Localizing the Lower Motor Unit: Nerve, Root, Plexus, NMJ & Muscle

When a patient presents with weakness, the first decision is vertical: is the lesion in the upper motor neuron (corticospinal tract) or somewhere below it — in the lower motor unit? Once the spasticity, hyperreflexia, and Babinski sign of an upper motor neuron lesion are excluded, the remaining territory runs from the anterior horn cell out through its root, the plexus, the peripheral nerve, the neuromuscular junction (NMJ), and finally the muscle. These do not all behave alike. Classic lower-motor-neuron lesions (anterior horn cell, root, plexus, peripheral motor nerve) produce hypotonia, atrophy with fasciculations, and hyporeflexia in the affected distribution. NMJ disorders instead cause fatigable weakness with normal sensation and usually normal reflexes. Myopathies cause proximal weakness with reflexes preserved until late. Each level carries a distinctive fingerprint. The clinician who reads the pattern of motor loss, the presence or absence of sensory involvement, and the behavior of the reflexes can localize with remarkable precision before a single electrode or needle is placed. This page walks down the motor system, level by level.

Anterior Horn Cell (Motor Neuron)

The anterior horn cell is the cell body of the lower motor neuron. A pure anterior horn cell process is purely motor — there is no sensory loss, because the sensory neurons live in the dorsal root ganglion, untouched. The hallmarks are weakness, prominent atrophy, and fasciculations, often with cramps. The distribution can be focal, asymmetric, and patchy rather than fitting any single root or nerve.

• Amyotrophic lateral sclerosis (ALS): the great mimic — combines lower motor neuron signs (atrophy, fasciculations) with upper motor neuron signs (hyperreflexia, spasticity) in the same regions, a combination that should always raise the question of motor neuron disease.
• Spinal muscular atrophy (SMA): inherited (SMN1) degeneration of anterior horn cells, classically with proximal weakness and areflexia.
• Poliomyelitis (and West Nile virus): acute viral destruction of anterior horn cells producing asymmetric flaccid weakness.

Pearl: Fasciculations plus atrophy plus normal sensation is anterior horn cell until proven otherwise; add brisk reflexes in a wasted limb and you are looking hard at ALS.

Radiculopathy (Nerve Root)

A root lesion announces itself with pain in a radicular, dermatomal band that shoots down the limb, accompanied by dermatomal sensory loss, myotomal weakness, and a depressed reflex that localizes to the level. Because the lesion sits at the root — before nerves recombine in the plexus — the deficits respect a single segmental map.

• C5–C6: diminished biceps/brachioradialis reflex, shoulder abduction and elbow flexion weakness, sensory loss over the lateral arm and thumb.
• C7: diminished triceps reflex, elbow extension and wrist weakness, sensory loss over the middle finger.
• L4: diminished knee jerk, quadriceps weakness; L5: foot/great-toe dorsiflexion weakness (foot drop with preserved ankle jerk); S1: diminished ankle jerk, plantarflexion weakness, lateral-foot sensory loss.

Symptoms are characteristically worsened by maneuvers that raise intraspinal pressure — coughing, sneezing, Valsalva — and by root-tension signs (straight-leg raise, Spurling maneuver).

Plexopathy (Brachial or Lumbosacral)

The plexus is where roots are reshuffled into peripheral nerves, so a plexus lesion produces deficits that span multiple roots and multiple peripheral nerves in a single limb — a distribution that fits neither one root nor one nerve. Sensory loss, weakness, and reflex changes are present but cross segmental boundaries.

• Brachial plexopathy: trauma (stretch/avulsion), Parsonage-Turner (neuralgic amyotrophy, often post-viral or post-vaccination with severe pain then patchy weakness), and neoplastic infiltration (e.g., Pancoast tumor of the lower trunk → Horner syndrome).
• Lumbosacral plexopathy: diabetic amyotrophy (Bruns-Garland syndrome) with painful proximal leg weakness, retroperitoneal hematoma, or pelvic malignancy.

Pearl: A limb that is weak and numb in a pattern "too big for one nerve and too scattered for one root" is plexus until the electromyographer says otherwise.

Mononeuropathy (Single Peripheral Nerve)

A single peripheral nerve lesion confines its deficit to that nerve's motor and sensory territory — clean borders that map onto a known nerve.

• Median nerve (carpal tunnel syndrome): nocturnal paresthesias of the thumb, index, middle, and radial half of the ring finger; thenar weakness/atrophy in advanced cases.
• Ulnar nerve (cubital tunnel at the elbow): little-finger and medial-hand sensory loss, intrinsic hand wasting, "claw hand."
• Radial nerve (spiral groove, "Saturday night palsy"): wrist drop and finger drop with preserved triceps if the lesion is distal to its branch.
• Common peroneal (fibular) nerve (fibular head): foot drop with sensory loss over the dorsum of the foot, and a preserved ankle jerk. A preserved ankle jerk helps argue against an S1 radiculopathy or a more extensive sciatic lesion, but it does not reliably exclude L5 radiculopathy — the ankle jerk is primarily S1 and is usually preserved in isolated L5 radiculopathy. Distinguish peroneal neuropathy from L5 radiculopathy instead by testing foot inversion (weak in L5 radiculopathy, spared in peroneal palsy), the hip-abduction/gluteal pattern, the sensory distribution, and EMG.
• Mononeuritis multiplex: several individual named nerves affected sequentially or simultaneously — think vasculitis and diabetes.

Polyneuropathy

Polyneuropathy is symmetric and length-dependent: the longest axons fail first, so symptoms begin in the toes and feet and ascend, producing the classic "stocking-glove" distribution (the hands become involved only once the legs are affected to about mid-calf). The picture is distal > proximal, with distal sensory loss, distal weakness, and lost ankle reflexes early.

• Axonal (most common; e.g., diabetic, alcoholic, toxic): slow, length-dependent, with conduction studies showing reduced amplitudes.
• Demyelinating (e.g., Guillain-Barré syndrome acutely, CIDP chronically, Charcot-Marie-Tooth type 1): slowed conduction velocities, often more proximal involvement and earlier diffuse areflexia.

Pearl: Diabetes is the perennial example because it can cause both the symmetric length-dependent polyneuropathy and the asymmetric mononeuritis multiplex.

Neuromuscular Junction

The NMJ has no sensory component, so by definition sensation is normal and reflexes are typically normal. Its signature is fatigable, fluctuating weakness with a predilection for ocular (ptosis, diplopia) and bulbar (dysarthria, dysphagia) muscles.

• Myasthenia gravis (postsynaptic): antibodies to the acetylcholine receptor (or MuSK) block transmission; weakness is worse with use and at the end of the day, with fatigable ptosis and diplopia. Repetitive nerve stimulation shows a decremental response.
• Lambert-Eaton myasthenic syndrome (presynaptic): antibodies to voltage-gated calcium channels reduce acetylcholine release; often paraneoplastic (small-cell lung cancer). Weakness paradoxically improves briefly with sustained or repeated effort ("facilitation"), depressed reflexes that augment after exercise, and frequent autonomic features (dry mouth). High-frequency repetitive stimulation produces an incremental response.

Muscle (Myopathy)

A primary muscle disorder produces symmetric, proximal weakness — difficulty climbing stairs, rising from a chair, and reaching overhead — with normal sensation and reflexes that are preserved until late (lost only when wasting is severe). Serum CK is often elevated, and conditions range from inflammatory (polymyositis, dermatomyositis, inclusion body myositis), to dystrophic, to toxic/metabolic causes.

Pearl: Proximal symmetric weakness with normal sensation and a high CK is myopathy; if it instead fatigues and hits the eyes, think NMJ.

The Localization Table