Ganong's Review of Medical Physiology, 23rd Edition

632 SECTION VIIRespiratory Physiology

NON-CHEMICAL INFLUENCES

ON RESPIRATION

RESPONSES MEDIATED BY RECEPTORS

IN THE AIRWAYS & LUNGS

Receptors in the airways and lungs are innervated by myeli-
nated and unmyelinated vagal fibers. The unmyelinated fibers
are C fibers. The receptors innervated by myelinated fibers are
commonly divided into slowly adapting receptors and rapid-
ly adapting receptors on the basis of whether sustained stim-
ulation leads to prolonged or transient discharge in their
afferent nerve fibers (Table 37–2). The other group of recep-
tors presumably consists of the endings of C fibers, and they
are divided into pulmonary and bronchial subgroups on the
basis of their location.
The shortening of inspiration produced by vagal afferent
activity (Figure 37–3) is mediated by slowly adapting recep-
tors, as are the Hering–Breuer reflexes. The Hering–Breuer
inflation reflex is an increase in the duration of expiration pro-
duced by steady lung inflation, and the Hering–Breuer defla-
tion reflex is a decrease in the duration of expiration produced
by marked deflation of the lung. Because the rapidly adapting
receptors are stimulated by chemicals such as histamine, they
have been called irritant receptors. Activation of rapidly
adapting receptors in the trachea causes coughing, broncho-
constriction, and mucus secretion, and activation of rapidly
adapting receptors in the lung may produce hyperpnea.

Because the C fiber endings are close to pulmonary vessels,

they have been called J (juxtacapillary) receptors. They are

stimulated by hyperinflation of the lung, but they respond as

well to intravenous or intracardiac administration of chemi-

cals such as capsaicin. The reflex response that is produced is

apnea followed by rapid breathing, bradycardia, and hypoten-

sion (pulmonary chemoreflex). A similar response is pro-

duced by receptors in the heart (Bezold–Jarisch reflex or the

coronary chemoreflex). The physiologic role of this reflex is

uncertain, but it probably occurs in pathologic states such as

pulmonary congestion or embolization, in which it is pro-

duced by endogenously released substances.

COUGHING & SNEEZING

Coughing begins with a deep inspiration followed by forced

expiration against a closed glottis. This increases the intra-

pleural pressure to 100 mm Hg or more. The glottis is then

suddenly opened, producing an explosive outflow of air at ve-

locities up to 965 km (600 mi) per hour. Sneezing is a similar

expiratory effort with a continuously open glottis. These re-

flexes help expel irritants and keep airways clear. Other as-

pects of innervation are considered in a special case (Clinical

Box 37–1).

AFFERENTS FROM PROPRIOCEPTORS

Carefully controlled experiments have shown that active and

passive movements of joints stimulate respiration, presumably

TABLE 37–2 Airway and lung receptors.

Vagal

Innervation Type

Location in

Interstitium Stimulus Response

Myelinated Slowly

adapting

Among airway

smooth muscle

cells (?)

Lung inflation Inspiratory time shortening

Hering–Breuer inflation and deflation reflexes

Bronchodilation

Tachycardia

Hyperpnea

Rapidly

adapting

Among airway

epithelial cells

Lung hyperinflation Cough

Exogenous and endogenous substances

(eg, histamine, prostaglandins)

Bronchoconstriction

Mucus secretion

Unmyelinated C

fibers

Pulmonary C

fibers

Bronchial

C fibers

Close to blood

vessels

Lung hyperinflation Apnea followed by rapid breathing

Exogenous and endogenous substanc-

es (eg, capsaicin, bradykinin, serotonin)

Bronchoconstriction

Bradycardia

Hypotension

Mucus secretion

Modified and reproduced with permission from Berger AJ, Hornbein TF: Control of respiration. In: Textbook of Physiology, 21st ed. Vol. 2. Patton HD, et al (editors). Saunders,
1989.