Motor control is a crucial function of the central nervous system (CNS) and plays a significant role in human interaction with the environment. It encompasses various activities such as running, reaching, eating, talking, writing, and reading. These activities require coordination of muscle contraction to maintain posture or transition between postures.

Damage or diseases affecting the nervous system often lead to motor abnormalities, making the assessment of motor function essential in clinical practice. The CNS devotes a substantial portion of its resources to motor control, involving the spinal cord, brainstem, cerebral cortex, cerebellum, and basal ganglia. The generation of signals from these areas coordinates muscle contraction throughout the body and head.

This chapter provides a detailed description of each major CNS region involved in motor control, starting with the spinal cord and progressing to the brainstem, cerebral cortex, cerebellum, and basal ganglia. The discussion concludes with eye movements, which are complex and require specialized circuits.

While each CNS area is discussed individually, it is important to note that these regions do not operate in isolation. Most movements result from the coordinated activity of multiple brain regions. Even spinal reflexes, which are mediated by local spinal cord circuits, can be influenced by descending motor commands. Similarly, voluntary movements originating from cerebral activity ultimately involve the activation of spinal cord circuitry or analogous brainstem nuclei for muscles in the head and face.

The principles of spinal cord organization are foundational in understanding motor control.
3.
What is a reflex, and why are reflexes useful for clinical and scientific understanding?
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What information about the state of the muscle is sensed by the muscle spindles, and what afferent fibers convey this information to the central nervous system (CNS)?
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How do γ motor neurons modulate the responses of the muscle spindle?
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What are the pathways and functions of the basic spinal reflexes?
7.
What is a central pattern generator, and what types of movements can it be used for?
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What distinguishes the pathways of the medial and lateral descending pathways in motor control?
9.
What is decerebrate rigidity, and what are its implications for the control of muscle tone?
10.
What distinguishes the cortical motor areas from each other?
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What motor parameters are coded for in the activity of neurons in motor cortex?
12.
How does the organization of the mossy and olivocerebellar (climbing) fiber afferent systems to the cerebellum differ in their origins, topography, and synaptic connections.
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What is the geometric relationship between the major cellular elements of the cerebellar cortex?
14.
What are simple and complex spikes in Purkinje cells?
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What are the direct and indirect pathways in the basal ganglia, and how does their activity influence movement?
16.
How is the balance of activity between the direct and indirect pathways altered in Parkinson’s disease and Huntington’s disease?
17.
How do the vestibuloocular and optokinetic reflexes act to stabilize gaze? How do they complement each other?
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What are the roles of saccades and smooth pursuit movements in visual tracking?
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What is nystagmus, and what types of sensory stimulation can drive nystagmus in a normal individual?
20.
What is the somatotopic organization of the different CNS regions involved in motor control.
ovements are the major way in which humans interact with the world. Most activities— including running, reaching, eating, talking, writing, and reading—ultimately involve motor acts. Thus motor control is a major task of the central nervous system (CNS), and from an evolutionary perspective, it is probably the reason that nervous systems first arose. Not surprisingly, a large amount of the CNS is devoted to motor control, which can be defined as the generation of signals to coordinate contraction of the musculature of the body and head, either to maintain a posture or to make a movement (transition between two postures).
Because large amounts of the nervous system are involved in motor control, damage or diseases of the nervous system often result in motor abnormalities. Conversely, particular motor symptoms help determine the location of the damaged or malfunctioning region; thus assessment of motor function is an important clinical tool.
In this chapter, each major CNS area involved in motor control is described, starting with the spinal cord and continuing with the brainstem, cerebral cortex, cerebellum, and basal ganglia. Eye movements are discussed at the end of the chapter because of their importance and the specialized circuits involved in their generation. Each CNS area is described separately; however, CNS regions do not function in isolation, and most movements result from the coordinated action of multiple brain regions. For example, even spinal reflexes, which are mediated by local circuits in the spinal cord, can be modified by descending motor commands, and virtually all voluntary movements, which arise from cerebral activity, are ultimately generated by activation of the spinal cord circuitry (or analogous brainstem nuclei for muscles in the head and face).
Principles of Spinal Cord Organization