Slow Motoneurons Research Articles

An elaborate tension receptor system highlights sensory complexity in the hind leg of the locust

The tibia of each leg of the locust is moved by two antagonistic muscles, the extensor and flexor tibiae. A variety of sense organs on and in each leg provide feedback about this joint's position and movement and about forces acting on the exoskeleton and muscles. One such organ is a muscle tension receptor found within the flexor tibiae muscle of the mesothoracic leg. We now show that an apparently homologous multipolar receptor is present in the hind leg, but that here it is associated with a specialised flexor muscle, the accessory flexor. This muscle comprises 13 fibres, innervated by five of the thirteen motor neurones that innervate the main flexor muscle and, since these are slow motor units, the response properties of the receptor are constrained. The multipolar receptor attaches to the muscle fibres near their proximal insertion onto the femoral cuticle. It generally has four primary dendrites, which do not branch extensively within the muscle. We show that the receptor responds strongly to active, isometric contractions but only poorly to imposed changes of accessory flexor muscle length (i.e. passive changes in tibial position). It does not respond to tension generated by the main flexor muscle or by the extensor muscle. The tension receptor causes short-latency (0.9­1.8 ms) excitatory inputs onto the three common inhibitory motor neurones and longer-latency (3.7­8.1 ms) inhibitory inputs onto the slow extensor tibiae motor neurone. In quiescent animals, it causes excitatory inputs onto flexor tibiae motor neurones (2.2­3.8 ms) but, in more active animals, its inputs onto these neurones are often inhibitory, with delays of 6­10 ms. The slow nature of the accessory flexor muscle and the pattern of central connections of the receptor suggest that together they are involved in the control of slow movements or posture, potentially acting through a servomechanism.

Intrinsic Differences in Axonal Growth from Crayfish Fast and Slow Motoneurons

The motoneurons innervating the fast and slow flexor muscles in the abdomen of crayfish form morphologically distinct motor terminals. Axons of the fast flexor (FF) motoneurons, which innervate the large (fast) flexor muscle, produce extensive motor terminal arbors with many branches. Axons of the slow flexor (SF) motoneurons, which innervate the thin (slow) flexor muscle, produce small terminal arbors with many fewer branches. To determine whether intrinsic factors contribute to these differences in terminal arbors, we compared regenerating axonal arbors from these two populations of motoneurons. We used an explant of the crayfish nerve cord in which axons from the FF and SF motoneurons regenerate on a homogeneous substrate. We found that regardless of the substrate, FF motor axons produced arbors with a greater total length and a greater number and density of branches than SF motor axons. These differences in regenerated arbors persisted in defined medium and in the absence of impulse activity, indicating that they result from intrinsic, neuron-specific factors. The greater branching of the FF motor axons may be related to differences in growth cones: growth cones of FF axons were significantly larger with more filopodia than growth cones of SF axons.

Evidence of incomplete neural control of motor unit properties in cat tibialis anterior after self-reinnervation.

1. The mechanical, morphological and biochemical properties of single motor units from the anterior compartment of the tibialis anterior muscle in adult cats were studied six months after the nerve branches to that compartment were cut and resutured in close proximity to the muscle. 2. In these self-reinnervated muscles, the maximum tetanic tensions were lower in slow than fast units, a relationship similar to that observed among motor units from control adult muscles. The maximum tetanic tensions produced by the fast units were larger than those produced by the same motor unit types in control muscles. Direct counts of muscle fibres belonging to a motor unit showed that factors controlling the number of muscle fibres innervated by a motoneurone type persist during the reinnervation process in that fast motoneurones reinnervated more muscle fibres than slow motoneurones. Thus, the number of muscle fibres reinnervated by a motoneurone principally accounted for the difference in the maximum tension outputs among motor unit types, a relationship similar to that observed in control tibialis anterior muscles. 3. Monoclonal antibodies for specific myosin heavy chains were used to differentiate fibre types. By this criterion, motor units from control muscles were found to contain a homogeneous fibre type composition. In contrast, a heterogeneous, yet markedly biased, fibre type composition was observed in each unit analysed from self-reinnervated muscles. 4. Although not all of the muscle fibres of a motor unit developed the same type-associated parameters after reinnervation, the relationships among myosin heavy chain profile, succinate dehydrogenase activity and the fibre size were similar in fibres of control and self-reinnervated muscles. 5. The processes which dictate both motor unit size and the matching between motoneurone and muscle fibre type during the reinnervation process must be interdependent and result from a hierarchy of decisions which reflects their relative importance. The mechanisms responsible for these two processes may be a combination of: (1) selective innervation which may or may not incorporate a pruning process if multiple synaptic connections are initially formed and/or (2) conversion of enough fibres of a motor unit to form a predominant type.

Cross-connections coordinate postural motoneuron activity in the crayfish abdomen

Intracellular recordings and dye injections were used to examine mutual coupling among slow abdominal postural motoneurons in the 4th abdominal ganglion in crayfish (Procambarus clarkii). Intracellular current injection into one motoneuron altered the spike firing rate of some of its synergists. Depending on the polarity of the injected current, the premotor effect on the synergists was excitatory or inhibitory. The magnitude of the effect was intensity dependent. No dye coupling was found among the motoneurons following injection of Lucifer yellow. The morphological basis of the coupling was examined by differential filling of motoneuron pairs, one with horseradish peroxidase and the other with Lucifer yellow. The stained motoneurons were simultaneously visualized under light microscopy to determine the proximity of their differently colored dendrites. It was thus possible to locate the site of the presumed monosynaptic contacts between them. Combined physiological and morphological evidence suggests that these neurons are mutually coupled, forming part of an integrative system for abdominal posture control in crayfish.

Motoneuron survival factors: Biological roles and therapeutic potential

The existence of factors that promote motoneuron survival in the spinal cord at critical stages of development was first deduced 50 yr ago. The large amount of work that has been put into characterizing such factors reflects both their biological importance and the hope that such molecules may be used therapeutically to slow motoneuron death in pathologies such as the spinal muscular atrophies and amyotrophic lateral sclerosis. Since 1990, several factors have been shown to have in vitro and/or in vivo activities that suggest they play a role in regulating motoneuron survival. Their physiological functions during motoneuron development are probably different and complementary. Several of them seem reasonable candidates for preclinical development, but many crucial experiments remain to be done.

The vasopressin-like immunoreactive (VPLI) neurons of the locust, Locusta migratoria. II. Physiology

The two vasopressin-like immunoreactive (VPLI) neurons of the locust, Locusta migratoria, have cell bodies in the suboesophageal ganglion and extensive arborizations throughout the CNS. One of the two peptides responsible for AVP-like immunoreactivity is a vasopressin-related peptide with putative 'diuretic hormone' properties. These neurons also have FLRF-like immunoreactivity, probably due to the FMRF-amide-related peptide. SchistoFLRF-amide, isolated from Schistocerca gregaria. This peptide has cardioinhibitory activity and a dual potentiation/inhibition of slow motoneuron induced muscle-twitch tension. Although haemolymph AVP-like peptide titre fluctuates under various conditions, the mechanism that regulates neurohaemal release of this peptide is not understood. Very little is known of the release of SchistoFLRF-amide. We have used intracellular recording from VPLI neurons in vivo to reveal synaptic inputs that lead to changes in their level of spiking activity, and probably, release of both the AVP-like peptides and SchistoFLRF-amide. This pair of neurosecretory cells has a major, common excitatory input whose sustained rate of activity is inversely related to light intensity; VPLI spiking activity, driven by this input, is greater in the dark than in light. This input is from a pair of descending brain interneurons. Their light-sensitivity persists after ablation of compound eyes, optic lobes and ocelli, showing them to be part of an extra-ocular photoreceptor system. Attempts to record from, and individually stain, the descending neuron have been unsuccessful, although its axon location and diameter in the circumoesophageal connective have been determined. Possible locations for its cell body have been identified; one region, close to the pars intercerebralis, is known to be photosensitive in some insects. Mechanosensory stimuli also lead to brief increases in VPLI spiking activity via the descending interneuron, though this modality rapidly habituates. We detect no changes in VPLI spiking activity that consistently correlate with the osmolality of perfusion salines; such changes might have been expected from their previously proposed role in water homeostasis. Alternative roles for VPLI cells are discussed.

Differences in H-reflex between athletes trained for explosive contractions and non-trained subjects

The efficacy of type Ia synapse on alpha-motoneurons of soleus and lateral gastrocnemius muscles has been investigated, using the H-reflex technique, in athletes engaged in sports requiring very rapid and intense contractions (sprinters and volley-ball players) as well as in non-trained subjects. It has been observed, in both muscles, that the ratio between the mean value of the maximal reflex response (Hmax) and the mean value of the maximal direct response (Mmax) elicited upon electrical stimulation of the tibial nerve is significantly smaller in athletes trained for explosive-type movements than in non-trained subjects. This difference in the Hmax: Mmax ratio was dependent on a smaller amplitude of Hmax and not on a greater amplitude of Mmax. No significant differences were observed between sprinters and volley-ball players. In both trained and non-trained subjects, soleus and lateral gastrocnemius muscles displayed significant differences in Hmax: Mmax ratio and Mmax amplitude but not in Hmax amplitude. Since the H-response is considered to be due mainly to activation of the smallest motoneurons in the motoneuronal pools, the difference in Hmax amplitude and Hmax: Mmax ratio between athletes and non-trained subjects could have been dependent on a lower incidence of these motoneurons in the athletes. This is in accord with the mechanical needs of muscles during explosive-type power training. Although this difference may have been wholly determined genetically, the possibility is discussed as to whether the lower incidence in sprinters and volley-ball players of small motoneurons could have been related to a training-induced transformation of small and slow motoneurons into large and fast ones.

Relative efficacy of slow and fast alpha-motoneurons to reinnervate mouse soleus muscle

Contractile and histochemical properties of reinnervated motor units in soleus muscles of C57BL/6J mice were examined 1 mo after sectioning the soleus nerve. Fifty-one motor units were isolated by the technique of ventral root splitting. Their sizes ranged from 0.4 to 13.6% of whole muscle tetanic tension (Po) with a mean size of 5.3% Po corresponding to 19 motor units. In control unoperated mice, the range was 2.2-8.6% Po, with a mean size of 4.8% Po corresponding to 22 motor units. Although no clear relationship between unit time to peak tension and size was seen in control units, it appeared that in the reinnervated muscle the large units were also slow contracting, whereas the smaller units were predominantly fast contracting. Adenosinetriphosphatase (ATPase) staining revealed an increase in the proportion of muscle area occupied by type I fibers in reinnervated soleus compared with control soleus. Immunohistochemical staining of reinnervated soleus with monoclonal antibodies against type I and IIa myosin showed the presence of hybrid fibers containing both myosins. It is concluded that during reinnervation most motoneurons reinnervate the soleus muscle of the mouse. The hypothesis that slow motoneurons are more adept at expanding their innervating field than fast motoneurons is also supported by the data.

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.

Intersegmental interneurons can control the gain of reflexes in adjacent segments of the locust by their action on nonspiking local interneurons

The gain of local reflexes of one leg of a locust can be altered by mechanosensory inputs generated by movements of or tactile inputs to an adjacent leg. Touching the mesothoracic tarsus, for example, increases the number of spikes that are produced by the metathoracic slow extensor tibiae motor neuron and enhances the depolarization of flexor tibiae motor neuron in response to imposed movements of the chordotonal organ in the ipsilateral hind femur. The sensory information from the middle leg is conveyed directly to nonspiking interneurons and motor neurons controlling the movements of the hindleg by a population of mesothoracic intersegmental interneurons (Laurent and Burrows, 1989). The metathoracic nonspiking interneurons receive direct inputs from receptors on a hindleg and are, therefore, a point of convergence for local and intersegmental inputs. We examine here the role of the connections between mesothoracic intersegmental interneurons and metathoracic nonspiking interneurons in controlling metathoracic local reflexes. The amplitude of synaptic potentials evoked in leg motor neurons by the stimulation of local afferents can be modulated by altering the membrane potential of an interposed nonspiking interneuron with current injected through an intracellular electrode. These imposed voltage changes mimic a mesothoracic input and show that the state of a nonspiking local interneuron is a determining factor in the expression of a local reflex. Inputs from mesothoracic intersegmental interneurons may cause large changes in the input conductance of nonspiking interneurons that can shunt a local afferent input. In some nonspiking interneurons, synaptic potentials caused by mesothoracic interneurons can be recorded, but no underlying conductance change can be detected at the recording site. Similarly, a particular nonspiking interneuron may receive synaptic inputs when two distinct regions of a middle leg are touched, but only one of these intersegmental inputs may be effective in reducing the amplitude of a synaptic potential caused by afferents from the hindleg. These results suggest that nonspiking local interneurons may be compartmentalized, with synaptic inputs and their associated conductance changes restricted to particular branches. In this way, an individual nonspiking neuron could contribute simultaneously to several local circuits. The inputs from different intersegmental interneurons could then modulate these pathways independently.

Spike afterpotentials in single, identified fast and slow motor neurons in the crab Pachygrapsus crassipes

1. 1. In the fast (FBE) and most slow (SBE) bender excitor axons the spike was followed by a depolarizing afterpotential (DAP). The DAP reversal potential was about 25 mV above resting in FBE and 7 mV above resting in SBE. 2. 2. The resting potential and action potential amplitude were larger in FBE, while the input resistance was larger in SBE. 3. 3. In both axons, warming or addition of 200 mM ethanol to the saline decreased spike amplitude and increased DAP amplitude. 4. 4. In both axons there was a space between the axon membrane and the adaxonal glial cell.

Glutamate-like immunoreactivity in identified neuronal populations of insect nervous systems

Glutamate is considered to be the most likely transmitter candidate at excitatory synapses onto skeletal muscles of insects. We investigated the distribution of glutamate-like immunoreactivity (Glu-LI) in identified motor neurons of glutaraldehyde-fixed metathoracic ganglia of the locust in paraffin serial sections. The presumably glutamatergic fast and slow extensor tibiae motor neurons show Glu-LI, whereas other cells, including the GABAergic common inhibitory motor neurons and the cluster of octopaminergic dorsal unpaired median cells, show rather low levels of staining. Immunoreactivity of the fast extensor tibiae motor neuron is located in soma, neurites, axon, and the terminal arborizations. A double-labeling experiment on sections of the locust metathoracic ganglion showed that antisera against glutamate and GABA discriminate between the presumably glutamatergic and GABAergic motor neurons and that GABA-LI-positive neurons are low in Glu-LI. The results suggest that Glu-LI can be used as a marker for detecting potential glutamatergic neurons in insects under the present conditions. Application of the glutamate antiserum to sections of the honeybee brain revealed Glu-LI in motor neurons but also in certain interneurons. The most prominent populations of Glu-LI-positive cells were the monopolar cells and large ocellar interneurons, which are first-order interneurons of the visual and ocellar system. Several groups of descending interneurons also showed Glu-LI. The distributions of Glu-LI and GABA-LI are complementary in locust and bee ganglia. The high level of Glu-LI in certain interneuronal populations, as well as in identified glutamatergic motor neurons, suggests that insect central nervous systems may contain glutamatergic neuronal pathways.

Positive feedback loops from proprioceptors involved in leg movements of the locust

1. Two campaniform sensilla (CS) on the proximal tibia of a hindleg monitor strains set up when a locust prepares to kick, or when a resistance is met during locomotion. The connections made by these afferents with interneurones and leg motor neurones have been investigated and correlated with their role in locomotion. 2. When flexor and extensor tibiae muscles cocontract before a kick afferents from both campaniform sensilla spike at frequencies up to 650 Hz. They do not spike when the tibia is extended actively or passively unless it encounters a resistance. The fast extensor tibiae motor neurone (FETi) then produces a sequence of spikes in a thrusting response with feedback from the CS afferents maintaining the excitation. Destroying the two campaniform sensilla abolishes the re-excitation of FETi. 3. Mechanical stimulation of a single sensillum excites extensor and flexor tibiae motor neurones. The single afferent from either CS evokes EPSPs in the fast extensor motor neurone and in certain fast flexor tibiae motor neurones which follow each sensory spike with a central latency of 1.6 ms that suggests direct connections. The input from one receptor is powerful enough to evoke spikes in FETi. The slow extensor motor neurone does not receive a direct input, although it is excited and slow flexor tibiae motor neurones are unaffected. 4. Some nonspiking interneurones receive direct connections from both afferents in parallel with the motor neurones. One of these interneurones excites the slow and fast extensor tibiae motor neurones probably by disinhibition. Hyperpolarization of this interneurone abolishes the excitatory effect of the CS on the slow extensor motor neurone and reduces the excitation of the fast. The disinhibitory pathway may involve a second nonspiking interneurone with direct inhibitory connections to both extensor motor neurones. Other nonspiking interneurones distribute the effects of the CS afferents to motor neurones of other joints. 5. The branches of the afferents from the campaniform sensilla and those of the motor neurones and interneurones in which they evoke EPSPs project to the same regions of neuropil in the metathoracic ganglion. 6. The pathways described will ensure that more force is generated by the extensor muscle when the tibia is extended against a resistance. The excitatory feedback to the extensor and flexor motor neurones will also contribute to their co-contraction when generating the force necessary for a kick.

Functional organization of crayfish abdominal ganglia: II. Sensory afferents and extensor motor neurons

Abdominal ganglia of crayfish contain identifiable neuropils, commissures, longitudinal tracts, and vertical tracts. To determine the functional significance of this ganglionic framework, we backfilled the following types of neurons with cobalt chloride: sensory hair afferents, slow and fast extensor motor neurons, the segmental stretch receptor neurons, and their inhibitory accessory cells. After the cobalt ions were precipitated and intensified, we studied the central projections of the filled neurons within the ganglionic structures. All of the axons of these neurons exit or enter each of the first five abdominal ganglia through the second pair of nerves. Our description of the central projections of the hair afferents is the first in the literature. These afferents innervate the large ventral horseshoe neuropil (HN) in the core of each ganglion. This neuropil is homologous to the insect ventral association centers, which also process sensory information. Furthermore, we discovered that some of the crayfish afferents innervate glomeruli within the HN. The slow and fast extensor motor neurons, the stretch receptor neurons, and the accessory cells branch mostly in the dorsal part of the ganglion. We reinterpret previous identifications of the extensor neurons that were based largely on soma position. Together with our previous descriptions of the flexor motor neurons, these results allow us to relate both rapid tail-flips and slower postural movements to the structure of the segmental ganglia.

Myofibrillar M-band structure and composition of physiologically defined rat motor units.

The isometric contraction time of 19 fast and slow rat motor units in the soleus and the anterior tibial muscles were recorded. The motor unit fibers, subsequently distinguished by glycogen depletion, were histochemically differentiated into fiber types and analyzed immunohistochemically for high molecular weight M-band proteins, as well as ultrastructurally for M-band fine structure, Z-disc width, and volume density of mitochondria. All fibers belonging to slow-twitch motor units in both the anterior tibialis and soleus muscles were histochemically classed as type 1. They lacked the Mr 165,000 M-protein, showed ultrastructurally a four-line M-band pattern, and had broad Z-discs, whereas the volume density of the mitochondria varied considerably. Muscle fibers belonging to the fast-twitch motor units were histochemically classed as types 2A and 2B in anterior tibialis and type 2A in soleus. They contained a three- or a five-line M-band pattern and medium-to-thin Z-discs in the anterior tibialis and a five-line M-band pattern and broad Z-discs in the soleus. Furthermore, the volume density of mitochondria showed considerable variation within and in between soleus and anterior tibialis type 2 fibers. As the differences in M-band composition and structure between fiber types overrode the intragroup variability in contraction times of slow and fast units within and between the two muscles, it is concluded that the M-band composition and structure is fundamentally related to whether the fiber is innervated by a slow or fast motor neuron, whereas other parameters such as contraction time, Z-disc width, and mitochondrial content of fibers of fast and slow units are relative and vary between muscles. Thus the M-band appearance can be used as a reliable marker to distinguish between fibers of slow- and fast-twitch motor units in rat leg muscles.

Hyperthyroidism selectively increases oxidative metabolism of slow-oxidative motor units

The effects of thyroid hormone on the NADH-tetrazolium reductase activity (oxidative metabolism marker) of soleus (slow-oxidative) and tensor fascia lata (fast-glycolytic) motoneurons were determined and compared with changes in a variety of enzyme activities in the corresponding muscle fibers. Histochemical assays have demonstrated a selective and qualitative conversion in muscle fiber ATPase and quantitative increases of NADH-tetrazolium reductase (oxidative) and mitochondrial α-glycerophosphate dehydrogenase (glycolytic) activities in the soleus muscle. Paralleling the selective action upon the soleus slow muscle fibers was a selective central nervous system effect of thyroid hormone on oxidative enzymes of soleus slow-oxidative motoneurons. This indicates that either thyroid hormones act directly and specifically on slow motoneurons or that conversion of the muscle fibers by thyroid hormones produces secondary changes in the motoneuron. These data strengthen the hypothesis that oxidative enzyme activities in motoneurons are tightly matched with oxidative enzyme activities in muscle fibers.

56] Regulation of parvalbumin concentration in mammalian muscle

Publisher Summary Parvalbumin (PA), an acidic Ca 2+ - and Mg 2+ -binding protein of approximately 12,000 M r , is thought to act in skeletal muscle as a cytosolic Ca 2+ - and Mg 2+ -buffering compound. In mammalian muscle, it has been detected exclusively in fast-twitch fibers. Several studies have shown that the PA concentration in skeletal muscle is subject to regulation by exogenous factors and may be altered experimentally by changes in contractile and/or motoneuron activity. It has also been shown that PA is reduced in hereditary mammalian muscle diseases. The influence of neural activity upon PA expression is clearly shown by denervation, cross-reinnervation, and nerve-stimulation experiments. Denervation of neonatal and adult fast-twitch muscles suppresses PA synthesis. Its concentration remains low in denervated, presumptive fast-twitch muscles of the newborn rabbit. PA synthesis is suppressed in fast-twitch muscle after cross-reinnervation with a nerve that normally supplies a slow-twitch muscle. In view of the different activity patterns of fast and slow motoneurons, it seems likely that PA expression is under the positive control of phasic, high-frequency activity as delivered by fast motoneurons.

Tailflipping ofMunida quadrispina (Galatheidae): conservation of behavior and underlying musculature with loss of anterior contralateral flexor motoneurons and motor giant

1. We analyzed tailflipping behavior, motor patterns, underlying musculature and flexor motoneurons inMunida quadrispina (Galatheidae) and compared them with these parameters in crayfish to reveal evolutionary changes in the nervous system which might have occurred in conjunction with loss of giant interneurons and change in body form and posture. 2. The form of tailflipping (videoanalysis) varies with respect to abdomen extension angle, amount of uropod promotion, and pereopod positioning (Fig. 1). In ‘full’ tailflips, the uropods are maximally promoted throughout active flexion; the first 3 pairs of pereopods are held forward. In ‘truncated’ tailflips, abdominal segments 4–6 do not extend, uropods do not promote and pereopods either move actively or trail passively. Flexion latency (=extension duration) covaries with extension period, but does not vary systematically with extension amplitude (Fig. 3C, D). 3. Flexor-burst latencies (electromyograms) in segments 2–4 vary independently of extensor-burst period; in segment 5 they covary with period (Figs. 2, 3 A, C). 4. M. quadrispina has slow flexor motoneurons identical to those in crayfish but lacks homologs of the motor giants and of any FAC motoneurons (Figs. 4, 5; Table 1). 5. Quantitative comparison of the relative sizes of fast flexor and extensor musculatures inM. quadrispina and in crayfish revealed only one significant difference between the 2 animals: the extensor muscles are approximately the same size in all segments in crayfish whereas inM. quadrispina they are largest in segment 5 (Fig. 6; Table 2). 6. The major conclusion is that whereas nongiant tailflipping behavior (overt behavior and motor pattern) and underlying musculature were conserved in evolution ofM. quadrispina, flexor motoneurons underwent reductions compared with crayfish (andGalathea strigosa). Loss of FAC motoneurons perhaps resulted from chance loss of certain neuroblasts which was tolerated because other motoneurons still innervated the affected muscles.

Slow-like electrostimulation switches on slow myosin in denervated fast muscle

Adult fast and slow skeletal muscles are composed of a large number of fibers with different physiological and biochemical properties that under neuronal control can respond in a plastic manner to a variety of stimuli. Although muscle cells synthesize muscle-specific contractile proteins in the absence of motoneurons, after innervation the neuron controls the particular set of isoforms subsequently synthesized. However, agreement has not been reached on the mechanism, either chemotrophic or impulse-mediated, by which the nerve influences gene expression in the muscle. Here we report the effect on isomyosins of continuous, low-frequency (a protocol mimicking the discharge pattern of the slow motoneuron) direct electrical stimulation of a permanently denervated fast muscle, the extensor digitorum longus of adult rat. After several weeks, unlike sham-stimulated muscle, the stimulated muscle showed a dramatic increase of the slow myosin light and heavy chains. Myosin light chains were identified by two-dimensional gel electrophoresis. The slow myosin heavy chain was clearly distinguished from fast and embryonic types by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and orthogonal peptide mapping. The myosin change could be restricted to a portion of the muscle by the position of the stimulating electrodes. Taking into account the morphologic appearance of the electrostimulated muscle and the large body of evidence demonstrating the absolute dependence of slow myosin on specific innervation, our observations indicate that at least the slow motoneuron influences the isomyosin genes' expression by the kind of activity it imposes on developing muscle fibers.

Assisting components within a resistance reflex of the stick insect, Cuniculina impigra

ABSTRACT. Rapid relaxation (shortening) of the femoral chordotonal organ in Cuniculina impigra Redtenbacher induces a depolarization followed by hyperpolarization of the fast and slow extensor tibiae motor neurons (FETi and SETi). The initial depolarization is caused by acceleration‐sensitive units of the chordotonal organ. The reverse sequence of responses is induced in flexor motor neurons. The common inhibitor neuron (CI) is depolarized by both lengthening (stretch) and relaxation of the chordotonal organ.The initial depolarization of FETi and SETi and the initial hyperpolarization of flexor motor neurons produced by rapid relaxation of the chordotonal organ and the depolarization of CI produced by lengthening of the chordotonal organ all oppose the resistance reflex response. However, these assisting components are weak compared to the resisting ones.