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

Two opposing muscle systems underlie abdominal contractions during escape swimming in crayfish. In this study we used extracellular and intracellular stimulation, recording and dye-filling to systematically identify each of the five deep extensor excitors and single inhibitor 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. Each excitor receives excitatory input from the tonic abdominal stretch receptors and the largest neuron also receives input from the phasic stretch receptor. The two largest excitors innervate the muscle bundle containing the fastest fibres and may be electronically coupled. The smaller neurons may also be electronically coupled and innervate the remaining deep extensor fibres which display dynamic characteristics from fast to medium-fast. The inhibitor does not receive input from the stretch receptors, but is strongly excited by tactile afferents. The implications of these findings for the current models of the control of abdominal tailflips and swimming are discussed. Accepted: 21 June 1998     

The whole-body shortening reflex of the medicinal leech: motor pattern,sensory basis,and interneuronal pathways

The leech whole-body shortening reflex consists of a rapid contraction of the body elicited by a mechanical stimulus to the anterior of the animal. We used a variety of reduced preparations — semi-intact, body wall, and isolated nerve cord — to begin to elucidate the neural basis of this reflex in the medicinal leech Hirudo medicinalis. The motor pattern of the reflex involved an activation of excitatory motor neurons innervating dorsal and ventral longitudinal muscles (dorsal excitors and ventral excitors respectively), as well as the L cell, a motor neuron innervating both dorsal and ventral longitudinal muscles. The sensory input for the reflex was provided primarily by the T (touch) and P (pressure) types of identified mechanosensory neuron. The S cell network, a set of electrically-coupled interneurons which makes up a fast conducting pathway in the leech nerve cord, was active during shortening and accounted for the shortest-latency excitation of the L cells. Other, parallel, interneuronal pathways contributed to shortening as well. The whole-body shortening reflex was shown to be distinct from the previously described local shortening behavior of the leech in its sensory threshold, motor pattern, and (at least partially) in its interneuronal basis.Abbreviations conn connective - DE dorsal excitor motor neuron - DI dorsal inhibitor motor neuron - DP dorsal posterior nerve - DP:B1 dorsal posterior nerve branch 1 - DP:B2 dorsal posterior nerve branch 2 - MG midbody ganglion - VE ventral excitor motor neuron - VI ventral inhibitor motor neuron     

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.     

Innervation and motor patterns of the abdominal superficial flexor muscles in larval lobsters

The pattern of innervation and motor program of the abdominal superficial flexor muscle was investigated electrophysiologically in larval lobsters (Homarus americanus). The muscle receives both excitatory and inhibitory innervation in the larval as well as in the embryonic stages. Individual muscle fibers receive a single inhibitory neuron (f5) and a maximum of three excitors. Based on spike heights these axons belong to either the small (f1 or f2) or large (f3, f4) motoneurons. While the small axons preferentially innervate the medial muscle fibers the large axons innervate medial as well as lateral fibers. This larval pattern of innervation resembles the pattern in the adult lobster. The resemblance extends to the firing patterns as well with both large and small excitors firing spontaneously. Furthermore, evoked activity in the larvae produces reciprocal (and occasionally cyclical) bursts of excitor and inhibitor neurons denoting abdominal extension and flexion and resembling the firing patterns in adults. Consequently motor programs employed in steering the pelagic larvae are reminiscent of the programs for maintaining posture in the benthic adult lobsters.     

The morphology,physiology and function of suboesophageal neck motor neurons in the honeybee

We report some of the neural and muscular circuitry that allows honeybees to control head movements. We studied neck motor neurons with cell bodies in the suboesophageal ganglion, axons in the first cervical nerve (IK1) and terminals in neck muscles 44 and 51 (muscle classification: Snodgrass in Smithsonian Misc Coll 103:1-120, 1942). We show that muscle 44 actually comprises five separate bundles of muscle fibres (subunits), while muscle 51 is split into two subunits. Eight motor neurons innervate muscles 44 and 51. Two motor neurons have cell bodies in the ventral-median cell body group (one innervates a subunit in muscle 44, the other a subunit in muscle 51). One motor neuron has a ventrally located contralateral cell body (innervating a subunit in muscle 44) and five have laterally located ipsilateral cell bodies. Of the five lateral cells, one innervates a subunit in muscle 51, three selectively innervate subunits in muscle 44 and one co-innervates a subunit in muscle 44 with the contralateral cell. Extracellular recordings revealed three types of visually driven, direction-selective cell-types in each IK1 tuned for leftward, rightward and downward motion over the eyes. The spatiotemporal tuning of the units is similar to that of other visual interneurons in the bee brain.     

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.     

Innervation and control of tension in abdominal muscles of Blaberus discoidalis and Gromphadorhina portentosa

In Blaberus discoidalis and Gromphadorhina portentosa, the distribution of motor axons to the muscles which control movements of the spiracular valves at both respiratory and non-respiratory spiracles is identical. Both fast and slowly contracting heads of the opener muscles are innervated by an excitatory motor axon. Physiological properties of the opener excitor axon correlate with valve function. The slowly contracting head of the opener muscle is, in addition, innervated by a common inhibitor which also occasionally innervates closer muscle fibers. Activation of the common inhibitor terminates contraction of slowly contracting opener muscle fibres and initiates a rapid relaxation of these fibres.     

Neuronal control of swimming in the medicinal leech

Summary The cell bodies and function of twelve neurons whose impulse pattern is clearly related to that of the swimming rhythm were identified in the segmental ganglion of the leech. These include excitatory and inhibitory motor neurons of the dorsal and ventral longitudinal muscles and the excitatory flattener motor neuron of the dorsoventral muscles. During swimming the membrane potential of these cells oscillates between a depolarized and a hyperpolarized phase. The activity of this ensemble of cells is sufficient to account for the contractile rhythm of the swimming animal. The following connections were found between these motor neurons. Electrotonic junctions link: (1) bilaterally homologous cells; (2) excitors of the dorsal longitudinal muscles; (3) excitors of the ventral longitudinal muscles; (4) inhibitors of both dorsal and ventral longitudinal muscles. The dorsal inhibitors project via an inhibitory pathway to the dorsal excitors, and the ventral inhibitor projects via an inhibitory pathway to the ventral excitors. The membrane potential oscillation of the excitors is at least partly attributable to the phasic inhibitory synaptic input which they receive from the inhibitors. The excitatory shortener motor neuron of the entire longitudinal musculature is maintained in an inactive state during swimming. This control is achieved by rectifying electrotonic junctions linking this neuron to the dorsal and ventral excitors. These junctions allow passage of only depolarizing current from the shortener to the dorsal and ventral excitors and of only hyperpolarizing current in the reverse direction. Furthermore, both dorsal and ventral inhibitors project via inhibitory pathways to the shortener neuron.We are greatly indebted to Ann Stuart for advice and help in this study, and for communicating to us some unpublished findings. We thank Elizabeth Mullenbach for excellent technical assistance.This research was supported by grant GB 31933 X from the National Science Foundation, and by Public Health Service Research grant GM 17866 and Training Grant GM 01389 from the Institute for General Medical Sciences.     

Penis-retractor muscle of Aplysia: excitatory motor neurons

Cobalt axonal iontophoresis and intracellular recordings were used to identify a cluster of several motor neurons innervating the penis-retractor muscle of Aplysia. Intracellularly recorded motor neuron action potentials elicited direct, one-for-one, constant latency excitatory junctional potentials (ejps) in individual muscle fibers. The axons of motor neurons could be recorded extracellularly in the penis-retractor nerve and stimulation of the nerve backfired the motor neurons. Perfusion of the ganglion, the muscle, or both with solutions of either increased Mg++/decreased Ca++ or increased Ca++ sea water indicated that the presumed motor neuron impaled was not a sensory cell and that interneurons were not intercalated in the pathway. Innervation of muscle fibers was found to be functionally polyneuronal and diffuse. The ejps were found to undergo marked facilitation with repetitive motor-neuron stimulation. The motor neurons were isolated in a distinct cluster in the right pedal ganglion. Their electrical activity was characterized by spontaneous irregular action potentials and a moderate input of postsynaptic potentials.     

The role of sensory input in motor neuron sprouting control

Primary sensory neurons project to motor neurons directly or through interneurons and affect their activity. In our previous paper we showed that intramuscular sprouting can be affected by changing the sensory synaptic input to motor neurons. In this work, motor axon sprouting within a peripheral nerve (extramuscular sprouting) was induced by nerve injury at such a distance from muscle so as not to allow nerve-muscle trophic interactions. Two different procedures were carried out: (1) sciatic nerve crush and (2) sciatic nerve crush with homosegmental ipsilateral L3-L5 dorsal rhizotomy. The number of regenerating motor axons innervating extensor digitorum longus muscle was determined by in vivo muscle tension recordings and an index of their individual conduction rate was obtained by in vitro intracellular recordings of excitatory postsynaptic end-plate potentials in muscle fibers. The main findings were: (1) there are more regenerated axons distally from the lesion than parent axons proximally to the lesion (sprouting at the lesion); (2) sprouting at the lesion was negatively affected by homosegmental ipsilateral dorsal rhizotomy; (3) the number of motor axons innervating extensor digitorum longus muscle extrafusal fibers counted proximally to the lesion increased following nerve injury and regeneration but this did not occur when sensory input was lost. A transient innervation of extrafusal fibers by &#110 motor neurons may explain the increase of motor axons counted proximally to the lesion.     

The role of sensory input in motor neuron sprouting control

Primary sensory neurons project to motor neurons directly or through interneurons and affect their activity. In our previous paper we showed that intramuscular sprouting can be affected by changing the sensory synaptic input to motor neurons. In this work, motor axon sprouting within a peripheral nerve (extramuscular sprouting) was induced by nerve injury at such a distance from muscle so as not to allow nerve-muscle trophic interactions. Two different procedures were carried out: (1) sciatic nerve crush and (2) sciatic nerve crush with homosegmental ipsilateral L3-L5 dorsal rhizotomy. The number of regenerating motor axons innervating extensor digitorum longus muscle was determined by in vivo muscle tension recordings and an index of their individual conduction rate was obtained by in vitro intracellular recordings of excitatory postsynaptic end-plate potentials in muscle fibers. The main findings were: (1) there are more regenerated axons distally from the lesion than parent axons proximally to the lesion (sprouting at the lesion); (2) sprouting at the lesion was negatively affected by homosegmental ipsilateral dorsal rhizotomy; (3) the number of motor axons innervating extensor digitorum longus muscle extrafusal fibers counted proximally to the lesion increased following nerve injury and regeneration but this did not occur when sensory input was lost. A transient innervation of extrafusal fibers by gamma motor neurons may explain the increase of motor axons counted proximally to the lesion.     

Sprouting and connectivity of embryonic leech heart excitor (HE) motor neurons in the absence of their peripheral target

The rhythmic pumping of the hearts in the medicinal leech,Hirudo medicinalis, is neurogenic and mediated by a defined circuit involving identified interneurons in a central pattern generator (CPG) and segmentally iterated motor neurons that drive the heart muscle. During early embryogenesis, presumptive heart excitor (HE) motor neurons extend many axon branches into the body wall; they later innervate the heart while retracting the supernumerary peripheral axons, and only much later in development receive synaptic input from the central pattern generator (Jellies, Kopp and Bledsoe (1992)J. Exp. Biol., 170, 71–92.)- In this study, HE motor neurons were deprived of an early interaction with the heart by surgical ablation of a circumscribed portion of body wall including the heart primordium. Anatomical and electrophysiological data were obtained using intracellular techniques to examine the hypothesis that peripheral interactions with the developing heart provide instructive cues for the final differentiation of these neurons. Target-deprived HE motor neurons continued to extend multiple axons in ventral, lateral and dorsal body wall throughout late embryonic and into postembryonic stages and they extended anomalous axons within the CNS. This resembles the early embryonic growth of HE motor neurons before heart tube differentiation. Furthermore, HE motor neurons deprived of heart contact exhibited tonic activity similar to the situation during early development before they are contacted by the CPG interneurons. In contrast, sham-operated and contralateral HE motor neurons oscillated normally. These results suggest that heart tube contact is specifically required for at least some aspects of HE development and provide a framework in which to identify cell-cell interactions that are involved in matching neurons and targets to generate behaviorally relevant neural circuits.     

Characterization of cardiac innervation in the nudibranch,Archidoris montereyensis

Summary The heart of the nudibranch mollusc Archidoris montereyensis is regulated by a small number of powerful effector neurons located in the right pleural and visceral ganglia. Two identifiable neurons in the pleural ganglion, a heart excitor (pl_HE) and a heart inhibitor (Pl_HI), are especially important regulators of cardiac function in that low levels of spontaneous activity in either cell significantly alters the amplitude and rate of heart contractions. These neurons have extensive dendritic arbors within the right pleural ganglion and branching axonal processes within the visceral ganglion. The visceral ganglion also contains a heart excitor neuron (V_HE) and at least two heart inhibitor neurons (V_HI cells), but their influence on cardiac activity is weaker than that of the pleural ganglion cells. All of these heart effector cells appear to be motor neurons with axons that terminate predominately in the atrio-ventricular valve region of the heart via the pericardial nerve. The simplicity and strength of these neuronal connections to the heart of Archidoris make this a favorable preparation for studies of cardiac regulation.Abbreviations Pl _HE pleural ganglion heart excitor neuron - Pl _HI pleural heart inhibitor neuron - V _HE visceral ganglion heart excitor neuron - V _HI cells, visceral heart inhibitor neurons - V _K visceral kidney excitor neuron - V _G visceral gill excitor neuron     

Centrally patterned rhythmic activity integrated by a peripheral circuit linking multiple oscillators

The central pattern generator for heartbeat in the medicinal leech, Hirudo generates rhythmic activity conveyed by heart excitor motor neurons in segments 3-18 to coordinate the bilateral tubular hearts and side vessels. We focus on behavior and the influence of previously un-described peripheral nerve circuitry. Extracellular recordings from the valve junction (VJ) where afferent vessels join the heart tube were combined with optical recording of contractions. Action potential bursts at VJs occurred in advance of heart tube and afferent vessel contractions. Transections of nerves were performed to reduce the output of the central pattern generator reaching the heart tube. Muscle contractions persisted but with a less regular rhythm despite normal central pattern generator rhythmicity. With no connections between the central pattern generator and heart tube, a much slower rhythm became manifest. Heart excitor neuron recordings showed that peripheral activity might contribute to the disruption of centrally entrained contractions. In the model presented, peripheral activity would normally modify the activity actually reaching the muscle. We also propose that the fundamental efferent unit is not a single heart excitor neuron, but rather is a functionally defined unit of about three adjacent motor neurons and the peripheral assembly of coupled peripheral oscillators.     

Slow motor neuron stimulation of locust skeletal muscle: model and measurement

The isometric force response of the locust hind leg extensor tibia muscle to stimulation of a slow extensor tibia motor neuron is experimentally investigated, and a mathematical model describing the response presented. The measured force response was modelled by considering the ability of an existing model, developed to describe the response to the stimulation of a fast extensor tibia motor neuron and to also model the response to slow motor neuron stimulation. It is found that despite large differences in the force response to slow and fast motor neuron stimulation, which could be accounted for by the differing physiology of the fibres they innervate, the model is able to describe the response to both fast and slow motor neuron stimulation. Thus, the presented model provides a potentially generally applicable, robust, simple model to describe the isometric force response of a range of muscles.     

Presynaptic inhibition and the participation of GABAB receptors at neuromuscular junctions of the crab Eriphia spinifrons

Presynaptic inhibition exerted by the common inhibitor on the closer and opener muscles and by the specific inhibitor on the opener muscle was investigated in the crab Eriphia spinifrons. In the closer muscle, activation of GABA_B receptors by baclofen reduced the mean quantal content of excitatory junctional currents by about 25%. Blocking GABA_B receptors with CGP 55845 diminished presynaptic inhibition at a similar percentage. GABA_B receptor-mediated presynaptic inhibition is linked to G proteins. Application of pertussis toxin eliminated about 25% of the inhibition exerted by the common inhibitory neuron. GABA_B receptors participate in presynaptic inhibition at release boutons of the slow and the fast closer excitor at a similar percentage. In the opener muscle, presynaptic inhibition of transmitter release from the same endings of the opener excitor was about 15% stronger with the specific inhibitor than with the common inhibitor. About 10% of the presynaptic inhibition produced by either one of the two inhibitors could be abolished by blocking GABA_B receptors. The amplitudes of the excitatory junctional currents in the opener were reduced in the presence of baclofen by about 25%, suggesting that synaptic terminals of the opener excitor are endowed with a similar percentage of GABA_B receptors as terminals of the slow and the fast closer excitors. Baclofen had no effect on postsynaptic inhibition, indicating that GABA_B receptors are not involved in postsynaptic neuromuscular inhibition. Accepted: 8 January 2000     

Sources of inputs to longitudinal muscle motor neurons and ascending interneurons in the guinea-pig small intestine

Light- and electron-microscopic studies were used to investigate connections between specific subgroups of neurons in the myenteric plexus of the guineapig small intestine. Inputs to two classes of calretinin-immunoreactive (IR) nerve cells, longitudinal muscle motor neurons and ascending interneurons, were examined. Inputs from calbindin-IR primary sensory neurons and from three classes of descending interneurons were studied. Electron-microscopic analysis showed that calbindin-IR axons formed two types of inputs, synapses and close contacts, on calretinin-IR neurons. About 40% of inputs to the longitudinal muscle motor neurons and 70% to ascending interneurons were calbindin-IR. Approximately 50% of longitudinal muscle motor neurons were surrounded by bombesin-IR dense pericellular baskets and 40% by closely apposed varicosities. At the electron-microscope level, the bombesin-IR varicosities were found to form synapses and close contacts with the motor neurons. Dense pericellular baskets with bombesin-IR surrounded 36% of all ascending interneurons, and a further 17% had closely apposed varicosities. Somatostatin-and 5-HT-IR descending interneurons provided no dense pericellular baskets to calretinin-IR nerve cells. Thus, calretinin-IR, longitudinal muscle motor neurons and ascending interneurons receive direct synaptic inputs from intrinsic primary sensory neurons and from non-cholinergic, bombesin-IR, descending interneurons.     

Effects of hypobaric hypoxia on the oxidative capacity of the extensor digitorum longus motor units in the rat

The fiber number, fiber type distribution, and succinate dehydrogenase activity were investigated from the fast-twitch extensor digitorum longus muscle of male rats exposed to 7 weeks of hypobaric hypoxia. The oxidative metabolic capacity of the motoneurons in the extensor digitorum longus neuron pool was also determined from quantitative histochemical analyses. The fiber number and oxidative enzyme activity of the muscle were not changed by hypoxia. An increase in the percentage of fast-twitch oxidative (FO) fibers and a concemitant decrease in the percentage of fast-twitch (F) fibers were observed in the hypoxic muscle. On the other hand, the oxidative capacity of small-to medium-sized alpha motoneurons (25–45 m average soma diameter) was increased. The increase in the oxidative capacity of small- to medium-sized motoneurons and the type shift of muscle fibers from F (low-oxidative) to FO (high-oxidative) indicate that hypoxia enhances the oxidative capacity of particular motor units in the neuron pool.     

Comparison of motor patterns in the intact and deafferented flight system of the locust

Summary The activity of flight interneurons was recorded intracellularly in intact, tethered flying locusts (Locusta migratoria) and after removal of sensory input from the wing receptors. Depolarization patterns and spike discharges were characterized and compared for the two situations.In general, depressor interneurons (n=6) showed only minor changes in their activity as a result of deafferentation (Fig. 1). Exceptions were interneurons 308 and 506 (Fig. 2). By contrast, all but one of the elevator interneurons (n=9) produced distinctly different depolarization patterns in intact locusts and following deafferentation. Three different groups of elevator interneurons were found (excluding the one exceptional neuron, Fig. 6). (i) One group of interneurons (n=4) produced different, superthreshold depolarizations in intact and deafferented animals (Fig. 3). Characteristic, biphasic depolarizations were recorded from these fibres at lower wingbeat frequencies in the intact situation but only single, delayed potentials were recorded after deafferentation. (ii) The second group of interneurons (n=3) exhibited distinct rhythmic activity only in intact animals. After deafferentation their depolarizations were small and often below the threshold for spike initiation (Fig. 4). (iii) One interneuron produced rhythmic flight motor oscillations only after deafferentation. In intact locusts the membrane potential of this neuron showed very small oscillations and remained subthreshold (Fig. 5).Four main conclusions emerge from these data. (i) The activity of elevator interneurons is under greater sensory control than that of the depressors. This confirms the results of our previous electromyographic and motoneuronal analyses, (ii) A considerable portion of elevator activity is generated as a result of phasic sensory feedback. An essential input is from the hindwing tegulae (Table 1; Pearson and Wolf 1988). (iii) The activity of depressor interneurons appears to be determined by central mechanisms to a major extent. (iv) Different sets of central neurons appear to be involved in flight pattern generation in intact and deafferented locusts —although the two sets share many common elements.Abbreviations EMG electromyogram - PSP postsynaptic potential (EPSP excitatory andIPSP inhibitory)     

Serial homologies of the motor neurons of the dorsal intersegmental muscles of the cockroach,Periplaneta americana (L.)

The types and locations of serially homologous motor neurons of the dorsal muscles in the cockroach Periplaneta americana remain rather constant regardless of the various adaptations of their muscles or the fusion of ganglia. However, the size and number of neurons do vary according to the development of the muscles they innervate. Neurons in four distinctive locations, two ipsisegmental and two antesegmental, innervate the dorsal longitudinal (DL) muscles in most segments. One of the ipsisegmental neurons (DLC) is common to all of the DL muscles of a segment and probably has a modulatory function. The dorsal oblique (DO) muscles of most segments have neurons in two antesegmental positions. One of these, an antesegmental, contralateral neuron, innervates both DO and DL muscles in each segment and is also probably modulatory. One neuron (DOC) of the prothoracic ganglion is the principal exception to the constancy of these serially homologous neurons. This neuron appears to be homologous to the DLC neurons of other segments but innervates the DO rather than the DL muscles.