Seasonal differences in the physiology and morphology of crayfish motor terminals

The physiology and morphology of identified crayfish motor terminals were compared at different seasons. We examined initial excitatory postsynaptic potential (EPSP) amplitudes, synaptic fatigue, and the frequency of synaptic varicosities along the motor terminals of an identified phasic motoneuron in animals collected over a period of 5 years. The physiology and morphology of identified crayfish motor terminals are different for animals collected in summer and winter. In winter animals, phasic axon motor terminals in the claw closer muscle produce large EPSPs initially, but show dramatic synaptic fatigue during repetitive stimulation. In summer animals, these terminals produce smaller initial EPSPs, but are more fatigue resistant. Due to their greater fatigue resistance, synaptic terminals have a greater over-all capacity for transmitter release in summer animals than do those of winter animals. Morphologically, terminals in summer animals have more synaptic varicosities, this result supports earlier studies that have shown that fatigue-resistant motor terminals have more synaptic varicosities. Experiments in which the electrical activity of the motoneuron was experimentally altered suggest that these differences in motor terminals may be due to seasonal differences in activity.     

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.     

Human leg model predicts ankle muscle-tendon morphology, state, roles and energetics in walking

A common feature in biological neuromuscular systems is the redundancy in joint actuation. Understanding how these redundancies are resolved in typical joint movements has been a long-standing problem in biomechanics, neuroscience and prosthetics. Many empirical studies have uncovered neural, mechanical and energetic aspects of how humans resolve these degrees of freedom to actuate leg joints for common tasks like walking. However, a unifying theoretical framework that explains the many independent empirical observations and predicts individual muscle and tendon contributions to joint actuation is yet to be established. Here we develop a computational framework to address how the ankle joint actuation problem is resolved by the neuromuscular system in walking. Our framework is founded upon the proposal that a consideration of both neural control and leg muscle-tendon morphology is critical to obtain predictive, mechanistic insight into individual muscle and tendon contributions to joint actuation. We examine kinetic, kinematic and electromyographic data from healthy walking subjects to find that human leg muscle-tendon morphology and neural activations enable a metabolically optimal realization of biological ankle mechanics in walking. This optimal realization (a) corresponds to independent empirical observations of operation and performance of the soleus and gastrocnemius muscles, (b) gives rise to an efficient load-sharing amongst ankle muscle-tendon units and (c) causes soleus and gastrocnemius muscle fibers to take on distinct mechanical roles of force generation and power production at the end of stance phase in walking. The framework outlined here suggests that the dynamical interplay between leg structure and neural control may be key to the high walking economy of humans, and has implications as a means to obtain insight into empirically inaccessible features of individual muscle and tendons in biomechanical tasks.     

Identified motor terminals in Drosophila larvae show distinct differences in morphology and physiology

In Drosophila, the type I motor terminals innervating the larval ventral longitudinal muscle fibers 6 and 7 have been the most popular preparation for combining synaptic studies with genetics. We have further characterized the normal morphological and physiological properties of these motor terminals and the influence of muscle size on terminal morphology. Using dye-injection and physiological techniques, we show that the two axons supplying these terminals have different innervation patterns: axon 1 innervates only muscle fibers 6 and 7, whereas axon 2 innervates all of the ventral longitudinal muscle fibers. This difference in innervation pattern allows the two axons to be reliably identified. The terminals formed by axons 1 and 2 on muscle fibers 6 and 7 have the same number of branches; however, axon 2 terminals are approximately 30% longer than axon 1 terminals, resulting in a corresponding greater number of boutons for axon 2. The axon 1 boutons are approximately 30% wider than the axon 2 boutons. The excitatory postsynaptic potential (EPSP) produced by axon 1 is generally smaller than that produced by axon 2, although the size distributions show considerable overlap. Consistent with vertebrate studies, there is a correlation between muscle fiber size and terminal size. For a single axon, terminal area and length, the number of terminal branches, and the number of boutons are all correlated with muscle fiber size, but bouton size is not. During prolonged repetitive stimulation, axon 2 motor terminals show synaptic depression, whereas axon 1 EPSPs facilitate. The response to repetitive stimulation appears to be similar at all motor terminals of an axon.     

Local inhibitor of the crayfish telson-flexor motor giant neurons: morphology and physiology

The motor circuits that control telson flexion in the crayfish (Procambarus clarkii) include a curiously arranged sub-circuit: a premotor 'command' neuron excites a motor neuron via a trisynaptic pathway, but also inhibits (and prevents firing of) the motor neuron via a shorter latency pathway (Kramer et al. 1981 a). The premotor and motor neurons in this circuit have been previously identified (Kramer et al. 1981 a; Dumont and Wine 1985a, b; see Fig. 1). We have now identified a local interneuron that inhibits the motor neurons. The cell we studied is called the 'C' cell because of its distinctive structure (Figs. 2, 3). A single pair of bilaterally homologous C-cells was found in the last (6th) abdominal ganglion. The C-cells are invariably dye coupled to one another following injections of lucifer yellow into either one of them, and are frequently dye coupled to smaller axons in the 2nd, 3rd, and 6th nerves. In addition, some of the extensive branches of the C-cell extend out into the 6th nerve, where they are in close proximity to the axons of the motor neurons they inhibit (Fig. 3). Two kinds of evidence established that the C-cell directly inhibits the motor neurons. First, when simultaneous recordings were made from the C-cell and the motor neurons, spikes in the C-cell, no matter how evoked, were invariably followed, within 1.5 ms, by depolarizing IPSPs in the motor neuron (Fig. 6). Second, when the C-cell was hyperpolarized so that it could not fire, that same IPSP in the motor neuron was abolished (Fig. 6). The inhibitory pathway to the motor neurons must be fired at short latency in order to prevent firing caused by the trisynaptic excitatory input (Fig. 1). The C-cells were fired at short latency (less than 3 ms) by impulses in either of the escape command cells (Fig. 4), and at even shorter latency by impulses in the Segmental Giant of the 6th ganglion (SG6) (Fig. 5). It has been established elsewhere that the SGs are a major output pathway of the escape command cells; our results suggest that they may be the pathway for command-evoked firing of the C-cell. The C-cells are also excited by two descending, non-giant, flexion premotor neurons, called I2 and I3 (Fig. 5). The EPSPs from a single I2 or I3 impulse were subthreshold, but temporal and spatial summation of EPSPs from the non-giant pathway sometimes fired the C-cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Some observations on the culture,physiology and morphology of some brown-redRhodospirillum-species

Summary From ditch- and canalwater 21 strains of brown-red, photo-synthetically active spirilla were isolated. These spirilla distinguished themselves fromRhodospirillum rubrum by the composition of their pigment complex and by their behaviour towards oxygen (strictly anaerobic). The strains, judged by morphological characteristics, represented three different species. Two of the species could be identified withRhodospirillum fulvum van Niel resp.Rhodospirillum photometricum Molisch. The third species was found to be a new one, for which the nameRhodospirillum Molischianum was proposed.Regarding the isolation, culture, morphological and physiological characteristics of these organisms detailed data were given. Various organic substances can function as H-donors for these spirilla. The absorption spectra of living cells of the three species show a striking conformity, but there is a distinct difference between the absorption spectra of the brown-redRhodospirillum-species and the absorption spectrum ofRhodospirillum rubrum. The results of a chromatographical analysis of the pigment complex of one of the strains ofRhodospirillum pholometricum make it probable that this difference is due to the absence of the carotenoid spirilloxanthin in the brown-red spirilla.     

The relationship between dense bodies and mitochondria in motor neurones

Summary An electron microscopic study was made of the hypoglossal nuclei in a series of rabbits which had been subjected to unilateral hypoglossal neurotomy. Particular attention was given to the dense bodies which were present in the cytoplasm of the motor neurones on both sides. The simplest forms of dense body showed a granular structure without a limiting membrane. Others showed a limiting membrane (with either one or two dense layers) and varying degrees of internal organization in the form of double membranes. Some of the more highly-organized dense bodies possessed electron-transparent areas and were very similar in appearance to mitochondria. The appearances seemed consistent with the possibility that different forms of dense body represented developmental stages leading to or from mitochondria.Counts made on micrographs confirmed that mitochondrial numbers were increased 10–11 days after axonal section and showed in addition that there was an associated increase in the number of dense bodies. These findings are interpreted as evidence in favour of the developmental relationship of dense bodies to mitochondria.Supported by Research Grants M-388 and B-782 from the National Institute of Mental Health and the National Institute of Neurological Diseases and Blindness, U.S. Public Health Service.On leave from the Department of Anatomy, The University, Bristol, England.     

Locust nymphal neurones in culture: a new technique for studying the physiology and pharmacology of insect central neurones

A new technique for studying the physiology and pharmacology of locust central neurones is described. Somata isolated from neurones in the meso and metathoracic ganglia of third instar locusts (Schistocerca gregaria) were maintained for up to 4 weeks in co-culture (monolayer) with embryonic locust neurones. Most of the cultured cells became multipolar but a few were monopolar like their in vivo counterparts. They had diameters of 40-80 microns and "clean" (glial free) surface membranes. Cells 6-14 days in vitro were depolarized by acetylcholine and usually hyperpolarized by gamma-aminobutyrate, taurine and glycine. L-Glutamate and L-aspartate were inactive but further pharmacological studies are required to confirm this. Cultured larval neurones should provide excellent opportunities to study the molecular basis for drug-receptor interactions and voltage-sensitive membrane channels using the patch clamp technique.     

Activity of neuromodulatory neurones during stepping of a single insect leg

Octopamine plays a major role in insect motor control and is released from dorsal unpaired median (DUM) neurones, a group of cells located on the dorsal midline of each ganglion. We were interested whether and how these neurones are activated during walking and chose the semi-intact walking preparation of stick insects that offers to investigate single leg-stepping movements. DUM neurones were characterized in the thoracic nerve cord by backfilling lateral nerves. These backfills revealed a population of 6-8 efferent DUM cells per thoracic segment. Mesothoracic DUM cells were subsequently recorded during middle leg stepping and characterized by intracellular staining. Seven out of eight identified individual different types of DUM neurones were efferent. Seven types except the DUMna nl2 were tonically depolarized during middle leg stepping and additional phasic depolarizations in membrane potential linked to the stance phase of the middle leg were observed. These DUM neurones were all multimodal and received depolarizing synaptic drive when the abdomen, antennae or different parts of the leg were mechanically stimulated. We never observed hyperpolarising synaptic inputs to DUM neurones. Only one type of DUM neurone, DUMna, exhibited spontaneous rhythmic activity and was unaffected by different stimuli or walking movements.     

The control of walking movements in the leg of the rock lobster

1. Experiments with rock lobsters walking on a treadmill were undertaken to obtain information upon the system controlling the movement of the legs. Results show that the position of the leg is an important parameter affecting the cyclic movement of the walking leg. Stepping can be interrupted when the geometrical conditions for terminating either a return stroke or a power stroke are not fullfilled. 2. The mean value of anterior and posterior extreme positions (AEP and PEP respectively) of the walking legs do not depend on the walking speed (Fig. 1). 3. When one leg is isolated from the other walking legs by placing it on a platform the AEPs and PEPs of the other legs show a broader distribution compared to controls (Figs. 2 and 3). 4. Force measurements (Fig. 4) are in agreement with the hypothesis that the movement of the leg is controlled by a position servomechanism. 5. When one leg stands on a stationary force transducer this leg develops forces which oscillate with the step rhythm of the other legs (Fig. 5). 6. A posteriorly directed influence is found, by which the return stroke of a leg can be started when the anterior leg performs a backward directed movement. 7. Results are compared with those obtained from stick insects. The systems controlling the movement of the individual leg are similar in both, lobster and stick insect but the influences between the legs seem to be considerably different.     

The coordination of force oscillations and of leg movement in a walking insect (Carausius morosus)

As in the preceding paper stick insects walk on a treadwheel and different legs are put on platforms fixed relative to the insect's body. The movement of the walking legs is recorded in addition to the force oscillations of the standing legs. The coordination between the different legs depends upon the number and arrangement of the walking legs and the legs standing on platforms. In most experimental situations one finds a coordination which is different from that of a normal walking animal.Supported by DFG (Cr 58/1)     

Lumbar back muscle activity during walking with a leg inequality

The influence of an artificial leg length discrepancy (= ALLD) on stride times, pelvic rotations and activity of the intrinsic lumbar back muscles (= ILBM) was investigated for 20 subjects. An ALLD was created by shoes with a raised sole. Walking with an ALLD produced an increase of the swing phase time and a decrease of the stance phase time for both feet. The influence of an ALLD on pelvic rotations in the sagittal and frontal plane and on ILBM-activity was small. Changes in pelvic rotations in the sagittal plane were too small to observe. The mean pelvic rotation angle in the frontal plane was changed 1.52 degrees when walking with an ALLD of 40 mm (6.9 degrees while standing with an ALLD of 40 mm with extended knees). Only small changes were found in activity time due to an ALLD (not in EMG-amplitude). The activity time of the ILBM around heel strike of the raised limb was increased and unilaterally shifted from toe off in the direction of heel strike with the raised limb.     

Octopaminergic modulation of locust motor neurones

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

Correlating physiology with gene expression in striatal cholinergic neurones

The expression of 34 transmitter-related genes has been examined in the cholinergic neurones of rat striatal brain slices, with the aim of correlating gene expression with functional activity. The mRNAs encoding types I, II/IIA, and III alpha subunits of the voltage-sensitive sodium channels were detected, suggesting the presence of these three types of sodium channel. Similarly, mRNAs encoding all four alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-type glutamate receptor subunits and the NR1 and NR2A, 2B, and 2D subunits of the NMDA-type glutamate receptors were detected, suggesting that various combinations of these subunits mediate the cellular response to synaptically released glutamate. Other mRNAs detected included the NK1 and NK3 tachykinin receptors, all four known adenosine receptors, and the GABA-synthesising enzyme glutamate decarboxylase. Subpopulations of these cholinergic neurones have been identified on the basis of the expression of the NK3 tachykinin receptor in 5% and the trkC neurotrophin receptor in 12% of the cells investigated.     

The walking gaits of some species of Pecora

A method is presented for defining the walking gaits of quadrupeds from films so that they can be compared in closely related species. Differences in walking patterns of 18 pecoran species belonging to four families are discussed with respect to anatomy and environment. Variation in the walk patterns of members within a species are assessed. They are found to vary often with the speed at which the walk is executed, with the terrain, with the presence of heavy horns or antlers and with age. The time taken for one walking stride increases with the increase in length of the legs, but the legs swing forward more rapidly than they would if they acted passively like cylindrical pendulums.