Motor Patterns Induced in the Antenna of the Rock Lobster by Cuticular Vibration on the Cephalothorax

In decapod crustaceans the neuronal basis of proprioceptive reflexes has been extensively studied since the work of Eckert (1959). Reflex patterns can be induced by joint chordotonal organs (Bush, 1962, 1965; Murayama, 1965; Muramoto, 1965; Evoy & Cohen, 1969; Clarac & Vedel, 1975) or by muscle receptor organs (Evoy & Cohen, 1971; Field, 1974; Vedel, Angaut-Petit & Clarac, 1975; Mill, 1976; Bush, 1981). Two kinds of proprioceptive reflex during passive joint movement now seem well established, the classical resistance reflex and the assistance reflex exciting the muscle being shortened (Wilson & Davis, 1965; Davis, 1969; Ayers & Davis, 1977, 1978; Di Caprio & Clarac, 1981; Vedel, 1980, 1982). Besides proprioceptors the decapod crustaceans have numerous external cuticular organs sensitive to mechanical events occurring in the environment of the animal. These mechanoreceptors respond to stimuli such as water currents, contact with hard objects or distortion of the cuticle. Some of them are very sensitive to cuticular vibrations, especially the so-called campaniform organs (Laverack, 1976; Barth, 1980; Bush & Laverack, 1982). Although some actions of cuticular mechanoreceptors on interneurones have been described in crustaceans (Kennedy, Calabrese & Wine, 1974; Fricke, Block & Kennedy, 1982), the eventual motor effects induced by the exteroceptive sensory organs have been very little studied, perhaps for the reason that it is not easy to stimulate selectively the different types of mechanoreceptor.

The present work is a study in the rock lobster, Palinurus vulgaris, of the motor patterns produced when mechanical vibrations are applied to different parts of the cuticle. It is shown that in many cuticular nerves some sensory units respond to vibrations imposed on the cuticle of the cephalothorax, and that these same vibrations induce various motor effects which can modulate the tonic discharge and reflex activity of the motoneurones innervating the muscles of the two distal joints of the antenna.

Experiments were performed on rock lobster (Palinurus vulgaris) of 400–500 g weight. The animal were fixed by rubber bands, dorsal side up, in a Perspex dish filled with oxygenated, refrigerated (16 °C) sea water.

The right or left antenna was fixed in a holder with the J2 joints fully extended (see Figs in Vedel 1980, 1982). The cuticle was cut from the dorsal side of the S2 segment around the tendon of the J2 flexor muscle. The attachment of the J2 flexor tendon was cut and the muscle was pulled back in order to isolate carefully the flexor and extensor nerves of the J3 muscles (muscles moving the flagellum). Activity in the extensor (E) and flexor (F) nerves was recorded by means of suction electrodes disposed ‘en passant’. The different motoneurones were identified according to criteria described in previous papers (Vedel, 1980, 1982).

The cuticle was vibrated by pulses applied to the cuticle of the cephalothoracic carapace by means of a stainless steel rod (diameter 2 mm) connected to an electromagnetic vibrator driven by a pulse generator. The duration of each cuticular shock was 2·5 ms and the pulse frequency used ranged from 5 to 70 Hz. Amplitude of the shock displaced the cuticle by about 0·1 mm.

Proprioceptive reflex responses of extensor and flexor motoneurones of the J3 antennal joint were induced by imposing passive sinusoidal extension/flexion movements of the flagellum by means of an electromechanical system similar to that described previously (Vedel, 1980). The flagellum was moved through 30° (midway within its full arc of 60°) at an angular velocity of 30° s⁻¹, these movements being monitored with a linear potentiometer. All recordings were stored on tape for subsequent analysis and photography.

Mechanical vibrations applied anywhere on the surface of the cephalothorax induced phasic activity in nearly all the cuticular sensory nerves. Generally they induced responses in some large units which fired in a one-to-one manner at frequencies of up to 40–60 Hz. Vibrations applied to one point of the cuticle diffuse to the whole cephalothorax and so are able to activate simultaneously a great number of sensory units.

Fig. 1 gives an example of a unit recorded in a nerve branch of the anterior part of the dorsal side of the cephalothorax which was sensitive to vibration of the cuticle and responded in a one-to-one manner at frequencies up to 40 Hz. This effect could be obtained when the cuticle was out of the sea water as well as when it was under watch demonstrating that the water movements induced by the vibratory system do not cause the response of the phasic unit. The cuticular organs sensitive to vibration could not be localized because they appeared highly sensitive to stimulation applied at distant sites. Observation of the cuticle of the cephalothorax by scanning electron microscopy (unpublished observations) did not reveal specialized structures similar to the campaniform organs observed in crustaceans by Shelton & Laverack (1968) and considered by the authors to be particularly sensitive to cuticular vibration.

During the study of the effects induced by vibratory stimuli on the tonic activity of antennal motoneurones, the afferent nerve of the chordotonal organ common to the J3 and J2 joints (Rossi-Durand & Vedel, 1982), and that of the muscle receptor organ sensitive to JI joint movements (Vedel & Monnier, 1983), were both cut, in order to avoid interference between proprioceptive and exteroceptive sensory activities during vibration. As some campaniform organs are located at the proximal end of the flagellum (J.-P. Vedel, in preparation), the two sensory nerves innervating this segment were also cut. It may be noted, however, that such nerve sections never modified the antennal motor effects induced by vibration of the cephalothoracic cuticle.

Motor effects induced by vibration pulses applied to the antero-dorsal cuticle of the cephalothorax in the muscle of the different joints of the antenna, J3, J2, J1 and J0 (Vedel, 1980; Vedel & Monnier, 1983) were studied by appropriate EMG and motoneuronal recordings. The vibratory stimulus had almost no effect on the motoneurones innervating the muscles of the two proximal joints, J1 and J0. Only a very slight and irregular facilitation was observed in some experiments. The most sustained effects concerned the innervation of the two distal joints, J2 and J3, and have mainly been studied by recording from the motoneurones of the J3 muscles responsible for the movements of the flagellum.

Two different motor effects were observed, whatever the cuticular site where the vibration was applied to the cephalothorax. The occurrence of one or other of these effects was apparently dependent on the prevailing level of reactivity of the animal, estimated by tonic motoneuronal discharge and the intensity of proprioceptive reflex responses.

In animals with low reactivity, cuticular vibration produced a reciprocal motor effect characterized by a decrease in the activity of the tonic extensor motoneurone of J3 and a facilitation of the tonic flexor motoneurone discharge (Fig. 2). Whatever the vibration frequency, the extensor motoneurone (E.J3) was completely silent during stimulation, apart from a brief motoneuronal burst in response to the first one or two shocks applied to the cuticle. The discharge rate of the flexor motoneurone (F.J3) increased with the vibration frequency. At 5 Hz and sometimes at 10 Hz its discharge could be driven in a ‘one-to-one’ manner by the vibration frequency, the motoneurone responding by one or several spikes to each vibration cycle. The inhibitory motoneurone (CI), common to the flexor and extensor muscles of J3 and J2, also fired in a one-to-one manner when the vibrations were applied at low frequency (5-10 Hz).

In more reactive animals, the same vibratory stimulation produced a diffuse facilitatory motor effect characterized by a simultaneous increase in the discharge both the flexor and the extensor motoneurones of J3 (Fig. 3). Generally, the extensor motoneurone activity was strongly increased, its discharge being for the most part unrelated to the vibration cycles. In some experiments, however, a one-to-one driving was observed at low frequency (Fig. 4). The diffuse facilitatory motor effect increased the discharge of the flexor excitatory motoneurone and of the common inhibitory motoneurone in the same way as during the reciprocal effect, and they were also driven in a one-to-one manner when cuticular vibrations were imposed at low frequency. From one experiment to another the delay between the cuticular shock and the motoneuronal response (recorded 4 cm from the brain) was relatively constant. Measured from the first shock applied to the cephalothorax it was about 17 ms (±2 ms) for the extensor and flexor motoneurones and about 19 ms (±2 ms) for the common inhibitory motoneurone. It should be noted that only the activity of the tonic motoneurones was affected during both the reciprocal and the diffuse motor effects. Activation of the phasic motoneurones was never obtained with this form of mechanical stimulation.

The reciprocal effect as well as the diffuse facilitatory effect could be modulated by other sensory inputs, such as those produced by water movements or by passive movement of various appendages. Fig. 4 illustrates the modifications obtained by moving the uropods laterally during a diffuse facilitatory effect induced by 10 Hz cuticular vibration. In this example both flexor (F.J3) and extensor (E.J3) motoneurones were driven in a one-to-one manner by the vibration frequency. Uropod movements produced an inhibition of the extensor motoneurone discharge and simultaneously increased the flexor activity. The common inhibitory motoneurone (CI), which was also driven in a one-to-one manner during the vibration, was only slightly facilitated by the uropod stimulation.

In preparations where the afferent nerve of the chordotonal organ common to the J2 and J3 joints was left intact, proprioceptive resistance reflex could be induced in the J3 motoneurones when imposing sinusoidal extension/flexion movements to the flagellum (Vedel, 1980). The reflex activation involved mainly the tonic excitatory motoneurones, with the flexor motoneurone discharging during extension and the extensor motoneurone firing during flexion (Fig. 5B). Cuticular vibration imposed on the cephalothorax in the condition where the reciprocal effect occurred (as in Fig. 5A cf. Fig. 2) produced a modulation of these proprioceptive reflexes, in the same way as its effect on the tonic motoneuronal activity. The extensor resistance reflex was strongly decreased but never completely abolished by the vibratory stimulus (Fig. 5B). Simultaneously the flexor resistance reflex increased, apparently as a result of summation of the activation of the tonic activity induced by vibration (Fig. 5A) with the reflex activation induced by the flagellar movement. The common inhibitory motoneurone (CI) was also activated during vibratory stimulation but its discharge appeared mainly related in a one-to-one manner to vibration frequency, and most of the time was only weakly modulated by the flagellum movement.

When the cuticular vibration inducing the diffuse facilitatory motor effect (Fig. 6A) was superimposed on flagellar extension/flexion movements, it simultaneously increased the flexor and extensor resistance reflexes (Fig. 6B). The extensor reflex was usually facilitated more than the flexor discharge which, as described above, seemed to combine tonic activation with the reflex response. As during the reciprocal effect, the common inhibitory motoneurone was activated by vibratory stimulation but was poorly modulated by flagellar movements.

The recordings from various cuticular nerve branches suggest that the cephalothorax has numerous mechanoreceptors which respond to vibration. These receptors are not sensitive to water movements and are mainly activated by direct cuticular stimulation. A single shock applied to one point on the cephalothorax diffuses to almost the whole cuticle of the animal body so that the vibration simultaneously activates a large number of sensory units; this can explain the sustained motor effects observed in the experiments. Scanning electron microscope observation of the cephalothoracic cuticle did not show the existence of structures similar to the campaniform organs which have been observed at the tip of the dactyl of the locomotor appendages and along the antennules in some decapod crustaceans (Shelton & Laverack, 1968; Laverack, 1976; Barth, 1980), and which are considered by these authors as receptors highly sensitive to vibration. Nevertheless, the present study did reveal various very small cuticular organs which could be involved in vibration detection, but their size does not permit their selective mechanical stimulation under stereomicroscopic control. It must therefore be considered that the presence on the cephalothoracic cuticle of structures sensitive to mechanical vibrations has so far only been demonstrated electrophysiologically, in terms of their reflex effects upon the motor behaviour of the antennae. The fine structure and precise afferent response characteristics of the sensory end-organs involved in vibration sensitivity studied here remain to be elucidated.

On the basis of their different experimentally observed characteristics, the motor effects induced by cuticular vibration of the cephalothorax in the motoneurone of the antenna do not appear as generalized actions of simple diffuse sensory inputs but rather as specific well organized sensori-motor patterns. The vibratory motor effects involve essentially the motoneurones of the two distal antennal joints, J2 and J3, which have the property of acting in the same plane, their combined action extending the flagellum forward from the cephalothorax or flexing it back towards the cephalothorax. Thus, it appears that the reflex effects of cuticular vibration mainly concern flagellar motor command rather than the postural motor system constituted by the two proximal joints, JI and JO (Vedel, 1980; Vedel & Monnier, 1983).

The fact that two different motor patterns, a reciprocal effect and a diffuse facilitatory effect, can be obtained with the same stimulation shows that the central actions of the sensory units activated by cuticular vibration can be modulated in accordance with the level of reactivity of the animal, which may also be responsible for the competition between resistance and assistance reflexes on the same motoneurones of the J2 and J3 joints (Vedel, 1980). So, if we consider the motor effects induced by vibrations as ‘exteroceptive reflexes’, it may be suggested that, as for the proprioceptive reflexes, their characteristics are modified by the central nervous state of the preparation.

In addition to this dependence of the different effects of vibratory stimulation upon the central state of the animal, it has been demonstrated that other sensory inputs, such as those evoked by moving the uropods, are also able to modify the effects induced by cephalothoracic vibrations. Thus, it has been observed in this study that the diffuse facilitatory effect produced by cuticular stimulation becomes a reciprocal effect when the uropod afferents are activated. These different possibilities of neuronal modulation enhance the diversity of the neural influences converging upon the antennal motoneurone pool.

The capacity for driving antennal motoneurones at low frequency (5–10 Hz) in a one-to-one manner shows that the sensory bursts induced by each mechanical shock on the cuticle are strong enough to evoke a motoneuronal response. Such an effect indicates a close relationship between the sensory and motor elements involved and supports the inference, noted above, that the effects obtained by mechanical vibration may indeed reflect definite cuticular-motor reflex patterns, and not merely diffuse sensory actions acting upon the antennal motor system.

These vibration-induced motor effects may be considered to constitute a system for modulation of the tonic discharge and reflex activity of the motoneurones controlling flagellar movements, this modulation being related to exteroceptive events. The types of activation of the sensory units underlying this modulation suggest that such cuticular reflexes could be involved in alarm behaviour, permitting the animal to react rapidly with its antennae against mechanical disturbances originating from the environment. Moreover, two motor effects, which could have different behavioural significance, are obtained depending upon the state of the central excitability of the animal. Thus, the reciprocal effect could be considered as a defensive reaction flexing back the flagellum towards the cephalothorax where threatening stimuli are applied. The diffuse facilitatory effect may be only one component of a general alerting reaction, perhaps involving arousal of the whole body and limbs.