VPB 121: Veterinary Physiology - II (2+1)

HERING - BREUER REFLEX

• The receptors for these reflexes are located in the lungs, particularly in the bronchi and bronchioles.
• The nerve impulses generated by these receptors are transmitted by fibers in the vagus nerves to the respiratory center.
• The effect of inflation-receptor stimulation is to inhibit further inspiration (stimulation of neurons in the dorsal respiratory group) and to stimulate expiratory neurons in the ventral respiratory group. However the tidal volume can be increased by a person with the modulation of pneumotaxic centre.
• There are other peripherally located receptors that modify the basic rhythm. Stimulation of receptors in the skin are excitatory to the respiratory center, and deeper than usual inspiration can be noted.
• Their excitation to the inspiratory area is through the apneustic area because inspiratory gasps are noted. Advantage is taken of these receptors when breathing stimulation is desired in newborn animals.

• Rubbing the skin with a rough cloth often initiates the breathing cycles. Several respiratory reflexes originate from receptors in the upper air passages.
• Stimulation of the mucous membranes in these regions causes reflex inhibition of breathing. Eg: inhibition of breathing that occurs during swallowing; also, in diving birds and mammals, there is a reflex inhibition of breathing when they submerge.

• Stimulation of the laryngeal mucous membrane in the unanaesthetized animal causes not only inhibition of breathing, but also usually powerful expiratory efforts (coughing and sneezing).
• Ordinary respirations proceed involuntarily. They can be altered voluntarily hastened, slowed, or stopped altogether, for a while. Phonation and abdominal pressure in the expulsive acts of defecation, urination, and parturition are examples of complete voluntary control of the respiratory movements.
• Afferent impulses from pressure receptors in the carotid and aortic sinuses have as their principal function a role in the regulation of circulation, but impulses from these receptors also go to the respiratory center.
• The impulses are inhibitory in nature-the higher the blood pressure, the greater the inhibition to respiration. Because of the influence of inspiration on return of blood to the heart, it can be seen that the reduction in inspirations would slow down the return flow of blood to the heart and thus help to lower blood pressure.