Where are neurons primarily found in the body?

Neurons are specialized cells responsible for transmitting nerve signals throughout the body, playing a critical role in the nervous system. They are primarily located in the brain and spinal cord, which comprise the central nervous system, and also in the peripheral nervous system, which includes nerves that extend throughout the body. This widespread distribution allows neurons to communicate between the central nervous system and various body parts, facilitating motor control, sensory perception, and many other essential functions.

The other options suggest much more limited areas for neuron presence, which does not accurately reflect the actual biological distribution of these cells. Neurons are not restricted to the skin, heart, lungs, or digestive tract; instead, they can be found throughout the entire nervous system and associated with many physiological processes. Thus, the correct answer emphasizes the broad presence of neurons across the brain, spinal cord, and peripheral nervous system.