PNS – Afferent Division Sensory Physiology Part 2

1. PNS – Afferent Division Sensory Physiology Part 2

2. Special Senses – External Stimuli • Vision • Hearing • Taste • Smell • Equilibrium Figure 10-4: Sensory pathways

3. Cross-Section of the Eye

4. Organization of the Retina Figure 17.6b, c

5. Vision • Light enters the eye through the pupil, diameter of pupil modulates light • Shape of lens focuses the light on the retina • Retinal rods and cones are photoreceptors • Reflected light translated into mental image Figure 10-36: Photoreceptors in the fovea

6. Pupils • Bright light they constrict to ~ 1.5 mm • Dark they dilate to ~ 8 mm. • Controlled by the autonomic nervous system, pupillary reflex

7. Image Projection • The image projected onto the retina is inverted or upside down. Visual processing in the brain reverses the image

8. Image Projection • Convex structures of eye produce convergence of diverging light rays that reach eye

9. Refraction of Light Figure 10-30a

10. Optics Figure 10-31a

11. Optics Figure 10-31b

12. Mechanism of Accommodation • Accommodation is the process by which the eye adjusts the shape of the lens to keep objects in focus Figure 10-32a

13. Mechanism of Accommodation Figure 10-32b

14. Common Visual Defects Figure 10-33a

15. Retina Photoreceptors Neurons Cone Rod Amacrine cell Horizontal cell Optic nerve fibers Ganglion cell Bipolar cell Pigmented epithelium Retina • Photoreceptors - rods and cones detect light stimulus • Bipolar - generate APs • Amacrine & Horizontal cells – local integration of APs • Ganglion cells converge form optic nerve

16. Photoreceptors • Rods - light-sensitive but don’t distinguish colors; monochromatic, night vision • Cones - Three types; red, green, & blue, distinguish colors but are not as sensitive, high acuity day vision

17. Rod Outer segment Disks Inside of disk Cell body cis isomer Light Enzymes Synaptic terminal Cytosol Retinal Rhodopsin trans isomer Opsin Photo-transduction • Each rod or cone contains visual pigments consisting of a light-absorbing molecule called retinal bonded to a protein called opsin

18. Phototransduction • Rods contain the pigment rhodopsin, which changes shape when absorbing light Retinal Changes Shape Retinal restored Opsin inactivated

19. (a) In darkness, rhodopsin is inactive, cGMP is high, and ion channels are open. (b) Light bleaches rhodopsin. Opsin decreases cGMP, closes Na+ channels, and hyperpolarizes the cell. (c) In the recovery phase, retinal recombines with opsin. Activated retinal Activates transducin Opsin (bleached pigment) Retinal converted to inactive form Pigment epithelium cell Disk Transducin (G protein) Inactive rhodopsin (opsin and retinal) Cascade Decreased cGMP cGMP levels high Na+ Retinal recombines with opsin to form rhodopsin. Na+ channel closes Na+ K+ K+ Membrane hyperpolarizes to -70 mV. Membrane potential in dark = -40mV Light Tonic release of neurotransmitter onto bipolar neurons Neurotransmitter decreases in proportion to amount of light. Photo-transduction • Photons "bleach" opsin, retinal changes shape and released, transduction cascade, decreased cGMP, Na+ channel closes, K+ opens , hyperpolarization reduces NT release

20. Light INSIDE OF DISK EXTRACELLULAR FLUID Active rhodopsin PDE Membrane potential (mV) Plasma membrane 0 Dark Light cGMP Transducin Inactive rhodopsin Disk membrane Hyper- polarization –40 GMP Na+ –70 Time CYTOSOL Na+ Photo-transduction

21. Light Responses Dark Responses Rhodopsin inactive Rhodopsin active Na+ channels open Na+ channels closed Rod depolarized Rod hyperpolarized Glutamate released No glutamate released Bipolar cell hyperpolarized Bipolar cell depolarized Photo-transduction • In the dark, rods and cones release the neurotransmitter glutamate into synapses with neurons called bipolar cells • Bipolar cells are hyperpolarized • In the light, rods and cones hyperpolarize, shutting off release of glutamate • The bipolar cells are then depolarized

22. Convergence and Ganglion Cell Function Figure 17.18

23. Fovea The Retina & Visual Acuity Light adapted eye has greatest visual acuity at the fovea - Photopic vision (cones) Dark adapted eye has least visual acuity at the fovea but has greater acuity in the parafoveal region Scotopic vision (rods)

24. Retinal cells 2x binocular vision plus accessory structures Retina Optic disk - blood supply optic nerve Visual Integration / Pathway

25. Vision Integration / Pathway • Optic nerve • Optic chiasm • Optic tract • Thalamus • Visual cortex Figure 10-29b, c: Neural pathways for vision and the papillary reflex

26. The Ear / Auditory Physiology

27. External Ear Structures & Functions • Pinna—Collects sound waves and channels them into the external auditory canal. • External Auditory Canal—Directs the sound waves toward the tympanic membrane. • Tympanic membrane—Receives the sound waves and transmits the vibration to the ossicles of the middle ear.

28. Sound and Hearing • Sound waves travel toward tympanic membrane, which vibrates • Auditory ossicles conduct the vibration into the inner ear • Movement at the oval window applies pressure to the perilymph of the cochlear duct • Pressure waves move through vestibular membrane through endolymph to distort basilar membrane • Hair cells of the Organ of Corti are pushed against the tectorial membrane Figure 17.28a

29. Cochlea and Organ of Corti

30. Organ of Corti • Ion channels open, depolarizing the hair cells, releasing glutamate that stimulates a sensory neuron. • Greater displacement of basilar membrane, bending of stereocilia; the greater the amount of NT released. • Increases frequency of APs produced.

31. Signal Transduction in Hair Cells • The apical hair cell is modified into stereocilia Figure 10-21a

32. Cochlea (uncoiled) Basilar membrane Apex (wide and flexible) 500 Hz (low pitch) 1 kHz 2 kHz 4 kHz 8 kHz 16 kHz (high pitch) Frequency producing maximum vibration Base (narrow and stiff) Pitch Discrimination • Different frequencies of vibrations (compression waves) in cochlea stimulate different areas of Organ of Corti • Displacement of basilar membrane results in pitch discrimination.

33. Sensory Coding for Pitch • Waves in basilar membrane reach a peak at different regions depending upon pitch of sound. • Sounds of higher frequency cause maximum vibrations of basilar membrane.

34. Vestibular Apparatus Vestibular apparatus provides information about movement and position in space Figure 10-23a, b: ANATOMY SUMMARY: Vestibular Apparatus

35. Vestibular Apparatus • Cristae are receptors within ampullae that detect rotational acceleration • Maculae are receptors within utricle and saccule that detect linear acceleration and gravity

36. Vestibular Apparatus: Semicircular Canals • Provide information about rotational acceleration. • Project in 3 different planes. Figure 10-23b

37. Semicircular Canals • At the base of the semicircular duct is the crista ampullaris, where sensory hair cells are located. • Hair cell processes are embedded in the cupula.

38. Semicircular Canals • Endolymph provides inertia so that the sensory processes will bend in direction opposite to the angular acceleration.

39. Rotational Forces in the Cristae Figure 10-24

40. Vestibular Apparatus • Cristae are receptors within ampullae that detect rotational acceleration • Maculae are receptors within utricle and saccule that detect linear acceleration and gravity

41. Otolith Organs: Maculae • The otolith organs sense linear acceleration and head position Figure 10-25a

42. Otolith Organs Figure 10-25a

43. Stereocilia and Kinocilium • When stereocilia bend toward kinocilium; membrane depolarizes, and releases NT • When bends away from kinocilium hyperpolarization occurs • Frequency of APs carries information about movement

44. Maculae of the Utricle and Saccule • Utricle: • More sensitive to horizontal acceleration. • During forward acceleration, otolithic membrane lags behind hair cells, so hairs pushed backward. • Saccule: • More sensitive to vertical acceleration. • Hairs pushed upward when person descends.

45. Taste (Gustation) • Taste Receptors - Clustered in taste buds • Associated with lingual papillae • Taste buds • Contain basal cells which appear to be stem cells • Gustatory cells extend taste hairs through a narrow taste pore

46. Taste (Gustation) • Epithelial cell receptors clustered in barrel-shaped taste buds • Each taste bud consists of 50-100 specialized epithelial cells. • Taste cells are not neurons, but depolarize upon stimulation and if reach threshold, release NT that stimulate sensory neurons.

47. Taste (continued) • Each taste bud contains taste cells responsive to each of the different taste categories. • A given sensory neuron may be stimulated by more than 1 taste cell in # of different taste buds • One sensory fiber may not transmit information specific for only 1 category of taste • Brain interprets the pattern of stimulation with the sense of smell; so that we perceive complex tastes

48. Taste Receptor Distribution • Salty: • Na+ passes through channels, activates specific receptor cells, depolarizing the cells, and releasing NT. • Sour: • Presence of H+ passes through the channel, opens Ca+ channels

49. Taste Receptor Distribution (continued) • Sweet and bitter: • Mediated by receptors coupled to G-protein (gustducin).

50. Summary of Taste Transduction Figure 10-16