Somatosensation

Unlike vision or hearing, somatosensation has no single sensory organ. Its receptors are distributed throughout the skin, muscles, tendons, joints, and viscera, and they fall into four functional groups: mechanoreceptors for pressure, vibration, and stretch, thermoreceptors for temperature, nociceptors for pain, and proprioceptors for limb position and movement. Each converts a physical stimulus into an action potential through transduction.

The cutaneous mechanoreceptors are usually learned as a set of four. Merkel cells detect fine detail and sustained pressure, Meissner corpuscles detect light touch and low-frequency vibration, Pacinian corpuscles detect deep pressure and high-frequency vibration, and Ruffini endings detect skin stretch. Receptor density varies enormously across the body, which is why two-point thresholds are small on the fingertips and lips but large on the back — and why those regions occupy a disproportionate share of the cortical map known as the sensory homunculus.

Signals travel to the brain along two main spinal pathways. The dorsal column–medial lemniscal pathway carries fine touch, vibration, and proprioception, while the spinothalamic tract carries pain and temperature. Both synapse in the thalamus before reaching the primary somatosensory cortex in the postcentral gyrus of the parietal lobe. Pain perception is not a simple readout of nociceptor firing: gate control theory holds that nonpainful input, such as rubbing an injured area, can inhibit pain transmission in the spinal cord, and descending signals from the brain modulate the same gate.

Somatosensation is tested in the MCAT psychological and social foundations section, usually within sensation and perception alongside absolute threshold, signal detection, and the other senses. USMLE Step 1 approaches the same material anatomically, asking which receptor or tract is responsible for a given modality and what deficits follow a lesion at a specific level of the spinal cord or cortex.