Morphology and neurophysiology of tarsal vibration receptors in the water strider Aquarius paludum (Heteroptera: Gerridae)

Substrate vibratory information receptors are extensively studied in insects and spiders, however for water surface dwelling species little data is available. We studied the vibration receptive organs in tarsi of the water strider Aquarius paludum, using light, transmission and scanning electron microscopes, and recorded the neural activity of the organs in response to vibrational stimuli, which were afterwards analysed with a custom made spike sorting program. We found that the tarsal chordotonal organ has one set of three scoloparia: one in the tarsomere I and two in the tarsomere II, all of which consisted of a few scolopidia. The chordotonal organ clearly responded to vibratory stimulation. Furthermore, we found that a pair of large subapical emergent dorsal setae, which had been deemed mechanosensory by previous authors, are not so. In turn, four ventral subapical trichobothria that are in direct contact with the water surface during locomotion, proved to be mechanosensory. The anatomical and ultrastructural observations support these electro-physiological results.

Different effects of spinally applied prostaglandin D2 on responses of dorsal horn neurons with knee input in normal rats and in rats with acute knee inflammation

Prostaglandin D2(PGD2) is the most produced prostanoid in the CNS of mammals, and in behavioral experiments it has been implicated in the modulation of spinal nociception. In the present study we addressed the effects of spinal PGD2 on the discharge properties of nociceptive spinal cord neurons with input from the knee joint using extracellular recordings in vivo, both in normal rats and in rats with acute inflammation in the knee joint. Topical application of PGD2 to the spinal cord of normal rats did not influence responses to mechanical stimulation of the knee and ankle joint except at a high dose. Specific agonists at either the prostaglandin D2 receptor 1 (DP1) or the prostaglandin D2 receptor 2 (DP2) receptor had no effect on responses to mechanical stimulation of the normal knee. By contrast, in rats with inflamed knee joints either PGD2 or a DP1 receptor agonist decreased responses to mechanical stimulation of the inflamed knee and the non-inflamed ankle thus reducing established inflammation-evoked spinal hyperexcitability. Vice versa, spinal application of an antagonist at DP1 receptors increased responses to mechanical stimulation of the inflamed knee joint and the non-inflamed ankle joint suggesting that endogenous PGD2 attenuated central sensitization under inflammatory conditions, through activation of DP1 receptors. Spinal application of a DP2 receptor antagonist had no effect. The conclusion that spinal PGD2 attenuates spinal hyperexcitability under inflammatory conditions is further supported by the finding that spinal coapplication of PGD2 with prostaglandin E2 (PGE2) attenuated the PGE2-induced facilitation of responses to mechanical stimulation of the normal joint.

Ultrastructure of tarsal sensilla and other integument structures of twoPseudocellus species (Ricinulei, Arachnida)

Ricinuleids are one of the least investigated groups of Arachnida. In particular, knowledge of their ultrastructure is poor. Observations of the distal tarsomeres of ricinuleids show differences in their shape and equipment of surface structures. Legs I and II are used by the Ricinulei to explore their surroundings with tentative movements. The tarsomeres of these legs show similarities in shape and surface structures that distinguish them from those of legs III and IV. In this study, 11 different structures of the tarsomere surfaces of two cave-dwelling species, Pseudocellus pearsei and P. boneti from México, were investigated for the first time with scanning and transmission electron microscopy and discussed regarding their possible function: 1) a single treelike ramifying seta resembles a no pore single-walled (np-sw) sensillum; 2) setae occurring in a small number and possessing a bipartite shaft represent terminal pore single-walled (tp-sw) sensilla. The surface of the proximal half of the shaft shows small branches. The distal half has a smooth surface; 3) long setae with conspicuous longitudinal lamellae show characteristics of chemoreceptive wall pore single-walled (wp-sw) sensilla; 4) frequent small wp-sw sensilla with flat and irregular lamellae; 5) very short wp-sw sensilla occurring solitary or in groups; 6) a few short setae with smooth surface correspond to wp-sw sensilla; 7) a single short clubbed seta articulating in a flat pit is considered to be a np-sw sensillum; 8) common long setae with a pointed tip show characteristics of mechanoreceptive np-sw sensilla; 9) ventral setae with adhesive and mechanosensory function are accompanied by multicellular "class III" glands; 10) slit organs with mechanoreceptive function; and 11) dome-like tubercles with no indication of sensorial function. Several of these sensilla form a sensory field on the dorsofrontal surface which is particularly pronounced on the distal tarsomeres of legs I and II.

Adjuvant-induced joint inflammation causes very rapid transcription of beta-preprotachykinin and alpha-CGRP genes in innervating sensory ganglia

Neuropeptides synthesized in dorsal root ganglia (DRG) have been implicated in neurogenic inflammation and nociception in experimental and clinical inflammatory arthritis. We examined the very early changes in response to adjuvant injection in a rat model of unilateral tibio-tarsal joint inflammation and subsequent monoarthritis. Within 30 min of adjuvant injection ipsilateral swelling and hyperalgesia were apparent, and marked increases in beta-preprotachykinin-A (beta-PPT-A) and alpha-calcitonin gene-related peptide (CGRP)-encoding mRNAs were observed in small-diameter L5 DRG neurones innervating the affected joint. This response was augmented by recruitment of additional small-diameter DRG neurones expressing beta-PPT-A and CGRP transcripts. The increased mRNA was paralleled by initial increases in L5 DRG content of the protein products, substance P and calcitonin gene-related peptide. Within 15 min of adjuvant injection there were increases in electrical activity in sensory nerves innervating a joint. Blockade of this activity prevented the rapid induction in beta-PPT-A and CGRP mRNA expression in DRG neurones. Increased expression of heteronuclear (intron E) beta-PPT-A RNA suggests that increases in beta-PPT-A mRNA levels were, at least in part, due to transcription. Pre-treatment with the protein synthesis inhibitor cycloheximide had no effect upon the early rise in neuropeptide mRNAS: This and the rapid time course of these changes suggest that increased sensory neural discharge and activation of a latent modulator of transcription are involved.

An electrophysiological investigation into the monosynaptic H-reflex in the rat

In order to set up a non-invasive, reliable and reproducible model for investigating alpha-motoneuronal activity, we studied the electrophysiological features of a monosynaptic H-reflex in anaesthetised intact rats, anaesthetised and non-anaesthetised rats transected at the level of the obex. Electrical stimulation of the tibial nerve at the ankle elicited an H-reflex, an F-wave and a direct motor (M) response in the plantaris muscles of all preparations. The H-reflex and F-wave exhibited very similar latencies. The H-reflex had a low threshold and a constant latency. Its amplitude increased as a function of stimulus intensity to reach a maximum value but then decreased when the stimulus intensity was further increased. It could follow high rates of stimulation without any change in shape or latency. The F-wave had a lower amplitude which together with its latency varied from one stimulus to the next. It appeared with intensities of stimulation that elicited an almost maximal M-response and did not decrease when the stimulation was increased. It did not appear systematically from one stimulus to the next. The H-reflex, but neither the F-wave nor the direct motor M-response, was depressed both by vibratory stimuli applied on the Achilles' tendon and following nociceptive stimulation of the flexor reflex afferents. This model could be used for assessing any potential direct effect on motoneurones of a physiological or pharmacological conditioning procedure.

The effect of stimulus pulse duration on selectivity of neural stimulation

Choice of stimulus parameters is an important consideration in the design of neural prosthetic systems. The objective of this study was to determine the effect of rectangular stimulus pulsewidth (PW) on the selectivity of peripheral nerve stimulation. Computer simulations using a cable model of a mammalian myelinated nerve fiber indicated that shorter PW's increased the difference between the threshold currents of fibers lying at different distances from an electrode. Experimental measurements of joint torque generated by peripheral nerve stimulation demonstrated that shorter PW's generated larger torques before spillover and created a larger dynamic range of currents between threshold and spillover. Thus, shorter PW's allowed more spatially selective stimulation of nerve fibers. Analysis of the response of a passive cable model to different duration stimuli indicated that PW dependent contributions of distributed sources to membrane polarization accounted for the observed differences in selectivity.

Projections of ankle joint afferents to the spinal cord and brainstem of the chicken (Gallus g. domesticus)

The projections of the ankle joint capsule afferents were studied by transganglionic transport of horseradish peroxidase injected directly into the ankle joint. The number and size of the labelled dorsal root ganglion cells were measured from synsacral nerves 2-9. In the dorsal root ganglia, all sizes of sensory neurones were labelled, and the largest number of labelled cells was in ganglia 5-7. The extensive sympathetic innervation of the ankle joint was identified by the large number of cell bodies labelled in the sympathetic ganglia of the paravertebral chain. Labelled afferent fibres projected to the spinal cord from the 2nd to the 8th synsacral nerves, with the rostral projection mainly via Lissauer's tract and the dorsal funiculus. Terminal labelling in the dorsal horn was identified in laminae I-III and VI, with a slight projection to V. Two areas of dense labelling, which did not correspond with the largest number of labelled dorsal root ganglion cells, were identified. A rostral area with the highest density of label was observed at the level of synsacral nerves 3-4 and a second slightly less dense area between synsacral nerves 7-8. In the caudal medulla, diffuse terminal labelling was observed in the nucleus gracilis et cuneatus, nucleus of the tractus solitarius, and the nucleus cuneatus externus. These results are discussed in a comparative context to identify similarities and differences between different primary afferent projections in birds and mammals and to highlight the possible functional significance of the avian articular afferent projection.

Effects of surgical denervation on substance P and calcitonin gene-related peptide in adjuvant arthritis

The occurrence of substance P (SP) and calcitonin gene-related peptide (CGRP) was assessed in the ankle joints and dorsal root ganglia (L2-L6) by immunohistochemistry and radioimmunoassay (RIA) after unilateral sciatic nerve section in adjuvant arthritis. Nerve section in arthritic rats had no clear-cut effect on warmth, redness, and swelling of ankle joints, but significantly affected the occurrence of SP and CGRP. Immunohistochemical analysis showed an almost complete disappearance of SP-positive fibers in the ipsilateral ankle joint in normal rats after axotomy, whereas in arthritic rats occasional SP-positive fibers remained. In dorsal root ganglia, only occasional SP-positive cells could be detected in normal and arthritic rats after axotomy. A similar but somewhat less pronounced effect of axotomy was noted for neuronal CGRP-LI. RIA showed a decrease in SP in ankle joints by 45% in normal rats and 58% in arthritic rats; the decrease in CGRP was 41% and 47%, respectively. In dorsal root ganglia, the decrease in SP after surgical denervation was 25% in normal rats and 54% in arthritic rats; the decrease in CGRP was 18% and 27%, respectively. The tissue concentrations of SP and CGRP in ipsilateral ankle joints and dorsal root ganglia were consistently correlated in normal as well as arthritic rats. The present study shows that an interruption of the nerve supply to joints cannot fully prevent the development of arthritis, although it significantly reduces the occurrence of sensory neuropeptides.

Early postnatal changes in the somatodendritic morphology of ankle flexor motoneurons in the rat

The development of locomotor function in the rat spans the first 3 postnatal weeks. We have studied morphological features of the soma and dendrites of motoneurons innervating the physiological flexor muscles of the ankle, tibialis anterior and extensor digitorum longus, by intracellular injection in vitro between the first and ninth postnatal days. We obtained serial optical sections of 96 adequately filled motoneurons in whole-mounted hemisected spinal cords by confocal microscopy, projected them onto a single plane and analysed them morphometrically. On the day after birth, the somatodendritic surfaces of most such motoneurons were covered in growth-associated spiny, thorny or hair-like appendages. These had disappeared from the soma by the fourth postnatal day and from most proximal dendrites by day 7, but were still common distally on day 9. During this period there was little or no net growth of either the soma (which was still much smaller than in the adult) or the dendritic tree. A dorsal dendritic bias was present and 'sprays' of long, loosely bundled dorsal dendrites were often seen. The mean number of primary dendrites remained constant at about eight, and their combined diameter was already significantly correlated with mean soma diameter, as in the adult cat. Thus, the critical neonatal period during which these ankle flexor motoneurons are known to change their electrophysiological properties and to be particularly sensitive to interference with neuromuscular interaction is characterized by major changes in the neuronal surface, presumably linked to synaptogenesis.

The effects of capsaicin, bradykinin, PGE2 and cicaprost on the discharge of articular sensory receptors in vitro

The responses of articular sensory receptors to capsaicin, bradykinin, PGE2, and the selective IP-receptor agonist cicaprost were studied in a rat isolated hindlimb in vitro preparation. Long-term maintenance of normal sensory receptor function was achieved in vitro under conditions of combined superfusion and slow perfusion. Response characteristics to mechanical or chemical stimuli on articular sensory receptors identified in this study did not differ to those reported in vivo. This preparation lacks complex effects mediated via spinal or central reflex mechanisms and allows greater control over the physiological environment of the receptors being studied. These results support the conclusion that the effects of capsaicin, bradykinin and the prostanoids are mediated by distinct pharmacological receptors associated with articular sensory nerve endings. The potent potentiating effects of cicaprost on bradykinin-induced excitation suggests that these actions are mediated via IP-receptors.

The locomotor discharge characteristics of ankle flexor gamma-motoneurones in the decerebrate cat

1. The discharge patterns of ankle flexor, tibialis anterior (TA), gamma-motoneurones were recorded during locomotion in the decerebrate cat. 2. At rest gamma-efferents had no background discharge. During locomotion two patterns of gamma activity could be distinguished. Most units (16) were phasically recruited with homonymous electroneurogram (ENG) activity, while the remainder (5) were tonically active throughout the step cycle. 3. The modulation of phasic units was greater (P < 0.01) than tonic neurones. Phasic units had lower (P < 0.02) mean, but higher (P < 0.01) peak, rates during the step cycle. 4. The discharge rate of both types of efferent increased around the onset of ENG activity and peaked during ENG activity, or shortly after its cessation. The conduction velocities of phasic and tonic units overlapped widely. 5. It is proposed, on the basis of muscle spindle afferent recordings during locomotion, that TA phasic and tonic units correspond to static and dynamic gamma-motoneurones, respectively. This correspondence is functionally advantageous for the role of ankle flexor muscles during locomotion. Thus phasic static gamma discharge during flexion would aid muscle contraction via increased Ia afferent activity, while tonic dynamic gamma firing would enhance Ia afferent stretch sensitivity throughout the step cycle. Such enhancement during flexion would oppose unexpected muscle lengthening while, during extension, it would contribute to reciprocal inhibition of ankle extensor muscles. 6. The results are discussed in relation to strategies of gamma usage during rhythmic movements. It is postulated that, for such behaviour, muscle contraction is accompanied by coactivity in static and dynamic gamma-motoneurones. A functional rationale is suggested for this strategy.

Control of torque direction by spinal pathways at the cat ankle joint

To study the biomechanics of the calcaneal tendon's complex insertion onto the calcaneus, we measured torque-time trajectories exerted by the triceps surae and tibialis anterior muscles in eight unanesthetized decerebrate cats using a multi-axis force-moment sensor placed at the ankle joint. The ankle was constrained to an angle of 110 degrees plantarflexion. Muscles were activated using crossed-extension (XER), flexion (FWR), and caudal cutaneous sural nerve (SNR) reflexes. Torque contributions of other muscles activated by these reflexes were eliminated by denervation or tenotomy. In two animals, miniature pressure transducers were implanted among tendon fibers from the lateral gastrocnemius (LG) muscle that insert straight into the calcaneus or among tendon fibers from the medial gastrocnemius (MG) that cross over and insert on the lateral aspect of calcaneus. Reflexively evoked torques had the following directions: FWR, dorsiflexion and adduction; SNR, plantarflexion and abduction; and XER, plantarflexion and modest abduction or adduction. The proportion of abduction torque to plantarflexion torque was always greater for SNR than XER; this difference was about 50% of the magnitude of abduction torque generated by tetanic stimulation of the peronei. During SNR, pressures were higher in regions of the calcaneal tendon originating from MG than regions originating from LG. Similarly, pressures within the MG portion of the calcaneal tendon were higher during SNR than during XER, although these two reflexes produced matched ankle plantarflexion forces. Selective tenotomies and electromyographic recordings further demonstrated that MG generated most of the torque in response to SNR, while soleus, LG, and MG all generated torques in response to XER.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic changes in the receptive field properties of spinal cord neurons with ankle input in rats with chronic unilateral inflammation in the ankle region

The aim of this study was to determine the discharge and receptive field properties of spinal cord neurons with ankle input in spinal segments L4-6 in the rat, both under control conditions and during the course of an adjuvant-induced unilateral inflammation in the ankle. The extent of receptive fields in the skin and deep tissue was assessed using brush, pinch and compression stimuli. Neurons were categorized as nociceptive-specific or wide-dynamic-range neurons on the basis of their response thresholds and responses to suprathreshold stimuli. At all stages of inflammation (2, 6, 13 and 20 days post inoculation) the population of neurons with ankle input showed differences from the population of neurons with ankle input in control rats. There was a reduction in the number of neurons that appeared as nociceptive specific and a concomitant increase in the number of neurons showing a wide-dynamic-range response profile. The receptive fields of the neurons with ankle input were markedly larger in rats with inflammation in the ankle region and mainly spread proximally on the ipsilateral hindlimb and also to the abdomen and tail in some cases. There was also an increase in the number of neurons with contralateral excitatory inputs. The mechanical thresholds at the ankle joint and proximal parts of the ipsilateral hindlimb were less in arthritic rats than in controls. The proportion of spontaneously active neurons was also increased in rats during the initial and later stages of inflammation, although there was no significant increase in the mean spontaneous discharge frequency. These data show that there are long-term changes in the receptive field and response properties of neurons in intact rats with chronic unilateral adjuvant-induced inflammation similar to those described previously in spinal cats with acute inflammation (Neugebauer and Schaible 1990). It is presumed that similar afferent and spinal mechanisms are at work under acute and chronic inflammatory conditions which produce hyperexcitability in spinal neurons with joint input.

Reinnervation of denervated extensor digitorum longus of the rat by the nerve of the soleus does not induce the type I myosin synthesis directly but through a sequential transition of type II myosin isoforms

The fast-contracting extensor digitorum longus (EDL) muscle of 1-month-old rats was denervated and reinnervated by the nerve innervating the slow-contracting soleus muscle. After variable periods of time, the myosin isoform content of the EDL was analyzed by sensitive electrophoretic techniques, which allowed to discriminate between the slow-type I and the three, IIA, (IID or IIX) and IIB, fast-type II myosin isoforms. Compared to the control EDL, which contains predominantly the IIB isoform, the operated muscles contained variable proportions of all the isoforms. Analysis of the results leads us to conclude that reinnervation of EDL induces a sequential transition of myosin isoforms: IIB----(IID or IIX)----IIA----I.

Effects of paracetamol and aspirin on neural activity of joint mechanonociceptors in adjuvant arthritis

1. The effects of paracetamol and lysine acetylsalicylate (L-AS) on high-threshold mechanonociceptors have been investigated by recording neural activity from the inflamed ankle joint in anaesthetized rats with mild adjuvant-induced monoarthritis. 2. Paracetamol (50 mg kg-1, i.v.) and L-AS (100 mg kg-1, i.v., equivalent to 50 mg kg-1 aspirin) both caused a maximal reduction of about 40% in mechanically-evoked discharge and of 30% in ongoing (spontaneous) activity by about 15 min after the injection: a second dose of either drug did not have any significant additional effect on discharge. 3. The prostanoid IP receptor agonist, cicaprost (0.1-0.5 micrograms), increased both mechanically-evoked and ongoing discharge to pre-paracetamol levels when injected close-arterially 30-50 min after paracetamol, whereas prostaglandin E2 (PGE2) was relatively ineffective at restoring activity. 4. The results suggest that prostacyclin (PGI2) contributes to the sensitization of high-threshold joint mechanonociceptors in adjuvant-induced monoarthritis, and that paracetamol and L-AS both act to reduce discharge by inhibiting the synthesis of prostacyclin in the joint capsule. 5. Paracetamol has a direct peripheral action affecting joint capsule mechanonociceptors in rat adjuvant-induced arthritis which is very similar to that of the soluble aspirin preparation, L-AS. These findings, together with the existing literature concerning the anti-arthritic effects of paracetamol, are relevant to the treatment of chronic inflammatory disorders such as rheumatoid arthritis.

Coordination of motor pools controlling the ankle musculature in adult spinal cats during treadmill walking

The coordination of the motor pools of two ankle plantar-flexor, i.e. the soleus (Sol) and medial gastrocnemius (MG), and an ankle dorsiflexor, i.e. the tibialis anterior (TA) was quantified by comparing the EMG amplitude relationships in muscle pairs in normal and trained adult spinalized cats during treadmill walking across a range of relatively slow speeds (0.1 to 1.0 m/s). The effects of increased tactile stimulation or loading on locomotor performance were also studied in the spinal cats. Joint probability density distributions in the spinalized cats showed a low level of MG activation relative to Sol which did not change as speed increased. In general, the coordination between Sol and MG was similar in normal and spinal cats. However, towards the final phase of the extensor burst, the MG EMG amplitude decayed prematurely in spinal cats, particularly at higher speeds. Preferential elevation of MG relative to Sol activity was seen as a result of tactile stimulation. An elevated load resulted in a higher level of MG activation relative to Sol, prolonged MG activity at the end of the extensor burst, and the reduction in the clonic pattern of EMG typical of spinal cats. Spinalized cats showed an increased incidence of Sol-TA coactivation, especially at the higher speeds, due in part to the tonic activity in the TA. However, the overall reciprocal relationship between these antagonists was maintained. This reciprocity was preserved, but the high level of coactivation was unaffected by tactile stimulation. An elevated load, however, resulted in less Sol-TA coactivation. These results suggest that the coordination between synergists (Sol-MG) and between antagonists (Sol-TA and MG-TA), as well as the level of activation are modulated in the adult spinal cat similar to that observed in the normal cat. Further, there are specific types of proprioceptive-cutaneous information that can affect selected phases of the step cycle such that full weight-supporting stepping is significantly improved.

Segmental reflex action in normal and decerebrate cats

1. The objective of this study was to evaluate the action of the stretch reflex on the ankle extensor muscles of normal and decerebrate cats. 2. Experiments were performed on nine freely standing, unrestrained cats and repeated after decerebration at the premammillary level. The length, force, and electromyograph (EMG) of the soleus (SOL) and lateral gastrocnemius (LG) muscles were recorded with the use of implanted transducers and electrodes. 3. The left ankle joint was unexpectedly and reproducibly dorsiflexed by briefly stimulating the common peroneal (CP) nerve with electrodes within an implanted nerve cuff. The ensuing twitch contractions of the ankle dorsiflexor muscles stretched the ankle extensor muscles by 0.3-2.0 mm. Lidocaine was infused into another nerve cuff proximal to the stimulation site, to reversibly block the central propagation of evoked volleys in the CP nerve. 4. Reflex action before and after decerebration was measured from the responses to perturbations of similar amplitude and duration delivered at approximately matched background values of muscle length and force. In most cats the temperature of the hindlimb was monitored with an implanted thermistor and was restored to normal values with radiant heat after decerebration. 5. A stretch imposed on the tonically active ankle extensor muscles immediately caused a considerable rise in the force recorded from the triceps tendon. Within 30-40 ms the triceps force peaked, reaching a value 10-20 N greater than background, and then rapidly declined while the extensor muscles were still lengthening. The initial rise in force preceded any change in triceps EMG. It was attributed to the intrinsic viscoelasticity of the stretched muscles and tendons. After decerebration the magnitude and timing of the initial force peak did not change. 6. A short-latency reflex EMG burst was typically recorded from both the SOL and LG muscles, starting 11-17 ms after stimulus onset. After decerebration the area of the reflex EMG burst increased in all nine cats, typically by a factor of 2 or 3. 7. After decerebration a second, smaller increase in force was typically observed starting 60-80 ms after onset of stretch. This later force rise, interpreted to be of reflex origin, was rarely apparent in normal cats. 8. Decerebration introduced consistent modifications in postural behavior that were revealed by pushing down on the back of quietly standing cats. In normal cats, after brief pushes that stretched the ankle extensor muscles by 1-2 mm, the EMG, force, and length quickly stabilized near their initial values.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction of Ib autogenetic inhibition in motoneurons during contractions of an ankle extensor muscle in the cat

1. Triceps surae and plantaris (Pl) motoneurons were recorded intracellularly in chloralose or pentobarbital sodium (Nembutal)-anesthetized cats during unfused tetanic contractions of gastrocnemius medialis muscle (GM) produced by stimulating either a cut branch of the GM nerve or the muscle directly. 2. In alpha-motoneurons, during a series of GM twitches at 10/s, contraction-induced inhibitory potentials, probably the result of input from Golgi tendon organs (autogenetic inhibition), rapidly subsided before the end of the series. In contrast, excitatory potentials, probably the result of the activation of spindle primary endings during relaxation from contraction, persisted. 3. In gastrocnemius lateralis-soleus (GL-S) and Pl motoneurons lacking an excitatory connection with Ia afferents from GM, the sustained contraction of this muscle also elicited a declining inhibition. Rapid reduction of contraction-induced autogenetic inhibition was also observed in homonymous gamma-motoneurons. During unfused tetanic contractions lasting 0.5-4s, inhibitory potentials quickly subsided, but an abrupt increase in contractile force elicited a new series of decreasing inhibitory potentials. 4. The assumption that the inhibition induced by GM unfused tetanic contractions was due to activation of homonymous Ib afferents was supported by observations of the effects of electrical stimulation of the GM nerve. In Pl motoneurons lacking an excitatory connection with Ia afferents from GM, repetitive trains applied to the GM nerve, at a strength just above threshold for group I fibers, elicited rapidly declining inhibitory potentials similar to those produced by GM contraction. It was verified that during such stimulation, the amplitude of the group I afferent volleys did not decrease. 5. Reduction of contraction-induced Ib inhibition during sustained GM contraction was still present after a low spinalization of the preparation. As GM tendon organ discharges were verified to persist throughout prolonged contractions, the observed decline of autogenetic inhibition is likely to depend on a spinal mechanism, possibly involving presynaptic inhibition of Ib afferents and/or mutual inhibition of Ib-inhibitory interneurons.

Factors determining segmental reflex action in normal and decerebrate cats

1. In the companion paper the gain of the stretch reflex in the ankle extensor muscles of normal cats was shown to increase after decerebration. The objectives of this study were 1) to identify the origin of the increased reflex and 2) to evaluate the contribution from afferents other than ankle extensor muscle afferents to the short-latency reflex. 2. Six cats were trained to stand unaided on four pedestals. Three cats were also trained to control the force exerted with the left hindlimb. The left soleus (SOL) and lateral gastrocnemius (LG) electromyogram (EMG), length, force, and temperature were recorded by chronically implanted electrodes and transducers. Measurements were taken before and after decerebration at the premammillary level. After decerebration limb temperature was returned to its normal range by the use of radiant heat. 3. Reproducible ramp-and-hold stretches and releases of the ankle extensor muscles were produced by a servo-controlled motor that rotated the left rear pedestal about the ankle joint. The length of the ankle extensor muscles changed by 2-3 mm within 30-35 ms after the onset of a ramp perturbation. Reflex responses before and after decerebration were compared at matched background values of muscle length and force. 4. In both the SOL and LG muscles, a short-latency EMG burst appeared 8-12 ms after stretch onset and lasted approximately 20 ms. After decerebration the onset of the rectified and smoothed EMG burst remained unchanged, but its area was increased by 36-89%. 5. The lateral gastrocnemius-soleus (LG-S) electroneurogram (ENG) was chronically recorded in two cats with a nerve cuff recording electrode implanted on the LG-S nerve. LG-S ENG activity started to increase soon after stretch onset and remained high during the entire ramp phase. The stretch-evoked LG-S ENG burst started approximately 8 ms earlier than the short-latency SOL and LG EMG bursts. It was interpreted to reflect mainly an increase in the activity of Group Ia and Ib muscle afferents, caused by increases in both muscle length and muscle force during the stretch. After the cats were decerebrated, for matched postural conditions, the area of the stretch-evoked LG-S ENG burst was increased by 29-35%. Because the length and force changes sensed by the muscle receptors before and after decerebration were similar, this suggests that the sensitivity of muscle spindles was increased as a consequence of altered activity in fusimotor neurons after decerebration.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential loss of neuromuscular connections according to activity level and spinal position of neonatal rabbit soleus motor neurons

We have tested whether the ability of synapses to compete for occupancy of endplates during neuromuscular synapse elimination is affected by differences in the spinal position or in the activity level of the parent motor neuron. To test the role of spinal position, the relative sizes of motor units for motor neurons from middle and extreme (rostral/caudal) positions in the rabbit soleus motor pool were determined at 3 postnatal ages: 4-5 d ("early" ages, when the soleus is heavily polyinnervated), 8-9 d ("intermediate"), and 11-15 d ("late," when the soleus has just reached singly innervated state). Average motor unit sizes from extreme ventral roots were similar to those from middle ventral roots in early-aged soleus muscles but were significantly smaller (by 18-27%) for both intermediate and late muscles. Thus, motor neurons from extreme positions evidently compete less effectively for retention of synapses than those from middle positions. To test the role of differential activity, inactive and active synapses were pitted directly against one another by implanting Silastic plugs laden with tetrodotoxin (TTX) into one of the spinal nerves containing a minority of the soleus motor axons. Differential activity was maintained during a period of extensive synapse loss, from the time of the implant at day 4 or 5 until the intermediate age (day 8-9). Motor unit twitch tensions were subsequently measured to determine the relative number of synapses retained by individual active and inactive motor neurons. The inactivated motor units were on average significantly larger (by more than 50%) than the corresponding group from normal and control-implanted animals. The abnormally large size of inactivated motor units persisted in animals allowed to recover from the TTX block and examined after multiple innervation had disappeared. Hence, the effect of the TTX block cannot be attributed to a simple slowing of synapse elimination specifically among the inactive motor neurons. We conclude that complete presynaptic inactivity improves the chances of survival relative to that for normal activity during synapse elimination in the neonatal rabbit soleus muscle. This difference in competitive ability may contribute to the development of an important characteristic of adult muscles, the correlation between motor unit size and recruitment threshold.

Excitability changes of ankle extensor group Ia and Ib fibers during fictive locomotion in the cat

The present study examines the modulation of gastrocnemius-soleus (GS) monosynaptic reflexes as well as the intraspinal threshold changes of GS group I primary afferent terminals ending in the intermediate and motor nuclei during fictive locomotion in high decerebrate cats. The amplitude of the monosynaptic reflexes (MSR's) evoked in the medial gastrocnemius by stimulation of the lateral gastrocnemius nerve was increased during the extensor (E) phase, decreased during the flexion (F) phase of the step cycle and remained transiently increased after spontaneous episodes of fictive stepping. The intraspinal threshold of populations and of single group Ia GS afferent fibers ending in the motor pool, as well as of single Ia and Ib fibers ending in the intermediate nucleus, showed a sustained reduction during the episodes of fictive locomotion with superimposed cyclic changes in phase with the step cycle. During fictive walking and trotting the reduction of the intraspinal threshold of both Ia and Ib fiber terminals was maximal during the middle or late portion of the F-phase. During fictive gallop elicited by stimulation of the superficial peroneus nerve, the decrease in the intraspinal threshold of the Ia afferent fibers occurred however in phase with the activity of the GS motoneurons. During episodes of fictive locomotion slow, sustained negative DC potential shifts lasting tents of seconds, reflecting an increase in the extracellular potassium concentration were recorded at the base of the dorsal horn and in the intermediate nucleus. The present findings support the existence of tonic and phasic depolarization of the intraspinal terminals of GS group Ia and Ib primary afferents during spontaneous fictive locomotion. It is suggested that accumulation of potassium ions in the extracellular space contributes mainly to the sustained depolarization of group I fibers. The phasic depolarization would be mostly due to the activation of specific sets of interneurons and may, in the case of Ia fibers, contribute to the cyclic modulation of the MRS elicited during fictive locomotion.

A computer-controlled system to perturb the ankle joint of freely standing cats trained to maintain a given force

A computer-based system was developed to (1) train freely standing cats to match various target forces with the left hindlimb, (2) perturb the left ankle joint when the cat was maintaining a desired force and (3) compare reflex responses before and after decerebration. Cats quickly learned to stand unaided on 4 pedestals. During a training session, a range of target force windows was presented to the cat. A successful trial consisted of maintaining the force applied on the left rear pedestal within the target window for a preset time period. To assist the cat, a light was turned on whenever the force was within the target window. A food pellet reward was delivered by the computer after each successful trial. To test reflex responses, the position of the left hindlimb could be briefly perturbed by activating a servo-controlled printed motor configured to rotate the pedestal about the axis of the ankle joint. Perturbations that either flexed or extended the ankle joint were presented pseudo-randomly by the computer. This approach has been used to quantify the magnitude of muscle afferent volleys and the reflex EMG in ankle extensor muscles of normal and decerebrated cats, in response to similar mechanical perturbations. It has also been used to study dynamic features in the electroneurogram recorded from a cutaneous nerve by implanted nerve cuff electrodes, and the correlations among the electroneurogram, the vertical contact force applied on the pedestal and the force recorded from muscle tendons by implanted transducers. This approach may have general applications in the study of postural control, including the study of the discharge patterns of individual motor, sensory or spinal cord neurons in freely standing cats.

Fine structure of tarsal sensory organs in the whip spider Admetus pumilio (Amblypygi, Arachnida)

The sensory organs on the tarsi of the antenniform first legs of the whip spider Admetus pumilio C. L. Koch (Amblypygi, Arachnida) were examined with the scanning and transmission electron microscope. At least four different types of hair sensilla were found: (1) thick-walled bristles, which have the characteristics of contact chemoreceptors (several chemoreceptive dendrites in the lumen plus two mechanoreceptors at the base); (2) short club sensilla, innervated by 4-6 neurons which terminate in a pore on the tip; they are possibly humidity receptors; (3) porous sensilla, which are either innervated by 20-25 neurons and have typical pore tubules, or they have 40-45 neurons but no pore tubules; both types are considered to be olfactory; (4) rod sensilla occur in clusters near segmental borders; they are innervated by only one large dendrite which branches inside the lumen. Other tarsal receptors are the claws, which correspond to contact chemoreceptors, and the pit organ which resembles the tarsal organ of spiders. Compared to other arthropod sensilla, the contact chemoreceptors are very similar to those of spiders, while the porous sensilla correspond structurally to olfactory receptors in insects; the club and rod sensilla seem to be typical for amblypygids.