M21: Orbit, Eye and Vision; Visual Pathways Flashcards
Orbit
-Eyes are located within either orbit in the skull
-Bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye.
What bones contribute to the orbit?
Roof: Frontal bone
Lateral wall: Zygomatic bone
Floor: Maxillary bone and palatine bone
Medial wall: Lacrimal bone and ethmoid bone
Posterior wall: Sphenoid bone
Eyebrows
-along the superior orbital ridge
-Prevent sweat from dripping in the eye
-Shade the eyes from sunlight
Eyelashes
-hairs on the margins of the eyelids
-Prevent large foreign objects from contacting the eyes
-Highly innervated → When something unexpected touches the eyelashes, it triggered a blink reflex
Eyelids (Palpebrae)
- Meet at medial and lateral angles i.e. corners of the eyes (canthi)
- Provide protection
Palpebral fissure
opening between eyelids, pace between upper and lower eyelid
Tarsal plates
- Connective tissue
- ensure eyelids maintain their shape
Tarsal glands
modified sebaceous glands
Functions:
1. lubricate the surface of the eye
2. prevent tear overflow
3. prevent the eyelids from sticking together
Conjunctiva
-Transparent mucous membrane made of stratified squamous epithelium
-extends over the white areas of the eye (the sclera), connecting the eyelids to
the eyeball
-Contains goblet cells → secrete mucus to lubricate the eye
Palpebral conjunctiva
Lines the inner surface of each eyelid
Bulbar conjunctiva
Lines the anterior eye
Trachoma
-Chronic conjunctiva
- Super inflamed which harms the surface of the eye
- Leading cause of blindness
Lacrimal Apparatus
-Produces, collects, and drains lacrimal fluid (tears) from the eye
—> Lubricates the anterior surface of the eye
—> Helps prevent bacterial infections→ antibiotic- like enzyme called lysozyme
—> Located in the superior lateral aspect of the orbit
Lacrimal fluid drainage pathway
- Lacrimal fluid is produced in the lacrimal gland
- Lacrimal fluid is dispersed across the eye surface when we blink.
- Lacrimal fluid drains into the lacrimal canaliculi via the lacrimal
puncta and collects in the lacrimal sac - Lacrimal fluid then drains through the nasolacrimal duct and..
- Enters the nasal cavity.
Extrinsic Eye Muscles
-Rectus muscles originate from a common tendinous ring in the posterior orbit and insert onto the outer surface of the eye
—> originate outside of the eye on the skull and insert onto the sclera of the eye
Medial rectus
- adduct eye
- originate in the posterior aspect of the orbit
at a common tendinous ring
-Innervated by CN III (oculomotor nerve)
Inferior rectus
-depress eye
-originate in the posterior aspect of the orbit
at a common tendinous ring
-Innervated by CN III (oculomotor nerve)
Superior rectus
- elevate eye
- originate in the posterior aspect of the orbit
at a common tendinous ring
-Innervated by CN III (oculomotor nerve)
Inferior oblique
-Elevates & abducts eye
- originates from the floor of the orbit and inserts into the inferolateral surface of the eye.
-When it contracts, it elevates (rotating superiorly) and abducts the eye
-innervated by CN III (oculomotor nerve)
Lateral rectus
- abduct eye - originate in the posterior aspect of the orbit at a common tendinous ring
- Only muscle innervated by CN VI (abducens nerve)
Superior oblique
- depresses & abducts
- originates at the posterior orbit, near the origin of the four rectus muscles.
- tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea meaning the contraction of the
superior oblique depresses (rotates inferiorly) and abducts (rotates laterally) the eye. - Only muscle innervated by CN IV (trochlear nerve)
Eye Structure
- Almost spherical organ ~2.5 cm in diameter
- Protect and support the photoreceptors
- Gather, focus, & process light into precise images
- Three principal layers form the wall of the eye (composed of 3 layers of tissue):
1. fibrous tunic
2. vascular tunic
4 retina
Fibrous tunic
The outermost layer of the eye including the sclera and cornea
Sclera
- white portion of the eye
- accounts for the majority of the surface of the eye, most of which is not visible
- site of attachment for the extraocular muscles
Cornea
- transparent, most anterior portion of the eye
- Allows light to enter the eye.
- Covered by stratified squamous epithelium
- Avascular, meaning it does not have its own blood supply
- Highly innervated → abundant free nerve endings so sensitive to pain
Vascular tunic
the middle layer of the eye including the choroid, ciliary body, and Iris
Choroid
- layers of highly vascularized connective tissue
- contains an extensive capillary network that delivers nutrients to the retina
Iris
- contains pigmented cells that give us our eye color
- contains intrinsic muscles of the eye, or the smooth muscles located completely within the eye that open or close the pupil
Retina
- the innermost layer of the eye
- contains the nervous tissue responsible for photoreception and the initial processing of visual stimuli
- Has two sublayers:
1. Pigmented layer
2. Neural layer
-Divided into two cavities:
1. Anterior cavity
2. Posterior cavity
Pigmented layer of the retina
absorbs light and, in response, undergoes
important biochemical interactions that trigger activity in the photoreceptors.
Neural layer of the retina
contains the photoreceptors as well as supporting cells and neurons that perform the first steps in visual processing and integrating visual information.
Types of photoreceptor cells
Rods
- responsible for vision in dim light
- Allow us to see in the dark
- low spatial acuity, meaning they are not capable of sharp, high-definition vision
- more highly concentrated toward the peripheral aspect of the retina
Cones
- responsible for color vision and high-acuity vision
- Allow us to see sharper, clearer color images
- Do not function well in dime light
- most concentrated in the central portion of the retina
Anterior Cavity
- Space between cornea and lens
Divided into:
- Anterior chamber: between the cornea & pupil
- Posterior chamber: between the iris & lens
- Both contain aqueous humor (watery fluid) → always being produced
Posterior Cavity
Vitreous Chamber: Contains vitreous body
Pupil
the hole at the center of the eye that allows light to enter
Pupil Diameter
-Sphincter pupillae: reduces the diameter of the pupil (pupils constrict), under autonomic control
- Dilator pupillae: increases the diameter of the pupil (pupils dilate), under autonomic control
Ciliary Body
-Changes lens shape
- Tension on suspensory ligaments caused by the ciliary muscles of the ciliary body
Lens
- Transparent, deformable, biconvex disc
Bends light rays that enter the eye - Accommodation: Changing the shape of the lens to focus on objects near or far
-Has sympathetic and parasympathetic input
Sympathetic input of the lens
- Relax ciliary muscle, tighten suspensory ligaments, flatten lens (lens thins)
- Distant vision (emmereopia) → helps us focus on something far away
Parasympathetic input of the lens
contract ciliary muscle, loosen suspensory ligaments, lens return to more rounded shape (lens thickens)
Near vision (accommodation) → helps us focus on something nearby
Ophthalmic Artery
- Supplies the inner 2/3 of the retina with oxygen and nutrients
- True end artery
Visual field
everything that we can see in a given moment
Visual Pathway
- Nasal fibers cross in the optic chiasm, after this
—-> Right occipital lobe processing left visual field info
—–> Left occipital lobe processing right visual field info - Most (90%) of optic tract fibers travel to the lateral geniculate nucleus of the thalamus
- Optic tract fibers are composed of retinal ganglia cells ← ON EXAM
- From the LGN, axons travel through optic radiations to the primary visual cortex in the occipital lobe
Occipital Lobe
- Posterior region of each hemisphere
- Primary visual cortex (area V1): receives & processes incoming visual information
—> Cortex of the calcarine fissure
—> Posterior calcarine cortex : high resolution
—> Anterior calcarine cortex: peripheral vision
—> Lower visual field → Upper retina → Lower calcarine cortex
—> Right visual cortex: Right half of each retina, LEFT visual field
—> Left visual cortex: Left half of each retina, RIGHT visual field
- Visual Association areas (BA 18): continue processing visual information
- 17 –Primary Visual Cortex
- 18, 19 –Visual Association Areas
Accessory Visual Pathways
Other 10% if axons in the optic tract travel to:
- Edinger-Westphal nucleus
- Superior colliculus
- Suprachiasmatic nucleus
Edinger-Westphal nucleus
- Midbrain
- Involved in pupillary reflex and lens control
- Tells us how bright it is outside
—> Axons originating in this nucleus travel with the fibers in the oculomotor nerve (cranial nerve III) to reach the ciliary ganglion.
—> The postganglionic parasympathetic fibers then project to the smooth muscle of the iris to constrict the pupil in the presence of brighter light and to the ciliary muscle to adjust the shape of the lens for near vision.
Superior colliculus
- Midbrain
- Involved in visual reflexes
—> Orienting movements of the head and eyes and reflexively directing the eyes to areas of interest
Suprachiasmatic nucleus
- Hypothalamus
- Light/dark info for the circadian rhythm (sleep/wake cycle)
- If light is not present (nighttime), it will send information to the pineal gland, signaling it to secrete melatonin, which makes us sleepy.
- If light is present (daytime), it will suppress the pineal gland and prevent it from secreting melatonin.
Visual Association Areas
- V1: primary visual cortex
- V2, V3 ← two main regions that surround the primary cortex
- Parts of temporal and parietal lobes
- “Vs” are retinotopic maps
—-> Each area processes the entire visual field in its own way
—-> Different elements of vision are added as the information passes from one area to the next - V4 = Color vision
- V5 = Motion sensitivity
Lesions of Visual Association Areas
- V4: impaired color discrimination
—-> Cerebral achromatopsia: no color sight, whole spectrum - V5: Cerebral Akinesia
—-> Impaired motion detection - Fusiform Face Area: Prosopagnosia
—-> Inability to recognize faces
—-> Very posterior temporal lobe adjacent to the occipital lobe - Inferotemporal Complex
—-> Inability to attach emotion to visual stimuli - Parietal regions: Visuospatial neglect
