Skeletal muscle tone and the misunderstood stretch reflex

Area of Science:

• Neuroscience
• Human Physiology
• Biophysics

Background:

• Traditional understanding of human muscle tone relies heavily on Sherrington's decerebrate animal models.
• This view posits muscle tone is primarily a monosynaptic stretch reflex, requiring tonic fusimotor activity for maintenance.
• Existing models suggest tonic muscle tone in antigravity muscles is crucial for posture.

Purpose of the Study:

• To revise long-held beliefs about the origins and mechanisms of human muscle tone.
• To present a more comprehensive model integrating mechanical and neural factors.
• To challenge the exclusive reliance on reflex mechanisms in explaining muscle tone.

Main Methods:

• Review of existing physiological data and research on muscle tone.
• Analysis of nonreflex mechanical contributions to resting muscle tone.
• Examination of spinal cord reflex plasticity and interneuronal activity.
• Investigation of long-latency transcortical responses to muscle stretch.
• Evaluation of inertial and viscoelastic forces in postural control.

Main Results:

• Nonreflex mechanical properties significantly contribute to resting muscle tone.
• Spinal reflex responses are dynamic, influenced by interneuronal activity and converging inputs.
• Long-latency transcortical pathways actively manage substantial muscle stretch displacements.
• Inertial and viscoelastic forces play a role in stabilizing posture against minor sway.

Conclusions:

• Human muscle tone is not solely reflex-driven but involves significant mechanical components.
• Muscle stretch responses are adaptable, not fixed, adjusting to situational demands.
• A revised understanding integrating mechanical and adaptable neural mechanisms is necessary for human muscle tone.