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Loading reflex to monitor tidal volume (AIIMS NOV.2011)

A Stretch receptors in bronchioles

B J receptors

C Thoracic muscle spindles

D Carotid and aortic bodies

Ans. C Thoracic muscle spindles

I. Regulation of Respiratory Activity

a. A rise in the PCO 2 or H+ concentration of arterial blood or a drop in its PO 2 increases the level of respiratory neuron activity in the medulla, and changes in the opposite direction have a slight inhibitory effect.

b. The effects of variations in blood chemistry on ventilation are mediated via respiratory chemoreceptors—the carotid and aortic bodies and collections of cells in the medulla and elsewhere that are sensitive to changes in the chemistry of the blood.

c. They initiate impulses that stimulate the respiratory center. Superimposed on this basic chemical control of respiration, other afferents provide non-chemical controls that affect breathing in particular situations.

Table 37–1 Stimuli Affecting the Respiratory Center.

Chemical control :-

CO2 (via CSF and brain interstitial fluid H+ concentration)



(via carotid and aortic bodies)

Non-chemical control :-

Vagal afferents from receptors in the airways and lungs

Afferents from the pons, hypothalamus, and limbic system

Afferents from proprioceptors

Afferents from baroreceptors: arterial, atrial, ventricular, pulmonary

II. J receptors :

a. They are stimulated by hyperinflation of the lung, but they respond as well to intravenous or intracardiac administration of chemicals such as capsaicin.

b. The reflex response that is produced is apnea followed by rapid breathing, bradycardia, and hypotension (pulmonary chemoreflex). A similar response is produced by receptors in the heart (Bezold–Jarisch reflex or the coronary chemoreflex).

c. 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 produced by endogenously released substances

III. Stretch receptors in bronchioles : cause Hering–Breuer reflexes

a. The shortening of inspiration produced by vagal afferent activity is mediated by slowly adapting Stretch receptors.

b. The Hering–Breuer inflation reflex is an increase in the duration of expiration produced by steady lung inflation, and the Hering–Breuer deflation reflex is a decrease in the duration of expiration produced by marked deflation of the lung.

c. They limit the inspiration and expiration therefore help in maintain normal tidal resting respiration (AIIMS nov 2010)

IV. Proprioceptors : Muscle spindles, Golgi tendon organs etc

a. Carefully controlled experiments have shown that active and passive movements of joints stimulate respiration

b. Presumably because impulses in afferent pathways from proprioceptors in muscles, tendons, and joints stimulate the inspiratory neurons.

c. This effect probably helps increase ventilation during exercise.

V. Muscle spindle cause Stretch reflex or the “Loading reflex” their stretching during inspiration leads to “loading” of spindle and help in sensing “inspiratory volume”