These carry the rhythmic bursts of impulses, relayed from the brain stem via the phrenic motor neurons in the spinal cord, which cause regular inspiration.
A rare variant affects the phrenic nerves and diaphragm.
Thymic tissue can span from the level of the diaphragm to the thyroid, making surgical dissection difficult (especially in light of the proximity of the phrenic and vagus nerve and other vital structures).
Origin
Early 18th century: from French phrénique, from Greek phrēn, phren- 'diaphragm, mind' (because the mind was once thought to lie in the diaphragm).