Download Sensory Physiology

Sensory Physiology
The Vision
Accommodation
Blind spot
Introduction…
The blind spot:
the optic disc is called the blind spot
because no photoreceptors are present ,
only neural tissue.
Rods and Cons : photoreceptors in
the retina.
Rods for night vision or dim vision
Cons acute vision and color vision
In order to achieve clear vision, light reflected from
objects within the visual field is focused onto the
retina of each eye.
The processes involved in producing a clear
image are:
Refraction of the light rays.
Changing the size of the pupils.
Accommodation .
1. Refraction of light rays
When light rays pass from
a medium of one density to
a medium of a different
density they are bent.
In our eyes the biconvex
lens bends and focuses
light rays.
2. Size of the pupils
Pupil size
influences
accommodation by
controlling the
amount of light
entering the eye.
3. Accommodation
What is accommodation?
Accommodation is to change the focal
length of the lens by changing the
curvature of the eye lens.
Why does our eye need accommodation?
Normally, when our ciliary muscles are
relaxed, parallel rays from distant objects
will converge into the retina.
If our eye is maintained at the above state,
and a near object is put before it, light rays
will converge behind the retina.
As the sharp image is behind the retina,
our brain can only detects a blurry vision.
To bring the image into focus, our eye does
accommodate.
How does are eye accommodate?
Our eye changes the curvature of the
lens, rather than the cornea.
The curvature of the cornea cannot
be changed.
In order to focus on near objects, accommodation
is required and the eye must make the following
adjustment:
Constriction of the pupils.
Convergence.
Changing the power of the lens.
Constriction of the pupils
Convergence
Changing the power of the lens
In normal resting state:
Our ciliary muscle is relaxed.
Suspensory ligaments are tensed.
Lens pulled into a thin shape.
When a near object is brought to your
eyes:
Contraction of ciliary muscles.
Relaxation of suspensory ligament.
Lens become thicker.
Increase in the refractive power.
Constriction of pupils due to sphincter-like
action of circular muscle fibers.
What is the near response?
It is the same as the accommodation
reflex.
During accommodation for near
vision, the pupils constrict and the
eyes converge, in addition to increase
in the convexity of the lens.
This 3-part response is called the
near response or the accommodation
reflex.
What is the far point and the near point?
Far point: it is the farthest point from
the eye at which an object is seen
clearly.
This point is infinity in the normal eye.
A distance of 6 meters from the eye is
considered as the practical far point
because light rays from this distance
are parallel.
Near point: the nearest distance from
the eye at which an object can be
seen clearly.
Presbyopia…
When we grow old,
our lens will turn
hard.
Our
accommodation
ability will decrease
and it will get more
and more difficult
to focus.
Presbyopia: (Greek for “old man’s
eyes” )
It is a refractive error of the eye in
which the amplitude of
accommodation is decreased.