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 visionCons 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.