In mammals and particularly in humans, touch is the oldest, most primitive, and pervasive sense. In the uterus as early as eight weeks of gestation, touch is the first of the five sensory systems to develop and respond to stimulation. Additionally, it is the first sense humans experience as infants. Touch helps babies to grow and bond with mothers, fathers, and other caregivers, while helping animals and humans to learn about the world around them. Touch also plays an integral role in biological, cognitive, and social development. It is the physical contact of the somatosensory system to the outside world. It allows people to learn shapes and hardness of objects.
There are several million points on the human body that register cold, heat, pain, or touch. These points that register the four basic cutaneous senses are mapped within the central nervous system to the somatosensory cortex, or the postcentral gyrus. Thus, touching an object can give the feeling of warmth, cold, pain, and pressure, and that information is sent to the brain for processing. There are many kinds of touch organs, called tactile corpuscles, in the skin and mucous membranes. These touch organs are found everywhere on and within the skin: near hair, in hairless areas (like the palm and fingertips), and in deeper tissues.
Anatomy and Physiology
The somatosensory system mediates many sensations received by the skin and body. Specifically for touch, it includes crude touch—with itch and tickle—and discriminative touch. Together these modalities make up haptic perception. The term haptic is derived from Greek, meaning “to touch.” Thus haptic perception integrates somatosensory information in recognizing objects. Texture, hardness, and temperature are material properties that are mediated through touch. It is also important to note that touch and proprioception are integrated sensations as they transmit their signals to the brain via the same pathway within the spinal cord and brainstem.
Touch is a peripheral nervous system function that transmits its information to the central nervous system. This means that the cell bodies of the neurons live in the dorsal root ganglion and the axon divides into a peripheral axon and a central axon. The peripheral axon ends in the joint, muscle, skin, or tendon while the central axon ends in the spinal cord of the central nervous system. The skin is the main touch receptor organ while joint, muscle, and tendon tissues are used in proprioception. The sensations of touch are represented by neurons that exhibit modality specificity. Modality specificity occurs when a somatosensory neuron is stimulated, which results in a perceived sensation that is specific to the information processed by the neuron.
Crude versus Discriminative Touch
The skin identifies two types of touch: crude (least sensitive) and discriminative (most sensitive) touch. The form of touch where localization is not possible is called crude touch. Crude touch or nondiscriminative touch is a sensory modality that allows the body to sense that something has touched it, without being able to localize exactly where the body was touched. For example, if a person were touched five inches below the left shoulder, the person would say he or she was touched on the back on the left side, but would not be able to give the exact location. Fine touch, then, is able to localize where the body was touched. Fine and crude touch do work in parallel, meaning a person will be able to localize touch until fibers carrying fine touch have been disrupted. When that happens, the body will feel the sensation, but will be unable to identify the exact location where it was touched.
The sensation of touch begins when an object comes in contact with the sense organ and presses it out of shape, or touches a nearby hair. The sense of touch is more sensitive in some parts of the body compared to other parts. The lips, tongue, fingers, feet, and genitals are the most sensitive. The least sensitive is the back. The reason for the difference is due to the fact that the end organs for touch are not scattered evenly over the body, but instead are arranged in clusters. This keenness of touch can easily be measured by an esthesiometer. This instrument looks like a drawing compass with two needlepoints. The tip of the tongue can feel both points when they are 1 millimeter apart. Less sensitive areas feel only one point at this distance. The back of the shoulders feels two points when the points are more than 60 millimeters apart. These differences show that certain body regions respond only to crude or fine touch. The nervous tissues from the sense organs then carry the sensation or nerve impulses to the brain.
Patricia A. Bloomquist
See also: Discriminative Touch; Homunculus; Mechanoreceptors; Sensory Receptors; Somatosensory Cortex; Somatosensory System
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