Examkin

Question

Difficulty: HardNervous Coordination and Sense Organs

When a person rapidly shifts their visual gaze from a distant object on the horizon to read fine print in a book held close to the face, which of the following physiological responses occurs within the mammalian eye to bring the near object into sharp focus on the retina?

  1. The ciliary muscles contract, causing the suspensory ligaments to slacken, which allows the elastic lens to become thicker and more convex.Answer
  2. B
    The ciliary muscles relax, causing the suspensory ligaments to tighten, which pulls the lens into a thinner and less convex shape.
  3. C
    The ciliary muscles contract, causing the suspensory ligaments to tighten, which pulls the lens into a thicker and more convex shape.
  4. D
    The ciliary muscles relax, causing the suspensory ligaments to slacken, which allows the lens to become thinner and less convex.

Answer

Near vision accommodation occurs when the ciliary muscles contract, tension on the suspensory ligaments decreases (slackens), and the crystalline lens becomes thicker and more convex to increase light refraction.
Accommodation for near vision requires the eye to increase its refractive power. When focusing on a near object, parasympathetic nerve signals cause the ciliary muscles to contract. Because the ciliary muscle forms a sphincter ring surrounding the lens, its contraction narrows the ring and reduces the outward pull on the suspensory ligaments (zonules of Zinn). With tension removed from the ligaments, the natural elasticity of the lens causes it to recoil into a thicker, more convex shape, sharply focusing divergent light rays onto the retina.

Step-by-Step Solution

1
Identify the visual requirement for focusing on a near object.
Light rays arriving from a near object are divergent and require stronger refraction (greater bending power) to converge accurately on the retina.
Near objects require a shorter focal length and higher optical refractive power.
2
Determine the required lens shape change for increased focal power.
The lens must become more spherical (thicker and more convex).
Increased curvature of the lens increases its refractive index and shortens the focal distance to focus light onto the retina.
3
Analyze the mechanical action of the ciliary muscles and suspensory ligaments.
Contraction of the sphincter-like ciliary body moves it closer to the lens, releasing tension on the suspensory ligaments and allowing lens bulge due to its natural elasticity.
Ciliary muscle contraction relaxes suspensory ligament strain, enabling maximum lens convexity.

Key Concept

Mechanism of visual accommodation in the mammalian eye
Rate this question

Topics

Nervous Coordination and Sense Organs

More questions from this topic