative disease causing both UMN and LMN signs due to protein aggregation

What is poliomyelitis?

An infectious anterior horn cell disorder (myelitis = spinal cord inflammation)

What is Kennedy disease?

A genetic disorder affecting motor neurons

What is spinal muscular atrophy (SMA)?

A genetic disorder causing degeneration of anterior horn cells

Where is the nerve root located anatomically?

Between the spinal cord and the point where spinal roots exit the vertebral column

What is the dorsal root ganglion?

A bundle of sensory neuron cell bodies that marks the boundary between nerve root and plexus

What is the function of the ventral root?

Carries motor output from the spinal cord

What is the function of the dorsal root?

Carries sensory input into the spinal cord

Does nerve root damage affect motor, sensory, or both?

Both

What type of weakness occurs in radiculopathy?

Myotomal weakness (limited to muscles innervated by the affected root)

What type of sensory loss occurs in radiculopathy?

Dermatomal numbness (limited to skin innervated by the affected root)

What happens to reflexes in radiculopathy?

Reflexes are decreased at affected levels

What are two hallmark signs of radiculopathy?

Myotomal weakness and dermatomal sensory loss

What is a common traumatic cause of radiculopathy?

Degenerative disc disease

What is a common infectious cause of radiculopathy?

Shingles (herpes zoster reactivation)

Where is the plexus located anatomically?

Between the nerve roots and the formation of final peripheral nerves

How many plexi are in the human body?

Four — one per limb (2 brachial, 2 lumbosacral)

What nerve roots form the brachial plexus?

C5 to T1

What nerve roots form the lumbosacral plexus?

L2 to S1

What nerves come from the lateral cord of the brachial plexus?

Musculocutaneous nerve and part of the median nerve

What nerves come from the posterior cord?

Axillary and radial nerves

What nerves come from the medial cord?

Ulnar nerve and part of the median nerve

What pattern of weakness and numbness occurs in plexopathy?

Involves multiple nerve root and peripheral nerve distributions — not limited to a single dermatome or myotome

What muscles are affected in upper plexus injuries?

Proximal muscles (e.g., shoulder, hip)

What muscles are affected in lower plexus injuries?

Distal muscles (e.g., hand, foot)

What happens to reflexes in plexopathy?

Reflexes are decreased at affected levels

What is brachial neuritis (Parsonage-Turner syndrome)?

An autoinflammatory cause of plexopathy

What is Erb’s palsy?

A congenital traumatic plexopathy

Where does the peripheral nerve begin and end?

From after the plexus finishes organizing into nerves → until nerves reach skin or muscle

What types of fibers are in a peripheral nerve?

Descending motor axons and ascending sensory axons

What cells myelinate most peripheral nerves?

Schwann cells

Which types of axons can be affected in neuropathy?

Motor, sensory, or both

Which nerve structures can be damaged in neuropathy?

Axons (axonal neuropathy), myelin (demyelinating neuropathy), or both

What is mononeuropathy?

Damage to a single peripheral nerve

What is polyneuropathy?

Damage to multiple peripheral nerves

What pattern of weakness occurs in neuropathy?

Weakness in the distribution of the affected peripheral nerve(s)

What pattern of sensory loss occurs in neuropathy?

Numbness in the distribution of the affected peripheral nerve(s)

What happens to reflexes in neuropathy?

Reflexes are decreased in affected distributions

What is a common metabolic cause of polyneuropathy?

Diabetic polyneuropathy

What is a common autoinflammatory cause of neuropathy?

Guillain-Barré syndrome

What is a common compressive cause of mononeuropathy?

Carpal tunnel syndrome

Where is the neuromuscular junction located?

At the synapse between a lower motor neuron axon terminal and the muscle end plate

What happens at the presynaptic membrane during normal function?

Action potential arrives → calcium channels open → acetylcholine is released

What happens at the postsynaptic membrane during normal function?

Acetylcholine binds receptors → sodium enters → muscle contracts

What is the effect of blocking or damaging NMJ signaling?

Motor neuron fires, but the signal does not reach the muscle → weakness

Does NMJ dysfunction cause numbness?

No — it causes weakness only

What is a hallmark feature of NMJ weakness?

Fluctuates with activity (fatigable or paradoxical improvement with exertion)

What cranial nerve symptoms may occur in NMJ disorders?

Diplopia, dysphagia, and dysarthria (due to CN-involved muscles)

What is the pathophysiology of myasthenia gravis?

Antibodies against postsynaptic acetylcholine receptors (autoimmune/paraneoplastic)

What is the pathophysiology of Lambert-Eaton myasthenic syndrome?

Antibodies against presynaptic calcium channels (autoimmune/paraneoplastic)

Where does the lesion occur in myopathy?

In the skeletal muscle cells (fibers) themselves

What happens in myopathy when the neuron signals the muscle?

The signal is received, but the muscle cannot contract properly

What is the primary symptom of myopathy?

Weakness without numbness

What is the typical distribution of weakness in myopathy?

Proximal and symmetric (e.g., shoulders > hands, hips > feet)

What is dermatomyositis?

An autoinflammatory muscle disorder

What is statin-induced myopathy?

A toxic muscle disorder caused by medication

What is Duchenne’s muscular dystrophy?

A genetic muscle disorder

What is the pattern of weakness in radiculopathy?

Myotomal (muscle group innervated by one nerve root)

What is the pattern of numbness in radiculopathy?

Dermatomal (skin area innervated by one nerve root)

Give an example of radiculopathy findings at C5–6

Shoulder abduction and elbow flexion weakness + numbness in shoulder/lateral arm/thumb

What pattern does plexopathy follow?

Mixed distribution involving multiple nerve roots or nerves

Why doesn't plexopathy follow a single dermatome or myotome?

Because nerve roots reorganize into multiple nerves within the plexus

What is the weakness pattern in neuropathy?

Follows a specific peripheral nerve distribution

What is the sensory loss pattern in neuropathy?

Matches the sensory territory of the affected peripheral nerve

Give an example of ulnar neuropathy findings

Hand weakness with fine motor tasks and numbness in digits 4–5