The document summarizes the key structures and functions of the special senses - photoreceptors, mechanoreceptors, and chemoreceptors. It describes the anatomy and physiology of the eye, ear, olfactory epithelium, and taste buds. Light is detected by rods and cones in the retina and sound waves are detected by hair cells in the cochlea. Odorants activate olfactory cells and different chemicals stimulate gustatory cells to detect the basic tastes.

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.