Swimmeret Beating Research Articles

Nitric oxide modulates a swimmeret beating rhythm in the crayfish.

The modulatory effects of nitric oxide (NO) and cAMP on the rhythmic beating activity of the swimmeret motor neurones in the crayfish were examined. Swimmerets are paired appendages located on the ventral side of each abdominal segment that show rhythmic beating activity during forward swimming, postural righting behaviour and egg ventilation in gravid females. In isolated abdominal nerve cord preparations, swimmeret motor neurones are usually silent or show a continuous low-frequency spiking activity. Application of carbachol, a cholinergic agonist, elicited rhythmic bursts of motor neurone spikes. The co-application of L-arginine, the substrate for NO synthesis with carbachol increased the burst frequency of the motor neurones. The co-application of the NO donor SNAP with carbachol also increased the burst frequency of the motor neurones. By contrast, co-application of a NOS inhibitor, L-NAME, with carbachol decreased beating frequency of the motor neurones. These results indicate that NO may act as a neuromodulator to facilitate swimmeret beating activity. The facilitatory effect of L-arginine was cancelled by co-application of the soluble guanylate cyclase (sGC) inhibitor ODQ suggesting that NO acts by activating sGC to promote the production of cGMP. Application of L-arginine alone or membrane-permeable cGMP analogue 8-Br-cGMP alone did not elicit rhythmic activity of motor neurones, but co-application of 8-Br-cGMP with carbachol increased bursting frequency of the motor neurones. Furthermore, application of the membrane-permeable cAMP analogue CPT-cAMP alone produced rhythmic bursting of swimmeret motor neurones, and the bursting frequency elicited by CPT-cAMP was increased by co-application with L-arginine. Co-application of the adenylate cyclase inhibitor SQ22536 ceased rhythmic bursts of motor neurone spikes elicited by carbachol. These results suggest that a cAMP system enables the rhythmic bursts of motor neurone spikes and that a NO-cGMP signaling pathway increases cAMP activity to facilitate swimmeret beating.

Asynchronous swimmeret beating during defense turns in the crayfish, Procambarus clarkii.

Swimmeret beating was monitored in freely moving specimens of the crayfish Procambarus clarkii as they exhibited defense turn responses to tactile stimuli. Analysis of videotape records revealed alterations in swimmeret beating during turning responses compared to straight, forward walking. During turns, swimmerets beat with shorter periods and smaller amplitude power strokes than during straight walking. Coordination between swimmerets also changed. Swimmerets on the side toward which the animal turned tended to lag behind their contralateral partners, rather than beat in synchrony as in straight walking, and ipsilateral coordination was loosened relative to straight walking. Asynchronous swimmeret beating accompanied asymmetric motions of the uropods in a manner similar to that observed during statocyst-dependent equilibrium reactions in P. clarkii, but removal of the statoliths did not eliminate turn-associated responses of the swimmerets. The coordinated action of the swimmerets and uropods may contribute to the torque that rotates the animal in the yaw plane. Implications of the observed changes in swimmeret coordination for understanding the underlying neuronal control system are discussed.

Intersegmental Coordination of Limb Movements during Locomotion: Mathematical Models Predict Circuits That Drive Swimmeret Beating

Normal locomotion in arthropods and vertebrates is a complex behavior, and the neural mechanisms that coordinate their limbs during locomotion at different speeds are unknown. The neural modules that drive cyclic movements of swimmerets respond to changes in excitation by changing the period of the motor pattern. As period changes, however, both intersegmental phase differences and the relative durations of bursts of impulses in different sets of motor neurons are preserved. To investigate these phenomena, we constructed a cellular model of the local pattern-generating circuit that drives each swimmeret. We then constructed alternative intersegmental circuits that might coordinate these local circuits. The structures of both the model of the local circuit and the alternative models of the coordinating circuit were based on and constrained by previous experimental results on pattern-generating neurons and coordinating interneurons. To evaluate the relative merits of these alternatives, we compared their dynamics with the performance of the real circuit when the level of excitation was changed. Many of the alternative coordinating circuits failed. One coordinating circuit, however, did effectively match the performance of the real system as period changed from 1 to 3.2 Hz. With this coordinating circuit, both the intersegmental phase differences and the relative durations of activity within each of the local modules fell within the ranges characteristic of the normal motor pattern and did not change significantly as period changed. These results predict a mechanism of coordination and a pattern of intersegmental connections in the CNS that is amenable to experimental test.

Modulation of Force during Locomotion: Differential Action of Crustacean Cardioactive Peptide on Power-Stroke and Return- Stroke Motor Neurons

Crustacean cardioactive peptide (CCAP) elicited expression of the motor pattern that drives coordinated swimmeret beating in crayfish and modulated this pattern in a dose-dependent manner. In each ganglion that innervates swimmerets, neurons with CCAP-like immunoreactivity sent processes to the lateral neuropils, which contain branches of swimmeret motor neurons and the local pattern-generating circuits. CCAP affected each of the four functional groups of motor neurons, power-stroke excitors (PSE), return-stroke excitors (RSE), power-stroke inhibitors (PSI), and return-stroke inhibitors (RSI), that innervate each swimmeret. When CCAP was superfused, the membrane potentials of these neurons began to oscillate periodically about their mean potentials. The mean potentials of PSE and RSI neurons depolarized, and some of these neurons began to fire during each depolarization. Both intensity and durations of PSE bursts increased significantly. The mean potentials of RSE and PSI neurons hyperpolarized, and these neurons were less likely to fire during each depolarization. When CCAP was superfused in a low Ca2+ saline that blocked chemical transmission, these changes in mean potential persisted, but the periodic oscillations disappeared. These results are evidence that CCAP acts at two levels: activation of local premotor circuits and direct modulation of swimmeret motor neurons. The action on motor neurons is differential; PSEs and RSIs are excited, but RSEs and PSIs are inhibited. The consequences of this selectivity are to increase intensity of bursts of impulses that excite power-stroke muscles.

Remote Control of the Swimmeret Central Pattern Generator in Crayfish (Procambarus Clarkii and Pacifastacus Leniusculus): Effect of a Walking Leg Proprioceptor

ABSTRACT An isolated preparation of the crayfish nervous system, comprising both the thoracic and the abdominal ganglia together with their nerve roots, has been used to study the influence of a single leg proprioceptor, the coxo-basal chordotonal organ (CBCO), on the fictive swimmeret beating consistently expressed in this preparation. Both mechanical stimulation of the CBCO and electrical stimulation of its nerve were used. In preparations not displaying rhythmic activity, electrical or mechanical stimulations evoked excitatory postsynaptic potentials (EPSPs) in about 30% of the studied motor neurones with a fairly short and regular delay, suggesting an oligosynaptic pathway. Such stimulation could evoke rhythmic activity in swimmeret motor nerves. The evoked swimmeret rhythm often continued for several seconds after the stimulus period. When the swimmeret rhythm was well established, electrical and mechanical stimuli modified it in a number of ways. Limited mechanical or weak electrical stimuli produced a small increase in swimmeret beat frequency, while more extreme movements of the CBCO or strong electrical stimuli had a disruptive effect on the rhythm. The effect of low-intensity stimulation on existing swimmeret beating was phase-dependent: it shortened the beat cycle when applied during the powerstroke phase and lengthened it when applied during the retumstroke phase. Rhythmic mechanical stimulation of CBCO or electrical stimulation of the CBCO nerve entrained the swimmeret rhythm within a limited range in relative or absolute coordination. Electrical stimuli also produced systematic effects on the whole metachronal pattern of the swimmeret rhythm, perturbing the interganglionic coordination.

The Coordinated Action of Abdominal Postural and Swimmeret Motor Systems in Relation to Body Tilt in the Pitch Plane in the Norway Lobster Nephrops Norvegicus

ABSTRACT The responses of the abdominal positioning system and the swimmerets of the Norway lobster Nephrops norvegicus to tilts in pitch and in other vertical planes have been investigated in intact and semi-intact preparations. Tilt of the intact animal in the pitch plane evoked abdominal flexion with the head up and extension with the head down. The response amplitudes were largely independent of tilt angle, and the null point for switching between flexion and extension responses was at the 10° head-up position, which corresponds to the normal posture of the animal. Intersegmental joint angles in the abdomen were significantly different for head-up and head-down tilts. These pitch responses were dependent upon the integrity of the statocysts. Tilts in different vertical planes demonstrated that the abdominal flexion/ extension response was restricted to a range of tilts within ±5° of true pitch. Outside this range, tilts in both directions elicited abdominal extension, which was accompanied by asymmetrical movements of the uropods and swimmerets. In semi-intact preparations, recordings from the motor supply to the abdominal superficial flexor muscles demonstrated a modulation of activity in response to tilt in the pitch plane: several small excitatory axons increased their discharge in the head-up position, while the inhibitory axon f5 fired when the head was down. With tilts in different vertical planes this pattern of firing persisted from pitch almost to roll. The effects of tilt in the pitch and roll planes on the temporal and spatial parameters of swimmeret beating were studied in intact animals. Swimmeret beating was bilaterally symmetrical in pitch: vigorous in the head-down position, when the powerstrokes were directed about 25° to the side, and weak in the head-up position, when the powerstrokes were directed to the rear. Responses in roll were asymmetrical, with the powerstroke of the upper swimmerets directed at about 41° to the side. Recordings of the tonic returnstroke motor neurone of the swimmeret demonstrated that its activity was modulated by tilt in the pitch plane: increasing and becoming rhythmic in the head-down position in active preparations, with the activity phase-locked to the tilt stimulus. With tilts in different vertical planes this unit showed a phase shift at intermediate angles between pitch and roll. Simultaneous recordings of motor activity to the abdominal superficial flexor muscles and the swimmeret tonic returnstroke unit demonstrate closely similar, but not completely coupled, patterns of firing. These results are discussed in terms of the nature of statocyst control of postural and rhythmic abdominal motor systems, the different relationships of motor activities and behavioural responses to tilts in pitch and other vertical planes, and possible neuronal mechanisms that control the interaction of the two abdominal motor systems.

Anatomical and physiological organization of the swimmeret system of the spiny lobsterJasus lalandii as adaptive components of the tail flick

1. The swimmerets ofJasus lalandii, in contrast to those well known in the nephropid lobsters (e.g.Homarus) and astacurans (crayfish), do not display spontaneous antero-posterior beating, but are either apposed actively to the ventral surface of the abdomen, or rotated outward (Fig. 2). These movements are imposed by the geometrical arrangement of the bicondylar joints at the base of the swimmeret (Fig. 3), and involve contraction of either the remotor muscle, or the promotor-rotator muscles (Figs. 2, 3). Each swimmeret includes a short, thick blade-like exopodite that contains two antagonistic muscles, a large curler and a small adductor muscle (Fig. 3). Each swimmeret is innervated by 80 motor neurons (MNs) which are disposed in two clusters in the ganglion. 2. The modulation of the tonic discharge of the muscles which maintain the swimmeret position at rest (remotor and curler) has been studied in two situations: body rolling (Fig. 4) and walking activity (Fig. 5). In the female, in which the most anterior pair of swimmerets are biramous, both endopodite and exopodite curler muscles display the same responses to body rolling (Fig. 4). In all these situations no overt swimmeret movement occurs. 3. Nevertheless, rhythmicity exists inJasus, but it is limited to the gravid female when the swimmerets bear the eggs (Fig. 6). In contrast to other decapod Crustacea, this swimmeret beating is not metachronous (Fig. 6). 4. Movement monitoring (Fig. 7) and EMG recordings (Figs. 9, 10) have demonstrated the involvement of the swimmerets in the three phases of the tail flick response (preparation, flexion, extension). During the preparatory phase, in response to mechanical stimulation of the legs, the swimmerets open on the stimulated side (on both sides in the case of a symmetrical stimulation) (Fig. 7). During the rapid abdominal flexion of the tail flick all swimmerets open fully regardless of the stimulus (Figs. 7, 8). Two different units in the rotator muscle EMG are responsible for swimmeret opening during the preparatory and the flexion phases of the tail flick (Figs. 9, 10). 5. The curler muscle of the endopodite in the female displays antagonistic activities to that of the exopodite during tail flicks (Fig. 10). 6. Selective swimmeret blockage demonstrates that they contribute to the thrust efficacy in tail flicks. In particular they are responsible for the variation of the maximal force produced at its onset. This effect could be interpreted as a consequence of force redistribution by the swimmerets acting on water flow (produced by the tail fan). This mechanism implies a functional role for the swimmerets in righting and steering responses (Fig. 11).

Phase-Dependent Modulation of Auxiliary Swimmeret Muscle Activity in the Equilibrium Reactions of the Norway Lobster, Nephrops Norvegicus L.

ABSTRACT The activity of swimmeret muscles of the lobster Nephrops norvegius during beating in the upright and tilted animal has been examined. The responses to tilt are produced primarily by stimulation of the statocysts. The anatomy of the swimmeret muscles is described. Although essentially similar to previous descriptions, important new aspects are presented. The arrangement of the main powerstroke and retumstroke muscle groups in relation to the peg-and-hook articulation of the swimmeret produces a segregation of action, with different muscles contributing progressively to force production. The auxiliary muscles of the basipodite, M9, M10 and M13 act to twist the swimmeret laterally. The auxiliary muscle Ml 1-12-14—15 acts to maintain a rearward powerstroke. The innervation of swimmeret muscles and the location of their motoneurone cell bodies in the abdominal ganglia have been revealed by cobalt staining. There is a clear segregation of powerstroke and returnstroke motoneurones. Intraganglionic fibre tracts as well as four interganglionic fibres are identified. Intracellular studies on the twister muscles show that M9 receives at least three excitatory units, and M10 at least six. No inhibitory activity was ever recorded in these muscles. M9 and the medial bundle of M10 receive tonic excitatory inputs, while the lateral bundle of M10 receives phasic inputs and is normally silent in the absence of lateral beating. In the absence of swimmeret beating, body roll about the long axis induces tonic motor activation of the lateral twister muscles (M9 and M10) and the return-stroke muscles in the swimmerets on the side tilted upwards. When beating occurs it is predominantly in the swimmerets tilted upwards, and there is an entrainment of phasic activity in M9, M10 and M13 to the powerstroke phase of the beats. Tonic units to the retumstroke muscle remain unaffected. The results are discussed with particular reference to the interaction of descending statocyst information with the central pattern generator for swimmeret beating.

Swimmeret Proprioceptors in the Lobsters Nephrops Norvegicus L. and Homarus Gammarus L.

ABSTRACT The morphology, sensory responses and reflex effects of two proprioceptive systems in the swimmerets of the Norway lobster Nephrops norvegieus are described. Two bipolar cells embedded in an elastic strand (strand B) which spans from the sternal rib to the proximal edge of the basipodite respond to stretch of the strand, applied directly or through swimmeret protraction. Powerstroke motoneurones are excited by a negative feedback reflex, and the transition from retumstroke to powerstroke movement is thereby sharpened. When protraction movements of the swimmeret are blocked, the intensity of beating is reduced both in the blocked swimmeret, and in neighbouring (particularly posterior) swimmerets. A second receptor strand, the twisting muscle receptor (TMR), stretches from the sternal rib wall to the proximal end of the twisting muscle M10 in both the lobsters Nephrops norvegieus and Homarus gammarus. It contains the sensory endings of two cells which have somata in the abdominal ganglion. The axons of these cells convey conventional spikes in response to strand stretch, which occurs on release of M10 from imposed extension or following active M10 contraction. They produce a specific activation of M10 motoneurones, which represents a positive feedback reflex. This reinforces the twist of the swimmeret blade, so that the beat is directed laterally to its greatest extent throughout the powerstroke. It is suggested that the TMR is homologous with the crayfish non-spiking swimmeret receptors, which also have central cell bodies. However, the receptors differ in their location, mode of afferent transmission and reflex actions. The discovery of these differences resolves anomalies between previous studies on lobsters and crayfish. The results are discussed in terms of the homologies of all limb proprioceptors with central cell bodies in decapod crustaceans, and of the proprioceptive control of swimmeret beating.

Intersegmental coordination of swimmeret rhythms in isolated nerve cords of crayfish

In crayfish,Pacifastacus leniusculus, abdominal ganglia that can generate the motor pattern normally associated with swimmeret beating continue to do so when the number of connected ganglia is reduced from six to two. The period and phase of the rhythm produced by these shortened chains of ganglia are the same as those produced by the full abdominal nerve cord. These results demonstrate that interactions between any two neighboring ganglia suffice to establish the metachronal phase-lag characteristic of the swimmeret rhythm.

Interrelations between posture and locomotion in response to body rotation in crayfish

1. Rhythmic beating of crayfish swimmerets and postural movement of the abdomen in response to body rotation in the pitch plane were analyzed quantitatively. 2. Abdominal extension and swimmeret beating were evoked frequently at the head-down position, and the incidence of spontaneous tail flip increased when the animal was in the upside-down position (Fig. 2). 3. Head downward rotation was mimicked by magnetically pulling iron grains which were used to replace the statoliths (Fig. 3). 4. Simultaneous nerve recordings were obtained from the 1st, 2nd, and superficial 3rd (S3rd) roots during the pitching response in an unre-strained animal. The response consisted of the discharge of F5 (flexion inhibitor) spikes accompanied by coactivation of extension motor neurons and rhythmic swimmeret bursts (Fig. 4). 5. Manual bending of the abdomen suppressed swimmeret beating (Fig. 5). 6. Spontaneous tail flip sometimes interrupted swimmeret beating in the pitching response. The swimmeret motor discharge was briefly suppressed when the tail flip was evoked by tap stimulation of the abdomen. There was, however, no marked phase shift in the beating rhythm after the tail flip interruption (Figs. 6 and 7). 7. When the LG was stimulated electrically during swimmeret beating in the immobilized preparation, however, no appreciable suppression of the swimmeret motor burst was observed (Fig. 8).

The Structural Basis of an Innate Behavioural Pattern

ABSTRACT The abdominal nervous system of the crayfish contains six serially homologous ganglia, each containing approximately 650 neurones. No two ganglia are identical, and the ganglia interact extensively. Studies confined to intraganglionic interactions thus yield limited and sometimes misleading information. Each ganglion contains intrinsic (local) interneurones, motor neurones and projecting interneurones in roughly equal numbers, except in the specialized terminal ganglion where the ratio of these cells is approximately 3:2: 1. Although the number of nerve cell bodies in a ganglion is small enough to be tractable, integration occurs in the neuropile, which contains terminals from interneurones and afferents that outnumber the neurones originating in the ganglion by at least ten to one. The abdominal nervous system responds almost exclusively to a variety of mechanosensory stimuli. It has very limited light sensitivity. Other modalities, notably chemosensitivity, are undescribed and may be lacking. The effectors of the abdomen consist of fast axial muscles (used for tailflip-powered escape), slow axial muscles (for setting abdominal posture), appendage muscles (for swimmeret beating), and slow muscles of the intestine and rectum (that control gut emptying). The fast and slow muscles of the tailfan are specialized homologues of the axial and appendage muscles. The abdominal nervous system represents only 3–4% of the 100000 neurones within the crayfish central nervous system (CNS). Most sensory information gathered in the abdomen is transmitted to the rostral CNS for processing, and many abdominal motor programmes are activated by descending commands. Nevertheless, a surprising degree of autonomy is present, and at least some motor programmes of every motor system can be activated in isolated abdomens. Tailflip escape behaviour illustrates the integrative properties of the crayfish nervous system. Ninety pairs of efferents and eighteen pairs of interneurones have been identified within the abdominal portion of the escape circuit. A cell-by-cell analysis has so far provided neurophysiological explanations, in varying states of completeness, for ethological concepts such as innate releasing mechanisms, spatial patterning of movement, serial order in behaviour, and alterations in responsiveness to a constant stimulus.

Influence of walking on swimmeret beating in the lobster Homarus gammarus.

Influence of walking on swimmeret beating in intact lobsters, Homarus gammarus, has been analyzed using a treadmill experimental device. Belt movement activates both leg stepping and swimmeret beating. The simultaneity of the onset of the two motor systems in this situation is demonstrated to be the result of a startle response initiated when the belt begins to move. This reaction consists of a non-specific motor activity involving several antagonist postural and dynamic muscles. Abdominal extension and vigorous swimmeret beating are the main features of this reaction. The main characteristics of the swimmeret beating as defined by Davis (1969) has been observed here in sequences without walking. However during long walking sequences a very different swimmeret beating pattern occurs. It is suggested that this slow swimmeret beating is completely subordinate to the walking rhythm during sequences of absolute coordination. In more rapid swimmeret beating a relative coordination with leg stepping is very common. The functional meaning of this linkage between legs and swimmerets is discussed.

Neural pathways of reflex-evoked behaviors and command systems in the abdomen of the crayfish

1. The lifting of a crayfish (Procambarus)clarkii) from the substrate results in reflexive abdominal extension often accompanied by metachronous swimmeret beating. Cinematographic analyses show that the maintained, maximally extended abdominal position is stereotypic from trial to trial for each animal. 2. Complete transections at various levels of the abdominal nerve cord abolish the reflex-evoked extension and swimmeret behavior in all ganglia caudal to the lesion (Figs. 1, 2). Therefore, some essential portion of the pathways mediating these reflexive behaviors originates rostral to the abdomen and projects caudally. 3. Transection of any one of the paired abdominal connectives (i.e., hemiconnectives), or transection of up to 4 unilateral hemiconnectives has no effect on the reflex-evoked behaviors. 4. Transection of two hemiconnectives, done on opposite sides in adjacent abdominal segments, eliminates reflexive behavior in all ganglia caudal to the more posterior cut (Fig. 3). Hence, there is no evidence that the extension drive in one hemiconnective can cross within abdominal ganglia and travel in the opposite hemiconnective. 5. Similar transections of hemiconnectives in adjacent segments reveal that the swimmeret drive can cross ganglion 2 and course in the opposite hemiconnective (Fig. 3). No such ability was evident in ganglion 4 (Fig. 3). 6. Chronic recordings from postural motoneurons were also made during reflex-evoked behaviors (Figs. 4, 5). Compared to the intact animal, ligation of either hemiconnective in the segment anterior to the recorded ganglionic root reduces by approximately 50% the activity level of the extension motor program, even though the degree of movement remains the same (Figs. 4, 5). 7. A map of abdominal extension-evoking interneurons was made in cross-section of a thoraco-abdominal hemiconnective. Over 83% of these interneurons were found to occupy six loci (Fig. 6). 8. At least three of the locus-specific units have uninterrupted axonal projections through the abdominal cord (Figs. 7, 8). 9. No excitatory interactions between nonhomologous interneurons on one side of the nervous system or between bilateral homologous units were evident (Figs. 8, 9). 10. These characteristics of extension-evoking interneurons correspond closely to those of reflex-evoked extension drives, suggesting that their activity underlies reflexive abdominal extension behavior.

Two coupled oscillators as a model for the coordinated finger tapping by both hands

Recently, it was found that rhythmic movements (e.g. locomotion, swimmeret beating) are controlled by mutually coupled endogeneous neural oscillators (Kennedy and Davis, 1977; Pearson and Iles, 1973; Stein, 1974; Shik and Orlovsky, 1976; Grillner and Zangger, 1979). Meanwhile, it has been found out that the phase resetting experiment is useful to investigate the interaction of neural oscillators (Perkel et al., 1963; Stein, 1974). In the preceding paper (Yamanishi et al., 1979), we studied the functional interaction between the neural oscillatory which is assumed to control finger tapping and the neural networks which control some tasks. The tasks were imposed on the subject as the perturbation of the phase resetting experiment. In this paper, we investigate the control mechanism of the coordinated finger tapping by both hands. First, the subjects were instructed to coordinate the finger tapping by both hands so as to keep the phase difference between two hands constant. The performance was evaluated by a systematic error and a standard deviation of phase differences. Second, we propose two coupled neural oscillators as a model for the coordinated finger tapping. Dynamical behavior of the model system is analyzed by using phase transition curves which were measured on one hand finger tapping in the preivous experiment (Yamanishi et al., 1979). Prediction by the model is in good agreement with the results of the experiments. Therefore, it is suggested that the neural mechanism which controls the coordinated finger tapping may be composed of a coupled system of two neural oscillators each of which controls the right and the left finger tapping respectively.

Reflex Organization in the Swimmeret System of the Lobster

ABSTRACT The dynamic properties of the intrasegmental swimmeret reflexes of the lobster Homarus americanas were studied by recording the discharge of the motor neurones while the swimmeret was moved sinusoidally in its natural arc over a wide range of frequencies. The reflex responses of the excitor neurones of both powerstroke (retractor) and returnstroke (protractor) muscles display hysteresis. In both cases the efferent response corresponding to a given limb position is usually greater during imposed retraction than during protraction. The cyclic efferent reflex response follows the sinusoidal movement stimulus at movement frequencies up to and beyond those which occur naturally during swimmeret beating, with no change in the position of maximum reflex activity in the cycle. The reflexes are therefore capable of influencing the motor output on a cycle-by-cycle basis. The strength of the reflex response is maximum between 1 and 3 Hz. of imposed movement, and declines to either side of this range. The dynamic properties of the reflexes are therefore adjusted so that the maximum amplification of the rhythmic central motor command occurs at the natural frequency of swimmeret beating.

Reflex Organization in the Swimmeret System of the Lobster

ABSTRACT The intrasegmental feedback reflexes in the swimmeret system of the lobster Homarus americanas were activated while recording the responses from the swimmeret nerves and muscles. Two main sources of sensory feedback were identified; proprioceptors in the coxal region of the swimmeret, and sensory setae on the edges of the two rami of each swimmeret. The reflexes activated by these inputs are described. Reflexive feedback from the powerstroke movement to the powerstroke excitatory motor neurones is positive, further reinforcing the movement. Intrasegmental reflexes capable of independently initiating or terminating the powerstroke activity are absent, however. Therefore the powerstroke movement of each cycle can begin and end only in response to a purely central nervous motor command. It follows that the intrasegmental swimmeret reflexes are incapable of contributing to the periodicity seen in the motor output pattern which underlies swimmeret beating. In addition to strengthening the powerstroke, the intrasegmental reflexes strengthen the linkage between the powerstroke and the returnstroke within each movement cycle. The reflexes may also reinforce the reciprocity between excitor and inhibitor axon activity to the main powerstroke and returnstroke muscles. It is shown, however, that these three features of the motor output pattern are programmed into the CNS independently of the sensory feedback. The intrasegmental reflexes thus act as subservient amplifying devices for cyclic motor patterns which are produced independently by purely central nervous mechanisms.

The neural control of swimmeret beating in the lobster.

ABSTRACT The neural basis of swimmeret beating in the lobster Homarus americanus was studied by recording the cyclic motor output to the swimmeret muscles of intact, unrestrained specimens. The force produced by each swimmeret is regulated by changes in the number of impulses within single motoneurones during each movement cycle, and by changes in the number of motoneurones which discharge during each cycle. Both of these parameters increase with the frequency of swimmeret beating. The burst duration of single swimmeret motoneurones is independent of the frequency of swimmeret beating. The duration of the total electrical activity of each muscle nevertheless increases with the cycle duration, owing to changes in the relative timing of the activities of the motoneurones which innervate the muscle. By this method the forces produced by each muscle are presumably distributed evenly throughout cycles of variable duration. As the frequency of swimmeret beating increases, the movement response progressively lags behind the motor nerve activity. At high frequencies of beating powerstroke motoneurones fire during the returnstroke of the preceding movement cycle because of this phase change, thereby actively braking the swimmeret. Tilting the lobster to one side changes the direction of the powerstroke of each swimmeret on the side tilted upward. This change is achieved by an increase in the activity of muscles which rotate the appendage outward on its long axis, by an increase in the activity of a muscle which directs the powerstroke out toward the side, and by a decrease in the activity of a muscle which directs the powerstroke to the rear. The latter decrease is accomplished through the use of a peripheral inhibitor axon. Under some conditions individual swimmeret motoneurones discharge in continuous trains rather than in bursts. This tonic activity is characterized by the occurrence of a disproportionate number of impulse pairs and triplets, a pattern to which the swimmeret muscles may be especially sensitive. During swimmeret beating, all of the swimmeret motoneurones discharge in cyclic bursts. The temporal structure of the bursts suggests that the underlying excitatory input oscillates sinusoidally with time at the same frequency as that of swimmeret beating.

Lobster righting responses and their neural control.

This paper reports a behavioural analysis of the movements and righting responses of the abdominal pleopods (swimmerets) and uropods (tail fans) of the lobsterHomarus americanus. The movements of individual swimmerets, studied from high-speed motion pictures, consist of a basic power stroke–return stroke cycle, upon which other cyclic power and control movements are superimposed. Tilting a lobster around the long axis of its body initiates bilaterally asymmetrical swimmeret beating. The swimmerets on the side tilted upward beat out toward the side, while those on the side tilted downward beat either straight to the rear, or not at all. These bilaterally asymmetrical swimmeret movements produce a cyclic righting torque around the long axis of the lobster’s body. The peak-to-peak amplitude of this torque is directly proportional to the frequency of swimmeret beating. The characteristics of the righting torque were related to the movements of individual swimmerets. Tilting a lobster around its long axis also evokes bilaterally asymmetrical uropod movements. The uropod on the side tilted downward is spread open, while the uropod on the side tilted upward is closed. It is argued that these movements also contribute to the production of righting torque. The righting responses of both the swimmerets and the uropods are controlled exclusively by the statocyst receptors. The position and acceleration receptors of the statocysts probably both participate in the control of the responses. Destroying one statocyst shifts the position of symmetry for the swimmerets and uropods from upright to 20° toward the injured side. Either the rightorleft statocyst can alone control the righting responses of the uropod and swimmerets of either side, however, even though the afferent responses of the two statocysts to roll in one direction are opposite. Neural models based on the bilaterally reciprocal organization of statocyst influences are proposed to explain these findings.

The neuromuscular basis of lobster swimmeret beating

AbstractThe anatomy and innervation of the swimmeret muscles of the lobster Homarus americanus were studied using histological and intracellular recording techniques. Each swimmeret is controlled by 12 muscles, each of which comprises from one to five anatomically‐distinguishable bundles of muscle fibers. The bundles which make up each muscle are usually merged along part of their lengths. Sometimes, however, bundles which share efferent innervation and which are therefore part of a single functional muscle are separate along their entire lengths. Thus, the concept of a muscle as a discrete anatomical unit is of limited use in the swimmeret system.The pattern of neuromuscular activity underlying swimmeret beating was determined by recording the rhythmic movements of a swimmeret simultaneously with the electrical activity of its muscles. Each muscle is active for a fixed fraction of every movement cycle, during which it discharges several muscle potentials. The 12 muscles of each swimmeret are recruited in the same temporal sequence during each cycle of swimmeret beating.