Physiology II Flashcards
What is needed for an object to be seen?
The pattern of the object must fall on the vision receptors (rods and cones in retina)
Brain must receive and interpret signals
Why must the amount of light entering the eye be regulated?
Too much light will bleach out the signals
What must the energy from waves of photons be transduced into for an object to be seen?
Electrical signals
What is the direct (vertical) pathway for signal transmission?
Photoreceptors to bipolar cells and finally to ganglion cells (light travels in the opposite direction from this)
What is the role of horizontal cells in the retina?
Receive input from photoreceptors and project to other photoreceptor and bipolar cells
What is the function of the amacrine cells in the retina?
Receive input from bipolar cells and project to ganglion cells
What do photoreceptors do?
Convert electromagnetic radiation to neural signals (transduction)
What are the four main regions of photoreceptors?
Outer segment, inner segment, cell body, synaptic terminal
What are the photoreceptors of the eye?
Rods and cones
What is the resting membrane potential of vertebrate photoreceptors?
Depolarised = resting Vm is more positive compared to other neurons (-20mV)
What happens to Vm on light exposure?
It hyperpolarises
What causes positive Vm?
The dark current = cGMP-gated Na channel that is open in the dark and closed in the light
What signal enables the brain to perceive objects in the visual field?
Change in Na due to closing/opening of dark current channel
How is the dark current modulated in the dark?
PNa = PK (Na channels in outer segment) = Vm between ENa and EK
How is the dark current modulated in the light?
PNa is reduced (outer segment channels close), PK > PNa, Vm > EK so hyperpolarise, change is local and graded
What are the visual pigment molecules?
Rhodopsin = retinal (vitamin A derivative) + opsin (G-protein coupled receptor)
Where is rhodopsin present?
In membrane folds (called discs in outer segment)
What effect does light have on 11-cis-retinal?
Converts it to its active form = all-trans-retinal
What does all-trans-retinal activate?
Transducin = causes molecular cascade which decreases cGMP, closing cGMP-gated Na channels
What does lowered Na entry result in?
Hyperpolarisation
How is transduction said to be a high gain process?
1 molecule of opsin gives 1000 molecules of transducin
What opens the dark current channel?
Binding of cGMP in response to light
What ion is the dark current channel permeable to?
Na = keeps photoreceptor Vm more positive that most neurons
What does the dark current channel ensure?
That there is a steady release of neurotransmitter
What is the relationship between glutamate and the levels of light?
More glutamate in the dark and less in the light
What is visual acuity?
Ability to distinguish two nearby points = determined largely by photoreceptor spacing and refractive power
What kind of light are rods and cones used to see in?
Rods used for dim light
Cones used for normal daylight
What is the convergence of the rod system?
High convergence = large spacing (low density), large ganglion cells
What is the convergence of the cone system?
Low convergence = high density, small ganglion cells
What is the benefit of the higher convergence of the rod system?
Increases sensitivity (however, decreases acuity)
What does light comprise of?
Discrete wavelengths
Why can humans not see UV or infra-red light?
Our photoreceptors are only activated in a small portion of the spectrum of electromagnetic waves
Which opsins respond to particular wavelengths of light?
Short wave cone = blue light
Medium wave cone = green light
Long wave cone = red light
What are some features of rods?
Achromatic, found in peripheral retina, high convergence, high light sensitivity, low visual acuity
What are some features of cones?
Chromatic, found in central retina (fovea), low convergence, low light sensitivity, high visual acuity
What does the visual system detect?
Local differences in light intensity (not absolute amounts of light)
What facilitates the detection of local differences in light intensity?
Physiological and structural adaptions on the retina
What is the monocular visual field?
+/- 45 degrees = each eye sees part of visual space
Why do the visual fields of each eye overlap extensively?
To provide a binocular visual field (+/-45 degrees)
Where is the retina divided in half?
Relative to the fovea = creates nasal and temporal hemiretina
What do the nerve fibres from the nasal half of the retina cross?
The optic chiasm
What do the resulting two optic tracts allow for?
Right and left visual fields to reach the right and left hemispheres separately = 60% (nasal retina) cross and 40% (temporal retina) do not
What is the striate cortex?
Part of the visual cortex that processes visual information
Where is the visual field mapped?
In the retina, LGN, superior colliculus and cortex
Why is the central visual field over-represented?
Magnification factor not constant
Why is a discrete point of light able to activate many cells in the target structure?
Due to overlapping receptive fields
What is perception based on?
The brain’s interpretation of distributed patterns of activation
Where do the signals form the right and superior visual fields go?
Right = left cortex Superior = lower cortex
What happens to eye inputs in the primary visual cortex?
Eye specific inputs are segregated in layer 4
What occurs in the primary visual cortex?
Both eyes project to each visual cortex but in the primary visual area they remain largely segregated into ocular dominance columns
Where do cells outside of layer 4 of the primary visual cortex receive input from?
Receive input from both eyes
What shapes visual perception?
Shaped by early experience
What do children with congenital cataracts struggle to see even after corrective surgery?
Have difficulty perceiving shape and form
What is amblyopia?
Cortical blindness caused by a variety of disorders where there is no issue in the eye itself but one eye has better vision than the other
What is one cause of amblyopia?
Uncorrected wandering eye (strabismus)
What is Hebb’s postulate?
When axon of cell A is near enough to excite cell B and repeatedly takes part in firing it, some growth process/metabolic change occurs in one/both cells so that A’s efficiency as a firing cell of B increases
What strengthens the connections between presynaptic and postsynaptic cells?
Correlated activity between them = cells that fire together, wire together
What does Hebb’s postulate apply to in memory?
Also used in learning and memory, where it is called long term potentiation (LTP)
What effect does monocular deprivation have on LGN axons?
Can rapidly change the terminal arborisations = lack of activity leads to less branching
