Respiration Reflex

Respiration is a reflex process. But it can be controlled voluntarily for a short period of about 40 seconds. However, by practice, breathing can be withheld for a long period. At the end of that period, the person is forced to breathe. Respiration is subjected to variation, even under normal physiological conditions. For example, emotion and exercise increase the rate and force of respiration. But the altered pattern of respiration is brought back to normal, within a short time by some regulatory mechanisms in the body. Normally, quiet regular breathing occurs because

of two regulatory mechanisms:

1. Nervous or neural mechanism

2. Chemical mechanism.


Nervous mechanism that regulates the respiration includes:

1. Respiratory centers

2. Afferent nerves

3. Efferent nerves


Respiratory centers are group of neurons, which control the rate, rhythm and force of respiration. These

centers are bilaterally situated in reticular formation of the brainstem. Depending upon the

situation in brainstem, the respiratory centers are classified into two groups:

A. Medullary centers consisting of

1. Dorsal respiratory group of neurons

2. Ventral respiratory group of neurons

B. Pontine centers

3. Apneustic center

4. Pneumotaxic center.


1. Dorsal Respiratory Group of Neurons


Dorsal respiratory group of neurons are diffusely situated in the nucleus of tractus solitarius which is

present in the upper part of the medulla oblongata. Usually, these neurons are collectively called

inspiratory center. All the neurons of dorsal respiratory group are inspiratory neurons and generate inspiratory ramp by the virtue of their autorhythmic property.


Dorsal group of neurons are responsible for basic rhythm of respiration (see below for details).

Experimental evidence

Electrical stimulation of these neurons in animals by using needle electrode causes contraction of inspiratory muscles and prolonged inspiration.

2. Ventral Respiratory Group of Neurons


Ventral respiratory group of neurons are present in nucleus ambiguous and nucleus retroambiguous.

These two nuclei are situated in the medulla oblongata, anterior and lateral to the nucleus of tractus solitarius. Earlier, the ventral group neurons were collectively called expiratory center.

Ventral respiratory group has both inspiratory and expiratory neurons. Inspiratory neurons are found in

the central area of the group. Expiratory neurons are in the caudal and rostral areas of the group.


Normally, ventral group neurons are inactive during quiet breathing and become active during forced breathing. During forced breathing, these neurons stimulate both inspiratory muscles and expiratory muscles.

Experimental evidence

Electrical stimulation of the inspiratory neurons in ventral group causes contraction of inspiratory muscles

and prolonged inspiration. Stimulation of expiratory neurons causes contraction of expiratory muscles and

prolonged expiration.


3. Apneustic Center


Apneustic center is situated in the reticular formation of lower pons.


Apneustic center increases depth of inspiration by acting directly on dorsal group neurons.

Experimental evidence

Stimulation of apneustic center causes apneusis. Apneusis is an abnormal pattern of respiration, charac terized by prolonged inspiration followed by short, inefficient expiration.

4. Pneumotaxic Center


Pneumotaxic center is situated in the dorsolateral part of reticular formation in upper pons. It is formed by neurons of medial parabrachial and subparabrachial nuclei. Subparabrachial nucleus is also called ventral parabrachial or K├Âlliker-Fuse nucleus.


Primary function of pneumotaxic center is to control the medullary respiratory centers, particularly the

dorsal group neurons. It acts through apneustic center. Pneumotaxic center inhibits the apneustic center so that the dorsal group neurons are inhibited. Because of this, inspiration stops and expiration starts. Thus, pneumotaxic center influences the switching between inspiration and expiration.

Pneumotaxic center increases respiratory rate by reducing the duration of inspiration.

Experimental evidence

Stimulation of pneumotaxic center does not produce any typical effect, except slight prolongation of

expiration, by inhibiting the dorsal respiratory group of neurons through apneustic center. Destruction or

inactivation of pneumotaxic center results in apneusis..


Efferent Pathway

Nerve fibers from respiratory centers leave the brainstem and descend in anterior part of lateral columns of spinal cord. These nerve fibers terminate on motor neurons in the anterior horn cells of cervical and thoracic segments of spinal cord. From motor neurons of spinal cord, two sets of nerve fibers arise:

1. Phrenic nerve fibers (C3 to C5), which supply the diaphragm

2. Intercostal nerve fibers (T1 to T11), which supply the external intercostal muscles.

Vagus nerve also contains some efferent fibers from the respiratory centers.

Afferent Pathway

Respiratory centers receive afferent impulses from:

1. Peripheral chemoreceptors and baroreceptors via branches of glossopharyngeal and vagus nerves

2. Stretch receptors of lungs via vagus nerve. By receiving afferent impulses from these receptors,

respiratory centers modulate the movements of thoracic cage and lungs through efferent nerve fibers.


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