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Chapter 72

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PSYC 2410
Boyer Winters

Chapter 7: Mechanisms of Perception 7.3 – Somatosensory system: Touch and Pain - Somatosensations – sensations from your body - Somatosensory system – system that mediates bodily sensations; consists of 3 separate but interacting systems - Proprioceptive system – monitors information about the position of the body; information comes from muscles, joints and organs of balance - Interoceptive system – provides general information about conditions within the body - Exteroceptive system – senses external stimuli applied to the skin; comprised of 3 divisions: perceiving mechanical stimuli (touch); thermal stimuli (temperature); and nociceptive stimuli (pain) Cutaneous receptors: - Free nerve endings – neuron endings with no specialized structures; simplest cutaneous receptors; particularly sensitive to temperature change and pain - Pacinian corpuscles – because they adapt rapidly, respond to sudden displacement of the skin but not to constant pressure; largest and deepest cutaneous receptors - Merkel’s disks and Ruffini endings – both adapt slowly and respond to gradual skin indentation and stretch - We’re often totally unaware of constant skin pressure (such as clothes) until we focus attention on it. When trying to ID things by touch we manipulate them in our hands so that the pattern of stimulation continually changes - Stereognosis – identification of objects by touch Dermatones: - Neural fibres that carry information from cutaneous receptors and other somatosensory receptors gather together in nerves and enter the spinal cord via the dorsal roots - Dermatone – the area of the body that is innervated by the left and right dorsal roots of a given segment of the spinal cord Two Major Somatosensory Pathways - Dorsal-column medial-lemniscus system – tends to carry information about touch and proprioception - Anterolateral system – tends to carry information about the pain and temperature - The separation of the two pathways is not complete Figure 7.12 – Dorsal-column medial-lemniscus system Figure 7.13 – Anterolateral system - Comprises 3 different tracts o Spinothalamic tract – projects to the ventral posterior nucleus of the thalamus o Spinereticular tract – projects to the reticular formation (then to the parafascicular nuclei and intralaminar nuclei of the thalamus) o Spinotectal tract – projects to the tectum (colliculi) - The 3 branches of the trigeminal nerve carry pain and temperature information from the face to the same thalamic sites ; the information is then distributed to the SI, SII, posterior parietal cortex and other parts of the brain - If both ascending somatosensory paths are completely transected by spinal injury the patient can feel no body sensation from below the level of the cut Cortical Areas of Somatosensation - Somatotopic – organized according to a map of the body surface; human primary somatosensory cortex (SI) is this way; somatosensory homunculus – common name for somatotopic map - The greatest proportion of SI is dedicated to parts of the body used to make tactile discrimination (hands, lips, etc) while only a small proportion receives input from large areas of the body Effects of Damage to the Primary Somatosensory Cortex - The effects are often remarkably mild – possibly because of several parallel pathways - Corkin, Milner and Rasmussen – assessed the somatosensory abilities of epileptic patients befoe and after a unilateral excision that included SI and two minor deficits were displayed: reduced ability to detect light touch and reduced ability to identify objects by touch Somatosensory System and Association Cortex - Biomodal neurons – neurons that respond to activation of two different sensory systems; respond to both somatosensory and visual stimuli The case of W.M. who reduced his scotoma with his hand - Hemianopsia – condition in which a scotoma covers half the visual field - He suffered a stroke. When tested with his left hand in his lap he could see little of his right visual field, with his left hand extended it was improved greatly and even further when he held a tennis racket in his extended left hand Somatosensory Agnosias - Two major types of agnosia: - Asteroagnosia – the inability to recognize objects by touch; usually occurs with presence of simple sensory deficits - Asomatoagnosia – the failure to recognize parts of one’s own body; usually unilateral affecting only the left side of the body; usually associated with extensive damage to the right posterior parietal lobe - Anosognosia – the failure of neuropsychological patients to recognize their own symptoms; asomatoagnosia often accompanied by this - Contralateral neglect – the tendency not to respond to stimuli that are contralateral to a right- hemisphere injury; asomatoagnosia is often a component of this Perception of pain - The perception of pain is paradoxical in three respects: - Adaptiveness of pain – an experience that seems so bad but is necessary for our survival; no special stimulus for pain but is a response to potentially harmful stimulation of any type - Lack of clear cortical representation of pain – no obvious cortical representation; painful stimuli activate many parts of the cortex; anterior cingulated cortex is the cortical area most frequently linked with pain but appears to be associated with the emotional reaction to pain not physical - Descending pain control – can be so effectively suppressed by cognitive and emotional factors - Gate-control theory – signals descending from the brain can activate neural gating circuits in the spinal cord to block incoming pain signals - Three discoveries led to identification of a descending pain-control circuit: - Periaqueductal gray (PAG) – electric stimulation to this has analgesic (pain-blocking) effects - Discovery that the PAG and other areas of the brain contain specialized receptors for opiate analgesic drugs such as morphine - Endorphins Neuropathic pain - Neuropathic pain – severe chronic pain i
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