Human Anatomy & Physiology

1. Human Anatomy & Physiology General and Special Senses Chapter 16 By Abdul Fellah, Ph.D.

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