The Special Senses

1. The Special Senses Eye and Ear Chapter 16

2. Introduction • Sensory Perception – processed by the nervous system - smell, taste, sight, and hearing (touch, large group of general senses) - receptors for equilibrium are located in the ear • Special sensory receptors are localized and confined to the head region • Receptors – neuronlike epithelial cells or small peripheral neurons - receptors are housed in complex sensory organs (eye or ear) or in epithelial structures (taste buds or olfactory epithelium) - sensory information travels to the brain via cranial nerves

3. The Eye and Vision • The optic system consists of the eye and its accessory structures - eyebrows, eyelids, conjunctiva, lacrimal apparatus, extrinsic eye muscles • The visual organ is the eye – spherical structure - surrounded by a protective cushion of fat - behind the eye, the posterior half of the orbit contains the optic nerve, the arteries and veins, and extrinsic eye muscles

4. Accessory Structures • Protect the eye or enable eye movement • Orbit of the skull surrounds the eye (frontal, lacrimal, ethmoid, zygomatic, sphenoid, and palatine bones) • Eyebrows, shade the eyes from sunlight and prevent perspiration from reaching the eyes • Eyelids (palpebrae), provide protection • Eyelashes, richly innervated by nerve endings, slight pressure will trigger reflexive blinking

5. Accessory Eye Structures • Conjunctiva, transparent membrane covers inner surface (palpebral conjunctiva) of the eyelids and folds back over to cover the anterior surface (bulbar conjunctiva) • Lacrimal Apparatus, keeps eye surface moist • Extrinsic (outer) eye muscle, control the movement of each eye and hold the eyes in the orbits

6. Accessory Eye Structures Fig 16.4 – 16.5

7. Lacrimal Apparatus • Lacrimal Apparatus includes the lacrimal gland, sac, & duct • Lacrimal glands secrete lacrimal fluid (tears) and are found in the superolateral orbit • Tears drain into two small openings (lacrimal puncta), through the lacrimal canaliculi, into the lactimal sac, to the nasolacrimal duct, to the nasal cavity

8. Lacrimal Apparatus Fig 16.5

9. Extrinsic Eye Muscles • Control eye movement and include: • Superior rectus (CN III) rotates eye upward & medially • Inferior rectus (CN III) rotates eye downward & medially • Medial rectus (CN III) rotates eye medially • Lateral rectus (CN VI) rotates eye laterally • Superior oblique (CN IV) rotates eye downward & laterally • Inferior oblique (CN III) rotates eye upward & laterally

10. Extrinsic Eye Muscles Fig 16.6

11. Anatomy of the Eyeball • Eyeball Structure - consists of 3 basic layers: the fibrous tunic, vascular tunic, & internal (sensory) tunic, and internal chambers • Fibrous tunic – tough outermost eyeball layer, consists of • Sclera - white of the eye composed of collagen & elastic fibers; optic nerve exits from sclera at back of eye. • Cornea - convex,clear part of sclera on the anterior eyeball. • Limbus – junction between the sclera & cornea; contains epithelial stem cells that renew the cornea.

12. Vascular tunic - consists of the choroid, ciliary body, & iris • Choroid - thin, dark vascular layer that lines the posterior 5/6th of the internal sclera; its blood vessels nourish the tunics • Ciliary body thick, anterior portion that forms an internal muscular ring toward the front of the eyeball; consists of: • Ciliary muscle, smooth muscle fibers, controlled by CN III & parasympathetic nerves • Suspensory ligaments connect ciliary muscles to the

13. Vascular tunic (con’t) • Lens - a thick, clear layer of protein fibers, which controls eye focus (accomodation) via contraction & relaxation of ciliary muscles. • Presbyopia is the loss of lens elasticity & accomodation. • Cataracts are a clouding of the lens. • Iris - colored, anterior part of the vascular tunic, continuous with the choroid • Consists of radial and circular smooth muscle fibers that regulate the amount of light in through the • Pupil - an opening in the center of the iris

14. Sensory tunic (Retina) - innermost eye layer; consists of an outer pigmented layer and inner neural layer that contains • Rods - photoreceptor cells on the peripheral posterior retina; respond to dim light for black & white vision • Cones - photoreceptor cells that provide color vision & surround a central depression called the fovea centralis • Rods & cones send impulses to ganglionic bipolar neurons that leave the eye as the optic nerve (CN II)  the optic chaisma  the optic tract  the thalamus  the white matter optic radiation  the visual cortex in the occipital lobe

15. Sensory tunic (retina) • The optic disk, found where the optic nerve exits, has no photoreceptors, thus is a blind spot. • The macula lutea, near the optic disk, contains mostly cones and a pit called the fovea centralis, which has only cones. • The ora serrata retinae is the posterior margin of the ciliary body, where the retina’s neural layer ends.

16. Internal Structure of the Eye Fig 16.7

17. Anterior Structures of the Eye Fig 16.8

18. Retina a) Light passes outward through the entire thickness of the retina before exciting the phostoreceptor cells; then electrical signals flow inward from neuron to neuron Fig 16.9

19. Cataract Clouding of the lens - milky and opaque due to inadequate nutrient delivery to the deeper lens fibers

20. Retina Through an Opthalmoscope

21. Optic Nerve • Axons of the ganglion cells form the optic nerve, which leaves the back of the eye at the optic disc

22. Visual Pathway • Visual information travels to the cerebral cortex through the main visual pathway • Axons of the ganglion cells exit the eye in the optic nerve • At the X-shaped optic chiasm, axons from the medial half of each eye decussate and continue in an optic tract to the thalamus • From the thalamus, the visual information projects to the visual cortex in the occipital lobe of the cerebrum

23. Visual Pathway

24. Cavities & Chambers of the Eyeball – the interior eye is separated by the lens into posterior & anterior cavities: • Posterior segment is filled with a gel-like vitreous humor • Anterior segment is filled with watery aqueous humor secreted by ciliary body epithelium • excess drains through the canal of Schlemm • excess fluid causes glaucoma. • The anterior cavity is further divided into: • Anterior chamber - between the cornea & iris • Posterior chamber - between the iris & lens

25. Anterior & Posterior Chambers

27. Visual Pathways • Other visual pathways include optic tracts to the: • Superior colliculi to control extrinsic eye muscles, • Pretectal nuclei in the midbrain to mediate pupillary light reflexes, • Suprachiasmatic nucleus in the hypothalamus to regulate daily biorhythms.

29. Sense of Hearing & Balance • Structures of the outer, middle, & inner ear are involved in hearing. The inner ear also has structures for sense of balance (equilibrium) • Outer (external) ear consists of the: • Auricle (pinna) composed of elastic cartilage & skin • External auditory canal - a fleshy tube within the external auditory meatus of the skull, that ends with the • Tympanic membrane (eardrum), a thin epithelial partition between the EAC and the middle ear tympanic cavity. • Excess external or internal pressure can cause a perforated eardrum

30. Ear Structure

31. Middle Ear Structure • Tympanic cavity - air-filled cavity in the petrous portion of the temporal bone • Posterior wall has an recessed area called the mastoid antrum that leads to the mastoid air cells • Anterior wall has an opening called the Eustachian (auditory or pharyngotympanic) tube (meatus) that leads to the nasopharynx • Medial wall has a bony partition with the oval and round window that separates the middle ear from the inner ear

32. Middle Ear Structure

33. Middle Ear Structure • Three auditory ossicles extend from the tympanic membrane to the vestibular window • Malleus (hammer) - articulates with tympanic membrane • Incus (anvil) - articulates with malleus & stapes • Stapes (stirrup) - articulates with incus & oval window • Otitis media is an inflammation of the middle ear, which can sometimes be alleviated by a myringotomy, in which tubes are inserted into the eardrum to drain excess fluid

34. Middle Ear Ossicles

35. Inner Ear Structure • Inner Ear - the labyrinth, consists of an outer bony labyrinth that surrounds and protects the inner membranous labyrinth. • Perilymph fluid circulates between the bony & membranous labyrinths • Endolymph circulates within the membranous labyrinth • Both fluids conduct vibrations involved in hearing & equilibrium • The bony labyrinth is divided into 3 main areas; the vestibule, semicircular canals, and cochlea

36. Inner Ear - Labyrinth

37. Inner Ear Structure • Vestibule - central part of the bony labyrinth; controls balance & equilibrium and contains the: • Vestibular (oval) window, into which the stapes fits, • Cochlear (round) window on the opposite end. • The membranous labyrinth within consists of 2 connected sacs, the • Utricle - larger sac, in the upper back of the vestibule, • Saccule - smaller sac

38. Membranous Labyrinth

39. Inner Ear Structure • Macula in both sacs contain: • Epithelial support cells • Receptor hair cells are embedded in an overlying otolithic membrane containing calcium carbonate otoliths • Together they sense equilibrium and linear acceleration and send impulses to • The vestibular nerve, which joins the cochlear nerve to form the vestibulocochlear nerve

40. Vestibular Macula

41. Inner Ear Structure • Semicircular canals - 3 bony canals posterior to the vestibule and positioned at right angles to each other. The membranous labyrinth contains the • Semicircular ducts, each of which has a • Membranous ampulla at one end and connects with the utricle and houses the cristae ampullaris, which contains: • Supporting cells • Receptor hair cells embedded in a gel-like cupula; these sense head rotation and send impulses to the vestibular nerve

42. Semicircular Canals & Ducts

43. Cristae Ampullaris

44. Inner Ear Structure • Cochlea (snail shell) - coiled 2½ times around bone, contains 3 chambers • Upper scala vestibuli begins at the vestibular window, extends to the end (apex) of the cochlea, and contains perilymph fluid • Lower scala tympani begins at the apex, ends at the cochlear window (secondary tympanic membrane), also contains perilymph • Both scalas are separated except at the cochlear apex, where they join

45. Cochlea Structure

46. Inner Ear Structure • Between the scalas is the cochlear duct (scala media), a triangular middle chamber that ends where the scalas join; it contains the • Vestibular membrane - roof of the duct • Basilar membrane - floor of duct • Endolymph fluid

47. Inner Ear Structure • Cochlear duct (con’t) • Organ of Corti (spiral organ) - sound receptors that transform mechanical vibrations into nerve impulses are found in the basilar membrane here • The epithelium consists of supporting cells & hair cells • The base of the hair cells are anchored in the basilar membrane and their tips embedded in the gelatinous tectorial membrane • Sound induces perilymph movement, which causes the hair cells in the tectorial membrane to bend, exciting sensory cells, which release neurotransmitter to the cochlear nerve

48. Cochlea & Sound

49. Auditory Pathway • Cochlear sensory neurons in the vestibulocochlear nerve (CN VIII) send impulses to the medulla, to the inferior colliculi, to the thalamus, to the auditory cortex, where the impulses are perceived as sound

50. Auditory Pathway