ANATOMY OF THE EYE

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Special senses [EYE]
These are organs in the body that performs specialised functions.

They contain sensory receptors which receives sensory impulses and pass them to the appropriate centers in the brain for interpretation. 

 ➡️ Types of Sensory Receptors - based on the type of stimuli they detect:

1. Mechanoreceptors - pressure receptors, stretch receptors, and specialized mechanoreceptors involved 
in movement and balance. 

2. Thermoreceptors - skin and viscera, respond to both external and internal temperature. 

3. Pain receptors - stimulated by lack of O2, chemicals released from damaged cells and inflammatory 
cells. 

4. Chemoreceptors - detect changes in levels of O2, CO2, and H+ ions (pH) as well as chemicals that 
stimulate taste and smell receptors 

5. Photoreceptors - stimulated by light

➡️ Distribution of Receptors in the body:
Special Senses
• mediated by relatively complex sense organs of the head, innervated by cranial nerves
• vision, hearing, equilibrium, taste and smell. 

General (somesthetic, somatosensory)
• receptors widely distributed in skin, muscles, tendons, joints, and viscera
• they detect touch, pressure, stretch, heat, cold and pain, blood pressure

DIVISION
The eye is divided into:
1. Accessory organ
2. Eye ball

ACCESSORY ORGANS OF THE EYE
Accessory  structures of the  eye are  those that  are  not directly related  the sense of vision,  but facilitate  the physiology  of the eyeballs.

➡️ Eyebrows –to shade the eyes  from sunlight  and to  prevent perspiration  from reaching  the eyes.

➡️ Eyelids-to protect  the  eyes  from foreign  objects  (e.g. dust particles)  and to prevent  desiccation  (drying) of the  eyes  by lubricating  fluid.

➡️ Eyelashes: are sparse hair present on the upper and lower eyelids. They help to prevent particles from entering the eyeballs. 

 ➡️ conjunctiva: a mucous membrane  on the  inner lining  of eyelids,  which produces lubricating  and cleansing  fluid  for the surface  of eye.

➡️ Lacrimal Apparatus: This is an exocrine gland that secretes a dilute saline solution called tears for moistening the eyes. [ Tears contain mucus, antibodies and antibacterial enzymes that protect the eye from infections. Emotional tears also contain enzymes that seem to help reduce stress levels].

THE EYE BALL
INTRO: The eye ball is a ovoid shaped organ responsible for the sense of sight.
LOCATION: It is located in the anterior compartment of the orbital carvity
SHAPE: almost spherical in shape
DIAMETER: 2.5 in diameter

LAYERS OF THE EYE
 The eye is composed of three layers 
1.  Outer  fibrous layer which consists of sclera and cornea
2.  Middle vascular layer which consists of choroid, ciliary body and iris 
3.  Inner layer consists of retina

➡️ FIBROUS LAYER : This is the outermost layer that consists  of  the  protective  sclera and the transparent  cornea
SCLERA: The sclera,  thick,  glistening white connective    tissue, is seen anteriorly as the  “white of the eye. It comprises of 4/5 of the total outermost layer of the eyeball.

It has a point of attachment of the extra ocular muscles at a point known as the Limbus.

FUNCTION
It helps in forming the shape of the eye and also serves as protection to the eyeball.

CORNEA - Is the transparent surface covering the iris and pupil, a clear, dome-shaped part of the sclera covering the front of the eye through which light enters the EYE. It makes up of 1/5 of the eyeball. 

➡️ VASCULAR LAYER : This is the  middle  layer  of  the  eyeball that consists of three distinguishable regions. 

Most posterior is the  CHOROID ,  a blood-rich nutritive tunic that contains a dark pigment.  The pigment prevents light from scattering inside the eye.

Moving anteriorly, the choroid is modified  to form two smooth muscle structures, the  CILIARY    BODY ,  to which the lens is attached by a suspensory ligament called the ciliary  zonule,  and  then the  IRIS . 

The  pigmented iris has a rounded OPENING known as the  PUPIL,  through which  light passes.  Circularly and radially arranged smooth muscle fibers form the iris,  which acts like the diaphragm of a camera.  That is,  it regulates the amount  of  light  entering  the  eye  so  that we  can  see as clearly as possible in the available light. 

In close vision and bright light,  the circular muscles contract, and  the pupil constricts.  In  distant vision and  dim light,  the radial fibers contract to enlarge (dilate) the pupil,  which allows more light to enter the eye.

➡️ RETINA:  The innermost  sensory layer  of the eye is the delicate two-layered  RETINA, which  extends  anteriorly  only  to  the  ciliary  body.

The outer  pigmented layer  of the retina is composed of pigmented cells that,  like those of the choroid,  absorb light and prevent light from scattering  inside the  eye.  They  also  act as  phagocytes to remove dead or damaged receptor cells  and store vitamin A needed for vision.

The transparent inner  neural layer  of the retina contains millions of receptor cells,  the  rods and  cones,  which are called  photoreceptors  because they respond to light .

PHOTORECEPTORS OF  THE RETINA 
➡️The  retina  contains  rods and cones  –photoreceptor  neurons that synapse  with bipolar  cells  .
 
➡️Rods are  responsible for  colorless vision in relatively  dim  light  . 
➡️Cones provide  color vision and  function  in day light  .

➡️ A light  sensitive  pigment  in rods (Rhodopsin) decomposes  in the  presence  of light  and triggers a complex  series  of reactions that  initiate  nerve impulses  on the  optic  nerve  . 

➡️3 sets of  cones (Red ,  blue ,  green) , provide color  vision . each set  contains  a different  light  sensitive  pigment  , and each  set is sensitive  to a  different  wave length  of light.  The color  perceived depends  on which set or  sets of cones  are stimulated  .

SPECIALISED AREA OF THE EYE 
➡️OPTIC DISK (blind spot)--no  vessels originate here. The vessels shadow the retina o Optic nerve fibers exit here o No photoreceptors.

➡️MACULA --area of the retina responsible for central vision and peripheral

  ➡️FOVEA CENTRALIS--center of the retina (where most of the cones are) 

➡️ LENS
the lens which is a transparent body located behind the iris. The lens is suspended by ligaments (called zonule fibers) attached to the anterior portion of the ciliary body. The contraction or relaxation of these ligaments as a consequence of ciliary muscle actions, changes the shape of the lens, a process called accommodation that allows us to form a sharp image on the retina.

➡️ACCOMMODATION , CLOSE & DISTANT VISION 

➡️ACCOMMODATION : As  the distant  object  moves closer  , the image  moves behind  the retina  to keep  the  image  sharply on the retina  , the lens  accommodates.

 ➡️ Close vision :  ciliary  muscles  contract,  lens ligaments (suspensory) relax,  and lens becomes  rounder (more convex).

 ➡️ Distant vision :  ciliary  muscles  relax,  lens ligaments (suspensory) contract,  lens becomes  less convex  (concave).

 Note  : with  age, the lens  hardens and is less able  to accommodate  . After age  55 , accommodation  is no longer possible  (presbyopia) , requiring  corrective  lenses  for reading  .
 Pupil  constriction :  During accommodation  , the  Iris also constricts  to narrow the pupil  , permitting  increased  depth of focus. For very close  objects  external  eye muscles  move  the eyeball  in word (converge) to  keep  sharp focus.

 ➡️ Convergence  :  The  movement  of each  eye  –ball  is controlled by six eye  muscles  that  allow the  eyes  to follow a moving object.

CLINICAL CORRELATIONS
➡️ Common eye defects include 
• myopia or nearsightedness where the eyeball is too long or the cornea is too  steep.

• hyperopia or far sightedness where the eyeball is short or lens cannot become round enough

• presbyopia where the muscles controlling the bulging of the lens become weak as we age

• cataracts where the lens becomes fogged

• nyctalopia or night blindness where vision is impaired in dim light  and in  the dark due to pigment rhodospin in the rods  not functioning properly External features of the eye

• Glaucoma where there is increased intra ocular pressure.

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