IDENTIFIED NONSPIKING INTERNEURONS IN LEG REFLEXES AND DURING WALKINGIN THE STICK INSECT

In the stick insect Carausius morosus identified nonspiking interneuro ns (type E4) were investigated in the mesothoracic ganglion during int ra- and intersegmental reflexes and during searching and walking. In t he standing and in the actively moving animal interneurons of type E4 drive the excitatory extensor tibiae motoneurons, up to four excitator y protractor coxae motoneurons, and the common inhibitor 1 motoneuron (Figs. 1-4). In the standing animal a depolarization of this type of i nterneuron is induced by tactile stimuli to the tarsi of the ipsilater al front, middle and hind legs (Fig. 5). This response precedes and ac companies the observed activation of the affected middle leg motoneuro ns. The same is true when compensatory leg placement reflexes are elic ited by tactile stimuli given to the tarsi of the legs (Fig. 6). Durin g forward walking the membrane potential of interneurons of type E4 is strongly modulated in the step-cycle (Figs.8-10). The peak depolariza tion occurs at the transition from stance to swing. The oscillations i n membrane potential are correlated with the activity profile of the e xtensor motoneurons and the common inhibitor 1 (Fig. 9). The described properties of interneuron type E4 in the actively behaving animal sho w that these interneurons are involved in the organization and coordin ation of the motor output of the proximal leg joints during reflex mov ements and during walking.