The information from these receptors in the lung is carried to the respiratory centre along the vagus nerve. The efferent nerves from the respiratory centre pass down the spinal cord to the diaphragm, intercostal muscles and accessory muscles of inspiration in the neck. The intercostal muscles are supplied by the segmental intercostal nerves that leave the spinal cord between TI and TI2. The accessory muscles in the neck are supplied from the cervical plexus. During normal breathing, inspiration is an active muscular process.
Expiration is passive and relies on the natural elasticity of the tissues to deflate the lung.
The most important muscle for inspiration is the diaphragm. Trauma to the cervical cord, above C3, is normally fatal for this reason. Anaesthetic drugs and respiration Opioid drugs, such as morphine or fentanyl, depress the respiratory centre's response to hypercarbia. These effects can be reversed by naloxone. Volatile anaesthetic agents depress the respiratory centre in a similar fashion, although ether has less effect on respiration than the other agents.