monosynaptiske

Monosynaptiske, or monosynaptic, refers to reflex circuits that involve a single synapse between a sensory neuron and a motor neuron within the spinal cord. The best-known example is the myotatic or stretch reflex, often tested as the knee-jerk (patellar) reflex. In this pathway, muscle spindle receptors in the quadriceps detect stretch and transmit via Ia afferent fibers to the dorsal horn. These afferents synapse directly on alpha motor neurons that innervate the same muscle, eliciting contraction. Because there is no interneuron at the primary synapse, the reflex latency is short, yielding a rapid and reliable response.

The activity is typically modulated by the level of spindle sensitivity, which is adjusted by gamma motor neurons, and by supraspinal input. Monosynaptic reflexes are fundamental for maintaining muscle length and posture and can participate in protective responses if needed, though basic monosynaptic stretch reflexes primarily oppose sudden muscle lengthening.

Clinical relevance: Changes in monosynaptic reflexes are used in neurology to assess the integrity of the spinal cord and motor neurons. Hyperreflexia or spasticity can accompany upper motor neuron lesions, while diminished or absent monosynaptic reflexes may indicate lower motor neuron damage or peripheral neuropathy.

Relation to polysynaptic reflexes: Monosynaptic reflexes involve a single synapse, whereas polysynaptic reflexes involve one or more interneurons in the reflex arc, producing longer latencies and greater complexity.