Which nerve transmits signals from the brain to the heart, lungs, and digestive tract?

The vagus nerve is the correct answer because it is a key component of the autonomic nervous system and plays a crucial role in transmitting signals from the brain to various internal organs, including the heart, lungs, and digestive tract. Specifically, the vagus nerve is responsible for parasympathetic control, which helps regulate bodily functions when at rest, such as lowering heart rate, promoting digestion, and facilitating respiratory functions.

In contrast, spinal nerves primarily convey signals to and from the peripheral nervous system, focusing on motor and sensory functions rather than direct control over internal organs. The sciatic nerve is mainly involved in the transmission of sensory and motor signals to and from the lower limbs, while the optic nerve is responsible for transmitting visual information from the retina to the brain, making them unrelated to the autonomic regulation of heart, lungs, and digestive systems. Thus, the vagus nerve's specific function in controlling involuntary bodily processes makes it the correct choice.