Muscle spindle responses in man to changes in load during accurate position maintenance

Area of Science:

• Neuroscience
• Human Physiology
• Motor Control

Background:

• Muscle spindles are sensory receptors crucial for proprioception and motor control.
• Understanding reflex responses to mechanical perturbations is key to deciphering motor system function.

Purpose of the Study:

• To investigate the role of muscle spindle activity and spinal reflexes in response to sudden and slow mechanical loads during voluntary muscle contraction.
• To determine if fusimotor activity contributes to reflex potentiation during muscle contraction.

Main Methods:

• Single and multi-unit recordings of muscle spindle afferent activity from peroneal nerves in human subjects.
• Subjects maintained constant ankle joint position while external loads on muscles were unexpectedly altered (increased or decreased).
• Electromyographic (EMG) activity was recorded to assess motor responses.

Main Results:

• Sudden load increases elicited similar muscle spindle discharge whether the muscle was relaxed or contracting.
• A spinal reflex motor response to sudden load increase only occurred during muscle contraction, indicating potentiation.
• Sudden load decreases caused a pause in spindle discharge and subsequent EMG activity, consistent with spinal reflexes.
• Slow load changes resulted in parallel adjustments in EMG and muscle spindle discharge, suggesting corticospinal modulation.

Conclusions:

• The potentiation of spinal reflexes during muscle contraction is not primarily mediated by fusimotor action.
• Distinct reflex responses to load increases and decreases highlight different neural control mechanisms.
• Voluntary control, likely via corticospinal pathways, adjusts motor output in parallel to alpha and gamma motoneurons to manage gradual load changes and maintain joint stability.