Sensory Receptors & Olfaction PDF

Intro to Sensory Receptors Sensation Conscious awareness of incoming sensory information Can only occur if the sensory input has reached the cerebral cortex Receptors...

Intro to Sensory Receptors Sensation Conscious awareness of incoming sensory information Can only occur if the sensory input has reached the cerebral cortex Receptors Respond to a stimuli and initiate sensory input to the CNS Range in complexity Stimuli Changes in the sensory information that our receptors detect Receptors as transducers Transducers change energy from one form to the next Original energy form is what is detected by the receptor Converted into electrical/chemical energy Two features Receptors establish and maintain a resting membrane potential across their plasma membrane Receptors contain modality-gated channels in their plasma membranes Receptive Field The area through which a stimulus is detected Receptive field size inversely correlates with the density of receptors The more receptors, the more frequent and smaller the fields The larger the receptive field, the less we can localize the exact spot of stimulation Tonic and Phasic Receptors Tonic Receptors Respond continuously to stimuli at a constant rate Respond to both the presence of and intensity of the stimulus Receptor sensitivity stay constant over time or slowly adaptive Phasic receptors Receptors that deal with temperature, pressure, and light touch Detect new stimuli or changes in the state of the detected stimulus Detect onset and offset of a stimulus Undergo rapid adaptation, a reduction in sensitivity to a continually applied stimulus General and Special Senses General sense receptor Typically simple in structure Somatic sensory receptor Housed within the skin monitoring tactile sensations Housed within muscles and joints monitoring stretch and movement Visceral sensory receptor Housed in the walls of the viscera Respond to temperature, chemicals, stretch and pain Special senses Specialized, complex sense organs Gustation, olfaction, audition, vision, equilibrium Classification by stimulus origin Exteroceptors Detect stimuli of external origin Located near body surfaces Tactile receptors in the skin Mucous membranes Interoceptors Detect stimuli of internal origin Visceral sensory receptors Detect changes within the visceral organs Report temperature, pressure, chemical changes, and perceived pain Somatic sensory receptors Part of musculoskeletal structures Position of bones and muscles Propioceptors Located in muscles, tendons and joints Detect body movement, skeletal muscle contraction and stretch Enable our awareness of body position Classifying general sensory receptors by modality Thermoreceptors Sensitive to changes in temperature Chemoreceptors Detect chemicals or specific molecules dissolved in fluid Mechanoreceptors Respond to touch, pressure, vibration, & stretch Specialized mechanoreceptors Baroreceptors detect changes in stretch or distention within body structures Osmoreceptors detect changes in solute concentration of bodily fluids Proprioceptors detect the position of body in space Nociceptors Respond to painful stimuli Detect chemical, heat, or mechanical damage Thermoreceptors Detect changes in temperature 6x more cold receptors than warm receptors Can not detect temperature below 10°C Receptors are transient receptor potential cation (TRP) channels Respond at different temperatures Allow Ca2+ to depolarize the cell Some channels also respond to chemicals Menthol activates TRPM8 (detects 25 and 28°C) Capsaicin activates TRPV1 & 2 (detect >43°C & >52°C, respectively) Nociceptors Subtype of free nerve endings Concentrated in areas more prone to injury Adapt very slowly or not at all Two primary types Visceral detect internal damage within the viscera Somatic detect chemical, temperature or mechanical changes at the body surface, joints, or skeletal muscles Respond to Cellular damage Noxious chemicals Cellular signals Classification by structure Encapsulated vs unencapsulated Unencapsulated has a free nerve ending Encapsulated has nerve ending is embedded within connective tissue Sensory cells coupled to neurons vs. neurons with peripheral processes Sensory cell possesses receptors to detect stimulus and stimulates peripheral neuron Neuron possesses receptors and responds directly to stimulus Tactile Receptors Most numerous type of receptor Mechanoreceptors Mostly located in dermis and subcutaneous layer Can be simple (unencapsulated) or complex (encapsulated) Tactile sensation Touch Provides information about location, texture, size, shape, and movement Pressure Results from deformation of deeper tissues Vibrations Rapid and repetitive sensory signals Unencapsulated tactile receptors Dendritic ends of sensory neurons lack a protective coat Three types Free Nerve endings Root hair plexuses Tactile Discs (Merkel discs) Abundant in epithelia and CT Most are unmyelinated Free nerve endings & root hair plexus Terminal branches of dendrites Least complex Slow to rapidly adapting Projections to the CNS are known as Aα, Aδ, and C fibers Lie close to the surface in stratum granulosum Around the hair follicles that respond to hair movement and gentle touch called root hair plexus Polymodal Detect temperature, touch, pressure, stretch and cell damage Merkel (or tactile) cells Flattened nerve endings associated with specialized sensory cells Abundant in the tips of fingers and lips Located in the stratum basale Tonic receptors Detect fine touch Distinguish texture and shapes of objects Encapsulated tactile receptors Wrapped by connective tissue or surrounded by glial cells Four types Tactile (Meissner’s) corpuscles Bulbous (Ruffini’s) corpuscles End bulbs (Krause’s corpuscles) Lamellated (Pacinian) corpuscles Almost all are mechanoreceptors Tactile corpuscles Meissner’s corpuscles Intertwined dendrites Enclosed by neurolemmocytes arranged as horizontal lamellae Surrounded by dense irregular CT Location Papillary dermal layer Lips, palms, eyelids, nipples, and genitalia Phasic receptors Detects Light touch Vibrations (10 – 50 MHz) Used to distinguish texture and shape of objects Bulbous corpuscles Ruffini’s corpuscles Spindle-shaped dendritic endings ensheathed in CT Nerve endings are intertwined with collagen fibers encased within a capsule Location Dermal layer Highest density around the fingernails to monitor the slippage of objects Detects skin distortion & continuous deep pressure Important for our ability to grasp and hold things with our fingers Tonic receptors Do not exhibit adaptation End bulbs Krause’s corpuscles Dendritic endings ensheathed in CT Locations Reticular dermal layer Mucous membranes of oral, nasal, vaginal, & anal cavity Detects Light pressure Temperature Lamellated corpuscles Pacinian corpuscles Capsule Outer CT sheath concentric layers of collagen fibers Inner core of neurolemmocyte Locations Hairless skin Subcutaneous layer of palms and soles, breasts and external genitalia Deep reticular layer of dermis Rapidly adapting Detects Deep pressure High frequency vibration Referred Pain Perception of sensory nerve signals of the viscera in dermatomes of the skin False sense of origin Believed to be the results from shared ascending tracts Useful in medical diagnosis Olfaction Sense of smell Detection of airborne chemicals by chemoreceptors within the nasal cavity Allows us to sample our environment For food Identification of other individuals Danger Human olfaction is not as sensitive or developed as other organisms Olfactory epithelium Found in the superior region of the nasal cavity Three distinct cell types Olfactory receptor cells An afferent neuron Detects odors Supporting cells Secrete mucus Support the olfactory receptor cells Basal cells Neural stem cell Give rise to new olfactory receptors cell (regenerate about every 40 – 60 days) Olfactory glands found within lamina propia Produces mucus Olfactory receptor cell Bipolar neurons Single axon that projects to CNS Olfactory nerve (CN I) Single dendrite projecting into the overlaying mucus Dendrites possesses olfactory hairs Olfactory hairs possess the chemoreceptors Axon Each chemoreceptor in that one cell are the same type Each chemoreceptor detects a specific chemical shape and thus a specific odorant Axons form fascicles of the olfactory nerves (CN I) Nerves project through the cribiform plate into the Dendrite olfactory bulb Each receptor responds to only one discrete Olfactory hairs component of the odor (odorant molecule) rather Odorant than the whole odor Mucus Olfactory bulb Terminal end of olfactory tract Olfactory nerves synapse with two types of secondary neurons Mitral and tufted cells Form the olfactory glomerulus About 2000 glomeruli Highly organized (arranged) within the olfactory bulb Secondary neurons form the olfactory tracts Project to primary olfactory cortex Project to Hypothalamus and Amygdala (limbic system) Do not project to thalamus Characteristics of an odor Consists of multiple molecules in various concentrations Molecules that are detected are called odorants In order for an odorant to be detected it must be: Volatile Easily vaporized Sufficiently water-soluble Must be able to dissolve in the mucus Detecting smells Most inhaled air does not pass across the olfactory epithelium Deep breathing Inhaled air becomes agitated as it passes over the nasal conchae Moves inhaled air towards the superior aspect of the nasal cavity Odorants Diffuse into the mucus overlying the ORCs Bound by odorant-binding proteins which assist in odorant-receptor coupling Olfactory transduction Involves a G-protein coupled receptor Called the odorant receptor About 1000 different genes accounting for the different types G-protein is termed Golf Stimulation of odorant receptor activates a cascade through Golf Results in opening ion channels Leads to depolarization of the olfactory receptor cell Stimulation the olfactory receptor cell Activation of a specific olfactory receptor varies by what can bind it. Some odorant molecules can bind various receptors with different affinities Some receptors can bind various odorants with different affinities The variation of odors give rise to different patterns of stimulation within the glomeruli The pattern of glomerular stimulation is what is submitted to the olfactory cortex and what gives rise to the perception of smell Olfactory detection All ORCs that possess the same olfactory receptors terminate in the same glomeruli Glomeruli are responsible for separating distinct components of the odor and organizing scent perceptions Mitral cells refine the smell signals & relay them to the brain for further processing Where olfactory signals go Olfactory bulb along the olfactory tract Cerebral Cortex Allows for conscious perception of smell Identification of smell Limbic system Hypothalamus Controls visceral reaction to smell Amygdala Recognition of odors Integrating odor to emotion