The surface of the skin can be mapped into distinct regions, each served by a single spinal nerve: these regions are called:
a. receptors
b. interneurons
c. synaptic clefts
d. dermatomes

The correct answer is D. The surface of the skin can be mapped into distinct regions, each served by a single spinal nerve: these regions are called dermatomes.

Explanation:

A dermatome is the area of skin innervated by a single spinal nerve and its spinal ganglion. The cutaneous nerves are those that reach the skin, picking up the sensitivity of the skin. Each cutaneous nerve is distributed in a certain area of skin, called a dermatome.

A pair of posterior or sensory roots and a pair of anterior or motor roots arise from each cord segment, joining laterally at the level of the intervertebral foramen to form a mixed spinal nerve. Each of these innervates a strip of skin called a dermatome, so the body surface can be considered a true mosaic of these.

In the extremities the arrangement of dermatomes is more complicated because of the embryological rotation of the limbs as they grow from the trunk.