15 Human Anatomy & Physiology

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(1)PowerPoint® Lecture Slides prepared by Barbara Heard, Atlantic Cape Community Ninth Edition College. Human Anatomy & Physiology. CHAPTER. 15. The Special Senses: Part B © Annie Leibovitz/Contact Press Images. © 2013 Pearson Education, Inc..

(2) Light And Optics: Wavelength And Color • Eyes respond to visible light – Small portion of electromagnetic spectrum – Wavelengths of 400-700 nm. • Light – Packets of energy (photons or quanta) that travel in wavelike fashion at high speeds – Color of light objects reflect determines color eye perceives. © 2013 Pearson Education, Inc..

(3) Figure 15.10 The electromagnetic spectrum and photoreceptor sensitivities. 10 nm 10 nm 1 nm –5. 10 nm. –3. Gamma rays. X rays. 3. UV. (109 nm =) 10 nm 1m 6. Infrared. 103 m. Micro- Radio waves waves. Light absorption (percent of maximum). Visible light. Blue cones (420 nm) 100. 50. 0 400 © 2013 Pearson Education, Inc.. Green Red Rods cones cones (500 nm) (530 nm) (560 nm). 450. 500 550 600 Wavelength (nm). 650. 700.

(4) Light And Optics: Refraction And Lenses • Refraction – Bending of light rays • Due to change in speed when light passes from one transparent medium to another • Occurs when light meets surface of different medium at an oblique angle. – Curved lens can refract light. © 2013 Pearson Education, Inc..

(5) Figure 15.11 Refraction.. © 2013 Pearson Education, Inc..

(6) Refraction and Lenses • Light passing through convex lens (as in eye) is bent so that rays converge at focal point – Image formed at focal point is upside-down and reversed right to left. • Concave lenses diverge light – Prevent light from focusing. © 2013 Pearson Education, Inc..

(7) Figure 15.12 Light is focused by a convex lens.. Point sources. Focal points. Focusing of two points of light.. The image is inverted—upside down and reversed. © 2013 Pearson Education, Inc..

(8) Focusing Light on The Retina • Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, photoreceptors • Light refracted three times along pathway – Entering cornea – Entering lens – Leaving lens. • Majority of refractory power in cornea • Change in lens curvature allows for fine focusing. © 2013 Pearson Education, Inc..

(9) Focusing For Distant Vision • Eyes best adapted for distant vision • Far point of vision – Distance beyond which no change in lens shape needed for focusing • 20 feet for emmetropic (normal) eye • Cornea and lens focus light precisely on retina. • Ciliary muscles relaxed • Lens stretched flat by tension in ciliary zonule © 2013 Pearson Education, Inc..

(10) Figure 15.13a Focusing for distant and close vision.. Nearly parallel rays from distant object. Sympathetic activation. Lens. Ciliary zonule Ciliary muscle. Inverted image Lens flattens for distant vision. Sympathetic input relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens.. © 2013 Pearson Education, Inc..

(11) 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 lenses • Constriction of pupils • Convergence of eyeballs. © 2013 Pearson Education, Inc..

(12) Focusing For Close Vision • Accommodation – 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. • Constriction – Accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye. • Convergence – Medial rotation of eyeballs toward object being viewed. © 2013 Pearson Education, Inc..

(13) Figure 15.13b Focusing for distant and close vision.. Parasympathetic activation Divergent rays Inverted from close object image. Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge.. © 2013 Pearson Education, Inc..

(14) Figure 15.13c Focusing for distant and close vision.. View Ciliary muscle Lens Ciliary zonule (suspensory ligament) The ciliary muscle and ciliary zonule are arranged sphincterlike around the lens. As a result, contraction loosens the ciliary zonule fibers and relaxation tightens them. © 2013 Pearson Education, Inc..

(15) Problems Of Refraction • Myopia (nearsightedness) – Focal point in front of retina, e.g., eyeball too long – Corrected with a concave lens. • Hyperopia (farsightedness) – Focal point behind retina, e.g., eyeball too short – Corrected with a convex lens. • Astigmatism – Unequal curvatures in different parts of cornea or lens – Corrected with cylindrically ground lenses or laser procedures. © 2013 Pearson Education, Inc..

(16) Figure 15.14 Problems of refraction. (1 of 3). Emmetropic eye (normal) Focal plane. Focal point is on retina. © 2013 Pearson Education, Inc..

(17) Figure 15.14 Problems of refraction. (2 of 3). Myopic eye (nearsighted). Eyeball too long Uncorrected Focal point is in front of retina.. Corrected © 2013 Pearson Education, Inc.. Concave lens moves focal point further back..

(18) Figure 15.14 Problems of refraction. (3 of 3). Hyperopic eye (farsighted). Eyeball too short Uncorrected Focal point is behind retina.. Corrected © 2013 Pearson Education, Inc.. Convex lens moves focal point forward..

(19) Functional Anatomy Of Photoreceptors • Rods and cones – Modified neurons – Receptive regions called outer segments • Contain visual pigments (photopigments) – Molecules change shape as absorb light. – Inner segment of each joins cell body. © 2013 Pearson Education, Inc..

(20) Synaptic terminals Rod cell body. Inner fibers Rod cell body. Nuclei. Cone cell body. Mitochondria. © 2013 Pearson Education, Inc.. Pigmented layer Inner Outer segment segment. Outer fiber. The outer segments of rods and cones are embedded in the pigmented layer of the retina.. Light. Lig ht. Process of bipolar cell. Ligh t. Figure 15.15a Photoreceptors of the retina.. Melanin granules. Connecting cilia. Apical microvillus Discs containing visual pigments Discs being phagocytized Pigment cell nucleus Basal lamina (border with choroid).

(21) Photoreceptor Cells • • • •. Vulnerable to damage Degenerate if retina detached Destroyed by intense light Outer segment renewed every 24 hours – Tips fragment off and are phagocytized. © 2013 Pearson Education, Inc..

(22) Rods • Functional characteristics – Very sensitive to light – Best suited for night vision and peripheral vision – Contain single pigment • Perceived input in gray tones only. – Pathways converge, causing fuzzy, indistinct images. © 2013 Pearson Education, Inc..

(23) Cones • Functional characteristics – Need bright light for activation (have low sensitivity) – React more quickly – Have one of three pigments for colored view – Nonconverging pathways result in detailed, high-resolution vision – Color blindness–lack of one or more cone pigments. © 2013 Pearson Education, Inc..

(24) Table 15.1 Comparison of Rods and Cones. © 2013 Pearson Education, Inc..

(25) Chemistry Of Visual Pigments • Retinal – Light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments – Synthesized from vitamin A – Retinal isomers: 11-cis-retinal (bent form) and all-trans-retinal (straight form) • Bent form  straight form when pigment absorbs light • Conversion of bent to straight initiates reactions  electrical impulses along optic nerve © 2013 Pearson Education, Inc..

(26) Phototransduction: Capturing Light • Deep purple pigment of rods–rhodopsin – 11-cis-retinal + opsin  rhodopsin – Three steps of rhodopsin formation and breakdown • Pigment synthesis • Pigment bleaching • Pigment regeneration. © 2013 Pearson Education, Inc..

(27) Figure 15.15b Photoreceptors of the retina.. Rod discs. Visual pigment consists of • Retinal • Opsin. © 2013 Pearson Education, Inc.. Rhodopsin, the visual pigment in rods, is embedded in the membrane that forms discs in the outer segment..

(28) Phototransduction: Capturing Light • Pigment synthesis – Rhodopsin forms and accumulates in dark. • Pigment bleaching – When rhodopsin absorbs light, retinal changes to all-trans isomer – Retinal and opsin separate (rhodopsin breakdown). • Pigment regeneration – All-trans retinal converted to 11-cis isomer – Rhodopsin regenerated in outer segments © 2013 Pearson Education, Inc..

(29) Figure 15.16 The formation and breakdown of rhodopsin.. 11-cis-retinal. 2H+ 1 Pigment synthesis: Oxidation 11-cis-retinal, derived from vitamin A, is Vitamin A 11-cis-retinal Rhodopsin combined with opsin to form rhodopsin. Reduction 2H+ 3 Pigment regeneration: Enzymes slowly convert all-trans-retinal to its 11cis form in cells of the pigmented layer; requires ATP.. Dark Light. 2 Pigment bleaching: Light absorption by rhodopsin triggers a rapid series of steps in which retinal changes shape (11-cis to alltrans) and eventually releases from opsin.. Opsin and All-transretinal. O All-trans-retinal © 2013 Pearson Education, Inc..

(30) Figure 15.17 Events of phototransduction.. Slide 1. Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race. 2nd Light Receptor G protein Enzyme messenger (1st messenger). 1 Retinal absorbs light and changes shape. Visual pigment activates.. Phosphodiesterase (PDE). Visual pigment. All-trans-retinal Light. cGMP-gated cation channel open in dark. 11-cis-retinal Transducin (a G protein). 2 Visual pigment activates transducin (G protein). © 2013 Pearson Education, Inc.. 3 Transducin activates phosphodiesteras e (PDE).. 4 PDE converts cGMP into GMP, causing cGMP levels to fall.. cGMP-gated cation channel closed in light. 5 As cGMP levels fall, cGMP-gated cation channels close, resulting in hyperpolarization..

(31) Figure 15.17 Events of phototransduction.. Slide 2. Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race. 1 Retinal absorbs light and changes shape. Visual pigment activates.. Visual pigment. All-trans-retinal Light. 11-cis-retinal Transducin (a G protein). © 2013 Pearson Education, Inc.. 2nd Light Receptor G protein Enzyme messenger (1st messenger).

(32) Figure 15.17 Events of phototransduction.. Slide 3. Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race. 1 Retinal absorbs light and changes shape. Visual pigment activates.. Visual pigment. All-trans-retinal Light. 11-cis-retinal Transducin (a G protein). 2 Visual pigment activates transducin (G protein). © 2013 Pearson Education, Inc.. 2nd Light Receptor G protein Enzyme messenger (1st messenger).

(33) Figure 15.17 Events of phototransduction.. Slide 4. Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race. 2nd Light Receptor G protein Enzyme messenger (1st messenger). 1 Retinal absorbs light and changes shape. Visual pigment activates.. Phosphodiesterase (PDE). Visual pigment. All-trans-retinal Light. 11-cis-retinal Transducin (a G protein). 2 Visual pigment activates transducin (G protein). © 2013 Pearson Education, Inc.. 3 Transducin activates phosphodiesteras e (PDE)..

(34) Figure 15.17 Events of phototransduction.. Slide 5. Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race. 2nd Light Receptor G protein Enzyme messenger (1st messenger). 1 Retinal absorbs light and changes shape. Visual pigment activates.. Phosphodiesterase (PDE). Visual pigment. All-trans-retinal Light. 11-cis-retinal Transducin (a G protein). 2 Visual pigment activates transducin (G protein). © 2013 Pearson Education, Inc.. 3 Transducin activates phosphodiesteras e (PDE).. 4 PDE converts cGMP into GMP, causing cGMP levels to fall..

(35) Figure 15.17 Events of phototransduction.. Slide 6. Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race. 2nd Light Receptor G protein Enzyme messenger (1st messenger). 1 Retinal absorbs light and changes shape. Visual pigment activates.. Phosphodiesterase (PDE). Visual pigment. All-trans-retinal Light. cGMP-gated cation channel open in dark. 11-cis-retinal Transducin (a G protein). 2 Visual pigment activates transducin (G protein). © 2013 Pearson Education, Inc.. 3 Transducin activates phosphodiesteras e (PDE).. 4 PDE converts cGMP into GMP, causing cGMP levels to fall.. cGMP-gated cation channel closed in light. 5 As cGMP levels fall, cGMP-gated cation channels close, resulting in hyperpolarization..

(36) Phototransduction In Cones • Similar as process in rods • Cones far less sensitive to light – Takes higher-intensity light to activate cones. © 2013 Pearson Education, Inc..

(37) Light Transduction Reactions • Light-activated rhodopsin activates G protein transducin • Transducin activates PDE, which breaks down cyclic GMP (cGMP) • In dark, cGMP holds channels of outer segment open  Na+ and Ca2+ depolarize cell • In light cGMP breaks down, channels close, cell hyperpolarizes – Hyperpolarization is signal! © 2013 Pearson Education, Inc..

(38) Information Processing In The Retina • Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs) • When light hyperpolarizes photoreceptor cells – Stop releasing inhibitory neurotransmitter glutamate – Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells – Ganglion cells generate APs transmitted in optic nerve to brain © 2013 Pearson Education, Inc..

(39) Figure 15.18 Signal transmission in the retina (1 of 2).. Slide 1. In the dark. 1 cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes.. Na+ Ca2+. 2 Voltage-gated Ca2+ channels open in synaptic terminals.. Photoreceptor cell (rod) −40 mV. 3 Neurotransmitter is released continuously.. Ca2+. 4 Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. 5 Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release.. Bipolar Cell. 6 No EPSPs occur in ganglion cell. 7 No action potentials occur along the optic nerve. © 2013 Pearson Education, Inc.. Ganglion cell.

(40) Figure 15.18 Signal transmission in the retina (1 of 2).. Slide 2. In the dark. 1 cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes.. Na+ Ca2+ Photoreceptor cell (rod) −40 mV. Bipolar Cell. Ganglion cell. © 2013 Pearson Education, Inc..

(41) Figure 15.18 Signal transmission in the retina (1 of 2).. Slide 3. In the dark. 1 cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes.. Na+ Ca2+. 2 Voltage-gated Ca2+ channels open in synaptic terminals.. Photoreceptor cell (rod) −40 mV Ca2+. Bipolar Cell. Ganglion cell. © 2013 Pearson Education, Inc..

(42) Figure 15.18 Signal transmission in the retina (1 of 2).. Slide 4. In the dark. 1 cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes.. Na+ Ca2+. 2 Voltage-gated Ca2+ channels open in synaptic terminals.. Photoreceptor cell (rod) −40 mV. 3 Neurotransmitter is released continuously.. Ca2+. Bipolar Cell. Ganglion cell. © 2013 Pearson Education, Inc..

(43) Figure 15.18 Signal transmission in the retina (1 of 2).. Slide 5. In the dark. 1 cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes.. Na+ Ca2+. 2 Voltage-gated Ca2+ channels open in synaptic terminals.. Photoreceptor cell (rod) −40 mV. 3 Neurotransmitter is released continuously.. Ca2+. 4 Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. Bipolar Cell. Ganglion cell. © 2013 Pearson Education, Inc..

(44) Figure 15.18 Signal transmission in the retina (1 of 2).. Slide 6. In the dark. 1 cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes.. Na+ Ca2+. 2 Voltage-gated Ca2+ channels open in synaptic terminals.. Photoreceptor cell (rod) −40 mV. 3 Neurotransmitter is released continuously.. Ca2+. 4 Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. 5 Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release.. Bipolar Cell. Ganglion cell. © 2013 Pearson Education, Inc..

(45) Figure 15.18 Signal transmission in the retina (1 of 2).. Slide 7. In the dark. 1 cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes.. Na+ Ca2+. 2 Voltage-gated Ca2+ channels open in synaptic terminals.. Photoreceptor cell (rod) −40 mV. 3 Neurotransmitter is released continuously.. Ca2+. 4 Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. 5 Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release.. Bipolar Cell. 6 No EPSPs occur in ganglion cell. Ganglion cell. © 2013 Pearson Education, Inc..

(46) Figure 15.18 Signal transmission in the retina (1 of 2).. Slide 8. In the dark. 1 cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes.. Na+ Ca2+. 2 Voltage-gated Ca2+ channels open in synaptic terminals.. Photoreceptor cell (rod) −40 mV. 3 Neurotransmitter is released continuously.. Ca2+. 4 Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. 5 Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release.. Bipolar Cell. 6 No EPSPs occur in ganglion cell. 7 No action potentials occur along the optic nerve. © 2013 Pearson Education, Inc.. Ganglion cell.

(47) Figure 15.18 Signal transmission in the retina. (2 of 2).. Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light 1cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes.. Light Light Photoreceptor cell (rod). 2 Voltage-gated Ca2+ channels close in synaptic terminals. −70 mV. 3No neurotransmitter is released. 4 Lack of IPSPs in bipolar cell results in depolarization. 5Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released.. Bipolar Cell Ca2+. Ganglion cell © 2013 Pearson Education, Inc.. 6EPSPs occur in ganglion cell. 7Action potentials propagate along the optic nerve.. Slide 1.

(48) Figure 15.18 Signal transmission in the retina. (2 of 2).. Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light Light Photoreceptor cell (rod) −70 mV. Bipolar Cell. Ganglion cell © 2013 Pearson Education, Inc.. 1cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes.. Slide 2.

(49) Figure 15.18 Signal transmission in the retina. (2 of 2).. Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light Light Photoreceptor cell (rod). 2 Voltage-gated Ca2+ channels close in synaptic terminals. −70 mV. Bipolar Cell. Ganglion cell © 2013 Pearson Education, Inc.. 1cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes.. Slide 3.

(50) Figure 15.18 Signal transmission in the retina. (2 of 2).. Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light Light Photoreceptor cell (rod). 2 Voltage-gated Ca2+ channels close in synaptic terminals. −70 mV. Bipolar Cell. Ganglion cell © 2013 Pearson Education, Inc.. 1cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes.. 3No neurotransmitter is released.. Slide 4.

(51) Figure 15.18 Signal transmission in the retina. (2 of 2).. Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light Light Photoreceptor cell (rod). 1cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. 2 Voltage-gated Ca2+ channels close in synaptic terminals.. −70 mV. 3No neurotransmitter is released. 4 Lack of IPSPs in bipolar cell results in depolarization.. Bipolar Cell. Ganglion cell © 2013 Pearson Education, Inc.. Slide 5.

(52) Figure 15.18 Signal transmission in the retina. (2 of 2).. Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light 1cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes.. Light Light Photoreceptor cell (rod). 2 Voltage-gated Ca2+ channels close in synaptic terminals. −70 mV. 3No neurotransmitter is released. 4 Lack of IPSPs in bipolar cell results in depolarization. 5Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released.. Bipolar Cell Ca2+. Ganglion cell © 2013 Pearson Education, Inc.. Slide 6.

(53) Figure 15.18 Signal transmission in the retina. (2 of 2).. Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light 1cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes.. Light Light Photoreceptor cell (rod). 2 Voltage-gated Ca2+ channels close in synaptic terminals. −70 mV. 3No neurotransmitter is released. 4 Lack of IPSPs in bipolar cell results in depolarization. 5Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released.. Bipolar Cell Ca2+. Ganglion cell © 2013 Pearson Education, Inc.. 6EPSPs occur in ganglion cell.. Slide 7.

(54) Figure 15.18 Signal transmission in the retina. (2 of 2).. Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light 1cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes.. Light Light Photoreceptor cell (rod). 2 Voltage-gated Ca2+ channels close in synaptic terminals. −70 mV. 3No neurotransmitter is released. 4 Lack of IPSPs in bipolar cell results in depolarization. 5Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released.. Bipolar Cell Ca2+. Ganglion cell © 2013 Pearson Education, Inc.. 6EPSPs occur in ganglion cell. 7Action potentials propagate along the optic nerve.. Slide 8.

(55) Light Adaptation • Move from darkness into bright light – Both rods and cones strongly stimulated • Pupils constrict. – Large amounts of pigments broken down instantaneously, producing glare – Visual acuity improves over 5–10 minutes as: • Rod system turns off • Retinal sensitivity decreases • Cones and neurons rapidly adapt. © 2013 Pearson Education, Inc..

(56) Dark Adaptation • Move from bright light into darkness – Cones stop functioning in low-intensity light – Rod pigments bleached; system turned off – Rhodopsin accumulates in dark – Transducin returns to outer segments – Retinal sensitivity increases within 20–30 minutes – Pupils dilate. © 2013 Pearson Education, Inc..

(57) Night Blindness • Nyctalopia • Rod degeneration – Commonly caused by vitamin A deficiency – If administered early vitamin A supplements restore function – Also caused by retinitis pigmentosa • Degenerative retinal diseases that destroy rods. © 2013 Pearson Education, Inc..

(58) Visual Pathway To The Brain • Axons of retinal ganglion cells form optic nerve • Medial fibers of optic nerve decussate at optic chiasma • Most fibers of optic tracts continue to lateral geniculate body of thalamus • Fibers from thalamic neurons form optic radiation and project to primary visual cortex in occipital lobes © 2013 Pearson Education, Inc..

(59) Visual Pathway • Fibers from thalamic neurons form optic radiation • Optic radiation fibers connect to primary visual cortex in occipital lobes • Other optic tract fibers send branches to midbrain, ending in superior colliculi (initiating visual reflexes). © 2013 Pearson Education, Inc..

(60) Visual Pathway • A small subset of ganglion cells in retina contain melanopsin (circadian pigment), which projects to: – Pretectal nuclei (involved with pupillary reflexes) – Suprachiasmatic nucleus of hypothalamus, timer for daily biorhythms. © 2013 Pearson Education, Inc..

(61) Figure 15.19 Visual pathway to the brain and visual fields, inferior view. Both eyes. onl. Righ t. e ye. eye. t Lef. only. Fixation point. y. Right eye Suprachiasmatic nucleus Pretectal nucleus. Left eye Optic nerve. Optic chiasma Optic tract. Lateral geniculate nucleus of thalamus Superior colliculus. Lateral geniculate nucleus Superior colliculus (sectioned) Uncrossed (ipsilateral) fiber Crossed (contralateral) fiber Optic radiation. Occipital lobe (primary visual cortex) The visual fields of the two eyes overlap considerably. Note that fibers from the lateral portion of each retinal field do not cross at the optic chiasma.. © 2013 Pearson Education, Inc.. Corpus callosum. Photograph of human brain, with the right side dissected to reveal internal structures..

(62) Depth Perception • Both eyes view same image from slightly different angles • Depth perception (three-dimensional vision) results from cortical fusion of slightly different images • Requires input from both eyes. © 2013 Pearson Education, Inc..

(63) Visual Processing • Retinal cells split input into channels – Color, brightness, angle, direction, speed of movement of edges (sudden changes of brightness or color) • Lateral inhibition decodes "edge" information – Job of amacrine and horizontal cells. © 2013 Pearson Education, Inc..

(64) Visual Processing • Lateral geniculate nuclei of thalamus – Process for depth perception, cone input emphasized, contrast sharpened. • Primary visual cortex (striate cortex) – Neurons respond to dark and bright edges, and object orientation – Provide form, color, motion inputs to visual association areas (prestriate cortices). © 2013 Pearson Education, Inc..

(65) Cortical Processing • Occipital lobe centers (anterior prestriate cortices) continue processing of form, color, and movement • Complex visual processing extends to other regions – "What" processing identifies objects in visual field • Ventral temporal lobe. – "Where" processing assesses spatial location of objects • Parietal cortex to postcentral gyrus. – Output from both passes to frontal cortex • Directs movements © 2013 Pearson Education, Inc..

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