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CHAPTER

37

Regulation of Respiration

OBJECTIVES

After studying this chapter, you should be able to:

■

Locate the pre-Bötzinger complex and describe its role in producing spontaneous

respiration.

■

Identify the location and probable functions of the dorsal and ventral groups of respi-

ratory neurons, the pneumotaxic center, and the apneustic center in the brain stem.

■

List the specific respiratory functions of the vagus nerves and the respiratory

receptors in the carotid body, the aortic body, and the ventral surface of the

medulla oblongata.

■

Describe and explain the ventilatory responses to increased CO

2

concentrations in

the inspired air.

■

Describe and explain the ventilatory responses to decreased O

2

concentrations in

the inspired air.

■

Describe the effects of each of the main non-chemical factors that influence respiration.

■

Describe the effects of exercise on ventilation and O

2

exchange in the tissues.

■

Define periodic breathing and explain its occurrence in various disease states.

INTRODUCTION

Spontaneous respiration is produced by rhythmic discharge of

motor neurons that innervate the respiratory muscles. This

discharge is totally dependent on nerve impulses from the

brain; breathing stops if the spinal cord is transected above

the origin of the phrenic nerves. The rhythmic discharges

from the brain that produce spontaneous respiration are reg-

ulated by alterations in arterial P

O 2

, P

CO 2

, and H

+

concentra-

tion, and this chemical control of breathing is supplemented by

a number of non-chemical influences. The physiological bases

for these phenomena are discussed in this chapter.

NEURAL CONTROL OF BREATHING

CONTROL SYSTEMS

Two separate neural mechanisms regulate respiration. One is
responsible for voluntary control and the other for automatic
control. The voluntary system is located in the cerebral cortex
and sends impulses to the respiratory motor neurons via the
corticospinal tracts. The automatic system is driven by a group
of pacemaker cells in the medulla. Impulses from these cells
activate motor neurons in the cervical and thoracic spinal cord

that innervate inspiratory muscles. Those in the cervical cord

activate the diaphragm via the phrenic nerves, and those in the

thoracic spinal cord activate the external intercostal muscles.

However, the impulses also reach the innervation of the inter-

nal intercostal muscles and other expiratory muscles.

The motor neurons to the expiratory muscles are inhib-

ited when those supplying the inspiratory muscles are active,

and vice versa. Although spinal reflexes contribute to this

reciprocal innervation,

it is due primarily to activity in

descending pathways. Impulses in these descending pathways

excite agonists and inhibit antagonists. The one exception to