Summary
Summary - Medical BioSciences (MBS214)
- Course
- Medical BioSciences (MBS214)
- Institution
- University Of The Western Cape (UWC)
Learning Outcomes for lectures 10 to 14
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MBS 214 LEARNING OUTCOMES:LECTURE 10: Part 1
● Describe the anatomy of the eye
The eye is shaped like a round ball, with a slight bulge at the front.
The eye has three main layers. These layers lie flat against each other and form the eyeball.
● The outer layer of the eyeball is a tough, white, opaque membrane called the sclera (the white of
the eye). The slight bulge in the sclera at the front of the eye is a clear, thin, dome-shaped tissue
called the cornea.
● The middle layer is the choroid. The front of the choroid is the coloured part of the eye called the
iris. In the center of the iris is a circular hole or opening called the pupil.
● The inner layer is the retina, which lines the back two-thirds of the eyeball. The retina consists of
two layers: the sensory retina, which contains nerve cells that process visual information and send it
to the brain; and the retinal pigment epithelium (RPE), which lies between the sensory retina and
the wall of the eye.
The inside of the eye is divided into three sections called chambers.
● Anterior chamber: The anterior chamber is the front part of the eye between the cornea and the
iris.
● The iris controls the amount of light that enters the eye by opening and closing the pupil.
● The iris uses muscles to change the size of the pupil. These muscles can control the amount
of light entering the eye by making the pupil larger (dilated) or smaller (constricted).
● Posterior chamber: The posterior chamber is between the iris and lens.
● The lens is behind the iris and is normally clear. Light passes through the pupil to the lens.
● The lens is held in place by small tissue strands or fibers (zonules) extending from the inner
wall of the eye.
● The lens is very elastic. Small muscles attached to the lens can change its shape, allowing the
eye to focus on objects at varying distances.
● Tightening (contraction) or relaxing these muscles causes the lens to change shape, allowing
the eyes to focus on near or far objects (accommodation).
● Vitreous chamber: The vitreous chamber is between the lens and the back of the eye.
● The back two-thirds of the inner wall of the vitreous chamber is lined with a special layer of
cells (the retina): millions of highly sensitive nerve cells that convert light into nerve
impulses.
● Nerve fibers in the retina merge to form the optic nerve, which leads to the brain. Nerve
impulses are carried through the optic nerve to the brain.
● The macula, near the center of the retina at the back of the eyeball, provides the sharp,
detailed, central vision for focusing on what is in front of you. The rest of the retina provides
side (peripheral) vision, which allows you to see shapes but not fine details.
● Blood vessels (retinal artery and vein) travel along with the optic nerve and enter and exit
through the back of the eye.
Fluid fills most of the inside of the eye. The chambers in front of the lens (both the anterior and posterior
chambers) are filled with a clear, watery fluid called aqueous humor. The large space behind the lens (the
vitreous chamber) contains a thick, gel-like fluid called vitreous humor or vitreous gel. These two fluids
press against the inside of the eyeball and help the eyeball keep its shape.
,The eye is like a camera. Light passes through the cornea and the pupil at the front of the eye and is
focused by the lens onto the retina at the back of the eye. The cornea and lens bend light so it passes
through the vitreous gel in the back chamber of the eye and is projected onto the retina. The retina
converts light to electrical impulses. The optic nerve carries these electrical impulses to the brain, which
converts them into the visual images that you see.
● Describe the properties of light
Light refraction:
● Light is bent or refracted when it passes from one medium to another with a different density
● Greatest amount of refraction occurs = when light passes from the air into the corneal tissue
● Has a density close to that of water
● Additional refraction occurs when light passes from the aqueous humor into the relatively dense
lens
● Lens provide the extra refraction needed to focus the light rays from the object towards a focal
point on the retina
Light refraction of lens:
● Image reversal
- Light from the objects is focused from the retina
- But the image arrived upside down
● Image formation
- Light rays projected from a horizontal object show why the image arrives with a left and
right reversal
- The image also arrives upside down
● Describe common eye disorders e.g. cataracts, glaucoma, retinitis pigmentosa and macula dense
degeneration
Cataracts: is a cloudy area in the lens of the eye
● Can result from:
- Injuries
- Radiation
- Reaction to drugs
● Senile cataracts: gradual, progressive thickening of the lens
- A natural consequence of aging
- Most common form
Glaucoma: group of eye diseases that can cause vision loss and blindness by damaging a nerve in the back
of your eye called the optic nerve
● Caused by an increase in intraocular pressure
● Caused by a narrowing or blockage of Schlemm’s canal
● Damage to the optic nerve leads to progressive, irreversible vision loss
Retinitis Pigmentosa: affects the retina
● Genetic disorder - generally inherited from a person’s parents
● Mutations in one of more than 50 genes are involved
● The underlying mechanism involves the progressive loss of rod photoreceptor cells in the back of
the eye
Macula densa degeneration:
● Also known as age related macular degeneration
● May result in blurred or no vision in the center of the visual field also known as “tunnel vision”
, LECTURE 10: Part 2
● Describe the structure of rods and cones
Rods and cones have photopigment-bearing regions (outer segments) composed of a large number of
pancakelike disks. In rods the disks are closed, but in cones the disks are partially open to the surrounding
fluid.
● Refers to the shape of the outer segment
● Discs of the outer segment in both rods and cones contain special organic compounds called visual
pigments (where light absorption occurs)
● In a cone: the discs are infoldings of the plasma membrane, and the outer segment tapers to a blunt
point
● In a rod: each disc is an independent entity, and the outer segment forms and elongated cylinder
● Cones: 3 types (red, green, blue)
● Describe the generation of receptor potentials in the rods and cones during light and dark
conditions
Light and dark adaptation
● After 30 minutes or more in the dark, almost all visual pigments will have recovered from
photo-bleaching and be fully receptive to stimulation
● Known as dark adapted state
● Dark
- Most visual pigments are fully receptive to stimulation
● Light
- Pupil constricts
- Bleaching of visual pigments occurs
● Describe the generation of action potentials in retinal ganglion cells
Retinal ganglion cells are the output neurons of the retina that transmit visual information to the brain via
action potentials. The generation of action potentials in retinal ganglion cells involves several steps:
Light absorption: Light is absorbed by the photopigments in the outer segments of the rod and cone
photoreceptor cells in the retina.
Hyperpolarization: This triggers a cascade of signaling events that leads to the hyperpolarization of the
photoreceptor cells, reducing the release of the neurotransmitter glutamate at their synapses with bipolar
cells.
Depolarization: The reduced glutamate release from photoreceptor cells leads to depolarization of the
bipolar cells, which in turn depolarizes the retinal ganglion cells.
Action potential generation: The depolarization of retinal ganglion cells to a certain threshold triggers the
opening of voltage-gated ion channels, leading to a rapid influx of sodium ions and an efflux of potassium
ions. This causes a rapid change in the membrane potential, generating an action potential.
Signal transmission: The action potential propagates down the axon of the retinal ganglion cell and is
transmitted to the brain via the optic nerve.
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