Respiratory Neurons

There are two types: I neurons and E neurons, located in the brain stem.

• I neurons discharge during inspiration.
• E neurons discharge during expiration.

Respiratory Centers

There are two defined groups of respiratory neurons in medulla. Dorsal group made up primarily of I neurons, located in and near the nucleus of the tractus solitarius.

Ventral group made up of both E neurons, at rostral and caudal ends, and I neurons, in the middle. It is a long column that extends through the nucleus ambiguus and the nucleus retroambiguus.

Pontine and Vagal Influences

Pneumotaxic center located in pons, made up of I and E neurons, as well as neurons that are active in both phases of respiration. Respiration becomes slower and tidal volume greater when this area is damaged.

Apneustic center located in the lower pons and seems to function as an inspiratory cut off switch. Apneustic breathing results when this area is damaged.

Vagus nerve (CN10) stretching of lungs during inspiration initiates vagal afferent to inhibit inspiratory discharge. After vagotomy, depth of breathing increases and apneusis (breath holding) develops.

Respiratory Motor Neurons

Phrenic motor neurons located in the ventral horns from C3 to C5.

External intercostal motor neurons located in the ventral horns throughout the thoracic cord.

Internal intercostal motor neurons same as above.

Source: Ganong W. Review of medical physiology. 19th ed. Norwalk, Conn. : Appleton & Lange, 1999.

Respiratory Center

Respiratory Neurons

Respiratory Motor Neurons

Cycle

Corollary Discharge

• During automatic breathing, the respiratory center discharges both motor output to motor neurons and its exact copy to sensory cortex.
• While breathing voluntarily, the motor cortex discharges both motor output directly to motor neurons and its exact copy to sensory cortex.
• These copies are called respiratory motor command corollary discharges. These are functional rather than structural; specific pathways and receptors have not been identified in human.
• The corollary discharges are thought to be important in shaping the sense of respiratory effort.