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GENERAL SENSORY RECEPTION The Sensory System • What are the senses ? • How sensory systems work • Body sensors and homeostatic maintenance • Sensing the external environment • Mechanisms and pathways to perception General Properties of Sensory Systems • Stimulus • Internal • External • Energy source • Receptors • Sense organs • Transducer • Afferent pathway • CNS integration General Properties of Sensory Systems Sensory Receptors • Somatic • Visceral -- Chemoreceptors (taste, smell) -- Thermoreceptors (temperature, pain) -- Photoreceptors (vision) -- Proprioreceptors (muscle stretch) --Mechanoreceptors (touch, pain, audition, balance). -- Chemoreceptors (chemicals in blood, osmoreceptors) -- Baroreceptors (bp) Sensory Receptor Types Special Senses – External Stimuli • Vision • Hearing • Taste • Smell • Equilibrium Special Senses – External Stimuli Somatic Senses – Internal Stimuli • Touch • Temperature • Pain • Itch • Proprioception • Pathway Somatic Pathways • Receptor • Threshold • Action potential • Sensory neurons • Primary – medulla • Secondary – thalamus • Tertiary – cortex • Integration • Receptive field • Multiple levels Somatic Pathways The Somatosensory System • Types of receptors - Mechanoreceptors: -- Proprioreceptors in tendons, ligaments and muscles body position -- Touch receptors in the skin: free nerve endings, Merkel’s disks and Meissner’s corpuscles (superficial touch), hair follicles, Pacinian corpuscles and Ruffini’s ending - Thermoreceptors: Warm receptors (30-45oC) and cold receptors (20-35oC) - Nociceptors: respond to noxious stimuli Touch (pressure) Skin touch receptors Sensory pathways • The sensory pathways convey the type and location of the sensory stimulus • The type: because of the type of receptor activated • The location: because the brain has a map of the location of each receptor Temperature • Free nerve endings • Cold receptors • Warm receptors • Pain receptors • Sensory coding: – Intensity – Duration Pain perception sharp and well localized, transmitted by myelinated axons • Fast pain: dull aching sensation, not well localized, transmitted by unmyelinated axons • Slow pain: • Visceral pain: not as well localized as pain originating from the skin pain impulses travel on secondary axons dedicated to the somatic afferents referred pain ALL THE PRECEDING MODALITIES CULMINATE IN THE PROPAGATION OF ACTION POTENTIALS Sensory transduction • Receptors transform an external signal into a membrane potential • Two types of receptor cells: - a nerve cell - a specialized epithelial cell Two types of sensory receptors Receptor adaptation • Tonic receptors -- slow acting, -- no adaptation: continue to for impulses as long as the stimulus is there (e.g., proprioreceptors) • Phasic receptors -- quick acting, adapt: stop firing when stimuli are constant (e.g., smell) Sensory coding • A receptor must convey the type of information it is sending the kind of receptor activated determined the signal recognition by the brain • It must convey the intensity of the stimulus the stronger the signals, the more frequent will be the APs • It must send information about the location and receptive field, characteristic of the receptor