(if med school is a Minnesota forest with millions of trees,
these are the red pines)
SOMATOSENSATION refers to the process that conveys information regarding the body surface and its interaction with the environment. It can be subdivided according to modality: thermosensation, mechanoreception and nociception. Even among these modalities, receptors are specialized to transduce specific stimuli, and have very different receptive field sizes and speeds of adaptation.
In many sensory systems, the transduction channel is a member of the transient receptor potential superfamily of ion channels. Cold and warm thermoreceptors adapt rapidly to changes in temperature, as well as specific chemical stimuli. Mechanoreceptors respond to deformation of the cell membrane. Nociceptors respond to heat, protons, and vanillinoids (esp. capsacin), and their responses can be potentiated by several compounds, including prostaglandins (blocking PG sensitization of nociceptors is a key mechanism of action of some analgesic drugs).
Somatosensory nerve fibres can be divided into several subcategories based on their conduction velocity (which is determined by their diameter and degree of myelination). This classification system is important for understanding pain and the mechanism of action of local anaesthetics.
Perception of pain is not simply due to activation of nociceptors, but is the outcome of modulation of both nociceptive and non-nociceptive inputs. According to the gate theory of pain, inhibitory interneurons regulate the transmission of ascending nociceptive information at the level of the second order neuron, allowing modulation of the signal (both increases and decreases in activity are possible). This modulation can explain phantom limb pain, as well as the success of TENS treatment and the actions of opioid analgesics.