A healthy person was suddenly frightened.

A. The main functions of sympathetic nerve are to dilate pupils, accelerate heartbeat, contract blood vessels of skin and viscera, dilate coronary arteries, increase blood pressure, relax bronchioles, weaken gastrointestinal peristalsis, relax bladder wall muscles, reduce saliva secretion, sweat secretion from sweat glands, and contract hairy muscles. From the stem, we know that this topic is the regulation of returning to normal after blood pressure rises (blood pressure drops). So "A" is not described correctly.

B. Autonomic nerves include sympathetic nerves and parasympathetic nerves. B description is not specific, wrong.

C, the role of parasympathetic nerve is opposite to that of sympathetic nerve. Its function has three aspects: ① promoting gastrointestinal activity, secretion of digestive glands, promoting defecation and maintaining the energy of the body; (2) narrowing the pupil to reduce irritation and promote the production of glycogen, thus saving energy; ③ Slow down the heartbeat and lower blood pressure. From the stem, we know that this topic is the regulation of returning to normal after blood pressure rises (blood pressure drops), so N should be parasympathetic nerve.

D, the sensory nerve is the afferent nerve, which corresponds to M in the picture, so D is wrongly described.

So, C.