Anatomy of the Sense of Touch

The skin is an organ which responds to touch and sends messages to the brain which can decipher the kind of touch experienced. Through haptic perception or touch, skin can detect between temperature, hot and cold, pressure, rough, smooth, itch and tickle, pain and vibrations among others. Touch is detected via receptors and nerve endings. 

Hand touching wheat iStock

Blind people are able to experience touch remotely through a cane or stick which helps them understand hazards. Some parts of the body are more sensitive to touch than others, including the hands and feet, lips and tongue and genitals. The organs and joints can also register pain to the brain. Scientists are still learning about touch, how signals are sent to the brain and how they are processed once there.

There are three layers to the skin: the epidermis is the top layer, which is protective and is able to regenerate fast. It contains pigment cells, keratin, it can make vitamin D when exposed to sunlight and also produces sweat which helps to regulate body temperature and dispose of unwanted chemicals in the body. The dermis lies beneath the epidermis and contains most of the skin receptors, capillaries, hair follicles and nerve endings. The last layer of the skin is the subcutaneous tissue, which helps with insulation and storing energy. This contains fat, larger blood vessels and connective tissue, which helps to attach the skin to the tendons and muscles below.

The network of receptors and nerves which help to form the sense of touch is known as the somatosensory system. A response is triggered to a stimulus of a receptor, carrying an electrical discharge to the nerve endings, the impulse travels up the spinal cord to the brain and sensation is registered at the somatosensory cortex part of the the brain, in the parietal lobe. The size of the stimulus relates to the amount of receptors triggered. Previous experiences can play a part in how people respond to touch.

Mechanoreceptors respond to forces, including push and pull, indicating  problems with the joints or muscles surrounding the joints. These receptors include the Pacinian corpuscles, sensitive to deep-pressure touch and high-frequency vibrations, while the Meissner’s corpuscles detect light touch and are found in places of high sensitivity, including the fingertips, body orifices, lips and nipples. Merkel’s discs are found in the fingertip ridges and detect pressure and texture.

Thermoreceptors are sensitive to temperature changes, Ruffini’s endings helps to detect heat and the end bulb of Krause helps to detect cold. They respond to temperatures in the range of 5-40ºC, reacting most frequently to 25ºC. Warm thermoreceptors react to the temperature range of 29-45ºC, being most active at 45ºC.

Chemoreceptors respond to substances produced by the skin itself. Nociceptors help to detect painful stimuli and can react to cuts or scrapes, burns or chemical stimuli such as an insect sting, responding to threatened or actual damage to the tissue cells. Damage to tissues releases chemicals which can make the nerve endings more sensitive or activate them.. They can also cause dull pain in an injured part of the body to ensure that it is rested and left alone. Proprioceptors sense the position of parts of the body in relation to one another and in the environment around them. These are sensitive to muscle length or tension changes, enabling people to get dressed or eat.

The brain is able to process sensations from many receptors at a time. The sense travels to the brain through the nerves. Neurons, the smallest parts of the nervous system are able to receive and transmit signals with other neurons to enable them to be sent to the brain. They also enable the brain to communicate with the rest of the body. If a part of the body should touch an object that is too hot, the receptors are activated, sending a signal to the nearest neuron, which is then passed from neuron to neuron. When it reaches the brain, it processes what has been touched and can send messages back about  whether it should stop touching it. Scientists are just beginning to realises the complexity and importance of touch and how essential it is to human well-being.

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Skin Section Model x70 Enlargement.

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