Human Anatomy and Physiology Tenth Edition Chapter 15 Part B The Special Senses Copyright © 2016 Pearson Education, Inc. All Rights Reserved

15.2 Focusing and Light Overview: Light and Optics Wavelength and color Electromagnetic radiation: all energy waves, from long radio waves to short X rays; visible light occupies a small portion in the middle of the spectrum Light has wavelengths between 400 and 700 nm Eyes respond only to visible light

Figure 15.10a the Electromagnetic Spectrum and Photoreceptor Sensitivities

Overview: Light and Optics (1 of 3) Wavelength and color Light: packets of energy (photons or quanta) that travel in wavelike fashion at high speeds When visible light passes through spectrum, it is broken up into bands of colors (rainbow) Red wavelengths are longest and have lowest energy, and violet are shortest and have most energy Color that eye perceives is a reflection of that wavelength Grass is green because it absorbs all colors except green White reflects all colors, and black absorbs all colors

Figure 15.10b the Electromagnetic Spectrum and Photoreceptor Sensitivities

Overview: Light and Optics (2 of 3) Refraction and lenses Refraction: bending of light rays Due to change in speed of light when it passes from one transparent medium to another and path of light is at an oblique angle Example: from liquid to air Lenses of eyes can also refract light because they are curved on both sides Convex: thicker in center than at edges Concave: thicker at edges than in center

Figure 15.11 Refraction

Overview: Light and Optics (3 of 3) Refraction and lenses Convex lenses bend light passing through it, so that rays converge at focal point Image formed at focal point is upside-down and reversed from left to right Concave lenses disperse light, preventing light from being focused

Figure 15.12 Light Is Focused by a Convex Lens

Focusing Light on the Retina (1 of 5) Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, and finally photoreceptors Light is refracted three times along path: (1) entering cornea, (2) entering lens, and (3) leaving lens Majority of refractory power is in cornea; however, it is constant and cannot change focus

Focusing Light on the Retina (2 of 5) Lens is able to adjust its curvature to allow for fine focusing Can focus for distant vision and for close vision

Figure 15.13a Focusing for Distant and Close Vision

Focusing Light on the Retina (3 of 5) Focusing for distant vision Eyes are best adapted for distant vision Far point of vision: distance beyond which no change in lens shape is needed for focusing 20 feet for emmetropic (normal) eye Cornea and lens focus light precisely on retina at this distance Ciliary muscles are completely relaxed in distance vision, which causes a pull on ciliary zonule; as a result, lenses are stretched flat

Figure 15.13b Focusing for Distant and Close Vision

Focusing Light on the Retina (4 of 5) Focusing for close vision Light from close objects (<6 m) diverges as approaches eye Requires eye to make active adjustments using three simultaneous processes: Accommodation of the lenses Changing lens shape to increase refraction Near point of vision Closest point on which the eye can focus Presbyopia: loss of accommodation over age 50

Focusing Light on the Retina (5 of 5) Constriction of the pupils Accommodation pupillary reflex involves constriction of pupils to prevent most divergent light rays from entering eye Mediated by parasympathetic nervous system Convergence of the eyeballs Medial rotation of eyeballs causes convergence of eyes toward object being viewed Controlled by somatic motor neuron innervation on medial rectus muscles

Figure 15.13c Focusing for Distant and Close Vision

Clinical – Homeostatic Imbalance 15.8 Problems associated with refraction related to eyeball shape: Myopia (nearsightedness) Eyeball is too long, so focal point is in front of retina Corrected with a concave lens Hyperopia (farsightedness) Eyeball is too short, so focal point is behind retina Corrected with a convex lens Astigmatism Unequal curvatures in different parts of cornea or lens Corrected with cylindrically ground lenses or laser procedures

Figure 15.14-1 Problems of Refraction

Figure 15.14-2 Problems of Refraction

15.3 Phototransduction (1 of 3) Functional Anatomy of Photoreceptors Photoreceptors (rods and cones) are modified neurons that resemble upside-down epithelial cells Consists of cell body, synaptic terminal, and two segments: Outer segment: light-receiving region Contains visual pigments (photopigments) that change shape as they absorb light Inner segment of each joins cell body Inner segment is connected via cilium to outer segment and to cell body via outer fiber

15.3 Phototransduction (2 of 3) Cell body is connected to synaptic terminal via inner fibers Plasma membrane of outer segment folds back to form many discs Photopigments are embedded in discs

15.3 Phototransduction (3 of 3) Photoreceptors are vulnerable to damage Degenerate if retina detached Destroyed by intense light Vision is maintained because outer segment is renewed every 24 hours Tips fragment off and are phagocytized

Figure 15.15a Photoreceptors of the Retina

Comparing Rod and Cone Vision (1 of 2) Rods are very sensitive to light, making them best suited for night vision and peripheral vision Contain a single pigment, so vision is perceived in gray tones only Pathways converge, causing fuzzy, indistinct images As many as 100 rods may converge into one ganglion

Comparing Rod and Cone Vision (2 of 2) Cones have low sensitivity, so require bright light for activation React more quickly than rods Have one of three pigments, which allow for vividly colored sight Nonconverging pathways result in detailed, high- resolution vision Some cones have their own ganglion cell, so brain can put together accurate, high-acuity resolution images

Table 15.1 Comparison of Rods and Cones Noncolor vision (one visual pigment) Color vision (three visual pigments) High sensitivity; function in dim light Low sensitivity; function in bright light Low acuity (many rods converge onto one ganglion cell) High acuity (one cone per ganglion cell in fovea) More numerous (20 rods for every cone) Less numerous Mostly in peripheral retina Mostly in central retina

Clinical – Homeostatic Imbalance 15.9 Color blindness: lack of one or more cone pigments Inherited as an X-linked condition, so more common in males As many as 8–10% of males have some form The most common type is red-green, in which either red cones or green cones are absent Depending on which cone is missing, red can appear green, or vice versa Rely on different shades to get cues of color

Visual Pigments (1 of 2) Retinal: key light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments Synthesized from vitamin A Four opsins are rhodopsin (found in rods only), and three found in cones: green, blue, red (depending on wavelength of light they absorb) Cone wavelengths do overlap, so same wavelength may trigger more than one cone, enabling us to see variety of hues of colors Example: yellow light stimulates red and green cones, but if more red are triggered, we see orange

Visual Pigments (2 of 2) Retinal isomers are different 3-D forms Retinal is in a bent form in dark, but when pigment absorbs light, it straightens out Bent form called 11-cis-retinal Straight form called all-trans-retinal Conversion of bent to straight initiates reactions that lead to electrical impulses along optic nerve

Figure 15.15b Photoreceptors of the Retina

Phototransduction (1 of 3) Phototransduction: process by which pigment captures photon of light energy, which is converted into a graded receptor potential Capturing light Deep purple pigment of rods is rhodopsin Arranged in rod’s outer segment Three steps of rhodopsin formation and breakdown: Pigment synthesis, pigment bleaching, and pigment regeneration Similar process in cones, but different types of opsins and cones require more intense light

Phototransduction (2 of 3) Capturing light Pigment synthesis Opsin and 11-cis retinal combine to form rhodopsin in dark Pigment bleaching When rhodopsin absorbs light, 11-cis isomer of retinal changes to all-trans isomer Retinal and opsin separate (rhodopsin breakdown) Pigment regeneration All-trans retinal converted back to 11-cis isomer Rhodopsin is regenerated in outer segments

Figure 15.16 The Formation and Breakdown of Rhodopsin (1 of 3)

Figure 15.16 The Formation and Breakdown of Rhodopsin (2 of 3)

Figure 15.16 The Formation and Breakdown of Rhodopsin (3 of 3)

Phototransduction (3 of 3) Light transduction reactions Light-activated rhodopsin activates G protein transducin Transducin activates PDE, which breaks down cyclic GMP (cGMP) In dark, cGMP holds cation channels of outer segment open enter and depolarize cell In light cGMP breaks down, channels close, cell hyperpolarizes Hyperpolarization is signal for vision!

Figure 15.17 Events of Phototransduction (1 of 5)

Figure 15.17 Events of Phototransduction (2 of 5)

Figure 15.17 Events of Phototransduction (3 of 5)

Figure 15.17 Events of Phototransduction (4 of 5)

Figure 15.17 Events of Phototransduction (5 of 5)

Information Processing in the Retina Photoreceptors and bipolar cells generate only graded potentials (EPSPs and IPSPs), not APs When light hyperpolarizes photoreceptor cells, they stop releasing inhibitory neurotransmitter glutamate to biopolar cells Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells Ganglion cells generate APs transmitted in optic nerve to brain

Figure 15.18-1 Signal Transmission in the Retina (1 of 7)

Figure 15.18-1 Signal Transmission in the Retina (2 of 7)

Figure 15.18-1 Signal Transmission in the Retina (3 of 7)

Figure 15.18-1 Signal Transmission in the Retina (4 of 7)

Figure 15.18-1 Signal Transmission in the Retina (5 of 7)

Figure 15.18-1 Signal Transmission in the Retina (6 of 7)

Figure 15.18-1 Signal Transmission in the Retina (7 of 7)

Figure 15.18-2 Signal Transmission in the Retina (1 of 7)

Figure 15.18-2 Signal Transmission in the Retina (2 of 7)

Figure 15.18-2 Signal Transmission in the Retina (3 of 7)

Figure 15.18-2 Signal Transmission in the Retina (4 of 7)

Figure 15.18-2 Signal Transmission in the Retina (5 of 7)

Figure 15.18-2 Signal Transmission in the Retina (6 of 7)

Figure 15.18-2 Signal Transmission in the Retina (7 of 7)

Light and Dark Adaptation (1 of 3) Rhodopsin is so sensitive that bleaching occurs even in starlight In bright light, bleaching occurs so fast that rods are virtually nonfunctional Cones respond to bright light So, activation of rods and cones depends on: Light adaptation Dark adaptation

Light and Dark Adaptation (2 of 3) Light adaptation When moving from darkness into bright light we see glare because: Both rods and cones are strongly stimulated Large amounts of pigments are broken down instantaneously, producing glare Pupils constrict Visual acuity improves over 5–10 minutes as: Rod system turns off Retinal sensitivity decreases Cones and neurons rapidly adapt

Light and Dark Adaptation (3 of 3) When moving from bright light into darkness, we see blackness because: Cones stop functioning in low-intensity light Bright light bleached rod pigments, so they are still turned off Pupils dilate Rhodopsin accumulates in dark, so retinal sensitivity starts to increase Transducin returns to outer segments Sensitivity increases within 20–30 minutes

Clinical – Homeostatic Imbalance 15.10 Nyctalopia (night blindness): condition in which rod function is seriously hampered Ability to drive safely at night is impaired Due to rod degeneration, commonly caused by prolonged vitamin A deficiency If administered early, vitamin A supplements restore function Can also be caused by retinitis pigmentosa Degenerative retinal diseases that destroy rods Tips of rods are not replaced when they slough off

15.4 Processing and Relaying of Visual Information (1 of 3) Visual Pathway to the Brain Axons of retinal ganglion cells form optic nerve Medial fibers from each eye cross over at the optic chiasma then continue on as optic tracts, which means each optic tract: Contains fibers from lateral (temporal) aspect of eye on same side and medial (nasal) aspect of opposite eye, and Each carries information from same half of visual field

15.4 Processing and Relaying of Visual Information (2 of 3) Most fibers of optic tracts continue on to lateral geniculate nuclei of thalamus From there, thalamic neurons form optic radiation, which projects to primary visual cortex in occipital lobes Conscious perception of visual images occurs here

15.4 Processing and Relaying of Visual Information (3 of 3) Other optic tract fibers send branches to midbrain One set ends in superior colliculi, area controlling extrinsic eye muscles A small subset of ganglion cells in retina contains melanopsin (circadian pigment), which projects to: Pretectal nuclei: involved with pupillary reflexes Suprachiasmatic nucleus of hypothalamus: timer for daily biorhythms

Figure 15.19a Visual Pathway to the Brain and Visual Fields, Inferior View

Figure 15.19 Visual Pathway to the Brain and Visual Fields, Inferior View

Depth Perception Both eyes view same image from slightly different angles Visual cortex fuses these slightly different images, resulting in a three-dimensional image, which leads to depth perception Requires input from both eyes

Clinical – Homeostatic Imbalance 15.11 Loss of an eye or destruction of one optic nerve eliminates true depth perception entirely Peripheral vision on damaged side is also affected If neural destruction occurs beyond optic chiasma, then part or all of opposite half of the visual field is lost Example: stroke can affect left visual cortex, which leads to blindness in right half of visual field

Visual Processing (1 of 3) Retinal cells split input into channels that include information about: Color and brightness, but also complex info such as angle, direction, and speed of movement of edges (sudden changes in brightness or color) Lateral inhibition decodes “edge” information Job of amacrine and horizontal cells

Visual Processing (2 of 3) Ganglions pass information to lateral geniculate nuclei of thalamus to be processed for depth perception, with cone input emphasized Primary visual cortex contains topographical map of retina Neurons here respond to dark and bright edges and to object orientation Provide form, color, motion inputs to visual association areas

Visual Processing (3 of 3) Info is also passed on to temporal, parietal, and frontal lobes, where objects are identified and location in space determined