The depressor trochanteris motoneurones and their role in the coxo-trochanteral feedback loop in the stick insect Carausius morosus

Abstract

The innervation pattern of the coxal part of the depressor trochanteris muscle is described. This muscle is located inside the coxa cavity and is innervated by motoneurones contained in nerve C2. Serial sections of nerve C2 reveal that nerve C2 contains 3 large neurones (8, 5, and 3 μm in diameter) in addition to many small neurones. In extracellular nerve recordings from nerve C2 3 large spikes could be recorded, which can easily be classified according to their amplitudes. Combined intracellular muscle recordings and extracellular nerve recordings revealed the physiological characteristics of these motoneurones, which are referred to here as the “fast depressor trochanteris” (FDTr) motoneurone and the spontaneously active “slow depressor trochanteris” (SDTr) motoneurone. The third motoneurone could be identified as an inhibitory motoneurone. Because this motoneurone was also found in nerves nl2, nl3, nl5 and in nerve C1 (to the levator trochanteris muscle) it is referred to here as the “common inhibitor” (CI) motoneurone.

The hypothesis that the trochanteral hairplate (trHP) is the only effective feedback transducer for the coxo-trochanteral control loop (Schmitz 1984, 1986) is confirmed by the nerve recordings from nerve C2, because no reflex response was measured after ablation of the trHP. In addition, shaving the trHP reduces the activity of the spontaneously active SDTr motoneurone.

The frequency responses of the excitatory depressor motoneurones show that the spontaneous activity of the SDTr motoneurone is modulated by the stimulus over a wide range of stimulus frequencies up to 100 Hz and that the FDTr motoneurone is reflexly activated during the same phase of the stimulus as the SDTr motoneurone. Up to 20 Hz the maximum of the motoneurone activity leads the maximum of the movement by about 60 to 80 deg. This shows that nonlinear highpass filter properties of the coxotrochanteral control system, described on the basis of force measurements in an earlier paper (Schmitz 1986), can be found already on the level of the motoneurones.