Neurophysiological processes involved in clothing comfort perception

The sensory perceptions of human skin are governed by mainly two types of nerve endings in the skin layers, i.e. corpuscular endings and free nerve endings. Corpuscular endings have small swelling on the nerve fibres and are responsible for different type of sensations, like touch, pressure, cold, heat, etc. The different types of nerve endings are Pacini’s corpuscles, Meissner corpuscles, Merkle’s nerve ending, Krause’s end bulb, Ruffini endings, hair follicle nerve ends and free nerve ends. The free nerve endings in subcutaneous fat are associated with pain fibre, and those projecting in to the epidermis may be associated with cold fibres or pain fibres.

Pacini’s corpuscles

These mechanoreceptor nerve endings are responsible for pain and pressure sensations and detect gross pressure changes and vibrations. These are rapidly adapting receptors in the human skin. Due to any deformation in the corpuscles the pressure sensitive sodium ion channels are opened, which allow the sodium ions inflow and create a receptor potential. Pacini’s corpuscles are capable to vibration and can sense any vibration even from few centimeters. Their optimal sensitivity is 250 Hz and this is the frequency range generated at the finger tips by textures of size less than 200 μms. These nerve endings respond when the skin is rapidly indented, but do not respond when the pressure is steady.

Meissner’s or tactile corpuscles

These mechanoreceptor nerve endings are responsible for light touch. These rapidly adaptive receptors have highest sensitivity (lowest threshold) when sensing vibration of lower frequency. The tactile corpuscles are distributed throughout the skin, but the concentration is very high to those places where the sensitivity is high at light touch, e.g. palms, lips, face, tongue, fingertips, etc. In case of any deformation, the Meissner’s corpuscles cause an action potential in the nerve. As these are quickly adapting mechanoreceptors, the action potential generated in the nerves decreases rapidly and ultimately ceases. Due to this action the wearer stops feeling his clothing after certain time. Due to their superficial location in the dermis these mechanoreceptors are particularly sensitive to touch and vibrations, but they cannot detect properly because they can only sense that something is touching the skin.

Merkel nerve endings

These mechanoreceptor nerve endings are responsible for providing information regarding pressure and texture and are classified as slowly adaptive type of mechanoreceptors. These nerve endings also have wide distribution in the human skin. These nerve endings are structurally rigid and are not encapsulated, which causes them to have a sustained response to mechanical deflection of the tissue of less than 1μm. Due to the sustained response to pressure these nerve endings are classified as slowly adapting. Merkel nerve ending is the most sensitive mechanoreceptor to vibrate at low frequency (within 5–15 Hz).

Krause’s end bulbs

The Krause’s end bulbs are the mechanoreceptors in the human skin and have the ability to detect low-frequency vibration. These can be found in some specific parts of human body, e.g. in the transparent lubricating mucous membrane that covers the eyeball and the area under the surface of the eyelid (conjunctiva), in the mucous membrane of the lips and tongue, etc.

Ruffini endings

This is a class of slowly adapting spindle-shaped mechanoreceptor and can be found only in the glabrous dermis and subcutaneous tissue of humans. It is sensitive to skin stretch and contributes to the kinesthetic sense, e.g. control of finger position and movement. These mechanoreceptors sense and monitor the slipping of objects along the surface of the skin, e.g. slipping of garment on one’s body. These are located in the deep layers of the skin and register mechanical deformation within joints more specifically very small angle change (up to 2 degree) at continuous pressure state.

Hair follicle nerve ends

These are the mechanoreceptors in the human skin present at the base of the hair follicle. These are sensory nerve fibres that wrap around each hair bulb. During bending or pulling of the hair these stimulate the nerve endings allowing a person to feel that the hair has been moved or pulled. One of the main functions of hair is to act as a sensitive touch receptor and it is done through this receptor. Greasy or oily glands are also associated with each hair follicle and these glands produce an oily secretion to help condition the hair and surrounding skin.

Free nerve endings

The free nerve endings are the most common type of nerve ending and are most frequently found in the skin. These nerve endings are responsible for conveying sensory information from the periphery of body to the brain. The free nerve endings act as skin sensory receptors and are mainly used to detect pain sensations. Unlike those found in Meissner’s or Pacini’s corpuscles, the free nerve endings are un-capsulated and have no complex sensory structures. They penetrate the epidermis and end in the stratum granulosum and can have different rate of adaptation, i.e. rapidly adapting, intermediate adapting and slowly adapting. Different free nerve endings work as thermoreceptors, mechanoreceptors and nociceptors. In other words, they express polymodality, i.e. having multiple stimulus modalities. They are responsible for sensing mechanical stimuli (e.g., touch, pressure, prick, stretch, etc.), temperature, or pain.