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The diaphragm, a crucial muscle for respiration, is primarily innervated by the phrenic nerve. This nerve emerges from the cervical spinal nerves, specifically from C3, C4, and C5, and extends down to the diaphragm. Its primary function is to control the contraction and relaxation of the diaphragm, facilitating inhalation and exhalation.

The phrenic nerve plays a vital role in the respiratory process, as it sends motor commands that lead to the diaphragm contracting, which in turn increases thoracic cavity volume and allows air to be drawn into the lungs. Understanding this illustrates the importance of the phrenic nerve in breathing and highlights its unique contribution compared to other nerves listed.

In contrast, the vagus nerve primarily affects heart rate and digestive tract functions, the intercostal nerves primarily serve the intercostal muscles for rib movement, and the accessory nerve primarily innervates certain neck and shoulder muscles rather than the diaphragm. Thus, the phrenic nerve’s specific role in stimulating the diaphragm for respiration is what makes it the correct answer.