A descending brain neuron elicits stridulation in the cricketGryllus bimaculatus (de Geer)

Abstract

of single descending axons [4]. In these experiments constant stimulus trains were sufficient to elicit singing. This is evidence for specific descending brain neurons in crickets, which might function as tonically active command neurons [5] for the control of singing behaviour. Corresponding neurons have recently been identified for the control of stridulation in acridid grasshoppers [6]. In summary, the experiments performed so far on cricket stridulation have left unresolved whether the chirp rhythm is determined by patterned activity from the brain or is controlled by descending tonic discharge. Also, the relevant neurons remain unknown. To answer these questions I sought to identify interneurons in the brain of the cricket Gryllus bimaculatus which evoke stridulation when stimulated. Intracellular recordings were obtained in the medial protocerebrum, a region in which electrical stimulation of the neuropil is known to elicit singing behaviour. Experiments were performed with tethered crickets, which were free to move their wings. Wing movements were measured with an optoelectronic system [7]. The movement also reflected the sound pattern during singing (Fig. 1B, inset). Hundreds of intracellularly impaled neurons in the protocerebrum of G. bimaculatus were depolarized during the course of the recordings, and the effect of the increased discharge rate on the behaviour was observed. An interneuron was encountered eight times which evoked stridulation of calling song when depolarized. Its arborization pattern in the brain was stained in three experiments. The interneuron (Fig. 1A) has a soma position close to the midline at the dorsal surface of the protocerebrum. The primary neurite projects ventrally and then turns posteriorly. At this point it sends off lateral arborizations. Other branches run almost parallel to the