Understanding the Respiratory System: Mechanisms and Control
Dorsal Respiratory Group Neurons
The dorsal respiratory group controls inspiration and breathing rate. It is located in the dorsal portion of the medulla and extends along it. It is mainly stimulated by inspiration. Most of its neurons are located in the nucleus of the solitary tract, where the vagus and glossopharyngeal nerves terminate. These nerves transmit signals to the nucleus from:
- Peripheral chemoreceptors
- Baroreceptors
- Several types of lung receptors
They are responsible for generating the respiratory rhythm at rest.
Ventral Respiratory Group of Neurons
The ventral respiratory group of neurons is in the ventrolateral medulla, about 5mm ahead and outside of the dorsal respiratory group. It functions in both inspiration and expiration, depending on which neurons are stimulated. Its neurons remain inactive during normal, quiet breathing. When the respiratory drive increases pulmonary ventilation, the ventral respiratory group functions as a mechanism for spreading hyperstimulation respiratory signals received from the dorsal respiratory group. This occurs when high levels of pulmonary ventilation are required, especially in forced expiration (during exercise). In this case, powerful signals are sent to the expiratory abdominal muscles.
Pneumotaxic Center
The pneumotaxic center is located dorsally in the nucleus parabranchialis of the upper pons. It helps control the rate and breathing pattern, influencing the duration of inspiration and increasing the respiratory rate. Its main effect is to control the extent of inactivation of the inspiratory ramp, regulating the filling phase of the pulmonary cycle. When the pneumotaxic signal is strong, inspiration could last only 0.5 seconds, and the filling is small. But when pneumotaxic signals are faint, inspirations can last 5 seconds or more, filling the lungs with a large excess of air.
The fundamental function of the pneumotaxic center is to limit inspiration. This has the side effect of increasing the respiratory rate because the limitation also shortens expiration and the entire respiratory period. A strong pneumotaxic signal may increase the respiratory rate to 30 to 40 breaths per minute, while a weak signal could reduce it to only 3 to 5 breaths per minute.
Inspiratory Ramp Signal
The nerve signal transmitted to the inspiratory muscles, mainly the diaphragm, is not an instantaneous discharge of action potentials. In contrast, in normal breathing, inspiration begins weakly and grows as a ramp over about 2 seconds. It suddenly stops for about 3 seconds, which interrupts the stimulation of the diaphragm and allows the elastic recoil of the chest wall and lungs, resulting in expiration. Then the inspiratory signal begins another cycle again, and so on, with expirations in between. Therefore, the inspiratory signal is a ramp signal.
Chemical Control of Breathing
The ultimate purpose of respiration is to maintain adequate concentrations of oxygen and carbon dioxide in tissues. Excess carbon dioxide in the blood primarily stimulates the respiratory center itself, greatly increasing the signal strength of the inspiratory and expiratory respiratory muscles. Oxygen has no significant direct effect on the respiratory center of the brain to control breathing. On the contrary, it acts almost exclusively on peripheral chemoreceptors located in the carotid and aortic bodies, and these, in turn, transmit appropriate nerve signals to the respiratory center for the control of breathing.
Respirations may be modified by many factors and external brain factors. Among the factors influencing respiration are:
- Cortical influences
- Inflation reflex
- Chemical stimuli such as oxygen and carbon dioxide
- Blood pressure
- Temperature
- Pain
- Irritation to the respiratory mucosa
The volume and frequency of breathing are determined by impulses from the respiratory center. These impulses are controlled by information received from different receptors in the body: the central receptors located near the respiratory center and peripheral receptors located in the carotid arteries.
Breathing Movements
After air leaves the lungs, it activates the inspiratory area, and nerve impulses are conducted through certain nerves to the intercostal muscles and diaphragm, causing their contraction and the start of inspiration. Stretch-sensitive receptors are located on the walls of the bronchi and bronchioles. These receptors send nerve impulses to the inspiratory area, causing its inhibition. This determines the relaxation of the intercostal muscles and diaphragm, causing expiration through this process. When air leaves the lungs, the stretch receptors are no longer stimulated; therefore, the inspiratory inhibition area ceases, and the phases of pulmonary ventilation can start again.
Respiratory Rate and Depth of Pulmonary Ventilation
The respiratory rate is the number of times a person breathes per minute. It can be measured when the person is at complete rest (count the times the chest rises). To take a person’s respiratory rate, one must:
- Be at rest, emotionally calm, and relaxed
- Not have eaten food for at least 15 minutes before
- Be in a comfortable position
- Place the right hand on the patient’s wrist (pretending to be controlling the pulse)
- Observe chest movement while having a one-minute timer