Figure 1.

Ia reciprocal connections to spinal motoneurons by antagonist muscle pairs. Rotating the joint to stretch flexor muscles provides disynaptic inhibition to extensor motoneurons and direct activation of the flexor motoneuron (A, solid lines). The opposite rotation stretches extensor muscles providing disynaptic inhibition to flexor motoneurons and direct activation of the extensor motoneuron (B, solid lines). The plus and minus signs in this figure are meant to indicate net depolarization (+) and net hyperpolarization (−). During these rotations the opposite conductance change occurs at each motoneuron type: extensor muscle shortening disfacilitates extensor motoneurons (A, black dashed lines) contributing to their hyperpolarization, and disinhibits flexor motoneurons (A, red dashed lines), contributing to their depolarization. Flexor muscle shortening disfacilitates flexor motoneurons (B, black dashed lines, hyperplolarization) and disinhibits extensor motoneurons (B, red dashed lines, depolarization).