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Tousif, role of receptors


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Tousif, role of receptors

  1. 1. SENSORY PERCEPTION, ROLE OF RECEPTORS <ul><li>DR Syed Tousif Ahmed Professor Department of Physiology </li></ul>
  2. 2.                                                                                                                                                                          
  3. 3.                                                                                                                                                                          
  4. 4. Sequence of Events in a Receptor Stimulus Basic Function Amplification Integration, perception Receptor Protein Activated Enzyme Cascade (in some cases) Receptor Ion Channels opened (or closed) Receptor Current Receptor Potential Modulated Impulse Frequency in Second Order Neuron Reception Transduction Transmission Modulated Impulse Frequency in Receptor Cell Axon Modulated Transmitter Release from Receptor Cell
  5. 5. Sensory Receptors
  6. 6. Sensory Receptor Types
  7. 7. Which receptor?
  8. 8. Peripheral Sensory Receptors <ul><li>Sensory receptors also classified according to: </li></ul><ul><ul><li>Location </li></ul></ul><ul><ul><li>Type of stimulus detected </li></ul></ul><ul><ul><li>Structure </li></ul></ul>
  9. 9. <ul><li>Receptors are structurally classified as either simple or complex </li></ul><ul><li>Most receptors are simple and include encapsulated and unencapsulated varieties </li></ul><ul><li>Complex receptors are special sense organs </li></ul>Receptor Classification by Structural Complexity
  10. 10. Sensory Receptor Types
  11. 11. Somatic Senses <ul><li>Touch </li></ul><ul><li>Temperature </li></ul><ul><li>Pain </li></ul><ul><li>Itch </li></ul><ul><li>Proprioception </li></ul><ul><li>Pathway </li></ul>
  12. 12. Encapsulated Nerve Endings <ul><li>Meissner’s corpuscles </li></ul><ul><li>Pacinian corpuscles </li></ul><ul><li>Ruffini’s corpuscles </li></ul><ul><li>Proprioceptors </li></ul>
  13. 15. Receptor Potentials <ul><li>All sensory receptors have one feature in common. </li></ul><ul><li>Whatever the type of stimulus excites </li></ul><ul><li>Change in electrical potential of the receptor. </li></ul><ul><li>This change in potential is called a Receptor potential . </li></ul>
  14. 16. Receptor potentials : Changes in the transmembrane potential of a receptor caused by the stimulus. Generator Potential : A receptor potential that is strong enough (reaches threshold) to generate an action potential. Remember that APs are all-or-none . The stronger the sitmulus (above threshold) the more APs are fired over a given time period; this is translated by the CNS as a strong sensation. Receptor/Generator Potential
  15. 17. Pacinian Corpuscle <ul><li>Central nerve fiber extending through its core. </li></ul><ul><li>Surrounding – multiple concentric capsule layers </li></ul><ul><li>Compression anywhere on the outside of the corpuscle will </li></ul><ul><ul><li>Elongate, </li></ul></ul><ul><ul><li>Indent or </li></ul></ul><ul><ul><li>Deform the central fiber </li></ul></ul>
  16. 18. <ul><li>Central fiber of the pacinian corpuscle </li></ul><ul><li>The tip of the central fiber - unmyelinated </li></ul><ul><li>The fiber - Myelinated </li></ul><ul><li>Deformed by compression </li></ul><ul><li>Sodium influx - a local circuit of current flow </li></ul><ul><li>Node of Ranvier, typical action potentials transmitted </li></ul>
  17. 19. Mechanisms of Receptor Potentials. <ul><li>By mechanical deformation </li></ul><ul><ul><li>Stretches the receptor membrane </li></ul></ul><ul><ul><li>Opens ion channels </li></ul></ul><ul><li>By application of a chemical </li></ul><ul><ul><li>Opens ion channels </li></ul></ul><ul><li>By change of the temperature of the membrane </li></ul><ul><ul><li>Alters the permeability of the membrane </li></ul></ul><ul><li>By the effects of electromagnetic radiation, such as light </li></ul><ul><ul><li>Allows ions to move </li></ul></ul><ul><li>Basic cause of the change in membrane potential is a change in membrane permeability of the receptor </li></ul>
  18. 20. Maximum Receptor Potential Amplitude. <ul><li>The maximum amplitude of most sensory receptor potentials </li></ul><ul><ul><li>100 millivolts </li></ul></ul><ul><li>Change in voltage when the membrane - maximally permeable to sodium ions. </li></ul><ul><li>When the receptor potential rises above the threshold for eliciting action potentials </li></ul><ul><ul><li>Action potentials occur </li></ul></ul><ul><li>More the receptor potential rises above the threshold level - Greater - action potential frequency </li></ul>
  19. 21. Classification by Modality <ul><li>Mechanoreceptors – respond to mechanical forces </li></ul><ul><li>Thermoreceptors – respond to temperature changes </li></ul><ul><li>Chemoreceptors – respond to chemicals in solution </li></ul><ul><li>Photoreceptors – respond to light – located in the eye </li></ul><ul><li>Nociceptors – respond to harmful stimuli that result in pain </li></ul>
  20. 22. Touch (pressure) <ul><li>Mechanoreceptors </li></ul><ul><li>Free nerve endings </li></ul><ul><li>Pacinian corpuscles </li></ul><ul><li>Ruffini corpuscles </li></ul><ul><li>Merkel receptors </li></ul><ul><li>Meisaner's corpuscles </li></ul><ul><li>Barroreceptors </li></ul>
  21. 23. Temperature <ul><li>Free nerve endings </li></ul><ul><li>Cold receptors </li></ul><ul><li>Warm receptors </li></ul><ul><li>Pain receptors </li></ul><ul><li>Sensory coding: </li></ul><ul><ul><li>Intensity </li></ul></ul><ul><ul><li>Duration </li></ul></ul>
  22. 24. Pain and Itching <ul><li>Nociceptors </li></ul><ul><li>Reflexive path </li></ul><ul><li>Itch </li></ul><ul><li>Fast pain </li></ul><ul><li>Slow pain </li></ul>
  23. 25. Special Senses – External Stimuli <ul><li>Vision </li></ul><ul><li>Hearing </li></ul><ul><li>Taste </li></ul><ul><li>Smell </li></ul><ul><li>Equilibrium </li></ul>
  24. 26. Three Types of Proprioceptors <ul><ul><li>Monitor stretch in locomotory organs </li></ul></ul><ul><ul><li>Three types of proprioceptors </li></ul></ul><ul><li>Muscle spindles – measure the changing length of a muscle </li></ul><ul><ul><li>Imbedded in the perimysium between muscle fascicles </li></ul></ul><ul><li>Golgi tendon organs – located near the muscle-tendon junction </li></ul><ul><ul><li>Monitor tension within tendons </li></ul></ul><ul><li>Joint kinesthetic receptors </li></ul><ul><ul><li>Sensory nerve endings within the joint capsules </li></ul></ul>
  25. 27. Classification by Location <ul><li>Exteroceptors – sensitive to stimuli arising from outside the body </li></ul><ul><ul><li>Located at or near body surfaces </li></ul></ul><ul><ul><li>Include receptors for touch, pressure, pain, and temperature </li></ul></ul><ul><li>Interoceptors – (visceroceptors) receive stimuli from internal viscera </li></ul><ul><ul><li>Monitor a variety of stimuli </li></ul></ul><ul><li>Proprioceptors – monitor degree of stretch </li></ul><ul><ul><li>Located in musculoskeletal organs </li></ul></ul>
  26. 28. Receptor Class by Location: Exteroceptors <ul><li>Respond to stimuli arising outside the body </li></ul><ul><li>Found near the body surface </li></ul><ul><li>Sensitive to touch, pressure, pain, and temperature </li></ul><ul><li>Include the special sense organs </li></ul>
  27. 29. Receptor Class by Location: Interoceptors <ul><li>Respond to stimuli arising within the body </li></ul><ul><li>Found in internal viscera and blood vessels </li></ul><ul><li>Sensitive to chemical changes, stretch, and temperature changes </li></ul>
  28. 30. Receptor Class by Location: Proprioceptors <ul><li>Respond to degree of stretch of the organs they occupy </li></ul><ul><li>Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles </li></ul><ul><li>Constantly “advise” the brain of one’s movements </li></ul>
  29. 31. Simple Receptors: Unencapsulated <ul><li>Free dendritic nerve endings </li></ul><ul><ul><li>Respond chiefly to temperature and pain </li></ul></ul><ul><li>Abundant in epithelia and underlying connective tissue </li></ul><ul><li>Monitor affective senses </li></ul><ul><li>Two specialized types of free nerve endings </li></ul><ul><ul><li>Merkel (tactile) discs </li></ul></ul><ul><ul><li>Hair follicle receptors </li></ul></ul>
  30. 32. Simple Receptors: Encapsulated <ul><li>Encapsulated nerve endings: dendrites with special supporting structures </li></ul><ul><li>Consist of one or more end fibers of sensory neurons </li></ul><ul><li>Enclosed in connective tissue </li></ul><ul><ul><ul><li>Mechanoreceptors </li></ul></ul></ul><ul><ul><ul><li>Meissner’s corpuscles (tactile corpuscles) </li></ul></ul></ul><ul><ul><ul><li>Pacinian corpuscles (lamellated corpuscles) </li></ul></ul></ul><ul><ul><ul><li>Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles </li></ul></ul></ul><ul><ul><ul><li>Joint kinesthetic receptors </li></ul></ul></ul>
  31. 33. Transduction of Sensory Receptors <ul><li>Transduction: The process by which an environmental stimulus becomes encoded as a sequence of nerve impulses in an afferent nerve fiber is called sensory transduction </li></ul><ul><ul><li>Sense orgrans transduce sensory energy into neural (bioelectrical) energy </li></ul></ul><ul><ul><li>Converting one type of energy into another type is the process of transduction </li></ul></ul><ul><ul><li>Your brain only deals with bioelectrical impulses so transduction must occur; what cannot be transduced cannot be a stimulus </li></ul></ul>
  32. 34. Each type of receptor is highly sensitive to one type of stimulus for which it is designed and yet is almost nonresponsive to normal intensities of other type of stimuli. The stimulus to which a given receptor has the lowest threshold is termed the adequate stimulus of the sensory receptor. . Adequate Stimulus of Sensory Receptors
  33. 36. Processing at the Receptor Level <ul><li>The receptor must have specificity for the stimulus energy </li></ul><ul><li>The receptor’s receptive field must be stimulated </li></ul><ul><li>Stimulus energy must be converted into a graded potential </li></ul><ul><li>A generator potential in the associated sensory neuron must reach threshold </li></ul>
  35. 38. Labeled line principle <ul><li>Receptor </li></ul><ul><li>Stimulation </li></ul><ul><li>Impulse </li></ul><ul><li>Nerve </li></ul><ul><li>Brain </li></ul><ul><li>Touch Pain Temp </li></ul>
  39. 42. Sensory Transduction <ul><li>Sensory transduction converts stimuli into graded potentials. </li></ul><ul><li>Such changes in receptor membrane potential are known as </li></ul><ul><li>the receptor potential and the generator potential . </li></ul>
  40. 43. Relation Between Stimulus Intensity and the Receptor Potential . <ul><li>Very intense stimulation </li></ul><ul><li>Progressively less and less additional increase in numbers of action potentials </li></ul><ul><li>Allows the receptors to have an extreme range of response, </li></ul><ul><ul><li>From very weak </li></ul></ul><ul><ul><li>To very intense </li></ul></ul>
  41. 44. Mechanism of Receptor adaptation <ul><li>Readjustments - structure of the receptor </li></ul><ul><ul><li>Pacinian corpuscle is a viscoelastic structure </li></ul></ul><ul><ul><li>Distorting force - transmitted by the viscous component </li></ul></ul><ul><ul><li>Elicit a receptor potential. </li></ul></ul><ul><ul><li>Within a few 100ths of a second- fluid within the corpuscle redistributes </li></ul></ul><ul><li>Electrical type of accommodation </li></ul><ul><ul><li>Progressive “inactivation” of the sodium channels </li></ul></ul>
  42. 45. Adaptation of different types of receptors <ul><li>Some sensory receptors adapt to a far greater extent than others </li></ul><ul><li>Mechanoreceptors adapt almost completely </li></ul><ul><li>Nonmechanoreceptors </li></ul><ul><ul><li>Chemoreceptors </li></ul></ul><ul><ul><li>Pain receptors, </li></ul></ul><ul><ul><li>never adapt completely. </li></ul></ul>
  43. 46. Slowly Adapting Receptors <ul><li>Detect Continuous Stimulus Strength </li></ul><ul><li>The “Tonic” Receptors. </li></ul><ul><li>Continue to transmit impulses to the brain as long as the stimulus is present/hours. </li></ul><ul><li>Keep the brain constantly apprised of the status of the body and its relation to its surroundings. </li></ul><ul><ul><li>Golgi tendon apparatuses </li></ul></ul><ul><ul><li>Macula in the vestibular apparatus </li></ul></ul><ul><ul><li>Baroreceptors and chemoreceptors </li></ul></ul>
  44. 47. Rapidly Adapting Receptors - <ul><li>Detect Change in Stimulus Strength </li></ul><ul><li>The Rate / Movement / Phasic Receptors </li></ul><ul><li>Cannot be used to transmit a continuous signal </li></ul><ul><li>React strongly while a change is actually taking place . </li></ul><ul><li>Pacinian </li></ul><ul><ul><li>Sudden pressure applied to the tissue excites this receptor for a few milliseconds </li></ul></ul><ul><ul><li>Transmits a signal again when the pressure is released…. </li></ul></ul><ul><ul><li>Importance </li></ul></ul>
  45. 48. Sensory Adaptation is one form of Integration Phasic receptors quickly adapt. The frequency of action potentials diminishes or stops if the stimulus is unchanging. Tonic receptors adapt slowly or not at all. Most exteroreceptors (receptors that monitor the external environment) are phasic receptors .
  46. 49. Rapidly Adapting “ The Rate Receptors” <ul><li>Also called as “Movement Receptors” or “Phasic Receptors”. </li></ul><ul><li>Detect change in stimulus strength </li></ul><ul><li>They react strongly when a change is actually taking place there fore called as rate receptors </li></ul>
  47. 50. Slowly Adapting “The Tonic Receptors” <ul><li>Macula in the vestibular apparatus </li></ul><ul><li>Pain receptors </li></ul><ul><li>Baroreceptors </li></ul><ul><li>Chemoreceptors of the carotid and aortic bodies </li></ul>
  48. 51. Encoding of Sensory Receptor The quality of the stimulus is encoded in the frequency of the action potentials transmitted down the afferent fibre and the number of sensory receptors activated.
  49. 52. Stretch Receptors: Weak stretch causes low impulse frequency on neuron leaving receptor. Strong stretch causes high impulse frequency on neuron leaving receptor. Time Membrane potential Frequency Code
  50. 53. Tonic Receptors vs. Phasic Receptors <ul><li>Slow or no adaptation </li></ul><ul><li>Continuous signal transmission for duration of stimulus </li></ul><ul><li>Monitoring of parameters that must be continually evaluated, e.g.: barorecptors __________ ? </li></ul><ul><li>Rapid adaptation </li></ul><ul><li>Cease firing if strength of a continuous stimulus remains constant </li></ul><ul><li>Allow body to ignore constant unimportant information, e.g.: ___________ ? </li></ul>
  51. 54. Adaptation of Receptors <ul><li>When a continuous sensory stimulus is applied , the receptor responds at a high impulse rate at first and then at a progressively slower rate until finally the rate of action potential decreases to very few or often to none at all </li></ul><ul><li>Some sensory receptors adapt to a far greater extent than others </li></ul><ul><li>While other sensory receptors adapt in hours or days to do so and are called non-adapting receptors </li></ul>
  52. 55. Receptive Field <ul><li>Is area of skin whose stimulation results in changes in firing rate of sensory neuron </li></ul><ul><ul><li>Area varies inversely with density of receptors </li></ul></ul><ul><ul><li>E.g. back, legs have low density of sensory receptors </li></ul></ul><ul><ul><ul><li>Receptive fields are large </li></ul></ul></ul><ul><ul><li>Fingertips have high density of receptors </li></ul></ul><ul><ul><ul><li>Receptive fields are small </li></ul></ul></ul>10-22
  53. 56. Two-Point Touch Threshold <ul><li>Is minimum distance at which 2 points of touch can be perceived as separate </li></ul><ul><ul><li>Measure of tactile acuity or distance between receptive fields </li></ul></ul>
  54. 57. 10-25
  55. 58. THE END