Final answer:
Damage to the cervical nerves can greatly impact the process of ventilation by affecting the diaphragm's motor control, resulting from impairment of the phrenic nerve connected at cervical levels 3 to 5. This can affect both normal and forced breathing.
Step-by-step explanation:
The respiratory action most affected by damage to the cervical nerves would be the process of ventilation, specifically the motor control of the diaphragm. The phrenic nerve, which is responsible for stimulating the diaphragm for breathing, is connected to the spinal cord at cervical levels 3 to 5. Since these motor fibers are crucial for the muscle contractions that drive ventilation, damage to the cervical nerves at these levels could impair the ability to breathe normally. Furthermore, damage to the cervical spinal cord below the mid-cervical level may not make ventilation impossible but could affect the visceral fibers from the diaphragm that enter the spinal cord at the same level as the somatosensory fibers from the neck and shoulder.
Damage to the cervical nerves could also have ramifications on the ability to perform forced breathing, which engages accessory muscles that are also innervated by nerves arising from the cervical and upper thoracic spinal cord. The medulla oblongata's dorsal and ventral respiratory groups regulate normal and forced breathing, respectively, and any impairment in the associated nerves could hinder these respiratory processes.