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. This is usually done on the basis of incoming sensory information, which will often be filtered by the sensory receptors and neurons closely associated with them to extract particular stimulus features. This must occur rapidly because a startled animal has only a few milliseconds left to it in which to initiate escape action. The relevant processing must be fed to the decision point with minimal delay. Once the decision to initiate an escape has been made, the second function is an executive one: the circuit must include connections that lead away from the decision point to excite those neurons which are involved in the escape movement and to inhibit other neurons involved in incompatible movements.

Early work in this area gave rise to the idea that there may be a special class of high-order interneurons that carry out these two functions. The term command neuron was applied to such interneurons by Wiersma & Ikeda (1964), who found that electrical stimulation of single interneurons elicited co-ordinated movements of the abdominal ventilatory appendages of crayfish (swimmerets), even when the system was deprived of sensory feedback. Subsequently, a variety of instances was analysed in which apparently normal behaviour could be elicited by stimulation of a single neuron, and the term command neuron came into general use. Following controversy as to exactly what a command neuron might be, Kupferman & Weiss (1978) suggested that a command neuron should be defined as a neuron that is both necessary and sufficient for the initiation of a particular behaviour pattern. In this chapter, three cases of startle behaviour in which identified interneurons have been studied in detail are examined. Each case shows that this definition of a command neuron cannot be strictly applied to any of the neurons examined. This gives rise to a situation that will be entirely familiar to ethologists: one must either be content to use the term 'command neuron' loosely, or abandon its use altogether.

The three examples of startle behaviour considered here are the tail flip of crayfish, the fast start of teleost fish, and escape running by cockroaches. These have a number of features in common, including the fact that giant interneurons play a key role in initiating the behaviour in each case. The crayfish tail flip is probably understood, in terms of its neuronal circuitry, more completely than any other behaviour pattern of comparable complexity. Part of the reason for this relatively complete understanding is that recording from identified neurons has gone hand in hand with an increasingly exact study of the responses of intact animals, using film and video techniques. This illustrates well how ethological and neurophysiological methods of analysis can mutually reinforce one another during the intensive study of a single system.

Giant Interneurons

Figure 3.1 Giant interneurons involved in crayfish (Procambarus) startle behaviour. (a) Crayfish showing the location of the central nervous system (in solid black), a chain of ganglia. Also shown are electrodes implanted to record neuronal activity in the freely moving animal. (b) Activity recorded by these electrodes during a startle response: a tap on the abdomen (stimulus) is followed by a spike in a lateral giant and a potential in the abdominal flexor muscles. (c) Transverse section of the connectives between two abdominal ganglia, showing the locations of the lateral and medial giants. (a modified after Schramek, 1970; b redrawn after Krasne & Wine, 1975; c redrawn after Krasne & Wine, 1977.)

Figure 3.1 Giant interneurons involved in crayfish (Procambarus) startle behaviour. (a) Crayfish showing the location of the central nervous system (in solid black), a chain of ganglia. Also shown are electrodes implanted to record neuronal activity in the freely moving animal. (b) Activity recorded by these electrodes during a startle response: a tap on the abdomen (stimulus) is followed by a spike in a lateral giant and a potential in the abdominal flexor muscles. (c) Transverse section of the connectives between two abdominal ganglia, showing the locations of the lateral and medial giants. (a modified after Schramek, 1970; b redrawn after Krasne & Wine, 1975; c redrawn after Krasne & Wine, 1977.)

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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