Figure 8.

Sample data and proposed model for motor circuitry. A, B, Tactile induced motor behavior consisting of coordinated rhythmic muscle contractions (A) and depolarizations in the membrane potential of the corresponding motoneuron (B). C, The rhythmic behavior is caused by waves of inhibition (magenta) and excitation (cyan) to the motoneuron. In the on cycles, excitation and inhibition are concurrently peaking, hence the motoneuron is receiving balanced input from the premotor network (arrow), whereas in the off cycles, there is relatively more inhibition than excitation, which is likely caused by the activation of reciprocal inhibition from the antagonist (arrow and shaded region). Data are adapted from Berg and Ditlevsen (2013) with permission. D, The proposed model: descending inputs from a rhythm-generating network provides drive to both a premotor network, where recurrent excitation (blue population) is balanced with recurrent inhibition (red population), as well as generic reciprocal inhibitory interneurons. The connectivity and number of E/I neurons in the network is unknown. MN, Motoneuron; Ia, reciprocal inhibitory premotor interneuron.