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1-6 Reflexes and Voluntary Movements

The most fundamental control system structure in the CNS is an individual reflex operating via the spinal cord or brainstem. A reflex is performed by sequential activities in a neuronal circuit that connects serially sensory receptor cells, an afferent path, a reflex center, an efferent path, and an effector (a muscle, set of muscles, or a secretory gland or glands). This is typically exemplified by the stretch reflex, in which motoneurons maintain the length of a muscle constant by using feedback from muscle spindles (Chapter 11, "Somatic and Autonomic Reflexes"). In this case, a group of motoneurons and associated segmental interneurons constitute a controller, whereas the motor apparatus composed of the muscle(s) and a joint provides the controlled object. Numerous reflexes of various types operate in the spinal cord and brainstem to control simple somatic and visceral functions of a living body. The operation of reflexes is usually automatic—that is, it does not reach the level of conscious awareness—but in ever-changing environments it is indeed modifiable by use of adaptive mechanisms of the cerebellum (Chapters 10–12).

We traditionally consider voluntary movements as a much higher order of movements than reflexes in the sense that they are controlled by "free will" and can be performed both automatically and at the level of conscious awareness, whereas reflexes are driven by peripheral stimuli and executed solely by automatic means. However, as our understanding advances for neuronal mechanisms underlying voluntary movements, distinctions between such movements and reflexes become blurred because many of the same neuronal circuits are employed for both types of movement (Prochazka et al., 2000 Hultborn, 2001). Practically speaking, however, we may still distinguish voluntary movements as initiated from the cerebral cortex, whereas reflexes operate largely within the spinal cord and brainstem. Typically, two cortical areas, the primary motor cortex and the frontal eye field, are involved in voluntary movements of the limbs and eyes, respectively (Chapters 13 and 14). In the systems control parlance emphasized in this volume, reflexes and voluntary movements may share neuronal circuits for their controller and controlled object structures, but they are separated from each other by the nature of the instruction signals that drive the controller. Instructions for reflexes arise from periphery, whereas voluntary movements are driven by "top down" instruction signals generated in higher centers of the cerebral cortex, including but not limited to the supplementary motor cortex and the anterior cingulate gyrus (see Chapter 13, "Voluntary Motor Control").

An interesting idea has been put forward to suggest that a central instruction causes a voluntary movement by an imitation or replacement of the peripheral stimulus that induces a reflex (the imitation hypothesis; Berkinblit et al., 1986). For instance, the CNS can voluntarily elicit a saccadic eye movement by means of the imitation of the visual signals that could elicit the saccadic movement reflexively. In this sense, the central instruction may imply an "afference copy" of the peripheral stimulus. Such a capability of imitating a peripheral stimulus might emerge during evolution to develop a neuronal mechanism of voluntary motor control. Neuronal mehanisms underlying the postulated capability of imitation are unknown, but one may suppose that a group of neurons memorize those signals of peripheral stimuli that evoke a motor behavior reflexively and reproduce the same signals whenever a similar motor behavior is to be generated voluntarily. Here, one may recall the "mirror" neurons, which are present in certain cerebral cortical areas and are activated during both observed and performed hand actions, as discussed below (Section 8 and also in Chapter 16, "Motor Actions and Tool Use," Section 5). These neurons appear to memorize perceptive signals representing certain successful motor actions performed by another individual and reproduce them as central instructions for their own body's motor actions. Admittedly, however, the neuronal sites and mechanisms underlying free will in the high cerebral centers are still an enigma (Wegner, 2002).

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