The accommodative triad and the pupillary light reflex

What is the Accommodative triad commonly referred to as?

The near reflex

Why is the accommodative triad a “triad”?

Involves 3 physiological responses that occur during accommodation (in a young eye)

What are the 3 physiological processes that occur during accommodation (in a young eye)?

Ciliary muscle contraction, pupil constriction, convergence

What is convergence?

Simultaneous contraction of MR in both eyes to focus on something in the center of the visual field

What are the 2 stimuli that trigger the accommodative reflex?

Blur cues and convergence

What are blur cues?

Blur presented to one or both eyes that induce BOTH eyes to accommodate

What do blur cues induce in the pupil?

Bilateral pupil constriction

When isolating convergence with base-out lenses (or monocularly) what does it induce?

Induces both eyes to accommodate and bilateral pupil constriction

The 3 physiological processes that comprise the accommodative reflex are blank but blank

Linked but separable

How are ciliary muscle contraction, pupil constriction, and convergence linked?

Neuronally coupled (neuronal pathways that instruct these muscles to function simultaneously)

What causes ciliary muscle contraction, pupil constriction, and convergence to be neuronally coupled?

Supranuclear connections

Blank fibers are triggered to induce contraction of blank for the accommodative reflex.

Distinct neural fibers; different effector muscles

How are the 3 physiological functions of the accommodative triad “separable”?

Any of the 3 may occur in the absence of the others in certain situations

What is the afferent path to the interneuron of the accommodative reflex?

1. Trigger: blur cues stimulate retina (for example)

2. Optic nerve axons (RGCs) project to and synapse in the lateral geniculate nucleus (LGN)

3. Neurons project from the LGN to the visual cortex and synapse on the interneurons

What is the path of the interneurons of the accommodative reflex?

1. Interneurons project from the visual cortex to the frontal eye fields and synapse

2. Neurons from the frontal eye fields project to the Edinger-Westphal nucleus (EW) and oculomotor nucleus

What is the efferent pathways of the accommodative reflex?

1. Neurons from the Edinger Westphal nucleus project to the ciliary ganglion along CN III and synapse

2. From ciliary ganglion neurons project to the ciliary muscle via short ciliary nerves and synapse

What ANS nerves comprise the efferent pathway of the accommodative reflex? What receptors do they stimulate on the ciliary muscle?

Parasympathetic; stimulates M3 receptors in the ciliary muscle

What is the parasympathetic innervation to the ciliary muscle similar to and for what?

“fast twitch” like for fine control

What is definition of convergence?

Simultaneous and synchronous adduction in both eyes

What is convergence stimulated/caused by? (related to the EOMs)

Contraction of medial recuts muscles

Convergence can be blank or blank.

Symmetrical or asymmetrical

Is convergence voluntary, involuntary, or both?

Can be both

Convergence is (voluntary/involuntary) for most use of eyes.

Involuntary

Neuronal pathway for convergence (also what neuronal couples ciliary muscle to MR):

1. The oculomotor nerve receives signals from the oculomotor nucleus to stimulate the medial rectus muscles

2. Supranuclear signals from the frontal eye fields and visual cortex couple ciliary muscle and medial rectus co-contraction

What are the three main functions of the pupil?

1. Control of retinal illumination

2. Reduction of optical aberrations

3. Improve depth of focus/field

What does the pupillary function of the control of retinal illumination aid in?

Optimizing retinal illumination to maximize visual perception

How does pupillary function control retinal illumination in dim light?

Dilation of the pupil maximizes the number of photons reaching the retina

How does pupillary function control retinal illumination in 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

What is spherical aberration?

Due to the fact the lens and cornea are not perfect spheres, light going through the peripheral portion of the lens and cornea are not going to be refracted to the same point as light entering the central portion of the cornea and lens.

What does a smaller aperture do to reduce optical aberrations?

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 pupillary function improve depth of focus/field

Small pupil produces the “pinhole effect” and increases the depth of field

What are the two pupillary muscles?

Dilator muscle and Sphincter muscle

What is the orientation of the dilator muscle?

Radially oriented

What is the dilator muscle associated with?

The pigmented epithelium

What does dilator contraction do?

Pulls pupillary margin toward ciliary body peripherally (mydriasis)

What is the sphincter muscle the major driver of?

Contraction induces miosis

What does the sphincter muscle encircle?

The pupillary margin

How does the sphincter muscle relate to the pigmented epithelium?

Separated from the pigmented epithelium

What ANS division are the major excitatory neurons of the sphincter muscle part of?

Parasympathetic

What are the parasympathetic excitatory neurons of the sphincter muscle mediated by?

Acetylcholine and muscarinic receptors inducing contraction and miosis

What shape/pattern does innervation occur in of the sphincter muscle?

Innervation occurs in a segmental distribution (~20 segments)

What ANS division are the major excitatory neurons of the dilator muscle part of?

Sympathetic

What are the sympathetic excitatory neurons of the dilator muscle mediated by?

Norepinephrine and alpha 1 receptors

What are the two distinct stimuli that comprise the pupillary response?

Light/brightness reflex and the near-reflex (accommodation)

What is the same about the pupil near reflex and the pupil light reflex?

The efferent pathway to control both types is the same

What does the efferent pathway to control the pupil near reflex and pupil light reflex entail?

Parasympathetic control of the iris sphicter and sympathetic control of the dilator

What pathway/control is different between the pupil near reflex and the pupil light reflex?

The afferent pathway which involves Supranuclear control over each response

What is the pupil near reflex?

Miosis occurs without change in retinal luminance

Where do the supranuclear neurons come from in the afferent pupil near reflex?

Cortical areas surrounding visual cortex and from the frontal eye fields and goes to Edinger-Westphal

What are the two components of the afferent pathway of the pupil light reflex?

Rod and cone photoreceptors, intrinsic photoreceptive retinal ganglion cells

What are intrinsic photoreceptive retinal ganglion cells (ipRGCs)

Retinal ganglion cells that can sense light themselves, independent of photoreceptors ,for the pupillary light reflex

What do intrinsic photoreceptive retinal ganglion cells (ipRGCs) involve?

Contain melanopsin, can trigger action potentials

Why can ipRGCs sense light?

They have their own opsin (Melanopsin)

What is the afferent pathway of the pupil light reflex?

Reintal ganglion cell axons project to the pretectal olivary nucleus and synapse (instead of the LGN)

What is the pathway of the interneuron in the pupil light reflex

From the pretectal olivary nucleus it projects to the Edinger-Westphal nucleus and synapses

What is the efferent pathway of the pupil light reflex?

From edinger westphal the neuron projects to the ciliary ganglion and synapses, from the ciliary ganglion the neuron projects to the iris sphincter

Which ANS division stimulates the iris sphincter, with what receptor, and what NT is involved in the pupil light reflex efferent pathway?

Parasymapathetic stimulation, Muscarinic receptor, acetylcholine

Is the pupil light reflex symmetric or asymmetric between the eyes?

Normally is symmetric between both eyes

Why is the pupil light reflex normally symmetric between both eyes?

Due to interneurons sending signals to the left and right EW

How does the amplitude of response of the pupil light reflex compare among normal individuals

Amplitude of response varies

What is the primary way to regulate pupil size?

Regulation of iris sphincter

With what signals is the iris sphincter contraction regulated by?

Two competing signals to the Edinger Westphal nucleus

What are the two competing signals that regulate iris sphincter contraction in the pupil light reflex?

1. Illumination induced increase in parasympathetic nerve stimulation

2. Continuous supranuclear inhibition of parasympathetic nerves (when we’re awake)

What happens to the pupil light reflex with significantly reduced light stimulus (while awake)?

The inhibitory pathway prevents sphincter contraction

Where does the inhibitory input for the inhibitory pathway preventing sphincter contraction in significantly reduced light stimulus coming from?

Originates in the CNS/brainstem

What does the inhibitory pathway that prevents sphincter contraction in significantly reduced light stimulus (while awake) block stimulation to?

Parasympathetic nerve to sphincter muscle not stimulated

What type of nerve transmits the inhibitory signal to the Edinger-Westphal nucleus?

Sympathetic nerve

What is the result of the pupil light reflex with significantly reduced light stimulus (while awake)?

Mydriasis

What happens to the pupil light reflex with some light stimulus?

Some competition to the inhibitory signal by a light induced, positive signal

Is there stimulation of the parasymapthetic nerve with the pupil light reflex with some light stimulus (while awake)?

Some stimulation along parasympathetic nerve to the iris sphincter

What is the general afferent pathway of the light induced, positive signal for the pupil light reflex with some light stimulus (awake)?

Optic nerve, olivary nucleus, Edinger-Westphal nucleus

What is the result of the pupil light reflex with some light stimulus (while awake)?

Some miosis occurs

What happens with the pupil light reflex with more light stimulus (while awake)

Even more competition to inhibitory signal and more stimulation along the parasympathetic nerve to the iris sphincter (light induced, positive signal overcomes inhibition)

What is the result of the pupil light reflex with more light stimulus (while awake)?

More miosis occurs

When can a lack of inhibition of the pupil light reflex occur (no inhibitory signal from CNS/brainstem at all)

1. Sleep (constant inhibition stopped)

2. Anesthesia (constant inhibition stopped)

3. Sympathetic inhibition is suppressed

What is the results of the pupil light reflex with a lack of inhibition?

Baseline positive level of positive signal from light-induced response causes miosis

The control of iris dilator is a blank of pupil size regulation.

Secondary mechanisms of pupil size regulation

What does the contraction of the iris dilator facilitate?

A super charge of mydriasis

Is control of the iris dilator a requirement for pupil dilation?

No (can be achieved with parasympathetic control alone)

What two things does control of iris dilator achieve?

Enhances speed of pupillary movement and causes maximal pupil diameter

What ANS division controls the iris dilator?

Sympathetic stimulation

Where do the sympathetic nerves that control the iris dilator muscle come from?

Superior cervical ganglion

What do the axons of the sympathetic nerves that control the iris dilator muscle pass through?

The ciliary ganglion

What are two sources of additional iris muscle regulation (aside form sympathetic innervation)?

1. Circulating catecholamines released from adrenal glands in the blood may act directly on the iris dilator causing mydriasis (independent of neural stimulation)

2. Sensory innervation to the iris

What nerve delivers sensory innervation to the iris?

CN V

What does the sensory innervation to the iris modify?

Modifies pupil diameter

What do cataract surgeons often observe of the sensory innervation to the iris?

Cataract surgeons often observe that mechanical and chemical irritation of the eye can cause a strong miotic response

Can autonomic drugs fix that strong miotic response cataract surgeons observe due to mechanical and chemical irritation?

Fails to reverse with autonomic drugs

Do we understand the mechanism of the strong miotic response cataract surgeons observe due to mechanical and chemical irritation?

Current mechanism unknown in humans

What do animal models suggest about the strong miotic response cataract surgeons observe due to mechanical and chemical irritation?

Suggest response is due to neuropeptide (cholecystokinin) release from neurons