Somatosensory System Touch mechanoreception Limb posture movements and forces proprioception Haptics object recognition through touch proprioception usually with the hand Readings Squires et al Ch 25 Supplementary reading Kandel et al Chs 21 23 http www youtube com watch v FKxyJfE831Q Hair follicle afferents 20 50 corpuscles afferent RA Slipping object large rf s High density SA1 PC Form braille Transmitted vibration Texture hardness textures roughness SA11 Stretch Most Sensitive to deformatio Reaction to slippage 65 msec 45 msec is peripheral nerve and motor response time Applied forces in response to shape of grasped object 100 msec Mechanoreceptors Differ in Morphology and Skin Location RA SA1 SAII merkel meissner ruffini pacinian Dorsal root ganglion cells serve a dual role of transduction and information transmission Multiple fibers from the axon branch to form a large receptive field Receptive fields in the skin overlap Large variation in two point threshold across the body surface The Spatial Resolution Varies Because the Density of Mechanoreceptors Varies RA mechanoreceptor responds to sinusoidal mechanical stimuli with a single action potential for each cycle The lowest stimulus intensity that evokes one action potential per cycle of the sinusoidal stimulus is called the receptor s tuning threshold Merkel 5 15 Hz Meissner 20 50 Hz Pacinian 60 400 Hz Tuning thresholds for vibration RA s lower thresholds 25 Hz vibratory stimulus Spatial characteristics of stimuli depend on activity across population of receptors Figure 22 8 The firing patterns of mechanoreceptors in the superficial layers of the skin encode the texture of obje Stimulus intensity in encoded by spike rate Neural response Perceived magnitude pressure Proprioceptors in muscle spindles and tendons dorsal column medial lemniscal system for tactile sensations and proprioception and anterolateral system for pain and temperature Figure 21 12 I nhibition of selected projection neurons in a sensory relay nucleus enhances the contrast b inhibitory pathways in the circuitry of the dorsal column nuclei the first relay in the system for touch The projection thalamus They receive excitatory input from touch receptor axons traveling in the dorsal columns These afferent fibe Serial parallel processing PET scan allow neurologists to image the somatotopic functioning of the cortex in individual patients While these imaging me thods are less precise tha microelectrode maps made in animals they are useful diagnostic tools in clinical neurology Figure 23 7 Each region of the somatic sensory cortex receives inputs from primarily one type of receptor A I n each of the four regions of the somatic sensory cortex Brodmann s areas 3a 3b 1 and 2 inputs from one type of receptor in specific parts of th Tactile information from the fingertips is used faster than can be readily explained by rate codes Johansson Birznieks NN 2004 Reaction to slippage 65 msec 45 msec is peripheral nerve and motor response time Applied forces in response to shape of grasped object 100 msec Receptive fields in area 3b of S1 Velocity invariance of cell s responses Receptive fields of neurons in S1 are larger than those of the sensory afferents Each of the hand figurines shows the receptive field of an individual neuron in areas 3b 1 2 and 5 based on recordings in alert monkeys Colored regions indicate the region where light touch elicits action potentials Neurons at later stages of cortical processing areas 1 and 2 have larger receptive fields and more specialized inputs than neurons in area 3b The neuron from area 2 is directionally sensitive to motion toward the fingertips Neurons in area 5 often have symmetric bilateral receptive fields at mirror image locations on the contralateral and ipsilateral hand Ablation of area 3b general losses Ablation of area 1 loss of texture discrim Ablation of area 2 loss of 3D form discrim Direction sensitivity emerges in Area 1 and 2 not present in 3b Orientation tuning in SII receptive fields Finger pads have similar tuning Results from lateral inhib not composition of inputs as in vision Ablation of SII Loss of discrim of shape and texture Doesn t affect SI responses Ablation of SI silences SII Responses modulated by attention Effect of lesion in area 2 loss of ability to coordinate fingers plus sensory deficits Somatotopic organization of SI http hebb mit edu courses 8 515 lecture3 sl d005 htm
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