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chapter 14.rtf

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Todd Ferretti

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Chapter 14: The Cutaneous Senses Somatosensory system- This includes: 1. The Cutaneous senses: responsible for perceptions such as touch and pain that are usually caused by stimulation of the skin (responsible for activities such as grasping objects and protecting against damage to the skin, but also for motivating sexual activity) 2. Proprioception: The ability to sense the position of the body and limbs 3. Kinesthesis: The ability to sense the movement of the body and limbs The perceptions we experience through are skin are crucial for: - carrying out everyday activity -protecting ourselves from injury -motivating sexual activity -are crucial for the survival of our species Overview of the Cutaneous System The skin: - Comel called the skin the "monumental facade of the human body" because it is the heaviest organ in the human body and if not the largest, it is certainly the most obvious, especially in humans -in addition to its warning function, the skin also prevents bodily fluids from escaping and at the same time protects us by keeping bacteria, chemical agents, and dirt from penetrating our bodies -our skin is a visible surface which is actually a layer of dead skin cells -the layer of dead cells is part of the outer layer of skin called the epidermis and below this is another layer called the dermis -It is in these two layers that we find the mechanoreceptors, receptors that respond to mechanical stimulation such as pressure, stretching and vibration. Mechanoreceptors: -There are four types of mechanoreceptors that are located in the epidermis and dermis -Two mechanoreceptors, the Merkel receptor and the Meissner corpuscle, are located close to the surface of the skin, near the epidermis. -The Merkel receptor fires continuously as long as the stimulus is on; the Meissner corpuscle fires only when the stimulus is first applied and when it is removed. -The type of perception that is associated with the Merkel receptor is sensing fine details, and with the Meissner corpuscle, controlling handgrip. -The other two mechanoreceptors the Ruffini cylinder and Pacinian corpuscle, are located deeper in the skin. The Ruffini cylinder responds continuously to stimulation and the Pacinian corpuscle responds only when the stimulus is applied and removed. -The Ruffini cylinder is associated with perceiving stretching of the skin and the Pacinian with sensing rapid vibrations and fine texture Pathways from Skin to Cortex: -Nerve fibres from receptors receptors in the skin travel in bundles called peripheral nerves that enter the spinal cord through the dorsal root. The nerve fibres then go up the spinal cord along two major pathways: the medial leminsical pathway and the Spinothalamic pathway. -The Leminsical pathway has large fibres that carry signals related to sensing the positions of the limbs and perceiving touch. -The Spinothalamic pathway consists of small fibres that transmit signals related to temperature and pain. *The case of Ian Waterman (beginning of chapter) illustrates this separation of function, because although he lost the ability to feel touch and to sense the positions of his limbs (leminsical pathway) he was still able to sense pain and temperature (spinothalamic pathway) -Fibres from both pathways cross over to the other side of the body during their upward journey to the thalamus. Most of these fibres synapse in the ventrolateral nucleus in the thalamus, but some synapse in other thalamic nuclei. *Remember that fibres from the retina and the cochela also synapse in the thalamus, in the lateral geniculate nucleus for vision and the medial geniculate nucleus for hearing. Maps of the Body on the Cortex" -From the thalamus, signals travel to the somatosensory receiving area (S1) in the parietal lobe of the cortex and possibly also to the secondary somatosensory cortex (S2). -An important characteristic of the somatosensory cortex is that it is organized into maps that correspond to locations on the body. The existence of a map of the body on S1 was determined by Wilder Penfield. He found that stimulating the ventral parts of S1 (lower parietal lobe) caused sensations on the lips and face, stimulating higher on S1 caused sensations in the hands and fingers and stimulating the dorsal S1 caused sensations in the legs and feet. Homunculus: latin for little man, shows that some areas on the skin are represented by a disproportionately large area of the brain. The area of the thumb is as large as the area devoted to the entire forearm. This is similar to the magnification factor in vision in which receptors in the fovea, which are responsible for perceiving visual details are allotted disproportionate area on the visual cortex. The Plasticity of Cortical Body Maps: -a basic principle of the cortical organization is that the cortical representation of a particular function can become larger if that function is used often. - Most of the early experiments that demonstrated experience dependant plasticity were carried out in the somatosensory system -William Jenkins and Michael Merzenich showed that increasing stimulation of a specific area of the skin causes an expansion of the cortical area receiving signals from that area of the skin. -After training monkeys to complete a task that involved extensive use of a particular location on one fingertip they found that the area representing part of the fingertip had greatly expanded after training. -Plasticity has created more cortical area for parts of the body that have been used more -The exact size of each area in the cortex representing different parts of the body is not fixed Four types of perception: 1. Perceiving detail: - One of the most impressive examples of perceiving details with the skin is provided by Braille -experienced readers can read at a rate of about 100 words per minute Tactile acuity- the ability to detect details on the skin- there are numerous ways to measure this -The classic method of measuring tactile acuity is the two point threshold. the minimum separation between two points on the skin that when simulated is perceived as two points. It is measured by gently touching the skin with two points and having the person indicate whether he or she feels one point or more. -The two point threshold was the main measure of acuity in most of the early research on touch -Grating acuity- is measured by pressing a grooved stimulus onto the skin and asking the person to indicate the orientation of the grating. Acuity is measured by determining the narrowest spacing for which orientation can be accurately judged. Receptor Mechanisms for Tactile Acuity: - the properties of the receptors are one of the things that determines what we experience when the skin is stimulated. See figure 14.8 for demonstration Cortical Mechanisms for Tactile Acuity: -Regions of high acuity, like the fingers and lips are represented by larger areas on the cortex -the map of the body on the brain is enlarged to provide the extra neural processing that enables us to accurately sense fine details with our fingers and other parts of the body. -another way to demonstrate the connection between cortical mechanisms and acuity is to determine the receptive fields of neurons in different parts of the cortical homunculus. * Remember that the receptive field for a neuron in the visual system is the area of the retina that, when stimulated, influences the firing rate of the neuron. 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. -cortical neurons representing parts of the body with better acuity, such as the fingers, have smaller receptive fields. -Having small receptive field of neurons receiving signals from the fingers translated into more separation on the cortex, which enhances the ability to feel two close-together points on the skin are two separate points. Perceiving vibration: - The mechanoreceptor that is primarily responsibl
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