The Insect Frontal Ganglion and Stomatogastric Pattern Generator Networks

Abstract

Insect neural networks have been widely and successfully employed as model systems in the study of the neural basis of behavior. The insect frontal ganglion is a principal part of the stomatogastric nervous system and is found in most insect orders. The frontal ganglion constitutes a major source of innervation to foregut muscles and plays a key role in the control of foregut movements. Following a brief description of the anatomy and development of the system in different insect groups, this review presents the current knowledge of the way neural networks in the insect frontal ganglion generate and control behavior. The frontal ganglion is instrumental in two distinct and fundamental insect behaviors: feeding and molting. Central pattern-generating circuit(s) within the frontal ganglion generates foregut rhythmic motor patterns. The frontal ganglion networks can be modulated in-vitro by several neuromodulators to generate a variety of motor outputs. Chemical modulation as well as sensory input from the gut and input from other neural centers enable the frontal ganglion to induce foregut rhythmic patterns under different physiological conditions. Frontal ganglion neurons themselves are also an important source of neurosecretion. The neurosecretory material from the frontal ganglion can control and modulate motor patterns of muscles of the alimentary canal. The current and potential future importance of the insect stomatogastric nervous system and frontal ganglion in the study of the neural mechanisms of behavior are discussed.

The role of the arthropod stomatogastric nervous system in moulting behaviour and ecdysis

“…The dynamics of the air-swallowing motor programme can be monitored by recording from foregut dilator muscles or from motor neurons in the FG (Hughes, 1980;Elliot, 1980;Zilberstein and Ayali, 2002). The results of such recordings support the exclusive control of the FG over airswallowing behaviour (see also Carlson and O'Gara, 1983;Bell, 1983;Miles and Booker, 1994;Miles and Booker, 1998) (reviewed by Ayali, 2004).…”

“…1B) on the subsequent behaviour and development of the insect (Penzlin, 1985) [see also Ayali (Ayali, 2004) and references therein]. Overall, from these many studies, one can deduce that the insect FG is instrumental in passing food through the foregut and in crop emptying.…”

“…Frontal ganglionectomy was reported to result in either failure by the insect to escape the old cuticle or failure to successfully expand body appendages, and specifically wings (e.g. Bell, 1983;Miles and Booker, 1994;Ayali, 2004). The dynamics of the air-swallowing motor programme can be monitored by recording from foregut dilator muscles or from motor neurons in the FG (Hughes, 1980;Elliot, 1980;Zilberstein and Ayali, 2002).…”

“…This temporal organization may reflect the precise timing of water swallowing, followed by an intense water uptake that results in the need for increased heart activity, which in turn is followed by the onset of ecdysis itself. One cannot avoid speculating that this temporal organization is similar in its general features and in the relative timing of events to insect air swallowing (Hughes, 1980;Zilberstein and Ayali, 2002;Ayali, 2004) (with the exception of the dramatic increase in haemolymph volume that is absent in insects).…”

The neurogenetics and evolution of food-related behaviour

“…Studies have often concentrated on small invertebrate circuits, leading to a wealth of knowledge about the neural basis for rhythmic behaviours such as respiration, locomotion and feeding. Systems used to model neural controls of feeding include the stomatogastric ganglion (STG) of the lobsters Panulirus and Homarus [9] and the crab Cancer [10], the buccal ganglia of the gastropods Aplysia and Lymnaea [11], and the subesophageal or frontal ganglia in insects such as the locust Locusta [12] or tobacco horn worm Manduca sexta [13]. These systems have been used to define how properties such as neuronal connections, neurotransmitter identities, membrane characteristics and inter-circuit communication contribute to specific feeding behaviours and to behavioural plasticity.…”