Fast Extensor Tibiae Research Articles

Matched Short-Term Depression and Recovery Encodes Interspike Interval at a Central Synapse

Reversible decreases in synaptic strength, known as short-term depression (STD), are widespread in neural circuits. Various computational roles have been attributed to STD but these tend to focus upon the initial depression rather than the subsequent recovery. We studied the role of STD and recovery at an excitatory synapse between the fast extensor tibiae (FETi) and flexor tibiae (flexor) motor neurons in the desert locust (Schistocerca gregaria) by making paired intracellular recordings in vivo. Over behaviorally relevant pre-synaptic spike frequencies, we found that this synapse undergoes matched frequency-dependent STD and recovery; higher frequency spikes that evoke stronger, faster STD also produce stronger, faster recovery. The precise matching of depression and recovery time constants at this synapse ensures that flexor excitatory post-synaptic potential (EPSP) amplitude encodes the presynaptic FETi interspike interval (ISI). Computational modelling shows that this precise matching enables the FETi-flexor synapse to linearly encode the ISI in the EPSP amplitude, a coding strategy that may be widespread in neural circuits.

Motoneurons, DUM cells, and sensory neurons in an insect thoracic ganglion: A tracing study in the stick insect Carausius morosus

Anatomical features of leg motoneurons, dorsal unpaired median (DUM) cells, and sensory neurons in stick insect mesothoracic ganglia were examined using fluorescent dye backfills of lateral nerves. Structures were analyzed in whole-mounts of ganglia and transverse sections. Numbers of motoneurons and details of their structure by far exceed previously published data. The general neuroanatomical layout of motoneurons matches the general orthopteran pattern. Cell bodies of excitatory motoneurons form clusters in the lateral cortex, dendrites branch mainly in the dorsal neuropil. We identified nine DUM cells, six of which have axons in nerve nl5. Most sensory fibers terminate in the ventral association center (VAC). Twenty-three small cell bodies located close to the soma of the fast extensor tibiae motoneuron likely belong to strand receptors. Labeled structures are compared with previously published data from stick insects and other orthopterous insects.

Front leg movements and tibial motoneurons underlying auditory steering in the cricket (Gryllus bimaculatusdeGeer)

Front leg movements in the cricket (Gryllus bimaculatus) were measured during phonotactic steering on a trackball together with electromyogram recordings of the tibial extensor and flexor muscles. Up-down leg movements clearly indicated the step cycle and were independent of auditory stimulation. By contrast, left-right movements of the front leg were dependent on sound direction, with crickets performing rapid steering leg movements towards the active speaker. Steering movements were dependent on the phase of sound relative to the step cycle, and were greatest for sounds occurring during the swing phase. During phonotaxis the slow extensor tibiae motoneuron responded to ipsilateral sounds with a latency of 35-40 ms, whereas the fast flexor tibiae motoneurons were excited by contralateral sound. We made intracellular recordings of two tibial extensor and at least eight flexor motoneurons. The fast extensor tibiae, the slow extensor tibiae and one fast flexor tibiae motoneurons were individually identifiable, but a group of at least four fast flexor tibiae as well as at least three slow flexor tibiae motoneurons of highly similar morphology could not be distinguished. Motoneurons received descending inputs from cephalic ganglia and from local prothoracic networks. There was no overlap between the dendritic fields of the tibial motoneurons and the auditory neuropile. They did not respond to auditory stimulation at rest. Neither extracellular stimulation of descending pathways nor pharmacological activation of prothoracic motor networks changed the auditory responsiveness. Therefore, any auditory input to tibial motoneurons is likely to be indirect, possibly via the brain.

Motor Control of Aimed Limb Movements in an Insect

Limb movements that are aimed toward tactile stimuli of the body provide a powerful paradigm with which to study the transformation of motor activity into context-dependent action. We relate the activity of excitatory motor neurons of the locust femoro-tibial joint to the consequent kinematics of hind leg movements made during aimed scratching. There is posture-dependence of motor neuron activity, which is stronger in large amplitude (putative fast) than in small (putative slow and intermediate) motor neurons. We relate this posture dependency to biomechanical aspects of the musculo-skeletal system and explain the occurrence of passive tibial movements that occur in the absence of agonistic motor activity. There is little recorded co-activation of antagonistic tibial extensor and flexor motor neurons, and there is differential recruitment of proximal and distal flexor motor neurons. Large-amplitude motor neurons are often recruited soon after a switch in joint movement direction. Motor bursts containing large-amplitude spikes exhibit high spike rates of small-amplitude motor neurons. The fast extensor tibiae neuron, when recruited, exhibits a pattern of activity quite different to that seen during kicking, jumping, or righting: there is no co-activation of flexor motor neurons and no full tibial flexion. Changes in femoro-tibial joint angle and angular velocity are most strongly dependent on variations in the number of motor neuron spikes and the duration of motor bursts rather than on firing frequency. Our data demonstrate how aimed scratching movements result from interactions between biomechanical features of the musculo-skeletal system and patterns of motor neuron recruitment.

Compensatory Plasticity at an Identified Synapse Tunes a Visuomotor Pathway

We characterized homeostatic plasticity at an identified sensory-motor synapse in an insect, which maintains constant levels of motor drive as locusts transform from their solitarious phase to their gregarious swarming phase. The same mechanism produces behaviorally relevant changes in response timing that can be understood in the context of an animal's altered behavioral state. For individual animals of either phase, different looming objects elicited different spiking responses in a visual looming detector interneuron, descending contralateral movement detector (DCMD), yet its synaptic drive to a leg motoneuron, fast extensor tibiae (FETi), always had the same maximum amplitude. Gregarious locust DCMDs produced more action potentials and had higher firing frequencies, but individual postsynaptic potentials (PSPs) elicited in FETi were half the amplitude of those in solitarious locusts. A model suggested that this alone could not explain the similarity in overall amplitude, and we show that facilitation increased the maximum compound PSP amplitude in gregarious animals. There was the same linear relationship between times of peak DCMD firing before collision and the size/velocity of looming objects in both phases. The DCMD-FETi synapse transformed this relationship nonlinearly, such that peak amplitudes of compound PSPs occurred disproportionately earlier for smaller/faster objects. Furthermore, the peak PSP amplitude occurred earlier in gregarious than in solitarious locusts, indicating a differential tuning. Homeostatic modulation of the amplitude, together with a nonlinear synaptic transformation of timing, acted together to tune the DCMD-FETi system so that swarming gregarious locusts respond earlier to small moving objects, such as conspecifics, than solitarious locusts.

Jumping and kicking in bush crickets.

Bush crickets have long, thin hind legs but jump and kick rapidly. The mechanisms underlying these fast movements were analysed by correlating the activity of femoral muscles in a hind leg with the movements of the legs and body captured in high-speed images. A female Pholidoptera griseoaptera weighing 600 mg can jump a horizontal distance of 300 mm from a takeoff angle of 34 degrees and at a velocity of 2.1 m s(-1), gaining 1350 microJ of kinetic energy. The body is accelerated at up to 114 m s(-2), and the tibiae of the hind legs extend fully within 30 ms at maximal rotational velocities of 13500 deg. s(-1). Such performance requires a minimal power output of 40 mW. Ruddering movements of the hind legs may contribute to the stability of the body once the insect is airborne. During kicking, a hind tibia is extended completely within 10 ms with rotational velocities three times higher at 41800 deg. s(-1). Before a kick, high-speed images show no distortions of the hind femoro-tibial joints or of the small semi-lunar groove in the distal femur. Both kicks and jumps can be generated without full flexion of the hind tibiae. Some kicks involve a brief, 40-90 ms, period of co-contraction between the extensor and flexor tibiae muscles, but others can be generated by contraction of the extensor without a preceding co-contraction with the flexor. In the latter kicks, the initial flexion of the tibia is generated by a burst of flexor spikes, which then stop before spikes occur in the fast extensor tibiae (FETi) motor neuron. The rapid extension of the tibia can follow directly upon these spikes or can be delayed by as long as 40 ms. The velocity of tibial movement is positively correlated with the number of FETi spikes. The hind legs are 1.5 times longer than the body and more than four times longer than the front legs. The mechanical advantage of the hind leg flexor muscle over the extensor is greater at flexed joint angles and is enhanced by a pad of tissue on its tendon that slides over a protuberance in the ventral wall of the distal femur. The balance of forces in the extensor and flexor muscles, coupled with their changing lever ratio at different joint positions, appears to determine the point of tibial release and to enable rapid movements without an obligatory co-contraction of the two muscles.

Neuromuscular plasticity in the locust after permanent removal of an excitatory motoneuron of the extensor tibiae muscle.

The capacity of the larval insect nervous system to compensate for the permanent loss of one of the two excitatory motoneurons innervating a leg muscle was investigated in the locust (Locusta migratoria). In the fourth instar, the fast extensor tibiae (FETi) motoneuron in the mesothoracic ganglion was permanently removed by photoinactivation with a helium-cadmium laser. Subsequently, the animals were allowed to develop into adulthood. When experimental animals were tested as adults after final ecdysis, fast-contracting fibers in the most proximal region of the corresponding extensor muscle, which are normally predominantly innervated by FETi only, uniformly responded to activity of the slow extensor tibiae (SETi) neuron. In adult operated animals, single pulses to SETi elicited large junctional responses in the fibers which resulted in twitch contractions of these fibers similar to the responses to FETi activity in control animals. The total number of muscle fibers, their properties as histochemically determined contractional types (fast and slow), and their distribution were not affected by photoinactivation of FETi. Possible mechanisms enabling the larval neuromuscular system to compensate for the loss of FETi through functionally similar innervation by a different motoneuron, i.e. SETi, are discussed.

The influence of proprioceptors signalling tibial position and movement on the kick motor programme in the locust

Four main proprioceptors monitor tibial position in the hindleg of the locust: the femoral chordotonal organ (FCO), the lump receptor, the suspensory ligament receptors and Brunner's organ. The influence of these proprioceptors on quantitative aspects of the kick motor programme has been investigated. The parameters measured were the duration of the initial flexion burst, the duration of co-activation of flexor and fast extensor tibiae (FETi) motoneurones, the number of FETi spikes during the co-activation, the interval between the kick and post-kick flexion, the number of FETi spikes occurring in this interval and the duration of post-kick flexion activity. The lump receptor and Brunner's organ have no detectable effect on any of these parameters. The FCO has highly significant effects on the duration of both initial flexion and post-kick flexion bursts, and on the number of FETi spikes occurring after the moment of tibial extension. The suspensory ligament receptors have significant effects upon the number of FETi spikes after the kick and the interval between the kick and the post-kick flexion. However, no proprioceptor had any influence upon the duration of co-activation or the number of FETi spikes during the co-activation. Thus, although elements of the kick motor programme preceding and following co-activation are strongly influenced by proprioceptors monitoring tibial position and movement, the co-activation stage, which is central to the effectiveness of the complete behaviour pattern, is not affected.

Excitatory interactions between antagonistic motor neurones underlying locust kicking and jumping during maturation after the adult moult.

There is a change in the synaptic connections between motor neurones that underlie locust kicking and jumping during maturation following the adult moult. The fast extensor tibiae (FETi) motor neurone makes monosynaptic excitatory connections with flexor tibiae motor neurones that have previously been implicated in maintaining flexor activity during the co-contraction phase of jumping, in which energy generated by the muscles of a hind leg is stored. The amplitude of the FETi spike decreases when repetitively activated, and this decrement is larger in locusts immediately following the adult moult than in mature locusts. The decrement in the FETi spike is correlated with a greater decrease in the amplitude of the flexor excitatory postsynaptic potential (EPSP) in newly moulted locusts and in turn with the failure of these locusts to kick or jump. The results presented here indicate that the developmental change in the connections between the motor neurones contributes to the change in behaviour following the moult.

Adaptive reconfiguration of a reflex circuit during different motor programmes in the locust

The cuticle strain which develops in the hindleg tibiae when a locust prepares to kick, or when the tibia thrusts against an obstacle, is detected by two campaniform sensilla, which reflexly excite the fast extensor tibiae motoneuron, some of the flexor tibiae motoneurons and nonspiking interneurons. The reflex excitation is adaptive for the extensor motoneuron during both co-activation and thrusting, but is only adaptive for the flexor motoneurons during co-activation, and is maladaptive during thrusting. We show that the femoral chordotonal organ, which monitors tibial position, controls the efficacy of the strain feedback. The campaniform sensilla-induced depolarization in the extensor motoneuron is about twice as large when the tendon is in mid position (reflecting a tibial-femoral angle of 90°) than when fully stretched (reflecting tibial flexion), while in the flexors the reverse is true. The amplitudes of excitatory postsynaptic potentials evoked by single campaniform sensilla spikes, are, however, not affected. Our data suggests that the chordotonal organ modulates the gain of the strain feedback onto the motoneurons by exciting interneuronal circuits whose output sums with the former. Thrusting typically occurs with the tibia partially extended, therefore the actions of the chordotonal organ support the production of a maximal thrusting force.

Peripheral control of the gain of a central synaptic connection between antagonistic motor neurones in the locust

The metathoracic fast extensor tibiae (FETi) motor neurone of locusts is unusual amongst insect motor neurones because it makes output connections within the central nervous system as well as in the periphery. It makes excitatory chemical synaptic connections to most if not all of the antagonist flexor tibiae motor neurones. The gain of the FETi-flexor connection is dependent on the peripheral conditions at the time of the FETi spike. This dependency has two aspects. First, sensory input resulting from the extensor muscle contraction can sum with the central excitatory postsynaptic potential (EPSP) to augment its falling phase if the tibia is restrained in the flexed position (initiating a tension-dependent reflex) or is free to extend (initiating a movement-dependent resistance reflex). This effect is thus due to simple postsynaptic summation of the central EPSP with peripheral sensory input. Second, the static tibial position at the time of the FETi spike can change the amplitude of the central EPSP, in the absence of any extensor muscle contraction. The EPSP can be up to 30 % greater in amplitude if FETi spikes with the tibia held flexed rather than extended. The primary sense organ mediating this effect is the femoral chordotonal organ. Evidence is presented suggesting that the mechanism underlying this change in gain may be specifically localised to the FETi-flexor connection, rather than being due to general position-dependent sensory feedback summing with the EPSP. The change in the amplitude of the central EPSP is probably not caused by general postsynaptic summation with tonic sensory input, since a diminution in the amplitude of the central EPSP caused by tibial extension is often accompanied by overall tonic excitation of the flexor motor neurone. Small but significant changes in the peak amplitude of the FETi spike have a positive correlation with changes in the EPSP amplitude, suggesting a likely presynaptic component to the mechanism of gain control. The change in amplitude of the EPSP can alter its effectiveness in producing flexor motor output and, thus, has functional significance. The change serves to augment the effectiveness of the FETi-flexor connection when the tibia is fully flexed, and thus to increase its adaptive advantage during the co-contraction preceding a jump or kick, and to reduce the effectiveness of the connection when the tibia is partially or fully extended, and thus to reduce its potentially maladaptive consequences during voluntary extension movements such as thrusting.

Quasi-reversible photo-axotomy used to investigate the role of extensor muscle tension in controlling the kick motor programme of grasshoppers.

The jump and kick of the grasshopper are behaviours which are potentially critical for the survival of the animal, and whose maximal performance depends upon optimizing the rate and level of tension development in the extensor tibiae muscle of the hind legs. In experimental conditions extensor tension control can be reduced to a single motoneuron, the fast extensor tibiae (FETi). The axon of FETi can be cut using dye-mediated laser photoaxotomy without damaging the central or peripheral portions of that neuron or any other neuron innervating the leg. The axotomy can be functionally reversed (i.e. the cut axon repaired) by an electronic axonal bypass which detects FETi spikes on the proximal side of the cut and stimulates the axon on the distal side of the cut. In this way motor spikes can either be allowed to reach the muscle or prevented from doing so (by switching the bypass on or off), and the motor programmes produced with and without extensor tension can be compared. The jump and kick are normally produced by a three-stage motor programme: (i) initial flexion brings the tibia into the fully flexed position; (ii) coactivation of extensor and flexor muscles allows the extensor muscle to develop maximal tension almost isometrically, while the simultaneous contraction of the flexor muscle holds the tibia flexed; (iii) sudden trigger inhibition of the flexor system (motoneurons and muscle) releases the tibia and allows the behaviour to be expressed. The grasshopper can produce fictive kicks with motor programmes which show each of these three major structural features of a normal kick, but without any extensor tension whatsoever. There is no significant difference in the frequency of FETi spikes, the duration of coactivation or the maximum depolarization of the flexor motoneurons between fictive and quasi-normal (i.e. reversed axotomy) kicks. The trigger inhibition of flexor motoneurons is shallower in fictive than in quasi-normal kicks. The significance of this is discussed in relation to the activity of the interneuron M, which is known to mediate trigger inhibition onto FITi motoneurons. There are two main conclusions from this study. First, the CNS does not need feedback from ETi muscle tension in order to produce the three-stage motor programme of the kick (and, by implication, the jump). Second, the CNS does not adjust the frequency or duration of FETi activity in response to unexpected changes in ETi tension. ETi tension appears to be under open-loop control in the kick motor programme.

The firing pattern of the locust ( Schistocerca gregaria americana) mesothoracic femoral motor axons in resistance reflexes and during walking on a treadmill

The firing pattern of the locust mesothoracid femoral motoneurones during passive extension of the tibia (resistance reflexes) has been examined. Of the flexor excitors, some are activated only by very fast tibial movements producing a few spikes. Others produce a discharge which lasts as long as the imposed tibial extension. The flexor slow motoneurones are the only spontaneously active neurones and their frequency increases and remains high as long as the tibial extension lasts. The inhibitors remain silent during active tibial flexion (or extension) and they are active only during maintained extension. From the mesothoracic extensor tibiae motor axons, the slow extensor tibiae fires during tibial flexion and the inhibitor during tibial extension. The fast extensor tibiae is silent during tibial movement. The activity of the femoral motoneurones was also studied in a tethered locust walking on a treadmill. First, the similarities in the firing motor pattern between a deafferented and a normal walking leg were established and then the activity of the flexor tibiae motoneurones, recorded from a deafferented leg of a walking locust, was analysed. A modification in the resistance reflexes during the active stage of the animal was also demonstrated. The resistance reflexes seemed to become assistant reflexes during walking.

Effects of temperature on a central synapse between identified motor neurons in the locust.

Changing the temperature from 10-40 degrees C modifies the transmission at an established monosynaptic connection between the fast extensor tibiae (FETi) and flexor tibiae motor neurons in the metathoracic ganglion of the locust Schistocerca gregaria (Forskål). Striking changes occur to the shape of the spikes, to membrane resistance, to the synaptic delay, and to the evoked synaptic potentials. In the presynaptic FETi motor neuron, raising the temperature reduces the amplitude of an antidromic spike recorded in the soma by a factor of 10 (40 mV to 4 mV), reduces the time taken to reach peak amplitude by 5 (3.5 to 0.7 ms) and decreases the duration at half maximum amplitude by 0.5. The conduction velocity of the spike in the axon is increased by 50% from 10 degrees C to 40 degrees C. Orthodromic spikes are affected by temperature in a similar way to the antidromic spikes. The membrane resistance of both pre- and postsynaptic motor neurons falls as the temperature is raised. The membrane resistance of FETi falls by a factor of 4 (about 4 M omega at 10 degrees C to 1 M omega at 40 degrees C). A contributory component to this fall could be the increase in the frequency of synaptic potentials generated as a result of inputs from other neurons. No temperature dependence could be demonstrated on the voltage threshold relative to resting potential for evoking orthodromic spikes, but because the resistance changes, the current needed to achieve this voltage must be increased at higher temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular Recordings From Interneurones and Motoneurones During Bilateral Kicks in the Locust: Implications for Mechanisms Controlling the Jump

ABSTRACT Intracellular recordings were made from the neuropile processes of thoracic neurones of Locusta migratoria during bilateral kicks of the hindlegs. Electromyographic (EMG) recordings showed that the pattern of flexor and extensor tibiae muscle activity during kicks in this extensively dissected preparation was similar to that seen during a jump. Intracellular recordings from hindleg flexor and extensor motoneurones and from 13 identified interneurones revealed additional features of the motor programme for jumping and kicking and of the mechanism which triggers these events. There was a discrete burst of activity in the fast extensor tibiae (FETi) moto-neurone at the end of the co-contraction phase, generated by a system that appeared to be separate from that triggering the kick. The excitatory connection from FETi to flexors was not responsible for initiating flexor activity and was of little functional importance in maintaining this activity during the co-contraction phase. The initial flexor excitation came from another, unidentified, central source. A pair of identified interneurones, the M-neurones, discharged with a high frequency burst just prior to the kick. Since these neurones inhibit hindleg flexor tibiae motoneurones, this observation provides further support for their proposed role as the neurones responsible for triggering kicks and jumps. Our data do not support the proposal that the activation of the M-neurones depends on them receiving progressively increasing proprioceptive input during the co-contraction phase. Throughout co-contraction, the M-neurones were hyperpolarized. Their activation was rapid and strong enough to cause them to discharge at rates as high as 400 spikes s−1. We suggest that the pulse-like activation of the M-neurones is produced centrally by a higher order system of interneurones. Another pair of previously identified interneurones, the C-neurones, were not necessary for the generation of the co-contraction phase of the motor programme. Their pattern of activity and their known connections indicated that they provide additional excitation to the flexors and extensors towards the end of co-contraction. Many other interneurones discharged either during co-contraction or when a kick was triggered. We conclude that the system generating the motor programme for a kick (jump) is more complex than proposed in previous studies.

Functional connections with foreign muscles made by a target-deprived insect motorneuron

Metathoracic limb buds were removed unilaterally from Locusta migratoria embryos at 30% of embryonic development, thereby depriving limb-innervating neurons of the opportunity of innervating their normal target muscles. The operated embryos were allowed to hatch and develop to adulthood, and then the connections between the identified limb motorneuron Fast Extensor Tibiae (FETi) and body wall muscles on the operated side of the segment were determined electrophysiologically. FETi innervated a number of foreign muscles in the ipsilateral body wall in limb-ablated locusts, showing that this neuron is not programmed to exclusively innervate its normal target muscle.

Axon growth from limb motorneurons in the locust embryo: The effect of target limb removal on the pattern of axon branching in the periphery

Metathoracic limb buds have been unilaterally ablated from locust embryos at 25 to 30% of embryonic development and the effect of this operation on the axon morphology of the motorneuron fast extensor tibiae (FETi) observed at later embryonic stages. In control embryos this neuron sends a single axon out the main leg nerve, nerve 5, to the extensor tibiae muscle in the femur. In limb ablated embryos the axon of FETi is found in a wide variety of aberrant peripheral nerve pathways and projects to a wide range of foreign muscles. There is a degree of apparent selectivity, but no rigid hierarchy, in the choice of pathway and muscle made by FETi. A high degree of variability is found between one embryo and another in the extent and pattern of axon branching. The axon of FETi is generally found in pathways that correspond to nerves in control embryos but on occasion grows along novel routes. An anteriorly directed dendritic branch, seldom seen in control FETi neurons, is frequently seen in experimental FETis. These findings are discussed in terms of the rules for specific axon growth in normal development.

Effects of temperature on identified central neurons that control jumping in the grasshopper

Grasshoppers, like many poikilotherms, are generally more active at warmer body temperatures. In particular, they jump more frequently when warm. To determine the neuronal basis of this increase in jumping activity, we investigated the effects of temperature on the properties of identified central neurons known to be involved in the control of the jump; these included the fast extensor tibiae (FETi) motoneuron and the C, G, and M interneurons. Heating did not result in a reduction in the current or voltage threshold for action potentials; in most cases, there was an increase in the current threshold with heating. At higher temperatures, the frequency-current relations of interneurons and motoneurons had steeper slopes. With strong current pulses, increasing the temperature resulted in an increase in the initial peak firing frequencies of central neurons and usually also in their steady state firing frequencies. A second temperature effect favoring increased CNS activity in warm grasshoppers was increased afferent input from the periphery. In a broad variety of sensory receptors, there was a dramatic increase in their sensitivity to sensory stimuli at both threshold and suprathreshold intensities. Various identified central neurons differed in the way in which some of their properties were influenced by temperature. The C and G interneurons showed a striking similarity in the unusual way in which their repetitive firing properties were influenced by heating. Since these neurons are sibling progeny of a single neuroblast, this shared physiological property is correlated with their developmental history.

Structural comparison of a homologous neuron in gryllid and acridid insects

The fast extensor tibiae (FETi) motor neuron is responsible for exciting the extensor tibiae muscle to produce most of the force for jumping in acridids. Because of its relatively large size and crucial role in jumping, FETi has been studied in an ever-increasing number of orthopteran species. Here we describe the structure of the metathoracic FETi neuron in six species of acridids and in two species of gryllids. The morphology of FETi within the respective groups is essentially equivalent, but marked differences are apparent between acridid and gryllid FETis. There are similarities in the size and location of the cell body and the course of the neurite through the ganglion. Differences are found in the number of large branches, density of branching, and the volume of neuropil receiving branches. We propose that the gryllid FETi is an intermediate form between slow extensor tibiae motor neurons involved in walking and acridid fast extensor tibiae motor neurons specialized for jumping.

The ultrastructure of identified locust motor neurones and their synaptic relationships.

Motor neurones in the thoracic ganglia of the locust were impaled with microelectrodes and identified according to the muscle they innervated and their other physiological properties. They were then labeled by intracellular injection of horseradish peroxidase and processed for electron microscopy. The nature and distribution of synapses on each motor neurone was examined and, by the use of reconstruction from serial sections, their spatial relationships revealed. The metathoracic fast extensor tibiae and mesothoracic tergosternal flight motor neurones have both output and input synapses on their neuropilar branches. These synapses are involved in serial, reciprocal, and recurrent relationships showing that the structural equivalent of a physiological synapse may be complex. The metathoracic slow extensor tibiae and anterior fast flexor tibiae motor neurones apparently have only input synapses within the neuropile.