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Psychophysics H > pupil response 1 - pupil constriction > Flashcards

pupil response 1 - pupil constriction Flashcards

1
Q

list the 4 structures associated with the iris and pupil

A
  • pupillary zone
  • collarette
  • ciliary zone
  • pupillary raphe
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2
Q

where is the pupillary zone of the iris located

A

closest to the pupil

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3
Q

where is the collarette located

A

the transition region where the pupillary and ciliary zones merge

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4
Q

where is the ciliary zone of the iris

A

closest to the sclera

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5
Q

what is the pupillary raphe

A

the pigmented epithelium at the rear of the iris

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6
Q

what is the average pupil diameter in low light levels i.e. when dilated

A

8mm

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7
Q

what is the average pupil diameter in bright light levels i.e. when constricted

A

2mm

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8
Q

what does bright light levels cause to the pupil

A

miosis/constriction

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9
Q

what does low light levels cause to the pupil

A

mydriasis/dilation

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10
Q

what 3 things is the resting pupil diameter influenced by

A
  • genetic factors
  • age (senile miosis)
  • ambient light levels
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11
Q

what problems does senile miosis cause

A

in low light levels it can become difficult to see e.g. reading at night
so higher light levels are needed to see

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12
Q

what is the name for a large resting pupil size

A

macrocoria

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13
Q

what is the name for a small resting pupil size

A

microcoria

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14
Q

after how long does the pupil respond to bright light and when is the maximal constriction response

A
  • after a latency of 200ms

- maximal constriction response is within 2.5s

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15
Q

how are the responses of the pupil in lower light levels and why

A

slower but completed sooner, as don’t have much to constrict in low light levels - the latency is longer

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16
Q

what are the direct pupil responses very similar to

A

the consensual response

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17
Q

describe how the pupil responds to prolonged bright light

A
  • pupil oscillates around an average steady state/rhythmic contractions i.e. gets bigger and smaller = hippus
  • the brighter the light the higher the hippus
  • pupil diameter rebounds, so it constricts and then dilates after the initial constriction - which is more apparent in lower light levels
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18
Q

what is hippus

A

when the pupil goes through rhythmic contractions of 2Hz, especially when looking at distance

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19
Q

what type of pupil reaction is more apparent in prolonged stimulation of lower light levels

A

pupil diameter rebounds, so it constricts and then dilates after the initial constriction

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20
Q

what are the two functions of the pupil responses

A
  • regulate the amount of light entering the eye and falling on the retina
  • constriction: which increases the image quality by increasing depth of focus and reducing amount of spherical aberration
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21
Q

how much difference is area is there between a fully constricted and a fully dilated pupil

A

only x16 in area

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22
Q

by how much does a difference in area of x16 alter the amount of light entering the eye and what does this mean

A

by only 1.2 log units
the pupil will not be as effective at regulating the amount of light coming into the eye
it is insignificant compared to the range of 12 log units under which the human visual system functions

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23
Q

what can the pupil not cope with

A

cannot cope with the big levels of light we’re exposed to and can only cope with small amounts of light

24
Q

by what two ways does the constriction of the pupil improve image quality

A
  • increasing the depth of focus: allows to see more at a larger range of distances
  • reduces amount of spherical aberrations: the smaller aperture means peripheral rays are cut out and only central rays (on central part of lens) will be focussing a sharper and clearer image at the back of the eye
25
Q

describe how by closing the pupil, spherical aberration can be reduced in relation to the lens

A
  • all lenses suffer from a degree of spherical aberration
  • if the whole lens is used to form an image, then theres a big blur circle and if the light rays are focussed everywhere = image will be blurred
  • light passing through the lens peripherally is usually focussed closer to the lens (and away from the retina) than more central rays which are focussed further away from the lens (and closer to the retina)
  • decreasing the aperture with a pupil inevitably leads to less spherical aberration and hence image quality cuts out peripheral rays and hence only uses the central part of the lens
26
Q

so what is the main function of the pupil, if it not to regulate light levels

A

it is to improve image quality

27
Q

what does accommodation to near object elicit

A

both convergence and pupillary constriction

28
Q

by how much does the pupil contract, to every dioptre of accommodation

A

pupil contracts by 0.3mm to every dioptre of accommodation

29
Q

explain how the elements of the near triad have a common cause

A

any one of the elements of the triad can occur in absence of the other 2 i.e. one doesn’t cause the other

30
Q

list how the three components of the near triad improves image quality

A
  • accommodation: focusses the image
  • convergence: puts the image on the fovea
  • pupil constriction: increases depth of field
31
Q

list the three things that the human pupil responds to

A
  • changes in illumination as a response to light
  • viewing near objects
  • cortical influences e.g. girls (preferentially)
32
Q

name the two smooth muscles that the iris contains

A
  • sphincter muscle: responsible for pupil constriction

- Dilatator muscle: responsible for pupil dilation

33
Q

what is the appearance of the sphincter muscle

A

circular muscle

34
Q

what is the appearance of the dilatator muscle

A

radial muscle

35
Q

what does the iris contain, which is the reason why light doesn’t get through the iris

A

a double pigment epithelium

36
Q

which part of the iris is the dilatator pupillae muscle closely associated with and what is it called

A

the anterior layer of the posterior iris pigmented double epithelium = myoepithelium

37
Q

what is the name of the posterior iris pigmented double epithelium

A

myoepithelium

38
Q

at which orientation does the iris dilatator pupillae run in the myoepithelium

A

in an up and down direction

39
Q

what is the iris dilatator muscle innervated by

A

the sympathetic branch of the autonomic nervous system

40
Q

what neurotransmitter does the sympathetic branch of the nervous system release

A

nor adrenaline on the iris dilatator muscle = pupil dilates

41
Q

what is the sphincter pupillae physically separated from in the iris

A

from the iris pigment epithelium

42
Q

where in the iris is the sphincter pupillae found

A

at the tip of the iris

43
Q

what is the sphincter pupillae innervated by

A

the parasympathetic branch of the autonomic nervous system

44
Q

what neurotransmitter does the parasympathetic branch of the nervous system release

A

acetylecholine on the smooth muscle to cause constriction of the pupil

45
Q

list all the things that take part in the neural pathway underlying light induced pupillary constriction

A
  • light has to be absorbed by retinal photoreceptors = rods and cones, which is what absorbs the light and triggers the pupil response
  • other photoreceptors which also respond to light and trigger a pupil response = photosensitive melanopsin containing retinal ganglion cells
  • so can respond to light without rods and cones
  • 1% of RGC’s contain photosensitive melanopsin and absorb light at higher levels which trigger a pupil response
  • most/the rest of RGC’s follow the main visual pathway (optic nerve - optic chiasm - LGN - optic radiations - V1) and mediate conscious vision
  • not all of the output of the eyes goes to V1 because not all vision is conscious i.e. responding to light without seeing the light - the pupil
  • this is the beginning of the sensory/afferent pathway for pupil constriction
46
Q

what is the role of the hypothalamus

A

regulation of the circadian rhythms

47
Q

what is the role of the pretectum

A

reflex control of the pupil and lens

48
Q

what is the role of the superior colliculus

A

orienting movements of head and eyes

49
Q

list the 4 main steps of the sensory pathway to pupil constriction (light induced) after the RGCs leave the retina

A
  1. some RGCs dont follow the main visual pathway, but instead branch off just before the LGN at the superior brachium
  2. some of these fibres go to the pretectum (reflex control of pupil and lens)
  3. there is direct bilateral (both eyes) retinal ganglion cell input to the olivary pretectal nucleus
  4. from the OPN, fibres go directly to the ipsilateral edinger westphal nucleus
    (there are also projections to the contralateral EWN to mediate consensual response via the posterior commissure projections which are bilateral, for direct and consensual)

= end of sensory/afferent part of the pupil pathway

50
Q

list the 5 step recap of the sensory/afferent part of the pathway of pupillary constriction as is in the sensory arm of the reflex arc

A
  1. light is absorbed by photoreceptors: rods, cones, melanopsin ganglion cells
  2. activity in the optic nerve
  3. partial decussation at the chiasm
  4. projection to the olivary pretectal nucleus
  5. bilateral projection to the edinger westphal nucleus
51
Q

what are the parasympathetic and sympathetic ganglia both a part of

A

the autonomic nervous system as both iris muscles are smooth muscles

52
Q

explain how the autonomic nervous system which contains the parasympathetic and sympathetic nervous system, is composed of a 2 neuron chain

A
  • first the neuron, the pre ganglionic neuron is in the spinal chord
  • its axons, the pre ganglionic axon synapses with the second motor neuron called the ganglionic neuron, in an autonomic ganglion outside of the CNS but very close to the spinal column still
  • the axon of the ganglionic neuron, called the post ganglionic axon, goes to the effector muscle
53
Q

where are the sympathetic ganglia located and what is it called

A

near the spinal chord called the superior cervical ganglion, which takes input to the eye
- they have pre ganglionic axons which leave the spinal chord and ganglia are always close to the spinal column, and they send fibres to the effector muscle

54
Q

where are the parasympathetic ganglia located and what is it called

A

by the effector muscle called the ciliary ganglion

- in the orbit, takes parasympathetic input to the eyes

55
Q

list the 4 main steps of the motor pathway to pupil constriction (light induced) from the edinger westphal nucleus

A

from the EWN:

  1. preganglionic parasympathetic fibres travel within/along the 3rd CN to the ciliary ganglion
  2. they synapse using acetylcholine as their neurotransmitter, with the short ciliary nerves which are the post ganglionic fibres
  3. the short ciliary ganglion nerves pierce through the back of the eye, the sclera and go through the choroid, to reach the iris sphincter
  4. this is where acetylcholine binds with muscarinic receptors

= pupil constriction

Psychophysics H (11 decks)

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