NRSC 2100 1st Edition Lecture 19 Somatic Sensation Responsible for touch hapsis pain and temperature nociception sensations Somatic sensory system Different from other systems Receptors Broadly distributed Respond to many types of stimuli Receptors located in hairy and glabrous eg hairless skin in inner dermis and outer epidermis skin Mechanoreceptors Most somatosensory receptors are mechanoreceptors Touch Hapsis Mechanoreceptors Sensitive to physical distortion bending or stretching Pacinian corpuscles Ruffini s endings Meissner s corpuscles Merkel s disks Krause end bulbs Mechanoreceptors Receptive Fields Small receptive fields a few millimeters or less Meissner s corpuscles Large receptive fields half the hand Pacinian corpuscles Two point discrimination sensitivity dependent on receptor field size and receptor density Mechanoreceptor Adaptation Some receptors adapt quickly respond only to onset or offset of stimuli Other receptors adapt more slowly respond throughout distortion Ending specializations corpuscles often responsible for responses Spinal Primary Afferent Axons of Somatosensory System A A A and C These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute C fibers mediate pain and temperature slow A mediates touch sensations fast Enter spinal cord through dorsal roots soma in dorsal root ganglia Dorsal Column Medial Lemniscal Pathway For hapsis Afferents produce 2 axon collaterals to dorsal horn and medullary dorsal column nuclei ipsilateral Dorsal column neurons contralateral medial lemniscus to ventral posterior VP thalamus VP Primary somatosensory cortex S1 Primary Somatosensory Cortex S1 Area 3b Adjacent areas Postcentral gyrus 3a 1 and 2 posterior to central sulcus Posterior Parietal Cortex 5 and 7 Columnar organization Primary Somatosensory Cortex Organization Contralateral body representation inversely and somatotopically organized Disproportional body representation receptor density in different body parts
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