Monosynaptinen

Monosynaptinen refers to a neural reflex pathway that involves a single synapse between a sensory neuron and a motor neuron within the spinal cord or brainstem. The term is used in neurology and physiology to distinguish this simple circuit from polysynaptic reflexes, which involve one or more interneurons.

The best-known example is the stretch reflex. When a muscle is stretched, muscle spindle Ia afferent fibers transmit a signal directly to the homonymous motor neurons in the ventral horn. The resulting EPSP in the motor neurons causes an immediate contraction of the same muscle. Because this arc contains no interneuron within the primary loop, the response is typically very fast. Supraspinal input can modulate the reflex, and reciprocal inhibition of antagonistic muscles arises from separate interneuronal circuits.

Other monosynaptic reflexes are present in certain cranial nerve circuits, such as the jaw-jerk reflex mediated by trigeminal pathways, but the knee-jerk reflex remains the classical example of a monosynaptic spinal reflex.

Clinical relevance: In neurological examinations, the integrity of monosynaptic pathways is assessed through reflex testing. Damage to nerves or the spinal segment involved can reduce or abolish the reflex (hyporeflexia or areflexia). Conversely, loss of inhibitory control from higher brain centers can lead to exaggerated reflexes (hyperreflexia) and spasticity, indicating involvement of supraspinal pathways.

See also: stretch reflex, patellar reflex, reflex arc.