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OBJECTIVES

Objectives

After studying this chapter, the student should be able to:

  • See how control of movement is instantiated in a hierarchical control system, much of which appears to involve the evolution of higher levels of more sophisticated control using lower levels as subsystems.

  • Understand how the pyramidal motor system originating in primary motor cortex permits learning of complex and subtle motor sequences.

  • See how goal execution proceeds from the most abstract planning stages in prefrontal cortex, through the supplementary motor and premotor global organizational areas to primary motor cortex.

  • Follow motor commands from primary motor cortex via upper motor neurons to the lower motor neuron spinal cord output.

  • See how the cortical motor output control of the body is laid out in a body map called a homunculus in a manner similar to that of the somatosensory system just across the central sulcus in neocortex.

  • Understand the role of the thalamus and other subcortical structures in motor control.

HIERARCHICAL CONTROL OF MOVEMENT

Movement distinguishes animals from plants. The sea squirt begins life as a free-swimming larva with sensory receptors like eyes, muscles that enable it to swim, and a small brain that mediates its behavior. At the end of the larval stage, it permanently adheres to a spot on the ocean bottom and commences to digest its eyes and nervous system. The nervous system exists to control movement. Movement must be directed toward some goal (planning), and the component muscles must be controlled to act in concert to achieve that goal.

Muscles, which produce movement by contraction, are called effectors. Different types of movement are subserved by 3 different primary muscle types: striated muscles for voluntary movement of skeletal bones around joints via attachments by tendons, smooth muscle that contracts the walls of organs like the stomach and esophagus, and unique cardiac muscle that has hybrid properties between striated and smooth muscle.

This chapter will focus on the cortical control of striated muscle underlying voluntary control of limb movement. All vertebrate striated muscles are activated by receipt of acetylcholine from motor neurons. Each motor neuron spike releases sufficient acetylcholine to elicit an action potential in the muscle cell. The rate of muscle cell action potentials is generally proportional to the contractile force the muscle generates. The motor neurons emanating from the spinal cord that activate muscles that control the limbs and limb appendages (hands and feet) are called lower or α motor neurons. Lower motor neurons are controlled, in turn, by neural circuits in the spinal cord, brainstem, and motor cortex.

Direct activation of lower motor neurons by neurons in primary motor cortex (upper motor neurons) is phylogenetically new and permits the learning of novel and complex motor sequences such as associated with manual praxis. In contrast, basic, highly evolutionarily conserved movement sequences such as walking and running are controlled by circuits in the spinal cord, ...

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