Essentials of Anatomy and Physiology

Hiccups, also a reflex, are spasms of the diaphragm.
The result is a quick inhalation that is stopped when
the glottis snaps shut, causing the “hic” sound. The
stimulus may be irritation of the phrenic nerves
or nerves of the stomach. Excessive alcohol is an irri-
tant that can cause hiccups. Some causes are simply
unknown.
Yet another respiratory reflex is yawning. Most of
us yawn when we are tired, but the stimulus for and
purpose of yawning are not known with certainty.
There are several possibilities, such as lack of oxygen

or accumulation of carbon dioxide, but we really do

not know. Nor do we know why yawning is conta-

gious, but seeing someone yawn is almost sure to elicit

a yawn of one’s own. You may even have yawned while

reading this paragraph about yawning.

CHEMICAL REGULATION

Chemical regulation refers to the effect on breathing

of blood pH and blood levels of oxygen and carbon

dioxide. This is shown in Fig. 15–10. Chemorecep-

The Respiratory System 359

Hypothalamus

Pneumotaxic

center

Apneustic

center

Stimulatory

Inhibitory

Cerebral cortex

External intercostal muscles

Baroreceptors in

lungs

Inspiration center Diaphragm

Expiration center

Medulla

Pons

A

B

C

Figure 15–9. Nervous regulation of respiration. (A) Midsagittal section of brain.

(B) Respiratory centers in medulla and pons. (C) Respiratory muscles. See text for descrip-

tion.

QUESTION:Which center directly stimulates inhalation? How can you tell from this

picture?