L15 Accommodation & Pupillary Function

accommodation

a dioptric change in optical power of the eye due to ciliary muscle contraction

What is the basic mechanism of accommodation?

ciliary muscle contraction moves the apex of the ciliary body towards the axis of the eye and releases resting zonular tension around the lens equator

when zonular tension is released, the elastic lens capsule molds the young lens into a more spherical and accommodated form

How is lens diameter affected by accommodation?

diameter decreases

How is the lens thickness affected by accommodation?

thickness increases

anterior lens surface moves anteriorly

posterior lens surface moves posteriorly

How are the anterior and posterior surface curvatures affected by accommodation?

anterior and posterior surface curvatures increase

*this increases the optical power of the lens and facilitates the ability of eye to focus on near objects

longitudinal fibers of the ciliary muscle

runs along inside of inside of sclera

radial fibers of the ciliary muscle

fan out from chamber angle toward ciliary processes

circular fibers of the ciliary muscle

runs parallel to Schlemm's canal

Where do the anterior fibers of the longitudinal/radial muscle apply the force?

applies force to the scleral spur and opens trabecular meshwork

Where do the posterior fibers of the longitudinal/radial muscle apply the force?

applies force to pars plane moving it anteriorly

How does the thickness of the ciliary muscles change upon contraction?

increase in thickness of the circular portion

decrease in thickness of the radial and longitudinal portions

How does contraction of the ciliary muscles move the choroid?

contraction pulls the anterior choroid forward, moves the apex of the ciliary processes towards the lens equator

How does the ciliary muscle change upon relaxation?

because the radial fibers are attached to the elastic choroid, the ciliary body is returned to its unaccommodated configuration through the elasticity of the choroid

What type of receptors are found on the ciliary muscle?

M3 receptors mediate contraction

B2 adrenergic mediates relaxation

T/F: one stimulus causes simultaneous contraction of all muscle groups of the ciliary muscle

true

Where is the smooth muscle of the ciliary body embryologically derived from?

ectoderm (other smooth muscle is derived from mesoderm)

What type of muscle fibers are found in the longitudinal fibers?

longitudinal fibers are similar to fast twitch muscles

have fewer mitochondria and more myofibrils

Which type of fibers of the ciliary muscle have the most mitochondria?

radial/circular portions have more mitochondria

What are the two functions of the ciliary zonules?

1. hold lens in place

2. transmit tensile forces for accommodative shape change of the lens

Where do zonules insert into the lens?

they embed into the superficial lens capsule

What are the three concentrations of zonule insertions?

anterior, equatorial, posterior

How are zonules synthesized during development?

synthesized by non-pigmented ciliary epithelial cells

What are zonule fibers primarily made up of?

fibrillin and fibrillin like proteins (strings and beads)

MAGP-1 (beads only)

T/F: zonules are elastic but non-collagenous and non-elastin

true

fibrillins

large cysteine-rich multi domain glycoproteins that polymerize in the extracellular space in a head-to-tail manner to form microfibrils

ectopia lentis

due to mutation in fibrillin gene

dislocated lines in one or both eyes

lens capsule

secreted by lens epithelial and fiber cells

thickest just anterior and posterior to equatorial region, thinnest posteriorly

What are the components of the lens capsule?

Collagen IV

laminin

heparin sulfate proteoglycans (perlecan, nidogen, and collagen XVIII)

T/F: ECM proteins of capsule turnover at a very slow rate compared to basement membrane of other epithelia

true, takes months to years

presbyopia

gradual age related loss of accommodative amplitude

due to age-related changes in accommodative apparatus

What are the age related changes of the lens capsule?

general increase in capsule thickness from birth through young adult

changes in aging adults occur regionally

thicker anteriorly

thinner posteriorly

non-enzymatic glycation of collagen IV and increases stiffness

How is the lens capsule affected by hyperglycemia?

an increase in aqueous humor glucose and can induce intra- and intermolecular cross-linking between lysine and amine groups

collagen IV in lens capsule is susceptible to glycation

lowered amplitude of accommodation exists in individuals with type I diabetes when compared with age-matched controls

What are the age related changes to zonules?

no elasticity change

zonular/capsular insertion distance to the lens equator increases with age

What are the age related changes to ciliary muscles?

loss of muscle fibers and increase in connective tissue

contractile force does NOT decrease

movement still occurs after accommodative loss

What are the age related changes in lens mass?

mass increases linearly with age

due to the increase in lens cell number without any cell loss

What is the lens paradox?

aged less appears to be similar in shape to an accommodated lens, yet there is near vision loss

What are the 2 reasons for the lens paradox?

1. thickness differences:

aging: thickness increases of cortical layers only

accommodation: thickness increase of entire lens

2. a decrease in refractive index near equatorial region with age

T/F: the lens gets more stiff as age increases

true

What is the primary reason for accommodative failure?

stiffness of lens

What is the accommodative triad?

1. ciliary muscle contraction

2. pupil constriction

3. convergence

What can stimulate the accommodative reflex?

blur cues

convergence

blur cues

blur presented to one or both eyes induce both eyes to accommodate

induces bilateral pupil constriction

How does convergence stimulate accommodative reflex?

isolating convergence with base-out lenses and induces both eyes to accommodate

induces bilateral pupil constriction

What are the afferent fibers of the accommodative reflex?

optic nerve axons project to and synapse in the lateral geniculate nucleus

neurons project from the LGN to visual cortex

Where are the interneurons of the accommodative reflex found?

from visual cortex to frontal eye fields

from frontal eye fields to Edinger-Westphal nucleus and oculomotor nucleus

What are the efferent fibers of the accommodative reflex?

from Edinger-Westphal to ciliary ganglion along CN III

from ciliary ganglion to ciliary muscle

parasympathetic fibers stimulate muscarinic receptors in muscle

"fast twitch" like for fine control

convergence

simultaneous and synchronous adduction in both eyes

stimulated by contraction of medial rectus muscle

What is the mechanism of convergence?

oculomotor nerve receives signals from the oculomotor nucleus to to stimulate muscles

supracnuclear signals from the frontal eye fields and visual cortex couple ciliary muscle and medial rectus contraction

What are the pupillary functions?

1. control of retinal illumination

2. reduction of optical aberrations

3. improves depth of focus/field

What is the pupillary function during dim light?

dilation of the pupil maximizes number of photons reaching the retina

What is the pupillary function during bright light?

pupil constriction reduces illumination by 1.5 log units within 0.5 seconds

How does the pupil reduce optical aberrations?

small pupil reduces the degree of chromatic and spherical aberration

smaller aperture limits light rays entering the optical system to central cornea and lens and avoids more peripheral portions of the cornea and lens where aberrations are greater

How does the pupil improve depth of focus/field?

small pupil produces "the pinhole effect" and increases the depth of field

dilator muscle

radially oriented

associated with pigmented epithelium

contraction pulls pupillary margin toward ciliary body peripherally

sphincter muscle

encircles the pupillary margin

separated from pigmented epithelium

contraction reduces pupil size

What are the major excitatory neurons of the sphincter muscle?

parasympathetic neurons using acetylcholine

What are the major excitatory neurons of the dilator muscle?

sympathetic neurons using norepinephrine

What receptors are found on the sphincter muscle?

muscarinic receptors

What receptors are found on the dilator muscle?

alpha 1 receptors

Holmes-Adi

neurological disorder characterized by a tonically dilated pupil that reacts slowly to light and accommodation

could be d/t:

viral or bacterial infection that causes inflammation and damage to neurons in the ciliary ganglion

lesion of the ciliary ganglion and may c cause loss of innervation within a subset of segments and preventing contraction in a few segments

synechiae

iris adheres to lens or cornea due to long term inflammation caused by uveitis

iris coloboma

congenital disorder

due to incomplete formation of ventral optic cup

iris is also malformed

What are the two distinct stimuli that pupillary responds to?

light/brightness

near reflex (accommodation)

How are the pupillary responses to light vs the near reflex the same/different?

same: efferent pathway

different: supranuclear control over each response

pupil near reflex

miosis occurs without change in retinal luminance

Where are the supranuclear neurons that control the pupillary near reflex?

supranuclear neurons come from cortical areas surrounding visual cortex and from the frontal eye fields and goes to the Edinger-Westphal nucleus

What cells are in the afferen tof the pupillary light reflex?

rod and cone photoreceptors

intrinsic photoreceptive retinal ganglion cells

What opsin do ipRGCs have?

melanopsin

What is the path of the pupillary light reflex?

afferent: retinal ganglion cell projects to pretectal olivary nucleus

interneuron: from pretectal olivary nucleus to EW nucleus

efferent: from EW nucleus to ciliary ganglion, from ciliary ganglion to iris sphincter

Why is the pupillary light reflex normal symmetric between both eyes?

due to interneurons sending signals to the left and right EW nucleus

What is the primary way to regulate pupil size?

regulation of the iris sphincter

What are the two competing signals for controlling pupil size?

illumination induced increase in parasympathetic nerve stimulation

continuous supranuclear inhibition of parasympathetic nerves

What is the neuron pathway for inhibiting sphincter contraction in a reduced light stimulus?

CNS/brainstem originating inhibitory input through activation of alpha 2 receptor in EW results in inhibition

parasympathetic nerve to sphincter muscle not stimulated

When is there lack of inhibition of the pupil light reflex?

sleep

anesthesia

sympathetic inhibition is suppressed

T/F: control of iris dilator is not a requirement for pupil dilation

true

What is the neuron pathway for stimulation of the iris dilator?

sympathetic nerves come from superior cervical ganglion

axons pass through ciliary ganglion

innervation of the dilator muscle

What are examples of additional iris muscle regulation aside from autonomic neuronal control?

1. circulating catecholamines released from adrenal glands cause mydriasis

2. sensory innervation to the iris from CN V modifies pupil diameter, mechanical and chemical irritation of the eye can cause strong miotic response