Fig. 7.

Some of the most important actions of PreI. A presynaptic inhibitory neuron (PreI₁) inhibits terminals of a Ia fiber (Ia₂) synapsing with a homonymous motoneuron (n₂); the latter is excited by a descending voluntary input (d₂) so as to activate a motor unit in a muscle (m₂). However, the PreI interneuron also inhibits Ia fibers (Ia₁, Ia₃) belonging to other muscles (m₁, m₃) that are not involved in the desired movement. At the left another PreI interneuron (PreI₂) receives inputs from cutaneous peripheral nerve fibers (Cut) and inhibits the terminals of these and other cutaneous afferents. Both PreI neurons are under descending control (d₁, d₃).