Chapter 12: Reflex & Voluntary Control of Posture & Movement

OBJECTIVES

After studying this chapter, you should be able to:

• Describe the elements of the stretch reflex and how the activity of γ-motor neurons alters the response to muscle stretch.

• Describe the role of Golgi tendon organs in control of skeletal muscle.

• Describe the elements of the withdrawal reflex.

• Define spinal shock and describe the initial and long-term changes in spinal reflexes that follow transection of the spinal cord.

• Describe how skilled movements are planned and carried out.

• Compare the organization of the central pathways involved in the control of axial (posture) and distal (skilled movement, fine motor movements) muscles.

• Define decerebrate and decorticate rigidity, and comment on the cause and physiologic significance of each.

• Identify the components of the basal ganglia and the pathways that interconnect them, along with the neurotransmitters in each pathway.

• Explain the pathophysiology and symptoms of Parkinson disease and Huntington disease.

• Discuss the functions of the cerebellum and the neurologic abnormalities produced by diseases of this part of the brain.

INTRODUCTION

Somatic motor activity depends ultimately on the pattern and rate of discharge of the spinal motor neurons and homologous neurons in the motor nuclei of the cranial nerves. These neurons, the final common paths to skeletal muscle, are bombarded by impulses from an immense array of descending pathways, other spinal neurons, and peripheral afferents. Some of these inputs end directly on α-motor neurons, but many exert their effects via interneurons or via γ-motor neurons to the muscle spindles and back through the Ia afferent fibers to the spinal cord. It is the integrated activity of these multiple inputs from spinal, medullary, midbrain, and cortical levels that regulates the posture of the body and makes coordinated movement possible.

The inputs converging on motor neurons have three functions: they bring about voluntary activity, they adjust body posture to provide a stable background for movement, and they coordinate the action of the various muscles to make movements smooth and precise. The patterns of voluntary activity are planned within the brain, and the commands are sent to the muscles primarily via the corticospinal and corticobulbar systems. Posture is continually adjusted not only before but also during movement by information carried in descending brainstem pathways and peripheral afferents. Movement is smoothed and coordinated by the medial and intermediate portions of the cerebellum (spinocerebellum) and its connections. The basal ganglia and the lateral portions of the cerebellum (cerebrocerebellum) are part of a feedback circuit to the premotor and motor cortex that is concerned with planning and organizing voluntary movement.

This chapter considers two types of motor output: reflex (involuntary) and voluntary. A subdivision of reflex responses includes some rhythmic movements such as swallowing, chewing, scratching, and walking, which are largely involuntary but subject to voluntary adjustment and control.

GENERAL PROPERTIES OF REFLEXES

The basic unit of integrated ...