Why is gamma motor neuron discharge increased in clonus, particularly in patients with upper motor neuron lesions?

Why Gamma Motor Neuron Discharge is Increased in Clonus

The Fundamental Misconception

Your premise is incorrect: clonus does NOT occur due to increased gamma motor neuron discharge. In fact, the opposite is true—clonus represents pure alpha motor neuron activity without gamma motor neuron involvement, distinguishing it fundamentally from normal voluntary movements and other movement disorders like Parkinsonian tremor. 1

The Actual Mechanism of Clonus

Pure Alpha Motor Neuron Activity

• Clonus is generated by stretch reflexes causing pure alpha contractions without alpha-gamma coactivation. 1
• During clonic oscillations, each afferent stretch discharge is regularly followed by a stretch reflex contraction, which on its falling phase elicits a new volley of impulses in the Ia afferent fibers, creating a self-perpetuating cycle. 1
• Intraneurally recorded spindle afferent activity during clonus shows NO contraction discharges (which would indicate gamma motor neuron activity), only stretch discharges. 1

The Self-Excitation Hypothesis

• The most widely accepted explanation is that hyperactive stretch reflexes in clonus are caused by self-excitation rather than central gamma motor neuron drive. 2
• Clonus arises when two conditions occur simultaneously: (1) the reflex pathway contains long delay times (typical of distal limb muscles), and (2) the excitability of alpha motoneurons is enhanced. 3
• The enhanced motoneuron excitability is mediated by a reduction in motoneuron firing threshold following upper motor neuron lesions, not by increased feedback gain from gamma motor neurons. 3

Contrast with Normal Movement and Tremor

Alpha-Gamma Coactivation in Normal Movement

• In healthy subjects during voluntary fast alternating movements, Ia afferent fiber discharge patterns show both stretch discharges AND contraction discharges (indicating gamma motor neuron activity). 1
• Parkinsonian tremor similarly demonstrates alpha-gamma coactivation, with contraction discharges present during the contraction phases. 1

Absence of Gamma Activity in Clonus

• Spastic patients with clonus show NO contraction discharges during clonic oscillations, confirming the absence of gamma motor neuron involvement. 1
• This distinguishes clonus as a pathological reflex phenomenon rather than a centrally organized motor pattern. 1

Clinical Characteristics Supporting This Mechanism

• Clonus is an involuntary rhythmic muscle contraction with a frequency of 5-8 Hz, occurring in muscles with hyperactive stretch reflexes following upper motor neuron lesions. 2
• The durations of clonus bursts are longer than medium-latency reflexes, allowing sufficient time for group II afferents and other spinal mechanisms to be involved along with Ia afferents. 2
• Sustained oscillations occur readily through self-reexcitation of the alpha motor neuron pool, reducing the need to propose a "central oscillator" or gamma motor neuron involvement. 3

Key Pitfall to Avoid

Do not confuse the hyperactive muscle spindle response in clonus with increased gamma motor neuron discharge. The hyperactivity reflects enhanced alpha motoneuron excitability and altered spinal reflex circuitry following upper motor neuron lesions, not fusimotor drive. The muscle spindles are responding normally to stretch; the pathology lies in the exaggerated alpha motor neuron response and the self-perpetuating reflex loop. 3, 1