Physiology of respiration, respiratory centres, effects of transection of brainstem.

Human respiration is controlled by two mechanisms.

  1. Voluntary control
  2. Automatic control

Voluntary control of respiration is by Cerebral cortex.Voluntary control is needed for respiration at one’s will as in whistling,singing etc.

Automatic control of respiration is via the Pontomedullary centers which enable the smooth and unconscious execution of respiratory activities.

Medullary centres of respiration.

  1. Dorsal respiratory group(DRG) -Located in dorsal portion of medulla,and causes spontaneous inspiration.
  2. Ventral respiratory group(VRG) – Located in ventrolateral part of medulla, causes forced expiration.

Pre -Botzinger Complex(pre-BOTC)

Rhythmic respiration is initiated by a small group of pacemaker cells in the pre Botzinger complex on either side of medulla between nucleus ambigus and lateral reticular nucleus.These neurons discharge rhythmically and produce impulse in phrenic nerve to cause respiration.

Medullary centres receive impulses from:

Carotid and aortic bodies(chemoreceptors) via Glossopharyngeal and vagus respectively.Hypoxia,hypercapnia and acidosis stimulate respiration through them.

Afferent impulses from pulmonary stretch receptors,carried by the vagus nerve.When lungs become overstretched, these receptors transmit impulses via vagi to the DRG, and they switch off inspiration.This is also called Hering-Breuer inflation reflex. This reflex also increase the rate of respiration.

Pontine centres of respiration

Pneumotaxic centre – Located dorsally in the nucleus parabrachialis in upper Pons,transmits signals to the inspiratory area. Pneumotxic centre reduces the duration of inspiration,ultimately leading to increased respiratory rate.

Apneustic centre: Excites the DRG and stimulates inspiration. Pnemotaxic centre and vagi send inhibitory impulses to apneustic centre and thus shortens inspiration.

Effects of transection at different brainstem levels:

1.A transection above pons has no effect on respration.

2.Midpontine transection with the vagi cut,inhibits inhibitory influences of both pneumotaxic centre and vagi on the apneustic centre and results in apneusis(Sustained gasping inspiration followed by short,inefficient expiration).

3.In transection at pontomedullary junction, respiration is maintained with slight irregularity.

4.Transection below medulla causes stoppage of respiration,completely.

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