Chapter 12 - Coordination and Response : Human Eye

External Structures of an Eye:

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• %%Tear gland%%:

  • Secretes tears which lubricate the eye, nourish the cornea and keeps it free from dust.

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• %%Eyelid:%%

  • Squinting is the partial closure of eyelids. This prevents excess light from entering the eye and damaging the light-sensitive tissues
  • Blinking spreads tears over the cornea and conjunctiva and wipes dust particles off the cornea

• %%Eyelashes:%%

  • Shield eyes from dust particles.

• %%Iris:%%

  • Contain %%radial and circular muscles%% that control the size of pupil
  • Pigment of iris gives the colour of eyes

• %%Pupil:%%

  • Allows light to enter the eye

• %%Sclera:%%

  • Tough white outer layer of connective tissue
  • Continuous with cornea

• %%Conjunctiva:%%

  • Thin, transparent mucous membrane that covers the sclera
  • Secretes %%mucous%% to lubricate the eye

• %%Cornea%%:

  • Transparent %%refractive layer%% covering the iris and pupil.
  • continuous with the sclera.

Internal Structure of Eye:

• %%Choroid:%%
  • Middle layer of the eyeball, between the sclera and retina.
  • Contains blood vessels that supply oxygen and nutrients, and remove metabolic waste products.
  • %%Pigmented black%% to prevent an internal reflection of light.
• %%Retina%%:
  • Innermost layer of the eyeball which contains photoreceptors, which are connected to the optic nerve.
• %%Lens:%%
  • Transparent biconvex structure that refracts light onto the retina.
  • The lens can change its curvature to focus light onto retina
• %%Fovea%%:
  • Also called %%yellow spot%%, a small depression in the retina where images are usually focused
  • The fovea contains the greatest concentration of cones, but no rods.
• %%Ciliary body:%%
  • Contains ciliary muscles which control the curvature of the lens.
  • It is also responsible for producing aqueous humour.
• %%Suspensory ligament:%%
  • Connects the ciliary body to the lens
• %%Aqueous chamber:%%
  • The space between the lens and the cornea.
  • Transparent %%aqueous humour%% keeps the front of the eyeball firm and helps to refract light %%into the pupil.%%
• %%Vitreous chamber:%%
  • The space behind lens
  • Transparent %%vitreous humour%% keeps the eyeball firm and helps to %%refract%% light %%onto the retina.%%
• %%Optic nerve:%%
  • Transmits signal from the retina to the brain.
  • There are no photoreceptors in the area of the retina where the optic nerve leaves. This area is called the %%blind spot%%

Photoreceptors:

%%Photoreceptors%% in the retina consist of rods and cones. The photoreceptors are connected to the nerve endings from the optic nerve.

CONES:

• Cones enable us to see colours in %%bright light.%%

• There are three types of cones, red, blue, and green (RBG) that allow us to see a wide variety of colours by containing different pigment which absorbs light of different wavelengths.

• Cones do not work well in dim light.

  

RODS:

• Rods enable us to see in %%dim light%%, but only in black and white.
• Rods are sensitive to light of low intensity as they contain pigment called visual purple. When the eye is exposed to bright light, all the visual purple is %%bleached%%.
• %%Visual purple%% must be reformed for a person to see in the dark. Therefore, it takes awhile for one to see in dark after being in a bright environment as time is taken for visual purple to reform.
• Formation of visual purple requires %%vitamin A.%%

How we see

• When light falls on an object, light rays are reflected from the object

• Light rays are refracted through the cornea and the aqueous humour onto the lens

• The lens causes further refraction and the rays are brought to a focus on the retina.

• The image on the retina stimulates the photoreceptors, either the rods or the cones, depending on the intensity of the light.

• %%Nerve impulses%% are produced and sent to the brain via optic nerve. The brain interprets the impulses and the person sees the object.

  

Accommodation:

A %%reflex action%% where the lens is adjusted so that %%clear images%% of objects at different distances are formed on the retina.

Focusing on a distant object:

• Light rays reflected off distant objects are %%nearly parallel%% and enter the eyes

• %%Ciliary muscles relax%%, causing suspensory ligaments to tighten

• %%Suspensory ligaments become taut%%, pulling on the edge of the lens.

• %%Lens becomes thinner%% and less convex, increasing its focal length, causing less refraction of the rays of light

• Light rays from the distant object are sharply focused on the retina.

  

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Focusing on a close object:

• Light rays reflected off close objects are %%diverging%% and enter the eyes

• %%Ciliary muscles contract%%, causing suspensory ligaments to become relax

• %%Suspensory ligaments slacken%%, relaxing their pull on the lens.

• %%The lens becomes thicker%% and more convex, decreasing its focal length, causing more refraction of the rays of light,

• Light rays from the near object are sharply focused on the retina.

  

Pupil Reflex:

• %%Reflex action%% (involuntary) where the pupil changes size in response to changes in light intensity.

• In low light intensity, pupils %%dilate%% allow more light to enter the eye for better vision.

• In high light intensity, pupils %%contract%% to restrict light to enter to prevent excessive light from damaging the retina.

  

• In dim light:

  • Radial muscles of the iris contract
  • Circular muscles of the iris relax
  • The pupil enlarges or dilates, increasing the amount of light entering the eye.

• In bright light:

  • Circular muscles of the iris contract
  • Radial muscles of the iris relax
  • The pupil becomes smaller or constricts, reducing the
    amount of light entering the eye.

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