What is the structure and function of an eye?

What is an Eye?

Anatomical Structure and Function

The eye is a specialized sensory organ that functions as an extension of the central nervous system, consisting of multiple integrated structures that capture, process, and transmit visual information to the brain for interpretation. 1, 2

Core Anatomical Components

The eye comprises several distinct anatomical regions that work as an integrated system:

External and Anterior Structures

• Eyelids and lashes protect the ocular surface and regulate light exposure 1
• Lacrimal apparatus produces and drains tears to maintain ocular surface health 1
• Conjunctiva and sclera form the outer protective layers, with the sclera providing structural integrity 1
• Cornea serves as the primary refractive surface, bending incoming light 1
• Anterior chamber contains aqueous humor and lies between the cornea and iris 1
• Iris controls pupil size to regulate light entry 1
• Lens provides additional focusing power and accommodation 1

Posterior Structures

• Vitreous cavity contains gel-like vitreous humor that maintains globe shape 1
• Retina contains photoreceptors (rods and cones) that convert light into electrical signals 3
• Macula provides central, high-acuity vision 1
• Optic nerve transmits visual information from retinal ganglion cells to the brain 4
• Choroid supplies vascular support to outer retinal layers 1

Functional Organization

Visual Processing Pathway

The eye operates through a three-stage neuronal organization 3:

1. Photoreceptor layer: Rods and cones decompose light into approximately 12 parallel information streams 3
2. Bipolar cell layer: Connects photoreceptor outputs to specific retinal ganglion cell types 3
3. Ganglion cell layer: Combines signals to create roughly 20 distinct encodings of visual information transmitted to the brain 3

Connection to the Brain

• Retinal ganglion cells are the only retinal neurons that send axons out of the eye, forming the optic nerve 5, 4
• The optic nerve (Cranial Nerve II) extends from the optic disc to the optic chiasm, containing glial cells and myelinated axons 4
• The retina functions as an extension of the CNS, sharing similar immune responses and cellular characteristics with brain and spinal cord tissue 2

Integrated Functional Systems

The Lacrimal Functional Unit

The ocular surface and tear-secreting glands operate as an integrated system that maintains tear film stability 1:

• Disease or dysfunction results in unstable tear film causing irritation and potential epithelial damage 1
• The ocular surface microenvironment includes tissues, cells, matrices, and microbiome working together to maintain integrity 1

Optical Properties

• The eye functions analogously to both a camera and microscope 1
• The cornea and lens act as objective lenses, focusing light onto the retina 1
• Light passes through multiple ocular structures (cornea, aqueous humor, lens, vitreous) twice—during illumination and on the return path to imaging devices 1

Nonvisual Functions

Beyond image formation, the eye performs critical nonvisual photoreception 6:

• Retinal photoreceptors entrain circadian rhythms through specialized ganglion cells containing photosensitive molecules like cryptochrome 1 6
• These nonvisual pathways influence the suprachiasmatic nucleus, the brain's primary pacemaker 6
• Nonvisual photoreception synchronizes periodic functions to environmental light cycles 6

Clinical Examination Framework

Comprehensive evaluation of eye structure and function includes 1:

• Visual acuity testing at distance and near with refraction 1
• External examination of eyelids, lashes, lacrimal system, and globe position 1
• Pupillary function assessment including size, light response, and afferent defects 1
• Slit-lamp biomicroscopy of anterior structures from lids through anterior vitreous 1
• Intraocular pressure measurement preferably with Goldmann applanation tonometry 1
• Fundus examination of vitreous, retina, vasculature, and optic nerve, optimally through dilated pupils 1

Developmental Considerations

In pediatric populations, eye examination techniques must be adapted 1:

• Binocular red reflex (Brückner) test screens for opacities, refractive errors, and asymmetries 1
• Cycloplegic refraction requires age-appropriate agents (cyclopentolate 0.2% with phenylephrine 1% for infants under 6 months) 1
• Examination order varies based on cooperation level, with sensory testing performed before dissociating techniques 1