D

A. In the skin, the cut activates primary afferents.
1. The neurons that relay pain are called nociceptors.
2. Large A-beta fibers and smaller A-delta fibers are stimulated and rapidly transmit neural impulses toward the spinal cord.
3. Although more difficult to stimulate, the cut would activate small C-fibers, and these neural impulses would also travel to the spinal cord.
B. Sensory afferents form nerves that travel toward the spinal cord, either in sensory nerves or mixed nerves.
C. Sensory input enters the dorsal horns of the spinal cord.
1. Many primary afferents synapse with secondary afferents in the dorsal horns, but other primary afferents continue to the brain.
2. The arrangement and interconnections of laminae of the dorsal horns receive sensory input, modulate it, and relay these messages toward the brain,
D. Sensory input travels to the brain by way of the spinal cord and crosses from the left side of the body to the right side of the brain.
E. Sensory neurons synapse in the thalamus, and then go to the right somatosensory cortex in the parietal lobe of the forebrain.
1. The primary somatosensory cortex maps those receptors in the hand, providing a representation of the hand in the brain.
2. The somatosensory cortex is involved with interpretation of the sensory input, most likely as pain.

C