Eye Anatomy and Visual Pathways

• Study notes on sensory stimuli, eye anatomy, development, and visual pathways.

Sensory Stimulus Definition

• Focuses especially on light, wavelength, visible spectrum, intensity, luminance, and illuminance.

Eye Importance in Survival

• Half of the sensitive receptors are in the eye.
• The cerebral cortex participates in visual information processing.

Electromagnetic Radiation

• Energy emitted as waves from the sun, including:
  • Gamma rays (shortest wavelength)
  • X-rays
  • Ultraviolet rays
  • Visible light
  • Infrared radiation
  • Microwaves (longest wavelength)

Electromagnetic Spectrum

• Represents the range of electromagnetic radiation

Wavelength

• Distance between two consecutive peaks.
• Can be either short or long.

Luminous Intensity

• Amount of light emitted by a source per unit solid angle.
• Measured in candelas.
• Depends on source power and emission direction.

Luminance

• Light emitted by a surface per unit area.
• Measured in candelas per square meter.
• It depends on source intensity and surface reflectivity.

Illuminance

• Amount of light incident on a surface per unit area.
• Measured in lux, equivalent to one candela per square meter.
• Depends on source intensity and distance from the source to the surface.

Visible Light

• Detected by the eye.
• Includes wavelengths between 400 and 700 nm.
• Colors vary depending on the wavelength; 400 nm corresponds to violet and 700 nm to red.

Photopic Vision

• Vision under high luminosity conditions.
• Cone cells in the retina detect colors and fine details.

Scotopic Vision

• Vision under low luminosity conditions.
• Rod cells in the retina detect movements and changes in luminosity.

Embryology of the Sensory System

• Focuses on when the sensory structure starts to be created during gestation.

Optic Cup and Lens Vesicle

• The eye appears around day 22 as shallow grooves on the sides of the developing brain.
• These vesicles contact the surface ectoderm, prompting the formation of the lens.
• The optic vesicles invaginate, forming the double-layered optic cup.

Optic Cup Layers

• Initially separated by a lumen, the intrarretinal space, but the lumen disappears, and the layers fuse.
• The invagination affects the lower part of the cup, forming the choroidal fissure, which allows the hyaloid artery to enter the posterior chamber of the eye.

Pupillary Development

• During the 7th week, the edges of the fissure fuse, creating a round opening that becomes the pupil.
• Cells of the surface ectoderm elongate and form the lens placode, which invaginates and becomes the lens vesicle.

Lens Development Timeframe

• Fifth week.
• Loses contact with the surface ectoderm, positioning itself inside the optic cup.

Retina, Iris, and Ciliary Body

• The outer layer of the optic cup becomes the pigmented layer of the retina, containing pigment granules.
• The inner layer differentiates into the optical portion of the retina, containing photoreceptor cells (rods and cones).

Fibrous Layer

• Located on the surface of the mantle layer.
• Contains axons from nerve cells that converge towards the optic stalk, allowing light impulses to pass through the retina.
• Anterior part of the inner layer forms the blind part of the retina, later dividing into the iridial and ciliary portions.

Iridial Portion

• Creates the inner layer of the iris.

Ciliary Portion

• Participates in forming the ciliary body.

Muscles of the Pupil

• Located between the optic cup and surface epithelium.
• Formed in a region of loose mesenchyme.
• Include the sphincter and dilator muscles.

Iris Structure

• Outer pigmented layer.
• Inner pigmented layer from the optic cup.
• Connective tissue layer with blood vessels containing pupillary muscles.

Ciliary Portion of the Retina

• Recognizable by its marked folding.
• Covered by mesenchyme, forming the ciliary muscle.
• Connected to the lens via elastic fibers called suspensory ligaments or zonules.

Ciliary Muscle Contraction

• Alters tension on the ligaments.
• Controls lens curvature.

Lens Development

• Vesicle cells elongate forward, filling the vesicle’s cavity.
• Fibers continuously added in the central lens region, adapting the eye to changing vision needs.

Choroid, Sclera, and Cornea Development

• Eye primordium surrounded by loose mesenchyme at the end of the fifth week.
• Tissue differentiates into an inner layer (choroid) and outer layer.

Inner Layer Development

• Forms the choroid, which is a pigmented, highly vascular layer.

Outer Layer Development

• Transforms into the sclera.
• Continuous with the dura mater around the optic nerve.

Anterior Chamber Development

• Forms via vacuolization in the mesenchyme, dividing it into two layers.

Layers of Mesenchyme Division

• Iridopupillary membrane: front of the lens and iris.
• Proper substance of the cornea: continuous with the sclera.

Layers of the Cornea

• Epithelial layer (from surface ectoderm).
• Proper substance or stroma.
• Endothelial layer, limiting the anterior chamber.

Posterior Chamber

• Space between the iris, the lens, and the ciliary body.

Anterior and Posterior Chambers

• Communicate through the pupil.
• Filled with aqueous humor, produced by the ciliary body.

Vitreous Humor

• Mesenchyme invades the optic cup's interior.
• Forms the hyaloid vessels that nourish the lens and the vascular layer on the inner retinal surface.
• It also creates a network of fibers between the lens and retina, filled with a clear, gelatinous substance.

Hyaloid Vessels

• Vessels and hyaloid disappear during fetal life.
• Hyaloid channel persists.

Optic Nerve

• The optic cup connects to the brain via the optic stalk.
• Contains a groove called the choroidal fissure, housing the hyaloid vessels.
• The fissure closes during the seventh week, forming a tunnel.

Optic Stalk Transformation

• It transforms into the optic nerve containing the central artery of the retina.
• Exteriorly enveloped by pia mater, arachnoid, and dura mater layers.

Molecular Regulation: PAX6 and Eye Development

• PAX6 plays a key role in the eye's development.
• Specifically in the formation of the optic cup and lens.

PAX6 Initial Expression

• Expressed in the anterior neural ridge of the neural plate before neurulation.
• Later, Sonic hedgehog (SHH) separates the optic field into two optic primordia.
• Positively regulates PAX2 in the optic field center and negatively regulates PAX6.

PAX6 Role in Lens Formation

• It is crucial in lens formation.
• Regulates genes like SOX2, LMAF, and PROX1.
• PAX6 expression in the surface ectoderm triggers SOX2 regulation and maintains PAX6 in the future lens ectoderm.

Vesicle Secretion

• Secretes BMP4.
• It undergoes positive regulation and maintains SOX2 and LMAF.

Combined Expression

• PAX6, SOX2, and LMAF triggers genes for lens formation, including PROX1.
• SIX3 inhibits crystallin gene.
• PAX6, with FOX3 mediation, regulates cell proliferation in the lens.

Anatomical Structures of Sensory Organs

• Focus is on the components and function of the human eye.
• Accessory structures include eyelids, eyelashes, eyebrows, lacrimal apparatus, and extrinsic muscles.

Eyelids

• Upper and lower.
• Close the eye during sleep.
• Protect against excessive light and foreign objects.
• Spread lubricating secretion over the eyeballs.

Upper Eyelid

• More mobile.
• Superior levator muscle is present.

Palpebral Fissure

• Space of the exposed ocular globe between the eyelids.

Angles of the Eye

• Lateral commissure: narrow and close to the temporal bone.
• Medial commissure: broad and close to the nasal bone.

Medial Commissure

• Contains the lacrimal caruncle, a small, reddish elevation containing:
  • Sebaceous glands
  • Sudoriferous glands
• These glands produce a whitish material.

Layers of the Eyelid

• From superficial to deep: epidermis, dermis, subcutaneous tissue, orbicular muscle fibers, tarsus, and conjunctival glands.

Tarsus

• Thick tissue providing shape and support.
• Contains tarsal glands.

Tarsal Glands

• Elongated sebaceous glands secreting fluid to prevent eyelids from sticking together.
• Infection of these results in chalazion.

Conjunctival Glands

• Thin mucous membrane lining the inner aspects of the eyelids and anterior eyeball.
• Composed of stratified columnar epithelium.

Sections of Conjunctival Glands

• Palpebral: inner aspect of the eyelids.
• Ocular: covers the sclera of the eyeball, excluding the cornea.

Eyelashes and Eyebrows

• Eyebrows: Arched transversely above the eyelids.
• Eyelashes: Borders of each eyelid.
• Protect the eyeball from foreign bodies, perspiration, and direct sunlight.

Ciliary Sebaceous Glands

• Sebaceous glands at the base of eyelash follicles.
• Release lubricating fluid into the follicles.
• Infection of these glands results in a stye.

Lacrimal Apparatus

• Structures producing lacrimal fluid.
• The lacrimal apparatus includes structures that produce and drain tears

Lacrimal Glands

• Similar in size and shape to an almond.
• Drain lacrimal fluid through 6 to 12 excretory ducts.
• Tears flow over the anterior surface of the eyeball and enter the lacrimal puncta (small openings); they then pass into the lacrimal canaliculi, to the lacrimal sac, to the nasolacrimal duct, and finally into the nasal cavity.

Bacterial Infection

• Bacterial infection of the lacrimal groove is known as dacryocystitis, leading to obstruction of the nasolacrimal ducts.

Lacrimal Glands Production

• Produces fluid containing salts, mucus, and lysozymes.
  • The fluid is intervened by parasympathetic fibers of the facial nerve 7 .
• Aims to clean, lubricate, and moisten the eyeball
• Each gland produces around 1 ML per day.
• Blink diminishes liquid.

Extrinsic Muscles

• Extrinsic muscles are located outside walls of the bony orbit to the sclera.
• These muscles produce eye movements that are smooth, precise, rapid, coordinated and coordinated and synchronized by nerve circuits of the brainstem and cerebrum

Orbit

• eyes located in two bone depressions in the skull point. Stabilize and secure in muscles.

Eye Muscles

• Rectus superior
• Recta inferior
• Rectus lateral
• Rectus medial
• Oblique superior
• Oblique inferior
• In adults, the eye measures 2.5 cm in diameter.

Eye Surface Exposure

• One-sixth.
• The remainder is protected by the orbit.
• Embedded within it - The eyeball contains three layers.

Fibrous Layer

• Superficial covering of the eyeball, including the cornea and sclera.

Cornea

• Transparent membrane covering the colored iris.
• Its curvature assists in focusing light onto the retina.
  • Anterior Surface formed by stratified non-keratinized pavimento epithelium.
  • Medial Surface formed by collagen fibers and fibroblasts
  • Posterior surface formed by flat pavimento epithelium,

Sclera

• Dense connective tissue composed of collagen fibers and fibroblasts.
• The rigid shape protects internal parts

Sclera and Cornea Interface

• Contains a venous sinus (Schlemm's canal).
• Fluid ("aqueous humor") drains from the body.

Vascular Layer (Uvea)

• Mid layer of the eyeball, consisting of the choroid, ciliary body, and iris.

Choroid

• Posterior vascular layer covering most of the sclera, containing melanocytes that produces melanin.
• The pigment helps absorb light, which reduces the inner reflection of the bulb

Anterior Part of the Vascular Layer

• The auxiliary body is joined.
• Extends serratal the light is from the anterior part of the auxiliary body.

Ciliary Body

• Internal processes produce folds to align internal parts or to contain capillaries in an aqueous humor fluid. From here extend sonular fibers, which adhere to crystalline.
• Ciliary muscle is composed of bands thet produce light or modified tension on the lens for distance vision

Iris

• The iris is the colored portion of the eyeball.
• it features a circular plane between the cornial area and connects and connects between externial processes which attach at the top borders of the eye

Melanocytes

• Produces melatonin.
• Defines the eye’s color.

Iris Fibers

• Radial.
• Circular.
• These are derived from smooth muscle.

Iris Regulation

• Regulates quantity.
• Passes through the pupil controlling center
• The reflexes regulate pupil by light levels

Pupil Reflections/Light level

• Brightness stimulates parasympathetic fibers to constrict to diminish diameter of pupil
• Dim-stimulates, but dim lights increases pupil diameter

Retina

• Innermost, vision component and optic receiver
• Three-fourths parts
• Starts the optic components
• Optic Disk where optic nerve transfers from eye
• Attatched artery distribute over the retinas entire anterior;
• The back: consists of:
• Layered: the layer contrains melalanin that absorbs rays
• Nervous: proccesses information and recieves impulses from optic nerves

Crystalline structure

• Located in the pupila
• Transparent proteins
• Surrounded by capusule
• Positioned by fibular
• Helps to faciliate view
• Divides occular section into a cavity behind and front filled with liquid

Vitreous body:

• Is gelatinous that maintains coraiods and a smooth surface for image precision
• once generated can not be replaced
• facillitates clarity
• Ganglion cellos information is transmitted: horizontals,bipolars, amacrines and photoreceptors.

Retina Characteristics

• Recovers the eyes surface
• connects the brain.
• it makes a nervous tissue and connevts

Retinal Component Layers

• neuronal sections
• synaptic interface
• interplexiform layers

Inner Nuclear Layers

• Inner cell bodies with cones, rods.
• Horizontal cells, horizontal bipolars
• Their dendrites extend and send synopsis via their extons

Additional Layers

• External layers.
• Ganglios are cell located.
• Plexiform: performs more synopsis, neuron
• They communicate info to make a network
• Phototropos transform luminious energy or chemicaly. It posseesses cones and basotones

Scotopic And Mesotopic

• Work in limited light
• detect movement

Photopic

• does nor works
• sensative to med light to color

Transduction

• send light through neuron by cells