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CHAPTER

16

Control of Posture

& Movement

OBJECTIVES

After studying this chapter, you should be able to:

■

Describe how skilled movements are planned and carried out.

■

Name the posture-regulating parts of the central nervous system and discuss the

role of each.

■

Define spinal shock and describe the initial and long-term changes in spinal reflex-

es that follow transection of the spinal cord.

■

Define decerebrate and decorticate rigidity, and comment on the cause and phys-

iologic significance of each.

■

Describe the basal ganglia and list the pathways that interconnect them, along

with the neurotransmitters in each pathway.

■

Describe and explain the symptoms of Parkinson disease and Huntington disease.

■

List the pathways to and from the cerebellum and the connections of each within

the cerebellum.

■

Discuss the functions of the cerebellum and the neurologic abnormalities pro-

duced 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 neu-

rons, the final common paths to skeletal muscle, are bom-

barded 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 neu-

rons 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 coor-

dinated movement possible.

The inputs converging on motor neurons subserve 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 corti-

cobulbar systems. Posture is continually adjusted not only

before but also during movement by descending brain stem

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 (cere-

brocerebellum) are part of a feedback circuit to the premotor

and motor cortex that is concerned with planning and orga-

nizing voluntary movement.