FACTORS AFFECTING RESPIRATORY CENTERS

FACTORS AFFECTING RESPIRATORY CENTERS

Respiratory centers regulate the respiratory movements by receiving impulses from various sources in the body.

1. Impulses from Higher Centers

Higher centers alter the respiration by sending impulses directly to dorsal group of neurons. Impulses from anterior cingulate gyrus, genu of corpus callosum, olfactory tubercle and posterior orbital gyrus of cerebral cortex inhibit respiration. Impulses from motor area and Sylvian area of cerebral cortex cause forced breathing.

2. Impulses from Stretch Receptors of Lungs:

Hering-Breuer Reflex

HeringBreuer reflex is a protective reflex that restricts inspiration and prevents overstretching of lung tissues. It is initiated by the stimulation of stretch receptors of air passage. Stretch receptors are the receptors which give response to stretch of the tissues. These receptors are situated on the wall of the bronchi and bronchioles. Expansion of lungs during inspiration stimulates the stretch receptors. Impulses from stretch receptors reach the dorsal group neurons via vagal afferent fibers and inhibit them. So, inspiration stops and expiration starts. Thus, the overstretching of lung tissues is prevented.

However, HeringBreuer reflex does not operate during quiet breathing. It operates, only when the tidal

volume increases beyond 1,000 mL.

Hering-Breuer inflation reflex and deflation reflex

The above mentioned reflex is called Hering-Breuer inflation reflex since it restricts the inspiration and

limits the overstretching of lung tissues. Reverse of this reflex is called Hering-Breuer deflation reflex and it takes place during expiration. During expiration, as the stretching of lungs is absent, deflation occurs.

3. Impulses from ‘J’ Receptors of Lungs

‘J’ receptors are juxtacapillary receptors which are present on the wall of the alveoli and have close contact with the pulmonary capillaries. AS Paintal discovered that these receptors are the sensory nerve endings of vagus. Nerve fibers from these receptors are nonmyelinated and belong to C type. Few receptors are found on the wall of the bronchi.

Conditions when ‘J’ receptors are stimulated

i. Pulmonary congestion

ii. Pulmonary edema

iii. Pneumonia

iv. Over inflation of lungs

v. Microembolism in pulmonary capillaries

vi. Stimulation by exogenous and endogenous chemical substances such as histamine, halo thane,

bradykinin, serotonin and phenyldiguanide.

Effect of stimulation of ‘J’ receptors

Stimulation of the ‘J’ receptors produces a reflex response, which is characterized by apnea. Apnea is followed by hyperventilation, bradycardia, hypotension and weakness of skeletal muscles.

Role of ‘J’ receptors in physiological conditions is not clear. However, these receptors are responsible

for hyperventilation in patients affected by pulmonary congestion and left heart failure.

4. Impulses from Irritant Receptors of Lungs

Besides stretch receptors, there is another type of receptors in the bronchi and bronchioles of lungs,

called irritant receptors. Irritant receptors are stimulated by irritant chemical agents such as ammonia and

sulfur dioxide. These receptors send afferent impulses to respiratory centers via vagal nerve fibers.

Stimulation of irritant receptors produces re flex hyperventilation along with bronchospasm. Hyperventilation along with bronchospasm prevents further entry of harmful agents into the alveoli.

5. Impulses from Baroreceptors

Baroreceptors or pressoreceptors are the receptors which give response to change in blood pressure.

Function

Baroreceptors in carotid sinus and arch of aorta give response to increase in blood pressure. Whenever

arterial blood pressure increases, baroreceptors are activated and send inhibitory impulses to vasomotor

center in medulla oblongata. This causes decrease in blood pressure and inhibition of respiration. However, in physiological conditions, the role of baroreceptors in regulation of respiration is insignificant.

6. Impulses from Chemoreceptors

Chemoreceptors play an important role in the chemical regulation of respiration. Details of chemoreceptors and chemical regulation of respiration are explained

later in this Chapter.

7. Impulses from Proprioceptors

Proprioceptors are the receptors which give response to change in the position of body. These receptors are situated in joints, tendons and muscles. Proprioceptors are stimulated during the muscular exercise and send impulses to brain, particularly cerebral cortex, through somatic afferent nerves. Cerebral cortex in turn causes hyperventilation by sending impulses to medullary respiratory

centers.

8. Impulses from Thermoreceptors

Thermoreceptors are cutaneous receptors, which give response to change in the environmental temperature. Thermoreceptors are of two types, namely receptors for cold and receptors for warmth. When body is exposed to cold or when cold water is applied over the body, cold receptors are stimulated and send impulses to cerebral cortex via somatic afferent nerves. Cerebral cortex in turn, stimulates the respiratory centers and causes hyperventilation.

9. Impulses from Pain Receptors

Pain receptors are those which give response to pain stimulus. Whenever pain receptors are stimulated, the impulses are sent to cerebral cortex via somatic afferent nerves. Cerebral cortex in turn, stimulates the respiratory centers and causes hyperventilation.

 

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