Synaptic responses produced in lobster abdominal postural motor neurons by mechanical stimulation of the swimmeret

1. Intracellular recordings were obtained from the somata of identified abdominal postural motor neurons in lobster to examine their subthreshold and suprathreshold responses to tactile stimulation of the swimmeret. 2. Pressure stimulation of the swimmeret surface evoked abdominal extension by producing tonic spiking in the extensor excitors and the synergistic flexor inhibitor (f5) and hyperpolarizing responses in the extensor inhibitor and antagonistic flexor excitors. These responses often continued for several seconds following the termination of the stimulus. The receptive fields of these motor responses extended over most of the swimmeret surface. 3. More localized tactile stimulation of the swimmeret surface elicited EPSPs in f5 and the extensor excitors, and IPSPs in the flexor excitors. The amplitude of these synaptic potentials decreased as the stimulus intensity was reduced. 4. Stimulation of feathered hair (both sexes) and smooth hair (female only) sensilla produced responses characteristic of extension whereas bristly spines on the male accessory lobe excited only two flexor excitors without affecting any of the other postural motor neurons. 5. Summed synaptic responses recorded from the motor neurons differed in their amplitudes and latencies according to the type of mechanoreceptor stimulated-cuticular receptors, feathered hairs or smooth hairs. Stimulation of the swimmeret cuticle produced the strongest responses (shortest latency, largest amplitude), while feathered hair stimulation initiated the weakest responses (longest latency, smallest amplitude). 6. The relatively long latencies (greater than 35 ms) and the complex form of the EPSPs and IPSPs indicate the involvement of multisynaptic interneuronal pathways in the reflex arcs.     

Tactile stimulation of the swimmeret alters motor programs for abdominal posture in the lobsterHomarus americanus

The influence of mechanosensory stimulation of a second segment swimmeret upon the abdominal postural program was examined in an isolated abdominal nerve cord-swimmeret preparation. The swimmeret was stimulated in several different ways to assess the extent of influence exerted on abdominal positioning. Localized tactile stimulation of the swimmeret surface with a mechanical probe usually generated flexion inhibition where the flexor inhibitor (f5) was activated while the small and medium flexor excitors were inhibited. Flexion inhibition was much stronger in females than males. In 50% of the animals a weak flexion excitation was seen. After 3-6 hours the response of one-third of these preparations changed to flexion inhibition. Strong manual stimulation of the swimmeret surface inhibited all of the flexor excitors (f1, f2, f3, f4, and f6) while exciting the inhibitor f5 and increasing extensor activity. Similar extension responses were observed in both sexes. Repeated tactile stimulation of the swimmeret surface elicited a response similar to that evoked during manual stimulation. The strongest extension response was produced at 2 Hz which falls within the normal range of swimmeret beating in intact lobsters. Similar extension responses were also obtained during spontaneous swimmeret beating and rhythmic manual movement of the swimmeret.(ABSTRACT TRUNCATED AT 250 WORDS)     

Output connections of a wind sensitive interneurone with motor neurones innervating flight steering muscles in the locust

Summary The output connections of a bilaterally symmetrical pair of wind-sensitive interneurones (called A4I1) were determined in a non-flying locust (Schistocerca gregaria). Direct inputs from sensory neurones of specific prosternai and head hairs initiate spikes in these interneurones in the prothoracic ganglion.The interneurone with its axon in the right connective makes direct, excitatory connections with the two mesothoracic motor neurones innervating the pleuroaxillary (pleuroalar, M85) muscle of the right forewing, but not with the comparable motor neurones of the left forewing. The connections can evoke motor spikes.The interneurones also exert a powerful, but indirect effect on the homologous metathoracic pleuroaxillary motor neurones (muscle 114), and a weaker, indirect effect on subalar motor neurones of the hindwings. No connections or effects were found with other flight motor neurones, or motor neurones innervating hindleg muscles, including common inhibitor 1 which also innervates the pleuroaxillary muscle.One thoracic interneurone with its cell body in the right half of the mesothoracic ganglion and with its axon projecting ipsilaterally to the metathoracic ganglion receives a direct input from the right A4I1 interneurone.These restricted output connections suggest a role for the A4I1 interneurones in flight steering.Abbreviations DCMD descending contralateral movement detector - EPSP excitatory postsynaptic potential - TCG tritocerebral commissure giant (interneurone)     

Sexually dimorphic mechanosensitive swimmeret sensilla affect abdominal posture in the lobster

The sensilla on the male and female second swimmerets are sexually dimorphic. Female swimmerets contain many long "smooth hairs" (long simple setae) on the coxa and rami. The endopodite of the male swimmeret has an accessory lobe covered with short "bristly spines" (serrate setae). In both sexes the swimmeret rami are lined by "feathered hairs" (plumose setae). The influence of mechanosensory stimulation of these sensilla upon abdominal tonic motor activity was analyzed in an in vitro swimmeret-nerve cord preparation. Movement of several clusters of smooth hairs produced an abdominal extension program by exciting the flexor inhibitor f5, inhibiting the flexor excitors, and activating several extensors. Stimulation of the male bristly spines excited the medium-sized flexor excitors f3 and f4. In both sexes the feathered hairs did not generate any response to mechanical stimulation. We infer that in nongravid females the smooth hairs are involved in receiving mechanosensitive cues to support abdominal extension. Bristly spines may contribute to postural adjustments that assist mating. The long latencies of these responses and their propagation to adjacent ganglia suggest that they are mediated by postural interneurons rather than by direct afferent terminations on postural motoneurons.     

Non-spiking local interneurones mediate abdominal extension related descending control of uropod motor neurones in the crayfish terminal abdominal ganglion

The role of non-spiking local interneurones in the synaptic interactions between abdominal extension-evoking descending interneurones and uropod motor neurones in the terminal abdominal ganglion of the crayfish Procambarus clarkii (Girard) was investigated electrophysiologically. Continuous electrical stimulation of the lateral region of the 3rd-4th abdominal connective that included abdominal extension evoking interneurones excited the opener motor neurones and inhibited the closer, reductor motor neurone. Spikes from a single descending interneurone evoked consistent and short latency (0.8–0.9 ms) excitatory postsynaptic potentials (e.p.s.ps) in the opener motor neurones, and evoked rather long-latency (1.5–2.7 ms) inhibitory postsynaptic potentials (i.p.s.ps) in the reductor motor neurone. Many non-spiking interneurones also received depolarizing p.s.ps (0.8–2.5 ms in latency) that were usually faster than i.p.s.ps of the reductor motor neurone if both neurones were recorded sequentially in the same preparation. Non-spiking interneurones received convergent inputs from several descending interneurones and made inverting connection with the reductor motor neurone. Elimination of descending inputs to a particular non-spiking interneurone could reduce the inhibitory response of the reductor motor neurone. These observations strongly suggested that descending inhibitory inputs to the closer, reductor motor neurone were mediated by non-spiking interneurones. Furthermore, some non-spiking interneurones made output connections with the opener motor neurones. The disynaptic pathway through non-spiking interneurones is significant to control and modulate the opening pattern of the uropod during abdominal extension. Accepted: 27 December 1996     

Octopaminergic modulation of locust motor neurones

The effect of octopamine on the fast extensor and the flexor tibiae motor neurones in the locust (Schistocerca gregaria) metathoracic ganglion, and also on synaptic transmission from the fast extensor to the flexor motor neurones, was examined. Bath application or ionophoresis of octopamine depolarized and increased the excitability of the flexor tibiae motor neurones. 1 mM octopamine reduced the amplitude of the fast extensor-evoked EPSP in the slow but not the fast flexor motor neurones, whereas 10 mM octopamine could reduce the EPSP amplitude in both. Octopamine broadened the fast extensor action potential and reduced the amplitude of the afterhyperpolarization, the modulation requiring feedback resulting from movement of the tibia. Octopamine also increased the frequency of synaptic inputs onto the tibial motor neurones, and could cause rhythmic activity in the flexor motor neurones, and reciprocal activity in flexor and extensor motor neurones. Octopamine also increased the frequency of spontaneous spiking in the octopaminergic dorsal unpaired median neurones. Repetitive stimulation of unidentified dorsal unpaired median neurones could mimic some of the effects of octopamine. However, no synaptic connections were found between dorsal unpaired median neurones and the tibial motor neurones. The diverse effects of octopamine support its role in mediating arousal.     

Parallel processing of proprioceptive information in the terminal abdominal ganglion of the crayfish

The processing of proprioceptive information from the exopodite-endopodite chordotonal organ in the tailfan of the crayfish Procambarus clarkii (Girard) is described. The chordotonal organ monitors relative movements of the exopodite about the endopodite. Displacement of the chordotonal strand elicits a burst of sensory spikes in root 3 of the terminal ganglion which are followed at a short and constant latency by excitatory postsynaptic potentials in interneurones. The afferents make excitatory monosynaptic connections with spiking and nonspiking local interneurones and intersegmental interneurones. No direct connections with motor neurones were found.Individual afferents make divergent patterns of connection onto different classes of interneurone. In turn, interneurones receive convergent inputs from some, but not all, chordotonal afferents. Ascending and spiking local interneurones receive inputs from afferents with velocity thresholds from 2–400°/s, while nonspiking interneurones receive inputs only from afferents with high velocity thresholds (200–400°/s).The reflex effects of chordotonal organ stimulation upon a number of uropod motor neurones are weak. Repetitive stimulation of the chordotonal organ at 850°/s produces a small reduction in the firing frequency of the reductor motor neurone. Injecting depolarizing current into ascending or non-spiking local interneurones that receive direct chordotonal input produces a similar inhibition.     

The organization of exteroceptive information from the uropod to ascending interneurones of the crayfish

The organization of exteroceptive inputs to identified ascending interneurones of the crayfish, Procambarus clarkii (Girard), has been analyzed by stimulation of hairs on the uropod and simultaneous intracellular recordings from ascending interneurones. The spikes of single afferent neurones which innervated hairs on the distal ventral surface of the exopodite were consistently followed by a depolarizing synaptic potential in many identified ascending interneurones with a constant and short central delay of 0.7–1.5 ms. The amplitude of the potentials depended on the membrane potential of the ascending interneurones. Each afferent neurone made divergent outputs onto several ascending interneurones and each ascending interneurone received convergent inputs from several afferent neurones. Certain ascending interneurones made inhibitory or excitatory connections with other ascending interneurones. These central interactions were always one-way, and the spikes from one ascending interneurone consistently evoked excitatory or inhibitory post-synaptic potentials in other interneurones which followed with a constant and short latency of 0.7–1.0 ms. The inhibitory postsynaptic potential was reversed by injection of steady hyperpolarizing current.Abbreviations EPSP excitatory post-synaptic potential - IPSP inhibitory post-synaptic potential     

Direct excitation of nonspiking local interneurones by exteroceptors underlies tactile reflexes in the locust

Summary Tactile stimulation of a leg of the locustSchistocerca gregaria can lead to specific reflex movements of that leg. At the same time nonspiking interneurones that are presynaptic to the participating motor neurones are excited or inhibited, suggesting that they are directly involved in these reflexes. The afferent pathways mediating these effects have been examined by recording from individual afferents and nonspiking interneurones.Afferent spikes fromtrichoid orcampaniform sensilla on specific regions of a leg evoke chemically-mediated EPSPs with a constant central latency of about 1.5 ms in certain nonspiking interneurones. The branches of an interneurone and the afferents from which it receives inputs overlap in the neuropil of the ganglion.No afferents have been found to evoke IPSPs directly in the nonspiking interneurones. Instead the inhibition is caused by a population of spiking local interneurones that are themselves excited directly by the afferents, and whose spikes evoke IPSPs in certain nonspiking interneurones.The tactile reflexes can involve movements about one or more joints of the leg, and these coordinated responses are explained by the participation of specific nonspiking interneurones that distribute the sensory inputs to the appropriate sets of motor neurones. For example, when hairs on the dorsal surface of a tarsus are touched, the tarsus is levated. This reflex involves nonspiking local interneurones which are excited directly by these hair afferents and which make direct excitatory connections with the single levator tarsi motor neurone.     

The central connections and actions during walking of tibial campaniform sensilla in the locust

Strain acting on the exoskeleton of insects is monitored by campaniform sensilla. On the tibia of a mesothoracic leg of the locust (Schistocerca gregaria) there are three groups of campaniform sensilla on the proximo-dorsal surface. This study analyses the responses of the afferents from one group, their connections with central neurones and their actions during walking.The afferents of the campaniform sensilla make direct excitatory connections with flexor tibiae motor neurones. They also make direct connections with particular spiking local interneurones that make direct inhibitory output connections with the slow extensor tibiae motor neurone.During walking extension movements of the tibiae during stance produce longitudinal tensile forces on the dorsal tibia that peak during mid stance before returning to zero prior to swing. This decline in tension can activate the campaniform sensilla. In turn this would lead to an inhibition of the extensor tibiae motor neurone and an excitation of the flexor tibiae motor neurones. This, therefore, aids the transition from stance to swing. During turning movements, the tibia is flexed and the dorsal surface is put under compression. This can also activate some of campaniform sensilla whose effect on the flexor motor neurones will reinforce the flexion of the tibia.     

Cholinergic transmission at mechanosensory afferents in the crayfish terminal abdominal ganglion

Electrical stimulation of mechanosensory afferents innervating hairs on the surface of the exopodite in crayfish Procambarus clarkii (Girard) elicited reciprocal activation of the antagonistic set of uropod motor neurones. The closer motor neurones were excited while the opener motor neurones were inhibited. This reciprocal pattern of activity in the uropod motor neurones was also produced by bath application of acetylcholine (ACh) and the cholinergic agonist, carbamylcholine (carbachol). The closing pattern of activity in the uropod motor neurones produced by sensory stimulation was completely eliminated by bath application of the ACh blocker, d-tubocurarine, though the spontaneous activity of the motor neurones was not affected significantly. Bath application of the acetylcholinesterase inhibitor, neostigmine, increased the amplitude and extended the time course of excitatory postsynaptic potentials (EPSPs) of ascending interneurones elicited by sensory stimulation. These results strongly suggest that synaptic transmission from mechanosensory afferents innervating hairs on the surface of the tailfan is cholinergic.Bath application of the cholinergic antagonists, dtubocurarine (vertebrate nicotinic antagonist) and atropine (muscarinic antagonist) reversibly reduced the amplitude of EPSPs in many identified ascending and spiking local interneurones during sensory stimulation. Bath application of the cholinergic agonists, nicotine (nicotinic agonist) and oxotremorine (muscarinic agonist) also reduced EPSP amplitude. Nicotine caused a rapid depolarization of membrane potential with, in some cases, spikes in the interneurones. In the presence of nicotine, interneurones showed almost no response to the sensory stimulation, probably owing to desensitization of postsynaptic receptors. On the other hand, no remarkable changes in membrane potential of interneurones were observed after oxotremorine application. These results suggest that ACh released from the mechanosensory afferents depolarizes interneurones by acting on receptors similar to vertebrate nicotinic receptors.Abbreviations ACh cetylcholine - mns motor neurones - asc int ascending interneurone     

The cord stretch receptors in the abdominal nerve cord of the crayfish Cherax destructor: physiology and relationships

The physiology and relationships of tonic cord stretch receptor neurons in the crayfish Cherax destructor were examined with intracellular and extracellular recording. Cord stretch evoked slow depolarisations leading to action potentials in tonic cord stretch receptor neurons. Intermittent post-synaptic potentials were also seen in cord stretch receptor neurons but were not the primary cause of the action potentials. Cord stretch still evoked action potentials in cord stretch receptor neurons when all synaptic activity, monitored at another known chemical synapse, was blocked using high [Mg(2+)] and low [Ca(2+)] in the bath. One source of facilitating excitatory post-synaptic potentials in the cord stretch receptor neurons was from mechanosensory hairs on the dorsal abdominal surface. Tonic cord stretch receptor neuron activity was associated with an increase in the activity of the abdominal slow extensor inhibitor motor neuron and at least one abdominal flexor excitor motor neuron in its segment, and reduced activity in the abdominal slow flexor inhibitor motor neuron. Activation of individual cord stretch receptor neurons produced a local resistance reflex. Cord stretch, activating many receptors, produced several other outcomes. One was the "extensor state" described in earlier literature. The tonic cord stretch receptor neurons of Cherax destructor appear to be stretch-sensitive interneurons that receive inputs from other elements of the abdominal control system and mediate polysynaptic reflex activity in postural motor neurons.     

Intersegmental modulation of abdominal postural responses initiated by mechanostimulation of the swimmeret in lobster

In a multiganglionic preparation of the lobster abdominal nerve cord, composed of the first through fifth ganglia (A1-A5) and attached second swimmeret, tactile stimulation of the cuticular surface of the swimmeret initiates a postural motor program in A2 for abdominal extension, whereas deflection of feathered hair sensilla that fringe the swimmeret rami does not affect postural motor activity recorded from A2 (Kotak and Page, 1986a). This report demonstrates that partial isolation of A2 from adjacent abdominal ganglia by sectioning the A1-A2 or the A2-A3 connectives both increases the strength of the extension response evoked by cuticular stimulation and disinhibits a postural flexion inhibition response initiated by feathered hair stimulation. Complete isolation of A2, by cutting the A1-A2 and the A2-A3 connectives, further increases the strength of these postural responses. Intersegmental inhibition of these responses originates in the ganglia adjacent to A2, since mechanoresponsiveness of A2 is not affected by resection of a more distant connective (A3-A4). These results provide evidence for the presence in adjacent abdominal ganglia of intersegmental interneurons that regulate the access of swimmeret sensory activity to the postural motor neurons in A2.     

Extensor motor neurons of the crayfish abdomen

Summary The somata of five deep extensor motoneurons of the third abdominal ganglion of the crayfish(Procambarus clarkii) were located and identified. The positions of these somata within the ganglion and their distal distribution to muscles have been mapped and were constant. The soma of the extensor inhibitor was noted to touch the soma of the flexor inhibitor. Three of the excitatory neurons were clustered near their exit route.Sensory and cord routes of activation of the extensor motoneurons were also found and were constant from preparation to preparation. Sub-threshold recording showed that these motoneurons exhibited radically different types of post-synaptic response to stimuli at different sites in the nervous system. No interaction between extensor motoneurons or between the extensor and flexor motoneurons was observed.     

Parallel effects of joint receptors on motor neurones and intersegmental interneurones in the locust

Summary At the distal end of a mesothoracic tibia of the locust,Schistocerca gregaria, is a chordotonal organ which monitors the position and movement of the tarsus relative to the tibia. It contains approximately 35 receptors that variously encode different spatial and temporal parameters (position, velocity and direction of movement). Some excite intersegmental interneurones that respond phasically or tonically, with directional sensitivity to active or imposed movements of the tarsus. Some of these interneurones are also excited by intrinsic movements of the tarsal segments. Others, besides being excited by tarsal proprioceptive inputs, are also excited by exteroreceptors on the tarsus.When stimulated mechanically or electrically, chordotonal afferents evoke excitatory postsynaptic potentials with a central latency of between 0.9 and 1.4 ms simultaneously in the intersegmental interneurones and in tarsal motor neurones. The central arborizations of the afferents, the intersegmental interneurones and the tarsal motor neurones overlap in certain neuropilar regions of the mesothoracic ganglion. Other afferents cause an inhibition of the motor neurones, with a longer and non-consistent latency suggesting the involvement of other intercalated interneurones.These results indicate that proprioceptive inputs from the tarsal joint receptors are transmitted in parallel and monosynaptically to tarsal motor neurones and to the intersegmental interneurones.     

The abdominal motor system of the crayfish, Cherax destructor. I. Morphology and physiology of the superficial extensor motor neurons

Using extracellular and intracellular stimulation, recording and dye-filling, we identified and studied the superficial extensor motor neurons of the crayfish, Cherax destructor. Functional associations of each neuron were characterised by recording its responses to sensory and abdominal cord inputs, its extensor muscle innervation pattern and its relationships with other neurons. Two clear associations were found among the six neurons of each segment. A medium-sized excitor (no. 3), that innervates a substantial percentage of extensor muscle fibres, and the largest excitor (no. 6), recruited during peak, excitation, were inhibited by input from unknown interneurons that excited the common inhibitor (no. 5). Likewise, these excitors received excitatory input when the inhibitor was silent. Another medium-sized neuron (no. 4) that innervates many muscle fibres was co-active with one of the small excitors (no. 2). The two medium-sized neurons were never active at the same time, and these two groupings may be determined by pre-motor interneurons. The implications of these findings for our understanding of motor control in this system are discussed. Accepted: 21 June 1998     

Thoracic output of crayfish giant fibres

Summary The thoracic homologue of the abdominal segmental giant neurone of crayfish Pacifastacus leniusculus is identified and described. It has a small cell body located in the anterior ventro-lateral quadrant of the ganglion and a large neuropil arborization, with dendrites aligned along the tracts of the giant fibres. The SG axon exits the ganglion within the major root which innervates the leg, usually in the anterior region of this root. Within 1–2 mm of the ganglion the axon terminates in a mass of fine branches, apparently randomly located within the base of the root.The SG receives suprathreshold input from the ipsilateral MG and LG fibres through rectifying electrical synapses. It makes output to FF motor neurones, also through electrical synapses. The SG also makes output to at least one corollary discharge interneurone. The SG receives depolarizing inhibitory synaptic potentials which can prevent its activation by the GFs. Some but not all of these synaptic potentials are common to similar potentials occurring in a large leg promotor motor neurone.Abbreviations AC anterior connective - GF giant fibre - IPSP inhibitory post-synaptic potential - LG lateral giant fibre - MG medial giant fibre - MoG motor giant neurone - PC posterior connective - PMM promotor motor neurone - r1 first root - r3 third root - rAD anterior distal root - rPD posterior distal root - rPM promotor muscle root - SG segmental giant neurone     

Synaptic connections between sensory afferents and the common inhibitory motoneuron in crayfish

The sensory inputs to the common inhibitory motoneuron that innervates every leg muscle of the crayfish Procambarus clarkii (Girard) were analyzed by performing intracellular recordings from its neurite within the neuropil of the 5th thoracic ganglion. Two types of sensory inputs involved in locomotion were studied, those from a movement coding proprioceptor (the coxobasal chordotonal organ) and those from sensory neu rons coding contact forces exerted at the tip of the leg on the substrate (the dactyl sensory afferents). Sinusoidal movements applied to the chordotonal organ strand induced a stable biphasic response in the common inhibitory motoneuron that consisted of bursts of spikes during release and stretch of the strand, corresponding to raising and lowering of the leg, respectively. Using ramp movements imposed on the chordotonal strand, we demonstrated that only movement-coding chordotonal afferents produce excitatory post-synaptic potentials in the common inhibitory motoneuron; these connections are monosynaptic. Mechanical or electrical stimulation of the dactyl sensory afferents resulted in an increase in the tonic discharge of the common inhibitory motoneuron through polysynaptic excitatory pathways. These two types of sensory cues reinforce the central command of the common inhibitory motoneuron and contribute to enhancing its activity during leg movements, and thus facilitate the relaxation of tonic muscle fibres during locomotion.Abbreviations ADR anterior distal root - A Lev anterior levator nerve - CB coxo-basipodite joint - CBCO coxo-basal chordotonal organ - CI common inhibitory motoneuron - Dep depressor nerve - DSA dactyl sensory afferents - EPSP excitatory post-synaptic potential - IN interneuron - MN motoneuron - PDR posterior distal root - P Lev posterior levator nerve - Pro promotor nerve - Rem remotor nerve     

Regulation of gamma- and alpha-motoneuron function of antagonistic muscles during cold tremor in cats

Activity of flexor and extensor γ- and α-motoneurons during cold tremor was investigated in anesthetized cats. General hypothermia and local cold stimulation or electrical stimulation of the dorsomedial region of the posterior hypothalamus were shown to induce primary activation of flexor and simultaneous inhibition of activity of extensor γ-motoneurons, followed by activation of flexor α-motoneurons (extensor muscles are not involved in temperature regulating activity). Electrical stimulation of the medial preoptic region during cold tremor led to primary inhibition of flexor α-motoneuron activity. It is concluded from these results that the development of temperature-regulating muscular activity is preceded by activation of γ-motoneurons. The use of cold tremor induced in anesthetized cats as a natural model with which to study the role of the fusimotor system in regulation of function of motor nuclei during postural activity is argued.