Intraocular Muscles

Reading

Robert A. Moses (ed), Adler's Physiology of the Eye, 7th Ed, Chapters 11, 12. Mosby (1981) RE67.A3

Accommodation

The visual stimulus for accommodation changes

Moses, chpt 11: "The normal eye is so constructed that when it is at rest, rays of light coming from a distant object or focused on the retina." "When the eye is adjusted for near vision, it is said to be accommodated..."

Blur circles & focus
1/p meters = P diopters, P + Q = F
look at a spot on a windowpane and see blur inside and outside the window.
the pupil constricts during accommodation the anterior pole of the lens moves forward, carrying the lens with it.
The anterior surface of the lens becomes more convex.
As the lens increases in axial thickness, it diminishes in diameter.

For one eye, there is only one clue about changing focus: the foveal image being out of focus. An image can be out of focus because it's too far away or too close. How does the eye know which way to accommodate the lens? Does it make a guess, see if the image becomes more or less blurred, then make another guess? Apparently not. Somehow the visual system knows which direction to change focus. Is size a cue?
But see Stark & Takahashi (1965), who found that the eye makes a correct initial response to blur only half the time...

For two eyes, there can be another clue: double vision due to the eyes verged on the target. Thus accommodative vergence involves a change in focus too.
The triad of the near reflex:
*vergence
*accommodation
*pupil constriction

The mechanics of changing accommodation

Engineering question: What is a way to change the focus of an optical system? In a camera a lens is moved back and forth, and some animals, like fish; a fixed shape lens is moved toward or away from the retina. (Birds of prey can vary the curvatrue of the cornea!) But in mammals the means of focus change (accommodation) is change in the shape of the lens. In fact most of the optical power of the human eye derives from the corneal-air interface. But some fraction of focus is due to the clear protein lens inside the eye. Note that the lens material must have a greater index of refraction than the vitreous humor fluid surrounding it. (n = 1.390 vs 1.334...not much of a difference!)

Hecht and Zajak, pages 138-145.

[Why you can't see well under water: the index of refraction of the cornea is too close to that of water...]

Which way should a fixed lens move to focus an object moving closer?
From your physics class you may recall the lens formula:


s is a distance measure from the center of the lens to the object in the real world or its image on the retina.
so if f is fixed and s-object decreases, then s-image must increase
What is the power of the human eye visual system? Say the image distance is about 20 mm. and that something at infinity is focused on the retina. then in terms of camera jargon, the eye has about a 20 mm lens (for focus at infinity). A kind of fisheye lens, corrected for by the curvature of the retina, I suppose. The optical power must increase for near vision.

Now what if the object is 10 cm = 100 mm from the eye? f = 100/6 = 16.6 mm lens, so the interior lens can contribute about "3.5 mm" worth of lens power.

There are [parasympathetic] ciliary muscles inside the eye (intraocular), for constricting pupil and for increasing power of lens. The lens does not move forward or backward, as it does in conventional optical systems; it changes curvature.

The ciliary muscle attaches to the suspensory ligament, which in turn 
is connected to the lens. The cililary muscle must be tangential. 
As the ciliary muscle contracts, the ring it forms becomes smaller 
and the zonal strands can relax, and the lens can collapse to a 
more spherical shape. No antagonist involved! contracting muscle 
simply results in zonal strands relaxing. On the outside, springy 
connective tissue (more ligament) tends to pull relaxed ciliary 
muscle bundle away from the lens, and flatten the lens. "The 
ciliary body is stretched backward and outward along the sclera by 
the elastic choroid." 

What kind of muscle is ciliary? smooth!
From where is it innervated? The ciliary ganglia, part of the PNS.





The ciliary muscles are for controlling the shape of the lens. See Patton et al. chpt 15. 

See the TIMOPTIC model EYEBALL! 

Structure of the lens: about 22,000 onion-like layers. The lens can become cloudy, and that is called a cataract; cataracts can also be pinpoint defects, that causes light to scatter (a common problem of premature birth). It is possible to put in an artificial lens, but it will not be able to change shape like the real lens. And the artificial lens may allow for UV light to get to the retina...see flowers in a different light!

Patton et al 206, "The lens is held in place behind the iris by the thin zonal strands of the suspensory ligament, which bridge between the elastic capsule of the lens and the ciliary body. In the relaxed eye the zonula is taut, holding the lens in a semiflattened shape. The tension of the zonula, however, is decreased by the contraction of the smooth muscle of the ciliary body. One set of ciliary fibers is circular; their contraction reduces the diameter of the ciliary body and thus slackens the zonula. Another set of muscle fibers is longitudinally arranged; their contraction pulls the attachments of the zonula closer to the base of the iris and reduces zonular tension. Contraction of both circular and longitudinal fibers thus relaxes the zonula and allows the elastic lens to bulge passively into a more nearly spherical shape; the magnitude of the change is a function of the strength ciliary contraction."

"The ciliary muscle is innervated by parasympathetic postganglionic neurons located in the ciliary ganglion. The preganglionic neurons are in Edinger-Westphal nucleus; their axons reach the ganglion via cranial nerve III. Like other parasympathetic postganglionic neurons, those supplying the ciliary muscle are cholinergic, and the acetylcholine receptors of the ciliary muscle can be readily blocked with atropine drops."

In aging, from 40 to 50, the flattened shape becomes harder to change into the spherical shape, and in that sense the flattened shape is the "relaxed" form; thus the need for reading glasses, to focus up close with the spherical shape.

1996: Nolte, page 299 ff: The NEAR reflex: ciliary contraction, ocular convergence, and pupillary constriction. Some axons from optic nerve go straight to pretectal nucleus. Pretectal neurons project to Edinger-Westphal. See Fig. 11.18. E-W axons are in the 3rd cranial nerve. Bilateral projection of pretectals to EWs accounts for consensual reflex (both eyes respond).

What happens to accommodation in the dark? Try it. It seems that you "relax" to far vision...

Control of pupil diameter

Chapter 12, H. Stanley Thompson: "The Pupil"

The visual stimulus for changing pupil diameter
"The iris contains two muscles, the sphincter pupillae and the dilator pupillae. They are among the very few muscles of the body derived from the neural ectoderm." "The iris sphincter shortens about 87% of its length..."

Thompson says the human pupil can go from 1 mm to 9 mm diameter, nearly 2 orders of magnitude area change...

"In adolescence the pupils are at their largest. They then become steadily smaller until about age 60..."
"Youth and excitement have always diated the pupils, and age and boredom still constrict them.

Atropine is from the belladonna family of flowering plants: The mischief caused by the ubiquitous jimsonweed is typical of this group of plants. ...atropine blocks parasympathetic activity."

%--------------
The "aperture stop" in camera lingo. How does a camera aperature work? Flat vanes collapse or expand in overlapping ways, based on a tangential mechanical action. See DEMO in class.

The pupil can change area by about a factor of 16 in human. 2 mm to 8 mm diameter. Far less than the 10 orders of magnitude over which the visual system is capable of sensing intensity changes! What is the stimulus for pupil diameter changes?

In general, the brighter the general illumination, the more constricted the pupil. Question: What kind of reaction causes pupil diameter dilation? In some cases it can be emotional! The eyes can be used to signal emotional responses to other humans. Thus during constant illumination the pupil diameter can change.

The parasympathetic system controls the sphincter (constricting) action, while sympathetic system controls pupil dilation. Atropine drops can dilate the eye (belladonna plant = "beautiful woman") used by Italian belles to appear more "romantic" since the Middle Ages.

A constricted pupil admits less light, but equally important may be that a constricted pupil blocks light that has entered from the outer edges of the cornea, and would degrade the image due to spherical and chromatic abberation. Increases the "depth of field", in camera lingo.

DEMO: pinhole "glasses."

The intensity level signalled by the retinal ganglion cells arrives at the nucleus of Edinger-Westphal. (Nolte, page 300) The route: Optic tract axons (from ganglion cells) project to the superior colliculis and thence to the "pretecal area". Pretectal neuons project to the nucleus of Edinger-Westphal. Axons from the n. EW travel in the III nerve as preganglionic parasympathetic fibers to the ciliary ganglion (different from the cilary body!), where they contact the postganglionic cells. Postganglionic fibers close the reflex loop by synapsing on smooth sphincter muscle that causes pupil constriction. Each pretectal area projects bilaterally to the n. EW, thus the yoked control.

Dowling Neural Networks book: A separate sympathetic pathway can cause dilation by activating a different set of radial smooth muscles. When we are interested or frightened our pupils dilate. (The "belladonna" signal?) The sympathetic pathway is not visual...

DEMO with atropine drops: takes at least 15-20 for full dilation...lasts a few hours! blocking the muscarinic cholinergic synapses so the pupil sphincter relaxes. vision v. blurry.

Mechanics of pupil diameter change

The two pupils are yoked:
Only one eye need by "stimulated" for both pupils to constrict.

more push-pull. both sym and parasym muscle control. sym causes dilation!

Patton et al. 207, "The iris has two sets of smooth muscles: a circular or sphincter muscle, which acts to constrict the pupillary aperture, and a radial component, contraction of which dilates the pupil. Pupillary size is determined by the balanced action of these two sets of muscles. The sphincter muscle is innervated by postganglionic parasympathetic neurons in the ciliary ganglion, the axons of which reach the iris through the short ciliary nerves in company with those destined for the ciliary muscles. The preganglionic neurons are located in Edinger-Westphal portion of the oculomotor nucleus and reach the ganglion via cranial nerve III. The radial muscle of the iris is innervated by sympathetic postganglionic neurons in the superior cervical ganglion; their axons reach the eye via the carotid plexus and the opthalmic branch of cranial nerve V."

It must be true that when the iris is "pulled back" in dilation it takes on a wrinkled appearance; this can be seen in some cross-section images of the iris during dilation. The dilating radial muscle should also reach all the way to insert at the inner edge of the iris.

Engineering: How does a camera "f-stop" aperture control mechanism work? see DEMO, and shutter control.

Superior cervical ganglion is part of the sympathetic nervous system and serves to dilate pupils and relax accommodation. Signals start at T1-2 of the cord. From there they project to SC ganglia just outside the cord. The nerve cells use ACh as transmitter. Post ganglionic cells use epinephrine. (Quote N&F top of page 110, "...promotes the organism's ability to expend energy...")

"...the dilator muscle of the pupil contracts. Thus the pupil widens." eyelids are raised too.
[Patton et al 144, and see Nauta & Feirtag p. 110 & 112]

Blinking and pupil diameter change: When you close your eyes, do your pupils dilate? If so, then when you open your eyes in bright light, do you see the pupils constricting?

Summary

* Accommodation: cues from blur or vergence.
* Mechanics of accommodation: tangentially oriented smooth muscle in the ciliary body contracts, shrinking the size of the ring attached to one side of the suspensory ligaments. The ligaments relax and the lens becomes more spherical.
* The intensity stimulus for pupil diameter changes.
* Sphincter muscle for constriction. Pathway through pretectal nucleus to the nucleus of Edinger-Westphal.
* Sympathetic pathway: emotional response
* Mechanical optical aperture mechanism: improve depth of field.