anterior horn

The spinal cord gray matter is organized in a butterfly/H-shaped pattern with dorsal (posterior), lateral, and ventral (anterior) horns. The anterior horn houses alpha motor neurons (and gamma motor neurons) that directly synapse on skeletal muscle fibers, forming the final common pathway for voluntary and reflexive movement. These are classic lower motor neurons, distinguished from upper motor neurons (which originate in the cortex and descend via corticospinal tracts to synapse on anterior horn cells).

Clinically, anterior horn cell pathology produces LMN signs: flaccid weakness, decreased/absent deep tendon reflexes, fasciculations, fibrillations, and muscle atrophy—contrasted with UMN signs (spasticity, hyperreflexia, Babinski sign, no significant atrophy).

Key diseases affecting anterior horn cells: - Poliovirus: selectively destroys anterior horn motor neurons, causing asymmetric flaccid paralysis, often affecting lower limbs. - Spinal muscular atrophy (SMA): autosomal recessive degeneration of anterior horn cells (SMN1 gene mutation), presenting in infancy as "floppy baby syndrome." - Amyotrophic lateral sclerosis (ALS): combined UMN and LMN degeneration; anterior horn cell loss produces LMN signs, while corticospinal tract degeneration produces UMN signs (mixed presentation is a hallmark). - West Nile virus: can also cause a poliomyelitis-like syndrome with anterior horn involvement.

Anatomically, the anterior horn is important on cross-sectional spinal cord imaging/pathology (e.g., anterior horn cell loss visible in polio/SMA), and lesions here spare sensory and autonomic (lateral horn) function, keeping sensation intact while causing pure motor deficits—useful for localizing lesions on exams.