The normal rhythmic cycle of breathing is involuntary- we don’t have to think about it. It continues when we are sleeping or even unconscious. However, we can voluntarily override the normal pattern and take deep breaths and breathe faster or slower if we wish. The centers for involuntary control of breathing lie in the brainstem, where two groups of neurons in the medulla oblongata and one group of neurons in the pons regulate breathing. The voluntary override of breathing is controlled by the primary motor area of the cerebral cortex.
Two bilateral groups of neurons compose the respiratory rhythmicity center in the medulla oblongata: the ventral respiratory group and the dorsal respiratory group. The ventral respiratory group (VRG) is responsible for the normal rhythmic cycle of breathing. It sends nerve impulses to the diaphragm and external intercostals causing them to contract, which results in inspiration. Inspiration continues as long as the VRG sends out the nerve impulses; but when nerve impulses cease, the muscles of inspiration relax and expiration occurs. This pattern of alternating neural activity and inactivity of the VRG produces the cyclic nature of inspiration and expiration. In quiet breathing, inspiration lasts about two seconds, and expiration lasts about three seconds.
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