Chapter 3

Chapter 3 - Chapter 3- The Somatosensory System: Touch,...

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Chapter 3- The Somatosensory System: Touch, Feeling, and Pain A. Neural Basis of Somatosensory Perception - Touch is distinctive among the senses in that its multiple attributes (pressure, vibration, warmth, cold) can interact to produce compound sensations (e.g. wetness, sponginess, etc.), having both objective (“where was I touched”) and subjective qualities (what did I touch/ how did it feel?), and yet they are all part of a single kind of sensory experience. - Touch is also distinctive in that its related receptors are located throughout the whole body. - Historically, 2 main ideas has been proposed to account for the way in which touch signals are generated and transmitted to the brain: o 1. The theory of receptor specificity – from Johannes Mueller’s doctrine of specific nerve energies, whereby individual receptors and nerve endings are selectively sensitive to a particular form of energy impinging on the skin. Max von Frey suggested that there were specific receptors for heat, cold, pressure, and pain. o 2. The pattern theory – in which the specificity of touch sensations was believed to arise not from individual receptors, but through overall pattern of activity across a broad spectrum of receptors. I.e. different types of touch generate different patterns of activity, which the brain interprets as a certain sensation. - It has been found that touch signals are indeed highly specific and selectively generated by a particular type of receptor, called mechanoreceptors . Although we accept this specificity, most forms of touch involve stimulation of multiple types of receptors. The skin and its receptors - Skin surface area = close to 15 square feet; weighs about 10 pounds. - Epithelial cells : closely packed cells that fit together to form continuous sheets found on the surface of the body or in the lining of its cavities. - Skin tissue is anatomically divided into 2 principal divisions: o Epidermis: outermost layer; serves as a protective shield; composed of several sublayers that are constantly being replenished. o Dermis: underlying layer; makes up the bulk of skin tissue; contains most of the mechanoreceptors and nerve endings that generate touch sensations. - Skin tissue can also be broadly classified into 2 general types: o Hairy o Hairless (palms, lips, etc.) Types of mechanoreceptors: - Mechanoreceptors can be classified into 3 types: o 1. Encapsulated receptors: have a specialized capsule surrounding a nerve ending. i.e. Pacinian, Meissner, and Ruffini corpuscles. The capsule itself serves specific functions of the particular type of touch. i.e. onion-like structure of Pacinian corpuscle acts like a mechanical filter, aiding in transmission of vibrational stimuli. Meissner- designed for transmitting light touch.
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Chapter 3 - Chapter 3- The Somatosensory System: Touch,...

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