physiology exam APRIL 2026 cairo university

Q1. Define:

a- Stiles-Crawford effect. (5 marks)

 The Stiles-Crawford effect is the directional sensitivity of the retina, in which light rays entering through the center of the pupil produce a greater visual response than rays of equal intensity entering through the pupil margin. This is because cone photoreceptors are oriented toward the center of the pupil and act as optical waveguides, accepting axial light and rejecting oblique light.

b- Purkinje shift phenomenon. (5 marks)

The Purkinje shift is the shift of the eye’s peak spectral sensitivity toward shorter wavelengths as illumination decreases. It occurs because vision changes from cone-mediated photopic vision to rod-mediated scotopic vision.

Q2: Applied physiology (explain the following observations):

• a- Fatigue is not a feature of extra-ocular muscles. (5 marks)
• b- It is not wise to measure the refraction of the eye in uncontrolled diabetes. (5 marks)

a- Fatigue is not a feature of extra-ocular muscles. (5 marks)

1. Many mitochondria (high oxidative metabolism).

2. Rich blood supply (many capillaries).

3. Small motor units (1 nerve fiber supplies 3–10 muscle fibers).

4. Fatigue-resistant muscle fibers.

5. They work continuously (fixation and eye alignment). 

b- It is not wise to measure the refraction of the eye in uncontrolled diabetes. (5 marks)

1. High glucose enters the lens and is converted to sorbitol by aldose reductase.
2. Sorbitol draws in water, so the lens swells and its refractive index changes (myopic shift).
3. Refraction fluctuates, so glasses become inaccurate. Refract after blood sugar is stable.

Q3. What are the functions of the different layers of the tear film? (10 marks)

The tear film consists of three main layers:

1. Lipid layer (meibomian glands): reduces evaporation and gives a smooth optical surface.
2. Aqueous layer (lacrimal glands): nourishes the cornea, is antibacterial (lysozyme, lactoferrin, IgA), and washes away debris.
3. Mucin layer (goblet cells): makes the corneal epithelium hydrophilic and stabilizes the tear film.

Q4) Describe the factors maintaining corneal transparency (10 marks)

1. Regular arrangement of stromal collagen: regular spacing of fibrils causes destructive interference of scattered light.
2. Corneal deturgescence: the endothelial pump keeps the stroma relatively dehydrated.
3. Absence of blood vessels and myelin: the cornea is avascular and its nerves are unmyelinated, reducing scatter.
4. Corneal crystallins: present in keratocytes, they reduce light scatter.
5. Smooth corneal surface: smooth epithelium and tear film give a regular optical surface.

Q5) What are the theories of accommodation? (10 marks)

1. Helmholtz (the most accepted): Ciliary muscle contracts → zonules relax → lens becomes more spherical → power increases.

2. Tscherning: Ciliary muscle contracts → zonules tighten → lens center bulges.

3. Schachar: Zonular tension increases → lens periphery flattens → center steepens.

4. Catenary: Pressure difference between aqueous and vitreous acts on the lens-zonule diaphragm.

Q6) What are the factors affecting aqueous outflow? (10 marks).

Two pathways:

1. Conventional (90%): Trabecular meshwork → Schlemm's canal → episcleral veins. It depends mainly on pressure.

2. Uveoscleral (10%): Ciliary muscle → suprachoroidal space. It is less pressure-dependent.

Factors:

1. Trabecular meshwork: Increased resistance (age, pigment, inflammation, steroids) reduces outflow.

2. Schlemm's canal: Blockage or collapse reduces outflow.

3. Episcleral venous pressure: High pressure reduces outflow.

4. Ciliary muscle: Contraction (pilocarpine) opens the meshwork and increases conventional outflow.

5. Prostaglandins: Increase uveoscleral outflow.

6. Angle closure: The iris blocks the meshwork, so outflow is reduced or stopped.

    Q7)  How can you diagnose Horner syndrome and the level of the lesion using pharmacological agents? (10 marks)

1. Confirm Horner syndrome

• Cocaine 4%: Normal pupil dilates; Horner pupil does not dilate.
• Apraclonidine 0.5-1%: Horner pupil dilates and anisocoria is reversed.

2. Locate the lesion

• Hydroxyamphetamine 1%:
  • Pupil dilates → 1st or 2nd order lesion.
  • Pupil does not dilate → 3rd order lesion.
• Phenylephrine 1%: Horner pupil dilates due to denervation supersensitivity.

Note: Phenylephrine dilates the pupil in a 3rd order lesion (supersensitivity), so it helps confirm a postganglionic lesion.

(8) Describe the modern techniques of perimetry and their cellular physiological basis. (10 marks)

1. Standard automated perimetry (white-on-white): All retinal ganglion cells.

2. Short-wavelength automated perimetry (blue-on-yellow): Small bistratified ganglion cells.

3. Frequency doubling technology: Magnocellular ganglion cells.

4. Flicker and motion perimetry: Magnocellular ganglion cells.

5. High-pass resolution perimetry: Parvocellular ganglion cells.

Q (9) What are the requirements and advantages of binocular single vision? (10 marks)

Requirements:

1. Clear media and similar vision in both eyes.
2. Normal retinal correspondence.
3. Straight eyes with coordinated muscles.
4. Fusion (sensory and motor).
5. Healthy visual pathways and cortex.

Advantages:

1. Stereopsis (depth perception).
2. Wider visual field.
3. Binocular summation (better vision).
4. Covers the blind spot.
5. If one eye is lost, the other still works.

END EXAM APRIL 2026

GOOD LUCK