Eye: Anatomy

The human eye is a sensory Sensory Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology organ whose primary function is vision Vision Ophthalmic Exam. The eye has a spheroidal shape and is structured in 3 layers: a supporting outer fibrous Fibrous Fibrocystic Change layer, a middle vascular layer, and an inner neural layer. The eye can also be subdivided into 3 compartments: the anterior, posterior, and vitreous chambers. Surrounding the eyeball itself are the extraocular muscles, the lacrimal apparatus, various nerves and vessels, and the bony structure of the orbit Orbit The orbit is the cavity of the skull in which the eye and its appendages are situated. The orbit is composed of 7 bones and has a pyramidal shape, with its apex pointed posteromedially. The orbital contents comprise the eye, extraocular muscles, 5 cranial nerves, blood vessels, fat, the lacrimal apparatus, among others. Orbit and Extraocular Muscles: Anatomy. Light travels through the compartments of the eye to focus Focus Area of enhancement measuring < 5 mm in diameter Imaging of the Breast on the retina, which is the location where photoreceptors convert the stimulus into a neural impulse that is carried by the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions to the brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification.

Last updated: 23 Jun, 2022

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Development

General

  • Develops from the forebrain forebrain The anterior of the three primitive cerebral vesicles of the embryonic brain arising from the neural tube. It subdivides to form diencephalon and telencephalon. Development of the Nervous System and Face
  • Regulated by the PAX6 gene Gene A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. Basic Terms of Genetics
  • Lens placode gives rise to the lens.
  • Optic cup gives rise to the retina.

Timeline

Embryologic development of the eyes

Embryologic development of the eye

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Gross Anatomy

General characteristics

The adult eye is a complex organ contained within the orbital cavity (composed of 7 bones). Each eye has multiple layers and chambers and is surrounded by 6 extraocular muscles.

Essential anatomy of the eye illustration

Anatomy of the eye

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Layers of the eye

The eye is composed of 3 layers ( fibrous Fibrous Fibrocystic Change, vascular, neural) and a transparent connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology covering (conjunctiva).

Conjunctiva:

  • Loose, transparent connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology that covers the eye (bulbar conjunctiva).
  • Adheres firmly to the limbus of the cornea
  • Reflects back on itself to form the inner layer of the eyelid ( palpebral conjunctiva Palpebral Conjunctiva Blepharitis)
  • Highly vascular
  • Maintains normal lubrication of the eye

Fibrous Fibrous Fibrocystic Change layer:

  • Cornea:
  • Sclera:
    • Continuous with cornea
    • Covers posterior aspect of eye
    • > 80% of fibrous Fibrous Fibrocystic Change layer
    • Seen as the white part of eye
    • Site of attachment Attachment The binding of virus particles to virus receptors on the host cell surface, facilitating virus entry into the cell. Virology for extraocular muscles
  • Limbus:
    • Border between cornea and sclera
    • Pathway of aqueous humor Humor Defense Mechanisms outflow
    • Site of surgical incision Surgical Incision Surgical Site Infections for cataract Cataract Partial or complete opacity on or in the lens or capsule of one or both eyes, impairing vision or causing blindness. The many kinds of cataract are classified by their morphology (size, shape, location) or etiology (cause and time of occurrence). Neurofibromatosis Type 2 and glaucoma Glaucoma Glaucoma is an optic neuropathy characterized by typical visual field defects and optic nerve atrophy seen as optic disc cupping on examination. The acute form of glaucoma is a medical emergency. Glaucoma is often, but not always, caused by increased intraocular pressure (IOP). Glaucoma surgery

Vascular layer:

  • Uvea, or uveal tract:
    • Composed of the iris, ciliary body, and choroid 
    • Lies between sclera and retina
    • Anterior portion is the iris and ciliary body.
    • Posterior portion is the choroid.
  • Choroid (posterior uvea):
    • Reddish brown layer between the sclera and retina
    • Contains large external vessels and an innermost capillary bed
    • Source of oxygen/nutrients to retina
    • Continuous anteriorly with ciliary body
  • Ciliary body:
  • Iris:
    • Pigmented diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm: Anatomy with central aperture ( pupil Pupil The pupil is the space within the eye that permits light to project onto the retina. Anatomically located in front of the lens, the pupil’s size is controlled by the surrounding iris. The pupil provides insight into the function of the central and autonomic nervous systems. Pupil: Physiology and Abnormalities)
    • On anterior surface of lens
    • Sphincter muscles within the iris adjust pupillary size in response to light and the autonomic nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. Nervous System: Anatomy, Structure, and Classification.
    • Light exposure/parasympathetic stimulation:
      • Narrows the pupil Pupil The pupil is the space within the eye that permits light to project onto the retina. Anatomically located in front of the lens, the pupil’s size is controlled by the surrounding iris. The pupil provides insight into the function of the central and autonomic nervous systems. Pupil: Physiology and Abnormalities ( miosis Miosis Pupil: Physiology and Abnormalities)
      • Sphincter pupillae muscle
    • Darkness/sympathetic stimulation:
      • Widens the pupil Pupil The pupil is the space within the eye that permits light to project onto the retina. Anatomically located in front of the lens, the pupil’s size is controlled by the surrounding iris. The pupil provides insight into the function of the central and autonomic nervous systems. Pupil: Physiology and Abnormalities ( mydriasis Mydriasis Dilation of pupils to greater than 6 mm combined with failure of the pupils to constrict when stimulated with light. This condition may occur due to injury of the pupillary fibers in the oculomotor nerve, in acute angle-closure glaucoma, and in adie syndrome. Glaucoma)
      • Dilator pupillae muscle
Iris muscle actions

The sphincter muscles of the iris are responsible for constricting (A) and dilating (B) the pupil Pupil The pupil is the space within the eye that permits light to project onto the retina. Anatomically located in front of the lens, the pupil’s size is controlled by the surrounding iris. The pupil provides insight into the function of the central and autonomic nervous systems. Pupil: Physiology and Abnormalities.

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Neural layer:

  • Visual retina:
    • Neural layer:
      • Light-receptive
      • Ends anteriorly along the ora serrata
    • Pigmented cell layer: 
      • Single layer of cells
      • Absorbs light to reduce scattering of light within eye
    • Optic disc or papilla: 
      • Known as the “blind spot”: no photoreceptors
      • Located in the fundus Fundus The superior portion of the body of the stomach above the level of the cardiac notch. Stomach: Anatomy of the retina, which is the location where sensory Sensory Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology fibers of the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions enter the eyeball.
    • Macula: 
      • Contains special photoreceptor cones for visual acuity Visual Acuity Clarity or sharpness of ocular vision or the ability of the eye to see fine details. Visual acuity depends on the functions of retina, neuronal transmission, and the interpretative ability of the brain. Normal visual acuity is expressed as 20/20 indicating that one can see at 20 feet what should normally be seen at that distance. Visual acuity can also be influenced by brightness, color, and contrast. Ophthalmic Exam
      • Lacks blood vessels
    • Fovea:
      • Located in center of macula
      • Responsible for high-acuity vision Vision Ophthalmic Exam
      • Densely saturated with cone photoreceptors
  • Nonvisual retina:
    • Portion of retina that is not sensitive to light
    • Anterior continuation of the pigment cell layer plus supporting cells over the ciliary body and posterior surface of the iris
Retina

Retina:
The fovea and macula lack blood vessels and are responsible for high-acuity vision Vision Ophthalmic Exam. The optic disc is the entrance point for the vasculature of the eye and does not contain photoreceptors.

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Characteristics of the retina and its layers:

Table: Characteristics of the retina and its layers
Layer of the retina (external to internal) Characteristics
Pigment epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium: Histology
  • Most external layer, closest to choroid, from which it supplies nutrition to photoreceptors
  • Contains melanin Melanin Insoluble polymers of tyrosine derivatives found in and causing darkness in skin (skin pigmentation), hair, and feathers providing protection against sunburn induced by sunlight. Carotenes contribute yellow and red coloration. Seborrheic Keratosis granules and stores vitamin A Vitamin A Retinol and derivatives of retinol that play an essential role in metabolic functioning of the retina, the growth of and differentiation of epithelial tissue, the growth of bone, reproduction, and the immune response. Dietary vitamin A is derived from a variety of carotenoids found in plants. It is enriched in the liver, egg yolks, and the fat component of dairy products. Fat-soluble Vitamins and their Deficiencies
  • Absorbs light and prevents reflection
Layer of rods and cones
  • Photoreceptor cells
  • Rods: slender, cylindrical cells containing rhodopsin, located around the fovea centralis, specialized for night vision Vision Ophthalmic Exam (dim light) and motion
  • Cones: flask-shaped cells containing photopsin/iodopsin, located within the fovea centralis, specialized for color vision Vision Ophthalmic Exam, bright light, object recognition, and visual acuity Visual Acuity Clarity or sharpness of ocular vision or the ability of the eye to see fine details. Visual acuity depends on the functions of retina, neuronal transmission, and the interpretative ability of the brain. Normal visual acuity is expressed as 20/20 indicating that one can see at 20 feet what should normally be seen at that distance. Visual acuity can also be influenced by brightness, color, and contrast. Ophthalmic Exam
External limiting membrane Supports the photoreceptor cells
Outer nuclear membrane Photoreceptors cell nuclei ( 1st-order neuron 1st-order neuron Originates in the hypothalamus and descends to the first synapse in the cervical spinal cord, located at levels C8–T2 (also called the ciliospinal center of Budge). Horner Syndrome)
Outer plexiform layer 1st synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission, between cones and rods and bipolar Bipolar Nervous System: Histology cells
Middle limiting membrane Supporting membrane
Inner nuclear layer Cell bodies and nuclei of bipolar Bipolar Nervous System: Histology cells ( 2nd-order neuron 2nd-order neuron Preganglionic pupillomotor fibers exit the spinal cord at T1, travel through the brachial plexus, over the lung apex, ascending to the superior cervical ganglion located near the angle of the mandible and the bifurcation of the common carotid artery. Horner Syndrome), transmit information to ganglion cells Ganglion Cells The Visual Pathway and Related Disorders
Inner plexiform layer Second synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission, between bipolar Bipolar Nervous System: Histology cells and ganglion cells Ganglion Cells The Visual Pathway and Related Disorders
Ganglion cell layer
  • Cell bodies and nuclei of ganglion cells Ganglion Cells The Visual Pathway and Related Disorders ( 3rd-order neuron 3rd-order neuron Pupillomotor fibers ascend along the internal carotid artery and enter the cavernous sinus where it is in close relation with the abducens nerve (cranial nerve (CN) VI). These fibers enter the orbit with the ophthalmic branch (V1) of the trigeminal nerve (CN V) via the long ciliary nerves, which innervate the iris dilator and Müller muscles. Horner Syndrome) combine here to form the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions (cranial nerve (CN) II).
  • Transmits information to the hypothalamus Hypothalamus The hypothalamus is a collection of various nuclei within the diencephalon in the center of the brain. The hypothalamus plays a vital role in endocrine regulation as the primary regulator of the pituitary gland, and it is the major point of integration between the central nervous and endocrine systems. Hypothalamus, pretectal nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles, lateral geniculate body Lateral Geniculate Body The Visual Pathway and Related Disorders, superior colliculus Superior Colliculus The anterior pair of the quadrigeminal bodies which coordinate the general behavioral orienting responses to visual stimuli, such as whole-body turning, and reaching. Cranial Nerve Palsies, and visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy
Optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions fibers Axons Axons Nerve fibers that are capable of rapidly conducting impulses away from the neuron cell body. Nervous System: Histology of ganglion cells Ganglion Cells The Visual Pathway and Related Disorders
Internal limiting membrane Most internal layer, closest to the vitreous humor Humor Defense Mechanisms

Chambers of the eye

  • The eye consists of 3 fused chambers:
    • Anterior: between cornea and iris
    • Posterior: between iris and lens
    • Vitreous: between lens and posterior eye wall
  • Aqueous humor Humor Defense Mechanisms:
    • Produced by the ciliary processes of the ciliary body
    • Travels to the anterior and posterior chambers
    • Absorbed via the lymphatics in the Schlemm canal
  • Vitreous body (also known as vitreous humor Humor Defense Mechanisms): 
    • Occupies the vitreous chamber
    • Encased in the vitreous membrane
    • Fluid-like at center and gel-like at edges
    • Attaches firmly to the serrata, optic disc, retinal vessels, lens capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides, and macula
Structure of limbal region

Diagram of the structure of the limbal region:
The aqueous humor Humor Defense Mechanisms is produced by the ciliary processes (1), circulates through the pupil Pupil The pupil is the space within the eye that permits light to project onto the retina. Anatomically located in front of the lens, the pupil’s size is controlled by the surrounding iris. The pupil provides insight into the function of the central and autonomic nervous systems. Pupil: Physiology and Abnormalities of the iris and into the anterior chamber (2), and finally through the Schlemm canal into the scleral venous sinus.

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Neurovasculature

Blood supply

The entire arterial supply of the eye is provided by branches of the ophthalmic artery and drained by a system of veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology that unite to form the central vein of the retina.

  • Ophthalmic artery, branch of the internal carotid artery Internal carotid artery Branch of the common carotid artery which supplies the anterior part of the brain, the eye and its appendages, the forehead and nose. Carotid Arterial System: Anatomy, in turn gives off branches:
    • Central retinal artery:
      • 1st branch of the ophthalmic artery
      • Supplies the inner layer of the retina and all nerve fibers Nerve Fibers Slender processes of neurons, including the axons and their glial envelopes (myelin sheath). Nerve fibers conduct nerve impulses to and from the central nervous system. Nervous System: Histology that form the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions
    • Lacrimal artery:
      • 2nd branch of the ophthalmic artery
      • Supplies the eyelids Eyelids Each of the upper and lower folds of skin which cover the eye when closed. Blepharitis, lacrimal gland, and conjunctiva
    • Ciliary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology:
      • Include the long and short posterior ciliary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology and anterior ciliary artery
      • Supply the choroid and iris
    • Supraorbital artery:
      • Bifurcates to form a superficial and deep branch
      • Supplies the upper eyelid, levator palpebrae muscle, skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions of the forehead Forehead The part of the face above the eyes. Melasma and scalp
    • Ethmoidal arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology:
      • Include the anterior and posterior ethmoidal arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology
      • Supply the ethmoid sinuses
    • Medial palpebral artery:
      • Also known as the internal palpebral arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology—superior and inferior
      • Arises from the ophthalmic artery near its terminal branches 
      • Supplies the eyelids
    • Muscular arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology:
      • Medial and lateral branches of the ophthalmic artery
      • Supply the extraocular muscles
    • Terminal branches:
      • Dorsal nasal artery: supplies the lacrimal sac Lacrimal Sac Dacryocystitis and tip of the nose Nose The nose is the human body’s primary organ of smell and functions as part of the upper respiratory system. The nose may be best known for inhaling oxygen and exhaling carbon dioxide, but it also contributes to other important functions, such as tasting. The anatomy of the nose can be divided into the external nose and the nasal cavity. Nose and Nasal Cavity: Anatomy
      • Supratrochlear artery: supplies the medial forehead Forehead The part of the face above the eyes. Melasma
  • Venous drainage: 
    • Central retinal vein and vortex veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology merge with superior and inferior ophthalmic veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology 
    • These veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology drain into the cavernous sinus, pterygoid venous plexus, and facial vein.

Innervation

  • Vision Vision Ophthalmic Exam:
    • Optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions (CN II)
    • Innermost retinal layer contains photoreceptors that convert light stimuli Light Stimuli Pupil: Physiology and Abnormalities into neural impulses that are transmitted by the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions:
      • Rods: best in dim light; about 120 million in each retina
      • Cones: color vision Vision Ophthalmic Exam; about 6 million in each retina
  • Miosis Miosis Pupil: Physiology and Abnormalities
    • Pupillary constriction 
    • Parasympathetic fibers of short ciliary nerves to the constrictor pupillae muscle
  • Mydriasis Mydriasis Dilation of pupils to greater than 6 mm combined with failure of the pupils to constrict when stimulated with light. This condition may occur due to injury of the pupillary fibers in the oculomotor nerve, in acute angle-closure glaucoma, and in adie syndrome. Glaucoma:
    • Pupillary dilation
    • Sympathetic fibers of long ciliary nerves to the radial dilator pupillae muscle
  • Accommodation Accommodation Refractive Errors:
  • Sensation: ophthalmic branch (V1) of the cranial nerve V ( trigeminal nerve Trigeminal nerve The 5th and largest cranial nerve. The trigeminal nerve is a mixed motor and sensory nerve. The larger sensory part forms the ophthalmic, mandibular, and maxillary nerves which carry afferents sensitive to external or internal stimuli from the skin, muscles, and joints of the face and mouth and from the teeth. Most of these fibers originate from cells of the trigeminal ganglion and project to the trigeminal nucleus of the brain stem. The smaller motor part arises from the brain stem trigeminal motor nucleus and innervates the muscles of mastication. The 12 Cranial Nerves: Overview and Functions)

Function

Vision Vision Ophthalmic Exam

  • The function of the eye is to provide the sense of sight.
  • Light rays course through the cornea, aqueous humor Humor Defense Mechanisms, pupil Pupil The pupil is the space within the eye that permits light to project onto the retina. Anatomically located in front of the lens, the pupil’s size is controlled by the surrounding iris. The pupil provides insight into the function of the central and autonomic nervous systems. Pupil: Physiology and Abnormalities, lens, and vitreous humor Humor Defense Mechanisms to focus Focus Area of enhancement measuring < 5 mm in diameter Imaging of the Breast onto the innermost layer of the retina.
  • The cornea and lens work together to focus Focus Area of enhancement measuring < 5 mm in diameter Imaging of the Breast light onto the retina.
  • The iris adjusts how much light enters the eye by adjusting the size of the pupil Pupil The pupil is the space within the eye that permits light to project onto the retina. Anatomically located in front of the lens, the pupil’s size is controlled by the surrounding iris. The pupil provides insight into the function of the central and autonomic nervous systems. Pupil: Physiology and Abnormalities in response to the intensity of light and the distance of the object.
  • The photoreceptor cells of the retina generate electrical and chemical impulses, which are transmitted to the brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification via the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions (CN II), resulting in an interpretation of the image.

Refractive media of the eye

Photoreceptor cells of the retina

  • Rods: 
    • Function best in dim light
    • Provide black-and-white vision Vision Ophthalmic Exam
    • 120 million rods in each retina
  • Cones: 
    • Function best in bright light
    • Provide color vision Vision Ophthalmic Exam, visual acuity Visual Acuity Clarity or sharpness of ocular vision or the ability of the eye to see fine details. Visual acuity depends on the functions of retina, neuronal transmission, and the interpretative ability of the brain. Normal visual acuity is expressed as 20/20 indicating that one can see at 20 feet what should normally be seen at that distance. Visual acuity can also be influenced by brightness, color, and contrast. Ophthalmic Exam
    • 6 million cones in each retina

Clinical Relevance

Disorders of visual perception Perception The process by which the nature and meaning of sensory stimuli are recognized and interpreted. Psychiatric Assessment

  • Refractive errors Refractive errors By refraction, the light that enters the eye is focused onto a particular point of the retina. The main refractive components of the eye are the cornea and the lens. When the corneal curvature, the refractive power of the lens, does not match the size of the eye, ametropia or a refractive error occurs. Refractive Errors: conditions in which the shape of the eye does not refract light correctly, resulting in blurry vision Vision Ophthalmic Exam. The main types of refractive errors Refractive errors By refraction, the light that enters the eye is focused onto a particular point of the retina. The main refractive components of the eye are the cornea and the lens. When the corneal curvature, the refractive power of the lens, does not match the size of the eye, ametropia or a refractive error occurs. Refractive Errors are myopia Myopia Refractive Errors ( nearsightedness Nearsightedness Refractive Errors), hyperopia Hyperopia Refractive Errors ( farsightedness Farsightedness Refractive Errors), presbyopia Presbyopia The normal decreasing elasticity of the crystalline lens that leads to loss of accommodation. Refractive Errors (loss of near vision Vision Ophthalmic Exam with age), and astigmatism Astigmatism Unequal curvature of the refractive surfaces of the eye. Thus a point source of light cannot be brought to a point focus on the retina but is spread over a more or less diffuse area. This results from the radius of curvature in one plane being longer or shorter than the radius at right angles to it. . Refractive Errors (mismatched curves of the cornea and lens).
  • Disorders of the visual pathway Visual pathway The primary visual pathway consists of a relay system, beginning at the retina, whose ganglion cell axons form the optic nerve. The optic nerve fibers from each eye hemidecussate in the optic chiasm (OC), with nasal fibers joining the temporal fibers of the contralateral nerve. Signals are then transmitted to the primary visual cortex of the occipital lobe. The Visual Pathway and Related Disorders: The structures responsible for disorders of the visual pathway Visual pathway The primary visual pathway consists of a relay system, beginning at the retina, whose ganglion cell axons form the optic nerve. The optic nerve fibers from each eye hemidecussate in the optic chiasm (OC), with nasal fibers joining the temporal fibers of the contralateral nerve. Signals are then transmitted to the primary visual cortex of the occipital lobe. The Visual Pathway and Related Disorders include the retina, optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions, optic chiasm Optic Chiasm The x-shaped structure formed by the meeting of the two optic nerves. At the optic chiasm the fibers from the medial part of each retina cross to project to the other side of the brain while the lateral retinal fibers continue on the same side. As a result each half of the brain receives information about the contralateral visual field from both eyes. The Visual Pathway and Related Disorders, optic tract Optic Tract Nerve fiber originating from the optic chiasm that connects predominantly to the lateral geniculate bodies. It is the continuation of the visual pathway that conveys the visual information originally from the retina to the optic chiasm via the optic nerves. The Visual Pathway and Related Disorders, optic radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma, and visual cortex Visual cortex Area of the occipital lobe concerned with the processing of visual information relayed via visual pathways. Cerebral Cortex: Anatomy of the occipital lobe Occipital lobe Posterior portion of the cerebral hemispheres responsible for processing visual sensory information. It is located posterior to the parieto-occipital sulcus and extends to the preoccipital notch. Cerebral Cortex: Anatomy. The site of damage is often localizable by ophthalmoscopy, pupil Pupil The pupil is the space within the eye that permits light to project onto the retina. Anatomically located in front of the lens, the pupil’s size is controlled by the surrounding iris. The pupil provides insight into the function of the central and autonomic nervous systems. Pupil: Physiology and Abnormalities reactions, and the pattern of visual-field defects.

Systemic diseases that affect Affect The feeling-tone accompaniment of an idea or mental representation. It is the most direct psychic derivative of instinct and the psychic representative of the various bodily changes by means of which instincts manifest themselves. Psychiatric Assessment the eye

  • Diabetic retinopathy Retinopathy Degenerative changes to the retina due to hypertension. Alport Syndrome: vascular disease of the retina affecting individuals with long-standing diabetes mellitus Diabetes mellitus Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus. Loss of vision Vision Ophthalmic Exam is typically slow and progressive, and it occurs in most individuals 2–3 decades after the onset of diabetes Diabetes Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus
  • Hypertensive retinopathy Hypertensive retinopathy Hypertension has many adverse effects on the eye, of which retinopathy is the most common presentation. Hypertensive retinopathy consists of retinal vascular changes that develop as a direct effect of elevated blood pressure. In acute increases of blood pressure, autoregulation results in retinal arteriolar narrowing. Hypertensive Retinopathy: retinal changes in individuals with long-standing hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension. Hypertensive retinopathy Hypertensive retinopathy Hypertension has many adverse effects on the eye, of which retinopathy is the most common presentation. Hypertensive retinopathy consists of retinal vascular changes that develop as a direct effect of elevated blood pressure. In acute increases of blood pressure, autoregulation results in retinal arteriolar narrowing. Hypertensive Retinopathy usually includes arterial narrowing and hemorrhage.

Miscellaneous disorders of the eye

  • Conjunctivitis Conjunctivitis Conjunctivitis is a common inflammation of the bulbar and/or palpebral conjunctiva. It can be classified into infectious (mostly viral) and noninfectious conjunctivitis, which includes allergic causes. Patients commonly present with red eyes, increased tearing, burning, foreign body sensation, and photophobia. Conjunctivitis: inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation of the conjunctiva due to an infectious Infectious Febrile Infant (including bacterial, viral, or fungal agents), allergic, or autoimmune reaction. Individuals present with conjunctival hyperemia, eye discharge that may be watery or purulent, discomfort, or a foreign-body sensation. 
  • Retinal detachment Retinal detachment Retinal detachment is the separation of the neurosensory retina from the retinal pigmented epithelium and choroid. Rhegmatogenous retinal detachment, the most common type, stems from a break in the retina, allowing fluid to accumulate in the subretinal space. Retinal Detachment: medical emergency in which a layer of retinal tissue peels off from the underlying supporting choroid layer. Retinal detachment Retinal detachment Retinal detachment is the separation of the neurosensory retina from the retinal pigmented epithelium and choroid. Rhegmatogenous retinal detachment, the most common type, stems from a break in the retina, allowing fluid to accumulate in the subretinal space. Retinal Detachment may be due to trauma or a high degree of myopia Myopia Refractive Errors, and individuals often have a family history Family History Adult Health Maintenance of this disorder. Retinal detachment Retinal detachment Retinal detachment is the separation of the neurosensory retina from the retinal pigmented epithelium and choroid. Rhegmatogenous retinal detachment, the most common type, stems from a break in the retina, allowing fluid to accumulate in the subretinal space. Retinal Detachment should be treated within 24–48 hours or it can lead to permanent loss of vision Vision Ophthalmic Exam.
  • Cataracts: decrease in vision Vision Ophthalmic Exam due to a cloudiness or loss of transparency of the lens, common in advancing age. Treatment of cataracts requires surgical extraction of the opaque lens and, in most cases, implantation Implantation Endometrial implantation of embryo, mammalian at the blastocyst stage. Fertilization and First Week of a new lens.
  • Glaucoma Glaucoma Glaucoma is an optic neuropathy characterized by typical visual field defects and optic nerve atrophy seen as optic disc cupping on examination. The acute form of glaucoma is a medical emergency. Glaucoma is often, but not always, caused by increased intraocular pressure (IOP). Glaucoma: condition of increased pressure within the eye as a result of inadequate drainage of aqueous humor Humor Defense Mechanisms, usually due to a blockage. Unless medically treated or surgically corrected, glaucoma Glaucoma Glaucoma is an optic neuropathy characterized by typical visual field defects and optic nerve atrophy seen as optic disc cupping on examination. The acute form of glaucoma is a medical emergency. Glaucoma is often, but not always, caused by increased intraocular pressure (IOP). Glaucoma can lead to gradual vision Vision Ophthalmic Exam loss owing to damage to the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions
  • Horner syndrome Horner syndrome Horner syndrome is a condition resulting from an interruption of the sympathetic innervation of the eyes. The syndrome is usually idiopathic but can be directly caused by head and neck trauma, cerebrovascular disease, or a tumor of the CNS. Horner Syndrome: This syndrome is caused by disruption of the sympathetic nerve supply to the eye and is characterized by ipsilateral miosis Miosis Pupil: Physiology and Abnormalities, partial ptosis Ptosis Cranial Nerve Palsies, enophthalmos Enophthalmos Recession of the eyeball into the orbit. Marfan Syndrome, and hemifacial anhidrosis.
  • Retinoblastoma Retinoblastoma Retinoblastoma is a rare tumor but the most common primary intraocular malignancy of childhood. It is believed that the condition arises from a neuronal progenitor cell. Retinoblastoma can be heritable or non-heritable. Retinoblastoma: intraocular malignancy Malignancy Hemothorax that begins in the retina. Retinoblastoma Retinoblastoma Retinoblastoma is a rare tumor but the most common primary intraocular malignancy of childhood. It is believed that the condition arises from a neuronal progenitor cell. Retinoblastoma can be heritable or non-heritable. Retinoblastoma typically manifests during childhood with an abnormal pupillary reflex to light, known as leukocoria Leukocoria Cataracts in Children (white pupil Pupil The pupil is the space within the eye that permits light to project onto the retina. Anatomically located in front of the lens, the pupil’s size is controlled by the surrounding iris. The pupil provides insight into the function of the central and autonomic nervous systems. Pupil: Physiology and Abnormalities).

References

  1. Bales, T.R., Lopez, M.J., Clark, J. (2021). Embryology, eye. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK538480/
  2. Silverman, M.A., Brenner, B.E. (2021). What is the conjunctiva and what is its function? Medscape. https://www.medscape.com/answers/797874-22279/what-is-the-conjunctiva-and-what-is-its-function
  3. Nguyen, J.D., Duong, H. (2021). Anatomy, head and neck, eye nerves. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK549919/
  4. Bird, B., Stawicki, S.P. (2021). Anatomy, head and neck, ophthalmic arteries. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK482317/
  5. Dahl, A.A., Gest, T.R. (2017). Retina anatomy. Medscape. https://emedicine.medscape.com/article/2019624-overview
  6. Gupta, N., Motlagh, M. Singh, G. (2021). Anatomy, head and neck, eye arteries. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK537063/
  7. Bardorf, C.M., Ing, E.B. (2019). Horner syndrome. Medscape. https://emedicine.medscape.com/article/1220091-overview
  8. Mahabadi, N., Khalili, Y.A. (2021). Neuroanatomy, retina. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK545310/
  9. Smith, A.M., Czyz, C.N. (2020). Neuroanatomy, cranial nerve 2 (optic). StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK507907/

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