1. Ventilatory mechanics
The filling of the lungs is related to the movements of the rib cage.
A ventilatory cycle is characterized by an inspiration, during which the rib cage rises and increases in volume, followed by an expiration bringing the rib cage back to its initial position.
The movements of the rib cage are linked to a set of muscles including the diaphragm and the intercostal muscles.
It is the contraction of these muscles which allows the lifting of the rib cage, and, consequently, the entry of air; inspiration is an active phenomenon.
The relaxation of these same muscles causes the return to the initial position, and, consequently, the exit of air; exhalation is a passive phenomenon.
2. Control of muscles by the nervous system
The activity of the respiratory muscles is controlled by the nervous system. All these muscles are in fact connected to the medulla oblongata via intercostal motor nerves for the muscles:
– Intercostal nerves;
– phrenic nerves for the diaphragm.
The parallel recording of the activity of these nerves, called an electroneurogram, and the respiratory activity, called a spirogram, makes it possible to follow the command of the respiratory activity.
At rest, the recorded respiratory activity, characterized by a tidal volume and a respiratory rate, follows a spontaneous nervous activity, consisting of a set of signals emitted at regular time intervals.
During exercise, the respiratory nerves are traversed by more numerous and more frequent trains of signals. This increase in nerve activity results in an increase in tidal volume and respiratory rate.
3. Control by the bulbar centers and the cerebral cortex
As with cardiac activity, the respiratory nerves are controlled by nerve centers located in the medulla oblongata.
These centers possess rhythmic spontaneous activity, responsible for automatic ventilation. An increase or decrease in their activity also makes it possible to increase or decrease the contractions of the respiratory muscles and thus control pulmonary ventilation.
However, it is possible to voluntarily modify its respiratory activity. In this case, the voluntary command from the cerebral cortex is preponderant and temporarily takes over the involuntary command.
The essential
Bulbar centers control lung ventilation through the respiratory nerves.
Sending messages to the muscles causes them to contract and inspiration. Between two emissions, the muscles relax which allows expiration.
