Lecture Human anatomy and physiology - Chapter 15: The special senses (part b)

In this chapter, students will be able to understand: Describe the events involved in the stimulation of photoreceptors by light, and compare and contrast the roles of rods and cones in vision; compare and contrast light and dark adaptation; trace the visual pathway to the visual cortex, and briefly describe the steps in visual processing.

Trang 1

PowerPoint ® Lecture Slides

prepared by Janice Meeking, Mount Royal College

C H A P T E R

15

The Special Senses:

Part B

Trang 2

Light

portion of the electromagnetic spectrum

(quanta) that travel in a wavelike fashion

wavelengths of the visible spectrum

Trang 3

(420 nm) Rods(500 nm)

Green cones

(530 nm)

Red cones

(560 nm)

X rays UV Infrared

Micro-waves Radio waves

Gamma rays

Trang 4

Refraction and Lenses

• Bending of a light ray due to change in speed when light passes from one transparent

medium to another

• Occurs when light meets the surface of a

different medium at an oblique angle

Trang 5

Refraction and Lenses

eye) is bent so that the rays converge at a

focal point

upside-down and reversed right to left

Trang 6

Point sources

(a) Focusing of two points of light.

(b) The image is inverted—upside down and reversed

Focal points

Trang 7

Focusing Light on the Retina

humor, lens, vitreous humor, neural layer of retina, photoreceptors

an image

Trang 8

Focusing for Distant Vision

parallel at the eye and need little refraction

beyond what occurs in the at-rest eye

no change in lens shape is needed for

focusing; 20 feet for emmetropic (normal) eye

zonule

Trang 9

Lens

Inverted image

Ciliary zonule Ciliary muscle

Nearly parallel rays

from distant object

(a) Lens is flattened for distant vision Sympathetic

input relaxes the ciliary muscle, tightening the ciliary

zonule, and flattening the lens.

Sympathetic activation

Trang 10

Focusing for Close Vision

approaches the eye; requires that the eye make active adjustments

Trang 11

Focusing for Close Vision

• Close vision requires

ciliary muscles to increase refractory power

bulge the lens can achieve

constricts the pupils to prevent the most divergent light rays from entering the eye

toward the object being viewed

Trang 12

Divergent rays

from close object

input contracts the ciliary muscle, loosening the

ciliary zonule, allowing the lens to bulge.

Inverted image

Parasympathetic activation

Trang 13

Problems of Refraction

the retina, e.g in a longer than normal eyeball

retina, e.g in a shorter than normal eyeball

different parts of the cornea or lens

implants, or laser procedures

Trang 14

Focal plane

Focal point is on retina.

Emmetropic eye (normal)

Trang 15

Concave lens moves focal point further back.

Eyeball too long

Trang 16

Eyeball too short

Trang 17

Functional Anatomy of Photoreceptors

• Outer segment of each contains visual

pigments (photopigments)—molecules that change shape as they absorb light

• Inner segment of each joins the cell body

Trang 18

Process of bipolar cell

Outer fiber

Apical microvillus Discs containing visual pigments

Melanin granules

Discs being phagocytized Pigment cell nucleus

Inner fibers Rod cell body Cone cell body

Synaptic terminals Rod cell body

Nuclei

Mitochondria Connecting cilia

Basal lamina (border with choroid)

The outer segments

of rods and cones

are embedded in the

Trang 19

Rods

• Very sensitive to dim light

• Best suited for night vision and peripheral vision

• Perceived input is in gray tones only

• Pathways converge, resulting in fuzzy and indistinct images

Trang 20

Trang 21

Chemistry of Visual Pigments

four proteins (opsin) to form visual pigments

all-trans-retinal (straight form)

initiates a chain of reactions leading to transmission

of electrical impulses in the optic nerve

Trang 22

Rod discs

Visual pigment consists of

(b) Rhodopsin, the visual pigment in rods, is embedded in

the membrane that forms discs in the outer segment.

Trang 23

Excitation of Rods

11-cis-retinal)

pigment)

Trang 24

11-cis-retinal

Bleaching of the pigment:

Light absorption

by rhodopsin triggers a rapid series of steps

in which retinal changes shape

(11-cis to all-trans)

and eventually releases from opsin.

2H +

Reduction Vitamin A

2

11-cis-retinal

All-trans-retinal

Trang 25

Excitation of Cones

colors of light absorbed: blue, green, and red

of more than one type of cone at the same

time

one or more of the cone types

Trang 26

Phototransduction

channels in the outer segments of

photoreceptor cells

• Na + and Ca 2+ influx creates a depolarizing dark potential of about 40 mV

Trang 27

from sodium channels

the membrane hyperpolarizes to about 70 mV

Trang 28

phosphodiester ase (PDE).

4

PDE converts cGMP into GMP, causing cGMP levels to fall.

5

As cGMP levels fall, cGMP-gated cation channels close, resulting in hyperpolarization.

Visual

pigment

Light

Transducin (a G protein)

All-trans-retinal

11-cis-retinal

Open cGMP-gated cation channel

Phosphodiesterase (PDE)

Closed cGMP-gated cation channel

Trang 29

Signal Transmission in the Retina

graded potentials (EPSPs and IPSPs)

them to stop releasing the inhibitory neurotransmitter glutamate

depolarize and release neurotransmitter onto

ganglion cells

the optic nerve

Trang 30

1

cGMP-gated channels open, allowing cation influx;

the photoreceptor depolarizes.

3 4

Hyperpolarization closes voltage-gated Ca 2+ channels, inhibiting neurotransmitter release

5

No EPSPs occur in ganglion cell.

Bipolar cell

Ganglion cell

In the dark

Trang 31

1

cGMP-gated channels are closed, so cation influx stops; the photoreceptor hyperpolarizes.

7

Photoreceptor cell (rod)

Bipolar cell

Ganglion cell

Light

Ca 2+

In the light

Trang 32

Light Adaptation

bright light

instantaneously, producing glare

• Pupils constrict

• Dramatic changes in retinal sensitivity: rod function ceases

Trang 33

Dark Adaptation

darkness

• The reverse of light adaptation

• Cones stop functioning in low-intensity light

• Pupils dilate

• Rhodopsin accumulates in the dark and retinal sensitivity increases within 20–30 minutes

Trang 34

Visual Pathway

nerve

the optic chiasma

lateral geniculate body of the thalamus

Trang 35

Visual Pathway

primary visual cortex in the occipital lobes

midbrain, ending in superior colliculi (initiating visual reflexes)

Trang 36

Visual Pathway

contain melanopsin (circadian pigment),

which projects to:

• Pretectal nuclei (involved with pupillary

reflexes)

the timer for daily biorhythms

Trang 37

Uncrossed (ipsilateral) fiber Crossed (contralateral) fiber

Optic nerve

Lateral geniculate

nucleus of

thalamus

(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.

Suprachiasmatic

nucleus

Trang 38

Depth Perception

different angles

results from cortical fusion of the slightly

different images

Trang 39

Retinal Processing

in doughnut-shaped receptive fields

for glutamate in the “on” and “off” fields

Trang 40

Stimulus pattern (portion of receptive field illuminated)

No illumination or

diffuse illumination

(basal rate)

Center illuminated

Surround illuminated

Response of off-center ganglion cell during period of light stimulus

Response of on-center ganglion cell during period of light stimulus

Trang 41

Thalamic Processing

• Segregate the retinal axons in preparation for depth perception

• Emphasize visual inputs from regions of high cone density

• Sharpen contrast information

Trang 42

Cortical Processing

cortex

regions

location

Định dạng
Số trang	42
Dung lượng	1,12 MB