Introduction

When an animal is suddenly attacked by a predator, it must respond with great urgency if it is to escape. The neuronal circuits that initiate such an escape response must be both straightforward and reliable in order to fulfil their biological function. A staightforward circuit is essential to ensure speed in initiating the escape, and a reliable circuit is needed not only to make sure the response occurs when required but also to avoid false alarms. These qualities of simplicity and reliability, which are of great survival value to the animal, are also of service to the neuroethologist exploring the role that nerve cells play in behaviour. Consequently, several of these startle responses have been studied in detail and they provide valuable insight into the flow of information through the nervous system from sensory inputs to muscular output.

Furthermore, these neuronal circuits often involve neurons that are exceptionally large and, because of this, are called giant neurons. The function of giant neurons is to conduct spikes rapidly along the body, but their size also makes them readily accessible to study with microelectrodes. The giant neurons therefore offer a major opportunity to investigate the role of individual nerve cells in behaviour.

Two main functions must be carried out by the neuronal circuit that initiates any behaviour pattern, including escape. First of all, a decision to initiate an activity must be made at some point in the circuit.