pupil response 1 - pupil constriction Flashcards
list the 4 structures associated with the iris and pupil
- pupillary zone
- collarette
- ciliary zone
- pupillary raphe
where is the pupillary zone of the iris located
closest to the pupil
where is the collarette located
the transition region where the pupillary and ciliary zones merge
where is the ciliary zone of the iris
closest to the sclera
what is the pupillary raphe
the pigmented epithelium at the rear of the iris
what is the average pupil diameter in low light levels i.e. when dilated
8mm
what is the average pupil diameter in bright light levels i.e. when constricted
2mm
what does bright light levels cause to the pupil
miosis/constriction
what does low light levels cause to the pupil
mydriasis/dilation
what 3 things is the resting pupil diameter influenced by
- genetic factors
- age (senile miosis)
- ambient light levels
what problems does senile miosis cause
in low light levels it can become difficult to see e.g. reading at night
so higher light levels are needed to see
what is the name for a large resting pupil size
macrocoria
what is the name for a small resting pupil size
microcoria
after how long does the pupil respond to bright light and when is the maximal constriction response
- after a latency of 200ms
- maximal constriction response is within 2.5s
how are the responses of the pupil in lower light levels and why
slower but completed sooner, as don’t have much to constrict in low light levels - the latency is longer
what are the direct pupil responses very similar to
the consensual response
describe how the pupil responds to prolonged bright light
- 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
what is hippus
when the pupil goes through rhythmic contractions of 2Hz, especially when looking at distance
what type of pupil reaction is more apparent in prolonged stimulation of lower light levels
pupil diameter rebounds, so it constricts and then dilates after the initial constriction
what are the two functions of the pupil responses
- 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
how much difference is area is there between a fully constricted and a fully dilated pupil
only x16 in area
by how much does a difference in area of x16 alter the amount of light entering the eye and what does this mean
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
what can the pupil not cope with
cannot cope with the big levels of light we’re exposed to and can only cope with small amounts of light
by what two ways does the constriction of the pupil improve image quality
- 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
describe how by closing the pupil, spherical aberration can be reduced in relation to the lens
- 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
so what is the main function of the pupil, if it not to regulate light levels
it is to improve image quality
what does accommodation to near object elicit
both convergence and pupillary constriction
by how much does the pupil contract, to every dioptre of accommodation
pupil contracts by 0.3mm to every dioptre of accommodation
explain how the elements of the near triad have a common cause
any one of the elements of the triad can occur in absence of the other 2 i.e. one doesn’t cause the other
list how the three components of the near triad improves image quality
- accommodation: focusses the image
- convergence: puts the image on the fovea
- pupil constriction: increases depth of field
list the three things that the human pupil responds to
- changes in illumination as a response to light
- viewing near objects
- cortical influences e.g. girls (preferentially)
name the two smooth muscles that the iris contains
- sphincter muscle: responsible for pupil constriction
- Dilatator muscle: responsible for pupil dilation
what is the appearance of the sphincter muscle
circular muscle
what is the appearance of the dilatator muscle
radial muscle
what does the iris contain, which is the reason why light doesn’t get through the iris
a double pigment epithelium
which part of the iris is the dilatator pupillae muscle closely associated with and what is it called
the anterior layer of the posterior iris pigmented double epithelium = myoepithelium
what is the name of the posterior iris pigmented double epithelium
myoepithelium
at which orientation does the iris dilatator pupillae run in the myoepithelium
in an up and down direction
what is the iris dilatator muscle innervated by
the sympathetic branch of the autonomic nervous system
what neurotransmitter does the sympathetic branch of the nervous system release
nor adrenaline on the iris dilatator muscle = pupil dilates
what is the sphincter pupillae physically separated from in the iris
from the iris pigment epithelium
where in the iris is the sphincter pupillae found
at the tip of the iris
what is the sphincter pupillae innervated by
the parasympathetic branch of the autonomic nervous system
what neurotransmitter does the parasympathetic branch of the nervous system release
acetylecholine on the smooth muscle to cause constriction of the pupil
list all the things that take part in the neural pathway underlying light induced pupillary constriction
- 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
what is the role of the hypothalamus
regulation of the circadian rhythms
what is the role of the pretectum
reflex control of the pupil and lens
what is the role of the superior colliculus
orienting movements of head and eyes
list the 4 main steps of the sensory pathway to pupil constriction (light induced) after the RGCs leave the retina
- some RGCs dont follow the main visual pathway, but instead branch off just before the LGN at the superior brachium
- some of these fibres go to the pretectum (reflex control of pupil and lens)
- there is direct bilateral (both eyes) retinal ganglion cell input to the olivary pretectal nucleus
- 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
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
- light is absorbed by photoreceptors: rods, cones, melanopsin ganglion cells
- activity in the optic nerve
- partial decussation at the chiasm
- projection to the olivary pretectal nucleus
- bilateral projection to the edinger westphal nucleus
what are the parasympathetic and sympathetic ganglia both a part of
the autonomic nervous system as both iris muscles are smooth muscles
explain how the autonomic nervous system which contains the parasympathetic and sympathetic nervous system, is composed of a 2 neuron chain
- 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
where are the sympathetic ganglia located and what is it called
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
where are the parasympathetic ganglia located and what is it called
by the effector muscle called the ciliary ganglion
- in the orbit, takes parasympathetic input to the eyes
list the 4 main steps of the motor pathway to pupil constriction (light induced) from the edinger westphal nucleus
from the EWN:
- preganglionic parasympathetic fibres travel within/along the 3rd CN to the ciliary ganglion
- they synapse using acetylcholine as their neurotransmitter, with the short ciliary nerves which are the post ganglionic fibres
- the short ciliary ganglion nerves pierce through the back of the eye, the sclera and go through the choroid, to reach the iris sphincter
- this is where acetylcholine binds with muscarinic receptors
= pupil constriction
