15 The Special Senses

1. 15 The Special Senses

2. Section 1: Olfaction and Gustation Learning Outcomes 15.1 Describe the sensory organs of smell, trace the olfactory pathways to their destinations in the cerebrum, and explain how olfactory perception occurs. 15.2 Describe the sensory organs of gustation. 15.3 Describe gustatory reception, briefly describe the physiologic processes involved in taste, and trace the gustatory pathway.

3. Section 1: Olfaction and Gustation Special senses introduction Special sense organs provide us with information about external environment Two types of receptors used Dendrites of specialized neurons Bind chemicals producing a depolarization of the cell or generator potential Example: olfactory (smell) receptors Specialized receptors that synapse with sensory neurons Stimulated receptor releases chemical transmitters that depolarize sensory neuron (generator potential) Small delay due to synapse Examples: vision, hearing, taste, equilibrium

4. Figure 15 Section 1 1 The function of olfactory receptors Stimulus removed Action potentials Stimulus Dendrites Threshold Generator potential Stimulus to CNS Specialized olfactory neuron

5. Figure 15 Section 1 2 The function of receptors for the senses of taste, vision, equilibrium, and hearing Receptor cell Stimulus removed Stimulus Threshold Receptor depolarization Axon Action potentials Stimulus Synaptic delay Stimulus to CNS Synapse Axon of sensory neuron Receptor cell Generator potential

6. Module 15.1: Olfaction Olfaction Provided by olfactory organs Located in nasal cavity, either side of nasal septum Cover: Inferior surface of cribiform plate Superior portion of perpendicular plate Superior nasal conchae of ethmoid

7. Module 15.1: Olfaction Olfactory pathway Sensory neurons in olfactory organ stimulated by chemicals Olfactory epithelium axons collect into 20 or more bundles penetrating cribiform plate of ethmoid bone Synapse with olfactory bulb Axons leaving bulb travel along olfactory tract to olfactory cortex, hypothalamus, and portions of limbic system Explains why smells can produce profound emotional and behavioral responses

8. Figure 15.1 1 Olfactory Pathway to the Cerebrum The sensory neurons within the olfactory organ are stimulated by chemicals in the air. Axons leaving the olfactory epithelium collect into 20 or more bundles that penetrate the cribriform plate of the ethmoid. The first synapse occurs in the olfactory bulb, which is located just superior to the cribriform plate. Axons leaving the olfactory bulb travel along the olfactory tract to reach the olfactory cortex, the hypothalamus, and portions of the limbic system. The distribution of olfactory information to the limbic system and hypothalamus explains the profound emotional and behavioral responses, as well as the memories, that can be triggered by certain smells. Cribriform plate of ethmoid Olfactory organ Olfactory epithelium Superior nasal concha

9. Module 15.1: Olfaction Olfactory organ composition Two layers Olfactory epithelium Olfactory receptor cells Each cell produces knob (base of 20 cilia) 10–20 million receptors in 5 cm2 area Supporting cells Basal (stem) cells Replace worn-out receptors One of the few examples of neuronal replacement Lamina propria Contains olfactory glands that produce mucus

10. Figure 15.1 2 A portion of an olfactory organ, which consists of the olfactory epithelium and the lamina propria Olfactory (Bowman) gland To olfactory bulb Olfactory nerve fibers Lamina propria Basal cell: divides to replace worn-out olfactory receptor cells Developing olfactory receptor cell Olfactory receptor cell Olfactory epithelium Supporting cell Mucous layer Knob Olfactory cilia: surfaces contain receptor proteins

11. Module 15.1: Olfaction Steps of olfactory reception Binding of odorant (dissolved chemical) to receptor protein Activates adenylyl cyclase (enzyme converting ATP to cAMP) cAMP opens sodium channels, depolarizing membrane With sufficient depolarization, an action potential may be generated and relayed to CNS

12. Module 15.1: Olfaction Odorants Generally small organic molecules Strongest smells associated with molecules with either high water or lipid solubilities As few as four odorant molecules can activate receptor

13. Figure 15.1 3 Step 3: If sufficient depolarization occurs, an action potential is triggered in the axon, and the information is relayed to the CNS. Step 1: The binding of an odorant to its receptor protein leads to the activation of adenylyl cyclase, the enzyme that converts ATP to cyclic-AMP (cAMP). Step 2: The cAMP then opens sodium channels in the plasma membrane, which, as a result, begins to depolarize. RECEPTOR CELL Sodium ions enter Active enzyme Inactive enzyme Depolarized membrane Closed sodium channel Odorant molecule MUCOUS LAYER The process of olfactory reception on the surface membranes of the olfactory cilia

14. Module 15.1 Review a. Describe olfaction. b. Which neurons associated with olfaction are capable of regenerating? c. Trace the olfactory pathway, beginning at the olfactory epithelium.

15. Module 15.2: Gustation Gustation or taste provides information about consumed food and liquids Taste (gustatory) receptors Found mainly on superior surface of tongue within taste buds Also some located in pharynx and larynx but decrease in importance and abundance with age

16. Module 15.2: Gustation Taste bud structure Gustatory cells Each has slender microvilli into surrounding fluids through narrow opening (taste pore) of taste bud Each only survives ~10 days Approximately 40–100 receptor cells/bud Basal cells Stem cells that divide and mature to produce more gustatory cells

17. Figure 15.2 3 – 4 Transitional cell Taste hairs (microvilli) The structure of taste buds Gustatory cell Basal cell Taste pore Diagrammatic view of a taste bud Taste buds Taste bud LM x 650 Taste buds LM x 280

18. Module 15.2: Gustation Taste bud location Recessed along epithelium lining tongue projections (lingual papillae; papilla, nipple-shaped mound) Papillae types Circumvallate (circum-, around + vallate, wall) papillae Large with deep folds containing ~100 taste buds Located in V-shape on tongue posterior Fungiform (fungus, mushroom) papillae Shaped like small buttons with shallow depressions Each contains ~5 taste buds Filiform (filum, thread) papillae Provide friction but contain no taste buds

19. Figure 15.2 1 – 2 Circumvallate Papillae Are relatively large and are surrounded by deep epithelial folds; each contains as many as 100 taste buds The lingual papillae on the superior surface of the tongue Water receptors (pharynx) Taste buds Umami Fungiform Papillae Circumvallate papillae Sour Bitter Salty Sweet Contain about five taste buds each Filiform Papillae Provide friction that helps the tongue move objects around in the mouth but do not contain taste buds

20. Module 15.2: Gustation Taste sensations Four primary sensations: sweet, salty, sour, and bitter Found in taste buds all over tongue Two other sensations Umami Meaty or savory Receptor binds amino acids Discovered in Japan Water receptors Demonstrated in human pharynx Information sent to hypothalamus to manage thirst

21. Module 15.2: Gustation Taste receptor sensitivity More sensitive to unpleasant stimuli 100,000× more sensitive to bitter, 1000× more sensitive to sour (acids) compared to sweet and salty May have survival value Toxic compounds are often bitter Acids can create chemical burns Overall sensitivity declines with age Number of taste receptors declines Number of olfactory receptors declines

22. Module 15.2 Review a. Define gustation. b. Describe filiform papillae. c. Relate the adaptive sensitivity of taste receptors for bitter and sour sensations, to sweet and salty sensations.

23. Module 15.3: Gustatory receptors and pathways Mechanism of gustatory reception Two types Chemically gated ion channels whose stimulation produces depolarization of the cell and release of neurotransmitters Salt and sour receptors Taste receptor activates G-proteins (gustducins) that activate 2nd messenger system to release neurotransmitters Sweet, bitter, and umami receptors

24. Figure 15.3 1 The mechanisms involved in gustatory reception Sweet, Bitter, and Umami Receptors Salt and Sour Receptors Salt receptors and sour receptors are chemically gated ion channels whose stimulation produces depolarization of the cell. Receptors responding to stimuli that produce sweet, bitter, and umami sensations are linked to G proteins called gustducins (GUST-doos-inz)—protein complexes that use second messengers to produce their effects. Receptor cells Sweet, bitter, or umami Sour, salt Membrane receptor Gated ion channel Resting plasma membrane Inactive G protein Active G protein Channel opens Plasma membrane depolarizing Plasma membrane depolarizing Active G protein Active 2nd messenger Inactive 2nd messenger Depolarization of membrane stimulates release of chemical neurotransmitters. Activation of second messengers stimulates release of chemical neurotransmitters.

25. Module 15.3: Gustatory receptors and pathways Gustatory information is relayed to the cerebral cortex along three different cranial nerves dependent on the location of the receptor Facial nerve (VII) – anterior 2/3 of tongue to line of circumvallate papillae Glossopharyngeal nerve (IX) – circumvallate papillae and posterior 1/3 of tongue Vagus nerve (X) – surface of epiglottis

26. Module 15.3: Gustatory receptors and pathways Gustatory pathway Receptors respond to stimulation Relay information to appropriate cranial nerve Sensory afferents synapse in solitary nucleus of medulla oblongata Postsynaptic neuron axons cross over at medial lemniscus with other somatic sensory information and relay to thalamus After synapse in thalamus, impulse is routed to appropriate area of primary sensory cortex

27. Figure 15.3 2 The components of the gustatory pathway After another synapse in the thalamus, the information is projected to the appropriate portions of the gustatory cortex of the insula. The axons of the postsynaptic neurons cross over and enter the medial lemniscus of the medulla oblongata. Cranial Nerves Carrying Gustatory Information The sensory afferents carried by these three cranial nerves synapse in the solitary nucleus of the medulla oblongata. The facial nerve (VII) innervates all the taste buds located on the anterior two-thirds of the tongue, from the tip to the line of circumvallate papillae. The glossopharyngeal nerve (IX) innervates the circumvallate papillae and the posterior one-third of the tongue. The vagus nerve (X) innervates taste buds scattered on the surface of the epiglottis. Start Receptors respond to stimulation.

28. Module 15.3: Gustatory receptors and pathways Central processing of gustatory sensations Conscious perception of taste occurs at the primary sensory cortex Taste sensation is analyzed with taste-related sensations “Peppery” or “burning hot” from afferents in trigeminal nerve (V) Olfactory stimulation significantly contributes to taste perception Central adaptation quickly reduces sensitivity to new tastes

29. Module 15.3 Review a. What are gustducins? b. Identify the cranial nerves that carry gustatory information. c. Trace the gustatory pathway from the taste receptors to the cerebral cortex.

30. Section 2: Equilibrium and Hearing Learning Outcomes 15.4 Describe the structures of the external, middle, and inner ear, and explain how they function. 15.5 Describe the structures and functions of the bony labyrinth and membranous labyrinth. 15.6 Describe the functions of hair cells in the semicircular ducts, utricle, and saccule.

31. Section 2: Equilibrium and Hearing Learning Outcomes 15.7 Describe the structure and functions of the organ of Corti. 15.8 Explain the anatomical and physiological basis for pitch and volume sensations for hearing. 15.9 Trace the pathways for the sensations of equilibrium and hearing to their respective destinations in the brain.

32. Section 2: Equilibrium and Hearing Equilibrium and Hearing Chemoreceptors compared to mechanoreceptors Olfactory and gustatory receptors are located in epithelia exposed to the external environment Olfactory receptors are modified neurons Gustatory receptors communicate with sensory neurons Equilibrium and hearing receptors are isolated and protected from external environment Located in inner ear Information is integrated and organized locally before forwarding to CNS

33. Figure 15 Section 2 1 Sensory receptors that are located within epithelia exposed to the external environment Gustatory receptor Olfactory receptor

34. Section 2: Equilibrium and Hearing Hair cell receptors of the inner ear Free surfaces covered with specialized processes 80–100 stereocilia (like long microvilli) May contain single large kinocilium Hair cells are mechanoreceptors that are not actively moved External forces push against processes causing distortion of cell membrane and neurotransmitter release Provide information about direction and strength of mechanical stimuli Complex inner ear structure determines what stimuli can reach different hair cells

35. Figure 15 Section 2 2 - 3 Inner ear Location of the receptors for equilibrium and hearing Displacement in this direction stimulates hair cell Displacement in this direction inhibits hair cell Stereocilia Kinocilium Receptors for equilibrium and hearing, which are isolated and protected from the external environment Hair cell Dendrite of sensory neuron Supporting cell A hair cell, the receptor located in the inner ear

36. Module 15.4: Ear regions and structures Three anatomical regions of the ear External ear – visible portion that collects and directs sound waves toward middle ear Auricle External acoustic meatus (passageway in temporal bone) Lined with Ceruminous glands (secrete waxy cerumen) Hairs Has some protection against entering foreign objects, insects, and bacteria

37. Figure 15.4 1 The ear’s three anatomical regions: the external ear, the middle ear, and the inner ear Middle Ear External Ear Inner Ear The visible portion of the ear; collects and directs sound waves toward the middle ear Site of sensory organs for hearing and equilibrium; receives amplified sound waves from the middle ear An air-filled chamber; is connected to the nasopharynx by the auditory tube Elastic cartilages Auditory ossicles Semicircular canals Petrous part of temporal bone Auricle Facial nerve (VII) Vestibulocochlear nerve (VIII) Bony labyrinth Tympanic cavity To nasopharynx Tympanic membrane (tympanum or eardrum) Auditory tube (pharyngotympanic tube or Eustachian tube) External acoustic meatus

38. Module 15.4: Ear regions and structures Three anatomical regions of the ear (continued) Middle ear Tympanic membrane (also tympanum or eardrum) Border between external and middle ear Thin, transparent sheet Auditory ossicles (3) Bones that connect tympanic membrane to receptor complexes of inner ear (smallest bones and synovial joints of body) Malleus (attached at three points to tympanum) Incus (middle ossicle) Stapes (bound to oval window of cochlea)

39. Module 15.4: Ear regions and structures Three anatomical regions of the ear (continued) Middle ear (continued) Auditory tube (pharyngotympanic or Eustachian tube) Connects middle ear to pharynx to equalize pressure on either side of tympanic membrane Also can lead to bacterial infection (otitis media) Middle ear muscles Tensor tympani muscle Connects to malleus and can dampen tympanic membrane vibrations Stapedius muscle (smallest muscle in body) Connects to stapes and reduces its movement at oval window

40. Figure 15.4 2 The structures of the middle ear Auditory Ossicles Stapes Malleus Incus Temporal bone (petrous part) Stabilizing ligament Oval window Branch of facial nerve VII (cut) Muscles of the Middle Ear Tensor tympani muscle External acoustic meatus Stapedius muscle Tympanic cavity (middle ear) Auditory tube Round window Tympanic membrane

41. Module 15.4: Ear regions and structures Sound impulse pathway Sound waves vibrate tympanic membrane Tympanic membrane and ossicles amplify and conduct vibrations to oval window of inner ear Vibrations can be dampened by actions of middle ear muscles Animation: The Ear: Balance Animation: The Ear: Ear Anatomy

42. Module 15.4 Review a. Name the three tiny bones located in the middle ear. b. What is the function of the auditory tube? c. Why are external ear infections relatively uncommon?

43. Module 15.5: Labyrinths of the inner ear Bony labyrinth Shell of dense bone Surrounds and protects membranous labyrinth Filled with perilymph (similar to CSF) Consists of three parts Semicircular canals Vestibule Cochlea

44. Module 15.5: Labyrinths of the inner ear Membranous labyrinth Collection of fluid-filled tubes and chambers Houses receptors for hearing and equilibrium Filled with endolymph Receptors only function when exposed to unique ionic composition

45. Module 15.5: Labyrinths of the inner ear Membranous labyrinth (continued) Consists of three parts Semicircular ducts (within semicircular canals) Receptors stimulated by rotation of head Utricle and saccule (within vestibule) Provide sensations of gravity and linear acceleration Cochlear duct (within cochlea) Sandwiched between pair of perilymph-filled chambers Resembles snail shell Receptors stimulated by sound

46. Figure 15.5 1 – 2 The structures of the inner ear Bony Labyrinth Surrounds and protects the membranous labyrinth and contains a fluid called perilymph Semicircular canals Vestibule Cochlea Receptor areas Membranous Labyrinth Houses the receptors for equilibrium and hearing and contains a fluid called endolymph Semicircular duct Utricle and saccule Cochlear duct Bony labyrinth Perilymph Membranous labyrinth KEY Endolymph Membranous labyrinth Bony labyrinth A cross section of a semicircular canal

47. Module 15.5 Review a. Identify the components of the bony labyrinth. b. Describe hair cells. c. Explain the regional differences among the receptor complexes in the membranous labyrinth.

48. Module 15.6: Receptors for equilibrium Receptors for equilibrium Semicircular ducts Structure Three ducts continuous with utricle and filled with endolymph Anterior Posterior Lateral Each contains an enlarged region (ampulla) with area (crista) housing receptors Hair cells with kinocilia and stereocilia embedded within gelatinous matrix

49. Figure 15.6 1 The location and structure of an ampulla, which contains receptors that respond to rotation The location of the ampullae within the inner ear Semicircular Ducts Ampulla Anterior Posterior Lateral Utricle Cupula Ampulla filled with endolymph Hair cells Crista Supporting cells Sensory nerve The structure of an ampulla

50. Module 15.6: Receptors for equilibrium Semicircular ducts Function Head rotating in plane of one duct causes endolymph movement and cupula bends, causing distortion of hair cells Movement one way causes stimulation Opposite movement causes inhibition Horizontal rotation (“no”) stimulates lateral duct receptors Nodding (“yes”) stimulates anterior duct receptors Tilting head stimulates posterior duct receptors