QS Study

Accommodation

The process of adjustment of optical apparatus for the near vision is called accommodation. The accommodation reflex occurs as we focus on a near object. In order to achieve a clear, precise projection onto the fovea of the retina, several different events must occur simultaneously:

  • Convergence of the eyes – to maintain a singular binocular vision of near object,
  • Thickening of the lens – to increase refractive power,
  • Constriction of the pupil – to sharpen the image by increasing the depth of focus

During accommodation, the following changes occur –

Convergence of eyeball, Contraction of the ciliary muscle, Relaxation of the suspensory ligament, Increase curvature of the lens, constriction of the pupil to increase the depth of focus etc.

Mechanism

  1. Contraction of ciliary muscles,
  2. Relaxation of the suspensory ligament,
  3. Relaxation of the lens capsule,
  4. ↑ anterior curvature of the lens,
  5. ↑ the refractive power,
  6. The lens becomes capable of focusing nearer object.

Structure involves in it:

  1. Ciliary muscle,
  2. Suspensory ligament,
  3. Lens capsule,
  4. Lens,
  5. Iris.

Fig.: Mechanism of accommodation (focusing).

Accommodation reflex

It comprises the 3 part:

  1. Accommodation,
  2. Constriction of the pupil,
  3. Convergence of the eyeball.

Pathway: light fall on the cornea → refractive media of eyeball → received by rods and cones → optic nerve → Optic chiasma → optic tract → lateral geniculate body → optic radiation → Visual cortex → Superior longitudinal fasciculus → Frontal eyebield → internal capsule → oculomotor nucleus, then,

→  Edinger Westphal nucleus → ciliary muscle → Contraction.

→ Ventral nucleus → medial rectus muscle → convergence of eyeball.