Human Anatomy: Sense Organs and Vision

Sense Organs

• Sense organs are the peripheral part of a receptor
• They receive information about the environment
• Vision, hearing, smell, taste, and touch are all sense organs

Organ of Vision Components

• Includes the eye, optic nerve, and accessory structures such as eyelids and lacrimal glands

The Eye

• Composed of the wall (several layers), chambers with liquid (aqueous humor), a lens, and the vitreous body

Layers of the Eye

• Outer fibrous layer: made of sclera (non-transparent posterior coating) and cornea (transparent anterior part)
• Middle vascular coat: consists of the iris (anterior part), choroid and ciliary body (posterior part)
• Inner sensory layer (retina): has a posterior photosensitive part and an anterior non-photosensitive part that covers the iris and ciliary body

Functions of the Peripheral Vision Organ

• Refraction: cornea, aqueous fluid, lens, and vitreous body
• Accommodation: iris and ciliary body
• Reception: retina

Fibrous Coating of the Eye: Sclera

• The sclera forms the spherical shape of the eye and consists of three layers
• External layer: episclera, a loose connective tissue adjacent to periorbital fat
• Substantia propria: sclera proper, also known as Tenon's capsule, consisting of a dense irregular connective tissue
• Inner layer: the layer adjacent to the choroid, containing collagen, elastic fibers, connective tissue cells, with melanocytes

Cornea Layers

• Corneal epithelium: stratified squamous nonkeratinized
• Anterior Limiting Membrane (Bowman's membrane): made of collagen fibers, acting as a barrier to infections, and does not regenerate
• Substantia Propria (corneal stroma): consists of around 60 thin lamellae of collagen fibers
• Posterior Limiting Membrane (Descement's): basal lamina of corneal endothelial cells
• Posterior Epithelium (corneal endothelium): simple squamous epithelium responsible for the exchange between the cornea and the anterior chamber

Vascular Middle Layer

• Composed of the choroid, ciliary body, and iris

Choroid

• Located between the sclera and retina, consisting of a highly vascularized coat with loose connective tissue rich in melanocytes, fibroblasts, macrophages, collagen, and elastic fibers
• Choroid function is nutrition for the retina
• The choroid firmly attaches to the sclera at the margin of the optic nerve
• Outer layer: composed of the suprachoroidal lamina and the choriocapillary layer (inner region rich in blood vessels)
• Bruch's membrane: thin, amorphous hyaline membrane; a multilaminar structure made of a basal lamina of endothelial cells of the choriocapillary layer, collagen fibers, elastic fibers, a second layer of collagen fibers, and the basal lamina of the retinal epithelial cells

Ciliary Body

• Thickened, anterior part of the vascular coat located between the iris and choroid
• The ciliary body has two layers, the inner vascular layer and the outer muscular layer
• The muscle layer is composed of smooth muscle fibers running in different directions, outer longitudinal, middle radial, and inner (circular)
• Muscles: loose connective tissue and are with blood vessels, elastic fibers, melanocytes
• Vascular Layer: includes the crown and 70-80 ciliary processes; made of loose connective tissue and an extensive network of capillaries
• Ciliary Processes: have two layers of epithelial cells
• Pigmental (outer layer): an extension of the retinal pigmental layer
• Non-pigmental (inner layer): cells produce a liquid that fills the chambers of the eye
• The ciliary body thickened edge of the choroid, controlling the shape of the lens

Suspensory Ligaments

• Hold the lens in place
• The lens focuses light rays on the retina

Iris

• Anterior portion of the vascular coat between the cornea and the lens
• Pupil: central aperture of the eye to let light in

Iris Layers

• Anterior layer: discontinuous with grooves and ridges, and contains melanocytes and fibroblasts
• Second layer: poorly vascularized connective tissue, containing fibroblasts and melanocytes
• Stroma of the iris: contains many blood vessels embedded in loose connective tissue, melanocytes, and two muscles of neuroectodermal origin
• Posterior pigmented epithelium: continuous with the epithelium covering the ciliary body

Iris Muscles

• Dilator pupillae: radially oriented myoepithelial cells
• Sphincter pupillae muscle: circumferentially oriented muscle cells near the pupillary margin

Corneoscleral Limbus

• Transition zone between the cornea and sclera
• Includes the iridocorneal angle, containing the canal of Schlemm
• Canal of Schlemm drains fluid from the anterior chamber and connects to the venous system

Aqueous Humor

• Secreted by the ciliary epithelium, filling the posterior and anterior chambers of the eye

Canal of Schlemm

• Located at the sclerocorneal junction of the eye
• Drains aqueous humor from the anterior chamber of the eye into veins draining the eyeball (episcleral veins)
• A structure similar to lymphatic vessels, basal lamina discontinuous

Glaucoma

• Condition in which connective tissue of the sclera and cornea proliferates with age, becomes more fibrous, and may obstruct the canal of Schlemm
• Causes an increase in intraocular pressure (normal levels 10-20 mm Hg)
• Untreated, damages ganglionic cells of the retina and the optical nerve, leading to blindness

Retina Components

• Anterior Part: covers the iris
• Middle Part: covers the ciliary body
• Posterior Part: photosensitive region

Photosensitive Retina

• Thin pigmental layer
• Neural Layer

Pigmental Layer

• Composed of low columnar pigmental cells with basal lamina oriented towards the choroid
• Internal surface houses 10-40 extensions surrounding cones and rods
• Adjacent cells have junctional complexes, forming the blood-retina barrier
• Functions to absorb light that passes through the neural retina, preventing reflection
• Isolates retinal cells
• Removes free radicals
• Phagocytose shed components from the adjacent rods and cones

Neural Layer

• Photosensitive part of the retina, containing a chain of three neurons
• External (photosensitive)
• Middle (interconnective)
• Inner (ganglionic)

Neurons in the Retina: Rods and Cones

• Nuclei in the external nuclear layer
• Bipolar Neurons are nuclei in the internal nuclear layer
  • Diffuse: synapses with two or more photoreceptors
  • Monosynaptic – contact with the axon of only one cone and only 1 ganglion cell
• Horizontal cells: contact between different photoreceptors and bipolar cells
• Amacrine cells: branch intensively to connect bipolar cells with several ganglion cells
• Supporting (Müller) cells: adhesion with photoreceptors is within the external limiting membrane

Neurons in the Retina: Ganglion Cells

• Synapse with bipolar cells and amacrine cells
• Axons of the ganglion cells originate the optic nerve fiber layer as they pass toward the optic disc (head of the optic nerve)
• Cell bodies are located in the ganglion cell layer

Layers of the Retina

• Pigmented epithelium
• Layer of cones and rods (photoreceptors)
• Outer (external) limiting membrane
• Outer (external) nuclear layer
• Outer (external) plexiform layer
• Inner (internal) nuclear layer
• Inner (internal) plexiform layer
• Ganglionic layer
• Nerve fiber layer
• Inner limiting membrane

Photoreceptors and Pigmented Epithelium

• Columnar cells with apical microvilli, where the cytoplasm contains melanin granules to absorb light and reduce reflection
• Rods: sensitive to low light intensity, located throughout the retina, outnumber cones
• Cones: less numerous than rods, sensitive to high-intensity light and respond to color, found in the fovea centralis

Neural Retina Layers

• External plexiform layer: contains synaptic contacts between photoreceptor axons and bipolar cell dendrites, and between photoreceptors and horizontal cells
• Internal nuclear layer: contains cell bodies of bipolar, horizontal, amacrine, and Müller cells
• Internal plexiform layer: synapses between bipolar cell axons and ganglion cell dendrites, as well as between amacrine and ganglion cells
• Ganglion cell layer: contains cell bodies of the ganglion cells
• Nerve fiber layer: contains collections of unmyelinated ganglion cell axons, which pass toward the optic disc to form the optic nerve
• Internal limiting membrane: the basal portions of the Müller cells

Photosensitive Layer Types of Neurons

• Rods
• Cones

Photosensitive Cells

• Outer segment - dendrite (cylindrical or conic)
• Inner segment (cell body)
• Axon with synaptic terminal
• Outer and inner segments are separated by a constriction

Rod Cell Structures and Functions

• Shorter outer segment, containing discs surrounded by the plasma membrane, along with visual pigment rhodopsin
• Detached disc borders and new discs formed by infolding of the plasma membrane
• Under light: rhodopsin conformation occurs producing membrane that changes the potential
• Membranous discs are shed constantly from the rod's end and phagocytosed by pigmental cells

Cone versus Rod

• Cones differ from rods in their shape, but structure is similar
• Discs are connected with the plasma membrane with different pigments
• Each cone responds maximally to one of these colors: red, green, or blue
• Pigment of the cones: iodopsin
• Discs of the cones are not shed under the influence of light

Photoceptor Concentration

• Rods concentrate at the retina's periphery
• Cones concentrate in the retina's optical centre

Interconnective Retinal Cells

• Horizontal cells: synapse with rods, cones, and bipolar cells
• Bipolar cells: dendrites synapse with photosensory cell axons, and axons with the dendrites of ganglionic cells
• Amacrine cells: branch intensively and connect bipolar cells with several ganglionic cells

Ganglionic Retina Layer

• Composed of large multipolar neurons, dendrites form synapses with axons of bipolar neurons, and axons form the optic nerve

Supporting Cells

• Müller cells (apical boundary of these cells make outer limiting membrane, basal lamina of these cells - inner limiting membrane)
• Microglia
• Astrocytes

Specialized Retina Areas

• Optic disc: axons of nerve fiber layer converge. Devoid of photoreceptors, so it creates a blind spot in the visual field
• Macula lutea: a yellow area located 2.5mm temporally from the optic disc consisting mainly of cones (fovea) which synapse with single bipolar and ganglionic cells, producing high resolution
• Peripheral retina: many photoreceptive cells are mapped to one ganglion cell thus producing low visual resolution

Retina Blood Supply

• RPE, Rods, Cones, Outer Limiting Membrane, Outer Nuclear Layer supplied by choriocapillaries
• Outer plexiform layer supplied by CRA + choriocapillaries
• Inner nuclear layer, Inner plexiform layer, Ganglion cell layer, Nerve fibre Layer, Internal Limiting Membrane supplied by Central Retinal Artery
• Macula Region supplied by the superior and inferior temporal branches of central retinal artery, in 20% cases the Cilioretinal Artery supplies macula, retaining visions during CRA Occlusion.

Lens Structure

• Diameter around 10mm and a width of 3.5–5mm
• Lens capsule: thickened basal lamina produced by subcapsular epithelium
• Subcapsular epithelium: presents only on the anterior surface of the lens, appearing simple cuboidal
• Lens fibers: derive from subcapsular epithelium primarily in the equatorial region, differentiating, they are highly becoming filled with crystallin proteins
• The lens capsule is composed of basal lamina of the lens epithelium
• The lens fibers are elongated epithelial cells (contain proteins, crystallins, in their cytoplasm)
• Contains no blood vessels or nerves

Lens Fibers

• Elongated columnar epithelial cells that contain cytoplasmic proteins
• Cytoplasmic proteins include (crystallins), filensin intermediate filaments, and a degenerated nucelus

Cataracts

• Opacities, causes diabetes, toxin or age.
• Clefts appear between lens fibres
• Pressure that swells and may obstruct and develop glaucoma happens as pressure grows and the fibres become dense and turn brown/black. The lens then gets surically removed as a result.

Vitreous Body

• Gel located between the lens and retina composed of more than ninety percent of water
• Consists of collagen, and hyaluronic acid
• Few cells create hyaluronic acid and collagen.

Eyelids

• Eyelids contain skin (anterior) and conjunctiva (posterior)
• Anterior surface is covered with the skin. Posterior surface (conjunctiva) is covered with stratified columnar epithelium, submucosa, and goblet cells which secrete mucous

Eyelid Glands

• Meibomian glands: sabeceous glands that are not connected with hair follicles, they are made in the tarsal plate of dense connective tissue
• Zeiss glands: smaller than meibomian and connected with hair follicles
• Moll glands: apocrine sweat glands
• Tarsal plates (plates of dense connective tissue) and Meibomian glands

Lacrimal Glands

• Located in the upper lateral margin of the orbit and the conjunctiva
• Have 10 small ducts
• Secrete serous fluid containing lysozyme

Ocular Stucture Embryology

• Optic vesicles approach the ectoderm, thickening it and leading to the formation of the lens placode
• Optic vesicles develop into optic cup double walled
• Retina develops from the inner layer of the cups, and pigmented epithelium from the outer

Detachment of the Retina

• Causes blindness separating neural from pigmented retinal
• If a separation occurs from oxygen and oxygen in the body'
• Caused by diabetic reasons that lead to intraoccular infection