Answer:
The answer is:
it isn't effected by most spinal cord injuries because it doesn't have the same source of innervation (supply)
Explanation:
Paralysis of the voluntary muscles below the neck due to this type of trauma would be a result of a severed spinal cord.
The heart gets innervation from the Vagus nerve (cranial nerve 10)
which doesn't come out of the spinal cord but rather the brain stem, which
is still protected in the skull where it branches off.
The diaphragm on the other hand receives innervation from
the Phrenic nerve, this originates at the cervical (the vertebra in your neck)