A patient with a spinal injury may still be able to use his or her diaphragm to breathe, but would lose control of the intercostal muscles if the spinal cord is injured:

• A. above the C3 level.
• B. between C1 and C2.
• C. below the C5 level.
• D. above the C5 level.

Answer: (C)

The muscles a person uses to breathe depend on where the spinal injury occurs. The diaphragm, which is the main muscle of breathing, is controlled by the phrenic nerve coming from C3 to C5. If the injury is below that level, the phrenic nerves can still function, so the diaphragm can continue to drive respiration. The intercostal muscles, on the other hand, are controlled by the thoracic nerves T1 through T11. An injury that disrupts the descending pathways to those thoracic segments (while leaving the cervical phrenic input intact) will paralyze the intercostals but spare diaphragmatic breathing.

So an injury that is below C5 can leave the diaphragm working but abolish intercostal muscle control, leading to breathing that relies on diaphragmatic (abdominal) effort with reduced chest wall expansion. If the injury were higher, such as above C3-C5, the diaphragm could be compromised as well, and if it were lower, the intercostals might still be affected depending on the exact level.