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Chapter 16 .

Chapter 16. Sense Organs. Sensory receptors properties and types General senses Chemical senses Hearing and equilibrium Vision. Properties of Receptors. Sensory transduction convert stimulus energy into nerve energy Receptor potential local electrical change in receptor cell

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Chapter 16 .

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  1. Chapter 16 .

  2. Sense Organs • Sensory receptors • properties and types • General senses • Chemical senses • Hearing and equilibrium • Vision

  3. Properties of Receptors • Sensory transduction • convert stimulus energy into nerve energy • Receptor potential • local electrical change in receptor cell • Adaptation • conscious sensation declines with continued stimulation

  4. Receptors Transmit Information • Modality - type of stimulus • Location • each sensory receptor receives input from its receptive field • sensory projection - brain identifies site of stimulation • Intensity • frequency, number of fibers and which fibers • Duration - change in firing frequency over time • phasic receptor - burst of activity and quickly adapt (smell and hair receptors) • tonic receptor - adapt slowly, generate impulses continually (proprioceptor)

  5. Receptive Fields

  6. Classification of Receptors • By modality: • chemo-, thermo-, mechano-, photo- receptors and nociceptors • By origin of stimuli • interoceptors - detect internal stimuli • proprioceptors - sense body position and movements • exteroceptors - detect external stimuli • By distribution • general senses - widely distributed • special senses - limited to head

  7. Unencapsulated Nerve Endings • Dendrites not wrapped in connective tissue • General sense receptors • for pain and temperature • Tactile discs • associated with cells at base of epidermis • Hair receptors • monitor movement of hair

  8. Encapsulated Nerve Endings • Dendrites wrapped by glial cells or connective tissue • tactile corpuscles - phasic • light touch and texture • krause end bulb - phasic • tactile; in mucous membranes • lamellated corpuscles - phasic • deep pressure, stretch, tickle and vibration • ruffini corpuscles - tonic • heavy touch, pressure, joint movements and skin stretching

  9. Somesthetic Projection Pathways • 1st order neuron (afferent neuron) • from body, enter the dorsal horn of spinal cord via spinal nerves • from head, enter pons and medulla via cranial nerve • touch, pressure and proprioception on large, fast, myelinated axons • heat and cold on small, unmyelinated, slow fibers • 2nd order neuron • decussation to opposite side in spinal cord or medulla/pons • end in thalamus, except for proprioception (cerebellum) • 3rd order neuron • thalamus to primary somesthetic cortex of cerebrum

  10. Pain • Nociceptors – allow awareness of tissue injuries • found in all tissues except the brain • Fast pain travels in myelinated fibers at 30 m/sec • sharp, localized, stabbing pain perceived with injury • Slow pain travels unmyelinated fibers at 2 m/sec • longer-lasting, dull, diffuse feeling • Somatic pain from skin, muscles and joints • Visceral pain from stretch, chemical irritants or ischemia of viscera (poorly localized) • Injured tissues release chemicals that stimulate pain fibers (bradykinin, histamine, prostaglandin)

  11. Projection Pathway for Pain • General pathway – conscious pain • 1st order neuron cell bodies in dorsal root ganglion of spinal nerves or cranial nerves V, VII, IX, and X • 2nd order neurons decussate and send fibers up spinothalamic tract or through medulla to thalamus • gracile fasciculus carries visceral pain signals • 3rd order neurons from thalamus reach primary somesthetic cortex as sensory homunculus • Spinoreticular tract • pain signals reach reticular formation, hypothalamus and limbic • trigger visceral, emotional, and behavioral reactions

  12. Pain Signal Destinations

  13. Referred Pain • Misinterpreted pain • brain “assumes” visceral pain is coming from skin • heart pain felt in shoulder or arm because both send pain input to spinal cord segments T1 to T5

  14. Referred Pain

  15. CNS Modulation of Pain • Intensity of pain - affected by state of mind • Endogenous opiods (enkephalins, endorphins and dynorphins) • produced by CNS and other organs under stress • in dorsal horn of spinal cord (spinal gating) • act as neuromodulators block transmission of pain

  16. Spinal Gating • Stops pain signals at dorsal horn • descending analgesic fibers from reticular formation travel down reticulospinal tract to dorsal horn • secrete inhibitory substances that block pain fibers from secreting substance P • pain signals never ascend • dorsal horn fibers inhibited by input from mechanoreceptors • rubbing a sore arm reduces pain

  17. Spinal Gating of Pain Signals

  18. Chemical Sense - Taste • Gustation - sensation of taste • results from action of chemicals on taste buds • Lingual papillae • filiform (no taste buds) • important for texture • foliate (no taste buds) • fungiform • at tips and sides of tongue • vallate (circumvallate) • at rear of tongue • contains 1/2 of taste buds

  19. Taste Bud Structure • Taste cells • apical microvilli serve as receptor surface • synapse with sensory nerve fibers at their base • Supporting cells • Basal cells

  20. Physiology of Taste • Molecules must dissolve in saliva • 5 primary sensations - throughout tongue • Sweet - concentrated on tip • Salty - lateral margins • Sour - lateral margins • Bitter - posterior • Umami - taste of amino acids (MSG) • Influenced by food texture, aroma, temperature, and appearance • mouthfeel - detected by lingual nerve in papillae • Hot pepper stimulates free nerve endings (pain)

  21. Physiology of Taste • Mechanisms of action • activate 2nd messenger systems • sugars, alkaloids and glutamates bind to receptors • depolarize cells directly • sodium and acids penetrate cells

  22. Projection Pathways for Taste • Innervation of taste buds • facial nerve (VII) - anterior 2/3’s of tongue • glossopharyngeal nerve (IX) - posterior 1/3 • vagus nerve (X) - palate, pharynx, epiglottis

  23. Projection Pathways for Taste • To solitary nucleus in medulla • To hypothalamus and amygdala • activate autonomic reflexes • e.g. salivation, gagging and vomiting • To thalamus, then postcentral gyrus of cerebrum • conscious sense of taste

  24. Chemical Sense - Smell • Olfactory mucosa • contains receptor cells for olfaction • highly sensitive • up to 10,000 odors • on 5cm2 of superior concha and nasal septum

  25. Olfactory Epithelial Cells • Olfactory cells • olfactory hairs neurons with 20 cilia • bind odor molecules in thin layer of mucus • axons pass through cribriform plate • survive 60 days • Supporting cells • Basal cells • divide

  26. Physiology of Smell • Molecules bind to receptor on olfactory hair • hydrophilic - diffuse through mucus • hydrophobic - transport by odorant-binding protein • Activate G protein and cAMP system • Opens ion channels for Na+ or Ca2+ • creates a receptor potential • Action potential travels to brain • Receptors adapt quickly • due to synaptic inhibition in olfactory bulbs

  27. Olfactory Pathway • Olfactory cells synapse in olfactory bulb • on mitral and tufted cell dendrites • in spherical clusters called glomeruli • each glomeruli dedicated to single odor

  28. Olfactory Pathway • Output from bulb forms olfactory tracts • end in primary olfactory cortex and thalamus • travel to insula and frontal cortex • identify odors • integrate taste and smell into flavor • travel to hypocampus, amygdala, and hypothalamus • memories, emotional and visceral reactions

  29. Olfactory Pathway • Feedback • granule cells in olfactory cortex synapse in glomeruli • food smells better when hungry

  30. Olfactory Projection Pathways

  31. The Nature of Sound • Sound - audible vibration of molecules • vibrating object pushes air molecules

  32. Pitch and Loudness • Pitch - frequency vibrates specific parts of ear • hearing range is 20 (low pitch) - 20,000 Hz (cycles/sec) • speech is 1500-4000 where hearing is most sensitive • Loudness – amplitude; intensity of sound energy

  33. Outer Ear

  34. Outer Ear • Fleshy auricle (pinna) directs air vibrations down external auditory meatus • cartilagenous and bony, S-shaped tunnel ending at eardrum • glandular secretions and dead cells form cerumen (earwax)

  35. Anatomy of Middle Ear

  36. Middle Ear • Air-filled tympanic cavity in temporal bone between tympanic membrane and oval window • continuous with mastoid air cells • Contains • auditory tube (eustachian tube) connects to nasopharynx • equalizes air pressure on tympanic membrane • ear ossicles • malleus • incus • stapes • stapedius and tensor tympani muscles attach to stapes and malleus

  37. Anatomy of Inner Ear

  38. Inner Ear • Bony labyrinth - passageways in temporal bone • Membranous labyrinth - fleshy tubes lining bony tunnels • filled with endolymph (similar to intracellular fluid) • floating in perilymph (similar to cerebrospinal fluid)

  39. Details of Inner Ear Fig. 16.12c

  40. Details of Inner Ear

  41. Anatomy of Cochlea • Scala media (cochlear duct) • separated from • scala vestibuli by vestibular membrane • scala tympani by basilar membrane • Spiral organ (organ of corti)

  42. Spiral Organ

  43. Spiral Organ • Stereocilia of hair cells attach to gelatinous tectorial membrane • Inner hair cells • hearing • Outer hair cells • adjust cochlear responses to different frequencies • increase precision

  44. SEM of Cochlear Hair Cells

  45. Physiology of Hearing - Middle Ear • Tympanic membrane • has 18 times area of oval window • ossicles transmit enough force/unit area at oval window to vibrate endolymph in scala vestibuli • Tympanic reflex – muscle contraction • tensor tympani m. tenses tympanic membrane • stapedius m. reduces mobility of stapes • best response to slowly building loud sounds • occurs while speaking

  46. Stimulation of Cochlear Hair Cells • Vibration of ossicles causes vibration of basilar membrane under hair cells • as often as 20,000 times/second

  47. Cochlear Hair Cells • Stereocilia of OHCs • bathed in high K+ • creating electrochemical gradient • tips embedded in tectorial membrane • bend in response to movement of basilar membrane • pulls on tip links and opens ion channels • K+ flows in – depolarization causes release of neurotransmitter • stimulates sensory dendrites at base

  48. Potassium Gates

  49. Sensory Coding • Vigorous vibrations excite more inner hair cells over a larger area • triggers higher frequency of action potentials • brain interprets this as louder sound • Pitch depends on which part of basilar membrane vibrates • at basal end, membrane narrow and stiff • brain interprets signals as high-pitched • at distal end, 5 times wider and more flexible • brain interprets signals as low-pitched

  50. Basilar Membrane Frequency Response Notice high and low frequency ends

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