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  • 3 parts of eyeball = Eye wall Humors Lens Humors = 2 types  aqueous and vitreous aqueous humor --> filtered from blood capillaries in the ciliary processes. Aqueous humor is drained by the canal of schlemm Function: provides nutrients and oxygen to the avascular cornea and avascular lensremoves metabolic wastes from the cornea and the lens also maintains the intraocular pressure (to prevent formation of glaucoma *aqueous humor is located in the anterior segment between the cornea and the lensvitreous humor is the thick fluid located in the posterior segment of the eye --> behind the lens and in front of the eye wall Vitreous humor  is formed during embyonic development and it lasts a lifetime Function: 1) vitreous humor supports the back of the lens 2) pushes the neural layer firmly against the pigmented layer to prevent retinal detachment *cells in pigmented layer provide nutrients to the photoreceptors in the neural layer
  • 3 rd part of the eye  the lens it is a biconvex, flexible, avascular, transparent structure held in an upright position by the suspensory ligaments composed of precisely folded clear proteins called CRYSTALINS When the cystallin proteins form climps due to lack of nutriends and oxygen  cataracts develop Function: lens refracts (or bends) light to focus on the photoreceptors in the retina (neural layer) =========================== Far distances  objects viewed will not require ACCOMODATION of the lens  accomodation of the lens refers to changes that lens undergoes to focus images on the retina --------------------------- Near objects  less than 20 ft  require accomodation of the lens  ciliary muscles in the ciliary body contract  causes the lens to bulge anteriorly and refract the image onto the retina Next slide for more info
  • Different types of lenses diverge/converge light before you see it to fix vision Problems with refraction of light by the lens Myopic Eye (nearsighted) eyeball too long, images focused in front of the retina (contains the photoreceptors) for correction  CONCAVE lens  diverges the image to focus on the retina Hyperopic eye (farsighted) *the eye can focus clearly on distant objects eyeball too short, so in close vision, images focused behind the retina for correction  CONVEX lens  converges the image onto the retina
  • Image processing and the route taken by light through the eye THIS IS IMPORTANT Cornea  Aqueous Humor  Pupil  Lens  Vitreous Humor  ganglion cells (light passes)  bipolar cells (neurons) (light passes)  photoreceptors (respond to light energy by producing electrical signals Electrical signals travel to  bipolar cells (neurons)  ganglion cells are the only cells in the eye that generate/transmit action potentials via their axons  axons bundle to form the OPTIC NERVE (Cranial Nerve II)
  • Optic nerves (one from each eye) cross over before entering the brain The point where the optic nerves cross over  OPTIC CHIASMA After the optic chiasma  OPTIC TRACTS as they are in the brain Because the crossing over, images from the left eye are received from the primary visual cortex located in the right occipital lobe and vice versa Information from the optic tracts (action potentials) are sent to the superior colliculi in the midbrain  visual reflex centers Visual information is relayed in the LGN (Lateral Geniculate Nucleus) in the thalamus  visual relay center From the LGN, the information is sent to the primary visual cortex for interpretation The visual association center  adds detail to the image Slide 34
  • EAR  3 regions
  • EAR  in search of the mechanoreceptor  HAIR CELLS 3 regions - External, Middle, and Inner External ear  consists of 2 parts  auricle (pinna) and the external auditory canal (external acoustic meatus) Auricle composed of elastic cartilage covered with skin function of the auricles (pinnae)  direct sound waves into the external auditory canal
  • EAR  3 regions  External, Middle, Internal External  Auricle = pinna and External auditory canal Auricle  composed of elastic cartilage External auditory canal  short canal carved in the temporal bone External auditory canal is lined with skin  sebaceous glands and specialized sweat glands called ceruminous glands  secrete Cerumen = earwax Cerumen keeps the tympanic membrane soft and supple (malleable) Cerumen also prevents the entry of foreign substances such as water or insects The tympanic membrane  cone-shaped flexible membrane that separates the external auditory canal from the middle ear ==========================
  • Middle Ear = Tympanic cavity air filled and contains/houses the smallest bones in the body called the auditory ossicles three auditory ossicles malleus - abuts tympanic membrane incus – located between the malleus and the stapes stapes – sits atop an oval window The middle ear is separated from the inner (=internal) ear by a bony wall with two openings The two openings  round window and the oval window The stapes sits on top of the oval window (the superior opening) Extending from the middle ear is the pharyngotympanic tube, which equalizes the air pressure in the middle ear with the air pressure in the external auditory canal (=atmospheric pressure) The air pressure in front of the tympanic membrane must be equal to the air pressure behind it to get optimum vibration of the tympanic membrane which sets off a series of events which eventually will be interpreted as hearing ======================== Inner/Internal Ear  Labyrinth Composed of 2 parts  bony and the membranous labyrinth Bony Labyrinth (3 parts)  carved inside the temporal bone and consists of meandering channels vestibule semicircular canals cochlea All 3 parts of the bony labyrinth contain a fluid called PERILYMPH = CSF Membranous Labyrinth – consists of a continuous series of ducts and sacs. These ducts and sacs contain the fluid called ENDOLYMPH (similar to intracellular fluid… high in K+) Hence, the membranous labyrinth structures (ducts and sacs) are surrounded by perilymph in the bony labyrinth. These same structures of the membranous labyrinth (ducts and sacs) contain endolymph ------------------------- Vestibule = bony labyrinth structure houses 2 membranous sacs called SACCULE and UTRICLE  contain equiilibrim receptors called MACULAE  used for static equilibrium for positioning of the head in space  the maculae is involved in posture Semicircular canals  bony labyrinth arranged in 3 planes of space = 3 semicircular canals: anterior semicircular canal posterior “ “ lateral “ “ Meandering through the semicircular canals are the membranous ducts called semicircular ducts  end in expanded structures called AMPULLAE (singular = ampulla)  house equilibrium structures called the CRISTAE AMPULLARES (crysta ampullares  singular) Cristae ampullares are used for rotational movements of the head; act to maintain the position of the head in space after rotational movements (think of a ballerina) ------------------- Cochlea  bony labrinth makes a 2 and a half turn around a bony pillar called the MODIOLUS Inside the cochlea is the membranous duct called the COCHLEAR DUCT  houses the spiral organ of Corti  rests on a flexible membrane called the BASILAR MEMBRANE The spiral organ of Corti is composed of supporting cells and the HAIR CELLS = MECHANORECEPTORS
  • Hair cells have stereocilia  microvilli stiffened by actin The stereocilia are trapped in a gel-like membrane called the TECTORIAL MEMBRANE Wrapped around the bases of the hair cells are the afferent fibers of the cochlear nerve The cochlear nerve is a division of the vestibulocochlear nerve = CN VIII
  • Route taken by sound through the ear Auricles (pinnae) direct sound waves into the external auditory canal Travels through the canal to the tympanic membrane The tympanic membrane vibrates and the sound wave is transmitted as vibrations to the ossicles in the middle ear in the order Malleus Incus Stapes The stapes vibrates and sets the fluids in the inner ear into motion  perilymph moves and the endolymph also moves This causes the BASILAR MEMBRANE in the spiral organ of corti to oscillate The hair cells located on top of the basilar membrane also oscillate which causes the stereocilia trapped in the tectorial membrane to bend The bending of the stereocilia causes the generation of electrical signals  conducted to the cochlear afferent fibers at the bases of the hair cells These afferent fibers undergo depolarization and the generation of action potentials These action potentials are transmitted via the vestibulocochlear nerve (CN VIII) to the structures in the brain Once inside the brain, we are looking at tracts. The action potentials are transmitted to the INFERIOR COLLICULI in the midbrain (inferior colliculi - auditory reflex centers) Action potentials are then transmitted to the MEDIAL GENICULATE NUCLEUS (MGN) = auditory relay center in the thalamus From the relay centers in the thalamus, the action potentials reach the primary auditory cortex located in the temporal lobes  INTERPRETATION OF SOUND AND THE QUALITY OF THE SOUND using the auditory association center Equilibrium receptors = mechanoreceptors Mechanoreceptors  Maculae and cristae ampullares both are composed of hair cells and supporting cells The hair cells in Maculae have stereocilia trapped in a gel-like membrane called OTOLITHIC membrane When the hair moves sideways (horizontally) the maculae in the utricle respond to maintain posture When the hair moves vertically (= ascending in an elevator), the maculae in the saccule respond to maintain posture Cristae ampullares  hair cells + supporting cells Hair cells have stereocilia trapped in cupula (a gel-like membrane) During rotational movements of the head (twirling) the hair cells in the cristae ampullares respond to maintain posture (prevents falling) --------------- Deafness  loss of hearing Conduction deafness  interference in the conduction of sound waves to the tympanic membrane EXAMPLES  impacted ear wav, perforated tympanic membrane SOUND WAVES DO NOT CAUSE VIBRATION OF THE TYMPANIC MEMBRANE Sensoneural deafness  damage to the hair cells or to the cochlear nerve or to the vestibulocochlear nerve or to the auditory cortex in the temporal lobe
  • Chemoreceptors  Olfactory and Gustatory cells -------------------------
  • The olfactory cells mediate OLFACtION  sense of smell In the roof of the nasal cavity  yellowish patch called the OLFACTORY EPITHELIUM composed of three types of cells Olfactory cells  bipolar neurons with ciliated dendrites. The cilia on the dendrites of the bipolar neurons are called OLFACTORY HAIRS OLFACTORY HAIRS are trapped (embedded) in a thin layer of mucus the olfactory cells (bipolar neurons) are unique because unlike other neurons, the olfactory cells do NOT exhibit longevity; they are replaced every 30-60 days and replaced by the next type of cell… THE BASAL CELLS Basal Cells  part of the olfactory epithelium and they differentiate to form new olfactory cells to replace the old ones every 30-60 days, as mentioned above. Supporting Cells  produce a yellowish pigment which gives the olfactory epithelium its yellowish tint (hue) Supporting cells produce mucus
  • Physiology of olfaction The chemical or odorant MUST dissolve in the thin coat of mucus covering the olfactory hairs Dissolved chemical binds to the olfactory hairs to cause depolarization and the formation of electrical signals The axons of the olfactory cells bundle up to form the OLFACTORY NERVE (CN I) If the depolarization currents  electrical signals are strong enough axons in the olfactory nerve will generate/transmit action potentials to another set of neurons called MITRAL CELLS located in the OLFACTORY BULB The axons of the mitril cells bundle up to form the OLFACTORY TRACT (this is unique)  When the action potentials reach the olfactory tract  transmitted to the two brain areas (MAMMILLARY BODIES and Mammillary bodies  located inferior to the hypothalamus  action potentials are transmitted to the amygdala for the recognition of the emotional aspect of smell
  • 12/6 Chemoreceptors  Gustatory cells used in Gustation = sense of taste Gustatory cells  specialized epithelial cells located in TASTE BUDS  located in papillae Papillae are the peg-like projections on the surface of the tongue 3 types of papillae contain taste buds Fungiform papillae Foliate papillae Circumvallate (Vallate) papillae Anatomy of a taste bud Each taste bud consists of 2 types of epithelial cells  Gustatory cells and Basal cells Gustatory cells are the chemoreceptors and they have long microvilli that extend through taste pores to the surface of the tongue long microvilli called GUSTATORY HAIRS are bathed in SALIVA Hence, any chemical can be tasted only when the chemical dissolves in saliva and the dissolved chemical attaches to the gustatory hairs  depolarization and development of electrical signals
  • 3 cranial nerves trasnmit action potentials from the gustatory cells in the taste buds when chemicals dissolved in saliva attach and depolarize the gustatory hairs Wrapped around the bases of the gustatory cells are the afferent fibers: Anterior 2/3 of the tongue  the fibers will be from the FACIAL NERVE (CN VII) Posterior 1/3 of the tongue is supplied by the GLOSSOPHARYNGEAL NERVE (CN IX) In the pharynx (throat) the gustatory cells are wrapped by the afferent fibers from the VAGUS NERVE (CN X) (refer to previous slide) Depolarization of the gustatory hairs  depolarization  generation of electrical signals  development/transmission of action potentials (=nerve impulse) via the facial nerve, glossopharyngeal nerve, vagus nerve  nerve impulse is relayed to the gustatory relay center in the ventral posterior medial nucleus (VPM nucleus)  nerve impulse is relayed from the VPM nucleus to the primary gustatory cortex in the INSULA The gustatory association areas provide quality and discrimination to taste

Transcript

  • 1. The Special Senses Photoreceptors Mechanoreceptors Chemoreceptors
  • 2. Photoreceptors
    • Photoreceptors = Rods and Cones
    • Sense organs - Eyes
  • 3. Figure 15.1a
  • 4. Figure 15.1b
  • 5. Figure 15.2
  • 6. Figure 15.3
  • 7. Internal Structure of the Eye (sagittal section)
  • 8. Figure 15.4a
  • 9. The Eye
    • Consists of 3 parts:
    • Eye wall
    • Humors ( bodily fluids)
    • Lens
  • 10. Eye Wall
    • Composed of 3 layers: Fibrous layer; Vascular layer; sensory layer
    • Fibrous layer : outer layer composed of tough dense fibrous CT
    • consists of 2 regions – posterior 5/6 th is the sclera ( continuous with the dura mater surrounding the brain); and the anterior 1/6 th is the cornea.
    • The sclera maintains the shape of the eye, protects the eye and serves attachments sites for the extrinsic muscles of the eye
    • Sclera forms the “WHITE” of the eye
    • The cornea allows light to enter the eye because it’s avascular and transparent
    • Vascular layer = Uvea : middle layer that’s highly vascularized
    • consists of 3 regions: posterior 5/6 th is the choroid , middle ciliary body , anterior iris
    • The choroid provides nutrients to the sclera and the sensory layer
    • The ciliary body ends in folds called the ciliary processes which contain blood capillaries that secrete the aqueous humor ; string-like structures extending from the ciliary processes called the ciliary zonule ( = suspensory ligament) which hold the lens in upright position in the eye
    • The iris is seen anteriorly as the “colored” part of the eye; the central opening in the iris called the PUPIL allows light to enter the lens; the iris contains 2 types of smooth muscle that control the size of the pupil; activation of the sympathetic nervous system and viewing distsnt objects result in pupillary dilation; activation of the parasympathetic nervous system and viewing close objects result in pupillary constriction.
  • 11. Pupillary Constriction and Pupillary Dilation
  • 12. Sensory layer
    • Innermost layer of the eye wall
    • Confined to the posterior wall ending at the ora serrata
    • Sensory layer consists of the pigmented layer and the neural layer
    • Pigmented layer – composed of a single layer of cells which provide nutrients to the neural layer; contain melanin which absorbs light and prevents it from scattering; contain vitamin A required for the synthesis of the light-absorbing pigment called RETINAL
    • Neural layer referred to as the R ETINA– extends anteriorly from the pigmented layer
      • Composed of 3 layers of neurons: PHOTORECEPTORS ; Bipolar neurons; Ganglion cells
      • PHOTORECEPTORS - abut the pigmented layer; they respond to light and generate electrical signals; 2 types of photoreceptors –
      • (i) rods – more numerous ( 150 million); more sensitive to light - used in dim-light and peripheral vision; provide images in shades of gray not used for color vision
      • (ii) cones – ( 80 million); operate in bright light, provide high- acuity color vision ;
      • 3 types of cones – blue, green and red cones
      • According to the TRICHROMATIC THEORY OF VISION - several colors are seen depending on which/how many of the three types of cones are activated
      • Bipolar neurons – receive electrical signals from the photoreceptors and conduct the signal to the ganglion cells
      • Ganglion cells – neurons that receive the electrical signals from the bipolar neurons; only the axons of the ganglion cells generate and transmit action potentials
  • 13. Neural Layer - Retina
  • 14. Photoreceptors – Rods and Cones
  • 15. The Electromagnetic spectrum and photoreceptor sensitivities
  • 16. Ganglion cells and the Optic Nerve
  • 17. Ganglion cells and the Optic Nerve
    • Ganglion cells are the only neurons in the retina that can generate and transmit action potentials
    • Bundle of axons of the ganglion cells = OPTIC NERVE (CN II)
    • The Optic Nerve exits the posterior wall of the eye through the OPTIC DISC = BLIND SPOT - because the optic disc lacks photoreceptors; lateral to the optic disc is the MACULA LUTEA and in its center is the FOVEA CENTRALIS ; the macula lutea contains mostly cones and the fovea centralis contains only cones and it’s used for hard focus
  • 18. The Eye
  • 19. Figure 15.7
  • 20. Aqueous and Vitreous Humors
  • 21. The Humors in the Eye
    • Humors = fluids in the body
    • 2 humors in the eye: Aqueous humor; Vitreous humor
    • Aqueous humor – filtered from blood capillaries in the ciliary processes into the anterior chamber in front of the lens; drained by the canal of Schlemm; formed and drained continually. If the rate of synthesis exceeds the rate of drainage, intraocular pressure rises causing damage to retina and the optic nerve resulting in GLAUCOMA
    • Function - supplies nutrients and oxygen to the lens and cornea; carries away metabolic wastes; maintains intraocular pressure to support the eyeball
    • Vitreous humor – gel-like fluid in the posterior segment behind the lens; formed in the embryo and lasts a lifetime.
    • Function – supports the posterior surface of the lens; pushes the neural layer against the pigmented layer; maintains the intraocular pressure.
    • Retinal detachment – the retina detaches from the pigmented layer and the vitreous humor seeps into the space – without their nutrient source, the photoreceptors in the retina die leading to blindness
  • 22. The Lens
  • 23. The Lens
    • Avascular, transparent, biconcave and flexible
    • Held in an upright position behind the pupil and the iris by the ciliary zonule
    • Composed of transparent proteins called crystallins
    • Function – focuses light on the retina; the flexible lens can change its shape to precisely focus light on the retina referred to as ACCOMODATION
    • Focusing for Distant Vision – the normal eyes are adapted for distant vision and therefore accommodation is not necessary; the far point of vision is the distance beyond which accommodation is not needed = 6m = 20ft (20/20 vision )
    • Focusing for Close Vision – less than 20ft; involves accommodation of the lens where the lens bulges to focus a close objects onto the retina; the near point of vision is the distance at which the lens can bulge maximally to focus the object on the retina = 10cm = 4in;
    • In addition, pupillary constriction occurs in close vision
  • 24. Figure 15.13
  • 25. Problems of Refraction
  • 26. Problems of Refraction
    • Myopia = nearsightedness – occurs when distant objects are focused in front of the retina; eyeball too long.
    • Correction – concave lenses to diverge the light before it enters the eye; flattened cornea using LASIK
    • Hyperopia = farsightedness – occurs when light from close objects are focused behind the retina; eyeball too short
    • Correction – convex lenses to converge the light onto the retina
  • 27. Refraction – spoon appears to be broken at the water-air interface
  • 28. Thickening and hardening of the crystallins in the lens leads to the formation of cataracts = clouding of the lens ct
  • 29. Path taken by light through the eye : cornea—aqueous humor--pupil—lens—vitreous humor—ganglion cells—bipolar neurons--photoreceptors
  • 30. Pathway of light through the Retina: Ganglion cells—bipolar neurons--Photoreceptors
  • 31. Transmission of electrical signals – light hits the photoreceptors and they generate electrical signals  Bipolar neurons-  Ganglion cells
  • 32. Visual pathway to the Brain
  • 33. Transmission of action potentials – impulse transmission
    • Axons of ganglion cells form the optic nerves; generate and transmit impulses via the optic nerves; medial fibers of optic nerves cross over to opposite sides at the OPTIC CHIASMA and continue on as the OPTIC TRACTS.
    • Impulses are transmitted to the visual reflex centers in the midbrain called the SUPERIOR COLLICULI
    • Impulses are transmitted to the visual relay center in the thalamus called the LATERAL GENICULATE NUCLEUS (LGN)
    • Impulses finally relayed to the primary visual cortex located in the occipital lobes
  • 34. Mechanoreceptors – Hair Cells Sense organs - Ears
  • 35. The EAR
  • 36. The Ear - 3 major regions
    • External ( outer) Ear = pinna( or auricle) + external auditory canal( external acoustic meatus)
    • Middle Ear – contains the 3 ossicles = malleus, incus, stapes; air-filled cavity
    • TYMPANIC MEMBRANE, cone-shaped membrane that separates the external year from the middle ear; the broad base faces the external auditory canal and the apex abuts the malleus.
    • Internal ( inner) Ear = labyrinth = Bony labyrinth and Membranous labyrinth; separated from the middle ear by a bony wall with an oval and round windows; the stapes sits atop the oval window
    • Bony labyrinth – Vestibule, Semicircular canals, Cochlea; all contain CSF-like fluid called PERILYMPH
    • Membranous labyrinth – consists on interconnecting sacs and ducts located inside the structures of the bony labyrinth; contains fluid called the ENDOLYMPH
    • Utricle and Saccule - membranous sacs located inside the vestibule; contain the equilibrium receptors that respond to the pull of gravity and head position
    • Semicircular ducts - membranous ducts located in the semicircular canals; expanded ends called AMPULLAE house equilibrium receptors that respond to the rotational movements of the head
    • Cochlear duct – membranous duct located in the cochlea
  • 37. The EAR
  • 38. Middle and Inner Ear
  • 39. The 3 ossicles in the Middle Ear
  • 40. Bony and Membranous Labyrinth
  • 41. Anatomy of the Cochlea
  • 42. The Organ of Corti
    • Located in the cochlear duct which contains endolymph
    • Rests on a flexible membrane called the BASILAR MEMBRANE
    • Composed of Supporting cells and HAIR CELLS , the mechanoreceptors; the apical surfaces of hair cells have stereocilia which are microvilli stiffened by actin; stereocilia are trapped in a gel-like membrane called the TECTORIAL MEMBRANE.
    • The afferent fibers of the cochlear nerve, a division of the vestibulocochlear nerve ( CN VIII), wrap around the bases of the hair cells
  • 43. Organ of Corti
  • 44. Photograph of cochlear Hair Cells with stereocilia Figure 15.33
  • 45. Route of Sound Waves through the Ear
  • 46. Route of sound waves through the Ear
    • Pinna  External Auditory canal  Tympanic membrane vibrates  Malleus  Incus  Stapes  Perilymph + Endolymph move  Basilar Membrane oscillates  Hair cells move  stereocilia bend  electrical signals develop  transferred to the cochlear nerve  cochlear nerve generates and transmits action potentials via the vestibulocochlear nerve  impulse transmitted to the auditory relay center in the thalamus called MEDIAL GENICULATE NUCLEUS (MGN)  primary auditory cortex in the Temporal lobes
    • Impulses also transmitted to the auditory reflex centers in the midbrain called the INFERIOR COLLICULI
  • 47. Figure 15.34 Auditory Pathway to the primary auditory cortex in the TEMPORAL LOBES in the cerebral hemispheres in the Brain
  • 48.
    • THE CHEMORECEPTORS
    • Olfactory cells in the Olfactory Epithelium
    • 2. Gustatory cells in the taste buds
  • 49. Olfactory Epithelium
  • 50. Olfactory Epithelium
    • Yellowish patch in the roof of the nasal cavity
    • Composed of Supporting cells, Basal cells and Olfactory cells
    • Olfactory cells are CHEMORECEPTORS
    • Olfactory cells are BIPOLAR NEURONS – their axons bundle up to form the OLFACTORY NERVE (CN I)
    • Olfactory cells are unique neurons because they do not exhibit longevity - olfactory cells are replaced every 60 days by the differentiation of the basal cells
    • Dendrites of olfactory cells are ciliated and the cilia are called olfactory hairs – trapped in a thin coat of mucus
  • 51. Olfaction – Sense of Smell
    • Chemicals or the odorants must meet 2 criteria – must be volatile and soluble in the thin coat of mucus covering the olfactory hairs ( cilia)
    • Dissolved chemicals attach to the cilia resulting in depolarization which eventually causes the axons of the olfactory nerve to generate and transmit action potentials ( the impulse)
    • Impulse is transmitted to a second type of neurons called MITRAL cells – a bundle of axons of the mitral cells form the OLFACTORY TRACT
    • Impulse is transferred from the olfactory tract to the olfactory relay centers in the thalamus and in the mammillary bodies
    • Impulses transmitted to the thalamus relayed to the primary olfactory cortex in the frontal lobe
    • Impulses transmitted in the mammillary bodies are sent to the temporal lobe, hypothalamus, amygdala responsible for the emotional aspect of odors
  • 52. Taste buds and Gustatory cells
  • 53. Taste Buds and the Gustatory Cells
    • Taste buds are located in peg-like projections of the tongue called PAPILLAE
    • 4 types of papillae – fungiform, foliate, vallate and filiform papillae
    • ( the filiform papillae lack taste buds)
    • Each taste bud consists of 2 types of epithelial cells- basal cells and Gustatory cells = CHEMORECEPTORS
    • Gustatory cells have long microvilli called gustatory hairs which extend to the surface of the tongue through taste pores
    • Gustatory hairs are bathed in saliva.
    • Gustatory cells are subjected to tremendous friction hence, they are replaced every 7 days by the differentiation of the basal cells into new gustatory cells
    • Afferent fibers coil around the gustatory cells – 3 types cranial nerves are involved in the gustatory pathway to the brain:
      • Chorda tympani, a branch of the facial nerve (CN VII)
      • Glossopharyngeal nerve( CN IX)
      • Vagus nerve ( CN X)
  • 54. Figure 15.24 The Gustatory Pathway to the Primary Gustatory Cortex in the Insula
  • 55. Gustation = Sense of taste
    • Activation of the gustatory cells – chemicals to be tasted, the tastant, must dissolve in saliva bathing the gustatory hairs
    • Taste sensations: sweet (sugars), salty(NaCl), sour (H+), bitter (alkaloids), “umami” ( monosodium glutamate =MSG)
    • Dissolved chemical binds to gustatory hairs – results in depolarization which is transferred to the afferent fibers coiled around the gustatory cells:
    • chorda tympani of the facial nerve (CN VII) generates and transmits action potentials from gustatory cells in the anterior two-thirds of the tongue
    • Glossopharyngeal nerve (CN IX) generates and transmits action potentials from the posterior third of the tongue and the superior part of the pharynx
    • Vagus nerve (CN X) generates and transmits action potentials from the inferior part of the pharynx
    • Impulses from these 3 cranial nerves are transmitted to the SOLITARY NUCLEUS in the medulla oblongata; then to the gustatory relay center in the thalamus called the VENTRAL POSTEROMEDIAL NUCLEUS; impulse finally relayed to the primary gustatory cortex located in the Insula
    • Taste is 80% smell – same chemicals activate both types of chemoreceptor = olfactory cells and gustatory cells
  • 56. www.jhu.edu/jhumag/996web/ taste .html
    • How come food doesn't taste good when I have a cold? Because most of what we call "taste" is in fact smell, triggered by odor molecules from our food and drink. Some molecules we smell in the air, from the plate or as the fork approaches; others vaporize as we chew, then rise into the nasal passages at the back of the mouth.
    • Tastebuds alone can detect only sweet, sour, salty, and bitter. "If you lick a pink ice cream cone," says Donald Leopold, an otolaryngologist at Hopkins's Bayview Medical Center, "your tongue tells you it's cold and sweet and smooth, but your sense of smell tells you it's strawberry. Probably 80 percent of what you eat, you appreciate through your sense of smell." That's why if you have a cold, you could mistake a bite of onion for apple.