Usually, motor neurons are not involved in the generation of rhythms for movements, but are driven through synapses from interneurons and sensory neurons. This arrangement allows motor neurons to be excited independently of each other, so that their muscles can be used in different movement patterns. Networks of neurons that can generate programs for movement are often called pattern generators. In general, rhythmical movements are generated by a number of different mechanisms operating together, which makes the pattern generator robust.
Phase-resetting experiments demonstrate whether a particular neuron is involved in generating rhythmical activity. In locust flight, the rhythm is generated by networks of interneurons, reinforced by cyclical feedback from proprioceptors such as the wing hinge stretch receptors, and from wind-sensitive neurons. There is good evidence for the existence of reverberating circuits of interneurons in the flight generator, but because so many interneurons are involved it is difficult to assign a precise role to a single neuron and to disentangle different circuits from each other.
There are a number of different ways in which the excitation of a rhythm-generating network can be sustained. In locust flight, sensory activity, particularly from wind-sensitive neurons, is important. Neuromodulators can play important roles by strengthening synaptic connections, as in Tritonia swimming, or by switching on bursting properties, such as in the pyloric rhythm of lobsters or, perhaps, locust flight. Finally, long-lasting postsynaptic potentials such as those mediated by NMDA receptors in tadpole swim motor neurons can also be important in sustaining a rhythm.
Circuits that generate programs for movement are not 'hard wired' and inflexible. One way in which this is evident is the manner in which sensory feedback works in controlling locust flight. At its simplest, sensory feedback allows a motor program to compensate for changing demands on muscles as the nature of the terrain alters. Another more subtle role is to select between patterns that differ in their effectiveness, for example in moving an animal along a straight path. Continual small variations in motor output allow the motor program to be updated continually, allowing compensation for changes in the mechanical properties of an animal's skeleton and muscles as it grows or is injured. Interneurons can also cause radical changes in the way that networks are configured. This is well illustrated by the way in which a single interneuron can reconfigure the lobster stomato-gastric ganglion so that the neurons participate in new groupings and patterns of activity.
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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.