Many different nerves control the muscles used for breathing, and each is complex in its own way. The phrenic nerve gets the most attention because it controls the diaphragm, which is arguably the most important muscle used for breathing. However, every muscle involved in the respiratory cycle is controlled by additional nerves that play essential roles.
The sternocleidomastoid and trapezius muscles are controlled by the accessory nerve, also known as cranial nerve eleven (CN XI). Although the name cranial nerve might suggest the central nervous system, the cranial nerves are twelve pairs of peripheral nerves that connect muscles and organs of the head and torso directly to the brain rather than through the spinal cord.
A group of nerves called the intercostal nerves control movement of the external intercostals, internal intercostals, innermost intercostals, the transversus thoracis, and the abdominal muscles. The external, internal, and innermost intercostal muscles are controlled by intercostal nerves three through six. The transversus thoracis muscle is controlled by intercostal nerves two through five. The abdominal muscles are controlled by intercostal nerves seven through eleven.
The scalene muscles are controlled by cervical spinal nerves three through eight. Despite the name spinal nerve, these are still peripheral nerves because they connect the spinal cord to structures outside the brain and spinal cord. The lateral pectoral nerve controls the pectoralis major muscles.
CMT can affect each of these nerves to varying degrees. The degree to which the muscles controlled by these nerves become affected can range from not at all to significantly affected. When these muscles do weaken, they weaken for the same reason that the muscles of the feet or hands do.
Not everyone who has CMT will experience breathing muscle weakness. For those who do, the weakness is not limited to the diaphragm.
