Co-ordinating interneurones of the locust which convey two patterns of motor commands: their connexions with flight motoneurones.

1. Some flight motoneurones receive two superimposed rhythms of depolarizing synaptic potentials when the locust is not flying; a slow rhythm which is invariably linked to the expiratory phase of ventilation, and a fast rhythm with a period of about 50 ms which is similar to the wingbeat period in flight. 2. By recording simultaneously from groups of motoneurones, the synaptic potentials which underly these rhythms have been revealed in 30 flight motoneurones in the three thoracic ganglia. The inputs occur in elevator motoneurones and some depressors but are of lower amplitude in the latter. The inputs have not been found in leg motoneurones. 3. The rhythmic depolarizations are usually subthreshold but sum with sensory inputs to evoke spikes in flight motoneurones at intervals equal to or multiples of the wingbeat period in flight. 4. Both rhythms originate in the metathoracic ganglion and are mediated by the same interneurones. They can be adequately explained by supposing that there are two symmetrical interneurones which each make widespread connexions with left and right flight motoneurones in the three ganglia. 5. The slow rhythm is coded in the overall burst of interneurone spikes during expiration and the fast rhythm in the interval between the spikes of a burst.     

Co-ordinating interneurones of the locust which convey two patterns of motor commands: their connexions with ventilatory motoneurones.

1. The interneurones which make widespread connexions with flight motoneurones also synapse upon ventilatory motoneurones so that in all 50 motoneurones receive synapses. They influence three aspects of ventilation; (a) the closing and opening movements of the thoracic spiracles, (b) some aspects of abdominal pumping movements and (c) the recruitment of some motoneurones controlling head pumping. 2. The two closer motoneurones of a particular thoracic spiracle receive the same excitatory synaptic inputs (EPSPs) during expiration. The EPSPs match those in appropriate flight motoneurones. 3. The closer motoneurones of each thoracic spiracle whose somata are in the pro-, meso- or metathoracic ganglia all receive the same excitatory synaptic inputs. These inputs are an adequate explanation of the pattern of spikes in the closer motoneurones. Both the slow ventilatory and fast rhythms of synaptic potentials are expressed as spikes; the slow as the overall expiratory burst of spikes and the fast as the groups of spikes within that burst. This establishes a ventilatory function for the interneurones. All thoracic closer motoneurones therefore receive the same excitatory commands which will tend to synchronize the movements of each spiracle. 4. Spiracular opener motoneurones are inhibited during expiration, their IPSPs matching the EPSPs in flight or closer motoneurones. Therefore the interneurones have reciprocal effects on the antagonistic motoneurones of the spiracles. 5. The interneurones synapse upon some motoneurones which control the pumping movements of the abdomen and which have their somata in the metathoracic or first unfused abdominal ganglion. Motoneurones in four separate ganglia therefore receive inputs from these interneurones. 6. The interneurones also synapse upon motoneurones which control an auxiliary form of ventilation, head pumping.     

Reflex activation of locust flight motoneurones by proprioceptors responsive to muscle contractions

 This report investigates the reflex activation of locust flight motoneurones following their spiking activity. As shown elsewhere, an electrical stimulus applied to a flight muscle produces multiple waves of delayed excitation in wing elevator and depressor motoneurones. Nerve ablation experiments show that this response is initiated by the mechanical movement of the stimulated muscle, and not the antidromic spike evoked in the motoneurone. The delayed excitation still occurs in the absence of inputs from the wing receptor systems, and also when all other sources of afferent feedback are abolished, excepting thoracic nerve 2. Following complete deafferentation, spikes in flight motoneurones had no influence on other flight motoneurones. Numerous afferents in the purely sensory nerve 2 are excited by flight muscle contractions. The responses are consistent for repeated contractions of the same muscle, but differ when other muscles are stimulated. During tethered flight, changes in the activation of single flight muscles are reflected in changes of the nerve 2 discharge pattern. Electrical stimulation of this nerve causes delayed excitation of flight motoneurones, and can initiate flight activity. It is suggested that internal proprioceptors, such as those associated with nerve 2, will contribute to shaping the final motor output for flight behaviour. Accepted: 24 April 1996     

The effect of forewing depressor activity on wing movement during locust flight

Locusts are passively yawed in the laminar air current of a wind tunnel (Fig. 1). In order to study the influence of depressor muscles of the forewing on its movement, electromyography is combined with true 3-dimensional inductive forewing movement recording. In quick response to the yaw stimulus, many kinematic parameters (e.g. shape of the wing tip path, amplitudes of wingstroke, ratios of downstroke to upstroke duration, time interval between beginning of downstroke and time of maximum pronation etc.) vary differently in both forewings (Figs. 3–5). Pronation changes in correlation to yawing reciprocally on both forewings with comparable differences of pronation angles (Fig. 5a). Maximum pronation is decreased on that side, to which the animal is-passively-yawed, whereas the slope of the wing tip paths remains almost constant. Therefore, decreasing pronation most probably indicates increasing thrust. The animal appears to perform a disturbance avoidance behaviour. Although the burst length of muscle firing is almost constant here, the onset of 8 depressor muscles (1 st basalar and subalar muscles of all 4 wings) varies in correlation to the stimulus (Figs. 6–8). The changing time intervals between the 1 st basalar muscle M97 and subalar muscle M99 are responsible for the alterations of forewing downstroke. Quantitative analysis of combined motor and movement pattern (Fig. 9) shows the following: (i) the maximum pronation and time interval between the onset of 1 st basalar muscle M97 as well as subalar muscle M99 and the beginning of downstroke are positively correlated (Figs. 10 and 12a and b). (ii) Maximum pronation is greatest, when muscles M97 and M99 act simultaneously (Fig. 12c). Thus, both muscles work synergistically, concerning pronation. Muscle M99 is of less importance than muscle M97. On failing activity of the depressor muscle M97, downstroke is greatly reduced. Some depressor as well as elevator muscles are switched on and off separately on each side (Fig. 11).     

Substrate utilization and flight speed during tethered flight in the locust

Mature laboratory locusts normally exhibit a characteristic pattern of change in flight speed with time. They fly at high speed for the first few minutes, during which carbohydrate forms the major fuel, but then slow to a cruising speed when lipid is used almost exclusively. Locusts flown for 30 min, rested for 2hr, and then reflown, exhibit an identical pattern of flight, even though they oxidise only half the amount of carbohydrate used in the first flight. The injection of adipokinetic hormone before the first flight elicits a low initial flight speed for 10 to 15 min but then the locusts accelerate to a constant higher speed. The injection of hormone before the second flight, when blood lipid levels are already high, reduces the utilization of carbohydrate by the flight muscles dramatically but results in constant high-speed flight.     

Functional peculiarities of stretch receptors of the flight apparatus in the cockroach Periplaneta americana]

Functional peculiarities of stretch receptors in wing articulations of the cockroach are considered and possible functional role of these receptors in maintenance of stable rhythm of flight is discussed. Stretch receptors of wing articulations are of phasictonic type they exhibit slow and incomplete adaptation, discharging when the wing goes upward. The pattern of impulse response of the receptor depends both on the angle and the velocity of displacement of wing platelet. The scheme of presumptive pacemaker of wing beats is discussed in detail.     

Cerebral neurosecretory cells and flight in the locust

The effect on flight performance of various superficial lesions of the pars intercerebralis in and around the area of the MNSC (median groups of cerebral neurosecretory cells) have been studied 18 hr after surgery. Only lesions involving areas immediately lateral to the MNSC produce an impairment of flight performance. The release of adipokinetic hormone during flight was studied in these locusts by measuring the changes in haemolymph lipid during flight. It has not been possible to identify any of the areas tested as being concerned with the control of the release of adipokinetic hormone since lipid mobilization was not prevented by any of the operations studied.The poor flight performance in locusts in which the MNSC were destroyed by cautery on day 1 of adult life can be prevented by regular topical application of a synthetic juvenile hormone analogue. It is argued that the effects of removal of the MNSC on the development of flight performance are most likely a consequence of reduced activity of the corpora allata.     

Modulation of transmitter release from the locust forewing stretch receptor neuron by GABAergic interneurons activated via muscarinic receptors.

The role of muscarinic receptors in the down-regulation of acetylcholine (ACh) release from the locust forewing stretch receptor neuron (fSR) terminals has been investigated. Electrical stimulation of the fSR evokes monosynaptic excitatory postsynaptic potentials (EPSPs) in the first basalar motoneuron (BA1), produced mainly by the activation of postsynaptic nicotinic cholinergic receptors. The general muscarinic antagonists scopolamine (10(-6) M) and atropine (10(-8) to 10(-6) M) caused a reversible increase in the amplitude of electrically evoked EPSPs. However, scopolamine (10(-6) M) caused a slight depression in the amplitude of responses to ACh pressure-applied to the soma of BA1. These observations indicate that the EPSP amplitude enhancement is due to the blockade of muscarinic receptors on neurons presynaptic to BA1. The muscarinic receptors may be located on the fSR itself and act as autoreceptors, and/or they may be located on GABAergic interneurons which inhibit ACh release from the fSR. Electron microscopical immunocytochemistry has revealed that GABA-immunoreactive neurons make presynaptic inputs to the fSR. The GABA antagonist picrotoxin (10(-6) M) caused a reversible increase in the EPSP amplitude, which does not appear to be due to an increase in sensitivity of BA1 to ACh, as picrotoxin (10(-6) M) slightly decreased ACh responses recorded from BA1. Application of scopolamine (10(-6) M) to a preparation preincubated with picrotoxin did not cause the EPSP amplitude enhancement normally seen in control experiments; in fact, it caused a slight depression. This indicates that at least some of the presynaptic muscarinic receptors are located on GABAergic interneurons that modulate transmission at the fSR/BA1 synapse.     

Interneuronal organization in the flight system of the locust

In this paper we describe the characteristics, connections, resetting properties and organization of some identified interneurones in the flight system of the locust. The major conclusions are that: (1) the flight rhythm is generated at the interneuronal level and the flight oscillator is not continuously active (2) the interneurones in the flight pattern generator are distributed within at least 6 segmental ganglia (three thoracic and three fused abdominal ganglia) and are not organized into two homologous groups for the separate control of the forewing and the hindwing (3) this distribution of flight interneurones has no obvious functional significance but could be a consequence of flight having evolved from a segmentally distributed motor behaviour (4) there may be a functional hierarchy among flight interneurones such that premotor interneurones are separate from those generating the rhythm.     

Localization of projectin in locust flight muscle

Projectin is a giant filamentous protein of arthropod striated muscle. By using immunofluorescence microscopy, projectin was shown to span between the I band and the A band in locust (Locusta migratoria) flight muscle sarcomeres. The N- and C-terminal regions of projectin molecules were localized in the I band and A band, respectively. This observation explains the controversial reports of previous studies that projectin is localized either in the I band or in the A band of locust flight muscle sarcomeres. It is also observed that the N-terminal region of projectin is located in the I band of locust leg muscle sarcomeres.     

Heat shock-induced thermoprotection of action potentials in the locust flight system.

There is increasing evidence that heat shock (HS) has long-term effects on electrophysiological properties of neurons and synapses. Prior HS protects neural circuitry from a subsequent heat stress but little is known about the mechanisms that mediate this plasticity and induce thermotolerance. Exposure of Locusta migratoria to HS conditions of 45 degrees C for 3 h results in thermotolerance to hitherto lethal temperatures. Locust flight motor patterns were recorded during tethered flight at room temperature, before and after HS. In addition, intracellular action potentials (APs) were recorded from control and HS motoneurons in a semi-intact preparation during a heat stress. HS did not alter the timing of representative depressor or elevator muscle activity, nor did it affect the ability of the locust to generate a steering motor pattern in response to a stimulus. However, HS did increase the duration of APs recorded from neuropil segments of depressor motoneurons. Increases in AP duration were associated with protection of AP generation against failure at subsequent elevated temperatures. Failure of AP generation at high temperatures was preceded by a concomitant burst of APs and depolarization of the membrane. The protective effects of HS were mimicked by pharmacological blockade of I(K+) with tetraethylammonium (TEA). Taken together, these findings are consistent with a hypothesis that HS protects neuronal survival and function via K+ channel modulation.