Steering and initiating flight

Flying animals require sensory mechanisms both for maintaining a certain flight course and for altering it so that they avoid collisions or predators. The DCMD neuron (see section 5.8) might be responsible for the ability of locusts to avoid colliding with each other in dense swarms; and locusts, like some other insects, possess neurons which respond to the high-frequency calls of insect-eating bats and which can probably cause a locust to steer its flight path away from a hunting bat (Robert, 1989; Baader, 1991).

More is known about neurons that play a role in maintaining a particular flight course, called DN neurons. These neurons are multimodal - they respond to a number of different sensory modalities in a way that could report details of deviations from a straight flight course. Many of them are excited by wind flowing over the head, which is detected by groups of short, curved hairs mostly situated dorsally, between the compound eyes. Each hair is attached to a bipolar sensory neuron which projects into the brain and probably makes synapses directly with some of the large interneurons. Other sense organs which affect the interneurons include the simple eyes or ocelli, the compound eyes, and proprioceptors that monitor movements of the neck joints.

One wind-sensitive interneuron is called the tritocerebral commissure giant (TCG), after the small nerve which contains its axon. The small nerve runs between the two connective nerves near to where they leave the brain, and this unusual anatomical feature has enabled experimenters to record spikes from the TCG in loosely tethered, flying locusts (Bacon and Mohl, 1983). When a locust is flying into a steady air stream, the TCG does not spike at a steady rate. Instead, it generates a brief burst of spikes to coincide with elevation during each wing-beat cycle. The reason is that movement of the wings and nodding motions of the head create their own air currents, which interact with wind caused by the locust's movement forward through the air. This interneuron is, therefore, rhythmically active during flight. Stimulating it artificially, to induce extra spikes, can reset the flight rhythm by lengthening or shortening the cycle in which the stimulus is delivered. Therefore, the TCG meets the same criteria for being part of the central pattern generator as some of the local, thoracic interneurons.

Two additional roles have been assigned to the TCG. First, it could initiate flight during a jump, when the TCG is excited by the rush of wind over the head. Stimulating the cut axon of a TCG is an effective way of initiating fictive flight (Bicker & Pearson, 1983). Second, the left and right TCGs could stabilise flight direction with respect to wind direction. If the locust yaws so that it is no longer facing directly into the wind, excitation increases to one TCG and decreases to the other (Mohl & Bacon, 1983). Stimulating one TCG electrically can induce yaw movements during tethered flight.

Another example of a DN neuron is DNC (Fig. 7.7a). Besides being excited by wind currents, DNC responds to rotation of the visual horizon. When the horizon rolls about the long axis of the locust, DNC is excited by movements which roll the animal towards the side contralateral to its axon (Fig. 7.7b), and inhibited by movements which roll it in the opposite direction. In experiments, the thorax of the locust is fixed, and a panorama painted on the inside of a sphere can be moved around the head to simulate movements of the horizon. Inputs from the wind-sensitive and visual pathways sum, so that when a locust is flying along a straight course, DNC is tonically excited by air currents. Rolling towards one side causes an increase in spike frequency, and rolling to the other side causes a reduction in spike frequency. Probably, two separate visual pathways converge on DNC: one from the compound eyes, and the other from the simple eyes, or ocelli, which have enormous fields of view and monitor changes in horizon position by measuring changes in the total amount of light they receive (Wilson, 1978). If a locust is carefully prepared to allow access to a short length of connective nerves in the neck, but is capable of moving its wings, abdomen and head freely, steering movements can be produced by stimulating DNC with trains of electrical pulses delivered through a micro-electrode in its axon (Hensler & Rowell, 1990). A train of spikes lasting a few hundred milliseconds causes a change in the relative latency of action potentials in motor neurons of steering muscles within one wing-beat cycle (Fig. 7.7c), and causes the head to roll after a slightly longer delay. Besides

Locust Neuron

Figure 7.7 Brain neuron DNC and its role in flight control. (a) Anatomy of the DNC neuron with its axon on the right of the nervous system. The cell body, on the left of the brain, is indicated with an arrow. In different experiments, intracellularly injected dye revealed the structure of DNC in the brain, and in the second and third thoracic ganglia. (b) Spikes recorded from the axon of the right DNC in response to a wind current and to rolling motion of an artificial visual horizon. Clockwise motion of the horizon - as if the locust was rolling away from the side of the DNC axon - excited this DNC. Rolling in the opposite direction inhibited it. (c) Electrical stimulation of a DNC axon induced a change in the relative latency of spikes in the left and right hindwing motor neurons, 127. Similar shifts in relative latency were also recorded in response to rolling movements of the visual horizon. (Modified after Hensler & Rowell, 1990.)

Figure 7.7 Brain neuron DNC and its role in flight control. (a) Anatomy of the DNC neuron with its axon on the right of the nervous system. The cell body, on the left of the brain, is indicated with an arrow. In different experiments, intracellularly injected dye revealed the structure of DNC in the brain, and in the second and third thoracic ganglia. (b) Spikes recorded from the axon of the right DNC in response to a wind current and to rolling motion of an artificial visual horizon. Clockwise motion of the horizon - as if the locust was rolling away from the side of the DNC axon - excited this DNC. Rolling in the opposite direction inhibited it. (c) Electrical stimulation of a DNC axon induced a change in the relative latency of spikes in the left and right hindwing motor neurons, 127. Similar shifts in relative latency were also recorded in response to rolling movements of the visual horizon. (Modified after Hensler & Rowell, 1990.)

the TCG and DNC neurons, between 20 and 30 other DN neurons on either side of the brain send their axons to the thoracic ganglia (Hensler, 1992). The ways in which these neurons generate steering movements are not clear, but involve some direct output connections with motor neurons as well as connections with local, thoracic interneurons.

Essentials of Human Physiology

Essentials of Human Physiology

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