23.13: Touch: the brain perceives pressure, temperature, and pain.

The last of the traditional five senses in humans is touch. Touch is actually a class of sensations generated by numerous different types of sensory neurons that are sensitive to pressure (mechanoreceptors), temperature (thermoreceptors), or pain (pain receptors). These sensory receptors are located throughout the body, and are found in particularly dense concentrations in places such as the fingertips in primates and the side of the body (called the lateral line) in most types of fish.

Whether they are mechanoreceptors or pain receptors or thermoreceptors—and any given part of the body usually has many of each of these sensory neuron types—they function similarly. External stimulation, such as the prick of a pin, the tickle from a feather, or the heat of a pan, can cause a change in the shape of the neuron’s membrane, momentarily altering its permeability. This change in permeability then increases or decreases the rate at which ions enter the cell, converting the stimulus into action potentials and, ultimately, sensations perceived by the brain (FIGURE 23-29).

Pain-killing drugs such as morphine reduce the ability of various pain receptors to send their signals to the brain. They do this by blocking the receptors’ ability to respond to stimuli, thereby blocking neurons in the brain from sensing stimuli that would normally be identified as pain. It’s as if the phone line between that part of the body and the brain is cut.

Conversely, and eerily, most amputees report that they sometimes experience tingling, prickly sensations or even shooting pains that come from where their amputated limb once was. Called “phantom pain,” these sensations are very real and occur when neurons in the brain that used to receive information from touch receptors in the lost limb are firing, usually due to stimulation from other, nearby neurons (FIGURE 23-30).