The cervical spinal nerve 4 (C4) is a spinal nerve

Spinal nerve

The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body...

 of the cervical segment..

It originates from the spinal column from above the cervical vertebra 4

Cervical vertebra 4

The cervical vertebra 4 is a vertebra of the spinal column.The cervical spinal nerve 4 passes out above it.It is also the level at which the common carotid artery bifurcates....

 (C4).

Its control of the thoracic diaphragm

Thoracic diaphragm

In the anatomy of mammals, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage. The diaphragm separates the thoracic cavity from the abdominal cavity and performs an important function in respiration...

 has inspired a medical mnemonic

Mnemonic

A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

: "Cut C4, breathe no more."

A duplicate copy in the cervical spinal nerve 4 shows a mutation originating from Indo-European mitochondrial groups, emerging in early Homo sapiens, most notably in Mongoloid mtDNA.