Focusing Light on the Retina & Aqueous Humor Flow
focusing light rays on the retina
- Optic Refraction
Optic refraction is the bending of light as it passes from one medium to another, and the cornea has the highest refractive power of any of structure within the eye – this process is addressed in a separate tutorial.
- To further focus light rays on the retina, three key adjustments occur:
- Lens accommodation (a change in refractory power).
- Pupillary Widening/Constriction.
- Eye convergence or divergence (neuroanatomical processes learned in detail elsewhere).
lens accommodation
Anatomy
- Anterior segment of the corneoscleral layer.
- Anterior uvea: the iris, ciliary body (ciliary muscle and ciliary process), and choroid.
- Lens
- Zonule
The ciliary body anchors suspensory ligaments, collectively called zonule, which either stretch or thicken the lens and alter its refractive power, so-called accommodation, which is a change in the refractory power of the lens as an object changes distance.
Physiology
- Lens accommodation is a change in the refractory power of the lens, which serves to help focus light on the retina.
- During near accommodation, the ciliary bodies contract (ie, shorten), the zonule relax and the lens rounds (ie, thickens), and the near object is brought into focus.
- During far vision, the ciliary bodies relax, the zonule stretch, and the lens flattens.
- Clinical correlation: In presbyopia, there is refractive error from loss of lens elasticity (lens hardening). This is an-age related process; individuals use "reading glasses" to help focus on near objects.
Lens accommodation Demonstration Exercise
To demonstrate lens accommodation for yourself, do the following. Extend your right index finger to represent a relaxed ciliary body. Then make a V with your left hand's thumb and index finger and touch their tips to the ends of the relaxed ciliary body. The V represents the taut zonular fibers. Imagine that your left hand is the lens; feel the pull of the zonule on your left hand and imagine the lens being flattened. Then, contract the ciliary body (ie, collapse your stretched right index finger); the zonule fold in on themselves, which causes your left hand to relax, and you can imagine the lens rounding.
pupillary constriction and relaxation.
Anatomy of the Superficial structures of the Eye
- The palpebra: the eyelid.
- The palpebral fissure is the distance between upper and lower eyelids.
- The iris.
- The sclera, itself (the "white of the eye") is the visualizable portion of the sclera.
- The pupil: the hole in the iris.
- The lacrimal caruncle at the medial corner of the eye, which produces whitish, oily secretion: "sleep in the eye".
Anatomy and Physiology of the Iris Muscles
- Iris sphincter muscles are:
- Circumferentially-arranged and narrow pupil size.
- They are parasympathetically-innervated muscles, which constrict pupil size in bright light.
- Iris dilator muscles are:
- Radially-arranged and enlarge pupil size.
- They are sympathetically-innervated muscles, which widen pupil size in low light.
- Sympathetic excitation causes upper eyelid elevation via activation of the superior tarsal muscle to maximize light absorption.
- Miosis (pupil constriction) from a loss of innervation to the dilator muscles.
- Ptosis (eyelid droop) from a loss of innervation to the superior tarsal muscle.
- Anhidrosis (loss of sweating) from loss of sympathetic innervation to sweat glands to the face.
Aqueous humor production and rebasorption
The ciliary process of the ciliary bodies produce aqueous humor, which is a low- protein, aqueous (ie, watery) fluid.
Anatomy
- The anterior eye cavity divides into:
- The anterior chamber, which lies anterior to the iris.
- The posterior chamber, which lies posterior to the iris.
- The Canal of Schlemm, which lies within the sclera at the corneoscleral junction.
Physiology
- The ciliary epithelia from the ciliary process actively secrete aqueous humor into the posterior chamber (the space between the lens and iris).
- The aqueous humor then flows through the pupil into the anterior chamber (the space between the iris and cornea.
- The aqueous humor is then reabsorbed at the iridocorneal filtration angle through the trabecular meshwork into the canal of Schlemm.
- Clinical correlation: Glaucoma occurs from aberrancy in aqueous humor flow.
- In narrow (or closed) angle glaucoma, the rarer form, there is apposition of key anterior structures, which trap the flow of aqueous humor. For instance, as we show here, there is abutment of the lens and iris and fluid is obstructed (alternatively, the iris could abut the cornea). Sudden closed angle glaucoma is painful and cause reddening of the eye – epinephrine will worsen the condition due to its mydriatic effect on the iris.
- In open angle glaucoma, there is normal space between the iris and lens, and fluid flows freely in between the iris and lens but its reabsorption is blocked via poor uptake at the trabecular meshwork or canal of Schlemm, such as from uveitis.
- Common pharmacologic therapies for glaucoma either reduce aqueous humor production or promote aqueous humor reabsorption.
Detailed Anatomy of the Internal Scleral Sulcus
- The trabecular meshwork and canal of Schlemm (along with the scleral spur) lie within the internal scleral sulcus, which sits at the inner surface of the sclera–corneal junction, called the limbus.
- The trabecular meshwork gates the reabsorption of aqueous humor through the canal of Schlemm, which drains directly into the venous system.
- An alternative reabsorption pathway also exists in which aqueous humor is directly reabsorbed through the ciliary body into the uveal vessels.