Sensory Organs (Part 3): Human Eye Anatomy

Sensory Organs (Part 3)

• The sclera, choroid, and retina are the three layers of the eye.
• The macula lutea and fovea centralis are densely packed receptor cells.
• The optic disc is the blind spot, as it lacks photoreceptors.
• The optic nerve carries visual information from the retina.
• The central artery and vein of the retina feed blood to the retinal tissue.

Anatomy & Physiology of the Human Eye

• The tunica fibrosa consists of the sclera and the cornea, which acts as a protective exterior layer.
• The tunica vasculosa (uvea) includes choroid, ciliary body, and iris.
• The choroid is richly vascular; it nourishes the retina.
• The ciliary body connects with the lens via suspensory ligaments.
• The lens focuses light onto the retina.
• The iris controls pupil size.
• The tunica interna includes the retina.
• The retina contains photoreceptors (rods and cones).
• Over 130 million rods and 6.5 million cones are present.
• The macula lutea and fovea centralis contain mostly cones.
• Fovea centralis are packed with cones.
• The optic disc (blind spot) is where the optic nerve exits the retina.
• The pupil's size is controlled by the iris.
• 130 million rods and 6.5 million cones are in the retina.
• Most astrocytes and other glial cells are in the retina.

Light and Vision

• Light is visible electromagnetic radiation, in wavelengths from 400 to 700 nm.
• Human vision: limited to wavelengths of light, from 400 to 700 nanometers.
• Ultraviolet and infrared radiation has too much or too little energy to cause photochemical reaction and are not visible to human eyes.
• Three types of cones respond to different wavelengths of light. These are short, medium, and long wavelengths.
• The wavelengths are for the short-wavelength cones peak sensitivity at 420 nanometers, for the medium-wavelength cones peak at 531 nanometers, and for the long-wavelength cones peak at 558 nanometers.

Accessory Structures of the Orbit

• Eye structures outside the eyeball are protective.
• Eyebrows help to prevent glare and perspiration, expression, and protect eyes.
• Eyelids (palpebrae) help protect the eye from foreign objects.
• Lacrimal caruncle and lacrimal glands keep the eye lubricated and provide moisture.
• Tarsal glands produce oil to prevent evaporation of tears.
• Eyelashes prevent debris from entering.
• Conjunctiva is a transparent mucous membrane that covers the anterior eyeball surface (except the cornea).
• Orbital fat supports and cushions the eye.

Accessory Structures of the Orbit (Parts 4)

• The lacrimal apparatus produces, distributes, and drains tears.
• Tears are produced by the lacrimal glands.
• Tears lubricate and nourish the surface of the eyes.
• Lysozyme helps prevent infections.
• Tears drain through the lacrimal puncta, lacrimal canaliculi, and nasolacrimal duct into the nasal cavity.

Accessory Structures of the Orbit

• Six extrinsic muscles control eyeball movement.
• These muscles include superior, inferior, lateral, and medial rectus muscles and superior and inferior oblique muscles.
• Cranial nerves IV, VI, and III innervate these muscles.

Innervation of the 6 Intrinsic (3 Pairs) Eye Muscles

• The six extrinsic eye muscles are responsible for eye movement, and they are innervated by cranial nerves.
• The superior oblique is controlled by cranial nerve IV.
• The lateral rectus is controlled by cranial nerve VI.
• The other four extrinsic muscles are controlled by cranial nerve III.

Anatomy of the Eye

• The eyeball has three main components: tunics (layers), optical components (refractive structures for light), and the neural component (retina and optic nerve).
• The tunics include the outer fibrous, middle vascular, and inner.
• The middle vascular tunic consists of the choroid, ciliary body, and iris.
• The inner tunic is the retina, containing photoreceptor cells and neural elements.
• The optical components are cornea, aqueous humor, lens, and vitreous body.
• Light passes through the optical compartments to focus on the retina.
• The retina and optic nerve transmit signals to the brain for visual processing.

The Tunics

• The tunica fibrosa is the outer layer of the eyeball.
• The sclera is part of the fibrous layer and is opaque.
• The cornea is part of the fibrous layer and is transparent.
• The tunica vasculosa (uvea) is the middle vascular layer.
• The choroid is highly vascular and pigmented layer.
• The ciliary body is a ring of smooth muscle around the lens.
• The iris contains pigment and controls pupil size.
• The tunica interna is the inner layer.
• The retina and optic nerve run along the inside of the globe, connecting to the brain.

The Optical Components

• Transparent structures that allow light to enter the eye.
• The cornea is the transparent anterior cover.
• The aqueous humor fills the anterior chamber.
• The lens focuses light on the retina.
• The vitreous humor fills the large posterior vitreous chamber.

The Optical Components

• Lens fibers are flattened, tightly compressed, transparent cells forming the lens.
• The ciliary body suspends the lens.
• The lens' shape changes to adjust focus.
• The vitreous body fills the vitreous chamber, providing support and maintaining eyeball shape.

The Neural Components

• The retina and optic nerve transmit visual information.
• The retina comes from the optic vesicle, a part of the brain.
• Attached to the eye at the optic disc (blind spot) and ora serrata.
• The retina is pressed against the eyeball to maintain correct shape.
• The macula lutea has multiple cells on the visual axis of the eye.
• Fovea centralis is a pit in the center of the macula lutea, containing packed receptor cells.
• Retinal blood vessels are visible in the opthalmoscope.

The Neural Components

• The optic disc is the blind spot on the retina where the optic nerve exits.
• It has no photoreceptors.
• The blind spot can be tested using special charts and dots.
• The brain fills in the information from the blind spot.

Cataracts and Glaucoma

• Cataracts involve the clouding of the lens fibers.
• The fluid-filled bubbles and clefts filled with debris appear between fibers.
• Causes of cataracts include age, diabetes, smoking, drugs, ultraviolet radiation, and certain viruses.
• Cataracts are treated by replacing the natural lens with a plastic one.
• Glaucoma is elevated pressure within the eye due to obstructed drainage of aqueous humor.

Cataracts and Glaucoma

• Glaucoma is caused by high intraocular pressure, leading to retinal cell death.
• Early symptoms of glaucoma can include illusory flashes of light, or halos around lights.
• Ultimately glaucoma causes vision loss, which cannot be recovered.
• Intraocular pressure is measured with a tonometer.

Formation of an Image

• Light passes through lenses, forming a tiny inverted image on the retina.
• The iris and its associated muscles control pupil diameter.
• Changes to the pupil's size due to changes to the light and/or objects the eyes are focusing on. This is known as the pupillary light reflex.
• Pupil's size is controlled by the parasympathetic and sympathetic nervous system.

Formation of an Image

• Pupillary constriction and dilation occur in response to changes in light intensity and gaze shifts.
• The photopupillary reflex involves the autonomic nervous system.
• Light intensity triggers signals to the pretectal region of the midbrain.
• Parasympathetic fibers in the oculomotor nerve then cause pupillary constriction.

Refraction

• Refraction is the bending of light rays as they pass from one medium to another.
• The speed of light is slower in materials other than a vacuum.
• Refractive index is a measure of how much light is slowed down.
• The cornea causes most of the bending and focusing of light.
• Light passing through the center of the cornea is not bent, while light striking off-center is bent toward the center.
• The aqueous humor and lens have minor effects on the path of light.
• The cornea has greater refractive effect than the lens.
• The lens becomes rounder when focusing on near objects to increase refraction.

The Near Response

• The near response involves adjustments to view closely situated objects.
• Emmetropia is where the eye is relaxed when focused on objects over 6 meters away.
• Light rays, coming from distant objects are essentially parallel.
• Light rays from closer objects are divergent.
• Three major adjustments to observe objects up close are; convergence of the eye, constriction of the pupil, and accommodation of the lens.
• Accommodation of the lens is when the shape of the lens changes to focus light on the retina.

The Near Response

• The near response involves convergence of the eyes, constriction of the pupil, and accommodation of the lens.
• The eyes orient their visual axes toward the object.
• The pupil constricts to reduce spherical aberration (blurry edges).
• The ciliary muscle contracts, slackening the suspensory ligaments, making the lens more rounded and thick.
• This increases light refraction to focus the image precisely on the retina.

Common Defects of Image Formation

• Image defects lead to vision problems, such as farsightedness (hyperopia) and nearsightedness (myopia).
• Hyperopia occurs when the eye's focusing power is too weak, requiring a convex (or converging) lens for correction.
• Myopia occurs when the eye's focusing power is too strong, needing a concave (or diverging) lens for correction.

Sensory Transduction in the Retina

• The retina converts light energy to action potentials.
• The pigment epithelium absorbs light to prevent degradation of images.
• Different cells in the retina from back to front include; photoreceptors, bipolar cells, and ganglion cells.
• Photoreceptor cells absorb light and generate chemical or electrical signals.
• Rods and cones are types of photoreceptor cells responsible for black and white and color vision respectively.
• Bipolar cells and ganglion cells transmit signals to the brain.

Sensory Transduction in the Retina (Light-absorbing cells)

• Rods are for night or scotopic vision.
• Rods are specialized for low-light conditions.
• Cones are specialized for day or photopic vision.
• Cones have separate sections for outer and inner segments and support other functions.

Sensory Transduction in the Retina (3)

• Cone cells (color vision) also have outer segments that taper to a point.
• Similar to the rod, outer segment discs contain plasma membrane.
• The structures of both rod and cone cells help in the light-sensitive mechanism.

Sensory Transduction in the Retina (Histology of the retina)

• Pigment epithelium, rods, cones, bipolar cells, and ganglion cells, and their interactions all form the retina.
• Histology looks at the tissues of the retina.
• The pigment epithelium is the posterior layer that absorbs stray light.
• Rods and cones are photoreceptor cells and the outermost layer of the retina.
• Bipolar cells transmit electrical signals to ganglion cells.
• Ganglion cells transmit signals to the brain.

Sensory Transduction in the Retina (Ganglion cells)

• Ganglion cells are large neurons that form the optic nerve, playing a role in transmitting information to the brain. Ganglion cells react to light using chemical and electrical signals.
• Ganglion cells absorb light using pigment melanopsin, and transmit signals to the brainstem.
• These signals regulate pupil intensity and circadian rhythms.
• These signals do not create visual images.

Sensory Transduction in the Retina

• Horizontal cells and amacrine cells are present but do not form separate layers.
• They connect cone, rod, and bipolar cells, facilitating contrast and edge perception.
• Most of the retina's mass consists of astrocytes and glial cells.

Visual Pigments

• Rods contain rhodopsin (visual purple), which absorbs light at 500 nm.
• Rhodopsin is composed of opsin (protein) and retinal (vitamin A derivative).
• Rods cannot distinguish colors.

Visual Pigments

• Cones contain photopsin (iodopsin), which absorbs different wavelengths, producing color vision.
• Retinal is the same in both rods and cones.
• Opsin molecules differ, generating color vision.
• The different kinds of cones have different absorption peaks at 420nm, 531nm, and 558nm.

Visual Pigments

• Visual pigments like rhodopsin in rods and photopsins in cones are light-sensitive.
• Rhodopsin is important in helping us see in dimmed light conditions.
• The pigment molecules (photopigments) are integral membrane proteins, with retinal as the chromophore (light-absorbing part).

Generating the Optic Nerve Signal (in Rods)

• In the dark, retinal is bent (cis-retinal), and opsin and retinal are bound together.
• In light, the retinal molecule straightens (trans-retinal), and retinal dissociates from opsin (bleaching).
• It takes about 5 minutes for rods to regenerate 50% of bleached rhodopsin.

Generating Visual Signals (in Cones)

• Three types of cones are named for their absorption peaks.
• The short-wavelength cone peaks sensitivity at 420 nm, medium wavelengths at 531 nm, and long wavelengths at 558nm.
• Ganglion cells are the only retinal cells that produce action potentials.

Generating the Optic Nerve Signal

• In darkness, rods release glutamate, an inhibitory neurotransmitter.
• When light strikes rods, glutamate secretion stops.
• Different types of bipolar cells react differently to the changes in glutamate concentrations.
• Some bipolar cells are excited when glutamate stops, which is caused by an increase in light intensity.
• Other bipolar cells are excited by glutamate and respond when light intensity decreases.

Generating the Optic Nerve Signal

Ganglion cells respond to the bipolar cell activity (directly or indirectly) with rising and falling frequency.

• The changes create visual signals that travel to the brain via the optic nerve.

Light and Dark Adaptation

• Light adaptation occurs when moving from darkness to light, and involves constriction of the pupil and decreased sensitivity of rods.
• Dark adaptation occurs when moving from light to darkness, and involves dilation of the pupil and increased sensitivity of rods.
• Rods quickly bleach and become nonfunctional, while cones take over.
• Retinal sensitivity to light changes.

The Dual Visual System

• The dual visual system explains how we have both high sensitivity and high resolution in different parts of the visual field.
• Rods are important for sensitive night vision, while cones are for high-resolution daytime vision.
• Rods have extensive neuronal convergence (one ganglion cell receives signals from many rods).
• Cones have a low degree of spatial summation. Thus, one ganglion cell receives signals from only one cone.

The Dual Visual System

• Fovea contains only cone cells (no rods), leading to high resolution.
• There is little spatial summation in the fovea, minimizing sensitivity to dim light.
• Primates have well-developed color vision (using three kinds of cones.)

Color Vision

• Color blindness is a hereditary alteration or lack of one or more photopsins.
• The most common type of color blindness is red-green color blindness.
• Red-green color blindness results from a deficiency in either L or M cones.

Stereoscopic Vision

• It is the ability to perceive depth through the overlap of visual fields using both eyes (eyes on same plane, not on sides of head.)
• Two eyes viewing an object from different angles. This information is processed by the brain to discern depth perception.
• Humans have stereoscopic vision by having eyes that overlap in their visual range.

The Visual Projection Pathway

• Bipolar cells and ganglion cells are first-order and second-order neurons, respectively.
• Optic nerves from each eye join to form the optic chiasm.
• Half the fibers cross over to the opposite side of the brain.
• Optic tracts continue to the lateral geniculate nucleus of the thalamus.

The Visual Projection Pathway

• The optic tracts pass laterally, around the hypothalamus.
• Third-order neurons arise from the lateral geniculate nucleus to form optic radiations.
• The optic radiations project to the primary visual cortex in the occipital lobe.
• A few optic nerve fibers project to the midbrain and terminate in superior colliculi and pretectal nuclei.
• Some processing starts in the retina, and adjusts for contrast, brightness, motion, and stereopsis.

The Visual Projection Pathway (4)

• Processing in visual areas like the primary visual cortex processes information from the visual field.
• Data from different areas (temporal and parietal) is processed and collected for information.
• Visual memories are stored in the occipital lobe.

This quiz focuses on the anatomy and physiology of the human eye, covering key components such as the sclera, choroid, retina, and their respective functions. Test your knowledge of the structure and roles of various eye parts, including the macula lutea, fovea, and optic nerve. Ideal for students learning about sensory organs in biology.