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Evolution of Movement

Movement (motion) is a fundamental and essential property of animal life. In simple, unicellular animals, motion depends on the contractility of protoplasm and the action of accessory organs: cilia, flagella, and so forth. Rudimentary multicellular animals possess primitive neuromuscular mechanisms; in more advanced forms of animal life, reflexive motion is based on the transmission of impulses from a receptor through an afferent neuron and ganglion cell to motor neurons and muscles. This same arrangement is found in the reflex arc of higher animals, including humans, in whom the spinal cord has further developed into a central regulating mechanism. Superimposed on these reflex circuits, the brain is concerned with the initiation and control of movement and the integration of complex motions.

Control of Movement in Humans

The motor system in humans controls a complex neuromuscular network. Commands must be sent to many muscles, and multiple ipsilateral and contralateral joints must also be stabilized. The motor system includes cortical and subcortical areas of gray matter; the corticobulbar, corticospinal, corticopontine, rubrospinal, reticulospinal, vestibulospinal, and tectospinal descending tracts; gray matter of the spinal cord; efferent nerves; and the cerebellum and basal ganglia (Figs 13–1 and 13–2). Feedback from sensory systems and cerebellar afferents further influences the motor system.

Figure 13–1

Schematic illustration of some pathways controlling motor functions. Arrows denote descending pathways.

Figure 13–2

A: Basal ganglia: major structures. MD, medial dorsal; VA, ventral anterior; VL, ventral lateral nuclei of thalamus. B: Major afferents to basal ganglia. C: Intrinsic connections. D: Efferent connections.

Movement is organized in increasingly complex and hierarchical levels.

Reflexes are controlled at the spinal or higher levels (Table 13–1; see also Chapter 5).

Table 13–1 Summary of Reflexes.

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