Regulation of Breathing

The regulation of breathing is based in the body's acid/base balance. The Central Chemoreceptors (CCR), primarily responsible for the breathing stimulation, are affected by the PaCO2. The responsiveness of the peripheral receptors is tied to the level of pH and PaCO2. Together these provide the ultimate in servo-control - sensors provide feedback that increase or decrease breathing.

Medullary Respiratory Center

The rhythmic cycle of breathing originates in the medulla. Higher brain centers (voluntary control), systemic receptors, and reflexes modify the medulla's output. However, no truly separate inspiratory and expiratory centers have been identified.

The medulla does contain several widely dispersed groups of respiratory-related neurons that form dorsal and ventral respiratory groups.

fig 9-1.jpg

Fig. 9-1.  Schematic illustration of the respiratory components of the lower brainstem (pons and medulla oblongata).
PNC = pneumotaxic center; APC = apneustic center; DRG = dorsal respiratory group; VRG = ventral respiratory group; CC = central chemoreceptors.

Dorsal respiratory groups (DRG)

Composed mainly of inspiratory neurons located bilaterally in the medulla, the DRG controls the basic rhythm of breathing by triggering inspiratory impulses.

These neurons send impulses to the motor nerves of diaphragm and external intercostal muscles.

DRG nerves extend into the VRG, but the VRG neurons do not extend into the DRG.

Vagus and glossopharyngeal nerves bring sensory impulses to the DRG from the lungs, airways, peripheral chemoreceptors, and joint proprioceptors.

Input modifies the breathing pattern.

Ventral respiratory groups (VRG)

Contain both inspiratory and expiratory neurons located bilaterally in the medulla and primarily active in exercise and stress.

VRG sends inspiratory impulses to

Laryngeal and pharyngeal muscles - (from neurons located in the nucleus ambiguus)

Diaphragm and external intercostals - (from neurons located in the rostral area of the nucleus retroambigualis)

Other VRG neurons send expiratory signals to abdominal muscles and internal intercostals - (from neurons in the caudal area)

 

Inspiratory ramp signal: Interaction between the DRG and VRG inspiratory neurons:

Signal starts low and gradually increases to produce a smooth inspiratory effort instead of a gasp.

fig 9-2.jpg

Fig.9-2.  Neural impulses from the respiratory center travel to the diaphragm by way of the right and left phrenic nerves. The cervical, thoracic, and lumbar motor nerves stimulate the external intercostal muscles (accessory muscles of inspiration).

Pontine Respiratory Centers

The pons modifies the output of medullary centers.

Two pontine centers are the apneustic and pneumotaxic.

Apneustic center

Stimulates the inspiratory neurons of the DRG and VRG.

Its function only identified by cutting connection to medullary centers (don't know what you've got till its gone!)

Over stimulation from the apneustic center results in apneustic breathing which is characterized by long gasping inspirations interrupted by occasional expirations.

Pneumotaxic center

Sends inhibitory signals to the inspiratory center of the medulla

Controls "switch-off," so controls inspiratory time.

Increased signals increase RR, while weak signals prolong IT and increase VT.