Fast Extensor Tibiae Research Articles

Interneurons coactivating hindleg flexor and extensor motoneurons in the locust

1. Using intracellular staining and recording techniques in the ventral nerve cord of locusts, we have identified a pair of large interneurons which monosynaptically excite hindleg flexor and extensor tibiae motoneurons. The somata and input processes of these interneurons are in the mesothoracic ganglion and each makes its connections to hindleg motoneurons via a large axon running in the meso-metathoracic connective contralateral to its soma. 2. We refer to these interneurons as C-neurons because single action potentials in one interneuron often caused the coactivation of the fast extensor tibiae motoneuron and a variable number of flexor tibiae motoneurons in one hindleg. The probability of activity in a C-neuron activating hindleg motoneurons depended on the position of the tibia, being highest when the tibia was flexed. 3. Coactivation of the fast extensor tibiae motoneuron and a number of flexor motoneurons could also be elicited by a variety of stimuli in intact animals when the tibiae were close to full flexion. This resulted in the tibiae being locked into full flexion and, very often, to the initiation of the co-contraction phase of the jump. We refer to this synchronous activation of hindleg flexor and extensor motoneurons in intact animals as the cocking response. 4. We propose that the C-neurons function to produce the cocking response. Consistent with this proposal is that the C-neurons receive input from all those sensory sources which evoke the cocking response. 5. A general feature of the interneuronal organization in insects derived from this and other investigations is the existence of individual interneurons for the production of single aspects of a behavior. By controlling transmission of sensory information to motoneurons via these unique interneurons, the same sensory stimulus can evoke a variety of motor response. This concept is discussed in relation to the jumping system of the locust.

Electrical Characteristics of the Membrane of An Identified Insect Motor Neurone

ABSTRACT The electrical properties of the membrane of an identified locust motor neurone, the fast extensor tibiae in the metathoracic ganglion, have been investigated to determine : the distribution of excitable and inexcitable membrane; the impulse initiation zone; and the conduction velocity of the spike in the ganglion and in the axon. The waveform of extracellularly recorded spikes indicates that the transition from inactive to active membrane occurs along the region of the neurite which bears many arborizations within the neuropile. Measurements of the delay between orthodromically or antidromically evoked spikes, recorded at the soma and other points along the neurite, place the impulse initiating zone close to the transition between active and inactive membrane. Within the ganglion, the spike is conducted at different velocities over different parts of the neurite. The average velocity within the ganglion is, however, only about a seventh of that in the axon (0· 54 m.s-1 against 4· 1 m.s-1).

Electrical properties of insect neurones with spiking and non-spiking somata: normal, axotomized, and colchicine-treated neurones.

ABSTRACT The paired motorneurones in the metathoracic ganglion of the locust have non-spiking somata whereas the dorsal unpaired media (DUM) neurones have spiking somata. We have studied the electrical properties of two identified neurones in the metathoracic ganglion that have spiking axons innervating the same muscle: the fast extensor tibiae (FETi) motomeurone has a non-spiking soma and the dorsal unpaired median extensor tibiae (DUMETi) neurone has a spiking soma. The inward current of the peripheral axon spikes of both DUMETi and FETi is carried predominantly by Na+, since the spikes are blocked only by removal of Na+ or addition of tetrodotoxin (TTX). The inward current of the soma spike of DUMETi is carried by Na+ and Ca2+, since it is blocked by either removal of Na+, addition of TTX, or addition of Co2+. The non-spiking soma of FETi shows delayed rectification. When some of the outward K+ current is blocked by TEA or 3-AP, the soma is capable of generating overshooting action potentials. The inward current of the TEA-induced soma spike of FETi is carried by Na+ and Ca2+, since it is blocked by either removal of Na+, addition of TTX or addition of Co2+. Axotomy or treatment with colchicine converts the non-spiking soma of FETi into a spiking soma within 4 days. The inward current of the soma spike is carried predominantly by Na+, since it is blocked only by removal of Na+. Five days after axotomy of one of the peripheral axons of DUMETi, Na+ is sufficient for the generation of the soma spike. Increased excitability is also observed in the neuropil of DUMETi after axotomy. When some of the outward K+ current is blocked by Ba2+, the normal and axotomized somata of FETi and DUMETi are all capable of generating long duration Ba2+ action potentials that are blocked by addition of Co2+. The overshooting Ba2+ action potentials in all cases are similar in amplitude and duration. It is concluded that the soma membrane of DUMETi and FETi normally contains both Na+ and Ca2+ inward current channels. The normal difference in excitability between these two somata may result in part or entirely from differences in the outward K+ current. It is suggested that axotomy or colchicine treatment cause an increase in the number of active Na+ channels in the soma membrane, which overcomes any differences in the outward K+ current and results in both cells being able to produce soma spikes.

Correlation of variability in structure with variability in synaptic connections of an identified interneuron in locusts.

AbstractNumerous reports have described variability in the morphology of identified neurons; none, however, has previously reported variability in the pattern of synaptic connection. In this report we describe variability in the synaptic connections of an identified interneuron in two species of locust (Locusta migratoria and Schistocerca gregaria) and show that this variability is associated with a large variation in the structure of the interneuron.The morphology of the descending contralateral movement detector (DCMD) interneuron was determined in the thoracic ganglia by intracellularly staining with cobalt sulphide followed by Timm's silver‐intensification of whole‐mount preparations. The striking characteristic of the structure of this interneuron was the variability from animal to animal. This was so large we were unable to describe a “normal” structure. However, five distinct branches of the interneuron were identified in the metathoracic ganglion. In each animal the structure of DCMD could be described by specifying which of these branches were present. One or more of these branches were usually absent, and there were differences in the probability of any particular branch being absent.Corresponding to the variation in the structure of DCMD there was a large variation in the synaptic connections made by this interneuron. DCMD can make monosynaptic connections to the fast extensor tibiae (FETi) motoneuron, fast and intermediate flexor tibiae motoneurons, and depressor and elevator flight motoneurons. These functional connections were not found in all animals. The absence of an EPSP from DCMD in FETi was associated with either the complete absence of a ventral branch or with the absence of an identifiable process of a ventral branch, and the absence of an EPSP from DCMD in flight motoneurons was associated with the absence of the dorsal branch. The connection of each of these specific branches of DCMD to FETi and flight motoneurons respectively was confirmed by double staining the motoneurons and DCMD in cases which had the functional connection. The implication of our findings is that the connections of interneurons in invertebrates may, in some cases, be far more variable than is generally believed at the present time.

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.

Isogenic locusts and genetic variability in the effects of temperature on neuronal threshold

Physiological variability was previously encountered while studying the effects of temperature on identified motorneurons. Of 22 locusts examined from within the breeding colony, 3 deviant animals were found in which the current threshold of the fast extensor tibiae (FETi) motorneuron did not decrease as was expected with increasing temperature (Fig. 1). We have used clones of isogenic locusts to study the genetic basis, physiological extent, and behavioral effects of a similar abnormal threshold response of FETi to increases in temperature. Over 30 clones, raised by parthenogenetic breeding, were used to isolate naturally-occurring genotypic variability. In this study we have focused our attention on the behaviorally abnormal clone 7 animals that have a low probability of jumping which is not altered by heating; and the behaviorally normal clone 8 animals that have a higher probability of jumping which increases with increasing temperature (Table 1). In clone 8 animals, the physiological properties of FETi show the normal steady-state responses to increases in temperature: the current threshold decreases (Fig. 1), the voltage threshold decreases, and the membrane resistance remains relatively unchanged. In clone 7 animals, the physiological properties of FETi show abnormal steady-state responses to increases in temperature: the current threshold increases (Fig. 1), the voltage threshold remains relatively unchanged (Fig. 2), and the membrane resistance decreases (Fig. 3), all of which are similar to the abnormal responses observed occasionally from the heterogenic breeding colony. These abnormalities in FETi of clone 7 animals are regarded as a neurophysiological correlate of the absence of an increase in jumpiness with increasing temperature. An abnormal response to heating in clone 7 animals is also found in AAdC (Fig. 6), ASFlTi, and the first basalar motorneuron. A normal response to heating in clone 7 animals is found in the spiracle closer motorneurons (Fig. 7), the DUM neurons, and the CI neuron.

Synapses upon motoneurons of locusts during retrograde degeneration.

Four interneurons of the ventral cord, the descending movement detectors (DMD) have symmetrical synapses upon the fast extensor tibiae (FETi) motoneurons on each side of the metathoracic ganglion. Each impulse in a DMD interneuron generates an excitatory post-synaptic potential (e.p.s.p.) of constant and similar amplitude in both FETi motoneurons of a normal locust. The symmetry provides inherent controls which makes this a convenient system to study the effect on inputs to a motoneuron caused by peripheral section of its axon. On the operated side the retrograde changes in the FETi motoneuron include, first an increased amplitude of the e.p.s.ps, then a brief period when they are variable, followed by a progressive reduction over a period of days. Other inputs to the FETi motoneurons from head, abdomen and tympanum also decline, but not at equal rates. Changes in e.p.s.p. amplitude are the opposite to those expected from simultaneous changes in the time constant. The observed changes in the e.p.s.ps are attributed to instability and then progressive loss of synapses upon the FETi motoneuron. The results show that the integrity of the motoneuron is essential for maintenance of its synaptic inputs.

Connections between descending visual interneurons and metathoracic motoneurons in the locust

Connections between the four DMD neurons and metathoracic motoneurons in the locustSchistocerca were examined by recording extracellularly from the interneurons in the pro-mesothoracic connectives and intracellularly from seventeen motoneurons. A DIMD or DCMD spike causes an EPSP in the fast extensor tibiae motoneuron, which can be modified by changing the membrane potential. The EPSP always follows spikes at frequencies up to 200 Hz and with a latency of 0.9 ms, suggesting that the connections are monosynaptic and chemically mediated. EPSPs from the DIMD or DCMD arrive at the same time, their axons having the same conduction velocity, and appear simultaneously in the fast extensor tibiae motoneurons on both sides of the ganglion. There is spatial and temporal summation between the inputs but on no occasion did the motoneurons spike. Three inhibitory neurons are depolarized by DMD inputs and may on occasion spike, but it is not known whether these connections are direct. Similarly the slow excitatory motoneuron to the anterior coxal adductor muscle is hyperpolarized by DMD input. Other leg, flight or ventilatory motoneurons examined received no inputs from the DMD neurons. The connections shown are consistent with the hypothesis that the DMD neurons are in some way involved with initiation of a jump, but to achieve this must act synergistically with other inputs.