Chapter 12 Notes

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University of Toronto St. George
Human Biology
Professor Niemeier

Chapter 12  Mechanoreceptors o Pacinian corpuscle – deep in derms  Large, most studied  Ruffini – in hair and glabrous skin  Meissner – in ridges of glabrous  Merkel’s disks – nerve terminal and flattened, non-neural epithelial cell o Epithelial is mechanically-sensitive part  Krause end bulbs – border regions of dry skin and mucous membrane  Pacinian – sensitive at 200-300Hz (loud music)  Meissner – at 50 Hz (rough texture) o Fluttering at lower frequencies  Ruffini –fluttering  Pacinian – onion-like layers make corpuscle insensitive to low-frequency stimuli o Indent – AP – reformation – removal – AP  Pacinian – layered capsule – sensitive to vibrating, high0firequency, NOT steady pressure  Two –point discrimination o Fingertips – highest resolution  1) more mechanoreceptors  2) small RFs in receptors  3) more brain tissue per square mm of fingertip  4) neural mechanisms for high resolution discriminations  Primary afferent axons enter spinal cord through dorsal roots (cell bodies in dorsal root ganglion) o Aalpha – Group 1 – 13 -20 Um – 50-120 m/s  Proprioceptors of skeletal muscle o Abeta – II – 5-12 um – 35-75 m/s  Mechanoreceptors o Agamma – III – 1-5um – 2-50 m/s  Pain, temperature o C – IV – 0.2 – 1.5 um – 0.5-2m/s  Temperature, pain, itch  Simpal cord o Cervical cord (8) o Thoracic cord (12) o Lumbar (5) o Sacral (5)  Dermatome – area of skin innervated by the L and R dorsal roots of a single spinal segment  Abeta axonnd– dorsal horn o 2 order sensory – unconscious reflex o Brain – conscious perception  Touch or vibration – dorsal column – medial lemiscal pathway o Decussation by meduall – info switches sides  Abeta – dorsal column – brain o (dorsal column nuclei) – through medal lemniscus to ventral posterior (VP) nucleu – SL  Trigeminal touch pathway o Trigeminal nerve – face, mouth, 2/3 tongue, dura mater  Axons from face – trigeminal nerve o Trigeminal nucleus (decussation) – VP nucleus of thalams 0 somatosensory (s1) cortex  S1 – Bridmann’s area 3b – postcentral gyrus  Central sulcus – postcentral gyrus – (3a, 3b, 1, 2)  Posterior parietyal cortex (5, 7)  3b o Receives input from VP nucleus o Neurons are responsive to somatosensory stimuli o Lesions impair somatic sensation o Evokes somatic sensory experiences when electrical stimulated  Area 3b – 1: texture information  Area 3b – 2: size and shape  Thalamus – layer III of S1 o Columnar organization  Slowly adapting sensory responses  Rapidly adapting sensory responses  Area for each digit of hand mapped on S1  Somatotopy – homunculus o Electrical stimulation of S1 o Record activity of a single neuron and determine site of its somatosensory receptive field on the body o More importance (density) on hands, mouth  Cluster of S1 neurons – barrels  Limbs – when limbs are lost, cortical areas go to activate face  Agnosia – inability to recognize objects (sight)  Asterognosia – inability to recognize objects (somatosensation) o Deficit contralateral to damage  Neglect syndrome o Part of body or part of visual fiend IGNORED  Nociceptor – free, branching, unmyelinated nerve endings that signal body tissue damage o Nociception – sensory process that triggers ain  Pain – perception of ouch  Nociceptors may fire continuously, but pain comes and goes o Pain can happen without nociception activity – brain instead  Nociceptor membrane stretches – activates mechanically gated ion channels o Depolarization – Aps generated  Damaged cells also release substances that cause ion channels ion nociceptor membranes to open o Protease, /K+, ATP
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