Cuticular receptor activation of postural motoneurons in the abdomen of the hermit crab, Pagurus pollicarus.

Displacement of the abdominal cuticle of the hermit crab, Pagurus pollicarus, activates motoneurons of the ventral superficial muscles that mediate posture and slow movements. Five excitatory motoneurons innervating the right ventral superficial muscle of the fourth abdominal segment were activated in a phasic stereotyped fashion in the isolated nervous system. Intracellular records from these motoneurons showed an initial monosynaptic burst, a period of inhibition in which inhibitory post-synaptic potentials were present and then a later period of increased spike frequency generated by excitatory post-synaptic potentials. The reflex response was maintained after severing all ganglionic roots from peripheral structures, isolating the nerve cord from peripheral feedback pathways. The two excitatory components of the response showed a dependence on strain that was much smaller than that found in sensory afferents. There was no relationship between the site of touch to the cuticle and the intensity or pattern of activation of the motoneurons. The reflex burst produced a transient activation of both longitudinal and transverse/circular layers of the muscle with forces that varied between 10% and 25% of the maximum muscle force. These results are consistent with a feedforward regulation of muscle stiffness.

Mechanoreceptors innervating soft cuticle in the abdomen of the hermit crab, Pagurus pollicarus.

Mechanoreceptors in the soft cuticle of the 4th abdominal segment of the hermit crab, Pagurus pollicarus, that are associated with reflex activation of abdominal postural motoneuron, were studied to determine whether their properties are consistent with a feedback control of abdominal stiffness. Three classes of receptors were identified: (1) setal dome receptors, (2) hypodermal receptors, and (3) funnel-canal receptors. The hypodermal receptors, which have the largest extracellular action potentials, were selected for further study. Their axons innervate the entire ipsilateral half of a segment; receptive fields of receptors with different amplitudes show extensive overlap. They are phasic and show significant adaptation; at higher frequencies they signal displacement rather than velocity. Although they are activated by changing muscle tension, their threshold for cuticular displacement is much lower than for forces generated by postural muscles. These features suggest that they are primarily involved in signaling cuticular displacement and shearing forces as they contact the columella of the shell in which the hermit crab lives.

Regulation of muscle stiffness during periodic length changes in the isolated abdomen of the hermit crab.

Reflex activation of the ventral superficial muscles (VSM) in the abdomen of the hermit crab, Pagurus pollicarus, was studied using sinusoidal and stochastic longitudinal vibration of the muscle while recording the length and force of the muscle and the spike times of three exciter motoneurons. In the absence of vibration, the interspike interval histograms of the two larger motoneurons were bimodal; cutting sensory nerves containing most of the mechanoreceptor input removed the short interval peak in the histogram, indicating that the receptors are important in maintaining tonic firing. Vibration of the muscle evoked a reflex increase in motoneuron frequency that habituated after an initial peak but remained above control levels for the duration of stimulation. Motoneuron frequency increased with root mean square (rms) stimulus amplitude. Average stiffness during stimulation was about two times the stiffness of passive muscle. The reflex did not alter muscle dynamics. Estimated transfer functions were calculated from the fast Fourier transform of length and force signals. Coherence was >0.9 for the frequency range of 3-35 Hz. Stiffness magnitude gradually increased over this range in both reflex activated and passive muscle; phase was between 10 and 20 degrees. Reflex stiffness decreased with increasing stimulus amplitudes, but at larger amplitudes, this decrease was much less pronounced; in this range stiffness was regulated by the reflex. The sinusoidal frequency at which reflex bursts were elicited was approximately 6 Hz, consistent with previous measurements using ramp stretch. During reflex excitation, there was an increase in amplitude of the short interval peak in the interspike interval histogram; this was reduced when the majority of afferent pathways was removed. A phase histogram of motoneuron firing during sinusoidal vibration had a peak at approximately 110 ms, also suggesting that an important component of the reflex is via direct projections from the mechanoreceptors. These results are consistent with the hypothesis that a robust feedforward regulation of abdominal stiffness during continuous disturbances is achieved by mechanoreceptors signalling the absolute value of changing forces; habituation of the reflex, its high-threshold for low frequency disturbances and the activation kinetics of the muscle further modify reflex dynamics.

Dynamics of reflex cocontraction in hermit crab abdomen: experiments and a systems model.

1. Both stretch and release of the ventral superficial muscles (VSM) in the abdomen of the hermit crab, Pagurus pollicarus, activate the VSM motoneurons in the intact animal and in the isolated abdomen. 2. This reflex was studied by recording intracellularly from muscle fibers innervated by single motoneurons during stretch and release of the VSM. The three motoneurons of the right fourth segment respond to both stretch and release with a phasic burst lasting approximately 250 ms. The burst in the two tonic motoneurons has two components, a short burst lasting 10-20 ms, with a latency from the beginning of stretch of 60-90 ms, and a longer burst of variable length, with a latency of 120 ms. Ramp stretches of different amplitudes and velocities were used to show that the first component is proportional to the absolute value of the second derivative of force and the second component to the absolute value of the first derivative of force. 3. Stretch and release of the VSM also simultaneously evoke phasic bursts in the motoneurons of the dorsal superficial muscles and the VSM circular muscles (functional antagonists of the longitudinal VSM), as well as in contralateral homologues of the same segment and in ipsilateral homologues of the next anterior segment. The effect of this coactivation is to stiffen the abdomen in response to perturbations in any direction. 4. Stretch or release of phasic mechanoreceptors in the VSM evokes this reflex. Isometric electrical stimulation of the isolated muscle also activates them, showing that they are transducing changes in force and suggesting that they operate to increase muscle stiffness by positive feedback. 5. A mathematical systems model of this reflex, composed of two parallel pathways activating the motoneurons, was constructed. The first pathway produces a signal proportional to the absolute value of the second derivative of force, the second pathway a signal proportional to the first derivative of force. The sum of the signals from the two pathways is filtered by an adaptation process, which is followed by a low-pass filter representing muscle activation kinetics. The muscle activation signal is then fed back to multiple muscle force. 6. Simulations using this model generate the phasic bursts to stretch and release as well as reproducing the frequency dependence of this reflex. The predominant action of this reflex is to enhance muscle stiffness.

Electromyographic analysis of male rat perineal muscles during copulation and reflexive erections.

Anatomical examination of the ventral bulbospongiosus (BS) muscle suggested that its proximal and distal portions may act during penile erection as a two-stage pump governing the intensity of glans erections. The coordination between these portions of the BS, and of the proximal BS with the ischiocavernosus (IC) muscle, was studied using electromyographic (EMG) recordings taken during copulation and reflexive erections. Mounts without intromission were accompanied by either strong IC activity with little or no proximal BS activity, or strong proximal BS activity preceding the onset of IC activity. Activity in the proximal BS during mounts was variable in both duration and amplitude but uniform in frequency. During mounts with intromission, EMG activity of the proximal BS consisted of two characteristic phases, an early phase of low-amplitude activity which was similar to proximal BS activity during nonintromissive mounts, followed by an intromissive phase of high-amplitude, high-frequency activity. During intromission patterns, IC activity reliably preceded proximal BS activity. Ejaculations were accompanied by stronger proximal BS activity than were other copulatory events and were followed by a series of proximal BS and IC bursts lasting for 10-20 seconds. During reflexive erections, EMG activity in the proximal BS was always fusiform and varied with the intensity of erection only in frequency. In contrast to the proximal BS, activity in the distal BS was similar in frequency and amplitude across copulatory and reflexive events. These findings suggest that: a) different motoneuron pools serve the different portions of the BS muscle; b) the distal BS does not differentially affect glans erection but may serve primarily to promote rigidity of the portion of the bulb that it surrounds, while the proximal BS acts as the variable aspect of a hypothetical two-stage pump, and c) activity in the IC must precede activity in the proximal BS to achieve intromission.

Kinematic analysis of the defense response in crayfish.

1. A threatening visual stimulus frequently elicits the defense response (DR) in crayfish, a behavior that comprises orienting the body to face the stimulus, raising the thorax, and extending and opening the claws. Although this behavior has been reported previously, its kinematics have not been characterized. This work employs kinematic analysis to provide a quantitative description of the claw (cheliped) as it is moved during the DR. 2. The cheliped was modeled as an open kinematic chain with three segments and 4 df. Simulations employing the model were compared with actual cheliped trajectories during the DR to ascertain the applicability of the model. The model was then employed to demonstrate the effects of individual joint rotations on the overall trajectory of the claw. 3. The individual segments of the cheliped were monitored during the DR, and spatial trajectories, tangential velocities, and intersegmental joint angles were calculated. 4. The joint angles assumed at the final position of the DR were highly stereotyped. This constancy in joint angle at the endpoint of the movement stands in contrast to the variability in both the angular and spatial trajectories of the cheliped as it was moved towards the final position. 5. Movement time was relatively constant. Larger amplitude movements were performed with a proportional increase in velocity similar to arm movements in primates. 6. The DR positions the cheliped in a fixed location in the workspace. Unlike the primate arm during reaching, the cheliped does not proceed towards the endpoint with a smooth controlled trajectory characteristic of a system with fine interjoint coordination. Instead, it appears that individual joint rotations are performed independently, thus precluding trajectory control although permitting an accurate specification of the endpoint.

Mechanics of stretch in activated crustacean slow muscle. II. Dynamic changes in force in response to stretch.

1. The mechanical dynamics of the ventral superficial muscles (VSM) of the abdomen of the hermit crab, Pagurus pollicarus, have been analyzed to develop a quantitative model of gradedly excitable arthropod muscle. Such a model is important for understanding the role of proprioceptive reflexes in posture and movement. 2. The decay in force produced after ramp stretch of both passive and active muscle was approximated by the use of regression equations involving a direct term and one to three exponential terms. A second-order equation produced an acceptable description of this decay over short (0.5 s) sampling durations. 3. The rate constants of the regression equation did not vary with stretch length, velocity, or activation level of the muscle. For the two-exponential-term model, the rate constants were approximately 90 and 9 s-1 for a sample duration of 0.3 s. An additional rate constant of approximately 1 s-1 was needed to adapt the model to longer sample times. 4. The direct term and the middle-order (9 s-1) residual were both functions of stretch length and activation level. The high-order (90 s-1) residual was primarily a function of stretch length and velocity. Transfer functions omitting the velocity dependence adequately described the mechanical dynamics of the muscle for physiological ranges of stretch velocity. 5. White-noise length perturbations were used to calculate spectral density functions of muscle force and length. These measurements confirmed the principal observations of the ramp stretch analysis: the frequency response of the muscle was independent of the level of activation; the magnitude of the stiffness increased over the stretch frequency range of 4-40 Hz and was then almost constant; and the phase response of the muscle became slightly positive over the same range of stretch frequency. 6. The speed of activation of the muscle to different stimulus frequencies was estimated by fitting a single exponential equation to the rise in isometric tension at the onset of stimulation of the motor nerve. The rate constant increased with stimulus frequency, but its maximum value was only 1.8 s-1, about one-fourth of the middle mechanical rate constant. 7. Because muscle activation is slower than the mechanical dynamics, it is unlikely that the nervous system can regulate muscle dynamics. However, it is possible that mechanical impedance could be regulated to maintain a desired time-averaged value.

Mechanics of stretch in activated crustacean slow muscle. I. Factors affecting peak force.

1. The active stiffness of ventral superficial abdominal muscle (VSM) of the hermit crab, Pagurus pollicarus, was measured with ramp stretches of different amplitudes and velocities. Active stiffness was calculated by subtracting the peak force produced by passive stretch and the isometric force just before stretch from the peak force produced by stretching active muscle. The result was then divided by stretch length to give stiffness. 2. The relationship between force just before stretch (the level of activation) and active stiffness was curvilinear and was found to apply under a variety of experiment conditions. For pooled data from eight experiments, active stiffness (GN.m-2.m-1) = 3.2*stress (MN/m2)-7.6*stress2. Decreasing the number of motor units or activating the inhibitor did not alter this relationship nor did the addition of proctolin, octopamine, or 5-HT to the bath. The relationship also applied during the rising phase of isometric tension. However, stiffness declined more rapidly than predicted by this relationship after the end of tetanus. 3. Active stiffness varied inversely with stretch amplitude for fast stretches, and the slope of this relationship increased with increasing muscle activation. At lower stretch velocities, the slope was much less than at rapid stretch velocities, so at low levels of activation and stretch velocity, active stiffness was essentially independent of stretch length. 4. Active stiffness covaried with muscle force as both were sampled at shorter and shorter lengths on the ascending limb of the length-tension curve.(ABSTRACT TRUNCATED AT 250 WORDS)

An inexpensive, microcomputer-based system for recording movements in real time.

An inexpensive, microcomputer-based system was developed for recording movements in real time. The positions of 5 serially illuminated, infrared emitting diodes fixed on the object of interest were tracked by a remote, position-sensing photodiode. Positional information from the photodetector was digitized by an LSI 11/23+ microcomputer (sampling rate = 50/s/channel) and stored for subsequent analysis. Any microcomputer equipped to perform analog-digital conversion is compatible for use with this system. The small size of the components that are mounted on the animal allows movement to occur virtually unimpeded and makes this recording system especially suited for small animal observation.

Reflex control of dynamic muscle stiffness in a slow crustacean muscle.

The properties of a stretch reflex in the ventral superficial muscle of the hermit crab abdomen were studied in an isolated abdominal preparation to determine how the reflex affects the mechanical properties of the muscle and whether the reflex is controlling length, force, or stiffness. The reflex was elicited by stretch of hypodermal mechanoreceptors in the cuticle and resulted in the activation of excitor motoneurons to both circular and longitudinal layers of the muscle, thus stiffening the abdomen. The medial motoneuron of the longitudinal layer of the right fourth segment was selected for detailed analysis. It was tonically active and responded to stretch with a phasic burst having a latency of 100 ms. Reflex muscle tension began to increase at 130 ms and reached a peak at 300 ms. Reflex-burst frequency increased slightly with stretch amplitude. Peak force was an approximately linear function of stretch amplitude. No tonic component to the reflex was found in the medial motoneuron, in the central motoneuron (the smallest excitor to the muscle), or in the medial motoneuron studied in intact animals. The reflex-burst frequency was a function of stretch velocity, increasing between two and one-half to four times for a 10-fold increase in stretch velocity. Peak force was essentially independent of stretch velocity over this range. The reflex-burst frequency was not a function of the initial length of the muscle on the ascending limb of the length-tension relation. Active peak force (between two and three times passive peak force) was relatively constant over this range. The dynamic active stiffness (the resistance to stretch of the muscle when the nervous system was intact) was separated into two components. One component is that due to the tonic frequency of the motoneurons, the other to the reflex burst. The reflex component makes up a substantial part of the total active stiffness. Dynamic active stiffness is relatively constant under the conditions of these experiments and, when normalized, is similar to that observed in mammalian myotatic reflexes. This constancy, however, cannot be due to negative feedback control of stiffness, as in mammals. It is suggested that constant reflex stiffness arises from the combination of the low-pass filter characteristics of the muscle and the high-pass filter characteristics of the reflex over a restricted range of velocities.(ABSTRACT TRUNCATED AT 400 WORDS)