Chapter 12 - Coordination and Response : Human Eye

External Structures of an Eye:

 

  • %%Tear gland%%:

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

  • %%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.

 

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.