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Figure 7.5 Synaptic relationships of some of the elements of the central pattern generator for flight in the locust. Excitatory connections are indicated as triangles and inhibitory connections as circles. Interneurons are numbered; and E and D indicate elevator and depressor wing motor neurons. (Modified after Robertson & Pearson, 1985.)

Figure 7.5 Synaptic relationships of some of the elements of the central pattern generator for flight in the locust. Excitatory connections are indicated as triangles and inhibitory connections as circles. Interneurons are numbered; and E and D indicate elevator and depressor wing motor neurons. (Modified after Robertson & Pearson, 1985.)

is relatively constant over a range of different wing-beat frequencies. This constancy can be explained by the actions of 301 in indirectly exciting these motor neurons while at the same time removing inhibition of them from 511. This creates a discrete time slot, following a burst of spikes in 301, during which it is possible to excite the depressor motor neurons. Another feature of the flight program, the constant delay between bursts in elevator and depressor motor neurons, can be explained by the dual action of 504 which excites the elevator motor neurons directly and the depressor motor neurons indirectly through interneuron 301.

Not all of the thoracic interneurons that are involved in flight participate in generating the rhythm. For example, some interneurons seem to play a role in initiating and maintaining flight, but not in the timing of wing beats (Pearson et al., 1985). The 204 interneurons are a small group of about four on each side of the second thoracic ganglion. They all have axons that describe a tight loop in that ganglion and then travel anteriorly, so they cannot make direct contact with most of the interneurons of the flight generator, and they are excited by air currents blown at the head. Injection of depolarising current into a single 204 neuron to excite it strongly can trigger Active flight, and injection of hyperpolarising current makes it less likely that air currents will trigger a flight. These interneurons may, therefore, be part of pathways that can start the flight motor pattern generator. During Active flight, they can spike tonically, so they neither contribute to nor receive timing information for wing-beat cycles. Some stimuli which initiate flight, such as wind to the tail end of a locust, do not excite the 204 interneurons, which means that these neurons are not obligatory for starting the flight motor program, so that different sensory pathways can trigger the program independently.

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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