lower motor neuron

**Anatomy/physiology** - Cell body: anterior horn of spinal cord (alpha motor neurons) or cranial nerve motor nuclei (brainstem). Axon exits via the ventral root, joins the peripheral nerve, and synapses on skeletal muscle at the NMJ (ACh acting on nicotinic receptors). - Final common pathway: it receives input from upper motor neurons (corticospinal/corticobulbar tracts), interneurons, and sensory afferents (stretch reflex arc). - Alpha motor neurons innervate extrafusal fibers. Gamma motor neurons innervate intrafusal fibers (muscle spindles). - A motor unit = one alpha motor neuron plus all the muscle fibers it innervates.

**Clinical findings in LMN lesions** - Flaccid paralysis or weakness, in the distribution of the affected neurons, roots, or nerve - Hypotonia - Hyporeflexia or areflexia (the reflex arc is interrupted) - Marked muscle atrophy (denervation atrophy) - Fasciculations (spontaneous firing of a motor unit, from irritable or dying neurons) and fibrillations (single muscle fibers, seen on EMG) - Negative (absent) Babinski sign, with a plantar flexor response - EMG: fibrillation potentials and positive sharp waves at rest. Later, large polyphasic motor unit potentials with reduced recruitment reflect reinnervation.

**Compare with UMN lesions** - UMN lesion: spastic paralysis, hyperreflexia, clasp-knife spasticity, positive Babinski, mild disuse atrophy only, and no fasciculations. Clonus may be present. - Distribution: UMN lesions usually involve groups of muscles or an entire limb or side. LMN lesions follow a segmental, root, or nerve pattern.

**Causes (high-yield)** - Poliomyelitis (poliovirus destroys anterior horn cells): asymmetric flaccid paralysis, fever, and a normal sensory exam. CSF shows lymphocytic pleocytosis with mildly elevated protein. - Spinal muscular atrophy (Werdnig-Hoffmann disease; SMN1 deletion on chromosome 5q): "floppy baby," tongue fasciculations, and LMN degeneration only. - ALS: combined UMN and LMN signs. It is the classic exam example (SOD1 mutations in familial cases; riluzole reduces glutamate excitotoxicity). Sensation is spared, and extraocular and sphincter functions are preserved. - Guillain-Barré syndrome: ascending demyelinating polyradiculopathy after infection (e.g., Campylobacter jejuni). Albuminocytologic dissociation in CSF. Treat with IVIG or plasmapheresis. - Peripheral nerve injury, such as Bell palsy (CN VII LMN facial palsy), radiculopathy (herniated disc), cauda equina syndrome, and diabetic or other neuropathies - Spinal root or anterior horn injury (e.g., syringomyelia affecting the anterior horn, causing LMN signs at the level of the lesion) - Note: NMJ disease (myasthenia gravis, Lambert-Eaton) and myopathies are not LMN lesions, though they also cause weakness.

**Key localization pearl: facial palsy** - LMN facial lesion (e.g., Bell palsy): paralysis of the entire ipsilateral face, including the forehead. - UMN lesion (e.g., stroke): contralateral lower face only, because the forehead receives bilateral cortical input.

**Management** - Treatment is directed at the cause: IVIG/plasmapheresis for GBS, nusinersen or gene therapy for SMA, riluzole for ALS, and corticosteroids for Bell palsy. Supportive care includes respiratory monitoring and PT.