Generation of the flight rhythm

An influential paper in the study of the neuronal control of movement was by Donald Wilson, published in 1960. This challenged the prevailing view that sequential activation of reflex loops generated rhythmical movements, such as locust flight and walking in vertebrates. Locust wings bear a large number of sense organs that report details of their movements. However, Wilson showed that when he removed the wings and destroyed the sense organs of the wing bases, a locust could still generate a pattern similar to flight. This consisted of regularly repeating, alternating spikes in wing elevator and wing depressor motor neurons. Either an air current directed at the head or a series of randomly timed electrical shocks to the connective nerves could elicit this flight-like activity. Flights are short in duration, and the wing-beat frequency is about half that of an intact locust. Wilson concluded that the basic program for generating flight movements is situated in the central nervous system. Generation of the rhythm, or the co-ordinated activity of different motor neurons in the correct sequence, does not require proprioceptors to report details of the movements caused by muscle contraction. Even if all the flight muscles and the head are removed, the thoracic ganglia are capable of generating a slow, repeating pattern of alternating excitation of elevator and depressor motor neurons.

Some of the neurons involved in generating the flight rhythm have been identified and characterised by intracellular recording. The thorax is opened dorsally to allow access to the thoracic ganglia, which are stabilised against movements by supporting them with a metal platform (Fig. 7.3a). Further stability is achieved by removing the legs and cutting most of the nerves that supply the flight muscles. The nerve to one muscle, usually one of the dorsal longitudinal muscles, is left intact so that EMGs recorded from this muscle provide a monitor of the flight rhythm. This nerve also carries the axons of most of the proprioceptors of the wing base. A locust prepared in this way will produce sequences of flight-like activity when wind flows over its head, or if the neurohormone octopamine is applied to the thoracic ganglia. There are many differences in the detail of the pattern from that generated by intact animals, and flight sequences are rather short in duration. However, the basic pattern of repeated, alternating activation of depressors and elevator motor neurons is present. As in an intact animal, there is a constant delay between activity in elevator and depressor motor neurons, and a delay of 5 ms between

Motor Neurons Locust

Figure 7.3 Intracellular recordings during fictive flight. (a) A method for preparing the locust, which is opened mid-dorsally and the body walls pinned, exposing the ganglia. Two glass capillary microelectrodes penetrate neurons in the ganglia while wire electrodes record EMGs from muscle 112. (b) Intracellular recordings from an elevator (el) and a depressor (dep) motor neuron, together with an EMG recording from muscle 112 (lower trace). (c) A phase-resetting experiment with interneuron 501. The upper trace is an intracellular recording from the neuron and the lower trace is the EMG from muscle 112 (when the neuron was stimulated with depolarising current, the intracellular recording could not be registered). The arrowheads indicate the times when bursts of spikes in the depressor muscle would have been expected to occur without any stimulus to the interneuron. (a and b modified after Robertson & Pearson, 1982; c modified after Robertson & Pearson, 1983.)

Figure 7.3 Intracellular recordings during fictive flight. (a) A method for preparing the locust, which is opened mid-dorsally and the body walls pinned, exposing the ganglia. Two glass capillary microelectrodes penetrate neurons in the ganglia while wire electrodes record EMGs from muscle 112. (b) Intracellular recordings from an elevator (el) and a depressor (dep) motor neuron, together with an EMG recording from muscle 112 (lower trace). (c) A phase-resetting experiment with interneuron 501. The upper trace is an intracellular recording from the neuron and the lower trace is the EMG from muscle 112 (when the neuron was stimulated with depolarising current, the intracellular recording could not be registered). The arrowheads indicate the times when bursts of spikes in the depressor muscle would have been expected to occur without any stimulus to the interneuron. (a and b modified after Robertson & Pearson, 1982; c modified after Robertson & Pearson, 1983.)

spikes in motor neurons of the hindwing and forewing depressor muscles. Flight-like activity in a dissected, immobile locust is called fictive flight.

At the start of a flight sequence, elevator motor neurons depolarise and generate spikes first. In an intact animal, this would open the wings. During flight, repeated smooth oscillations in membrane potential, up to 25 mV in amplitude, are recorded from flight motor neurons (Fig. 7.3b). Depolarisation of the elevator and depressor motor neurons alternates and sometimes a motor neuron might hyperpolarise from its resting potential between cycles of depolarisation.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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