Thoracic-coxal Muscle Receptor Organ Research Articles

Information transfer rate of nonspiking afferent neurons in the crab.

The thoracic-coxal muscle receptor organ (TCMRO) is the only proprioceptor at the thoracic-coxal joint in the crab leg. The S and T afferent neurons of the TCMRO convey signals to the CNS solely by means of graded changes in membrane potential. The rate of information transfer of these afferents was determined by measuring the signal-to-noise ratio (SNuR) of these cells after repeated stimulation of the receptor with identical sequences of random movement and applying the Shannon formula for the information capacity of a Gaussian channel. Intracellular recordings were made from the S and T afferents adjacent to the transduction site at the origin of the receptor and along the axon 5-7 mm distal to this site. These nonspiking afferents transduce receptor movement and transmit this information with extremely high fidelity. The SNR of both neurons near the transduction site was >1000 over most of the 200 Hz stimulation bandwidth, and the mean information transfer rate was approximately 2,500 bits/s. When calculated over a wider bandwidth of 500 Hz, the information rate was >4,600 bits/s. The effect of axonal cable properties on the information rate was evaluated by determining the SNR from membrane potential recordings made 5-7 mm distal to the transduction region. The major effect of graded transmission along the axon was attenuation and low-pass filtering of the sensory signal. The consequent reduction in signal power and bandwidth decreased the information transfer by approximately 10-15% over 200 Hz and approximately 30% over a 500 Hz bandwidth.

Nonspiking and spiking proprioceptors in the crab: nonlinear analysis of nonspiking TCMRO afferents.

The proprioceptor that signals the position and movement of the first joint of crustacean legs provides an excellent system for investigating information processing and transmission in neurons that function in a graded (nonspiking) manner in the context of a simple motor system. The thoracic-coxal muscle receptor organ (TCMRO) spans the thoracic-coxal joint and transmits graded signals to the CNS via two large nonspiking axons. The response characteristics and nonlinear models of the input-output relationship for the two nonspiking TCMRO afferents (S and T fibers) were determined using white noise analysis (Wiener kernel) methods. The best-fitting linear responses of these neurons was similar, as were their second-order kernels. The gains of the afferents slowly increased with increasing frequency and reached a maximum at approximately 40-60 Hz for the S fiber and 60-80 Hz for the T fiber. Above this corner frequency, the gains of both afferents decreased at approximately 20 dB/decade for the remainder of the 220-Hz stimulus bandwidth. The shape of the first-order kernels, and hence the corresponding (linear) gain functions, of both afferents were similar when driven with different amplitudes of noise, covering a 40-fold amplitude range. Predictions of the S fiber response based on the first two Wiener kernels were accurate, with the second-order model producing a mean square error of 6-8%. Second-order Wiener models for the T fiber were less accurate with a mean square error of approximately 22-26%, but this accuracy improved to 10-16% with the incorporation of the third-order term in the Wiener expansion. The effect of cable properties on the transmission of the sensory potentials to the CNS was evaluated by determining the system characteristics using membrane potentials 5-7 mm distal to the transduction site. The major change after transmission along the axon was a low-pass filtering of the sensory signals and consequent reduction in signal bandwidth.

Nonspiking and spiking proprioceptors in the crab: white noise analysis of spiking CB-chordotonal organ afferents.

The proprioceptors that signal the position and movement of the first two joints of crustacean legs provide an excellent system for comparison of spiking and nonspiking (graded) information transfer and processing in a simple motor system. The position, velocity, and acceleration of the first two joints of the crab leg are monitored by both nonspiking and spiking proprioceptors. The nonspiking thoracic-coxal muscle receptor organ (TCMRO) spans the TC joint, while the coxo-basal (CB) joint is monitored by the spiking CB chordotonal organ (CBCTO) and by nonspiking afferents arising from levator and depressor elastic strands. The response characteristics and nonlinear models of the input-output relationship for CB chordotonal afferents were determined using white noise analysis (Wiener kernel) methods. The first- and second-order Wiener kernels for each of the four response classes of CB chordotonal afferents (position, position-velocity, velocity, and acceleration) were calculated and the gain function for each receptor determined by taking the Fourier transform of the first-order kernel. In all cases, there was a good correspondence between the response of an afferent to deterministic stimulation (trapezoidal movement) and the best-fitting linear transfer function calculated from the first-order kernel. All afferents also had a nonlinear response component and second-order Wiener kernels were calculated for afferents of each response type. Models of afferent responses based on the first- and second-order kernels were able to predict the response of the afferents with an average accuracy of 86%.

Morphology and organization of muscle fibres in the thoracic coxal muscle receptor organ and the associated promotor muscle, in a crayfish, Cherax destructor, and mud crab, Scylla serrata

AbstractUsing confocal laser scanning and conventional light microscopy, the morphology and organization of the muscle fibres in a proprioceptor, the thoracic coxal muscle receptor organ (TCMRO), and the associated ‘extrafusal’ promotor muscle were investigated in two species of decapod crustacea, the crayfish Cherax destructor and the mud crab Scylla serrata. The diameter of the TCMROs was shown to increase distally, with an increase up to 350% recorded for the crayfish. The tapered shape of the crayfish TCMRO was demonstrated to amplify movements mechanically at the transducer region where the afferent nerves attach. Serial sectioning of the TCMROs, showed that the fibre number increased in the proximal to distal direction from 14 to 30 fibres in the crayfish and from 7 to 20 in the crab. Optical sectioning with the laser scanning confocal microscope revealed that the increase in fibre numbers was the result of muscle fibres branching in the distal third section of the TCMRO. The percentage of muscle tissue in the cross‐sectional area in the TCMRO was found to be only 35.2% and 64.6% in the crayfish and crab, respectively. Longitudinal sectioning using laser scanning confocal microscopy revealed the average sarcomere length of the TCMRO muscle fibres of both species to be in the intermediate range for crustacean muscle fibres (4.1 ± 0.1 µm and 4.55 ± 0.34 µm for the crayfish and crab) compared with the long sarcomere muscle fibres in the associated promotor muscles (7.87 ± 0.2 and 10.6 ± 0.6 µm). The distinct morphology of the TCMRO muscle fibres – smaller diameter, intermediate sarcomere length and branching of fibres compared to the larger, long sarcomere promotor fibre muscle fibres – suggest that the TCMRO muscle fibres are specialized in their role of proprioception.

Morphology and organization of muscle fibres in the thoracic coxal muscle receptor organ and the associated promotor muscle, in a crayfish, Cherax destructor, and mud crab, Scylla serrata

Morphology and organization of muscle fibres in the thoracic coxal muscle receptor organ and the associated promotor muscle, in a crayfish, Cherax destructor, and mud crab, Scylla serrata

Ca2+- and Sr2+-activation properties of muscle fibres from a muscle receptor organ and the associated extrafusal muscle of the crab and crayfish.

In this study on decapod crustaceans, we examined the Ca2+- and Sr2+-activation properties of skeletal muscle fibres from an identified proprioceptor, the thoracic coxal muscle receptor organ (TCMRO) and its extrafusal promotor muscle fibres. Proprioceptors and extrafusal muscles were isolated from a walking leg from the crayfish (Cherax destructor) and the rear swimming leg of the mud crab (Scylla serrata). The crayfish and mud crab TCMROs had very low Hill coefficient (nCa) values (1.86 +/- 0.08 and 1.64 +/- 0.03, respectively). In comparison to other skeletal muscle fibre types these low Hill coefficients would enable the length of the receptor muscles to be finely controlled over a wide range of [Ca2+]. Maximum force was found to be significantly lower in the TCMROs (crayfish: 5.76 +/- 0.98; crab: 4.80 +/- 0.56 Ncm(-2)), compared to their associated extrafusal promotor muscle fibres (crayfish: 10.69 +/- 1.63; crab: 20.07 +/- 1.98 Ncm(-2)), which is consistent with their sensory role. The muscle fibres of the crayfish TCMRO had faster contractile properties than the mud crab TCMRO, we discuss how these contractile properties relate to the type of locomotion undergone by each leg. The mud crab 'red' promotor and all crayfish promotor fibres were characterised as slow with low Hill coefficients (nCa: crayfish: 3.22 +/- 0.29; crab: 3.34 +/- 0.29) and a contractile apparatus with a high sensitivity to Ca2+ (pCa50: crayfish: 6.42 +/- 0.03; crab: 6.18 +/- 0.03). In contrast the 'white' mud crab promotor fibres from the swimming leg had contractile properties that were characteristic of fast fibres with a high mean Hill coefficient (nCa: 5.27 +/- 0.76) and a lower Ca2+ sensitivity (pCa50: 6.03 +/- 0.03). The sensitivity of the contractile apparatus to Sr2+ was very low (range of mean pSr50: 4.23 +/- 0.03-3.48 +/- 0.06) and low force levels were produced in comparison to that produced with Ca2+. The results of this study show that the muscle fibres of the sensory receptor, produce less force and have been adapted to enable the length of the receptor to be finely set in relation to the length of the extrafusal muscle. We discuss how the striated fibres of the receptor have been adapted to perform a sensory role and how this is related to the type of locomotion undergone by the legs. We also discuss how the fibre types of the extrafusal muscle have adapted to the mode of locomotion.

Sensory feedback and central afferent interaction in the muscle receptor organ of the crab, Carcinus maenas.

1. An interaction exists between two proprioceptive afferent neurons innervating the thoracic-coxal muscle receptor organ (TCMRO) of the crab, Carcinus maenas. Intracellular recordings were made from the extraganglionic regions of the afferents in order to characterize this interaction and its effects on sensory feedback. 2. A current-induced depolarization of the nonspiking T fiber of the TCMRO results in a depolarization of the P fiber, a small-diameter (7 microns) neuron innervating the same receptor. This interaction is graded in amplitude, and may result in a single action potential being superimposed on the graded response of the P fiber. A hyperpolarization of the T fiber has a smaller effect on the P fiber than a depolarization of similar amplitude. The interaction is rectified in a T- to P-fiber direction, and has a minimum central delay of approximately 3.6 ms. 3. The site of the interaction between the afferents is situated centrally, within the thoracic ganglion. Action potentials evoked in the P fiber by a T-fiber depolarization propagate actively and antidromically to the periphery. 4. Central modulation of the interaction occurs, because the amplitude of a T-fiber-induced depolarization is reduced in the P fiber during centrally generated spontaneous bursts of activity in the motoneurons of basal leg muscles. 5. Because of the interaction between T and P fibers, action potentials recorded from the peripheral portion of the P fiber during receptor stretch may be either orthodromic, resulting directly from the effects of the stretch on the sensory endings of the P fiber, or antidromic, resulting from the central input from the T fiber. 6. The T- to P-fiber interaction may serve to extend the dynamic sensitivity range of the P fiber, in particular by amplifying its sensory response at short receptor lengths and low velocities of stretch.

Sensory characteristics of the P afferent neurone of the crab thoracic-coxal muscle receptor organ.

Intracellular recordings were made from the P fibre, the smallest of the three afferent neurones innervating the thoracic-coxal muscle receptor organ of the crab (Carcinus maenas). While the two larger afferents are nonspiking, the response of the P fibre to a trapezoidal change in receptor muscle length consists of a single action potential signalling the onset of stretch superimposed on a graded amplitude receptor potential. The P fibre is sensitive to the velocity of the applied stretch, but is insensitive to static joint position, stretch amplitude and the velocity of the release phase. The presence and amplitude of the action potential depends on the initial length of the receptor muscle, the tension caused by efferent activation of the receptor muscle prior to receptor stretch, and on the velocity of stretch. Length constant (1.9 mm) and specific membrane resistance (76 K omega x cm2) values obtained for the P fibre, together with its small diameter (7 microns) suggest that this neurone is less well adapted to conveying passive signals to the thoracic ganglion than are the S and T fibres. It is likely that the P fibre complements the length sensitivity of the S fibre and the tension and velocity sensitivity of the T fibre by signalling the onset of receptor stretch via single action potentials.

Proprioceptive Input From two Basal Joint Stretch Receptors to Leg Motoneurones in the Isolated Thoracic Ganglion of the Shore Crab

ABSTRACT The reflex effects and interactions of two proprioceptors upon motoneurones supplying the four basal leg muscles of the shore crab Carcinus maenas have been studied in a new in vitro preparation consisting of the thoracic-coxal muscle receptor organ (TCMRO) and the coxo-basal chordotonal organ (CBCO) isolated together with the whole thoracic ganglion complex to which they were still connected by their afferent nerves. Each receptor strand was stimulated mechanically, while recording intracellularly from motoneurones in the ganglion, and extracellularly from the cut motor nerves innervating the promotor and remotor muscles of the thoracic-coxal (T–C) joint and the levator and depressor muscles of the coxo-basal (C–B) joint. Stretch of the TCMRO evoked reflex firing in several units in the promotor motor nerve, confirming previous studies. In addition to this ‘intrajoint’ reflex, however, TCMRO stretch also elicited ‘interjoint’ reflex responses in moto-neurones of both the levator and depressor muscles. Similarly, stretch and release of the CBCO produced intrajoint resistance reflexes in levator and depressor motoneurones, respectively, as well as interjoint reflexes in promotor and remotor motoneurones. In general, the CBCO produced stronger reflex effects in all four motor nerves than did the TCMRO. Intracellular recordings from individual motoneurones of all four muscles revealed that the majority of them received convergent input from both proprio-ceptors. The importance of such convergent input in vivo is discussed.

Interaction between afferent neurones in a crab muscle receptor organ

Two of the three mechanoreceptor neurones which innervate the crab thoracic-coxal muscle receptor organ are non-spiking. The third, the small diameter P neurone, employs graded amplitude receptor potentials with either spikes or graded amplitude active membrane responses superimposed. An excitatory synaptoc connection between one of the larger non-spiking afferents and the P neurone has now been detected within the thoracic ganglion. This novel connection between two afferents innervating the same receptor strand effectively extends the dynamic sensitivity range of the P neurone.

Reflex actions of one proprioceptor on the motoneurones of a muscle receptor and their central modulation in the shore crab.

1. Reflex efferent control of a muscle stretch receptor by a joint proprioceptor of the same limb was studied in an isolated CNS preparation from the shore crab. The influence of 'fictive locomotor' activity on this interjoint reflex was also examined. 2. The thoracic-coxal muscle receptor organ (TCMRO) and the coxo-basal chordotonal organ (CBCO), which monitor movement and position of the first and second joints of the posterior leg, were isolated together with the whole thoracic ganglion complex. The TCMRO, functionally analogous to a mammalian muscle spindle, has two receptor motoneurones. RM1 innervating the receptor muscle alone and RM2 which also supplies the 'extrafusal' promotor muscle. The CBCO is a typical arthropod elastic strand organ, with many sensory neurones but lacking an efferent supply. The TCMRO was fixed at its mid-length, and stretch-hold-release stimuli were applied to the CBCO. Efferent activity was recorded from the cut nerve roots of the four basal limb muscles and intracellularly as excitatory junction potentials (EJPs) from the receptor muscle. 3. A dynamic increase in the frequency of action potentials in RM1 occurred on both stretch and release of the CBCO. During the hold phase the RM1 activity declined from the dynamic response but remained elevated compared to the resting tonic discharge. RM2, identified by EJPs occurring 1:1 with a unit in the promoter nerve, responded in a similar way. 4. One or more promotor motoneurones were usually co-activated with the two receptor efferents in response to input from the CBCO. In a typical example the average spike frequency of RM1 rose from 0 to 27 Hz during the dynamic phases (stretch and release) of the CBCO stimulus, falling to 2.5 Hz during the hold phase, while the corresponding promotor spike frequencies were 25 and 7.5 Hz, respectively. The other three muscle nerves recorded from generally also showed reflex driving by the CBCO. 5. The totally isolated thoracic ganglion could produce a rhythmic, bursting motor output in the absence of any sensory input. During this centrally generated activity the receptor motor innervation was strongly co-activated with the promotor bursts, and the reflex input from the CBCO was overridden or modulated in a phase-dependent manner. 6. The proximally directed interjoint reflex to the receptor muscle probably functions to maintain the tension on the sensory endings of the TCMRO, and so enable them to respond effectively at all times to movements of the basal leg joint.(ABSTRACT TRUNCATED AT 400 WORDS)

Proprioceptive reflex interactions with central motor rhythms in the isolated thoracic ganglion of the shore crab

Experiments were carried out on an isolated central nervous system preparation of the shore crab,Carcinus maenas, comprising the fused thoracic ganglion complex with two proprioceptors of one back leg still attached. These, the thoracic-coxal muscle receptor organ and the coxo-basal chordotonal organ, monitor movement and position of the first and second joints, respectively. Motor activity was recorded extracellularly from the central cut ends of the nerves innervating the promotor and remotor muscles of the thoracic-coxal joint, and the levator and depressor muscles of the coxal-basal joint of the same leg. Simultaneous intracellular recordings were made from central processes of individual motoneurones of each muscle.

Action potentials in a ‘non-spiking’ neurone: graded responses and spikes in the afferent P fibre of the crab thoracic-coxal muscle receptor organ

The small diameter P neurone (7 μm) is here characterized as an afferent of the crab thoracic-coxal muscle receptor organ (TCMRO), in addition to the S and T neurones (50–60 μm). The P neurone employs both passively conducted, graded amplitude receptor potentials and, superimposed upon these, either graded active membrane responses or constant amplitude action potentials. Intracellularly recorded receptor potential waveforms reflect the dynamic component of stretch more strongly than receptor length, while action potentials at the onset of stretch accentuate the phasic response characteristics of this neurone.

After-effects following responses of a muscle stretch receptor of the shore crab, Carcinus maenas

Stretch evoked responses of the thoracic-coxal muscle receptor organ in the crab are shown to be time dependent. Thus, by varying the interstretch interval, the duration of the T fibre dynamic component of the response to a ramp-and-hold stretch was found to be directly related to the inter-stretch interval. With long interstretch intervals (e.g. 10 min), the T fibre dynamic component of the response to stretch was indistinguishable from the response when pre-conditioned with stimulation of the receptor motor nerve. These observations are consistent with the hypothesis of spontaneous cross-bridge formation when the receptor is quiescent. Such properties lead to a source of ambiguity of the muscle afferent response, which are presumably overcome by maintaining tonic efferent activation of the receptor.

Intersegmental Coordination of Central Neural Oscillators for Rhythmic Movements of the Walking Legs in Crayfish, Pacifastacus Leniusculus

ABSTRACT The neuronal circuits underlying rhythmical movements of the walking legs in crayfish Pacifastacus leniusculus are organized into central oscillators within each thoracic hemiganglion. These segmental networks are coupled in different modes of coordination during activities such as walking and limb waving. This paper examines the nature of the coupling between hemisegmental oscillators in the absence of sensory feedback, and the effect of phasic input from a major limb proprioceptor. In isolated preparations of the thoracic nerve cord, adjacent ipsilateral hemiganglia can generate a rhythmic motor output pattern in which homologous motor roots discharge approximately synchronously (the ‘in-phase’ rhythm). In contrast, contralateral hemiganglia show no evidence of any such phase-locking in their activity. A single proprioceptor at the base of each limb, the thoracic-coxal muscle receptor organ (TCMRO), can influence the timing and intensity of the rhythmic output of two or more ipsilateral thoracic ganglia. Rhythmical stretch-release of the TCMRO of the fourth ganglion on one side can entrain the motor rhythms of both the third and fourth ipsilateral hemiganglia, in the in-phase pattern of coordination. The TCMRO also elicits intra- and intersegmental reflexes during expression of the in-phase motor rhythm. These reflexes are centrally modulated in a phasedependent manner. Thus mechanical stimulation of the TCMRO co-activates homologous sets of motoneurones in adjacent ipsilateral hemiganglia only during their active (spiking) phase in the motor rhythm. We discuss the implications of these results for the generation and coordination of walking and other rhythmic behaviour of the thoracic limbs in decapod Crustacea. Our data shed further light on the properties of the motor rhythm generator for walking in crayfish, and suggest that the concept of a central pattern generator cannot be rigorously applied in this system.

Slow Active Potentials in Walking-Leg Motor Neurones Triggered by Non-Spiking Proprioceptive Afferents in the Crayfish

ABSTRACT Intracellular recordings have been made from walking-leg motor neurones of the crayfish, Pacifastacus leniusculus, in isolated preparations of the thoracic ganglia. Some motor neurones display slow depolarizations that can drive bursts of spikes and resemble ‘plateau’ potentials described in other invertebrate and vertebrate neurones. Evidence is presented which suggests that the potentials are regenerative and endogenous to the motor neurones, and are not the result of feedback from a neural network. These potentials can be induced by synaptic inputs from the non-spiking afferent neurones of the thoracic-coxal muscle receptor organ, a basal limb proprioceptor. Reflex input from this receptor is augmented during the active depolarization of the motor neurone. The results are discussed in terms of the control of rhythmic motor output and the central modulation of reflexes in the crayfish’s thoracic nervous system.

Two identified afferent neurones entrain a central locomotor rhythm generator

Sensory feedback can modulate the intensity and timing of the central rhythms underlying locomotion and adjust them to compensate for natural or experimental perturbations1–4. However, because of the complexity of the neural systems involved, the role of specific sense organs and the function of individual afferent neurones are poorly understood. The thoracic-coxal muscle receptor organ (TCMRO), a proprioceptor of the crayfish walking leg, has only two afferent neurones, the non-spiking S and T fibres. Their receptor potentials encode, respectively, the magnitude and velocity of receptor muscle stretch, which occurs during limb remotion (retraction). Rhythmically stretching the TCMRO entrains a central locomotor rhythm, produced by the thoracic ganglia, in which remotor and promotor motoneurones of the leg discharge in alternation. Intracellular stimulation of the S and T fibres can trigger promotor and remotor bursts, respectively. Here we propose a mechanism for proprioceptive entrainment in terms of the opposite feedback effects of these two afferents. The possible role of these effects in the feedback control of locomotion is also discussed.

Reversal of an Intersegmental Reflex Elicited by A Muscle Receptor Organ

ABSTRACT One of the most common reflexes observed in invertebrates is the resistance reflex, in which a proprioceptor at a given joint excites the motor neurones innervating the muscles controlling the joint so as to resist imposed movement (Bush, 1962, 1963, 1965; Barnes, Spirito & Evoy, 1972). As well as these intrasegmental reflexes, proprioceptive reflexes in which sensory information is used to control muscles at joints other than the one spanned by a given proprioceptor have been reported (Ayers & Davis, 1977; Clarac, Vedel & Bush, 1978). These intersegmental reflexes may also control the state of excitation of other receptors (Moody, 1970; Clarac et al. 1978). Resistance reflexes and intersegmental reflexes are presumed to have a role in postural maintenance and load compensation (op. cit.; for review see Mill, 1976). However, resistance reflexes can be suppressed during locomotion (Barnes et al. 1972) and intersegmental reflexes may be modified during walking (Ayers & Davis, 1977). In addition, we have recently shown that a resistance reflex elicited by the thoracic-coxal muscle receptor organ (TCMRO) in the crab can be reversed to become a positive feedback assistance reflex (DiCaprio & Clarac, 1981). In this report we demonstrate that an intersegmental reflex activation of the basipodite levator and depressor motor neurones mediated by the TCMRO can also be reversed with respect to movement of the thoracic-coxal (TC) joint.

Dual reflex motor control of non-spiking crab muscle receptor

The thoracic-coxal muscle receptor organ (T-C MRO) in crabs mediates concurrent reflex excitation (positive feedback) and inhibition (negative feedback) of the one of its two motoneurones with the greater contractile effect on the receptor muscle (RM), viz. the specific receptor motoneurone, Rm1 (analogous to mammalian γ motoneurones). 1. In most preparations the tonic firing frequency of Rm1 declines progressively with increasing RM length over the physiological range (Figs. 1–3). 2. Selectively ablating the displacement sensitive afferent, the S fibre, or damaging its membrane, reverses this RM length-dependent tonic decline, and enhances the dynamic Rm1 response to receptor stretch (Fig. 4). 3. Conversely, ablation of the tension-sensitive T fibre abolishes, and T fibre membrane damage reduces, the Rm1 reflex response to receptor stretch. 4. Intracellular injection of constant current pulses confirms the excitatory drive of Rm1 by the T fibre and the inhibitory influence of the S fibre. Thus while depolarisation of the T fibre excites Rm1, S fibre depolarisation inhibits it; conversely hyperpolarisation of the T fibre abolishes ongoing tonic activity whereas S fibre hyperpolarisation (at extended RM lengths) enhances tonic activity of Rm1 (Figs. 5, 6). 5. Ramp-function 1 mm stretches of constant velocity applied at different RM lengths over the full in vivo range of 5 mm result in invariant Rm1 dynamic reflex frequencies. The dynamic positive feedback reflex is thus stabilized with respect to RM length (Fig. 8). 6. Experiments involving current-induced S fibre hyperpolarisation concurrent with rampfunction stretch stimuli, demonstrate unequivocally the inhibitory role of the S fibre in effecting this dynamic reflex stabilization (Fig. 9). 7. The small dynamic sensitivity of the S fibre (Table 1) does not significantly affect the reflex expression of T fibre dynamic responsiveness, so that Rm1 reflex output remains a direct function of the logarithm of stretch velocity at any given initial RM length (Figs. 7, 9).

Positive feedback to a muscle receptor stabilized by concurrent self-inhibition

The two non-spiking afferent neurones (T and S fibres) of the crab's thoracic-coxal muscle receptor organ mediate antagonistic reflex controls upon the specific receptor motoneurone (Rm1). Depolarization of the 'in-series' T fibre, by receptor muscle stretch or intracellular current injection, reflexly excites Rm1, whereas length- or current-induced S fibre depolarization inhibits Rm1. The latter, autogenetic negative feedback stabilizes the concurrent positive feedback over the physiological range of receptor muscle lengths.