What is the effect of the accessory nerve on the body?

The accessory nerve, also known as cranial nerve XI, primarily affects the muscles of the neck and back, which is why option B is the correct choice. This nerve is responsible for innervating the sternocleidomastoid and trapezius muscles, which are crucial for movements such as turning the head and shrugging the shoulders. The proper functioning of these muscles is essential for maintaining posture and assisting in various upper body movements.

In contrast, the other options are associated with different cranial nerves or systems. For example, controlling facial expressions is primarily the function of the facial nerve (cranial nerve VII). Regulating heartbeats involves the autonomic nervous system, particularly the vagus nerve, and is not related to the accessory nerve. Similarly, while the diaphragm is mainly responsible for breathing, it is innervated by the phrenic nerve rather than the accessory nerve. Understanding these distinctions can clarify the specific roles of different cranial nerves and how they contribute to bodily functions.