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Physiology & Science: Neuroscience II > Visual physiology > Flashcards

Visual physiology Flashcards

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1
Q

Describe the resting membrane state in terms of ions of photoreceptors

A
  • Sodium channels open at rest (Na in)
  • Potassium channels open at rest (more K out)
  • -40mV
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2
Q

What happens to the photoreceptor membrane potential when light increases (in basic terms)?

A
  • Sodium channels CLOSE
  • The membrane potential therefore drops down
  • Hyperpolarisation
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3
Q

What happens to the photoreceptor membrane potential when light decreases (in basic terms)?

A
  • Depolarisation
  • Opens extra sodium channels (More Na in)
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4
Q

How is the sodium channel held open at resting state?

A

By 2 molecules of cGMP that bind to it and hold it open, allowing sodium influx at rest.

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5
Q

The light response acts at the photoreceptor membrane and specifically in the membrane disc in the outer segment. What molecules form the resting/inactive photopigment?

A

Opsin (the structural protein) and 11-cis-retinal, which binds to it.

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6
Q

How is the photopigment activated and how does this in turn result in hyperpolarisation?

A
  • Light photon hits
  • Cis-retinal becomes trans-retinal due to weak bond
  • Trans-retinal reduces cGMP
  • cGMP diffuses away from sodium channels
  • Sodium channels close
  • Less sodium influx -> hyperpolarisation
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7
Q

How is the transudction response terminated?

A

Sequence of events take place, which removes used trans-retinal, terminates biochemical cascade and returns original levels of cGMP.

New molecule of 11-cisretinal attached to photopigment molecule, ready to go again.

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8
Q

What happens to the photoreceptor as it is exposed to prolongued light?

A

Adaptation occurs, so membrane potential can restore allowing continuous response to light stimuli changes -> allows us to see over a huge range of light levels.

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9
Q

What is retinitis pigmentosa?

A

A degenerative disease from the result of defects in necessary proteins in the eye

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10
Q

What layer divides the choroid (blood supply) and the photoreceptors?

A

Retinal pigment epithelium

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11
Q

What are the functions of retinal pigment epithelium?

A
  • blood-retinal barrier between the outer segments + choroid
  • regenerates 11-cis retinal
  • helps to renew outer segment membranes
  • holds retina in place
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12
Q

What molecules cause photo-oxidation of phospholipids and proteins in the RPE - why is this a problem?

A

Electromagnetic radiation (light) and oxygen

This is bad as phospholipids and proteins (high conc in eye) are easily photo-oxidised, so have to be renewed at regular intervals to get rid of any damaged molecules.

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13
Q

How does the RPE become clogged with age?

A

Intracellular debris (‘lipofuscin’) accumulates, secreted onto the basal membrane and forms fatty plaques between choroid and RPE, in turn disrupting the oxygen supply to the photoreceptor layer.

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14
Q

What are the four main types of ganglion cell?

A
  • OFF cell (+ by decreased light)
  • ON cell (+ by increased light)
  • PARVOCELLULAR GCs (high res + colour)
  • MAGNOCELLULAR GCs (fast moving, low contrast)
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15
Q

There is a 2% prevelance of red-green colourblindness, who is it common in and why?

A

Genes for red and green photopigments are on the X chromosome - hence common in boys.

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16
Q

What do the retina and lateral geniculate nuclei cells encode for? How is this different to the function of the primary visual cortical cells?

A
  • Retina/LGN -> Contrast (the edges of things) and wavelength
  • Primary vis cortex does:
  • orientation of edges
  • presence of corners etc
  • direction of motion
  • binocular disparity
17
Q

What are binocular receptive fields important for?

A

Generating depth perception - how we see 3D,

18
Q

What is the inferior temporal cortex associated with in terms of visual physiology?

A

Determining shape, colour, meaning and identity.

19
Q

What is the frontal cortex associated with in terms of visual physiology?

A

Uses incoming information from the primary visual cortex and combines it with pre-existing info.

20
Q

What is the parietal cortex associated with in terms of visual physiology?

A

Processing location and movement - where something is and where it is going, how it relates to other objects and self-movement.

For self-movement, the parietal cortex then projects to motor adn somatosensory areas.

21
Q

What is meant by conjugate and disconjugate eye movements?

A

Conjugate - when the eyes move in the same way

Disconjugate - converging eyes to look at something nearby (so both go in diff directions)

22
Q

Describe the reflexive saccadic eye movement

A
  • Conjugate
  • Something appears away from fovea and eyes ‘jump’ to it
  • Controlled by superior colliculus
  • Sends instructions to brainstem centres -> control movement of eyes via cranial nerves
  • Involves vertical and horizontal gaze centres
23
Q

Describe the exploratory saccadic eye movement

A
  • Conjugate
  • Not random, looks at important part of image
  • Involves parietal cortex projecting to superior colliculus and gaze centres
24
Q

Describe the voluntary saccadic eye movement

A
  • Conjugate
  • eg. looking at a clock
  • Frontal eye fields project to superior colliculus and gaze centres
25
Q

What is the other type of eye movement apart from saccadic? How do they work?

A

(smooth) pursuit movements - work through pontine nuclei, which send instructions to eye muscles via vestibular nuclei, loop in the cerebellum.

Vestibular nuclei bring together instructions to move eyes with info about movements of the head.

26
Q

How do disconjugate eye movements work?

A
  • Signals sent to vergence centre
  • Activates oculomotor nucleus
  • Motor signals sent to medial rectus muscles
  • converge the eyes
27
Q

What is the role of the Edinger-Westphal nucleus in converging the eyes?

A
  • Vergence centre stimulates Edinger-westphal nucleus
  • Activates ciliary ganglion -> short ciliary nerves
  • Ciliary muscle and sphincter pupillae -> constrict pupils

So the E-W nucleus helps to plump up (accommodate) the eyes to view nearby objects along with convergence.

Physiology & Science: Neuroscience II (18 decks)

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