Final answer:
The thumb's dermatome is innervated by the C6 spinal cord segment, tested through sensory exams for touch perception, with sensory and motor components arising from the dorsal root ganglia and the anterior horn of the spinal cord, respectively.
Step-by-step explanation:
The dermatome of the thumb is innervated by nervous elements that arise from the C6 spinal cord segment. Specifically, these sensory fibers are axons of sensory neurons located in the dorsal root ganglia and enter the spinal cord through the dorsal nerve root, while motor neurons located in the anterior horn of the spinal cord emerge through the ventral nerve root to send action potentials that cause skeletal muscles to contract. The sensory exam for dermatomes involves testing regions that connect to the postcentral gyrus where somatosensation is consciously perceived; the eighth cervical nerve is tested for sensation on the medial forearm and fingers. The radial nerve, for example, contains fibers that provide cutaneous sensation in the arm and motor fibers for muscle movement.