NBCE Exam 2025 – 400 Free Practice Questions to Pass the National Board of Chiropractic Examiners

Clonus is characterized by involuntary, rhythmic muscle contractions, which can often be observed during a neurological examination. This phenomenon typically arises from a dysregulation within the upper motor neuron systems, specifically associated with pyramidal tract lesions.

When there is damage to the pyramidal tract—pathways that carry motor signals from the cortex to the spinal cord—there is a loss of inhibitory control over the motoneurons, leading to hyperexcitability of these pathways. This hyperexcitability manifests as clonus, particularly at the ankle or knee, and indicates an upper motor neuron lesion.

In contrast, conditions like cerebellar dysfunction primarily affect coordination and balance without causing clonus, while sensory neuron damage and peripheral neuropathy typically involve sensory deficits rather than motor control issues, such as involuntary muscle contractions.

Thus, the presence of clonus during an examination is specifically indicative of pyramidal tract lesions, where the balance of excitation and inhibition in the motor pathways is disrupted.