week 1 physiology 2

Question 1

what is needed in order to see an object?

Answer 1

1- the pattern of the object must fall on the vision receptors (rods and cones in the retina) accommodation

2- the amount of light entering the eye must be regulated (too much light will “bleach out” the signals)

3- the energy from the waves of photons must be transduced into electrical signals

4- The brain must receive and interpret the signals

Question 2

what is meant by the direct/vertical for signal transmission?

Answer 2

photoreceptors →bipolar cell →ganglion cells →ganglion cell axons forgetting to forebrain →optic nerve/CN II

Question 3

what is meant by the lateral connections influencing the single processing?

Answer 3

• Horizontal cells, Receive input from photoreceptors and project to other photoreceptors and bipolar cells
• Amacrine cells, Receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells

Question 4

what is the act of converting electromagnetic radiation to neural signals called?

Answer 4

transduction

Question 5

what are the 4 main regions of photoreceptors?

Answer 5

Outer segment (membranous disks containing phtopigment)
Inner segment
Cell body (middle of inner segment)
Synaptic terminal

Question 6

what are the different types of photoreceptor?

Answer 6

rods and cones

Question 7

what is the basis of phtotransduction?

Answer 7

1. Photoreceptors have a depolarized rmp (Vm) [Compared to other neurons, resting Vm is more positive (~ -20mV)]
   2) With light exposure, Vm hyperpolarizes!

Question 8

what is the dark current

Answer 8

the residual electric current flowing in a photoelectric device when there is no incident illumination.

Question 9

when is the dark current open/closed?

Answer 9

open when dark and closed when light

Question 10

what channel controls the dark current?

Answer 10

A cGMP-gated Na+ channel that is open in the dark and closes in the light

Question 11

what allows the brain to perceive objects in the visual field

Answer 11

This change in Na+ with light (due to cGMP-gated Na+ channel ) - in light there is reduced permeability of NA+ meaning that the membrane potential drifts to the K+

Question 12

name the visual pigment molecule found in rods? what is it made up of?

Answer 12

Rhodopsin

Retinal (Vitamin A derivative) + Opsin (G- protein coupled receptor)

Question 13

how do the rods work/what is the basis of Phototransduction?

Answer 13

Light converts 11-cis-Retinal to all-trans-Retinal (activated form)→opsin becomes activated→G=protein (tranducing)→molecular cascade→activated G-protein→activated cGMP PDE→PDE hydrolyses cGMP→reduces cGMP concentration→leads to closure of cGMP-gated Na+ channel→Lowered Na entry results in hyperpolarization

THE Modulation of the Dark Current is the basis of Phototransduction!!!

Question 14

THE DARK-CURRENT CHANNEL key facts

Answer 14

• Open in the dark
• Closes in response to light
• Nucleotide-gated channel (opened by cGMP)
• Permeable to Na+
• Keeps photoreceptor Vm more positive than most neurons meaning steady release of neurotransmitter

More glutamate in the dark/Less glutamate in the light

Question 15

what facilitates high acuity?

Answer 15

• photoreceptor spacing

- refractive power

Question 16

what can rods do well, what can cones do well?

Answer 16

Rods – seeing in dim light

Cones – seeing in normal daylight

Question 17

how are rods and cones different structurally?

Answer 17

More convergence in rod system, increasing sensitivity while decreasing acuity (one large ganglion for many rods vs small ganglion for a few cones)

Question 18

how can we see colour?

Answer 18

Different opsins for discrete wavelengths

red, green, blue= long wave to short wave

Question 19

compare the two types of photoreceptors

Answer 19

rods = achromatic, peripheral retina, high convergence, high light sensitivity, low visual acuity

cones=chromatic, central reina/fovea, low convergence, low light sensitivity, high visual acuity

Question 20

hw does the retina process things?

Answer 20

CONTRAST - Our visual system detects (+enhances) local differences in light intensity, not the absolute amounts of light.

Question 21

why is vital field representation on the retina needed?

Answer 21

because image arrives at brain a Left/RightTop/Down Inverted representation

Question 22

what part of vision cross over at optic chiasma

Answer 22

nasal hemiretina

Question 23

what is the visual field mapped by?

Answer 23

the retina, LGN (lateral geniculate nucleus), superior colliculus, & cortex

Question 24

as image is swapped over where is the right visual field, the superior visual field processed

Answer 24

left cortex, lower cortex

Question 25

In the primary visual cortex, where are the eye specific inputs are segregated. when does it being to merge?

Answer 25

in layer 4

information starts to merge after layer 4C

Question 26

what is Amblyopia

Answer 26

cortical blindness, is a term that refers to a variety of visual disorders when there is no problem with the eye (the optics and retina are fine), but one eye has better vision than the other.

Question 27

what causes Amblyopia

Answer 27

can be caused by strabismus (wandering eye) if it is not corrected in infancy - must intervene to prevent. can be corrected surgically but not until baby is old enough

Question 28

why to treat Amblyopia

Answer 28

patch over one eye and swap between eyes - This helps the brain develop properly to process signals coming from both eyes.

Question 29

what is Hebb’s postulate?

Answer 29

When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased (strong eye becomes relied upon more than weak one)

Cells that fire together, wire together - strengthens synaptic connections.

Question 30

what can happen to the Terminal Arborizations of LGN Axons in Monocular Deprivation

Answer 30

Axons Can Change Rapidly.

reduced number of branches in the deprived eye