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PSY290 - YEOMANS - "How You Know the World": Somatosensory Systems - Oct. 8

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John Yeomans

HOW YOU KNOW THE WORLD - LEC 5 - Oct. 8, 2013 Today’s overview: Input to thalamus and limbic system:  Taste → as visceral system  Olfaction → sense of emotions; how we’ve evolved beyond chemical detection Emphasis on complexity of sensory systems We use sense of taste & touch especially → therefore emphasis on fingers (human touch) Emotions and feeling (senses) No emphasis on auditory & visual cortices (though they are main sensory systems) Midterm (in-class): MC, 4/5 definitions, short answers, essay (1hr) → audition → only overview covered in lecture Cutaneous sensations (from skin → brain): → 4 major receptors in fingers Meniscal and spinal phylanic Must understand interactions b/w pathways →perception Finally, olfaction system: → more receptors, diversity in systems, types of organization → leads to evolution of Telencephalon In general, receptors, thalamus, cortex (perceived system). Receptors are in cochlea (“snail-like structure” → basalar membrane → auditory nerve → brain Most need several more relays. Auditory membrane (midbrain visual system) Medial genicula →auditory thalamus Cortex (auditory) →temporal lobe (below lateral fissure) → oversimplified pathway (there are many more intermediate ones, e.g. Superior olives, lateral somethingsomething) →usually don’t go directly from receptor--thalamus (2-3 more relays, usually) Current model: →series of connections/cross-connections →several cortical relays (excludes brain stem relays) →even still, very simplified Cortical areas=most plastic areas of brain, along w/limbic system →maps Δ w/injury/reorganization → go back to reorganize thalamus  Missing recurrent pathways (retrograde pathways)  Are complex networks involving plasticity, feedback networks, complex reorganizing processes** TOUCH: →receptors to spinal chord (straight) →Trigeminal nerve (straight from face→thalamus) →much less processed  Small # receptors (7 types) →memorize & associate w/precise functions  Skin’s specialized endings:  Generally, fibre extending from skin  Tip of fibre--sensor responds to mechanical movements of skin, also chemical signals (pain)  Simple mechanoreceptors →stretching & moving skin activates receptors  Middle layer of skin (dermis)  Fatty laters = subdural layers, upper layers = epidermal  Hair cells (have receptors wrapping around base)  Emphasize non-hairy skin* →maximal touch sensations, especially sensitive**  More sensitivity, higher density, more “interesting” touch sensations  Receptors:  Some naked, bare endings (free nerve endings) - unmyelinated/lightly myelinated = pain & temperature  4 encapsulated endings:  Pacinian corpuscles (deep)  Ruffinian ends (deep)  Merkel’s disks  Meissner’s corpuscles  All easily distinguishable, physically  Ones w/small receptive fields (2 of them); other two have larger receptive fields →suborganized into fast v slow adapting  Receptive field = part of environment to which a neuron responds (excitation +, or inhibitory -)  Shown w/excitatory centre (red) →w/extracellular recording (electrodes)  Touching outside inhibits cell→slows down firing  Electrophysicists connect to noise generator → method of detecting neuron stimulation (can hear when turning on--firing more-- or off--firing less)  Method also works for other sensory systems (i.e. Vision-- activation of retina, or place on screen in front of observed animal--map part of environment either on screen and retina, because point of connection b/w them)  Every neuron has individual receptive field →E.g. basilar membrane →auditory neuron  Adaptations (other property)  Sometimes maintain firing, sometimes adapt slowly,  Slowly adapting neuron:  Only when you touch particular coral on fingertip do you excite the neuron  Strong stimulus→Greater firing, and keeps firing →slowly adapting n euron  Fast adapting neuron→fires at start w/burst, then slows and stops quickly, then when you turn stimulus off it fires again (fires an offset)  4 types of encapsulated endings in fingerslarge axons: →Each w/unique psychological impression →unique psychological impressions, each  From Meissner’s corpuscles→fast adapting  Respond to touch and offset  More of a buzzing, unnatural sensation  Merkel’s Disks →good for light-touch  Good for tapping on/off  Small fields; slow-adapting  Deep ones: Pacinian corpuscles (10xbigger than axons)  Deep, subdermal layers, muscles  Deep vibrations in skin, massages →feeling of relaxation  Fast-adapting (like being turned on/off) →better for vibration  Ruffini endings →for handy animals  Connected to connective tissue  Respond to pulling of skin → grabbing things, moving things  Allows humans to control objects & body movements  Palm/finger →deep tugging, control grip  Sense contact →slow adapting  Hair endings →breezes Nerves & Dermatomes:  8 cervical nerves, 12 thoracic, 5 lumbar & sacral, 1 coccygeal  Lumbar →back pain  Sacral→hip region  Coccygeal→tail→anus  C2-8 = control of arms (goi
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