Chapter 14 The Cutaneous Senses I Introduction a Somatosensory System i The Cutaneous Senses responsible for perceptions such as touch and pain that are usually caused by stimulation of the skin ii Proprioception the ability to sense the position of the body and iii Kinesthesis the ability to sense the movement of the body and limbs limbs b The perceptions we experience through our skin are crucial for carrying out everyday activities protecting ourselves from injury and motivating sexual activity II Overview of the Cutaneous System a The Skin i The heaviest organ in the human body ii Functions b Mechanoreceptors i Merkel Receptor 1 Warning function 2 Prevents body fluids from escaping 3 Protects us by keeping bacteria chemical agents and dirt from penetrating our bodies 4 Provides us with information about the various stimuli that contact it iii Visible surface layer of tough dead skin cells iv Epidermis the outer layer of skin v Dermis layer of the skin below the epidermis vi Mechanoreceptors found within these two layers receptors that respond to mechanical stimulation such as pressure stretching and vibration 1 Located close to the surface of the skin near the 2 Fires continuously as long as the stimulus is on 3 Associated with sensing fine details ii Meissner Corpuscle 1 Located closed to the surface of the skin near the epidermis epidermis removed 2 Fires only when the stimulus is first applied and when it is 3 Associated with controlling handgrip iii Ruffini Cylinder 1 Located deeper in the skin 2 Responds continuously to stimulation 3 Associated with perceiving stretching of the skin iv Pacinian Corpuscle 1 Located deeper in the skin 2 Responds when the stimulus is applied and removed 3 Associated with sensing rapid vibrations and fine texture c Pathways from the skin to the cortex i Nerve fibers from receptors in the skin travel in bundles called ii Peripheral nerves go up the spinal cord along two major peripheral nerves pathways 1 Medial Leminiscal Pathway large fibers that carry signals related to sensing the positions of the limbs and perceiving touch 2 Spinothalamic Pathway consists of smaller fibers that transmit signals related to temperature and pain iii Fibers from both pathways cross over to the other side of the body during their upward journey to the thalamus 1 Fibers synapse in the ventrolateral nucleus in the thalamus 2 Signals originating from the left side of the body reach the thalamus in the right hemisphere signals from the right side of the body reach the left hemisphere i From the thalamus signals travel to the somatosensory receiving area S1 in the parietal lobe of the cortex ii Signals also possibly travel to the secondary somatosensory cortex S2 iii The somatosensory cortex is organized into maps that correspond to locations on the body stimulating different points of S1 caused sensations in different parts of the body iv Homunculus body map that shows some areas on the skin are represented by a disproportionately large area of the brain 1 The area devoted to the thumb is as large as the area devoted to the entire thumb 2 Fingers also allotted a disproportionate area on the d Maps of the body on the cortex e The plasticity of cortical body maps somatosensory cortex i The cortical representation of a particular function can become larger if that function is used often 1 For example a violin player who uses the finger of his left hand to finger the strings will have a greater than normal cortical representation for the fingers on his left hand ii Thus the exact size of the area representing each pat of the body is not totally fixed III Perceiving Details a Impressive example of perceiving details with the skin Braille reading system b Measuring tactile detail perception i Tactile Acuity the ability to detect details on the skin ii Two Point Threshold the classic method of measuring tactile acuity 1 Measured by gently touching the skin with two points such as the points of a drawing compass and having the person indicated whether he or she feels one point or two 2 Main measure of acuity in most of early research on touch iii Grating Acuity measured by pressing a grooved stimulus onto the skin and asking the person to indicate the orientation of the grating is the grating vertical or horizontal 1 Acuity is measured by determining the narrowest spacing within the grating for which orientation can be accurately judged c Receptor mechanisms for tactile acuity i Merkel receptor is sensitive to details the firing of the Merkel receptor s fiber signals details ii High density of Merkel receptors in the fingertips because the fingertips are the parts of the body that are most sensitive to details iii Better acuity is associated with less spacing between Merkel Interestingly the spacing of the Merkel receptors is the same on all fingertips so how do we explain why the top of the index finger is more sensitive to touch than is the top of the little finger d Cortical mechanisms for tactile acuity i Regions of high acuity are represented by larger areas on the receptors 1 cortex ii The magnification of the representation on the brain of parts of the body such as the fingertips parallels the magnification factor in vision iii The receptive field for a neuron in the cutaneous system is the area on the skin that when stimulated influences the firing rate of the neuron iv Cortical neurons representing parts of the body with better acuity such as the fingers have smaller receptive fields 1 Having small receptive fields of neurons receiving signals translates into more separation on the cortex which enhances the ability to feel two close together points in the skin as two separate points thus it enhances tactile acuity IV Perceiving Vibration a Pacinian Corpuscle PC the mechanoreceptor that is primarily responsible for sensing vibration i Consists of a series of layers with fluid between each layer ii Transmits rapidly applied pressure like vibration to the nerve fiber but does not transmit continuous pressure the PC thus causes the nerve fiber to receive rapid changes in pressure but not to receive continuous pressure iii Lowenstein 1960 concluded that properties of the PC cause the fiber to respond poorly to continuous stimulation but to respond well to changes in stimulation that occur at the beginning and end of a pressure stimulus or when stimulation is changing rapidly as occurs in vibration V Perceiving Texture a David Katz 1925 proposed that our perception