Special senses - Eyes
What spectrum consists of visible light?
- 400-700 nm wavelength - Color is due to light wavelength absorption (white objects reflect all light wavelengths; black objects absorb all light wavelengths)
What will you find in hypertensive retinopathy?
- AV nicking: a small artery (arteriole) is seen crossing a small vein (venule), which results in the compression of the vein with bulging on either side of the crossing. - Cotton-wool spots - Copper-wiring: the blood vessel walls will be overly defined due to arteriole wall thickening
What adjustments do our eyes make to focus on close objects?
- Accommodation of the lenses - Pupillary constriction - Convergence of eyeballs (mediated by the medial rectus muscle) - All 3 must occur simultaneously - Blurring of retinal images stimulates these adjustments - Lens bulges → suspensory ligaments become less taut → ciliary muscles contract - The process is mediated by the parasympathetic NS and oculomotor nerve (CN III)
Glaucoma: definition, detection, treatment
- An increase in intraocular pressure - Can lead to a compression of the retina and optic nerve; may cause blindness - Intraocular pressure can rise as high as 70 mmHg (20-30 mmHg can cause visual loss) - Detected by measuring the amount of corneal deformation after a puff of air hits them - Treatment includes: eye drops that reduce secretion or increases absorption of aqueous humor and surgical interventions to reduce the pressure
What is light refraction?
- Bending of light rays - Occurs due to changes of speed when light passes through different mediums at variable angles - Our lens bends light in an attempt to concentrate rays on the fovea
What will you notice in a healthy retina?
- Blood vessels should look healthy and highly vascularized - A defined macula; dimmer with the fovea outlined even further - a well defined and visible optic disc
What cranial nerves control the extraocular eye muscles?
- CN III (Oculomotor): Sup & inf rectus. Most eye movements - CN IV (Trochlear): Superior oblique, downward or inward - CN VI (Abducens): Lateral rectus, Upward or lateral deviation **LR6SO4AO3**
Where are the vitreous and aqueous humor located and what is their function?
Vitreous humor: gelatinous fluid that fills the posterior cavity, transmits light and supports the posterior surface of the lens and retina, and contributes to the intraocular pressure Aqueous humor: a less gelatinous fluid that fills the anterior cavity, produced and drained at the same time, functions to supply nutrients & oxygen to the lens, cornea, parts of the retina, helps rid the eye of wastes
Cataracts: definition, causes, and treatment
- Clouding of the lens due to clumping of crystalline proteins - Results in an inadequate delivery of nutrients to the eye - May be congenital or acquired (medications can cause this) - Treatment includes surgical removal/replacement of lens
Concave vs. Convex lenses
- Convex lenses: thickest in the center, light rays converge at a focal point - Concave lenses are thicker on the edges, light rays bend outward
Define esotropia and exotropia
- Esotropia: inward misalignment of the eyes - Exotropia: outward misalignment of the eyes
Hyperopia: definition, causes, and treatment
- Far-sightedness - Objected are focused behind the retina - the eyeball is too short - Corrected with convex lenses
How are photoreceptor disks replaced?
- Formation of new disks at the inner edge of the segment - Phagocytosis of old disks from the outer tip by cells of the pigmented retina
How does fusion of visual images from each eye occur?
- Function of the visual cortex - Two retina transfer visual signals to the lateral geniculate body - Signals are relayed to neurons in the visual cortex - Cortical neurons recognize visual perceptions are not fused and perform convergence/divergence so fusion can be established
Night blindness definition and causes
- Impairment of rod function - Causes include malnutrition, vitamin A deficiency
The retina (neural layer): location, function
- Inner layer of the retina; transparent - Contains millions of photoreceptors (rods & cones) which process light stimuli - Contains bipolar cells, ganglion cells, horizontal cells, and amacrine cells
Strabismus: definition, causes, and treatment
- Lack of fusion of eyes in one or more visual coordinates: horizonal, vertical, rotational - One eye fixates satisfactorily; the other does not - Due to abnormal fusion mechanism/extraocular muscle deficiency - Can be congenital or be due to an oculomotor lesion
What muscles are responsible for eye movement? What direction do they move the eye?
- Lateral & Medial Rectus: Side to side movement - Superior & Inferior Rectus: Upward & downward movement - Superior & Inferior Oblique: Rotate eyeballs
The retina (pigmented layer): location, function
- Located adjacent to the choroid - Functions in absorbing light - Contains phagocytes that remove damaged photoreceptors - Stores vitamin A
Presbyopsis: definition and cause
- Loss of the ability to accommodate - Causes by decreased elasticity of the lens
What is accomodation and what changes does the lens make to accomodate?
- Mechanism by which the eye changes its refractive power - Dependent on CN II (afferent) and CN III (efferent) - Alters the shape of the lens to focus on objects at different distances: flattens when focusing far away; thickens when focusing near
What are rods and what are they responsible for interpreting?
- Most numerous in retina - Function in dim light (night vision) and peripheral vision - More sensitive to light than cones but do not provide sharp or color images
Myopia: definition, causes, and treatment
- Near-sightedness - Objects are focused in front of the retina - the eyeball is too long - Corrected with concave lenses
Emmetropia: definition
- Normal eye - Objects in distance are sharply focuses on the retina when the ciliary muscle is completely relaxed - Appropriate accommodation when focusing on close objects
What is normal intraocular pressure and what does it measure?
- Normal intraocular pressure is 12-20 mmHg and should remain +/- 2mmHg - It's determined by the resistance to outflow of aqueous humor - If the pressure rises above 15 mmHg, the tiny spaces in the trabeculae enlarge to decrease the excess pressure
What are cones and what are they responsible for interpreting?
- Operate in bright light - Less sensitive than rods - Responsible for high acuity color vision - Unique cones absorb different wavelengths: Blue: 425-550 nm Red: 500-700 nm Green: 450-650 nm When all cones are stimulated equally you will see white light
Describe the neural physiology of photoreceptors
- Photoreceptors detect visible light: rods tell the CNS that photons are present, cones provide the CNS with the specific wavelength - Visual pigments in rods & cones absorb photons; the pigment is derived from rhodopsin (opsin = protein). Rods have 1 opsin, cones have several opsins. - The photon strikes the rhodopsin, opsin is activated and the photon is absorbed. The rhodopsin breaks down when it is not stimulated; causes bleaching as it is breaking down
Retinal detachment: definition, causes, treatment
- Pigmented and neural retinal layers detach; vitreous humor will flow between them - Photoreceptors die due to nutrient deprivation; leads to blindness - Caused by traumatic blows to the head or sudden jerky head movements, diabetics and myopic patients are at an increased risk for detachment - Patients may complain of the "curtain being pulled over my eyes" or flashes of light as the light rays hit the bulging parts of the retina - Treatment includes laser intervention
Where is aqueous humor produced and how does it diffuse through the eye?
- Produced by the ciliary body - First released into the posterior chamber and exits via trabeculae outflow to the anterior chamber; exits via Schlemm's canal - Produced and drains continuously at the same rate
Describe the following and identify them on a diagram: Sclera, cornea, choroid, retina, lens, suspensory ligament, ciliary body, iris, pupil, vitreous and aqueous humor
- Sclera: outer protective layer; - Cornea: transparent layer where light enters; - Choroid: layer beneath sclera with rich vascular supply; - Retina: neural tissue containing receptor cells; - Lens: transparent structure where light passes; - Suspensory ligament: holds lens in place, is attached to ciliary body; - Ciliary body: composed of muscle fibers; function in accommodation, aqueous humor production, and anchors lens in place. - Iris: contains circular muscle fibers that constrict radial fibers and dilate pupil. - Pupil: controls amount of light reaching retina (like the aperture of a camera). - Vitreous humor: clear gelatinous material that fills post cavity; - Aqueous humor: clear liquid that nourishes cornea and lens, produced by ciliary body.
What will you find in diabetic retinopathy?
- Small hemorrhages in the retina (spotting) which can lead to large hemorrhages over time - Cotton-wool spots due to neurologic damage - Neovascularization due to poor circulation: large but very thin-walled vessels
Understand the visual pathways and the deficits due to specific optic nerve lesions
- Some optic nerve fibers will cross at the optic chiasm (at the level of the pituitary gland) and end up on the opposite side of the occipital cortex - The crossing over allows for binocular vision and depth perception - In A: in an optic nerve lesion, you won't have any left sided vision and the right side will be OK - In B: note that the part that is blocked at the optic chiasm are the fields from the periphery (consider a pituitary adenoma) - In C: a lesion is in the optic tract; left eye nasal field and peripheral vision in right eye will be diminished/lost - In D: a lesion in the geniculocalcarine tract will the same deficit in D with vision in the central part of the field of vision due to maculo-sparing (the macula is not affected by a lesion in this area)
What muscles/ligaments are responsible for accomodation?
- The ciliary muscle and suspensory ligaments - The ciliary muscle contracts, the suspensory ligaments relax, and the lens becomes more convex (thickens; near vision) - The ciliary muscle relaxes, the suspensory ligaments are pulled tight, and the lens flattens (far vision)
Where is visual acuity the highest and how do you measure it
- The fovea centralis in the macula is the area of the retina that has the sharpest acuity: the only place in the eye that has cones - Normal vision is 20/20: a person can read at 20 feet what a normal eye is able to read at 20 feet (20/200 = at 20 feet the patient is reading a line that a normal eye can see at 200 feet)
How are photoreceptors designed and work?
- The outer segment is composed of stacks of flattened saccules (cones) or disks (rods) that contain photosensitive compounds to light. - The inner segment is rich in mitochondria and provides the energy needed - When light hits the disk or saccule, proteins respond to the light and initiate an action potential by opening sodium channels
What is the optic disk?
- The point where the optic nerve leaves the eye and the retinal blood vessels enter it - No visual receptors present - Also called the blind spot
Astigmatism: definition, causes, and treatment
- Unequal curvature of the lens, cornea - Results in blurred images - Cylindrical ground lenses are used to correct problem
Describe the lens
- biconvex, transparent, flexible structure - changes shape to allow for precise focusing of light on the retina - with aging, the lens becomes more dense (which affects its ability to change shape)
In what orientation does our lens project images on our retina?
Upside down
