The human skin, along with the hair and nails is a part of the integumentary system. It acts as a protective barrier to all our internal organs, holding everything in place. It's primary functions are reacting to sensations of touch, vibrations, and pressure, regulating body temperature, and responding to sensations of heat and cold. The sense of touch encompasses senses related to pain, pressure, and temperature. The human skin contains millions and millions of receptors, and we sense and respond to the surrounding environment because of them. They send signals to our brain and spinal cord by reacting to sense of touch.The somatosensory system is a complex system which includes receptors, and the somatosensory cortex, which includes the postcentral gyrus, processes all the touch information that is received by the body. The system reacts to all changes and internal and external stimuli with the help of different types of receptors, such as mechanoreceptors, thermoreceptors, nociceptors, and chemoreceptors. Except for chemoreceptors, the others act as receptors for the sense of touch.
About Mechanoreceptors
Based on the way they adapt to stimuli, mechanoreceptors are classified as slowly adapting, moderately adapting, and rapidly adapting. On receiving a stimuli, mechanoreceptors react to it by transmitting an impulse at a certain frequency. The rate of adaptation implies how slowly or quickly receptors will adapt to that stimulus. Phasic, or rapidly adapting receptors adapt quickly and their pulse rate quickly subsides to a normal rate. Tonic, or slow adapting receptors, adapt at a much slower rate and their pulses sink back to the normal rate much later. Rapidly acting receptors are found in skin which is extremely sensitive to touch and vibration, whereas slowly adapting receptors are found in skin where the reaction to stimuli takes a very long time, producing pulses as long as the stimulus is sustained.
Types of Mechanoreceptors that Respond to Touch
The different types of mechanoreceptors responsible for the sense of touch (cutaneous mechanoreceptors) are described below.
Ruffini's End Organs
Present in the glabrous (hairless) skin of humans, these are also sometimes referred to as Bulbous or Ruffini corpuscles. These are spindle-shaped, and perceive the sense of touch, pressure, and stretch deep inside the skin. Located in the deeper layers of the skin, these rapidly adapting corpuscles monitor movement of objects along the skin, which helps us to grip and hold an object.
Meissner's Corpuscles
These mechanoreceptors are also known as tactile corpuscles. Distributed all over the skin, these are found in heavy concentration in areas such as lips, nipples, palms and soles. Located in the superficial layers of glabrous skin, these rapidly acting corpuscles are highly responsive to light touch, and sense vibrations lower than 50 Hz.
Pacinian Corpuscles
These nerve endings respond to senses of vibration and deep pressure touch on our skin. These corpuscles are enclosed in a layer of connective tissue and are approximately 1 mm in length. Extremely sensitive to even the lightest of vibrations, they can sense vibrations even a couple of inches away. These corpuscles are located in glabrous skin.
Merkel's Disks
These slowly adapting mechanoreceptors provide pressure and texture information to the brain. Merkel's disks occur in the superficial layers of the skin, and are present in both glabrous and hairy skin. They can sense vibrations in the range of 5-15 Hz.
Free Nerve Endings
There are various types of free nerve endings, those which detect changes in temperature (thermoreceptors), pressure and touch (mechanoreceptors), and pain (nociceptors). Thermoreceptors sense changes in environmental temperature which are not likely to cause any physical harm. Internal body temperature is regulated by thermoreceptors. Changes in temperature which can prove harmful to our bodies are detected by receptors called thermal nociceptors, whereas silent nociceptors get activated after the surrounding tissue gets inflamed. Mechanical nociceptors react to stimuli which causes extreme pressure or scratches that run deep into the surface of the skin. Nociceptors send signals to the brain and spinal cord and produce the sensation of pain, and number more than any other receptor.
Hair Follicle Receptors
Present in hair follicles, these detect any change and displacement occurring in the hair. These are moderately adapting receptors. These are very sensitive to light touch and can detect even the slightest movement in hair, even if the skin is not touched directly.
The sense of touch is one of the most important methods (and also the most primitive) of collecting information by our bodies, which is then processed by the brain, and used to control our environment. People for whom any of the other four senses are not functioning optimally, rely heavily on their sense of touch to manipulate and gain control over their surroundings. Without the sense of touch, we wouldn't be feeling warm or cold, wouldn't know if our hands and feet have touched any surface, wouldn't even know if we have hurt ourselves! And who can deny how important the sense of touch is, when a much needed hug from a friend or loved one can work wonders and brighten our day!