Understanding the mechanisms which generate and control locomotory movements is fundamental to a complete knowledge of the neuronal control ofbehaviour.We canregardlocomotion, such as jumping, walking or flying, as basic building blocks for much of an animal's behavioural reper-toire;and we can pose three basic questions about the control of such movements. First, what mechanisms ensure that muscles contract in the appropriate sequence? In walking, for example, the basic pattern is repeated flexion and then extension of each leg, with flexion of the left leg coinciding with extension of the right. Second, how does a nervous system select, initiate and terminate a particular type of movement? For example, what initiates the pattern of walking;and how is walking rather than running or swimming selected? Third, how is the basic pattern for movement modulated appropriately? Stride pattern changes, for example, when a person walks up a flight of steps or turns a corner.
Experimental approaches to these questions have often involved work on invertebrates and lower vertebrates, animals in which the parts of the nervous system that generate programs for movement contain a limited number of neurons. This offers the opportunity to identify and characterise all the components involved in generating a particular movement. A specific question that has occupied many investigators is how to determine the source of rhythmical activity that underlies many regularly repeated movements, such as walking or flying. One possible source is proprioceptive reflexes, and muscles could be activated in a particular sequence by joining reflexes in a chain. This view originated from Sherrington's elucidation of spinal reflexes in mammals and predominated for the first half of the twentieth century. However, a number of experiments, particularly on insects from 1960 onwards, showed that proprioceptive reflexes are not required for the co-ordination of quite long, complex sequences of movements. From these experiments emerged the concept that the central nervous system contains central pattern generators, responsible for generating programs for movement.
Nowadays, it is widely acknowledged that timing cues for movements are generated both within the central nervous system and by feedback from proprioceptors. Because more than one mechanism operates, the overall control of a rhythmical movement is rugged. Even within the central nervous system, it now appears to be usual that central pattern generators employ several mechanisms that operate in parallel to generate rhythms of activity. One mechanism is by circuits, such as pairs of neurons that inhibit each other so that excitation alternates between the two neurons. Another way is by pacemaker neurons, which have the intrinsic property of generating regular, clock-like waves of depolarisation and repolarisation of their membrane potentials.
A recent development is the realisation that circuits of neurons responsible for generating movement patterns are often quite plastic. In the locust flight system, for example, such plasticity allows the animal to adapt its movements in the best way for maintaining its course. Another aspect of plasticity is found when neurons participate in more than one type of movement. This has been most extensively investigated in a small group of neurons responsible for controlling movements of the teeth and foregut of spiny lobsters and crabs, which have shown how neuronal circuitry can be dramatically and rapidly reconfigured.
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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.