Central Control of An Insect Sensory Interneurone

A previous paper (McKay, 1969) described the response characteristics of the large auditory T fibre of the tettigoniid Homorocoryphus. This neurone is both frequency-sensitive and amplitude-sensitive, and does not respond to the species song. It responds preferentially to short, high-frequency sounds, which, together with the large size of the axon, suggests that it may function as a warning neurone. Recent work by Rowell & McKay (1969) indicates that the responsiveness of insect sensory interneurones can be controlled by auxiliary inputs, often inhibitory, which are central in origin. Rowell (1964) found that lesions in the central nervous system of Schistocerca increased the frequency of a behavioural reflex controlled by the prothoracic ganglion. Increased responsiveness was correlated with a decrease in the input to the ganglion, and increased output from it (Rowell, 1969), indicating that inhibitory influences had been interrupted by the lesions. Rowell & McKay (1969) found that disconnecting the head ganglia of Gastrimargus from the rest of the central nervous system markedly increased the responsiveness of the alpha auditory intemeurone, while subsequent lesions had little or no effect.

Habituation is also controlled by central factors. Habituation is the waning of a response to a repeated stimulus, and is seen in many vertebrate sensory interneurones (Horn, in press) and in a growing number of invertebrate sensory interneurones (Horridge, Scholes, Shaw & Tunstall 1965; Horn & Rowell, 1968, Rowell & McKay, 1969). Habituation is characterized by stimulus specificity, recovery after a time lapse, and sometimes by dishabituation by another modality of stimulus (Horn, 1967). It is an important property of interneurones in that it enables repetitive, irrelevant stimuli to be ignored, and novel stimuli to be recognized easily. Visual units in the tritocerebrum and ventral nerve of Schistocerca (Horn & Rowell, 1968; Rowell & Horn, 1968; Palka, 1967) show complex habituation phenomena which may last for several hours. Similar characteristics have been observed in the alpha auditory intemeurone of Gastrimargus (Rowell & McKay, 1969). The habituation rate of this intemeurone is decreased by the removal of the head ganglia, but other lesions to the central nervous system have very little effect.

This paper describes the effect of lesions to the central nervous system on the responsiveness and habituation rate of the large auditory T fibre of the tettigoniid, Homorocoryphus. The results suggest that the response level of the intemeurone is controlled by an inhibitory input or inputs, which originate within the posterior thoracic ganglia and act synaptically within the prothoracic ganglion. Habituation of the T fibre is found to be less complex than habituation of other insect interneurones described to date. The rate of habituation is also under the control of central factors, which may be different from those which affect the response level of the intemeurone.

The response of the T fibre to repetitive acoustic stimuli was investigated using methods described in a previous paper (McKay, 1969). The stimulus train consisted of 20 pulses of 10 kc/s, each of 40 msec duration. The amplitude of each pulse was + 94 dB (odB = 0·0002 dyn/cm²). Response curves of the intemeurone were recorded in the neck connectives by means of silver wire electrodes. Recordings were made from the intact animal, and then after the head ganglia had been disconnected by cutting the neck connectives. A final response curve was recorded from each animal after the prothoracic ganglion had been completely isolated from the rest of the central nervous system by cutting the pro-mesothoracic connectives. In some animals, the order of lesions was as follows: the pro-mesothoracic connectives were cut after recording from the intact animal, and the prothoracic ganglion was subsequently isolated from the rest of the central nervous system by cutting the neck connectives (Fig. 1).

The results presented below were obtained from a total of 20 adult insects. No differences were observed between the responses of male and female insects.

When a train of pulses is played to the intact animal, the response to each pulse is phasic and shows little habituation. The response to the first pulse of the train is largest, and subsequent responses decline to a plateau but never reach zero, even when the initial response level is low (Fig. 2). The T fibre response recovers completely within 10 min. Response curves were obtained for the lesion conditions described above (see also Fig. 1). The response decrement (Fig. 2) was seen to be approximately linear, unlike the exponential decay seen in the alpha neurone of Gastrimargus (Rowell & McKay, 1969). Accordingly, the data for each trial was fitted to regression equations of the type y = a + bx, where y = number of spikes/response, and x = stimulus presentation number. The correlation coefficient, r, was also obtained for each of these fits, and gives a measure of how closely the assumption that the decrement is linear is justified. The results (Table 1) suggest that this assumption is a reasonable one. The parameters a and b are measures of the response level and habituation rate respectively (see Rowell & McKay 1969, for further discussion). The values of a, b, and r were calculated for the response curves obtained from the intact animal, and for all lesion conditions, i.e. with the neck connectives cut, the pro-mesothoracic connectives cut, and the prothoracic ganglion isolated from the rest of the central nervous system. These values are set out in Table 1. The significance of difference between the values of both a and b for different lesion conditions was tested by the Walsh test for matched pairs, the results from individual animals supplying the pairs.

There is no significant difference in the rate of habituation between the intact animal and the animal with neck connectives cut. There is, however, a significant increase (P ⩽ 0·05) in the rate of habituation in the animal in which the pro-mesothoracic connectives have been cut. When the prothoracic ganglion is isolated by cutting the pro-mesothoracic connectives in an animal in which the neck connectives had already been cut, there is a significant increase in the rate of habituation (P ⩽ 0·05) over the condition in which the neck connectives only have been cut. When the prothoracic ganglion is isolated by cutting the neck connectives in an animal in which the promesothoracic connectives have previously been cut, there is no significant change in the rate of habituation. These results are shown diagrammatically in Fig. 3. The intrinsic rate of habituation of the T fibre, indicated by the values of b in the isolated ganglion, is seen to be low (Table 1). The lesion experiments indicate that the rate of habituation is reduced still further in the intact animal by inputs to the prothoracic ganglion which arise within the posterior thoracic ganglia. The head ganglia appear to exert no effect on the rate of habituation.

The response level of the T fibre depends on the frequency and amplitude of the stimulus (McKay, 1969). The present results show that, when these parameters of the stimulus are held constant, the response level is affected by inhibitory inputs from within the central nervous system. There is no significant change in the response level (parameter a) over that in the intact animal when the neck connectives are cut. There is a significant increase (P ⩽ 0·05) from the intact response level when the promesothoracic connectives are cut. When the neck connectives are cut in a preparation in which the pro-mesothoracic connectives have already been cut, there is no further increase in the response level. When the prothoracic ganglion is isolated in an animal in which the neck connectives have been cut, by cutting the pro-mesothoracic connectives, there is a significant increase in the response level (P ⩽ 0·05, see Fig. 3).

These results indicate that, in the intact animal, the response level is maintained at a low level by an inhibitory input or inputs to the T fibre in the prothoracic ganglion. These inputs arise within the posterior thoracic ganglia. This data shows that the head ganglia do not affect the response level of the T fibre. These inhibitory influences are not mediated by any sensory input, as in Locusta (Yanagisawa, Hashimoto & Katsuki, 1967). When the thoracic ganglia are de-afferented (Fig. 1) there is no change in the response level with reference to that in the intact animal, and an increase in level does not occur until the pro-mesothoracic connectives are cut. This indicates that the inhibitory influences arise within the central nervous system, within the posterior thoracic ganglia, and that they are not a consequence of extratympanal sensory input.

The rate of habituation is loosely correlated with the response level. There is a small increase in the rate of habituation with the increase in response level, but the calculated regression (r = − 0·56) indicates that the correlation of the two characters is not a close one.

Habituation of insect sensory interneurones to repetitive stimuli has been observed by several workers (Horridge, et al. 1965; Palka, 1967; Horn & Rowell, 1968; Rowell & McKay, 1969), and is important in that it provides a mechanism whereby an animal can ignore irrelevant stimuli, and respond more effectively to novel occurrences in the environment. It differs in this respect from the visual units of the tritocerebrum of Schistocerca (Horn & Rowell, 1968) and the alpha auditory interneurone of Gastrimargus which habituate rapidly. The slow habituation rate of the T fibre may be correlated with its probable function as a warning neurone (McKay, 1969). The rate of habituation of the T fibre is increased by lesion of the pro-mesothoracic connectives, suggesting that, in the intact animal, the rate of habituation is maintained at a low level by accessory inputs from the posterior thoracic ganglia to the prothoracic ganglion. This may be contrasted with the situation which is seen in Gastrimargus (Rowell & McKay, 1969), where the habituation rate is high in the intact animal and is decreased by dis-connecting the head ganglia from the rest of the CNS by lesion of the neck connectives. The rate of habituation of the T fibre and the alpha neurone are, therefore, controlled by accessory inputs to the ganglion in which impulse initiation occurs. The origin and effect of these accessory inputs is different for the two neurones. It is suggested that the rate of habituation of any intemeurone is a result of selection, and that the habituation rate reflects the role of the neurone in controlling the behaviour of the animal. Neurones with a rapid rate of habituation will be concerned with the recognition of novel stimuli, e.g. the alpha neurone of Gastrimargus and visual units of Schistocerca, while those with a low rate of habituation, e.g. the T fibre of Homorocoryphus, will be concerned with the recognition of repetitive stimuli which cannot be ignored. The T fibre of Homorocoryphus seems to serve as a warning neurone. The modification of the rate of habituation of a neurone by accessory inputs suggests that individual neurones do not have sufficient plasticity to maintain extreme rates of habituation.

The response level of the T fibre is also reduced in the intact animal by an inhibitory input or inputs to the prothoracic ganglia from the posterior thorax. The head ganglia are seen to have no effect on the response level of the T fibre. The T fibre sends a descending branch from the prothoracic ganglion to the posterior thorax, and dis-charge by this fibre may initiate a feedback loop which controls subsequent inhibitory output from the pterothorax (Fig. 4) maintaining the response level of the T fibre at a low level. Stabilization of the T fibre response at a low level would prevent over-loading of the system at high stimulus amplitude, which could lead to the confusion of pulses and loss of directional information. The inhibitory input maintains a phasic response, thus rapid pulses are not confused.

The observed correlation between response level and habituation rate suggests that a further function of feedback stabilization of the response at a low level may be to minimize habituation. This is in keeping with the hypothesis that habituation reflects a response decrement brought about by the depletion of transmitter substances at the active synapses (Bruner & Tauc, 1966; Horn, 1967). The correlation between the response level and rate of habituation, although quantitatively valid, is not particularly close (r = 0·56). This suggests that these parameters of the response are either non-linearly related by common synaptic events, or, alternatively, they are controlled by different accessory inputs from the posterior thorax. The synaptic system affecting the T fibre is complex (McKay, 1969), and while changes in response level and habituation are a result of changes at one synapse only, it is more likely that changes in sensitivity at several synapses are involved. The inhibitory input from the posterior thorax to the prothoracic ganglion is not influenced by any peripheral input to the central nervous system, as demonstrated by experiments in which the thoracic ganglia were de-afferented. The inhibitory input to the prothoracic ganglion may act pre-synaptically or postsynaptically, but presynaptic inhibition would give the fine control required of a negative feedback system.

The auditory systems of the Orthoptera are seen to be far from simple, as was once thought. The response properties of each intemeurone may be correlated with the role, or probable role, it plays in controlling the behaviour of the animal, and in the case of the T fibre the properties are in part maintained by synaptic systems within the central nervous system, which are independent of peripheral inputs.