Clonic

Clonus
Classification and external resources
Specialty	Neurology
[]

Clonus (from the Greek for "violent, confused motion") is a series of involuntary, rhythmic, muscular contractions and relaxations. Clonus is a sign of certain neurological conditions, particularly associated with upper motor neuron lesions involving descending motor pathways, and in many cases is, accompanied by spasticity (another form of hyperexcitability). Unlike small, spontaneous twitches known as fasciculations (usually caused by lower motor neuron pathology), clonus causes large motions that are usually initiated by a reflex. Studies have shown clonus beat frequency to range from three to eight Hz on average, and may last a few seconds to several minutes depending on the patient’s condition.

Clonus is most commonly found at the ankle, specifically with a dorsiflexion/plantarflexion movement (up and down). Some case studies have also reported clonus in the finger, toe, and laterally in the ankle (as opposed to the typical up and down motion).

Clonus is typically seen in people with Cerebral Palsy, stroke, multiple sclerosis, spinal cord damage and hepatic encephalopathy. Clonus has also appeared after ingesting potent serotonergic drugs, where ingestion strongly predicts imminent serotonin toxicity (serotonin syndrome).

The self re-excitation of hyperactive stretch reflexes theory involves a repetitive contract-relax cycle in the affected muscle, which creates oscillatory movements in the affected limb. In order for self re-excitation to exist, both an increase in motor neuron excitability and nerve signal delay are required. Increased motor neuron excitability is likely accomplished by alterations to the net inhibition of neurons occurring as a result of injury to the CNS (stroke/ spinal cord injury). This lack of inhibition biases neurons to a net excitatory state, therefore increasing total signal conduction. Signaling delay is present due to an increased nerve conduction time. Long delays are primarily due to long reflex pathways, which are common in distal joints and muscles. This may therefore explain why clonus is typically found in distal structures like the ankle.Frequency of clonus beats have been found to be directly proportional to the length of the reflex pathway it is found in.

...
Wikipedia