EL EFECTO FINANCIERO DEL IMPUESTO AL VALOR AGREGADO

Neurophysiological processes in clothing comfort

3.1 Neurophysiological perceptions 3.1.1 Sensory system of human skin

The structure of human skin is very complex. Figure 3.1 shows the structure of hairy skin which covers most of the human body. The skin has several layers. The overlaying outer layer is called epidermis, which consists of several layers of dead cells on top of a single living cell. The layer below epidermis is called dermis, which contains a network of blood vessels, hair follicle, sweat gland and sebaceous gland. Beneath the dermis are subcutaneous fatty tissues. The layers of epidermis are as follows: Stratum Germinativum, i.e. growing layer; Malpighion layer, i.e. pigment layer;

Stratum Spinosum, i.e. prickly cell layer; Stratum Granulosum, i.e. granular layer; Stratum Lucidum and Stratum Corneum, i.e. horny layer [1]. The basic functions of human skin are [2]

● To protect from external stimuli like light, heat, cold and radiation;

● To check of body fluids and tissues;

● Reception of stimuli like pressure, heat, pain, etc.;

● Biochemical synthesis;

● Metabolism and disposal of biochemical wastes;

● Regulation of body temperature;

● Controlling of blood pressure;

● Prevent penetration of noxious foreign material and radiation;

● Cushions against mechanical shock;

● Interspecies identification.

Skin is the interface between the body and its environment and it is highly stimulated and contains specialized sensory receptors to sense different external stimuli. There are mainly three types of stimuli, i.e.

mechanical interactions with external objects, thermal interactions due to heat flow to or from the body surface, and damaging (traumatic and chemical) insults. In responding to these stimuli, the skin sensors generate different sensations, like touch, pressure, pain, warm, cold, etc.

The Pacini’s corpuscle detects rapid vibration (200–300 Hz). Ruffini’s endings are responsible for detecting tension deep in the skin. The main functions of Meissner’s corpuscles are to detect and adapt the changes in texture. Merkle’s nerve endings detect sustained touch and pressure [3, 4]. Hair follcle’s nerve ends and free nerve ends are also mechanoreceptors.

Hair follcle’s nerve ends sense the changes in position of hairs and the free nerve ends sense touch, pressure and stretch.

The thermoreceptors are the sensory receptors which code the absolute and the relative changes in temperature, primarily within the safe temperature range and also respond to both constant and fluctuating skin temperatures. There are two types of thermoreceptors: cold receptors and warm receptors. The cold receptors have a peak sensitivity of around 25–

30°C and are excited by reduction in temperature. The warm receptors have a peak sensitivity of around 39–40°C and are sensitive to increase in skin temperature [4, 5].

The nociceptors are the sensory receptors which are responsible for sensing the pain, like heating the skin, strong pressure, or contact with sharp or damaging objects. These receptors have relatively high thresholds to act as warning devices that enable the organism to take protective action in time [5]. They react to potentially damaging stimuli by sending nerve signals to the spinal cord and brain.

3.1.2 Nerve endings in human skin

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. Figure 3.3 illustrates the different types of nerve endings and nerve fibres in the skin layers. 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 [6].

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

3.3 Different nerve endings in the skin layers.

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 [7]. These nerve endings respond when the skin is rapidly indented, but do not respond when the pressure is steady [8].

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 [9].

Pacini’s corpuscles Meissner corpuscles Merkle’s nerve ending Krause’s end buld

Ruffinl’s endings Hair follicle nerve ends Free nerve ends

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) [7].

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 [10]. 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 [11].

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 [12] 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.