Brainstem Respiratory Centers
| Overview |
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- The Brainstem Respiratory Centers are a functionally-defined collection of anatomical loci within the brainstem that coordinate breathing through modulation of the inspiratory and expiratory muscles described in breathing biomechanics. While the brainstem centers can operate independently to maintain normal, quiet breathing, they also receive significant afferent input from a variety of sensory neurons that provide information regarding the partial pressures of arterial oxygen, arterial carbon dioxide, and blood pH. This afferent information is integrated within the brainstem centers and the breathing pattern is coordinated in such a way as to modulate alveolar ventilation so that normal values of arterial oxygen, carbon dioxide, and pH are maintained. Below we detail the principal brainstem respiratory centers and discuss their individual contributions to respiratory control.
| Locations and Regulation |
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- Overview
- The Brainstem respiratory centers are spread relatively diffusely throughout the medulla and pons; however, several distinct anatomical loci can be described that appear to display unique functional contributions to the control of respiration.
- Inspiratory Center
- The Inspiratory Center is located within the dorsum of the medulla and appears to be the physiologically most important brainstem center responsible for coordinating respiration. In the absence of any afferent input, networks of neurons within the inspiratory center intrinsically display a rhythmic pattern of discharge that is transmitted to the diaphragm via the phrenic nerve and is responsible for normal, quiet breathing. This pattern of rhythmic discharge displays a steady ramping phase over 2 seconds which abruptly ends leaving several seconds of inactivity. The ramping phase is likely responsible for the smooth, non-gasp-like nature of inspiration while the abrupt quiet phase allows relaxation of the diaphragm which provides time for passive expiration (See: Breathing Biomechanics).
- However, the Inspiratory Center is also the primary locus that receives sensory afferents from the chemoreceptors as well as lung receptors such as the pulmonary stretch receptors, irritant receptors, and J receptors. These sensory centers likely influence the inspiratory rate by modulating the ramping time described above and may promote the inspiratory center to recruit additional inspiratory muscles (See: Breathing Biomechanics). Intense afferent stimulation of the inspiratory center may also spill out into the other brainstem areas described below, allowing for additional mechanisms of respiratory control to be activated.
- Pneumotaxic Center
- The pneumotaxic center is located in the upper portion of the pons and primarily acts to inhibit the length of ramping by the inspiratory center. Pneumotaxic inhibition of inspiratory center ramping time results in shallower and more frequent breaths, thus raising the respiratory rate but reducing inspiratory volume.
- Expiratory Center
- The Expiratory Center is located in the ventrum of the medulla and appears to primarily activate the rectus abdominus and other expiratory muscles detailed in breathing biomechanics. The expiratory center is normally quiescent during quiet breathing but can be activated by intense stimulation of the inspiratory center, thus providing active expiration of the lungs when inspiration is highly stimulated.