Physiology-Case study

We will study the physiology of eyes base on case study;

1. Normal sight

2. Change the nature of object’s light.

3. Dust and water

4. Strong sun light

5. Pathogens attack

6. “Blind spot”

7. “Hollow hand”

8. Optical illusion

Physiology-Case study

Normal Sight

When we see an object, the light entering our eyes. The light then refracted when

passing through cornea and anterior cavity. Then, the light passing through the lens and

refracted again to focus onto fovea centralisi on retina. Then when reach the retina, the

photoreceptor receive the light, transduce the light energy( electromagnetic spectrum) into

nerve impulseii.

Change the nature of object

1. Change the distance from actual location.

The eye need to maintain the image of object focussing on fovea centralis called

accommodation. When we put the object closer to the eye, the cilliary muscle around the

lens will be contractediii, hence the lens become thicker; increase the refraction angleiv of

light. When we put the object away from eye, the cilliary muscle will be relaxed and the

lens become thinner, decrease the refraction angle of the light.

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Fovea centralis

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Physiology-Case study

2. Change the brighteners of object

The iris compose of 2 muscles, circular(11) and radial muscle(7). In bright light, the

circular muscle will be contracted, and the pupil become smaller. This is parasympathetic

reaction, the motor nerve from superior colliculus (9) (ocumulator nerve-10). In dim light,

the radial muscles wil be contracted, and the pupil become bigger. This is sympathetic

reaction, the motor nerve from superior cervical.

i Fovea centralis is the sharpest visual field. There are 3 factors that contribute to the sensitivity of fovea centralis;

1. Location on retina straight to the lens at posterior eyeball, at the yellow part called macula lutea.

2. Contain of rods rather than cods. Rods is shaper receptors than cods.

3. Shape; a pit if retina cause by displacement of neural layers, light fall down directly on pigmented epithelium.

ii Caused by chemical change in molecules in pigment epithelium and photoreceptors.

iii When the light reach the retina, photoreceptors alerts to the changes and sends the impules to the optic nerve, CN II. Optic nerve transmits the impulse to the virtual reflex. Then, the superior colliculus at visual reflexes sebd the signals to the motor neurons to the cilliary muscle. This reaction is parasympathetic. But dilation of lens (relaxation) are sympathetic reaction; the nerve will reach to virtual cortex, not virtual reflex.

iv Same as refractive index concepts. To reflex the light, we need 2 medium with different refractive index. At atmosphere, the refractive index is 1.00, cornea 1.38, anterior chamber1.33 and lens 1.40. The bigger the differentness, the higher the refraction angle.

Physiology-Case study

Physiology-Case study

3. Change the location of object at same distance.

There are six muscles that responsible to control the location of our pupil. There are

4 rectus and 2 oblique muscle.The nerve that supply the signals are CN VI for lateral

rectus, CN IV for superior oblique and CN III for others muscles. (CN VI- Abducent

nerve, CN IV- Trochlea nerve, CN III- Oculomotor nerve ). When the object move

upwards, the oculumotor nerve supply signals to the superior rectus , and cause

contraction hence the pupil move upwards. I f the object move downwards, the

ocumulator nerve supply signals again to the inferior rectus cause contraction and the

pupil move downwards. If the object move laterally, the abducent nerve supply signals to

the lateral rectus and cause contraction and the pupil move laterally. If the object move

medially, the ocumulator nerve will supply signals to medial rectus cause contraction and

the pupil move medially. How about oblique muscles? The oblique muscles balance the

medial pull of the superior and inferior rectus, to move the pupil laterally (abduction).The

inferior oblique directs the pupil laterally and superiorly, therefore when it works

synergistically with superior rectus, pure elevation of the pupil occurs. Similarly, the

superior oblique directs the pupil inferiorly and laterally, therefore when it works

synergistically with superior rectus, pure depression of the pupil occurs. The inferior

oblique contract when receive signals from ocumulator nerve while superior oblique

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Physiology-Case study

contract when receive signals from trochlea nervev. To easier remembering the nerve

supply to respective muscle; LR6,SO4,AO3 (Lateral rectus CN6, Superior Oblique

CN4,All others CN3.

v The nerve named as “trochlea” as close to the trochlea ( bone structure) that hook the tendon of superior oblique. This is to sure the direction force of superior oblique for pure elevation.

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Physiology-Case study

Dust and water

While we are playing the sand at the shore, our eyes are easily exposed to teh

sand and salty water. How can we prevent this happen? The eyebrow and eyelash

prevent the sand and water from touch the eyeball-conjunctiva mechanically. It is not

impossible for the sand and water reach the eyeball. The lacrimal gland will release

the lacrimal fluidvi-tears and slip away the dust and water. However, the changing of

npressure on cornea, cause the signals will be transmitted to the orbicularis oculi

muscle, closing the eyelid due to contraction of those musclevii.

vi Nerve that give signals for releasation of fluid is ophthalmic nerve CN5.

vii Opthalmic nerve, CN5 also branched to frontal nerve that supply signals to orbicularis oculi muscles.

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Physiology-Case study

Strong sunlight

We are not advised to see the sun directly. Our retina have limitation to detect

the lightviii(P7). Strong uv light can damage our retina (also the x-rays and gamma

rays). Hence, when we expose to the sunlight, the retina will esnd impulse via CN2

DIRECT TO SUPERIOR COLLICULUS. Then from superior colliculus, the signal

will transmited via oculomotor nerve (parasympathetic

reaction), contracting the circular muscle of iris, smaller the pupil. Otherwise,

the signals also supply to frontal nerve and cause contraction of orbicularis muscle,

cause the closing of eyelid.

viii The light can classified by electromagnetic spectrum. The lower the wavelength, the higher the ability to damaging the retina.

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