Innervation of the limb accessory flexor muscle in several decapod crustaceans. II. Electrophysiology

The innervation of the distal and proximal heads of the accessory flexor muscle in three portunid crabs and two non-portunid decapods was studied electrophysiologically. In all species studied, the proximal head received only the two previously reported accessory flexor axons, an excitor and an inhibitor. The same two axons also innervated the distal head in all species, but in the portunids the distal head also received excitation from at least three, and probably sometimes four, of the main flexor excitor efferents. The accessory inhibitor exerted very strong effects in the tonic muscle fibers found in the proximal head and in the most proximal bundle of the distal head. The newly described inhibitory and excitatory distributions may have important implications for locomotory behavior.     

Regeneration of motorneurones to a cricket muscle after partial or complete denervation

The metathoracic extensor tibiae muscle of the cricket Teleogryllus oceanicus is innervated by two excitatory axons; one of which leaves the metathoracic ganglion through nerve 5, the other through nerve 3. Axons in nerve 5 frequently regenerate to reinnervate the extensor tibiae if the nerve is sectioned in a late nymphal stage; functional reinnervation is rare if the nerve is sectioned in young adults. The muscle may become reinnervated by several axons regenerating through nerve 5, and individual muscle fibres may receive inputs from two regenerated axons. Axons regrowing through nerve 5 to a partially-denervated extensor tibiae preferentially innervate fibres in the central portion of the muscle, which is the normal innervation field of nerve 5. If the muscle is totally denervated by transection of both nerve 5 and nerve 3b, reinnervation is less specific and fibres throughout the muscle may be reinnervated by axons in either nerve. Reinnervation by regenerating axons is progressive. The proportion of muscles which are functionally reinnervated by regenerated axons increases with survival time as does the proportion of fibres within a muscle with reinnervation. The amplitude of excitatory junctional potentials and of muscle contraction evoked by regenerated axons both increase with survival time.     

The pattern of innervation in serially duplicated axolotl limbs: further evidence for the existence of local pathway cues?

The innervation of the biceps muscle was examined in regenerated and vitamin A-induced serially duplicated axolotl forelimbs using retrograde transport of horseradish peroxidase. The regenerated biceps muscle becomes innervated by motor neurones in the same position in the spinal cord as the normal biceps motor pool. In previous experiments in which the innervation of a second copy of a proximal limb muscle was examined in serially duplicated limbs (Stephens, Holder & Maden, 1985), the duplicate muscle was found to become innervated by motor neurones that would normally have innervated distal muscles. In the present study, the innervation of the second copy of biceps was examined under conditions designed to encourage nerve sprouting from 'correct' biceps axons. Following either partial limb denervation or denervation coupled with removal of the proximal biceps, the second copy of the muscle was still innervated by inappropriate motor neurones, which again would normally innervate distal limb muscles. These results are interpreted as evidence for the necessity for an appropriate local environment for axonal growth to allow reformation of a correct pattern of motor innervation in the regenerated limb.     

Motor patterns during kicking movements in the locust

Locusts (Schistocerca gregaria) use a distinctive motor pattern to extend the tibia of a hind leg rapidly in a kick. The necessary force is generated by an almost isometric contraction of the extensor tibiae muscle restrained by the co-contraction of the flexor tibiae (co-contraction phase) and aided by the mechanics of the femoro-tibial joint. The stored energy is delivered suddenly when the flexor muscle is inhibited. This paper analyses the activity of motor neurons to the major hind leg muscles during kicking, and relates it to tibial movements and the resultant forces.During the co-contraction phase flexor tibiae motor neurons are driven by apparently common sources of synaptic inputs to depolarized plateaus at which they spike. The two excitatory extensor motor neurons are also depolarized by similar patterns of synaptic inputs, but with the slow producing more spikes at higher frequencies than the fast. Trochanteral depressors spike at high frequency, the single levator tarsi at low frequency, and common inhibitors 2 and 3 spike sporadically. Trochanteral levators, depressor tarsi, and a retractor unguis motor neuron are hyperpolarized.Before the tibia extends all flexor motor neurons are hyperpolarized simultaneously, two common inhibitors, and the levator trochanter and depressor tarsi motor neurons are depolarized. Later, but still before the tibial movement starts, the extensor tibiae and levator tarsi motor neurons are hyperpolarized. After the movement has started, the extensor motor neurons are hyperpolarized further and the depressor trochanteris motor neurons are also hyperpolarized, indicating a contribution of both central and sensory feedback pathways.Variations in the duration of the co-contraction of almost twenty-fold, and in the number of spikes in the fast extensor tibiae motor neuron from 2–50 produce a spectrum of tibial extensions ranging from slow and weak, to rapid and powerful. Flexibility in the networks producing the motor pattern therefore results in a range of movements suited to the fluctuating requirements of the animal.     

The distribution of the common inhibitory neuron in brachyuran limb musculature

Summary In the walking legs of two common crabs, antidromic stimulation of the common inhibitory axon (CI) from either opener or closer nerve produces inhibitory potentials in certain fibers of every muscle distal to the ischiopodite. In particular, CI inhibits the flexor and accessory flexor muscles of the meropodite and abolishes or reduces contractile force in the flexor. The specific opener inhibitor, OI, formerly believed to innervate the flexor, has no electrical or mechanical effect on this muscle. The brachyuran inhibitory limb innervation thus appears to be the same as that accepted for the anomurans, comprising one universally distributed common inhibitor and two truly specific inhibitors serving the opener and stretcher muscles.Abbreviations CI common inhibitor - OI opener inhibitor - SI stretcher inhibitor - FI flexor inhibitor - FE flexor excitor(s)     

Proprioceptive feedback in locust kicking and jumping during maturation

Campaniform sensilla monitor the forces generated by the leg muscles during the co-contraction phase of locust (Schistocerca gregaria) kicking and jumping and re-excite the fast extensor (FETi) and flexor tibiae motor neurones, which innervate the leg muscles. Sensory signals from a campaniform sensillum on the proximal tibia were compared in newly moulted locusts, which do not kick and jump, and mature locusts which readily kick and jump. The activity pattern of FETi during co-contraction was mimicked by stimulating the extensor tibiae muscle. Less force was generated and the spike frequency of the sensory neurone from the sensillum was significantly lower in newly moulted compared to mature locusts. Depolarisation of both FETi and flexor motor neurones as a result of sensory feedback was consequently less in newly moulted than in mature locusts. The difference in the depolarisation was greater than the decrease in the afferent spike frequency suggesting that the central connections of the afferents are modulated. The depolarisation could generate spikes in FETi and maintain flexor spikes in mature but not in newly moulted locusts. This indicates that feedback from the anterior campaniform sensillum comprises a significant component of the drive to both FETi and flexor activity during co-contraction in mature animals and that the changes in this feedback contribute to the developmental change in behaviour.Abbreviations aCS anterior campaniform sensillum - ETi extensor tibiae - FETi fast extensor tibiae motor neurone - FlTi flexor tibiae - pCS posterior campaniform sensillum     

Anatomical partitioning of three multiarticular human muscles.

To examine neuromuscular partitioning within human muscles, the innervation patterns and muscle fiber architecture of the flexor carpi radialis (FCR), extensor carpi radialis longus (ECRL) and lateral gastrocnemius (LG) muscles were examined. Consistent patterns of innervation between specimens were found within each of the three muscles. The nerve to the FCR clearly innervates three major architectural divisions of the muscle. The ECRL is innervated by two different muscle nerves. Branches of these nerves innervate at least two distinct anatomical subvolumes. However, the subvolumes of the ECRL defined by muscle architecture are not totally congruent with those defined by the innervation pattern. In the LG, the single muscle nerve branches into two main divisions, and these subsequently divide into branches which supply the three heads. However, each head does not receive a completely private nerve. These results indicate that human muscles are partitioned in a manner roughly similar to the divisions of the same muscles in cats and rats, but with less congruency of architecture and innervation.     

Structure and physiology of the locust femoral chordotonal organ

The connective chordotonal organs (COs) in the femora of the prothoracic and mesothoracic legs of the locust Schistocerca gregaria are divided into two parts, the proximal and the distal scoloparia. The proximal scoloparium contains about 150 small neurons and is anchored to the femoral cuticle. The distal scoloparium contains about 50 larger neurons and is connected at its proximal end to both the cuticle and the flexor tibiae muscle.Records were made from the distal scoloparium, classifying units by spike size. The tibial position/total activity response curve is ∪-shaped but when a small number of units is selected the responses occur only when the tibia is on one side of its centre position. The tonic responses display considerable hysteresis and a degree of adaptation which varies with the tibial angle. Units with phasic and phasic-tonic responses are common and their responsiveness depends on the range of angles the tibia is moved through. The same units respond strongly to flexor tibiae contraction with the tibia either fixed or free, and so may serve as receptors for tension in that muscle.The CO mediates phasic resistance reflexes in all three extensor tibiae motoneurons and tonic reflexes in the extensor ‘slow’ neuron. It is suggested that the very detailed information furnished by the CO is used in a complex way in the control of the femoral muscles.     

Reflex modulation of motoneurone activity in the cheliped of the crayfish Astacus leptodactylus.

1. The reflex activity elicited by movement of the mero-carpopodite (M-C) joint in the cheliped of the crayfish Astacus leptodactylus is investigated and the role of the different proprioceptors (chordotonal and myochordotonal organs) separately studied. 2. The reflex discharge involves mainly the tonic motoneurones of the extensor (E), the flexor (F) and the accessory flexor (AF) muscles. 3. M-C joint posture is also regulated by the cuticular stress detector (CSD2) afferents: they increase mainly the F discharge and secondarily the AF command. 4. The activity of the motor axons supplying the muscles of the meropodite can be also influenced by a variety of natural stimuli applied to other appendages. The effect usually produced is a general flexion reaction which is characterized by a reciprocity between E and F involving both central and peripheral mechanisms. 5. The AF muscle is innervated by two antagonistic motoneurones, an excitatory neurone functionally linked in its discharge with one of the four excitors supplying F and an inhibitory motoneurone, common with E. The resulting competitive effect between these two neurones has been recorded intracellularly in AF muscle fibres. 6. The role of the myochordotonal organ (MCO) in the crayfish is discussed. In particular the modulation of the AF command in relation to the discharges of the motor nerves to the main muscle E and F is studied.     

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.     

Muscle type-specific myosin isoforms in crustacean muscles

Differential expression of multiple myosin heavy chain (MyHC) genes largely determines the diversity of critical physiological, histochemical, and enzymatic properties characteristic of skeletal muscle. Hypotheses to explain myofiber diversity range from intrinsic control of expression based on myoblast lineage to extrinsic control by innervation, hormones, and usage. The unique innervation and specialized function of crayfish (Procambarus clarkii) appendicular and abdominal musculature provide a model to test these hypotheses. The leg opener and superficial abdominal extensor muscles are innervated by tonic excitatory motoneurons. High resolution SDS-PAGE revealed that these two muscles express the same MyHC profile. In contrast, the deep abdominal extensor muscles, innervated by phasic motoneurons, express MyHC profiles different from the tonic profiles. The claw closer muscles are dually innervated by tonic and phasic motoneurons and a mixed phenotype was observed, albeit biased toward the phasic profile seen in the closer muscle. These results indicate that multiple MyHC isoforms are present in the crayfish and that differential expression is associated with diversity of muscle type and function.     

The structure and function of serially homologous leg motor neurons in the locust. I. Anatomy

Twenty-one prothoracic and 17 mesothoracic motor neurons innervating leg muscles have been identified physiologically and subsequently injected with dye from a microelectrode. A tract containing the primary neurites of motor neurons innervating the retractor unquis, levator and depressor tarsus, flexor tibiae, and reductor femora is described. All motor neurons studied have regions in which their dendritic branches overlap with those of other leg motor neurons. Identified, serially homologous motor neurons in the three thoracic ganglia were found to have: (1) cell bodies at similar locations and morphologically similar primary neurites (e.g., flexor tibiae motor neurons), (2) cell bodies at different locations in each ganglion and morphologically different primary neurites in each ganglion (e.g., fast retractor unguis motor neurons), or (3) cell bodies at similar locations and morphologically similar primary neurites but with a functional switch in one ganglion relative to the function of the neurons in the other two ganglia. As an example of the latter, the morphology of the metathoracic slow extensor tibiae (SETi) motor neurons was similar to that of pro- and mesothoracic fast extensor tibiae (FETi) motor neurons. Similarly the metathoracic FETi bears a striking resemblance to the pro- and the mesothoracic SETi. It is proposed that in the metathoracic ganglion the two extensor tibiae motor neurons have switched functions while retaining similar morphologies relative to the structure and function of their pro- and mesothoracic serial homologues.     

The structure and function of serially homologous leg motor neurons in the locust. II. Physiology

Simultaneous intracellular recordings were made from pairs of motor neurons in the pro- or mesothoracic ganglion of the locust. Though central connections were sought between pairs of motor neurons, none were found. This is in sharp contrast to the findings that flexor and extensor tibiae neurons in the metathoracic ganglion make certain connections between themselves (Hoyle and Burrows, 1973; Heitler and Burrows, 1977a). As the previously mentioned authors believed that the metathoracic flexor-extensor connections were used as part of the motor program for jumping and kicking, the present results strongly support their hypothesis. Common PSPs have been found in a variety of pairs of motor neurons. Of note are common PSPs of the same sign to antagonists. Different innervation patterns have been found for the flexor and extensor muscles. It is proposed that serially homologous motor neurons serving similar functions are, to a first approximation, similar in the locust. Serially homologous motor neurons serving different functions will, in most cases, have altered structures and/or functions.     

Changes in the electrical activity of the rabbit proximal colon in vivo by stimulation of the vagus and splanchnic nerves]

1. Using extracellular electrodes placed on the serosa, we recorded the modifications of the electrical activity of the colonic muslce fibers caused by the stimulation of vagal and splanchnic nerve fibers. 2. Vagal stimulation produces two types of junction potentials: excitatory junction potentials (EJPs) and inhibitory junction potentials (IJPs). The IJPs are elicited by stimulation of vagal fibers which innervate intramural non-adrenergic inhibitory neurons. 3. The conduction velocity of the nerve impulse along the vagal pre-ganglionic fibers is 1.01 m/sec for excitatory fibers and 0.5. m/sec for inhibitory fibers. 4. Splanchnic fiber stimulation causes EJP disappearance, blocking transmission between preganglionic fibers and intramural excitatory neurons, and a decrease in IJP amplitude that most likely indicates a previous hyperpolarization of the smooth muscle. 5. IJP persistence during splanchnic stimulation proves that sympathetic inhibition does not modify the transmission of the vagal influx onto the non-adrenergic inhibitory neurons of the intramural plexuses. 6. Through a comparative study of proximal and distal colonic innervation, we are able to show that there is a similar organization of both regions, that is a double inhibitory innervation: an adrenergic one of a sympathetic origin, and a non adrenergic one of a parasympathetic origin.     

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.     

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. Accepted: 28 April 1997     

Afferents from a single muscle of the forelimbs and fastigial neurons of the cerebellum

Responses of neurons in the medial nucleus of cerebellum (CBM) were studied on stimulation of ipsilateral and contralateral homonymous muscles, in decerebrated cats. The aim was to find out to what extent information from homonymous muscles of the forelimbs converge on the same CBM neurons and whether the probability of such a convergence depends on location (axial, proximal, distal) or function (flexor, extensor) of the tested muscles. The analysis was limited to the neurons belonging to the rostral part of the nucleus which is known to control the ipsilateral muscle periphery. Neuronal activity was recorded extracellularly using tungsten microelectrodes (5-12 M omega) and muscle stimulation was performed by bipolar coated steel electrodes, with the exception of the tip. At least 6 pairs of homonymous muscles were generally stimulated: two axial, two proximal and two distal in both forelimbs. Care was taken that, when a muscle was stimulated, the others were not activated either directly or in a reflex way. Out of the 65 neurons studied, 60 (92%) were responsive to muscle stimulation. It was specifically observed that a high percentage of cells reacted to stimulation of distal muscles (74% to ipsilateral and 71% to contralateral ones). More than half (55%) of the neurons were responsive to activation of a pair of homonymous distal muscles and about one third of them (31%) to both the pairs of distal muscles. On the contrary the percentage of responses to proximal muscles was reduced foremost in the ipsilateral ones (23%) and only an exiquous percentage of cells (15%) received information from the homonymous proximal muscles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reformation of the pattern of neuromuscular connections in the regenerated axolotl hindlimb

Retrograde neuronal tracing with horseradish peroxidase (HRP) was used to determine the position in the spinal cord of motor neurone pools innervating muscles in the regenerated axolotl hindlimb. This method allows a detailed analysis of the accuracy of reformation of neuromuscular connections. The results show that regenerated distal limb muscles are reinnervated by motor neurones in the same region of the cord as those that innervate normal control distal limb muscles but that proximal muscles are innervated by a mixture of motor neurones in a normal position and motor neurones in a region of the spinal cord that normally supplies innervation to distal limb muscles. This difference between the reinnervation of proximal and distal limb muscles suggests that axons destined for proximal muscles may not enter distal limb territory during reinnervation of the regenerated limb.