Reciprocal and Renshaw (recurrent) inhibition are functional in man at birth

Area of Science:

• Neuroscience
• Developmental Biology
• Motor Control

Background:

• Group Ia reciprocal inhibition and Renshaw cell function are crucial for motor control.
• Understanding their developmental trajectory in humans is essential for comprehending motor skill acquisition.

Purpose of the Study:

• To determine the onset of Group Ia reciprocal and Renshaw inhibition during human postnatal development.
• To investigate the relationship between reciprocal inhibition and the disappearance of Group Ia excitatory reflexes.

Main Methods:

• Surface electromyography (EMG) recorded biceps brachii (Bi) reflexes evoked by triceps brachii (Tri) tendon taps in 99 subjects (1 day to 31 years).
• Longitudinal study of 29 infants from birth to 12 months.
• Postmortem analysis of human spinal cord for Renshaw cell morphology and markers.

Main Results:

• Reciprocal excitatory reflexes were present in nearly all neonates.
• Reciprocal inhibition was observed in only 25% of neonates, likely masked by excitation, but present in all subjects after 9 months.
• Renshaw cells showed mature morphology by 35 weeks post-conceptional age; evidence of Renshaw inhibition was observed in neonates.

Conclusions:

• Group Ia reciprocal inhibition matures postnatally, becoming consistently demonstrable after 9 months of age.
• Renshaw cells are present early in development and contribute to motor control.
• Early development of inhibitory circuits, including reciprocal and Renshaw inhibition, is critical for normal motor function.