The Special Senses

1. The Special Senses PART 1

2. The Special Senses • Taste, smell, sight, hearing, and balance • Special sensory receptors • Localized – confined to the head region • Receptors are not free endings of sensory neurons • Special receptor cells

3. The Chemical Senses: Taste and Smell • Taste – gustation • Smell – olfaction • Receptors – classified as chemoreceptors • Respond to chemicals

4. Taste – Gustation • Taste receptors • Occur in taste buds • Most are found on the surface of the tongue • Located within tongue papillae

5. Taste Buds • Collection of 50-100 epithelial cells • Contain three major cell types • Supporting cells • Gustatory cells • Basal cells • Contain long microvilli – extend through a taste pore

6. Taste Buds Figure 16.1a, b

7. Taste Sensation and the Gustatory Pathway • Four basic qualities of taste • Sweet, sour, salty, and bitter • A fifth taste – umami – “deliciousness” • No structural difference among taste buds

8. Gustatory Pathway • Taste information reaches the cerebral cortex • Primarily through the facial (VII) and glossopharyngeal (IX) nerves • Some taste information through the vagus nerve (X) • Sensory neurons synapse in the medulla • Located in the solitary nucleus

9. Gustatory Pathway from Taste Buds Figure 16.2

10. Smell (Olfaction) • Receptors are part of the olfactory epithelium • Olfactory epithelium composed of: • Cell bodies of olfactory receptor cells • Supporting cells – columnar cells • Basal cells – form new olfactory receptor cells

11. Smell (Olfaction) • Axons of olfactory epithelium • Gather into bundles – filaments of the olfactory nerve • Pass through the cribriform plate of the ethmoid bone • Attach to the olfactory bulbs

12. Olfactory Receptors Figure 16.3a, b

13. The Eye and Vision • Visual organ – the eye • 70% of all sensory receptors are in the eyes • 40% of the cerebral cortex is involved in processing visual information

14. Accessory Structures of the Eye • Lacrimal apparatus – keeps the surface of the eye moist • Lacrimal gland – produces lacrimal fluid • Lacrimal sac – fluid empties into nasal cavity Figure 16.5b

15. The Fibrous Tunic • Most external layer of the eyeball • Composed of two regions of connective tissue • Sclera – posterior five-sixths of the tunic • White, opaque region • Provides shape and an anchor for eye muscles • Cornea – anterior one-sixth of the fibrous tunic • Limbus – junction between sclera and cornea • Scleral venous sinus – allows aqueous humor to drain

16. Medial View of the Eye Figure 16.7a

17. The Vascular Tunic • The middle coat of the eyeball • Composed of choroid, ciliary body, and iris • Choroid – vascular, darkly pigmented membrane • Forms posterior five-sixths of the vascular tunic • Brown color – from melanocytes • Prevents scattering of light rays within the eye • Choroid corresponds to the arachnoid and pia maters

18. Posterior View of the Anterior Half of the Eye Figure 16.9a

19. The Vascular Tunic • Ciliary body – thickened ring of tissue – encircles the lens • Composed of ciliary muscle • Ciliary processes – posterior surface of the ciliary body • Ciliary zonule (suspensory ligament) • Attached around entire circumference of the lens

20. The Vascular Tunic Figure 16.8

21. The Iris • Visible colored part of the eye • Attached to the ciliary body • Composed of smooth muscle • Pupil – the round, central opening • Sphincter pupillae muscle (constrictor or circular) • Dilator pupillae muscle (dilator or radial) • Act to vary the size of the pupil

22. Pupillary dilation and constriction

23. The Sensory Tunic (Retina) • Retina – the deepest tunic • Composed of two layers • Pigmented layer – single layer of melanocytes • Neural layer – sheet of nervous tissue • Contains three main types of neurons • Photoreceptor cells • Bipolar cells • Ganglion cells

24. Microscopic Anatomy of the Retina Figure 16.10a

25. Photoreceptors • Two main types • Rod cells – more sensitive to light • Allow vision in dim light • Cone cells – operate best in bright light • Enable high-acuity, color vision • Considered neurons

26. Photoreceptors Figure 16.11

27. Regional Specializations of the Retina • Macula lutea – contains mostly cones • Fovea centralis – contains only cones • Region of highest visual acuity • Optic disc – blind spot

28. Medial View of the Eye Figure 16.7a

29. Blood Supply of the Retina • Retina receives blood from two sources • Outer third of the retina – supplied by capillaries in the choroid • Inner two-thirds of the retina – supplied by central artery and vein of the retina Figure 16.10c

30. Internal Chambers and Fluids • The lens and ciliary zonules divide the eye • Posterior segment (cavity) • Filled with vitreous humor • Clear, jelly-like substance • Transmits light • Supports the posterior surface of the lens • Helps maintain intraocular pressure

31. Internal Chambers and Fluids • Anterior segment • Divided into anterior and posterior chambers • Anterior chamber – between the cornea and iris • Posterior chamber – between the iris and lens • Filled with aqueous humor • Renewed continuously • Formed as a blood filtrate • Supplies nutrients to the lens and cornea

32. Internal Chambers and Fluids Figure 16.8

33. The Lens • A thick, transparent, biconvex disc • Held in place by its ciliary zonule

34. Lens, Zonule Fibers, & Ciliary Muscles

35. Lens Epithelium capsule epithelium fibers

36. Eye Dissection Video http://sam.ntpi.spcollege.edu/spjc/default.jhtml

37. The Eye as an Optical Device • Structures in the eye bend light rays • Light rays converge on the retina at a single focal point • Light bending structures (refractory media) • The lens, cornea, and humors • Accommodation – curvature of the lens is adjustable • Allows for focusing on nearby objects

38. Visual Pathways • Most visual information travels to the cerebral cortex • Responsible for conscious “seeing” • Other pathways travel to nuclei in the midbrain and diencephalon

39. Visual Pathways to the Cerebral Cortex • Pathway begins at the retina • Light activates photoreceptors • Photoreceptors signal bipolar cells • Bipolar cells signal ganglion cells • Axons of ganglion cells exit eye as the optic nerve

40. Visual Pathways to the Cerebral Cortex • Optic tracts send axons to: • Lateral geniculate nucleus of the thalamus • Synapse with thalamic neurons • Fibers of the optic radiation reach the primary visual cortex

41. Visual Pathways to the Brain and Visual Fields Figure 16.15a

42. Visual Pathways to Other Parts of the Brain • Some axons from the optic tracts • Branch to midbrain • Superior colliculi • Pretectal nuclei • Other branches from the optic tracts • Branch to the suprachiasmatic nucleus

43. Normal Opthalmoscopic View of Eye

44. Disorders of the Eye and Vision: Macular Degeneration • Age-related macular degeneration (AMD) • Involves the buildup of visual pigments in the retina Wet Dry

45. Macular Degeneration Simulation

46. Disorders of the Eye and Vision: Retinopathy • Retinopathy in diabetes • Vessels have weak walls – causes hemorrhaging and blindness

47. Disorders of the Eye and Vision: Trachoma • Trachoma – contagious infection of the conjunctiva

48. The Ear: Hearing and Equilibrium • The ear – receptor organ for hearing and equilibrium • Composed of three main regions • Outer ear – functions in hearing • Middle ear – functions in hearing • Inner ear – functions in both hearing and equilibrium

49. The Outer (External) Ear • Composed of: • The auricle (pinna) • Helps direct sounds • External acoustic meatus • Lined with skin • Contains hairs, sebaceous glands, and ceruminous glands • Tympanic membrane • Forms the boundary between the external and middle ear

50. The Outer (External) Ear Figure 16.17a