The sense of touch is located in the skin, which is composed of three layers: the epidermis, dermis, and hypodermis. Different types of sensory receptors, varying in size, shape, number, and distribution within the skin, are responsible for relaying information about pressure, temperature, and pain.
The largest touch sensor, the Pacinian corpuscle, is located in the hypodermis, the innermost thick fatty layer of skin, which responds to vibration. Free nerve endings—neurons that originate in the spinal cord, enter and remain in the skin—transmit information about temperature and pain from their location at the bottom of the epidermis.
Hair receptors in the dermis, which are wrapped around each follicle, respond to the pressure produced when the hairs are bent. All the sensory receptors respond not to continued pressure but rather to changes in pressure, adapting quickly to each new change, so that, for example, the skin is unaware of the continual pressure produced by clothes.
Once stimulated by sensation, the receptors trigger nerve impulses which travel to the somatosensory cortex in the parietal lobe of the brain, where they are transformed into sensations. Sensitivity to touch varies greatly among different parts of the body. Areas that are highly sensitive, such as the fingers and lips, correspond to a proportionately large area of the sensory cortex.
Sensory receptors encode various types of information about objects with which the skin comes in contact. We can tell how heavy an object is by both the firing rate of individual neurons and by the number of neurons stimulated. (Both the firing rate and the number of neurons are higher with a heavier object.)
Changes in the firing rate of neurons tell us whether an object is stationary or vibrating, and the spatial organization of the neurons gives us information about its location.
The temperature of human skin is usually about 89°F (32°C). Objects or surroundings at this level— known as physiological zero—produce no sensation of temperature. Warmth is felt at higher temperatures and coldness at lower ones. Some of the sensory receptors in the skin respond specifically to changes in temperature.
These receptors are further specialized, as certain ones sense warmth and increase their firing rates in temperatures of 95 to 115°F (33 to 46°C), while others sense cold. Sensations of warmth and coldness are differentiated on a skin area as small as one square centimeter. Within that area, cold will be felt at about six points and warmth at two.
When cold and warm stimuli are touched at the same time, a sensation of extreme heat is felt, a phenomenon known as “paradoxical hotness.” Touch and temperature interact in some sensors, producing phenomena such as the fact that warm and cold objects feel heavier than those at moderate temperatures.
With free nerve endings as receptors, pain carries information to the brain about a real or potential injury to the body. Pain from the skin is transmitted through two types of nerve fibers. A-delta fibers relay sharp, pricking types of pain, while C fibers carry dull aches and burning sensations.
Pain impulses are relayed to the spinal cord, where they interact with special neurons that transmit signals to the thalamus and other areas of the brain. Each neuron responds to a number of different pain stimuli. Pain is carried by many types of neurotransmitters, a fact that has made it possible to develop numerous types of pain-relieving medications.
Many factors affect how pain is experienced. Pain thresholds vary with the individual and the occasion. Intensely concentrated activity may diminish or even eliminate the perception of pain for the duration of the activity.
Natural mechanisms, including replacement by input from other senses, can block pain sensations. The brain can also block pain by signals sent through the spinal cord, a process that involves the neurotransmitter serotonin and natural painkillers known as endorphins.