Chapter 14 Sensory Processes part2

Question 1

Vision is

Answer 1

Sensory detection of light

Question 2

What do photoreceptors do?

Answer 2

transduce photons of light into electrical signals

Question 3

Opsins

Answer 3

Specialized proteins that have been highly conserved in evolution………

Question 4

Numerous strategies have evolved for vision; from simple eyespots (a) to the vertebrate eye (e) which incorporates both the pinhole (b) and refracting lens (c) elements of eye design

Answer 4

Question 5

Photoreception

• Photopigments are made up of a protein (opsin) and nonpeptide organic molecule, a chromophore
• A chromophore utilized by bacteria and every animal (so far) is retinal
• Retinal + opsin = rhodopsin
  
  • Most animals use 11-cis retinal
  • Insects use 3-hydroxy-11-cis retinal
  • Some freshwater vertebrates use 3-dehydro-11-cis retinal

Answer 5

Question 6

Photorceptione prt 2

• Rhodopsin is a G-protein-coupled receptor
  
  • Like others, it has 7-transmembrane domains
• Rhodopsin is localized in the disc membrane of photoreceptive cells
• Absorption of light by 11-cis-retinal changes it into all-trans retinal (1st step)
  
  • The intracellular cascade is species specific

Answer 6

Question 7

Phototransduction in Drosophila

• A compound eye is used
  
  • Contains ommatidia (ium is singular)
    
    • Each has its own lens and photoreceptors, called retinular cells
• Microvilli arranged in arrays called rhabdomeres

Answer 7

Question 8

Phototransduction in Drosophila prt 2

• Very rapid and sensitive
  
  • 1 photon can depolarize the cell several mV

Answer 8

Question 9

The Compound vs. Vertebrate Eye

• The compound eye of arthropods is made up of units called ommatidia, each of which works like an independent lens.

Answer 9

The Compound vs. Vertebrate Eye

• The vertebrate eye only has one lens that is shared by all of its receptors.

Question 10

Compound eye

Answer 10

Vertebrate eye

Question 11

Mammalian Eye

• In the mammalian eye, light is refracted twice: first at the cornea (not shown); next at the lens.
• The resulting image is inverted on the retinal surface (the location of the photoreceptors).

Answer 11

Question 12

Mammalian Retina

Answer 12

cones- colored light (red, blue, green)

rods- black and white light (night vision)

Question 13

Vertebrate Photoreceptors

• Rods (left) & cones (right)
• Rods function better in dim light, but do not have good resolution
• Cones function better in bright light and have high resolution
• The photoreceptor-rich membranes are arranged in flattened stacks or lamellae

Answer 13

Question 14

Phototransduction in Vertebrates

Light causes hyperpolarization!

Answer 14

Question 15

Dark Currents

• In the dark, the photoreceptor membrane is equally permeable to Na+ and K+; therefore, its resting membrane potential is much more positive than an average neuron
• The Na+ channels are kept open by cGMP
• The current carried by the Na+ ions is called the “dark” current.

Answer 15

Question 16

Light Transduction

• Light strikes the visual pigment rhodopsin which has two main components: opsin and retinal (retinene)
• The triggering event in photoreceptor activation is the absorption of photon by the vitamin A-derived molecule retinal
• Light absorption causes a conformational change from cis-retinal to trans-retinal

Answer 16

Question 17

Light Transduction prt 2

• Rhodopsin causes transducin (a G-protein) to cause phosphodiesterase (PDE) to break down cGMP which is needed to keep the “dark current” channels open
• The closing of these channels hyperpolarizes the disk membrane

Answer 17

Question 18

Photoreceptor Electrophysiology

• Light hyperpolarizes vertebrate photoreceptors

Answer 18

Question 19

Visual Pathway (Essay)

Answer 19

Question 20

Color Vision

Cones are the color receptors

Different classes of opsins have different light absorption characteristics, called their “absorptive spectra” (retinal retains its structure in all of these classes)

Absorption (a), electrical activity (b), and %-age activity (c) of cones from a goldfish retina showing three distinct peaks corresponding to wavelengths in the blue, green, and red parts of the visual light spectrum

Answer 20

Question 21

Visual Sensory Processing

• Although photoreceptors respond to light, the visual system responds to contrast
  
  • Occurs in retina and brain
• Ganglion cells respond to stimulation over a large area (the receptive field)
  
  • On-center cell responses
  • Off-center cell responses

Answer 21

Question 22

Visual Sensory Processing prt 2

This is the path for the center

Answer 22

Visual Sensory Processing prt 2

This is the path for the surround

Question 23

Visual Sensory Processing prt 3

Answer 23

Question 24

Vision Processing

• Ganglion cells
  
  • Optic nerve
  • Optic chiasm
  • Optic tract
• Synapses in LGB
• Optic radiations
• Synapses in visual cortex
• Right visual field to left visual cortex & left visual field to right visual cortex

Answer 24

Question 25

Lateral-Line Systems

• Fish and amphibians have lateral-line systems to detect movement in surrounding water.
• The cupula is an accessory structure found in the lateral line which bends hair cell cilia when it is displaced.

Answer 25

Question 26

Electroreceptors

This is a weakly electric fish with an electric organ

Extremely sensitive electroreceptor cells (b) spontaneously depolarize, releasing neurotransmitters and causing action potentials

Electroreceptors detect minute changes in current flow (c) sending information to the fish’s enlarged cerebellum about its immediate environment or for communication with one another

(because it’s dark)

Answer 26

Question 27

Other Specialized Senses

Thermoception- hot and cold

Nociception- pain

Magnetoception- magnetic field