clonus

Meaning: Clonus is a repeating cycle of contraction and relaxation in a muscle after it is suddenly stretched and held. A common example is the ankle beating back and forth after the foot is briskly flexed upward. The stretch reflex is a simple spinal arc: a muscle spindle sensory neuron synapses on a motor neuron in the spinal cord, which contracts the same muscle. Descending pathways from the brain normally dampen this arc. When those pathways are damaged (upper motor neuron damage), the arc becomes hyperexcitable. The muscle contracts, relaxes, and the maintained stretch stimulates the spindles again, so the cycle repeats.

On the MCAT: Clonus is not a psychology or sociology concept. If it appears, it is usually in a nervous-system passage about reflex arcs and the spinal cord. The reasoning to predict is the direction of change. Losing descending input from the brain tends to exaggerate reflexes (hyperreflexia). Damage to the sensory or motor neuron within the arc itself tends to weaken or abolish the reflex.

Common confusion: Clonus is not a resting tremor. It is elicited by a stretch stimulus, and it signals too much reflex activity, not too little. The reflex arc itself runs through the spinal cord without needing the brain, which is why brain or cord damage can release it rather than abolish it.