The Five Senses: Touch

The Five Senses

Touch: The physiological sense by which external objects or forces are perceived through contact with the body (Www.Thefreedictionary.Com, p. touch).

Touch is a perception resulting from activation of neural receptors, generally in the skin including hair follicles, but also in the tongue, throat, and mucosa (Http://en.Wikipedia.Org/Wiki/, p. Somatosensory_system).

Our sense of touch is controlled by a huge network of nerve endings and touch receptors in the skin known as the somatosensory system. It is a diverse sensory system composed of the receptors and processing centers to produce the sensory modalities such as touch, temperature, proprioception (body position), and nociception (pain). The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system. While touch is considered one of the five traditional senses, the impression of touch is formed from several modalities.

Anatomy of Touch

The somatosensory system is responsible for all the sensations we feel - cold, hot, smooth, rough, pressure, tickle, itch, pain, vibrations, and more. Within the somatosensory system, there are four main types of receptors: mechanoreceptors, thermoreceptors, pain receptors, and proprioceptors (Http://Www.Hometrainingtools.Com/Skin-Touch/a/1388/).

Mechanoreceptors perceive sensations such as pressure, vibrations, and texture. There are four known types of mechanoreceptors whose only function is to perceive indentions and vibrations of the skin: Merkel's disks, Meissner's corpuscles, Ruffini's corpuscles, and Pacinian corpuscles.

The most sensitive mechanoreceptors, Merkel's disks and Meissner's corpuscles, are found in the very top layers of the dermis and epidermis and are generally found in non-hairy skin such as the palms, lips, tongue, soles of feet, fingertips, eyelids, and the face. Merkel's disks are slowly adapting receptors and Meissner's corpuscles are rapidly adapting receptors so the skin can perceive both when we are touching something and how long the object is touching the skin. Our brain gets an enormous amount of information about the texture of objects through our fingertips because the ridges that make up our fingerprints are full of these sensitive mechanoreceptors.

Located deeper in the dermis and along joints, tendons, and muscles are Ruffini's corpuscles and Pacinian corpuscles. These mechanoreceptors can feel sensations such as vibrations traveling down bones and tendons, rotational movement of limbs, and the stretching of skin. This greatly aids our ability to do physical activities such as walking and playing ball.

Thermoreceptors perceive sensations related to the temperature of objects the skin feels. They are found in the dermis layer of the skin. There are two basic categories of thermoreceptors: hot and cold receptors.

Cold receptors start to perceive cold sensations when the surface of the skin drops below 95º F. They are most stimulated when the surface of the skin is at 77º F and are no longer stimulated when the surface of the skin drops below 41º F. This is why feet or hands start to go numb when they are submerged in icy water for a long period of time.

Hot receptors start to perceive hot sensations when the surface of the skin rises above 86º F and are most stimulated at 113º F. But beyond 113º F, pain receptors take over to avoid damage being done to the skin and underlying tissues.

Thermoreceptors are found all over the body, but cold receptors are found in greater density than heat receptors. The highest concentration of thermoreceptors can be found in the face and ears (hence why nose and ears always get colder faster than the rest of the body on a chilly winter day).

Pain receptors (nociceptors). "Noci-" in Latin means "injurious" or "hurt": these receptors detect pain or stimuli that may cause damage to the skin and other tissues of the body. There are over three million pain receptors throughout the body, found in skin, muscles, bones, blood vessels, and some organs. They can detect pain that is caused by mechanical stimuli (cut or scrape), thermal stimuli (burn), or chemical stimuli (poison from an insect sting).

These receptors cause a feeling of sharp pain to encourage us to quickly move away from a harmful stimulus such as a broken piece of glass or a hot stove stop. They also have receptors that cause a dull pain in an area that has been injured to encourage us not to use or touch that limb or body part until the damaged area has healed. They play an important role in keeping the body safe from serious injury or damage by sending these early warning signals to the brain.

Proprioceptors: In Latin, the word "proprius" means "one's own": these receptors sense the position of the different parts of the body in relation to each other and the surrounding environment. Proprioceptors are found in tendons, muscles, and joint capsules. Their location in the body allows these special cells to detect changes in muscle length and muscle tension. Without proprioceptors, we would not be able to do fundamental things such as feeding or clothing ourselves.

Some areas of the body are more sensitive than others because they have more nerve endings. For example, the sides of the tongue have a lot of nerve endings that are very sensitive to pain. However, the tongue is not as good at sensing hot or cold. The fingertips are also very sensitive. For example, people who are blind use their fingertips to read Braille by feeling the patterns of raised dots on their paper.

Touch is also vital for our health from a different perspective. As humans we need touch. Babies who do not receive enough touch may not live, or may develop psychological problems. Babies with more touch will grow to be healthier both physically and psychologically ("Touching your children is a wonderful way to talk").

Physiology of Touch

Transmission of information from the receptors passes via sensory nerves through tracts in the spinal cord and into the brain. Processing primarily occurs in the primary somatosensory area in the parietal lobe of the cerebral cortex.

When the touch, pain or heat sensors in the skin are stimulated, they send electrical pulses to the neurons, special cells that relay electrochemical impulses (A.D.A.M.). The sensory neurons then act as a relay team, passing along the electrical pulse from neuron to neuron until it reaches the spinal cord. The spinal cord takes the incoming signal and sends it to the brain. Once the brain receives the signal