Explain the pupil reflex in terms of the antagonistic action of circular and radial muscles in the iris

The pupil reflex is a critical mechanism that regulates the amount of light entering the eye, and it operates through the antagonistic action of two sets of muscles in the iris: the circular muscles and the radial muscles.

Antagonistic Muscles in the Iris

1. Circular Muscles (Sphincter Pupillae)
   • These muscles are arranged in a circular pattern around the pupil.
   • When they contract, they cause the pupil to constrict (reduce in diameter), allowing less light to enter the eye. This contraction occurs in response to bright light conditions, protecting the retina from excessive light exposure.
2. Radial Muscles (Dilator Pupillae)
   • The radial muscles run outward from the pupil like spokes on a wheel.
   • When these muscles contract, they cause the pupil to dilate (increase in diameter), permitting more light to enter the eye. This dilation occurs in low-light conditions, enhancing visibility by maximizing light entry.

Mechanism of Action

• In Bright Light:
  • Light receptors in the retina detect increased light intensity.
  • The circular muscles contract while the radial muscles relax. This results in pupil constriction (miosis), which limits light entry and protects sensitive retinal cells.
• In Dim Light:
  • When light levels decrease, the radial muscles contract while the circular muscles relax.
  • This action leads to pupil dilation (mydriasis), allowing more light to enter and improving vision under low-light conditions.

Summary of Pupil Reflex Actions