Three Classes of Receptors Are Associated With Lung Reflexes

Pulmonary receptors can be divided into three groups: slowly adapting receptors, rapidly adapting receptors, and C fiber endings. Afferent fibers of all three types lie predominantly in the vagus nerves, although some pass with the sympathetic nerves to the spinal cord. The role of the sympathetic afferents is uncertain and is not considered further.

Mechanisms Pulmonary Ventilation

value of the difference is taken as an input by the CNS and passed on to respiratory muscles as new minute ventilation. The loop is completed as the new ventilation alters blood gas composition through the mechanism of lung-blood gas exchange.

Negative-feedback control of arterial CO2.

Variations in CO2 production lead to changes in arterial CO2 that are sensed by chemoreceptors. The chemore-ceptor signal is subtracted from a reference value. The absolute value of the difference is taken as an input by the CNS and passed on to respiratory muscles as new minute ventilation. The loop is completed as the new ventilation alters blood gas composition through the mechanism of lung-blood gas exchange.

Juxtapulmonary Receptor

An optimization controller. The components inside the dashed box constitute the controller. In this strategy for breathing, the conflicting needs to maintain chemical homeostasis and to minimize respiratory effort are resolved by selecting an optimal ventilation. The muscle use and

CO2 tolerance couplers convert neural drive and the output of the chemoreceptors to a form interpreted by the neural optimizer as a cost to be minimized. (Modified from Poon CS. Ventilatory control in hypercapnia and exercise: Optimization hypothesis. J Appl Physiol 1987;62:2447-2459.)

An optimization controller. The components inside the dashed box constitute the controller. In this strategy for breathing, the conflicting needs to maintain chemical homeostasis and to minimize respiratory effort are resolved by selecting an optimal ventilation. The muscle use and

CO2 tolerance couplers convert neural drive and the output of the chemoreceptors to a form interpreted by the neural optimizer as a cost to be minimized. (Modified from Poon CS. Ventilatory control in hypercapnia and exercise: Optimization hypothesis. J Appl Physiol 1987;62:2447-2459.)

Slowly Adapting Receptors. The slowly adapting receptors are sensory terminals of myelinated afferent fibers that lie within the smooth muscle layer of conducting airways. Because they respond to airway stretch, they are also called pulmonary stretch receptors. Slowly adapting receptors fire in proportion to applied airway transmural pressure, and their usual role is to sense lung volume. When stimulated, an increased firing rate is sustained as long as stretch is imposed,- that is, they adapt slowly. Stimulation of these receptors causes an excitation of the inspiratory off-switch and a prolongation of expiration. Because of these two effects, inflating the lungs with a sustained pressure at the mouth terminates an inspiration in progress and prolongs the time before a subsequent inspiration occurs. This sequence is known as the Hering-Breuer reflex or lung inflation reflex.

The Hering-Breuer reflex probably plays a more important role in infants than in adults. In adults, particularly in the awake state, this reflex may be overwhelmed by more prominent central control. Because increasing lung volume stimulates slowly adapting receptors, which then excite the inspiratory off-switch, it is easy to see how they could be responsible for a feedback signal that results in cyclic breathing. However, as already mentioned, feedback from vagal afferents is not necessary for cyclic breathing to occur. Instead, feedback modifies a basic pattern established in the medulla. The effect may be to shorten inspiration when tidal volume is larger than normal. The most important role of slowly adapting receptors is probably their participation in regulating expiratory time, expiratory muscle activation, and functional residual capacity (FRC). Stimulation of slowly adapting receptors also relaxes airway smooth muscle, reduces systemic vasomotor tone, increases heart rate, and, as previously noted, influences laryngeal muscle activity.

Rapidly Adapting Receptors. The rapidly adapting receptors are sensory terminals of myelinated afferent fibers that are found in the larger conducting airways. They are frequently called irritant receptors because these nerve endings, which lie in the airway epithelium, respond to irritation of the airways by touch or by noxious substances, such as smoke and dust. Rapidly adapting receptors are stimulated by histamine, serotonin, and prostaglandins released locally in response to allergy and inflammation. They are also stimulated by lung inflation and deflation, but their firing rate rapidly declines when a volume change is sustained. Because of this rapid adaptation, bursts of activity occur that are in proportion to the change of volume and the rate at which that change occurs. Acute congestion of the pulmonary vascular bed also stimulates these receptors but, unlike the effect of inflation, their activity may be sustained when congestion is maintained.

Background activity of rapidly adapting receptors is inversely related to lung compliance, and they are thought to serve as sensors of compliance change. These receptors are probably nearly inactive in normal quiet breathing. Based on what stimulates them, their role would seem to be to sense the onset of pathological events. In spite of considerable information about what stimulates them, the effect of their stimulation remains controversial. As a general rule, stimulation causes excitatory responses such as coughing, gasping, and prolonged inspiration time.

C Fiber Endings. C fiber endings belong to unmyelinated nerves. These nerve endings are classified into two populations in the lungs. One group, pulmonary C fibers, is located adjacent to alveoli and is accessible from the pulmonary circulation. They are sometimes called juxtapulmonary capillary receptors or J receptors. A second group, bronchial C fibers, is accessible from the bronchial circulation and, consequently, is located in airways. Like rapidly adapting receptors, both groups play a protective role. They are both stimulated by lung injury, large inflation, acute pulmonary vascular congestion, and certain chemical agents.

Pulmonary C fibers are sensitive to mechanical events (e.g., edema, congestion, and pulmonary embolism), but are not as sensitive to products of inflammation, whereas the opposite is true of bronchial C fibers. Their activity excites breathing, and they probably provide a background excitation to the medulla. When stimulated, they cause rapid shallow breathing, bronchoconstriction, increased airway secretion, and cardiovascular depression (bradycardia, hypotension). Apnea (cessation of breathing) and a marked fall in systemic vascular resistance occur when they are stimulated acutely and severely. An abrupt reduction of skeletal muscle tone is an intriguing effect that follows intense stimulation of pulmonary C fibers, the homeostatic significance of which remains unexplained.

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Responses

  • bessie
    Are chemoreceptors slowly adapting receptors?
    8 years ago
  • andrea mayer
    What do c fibers j receptors do?
    8 years ago

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