Accomodation Reflex

ACCOMMODATION REFLEX

Accommodation is a reflex action. When a person looks at a near object after seeing a far object, three

adjustments are made in the eyeballs:

1. Convergence of the eyeballs due to contraction of the medial recti

2. Constriction of the pupil due to the contraction of constrictor pupillae of iris

3. Increase in the anterior curvature of the lens due to contraction of the ciliary muscle.

Thus, the accommodation reflex involves both skeletal muscle (medial recti) and smooth muscle (ciliary muscle and sphincter pupillae). During accommodation, all the adjustments are carried out simultaneously. Although accommodation is a reflex action, it can be controlled by willpower to a certain extent.

PATHWAY FOR ACCOMMODATION REFLEX

Afferent Pathway

Visual impulses from retina pass through the optic nerve, optic chiasma, optic tract, lateral geniculate

body and optic radiation to visual cortex (area 17) of occipital lobe. From here, the association fibers carry

the impulses to frontal lobe.

Center

The center for accommodation lies in frontal eye field (area 8) that is situated in the frontal lobe of cerebral Cortex 

Efferent Pathway

1. Efferent fibers to ciliary muscle and sphincter pupillae From area 8, the corticonuclear fibers pass via

internal capsule to the Edinger-Westphal nucleus of third cranial nerve. From here, the preganglionic fibers pass through the third cranial nerve to ciliary ganglion. Postganglionic fibers from ciliary ganglion pass via the short ciliary nerves and supply the ciliary muscle and the constrictor pupillae.

2. Efferent fibers to medial rectus Some of the fibers from frontal eye field terminate in

the somatic motor nucleus of oculomotor nerve. The fibers from motor nucleus supply medial rectus.

RANGE AND AMPLITUDE OF ACCOMMODATION

The farthest point from the eye at which the object can be seen is called far point or punctum remotum. In the normal eye, it is infinite, i.e. at a distance beyond 6 meters or 20 feet. It is limited only by the size of

object, clearness of the atmosphere and the curvature of earth. The nearest point from eye at which the object is seen clearly is called near point or punctum proximum. It is about 7 to 40 cm, depending upon the age. Distance between far point and near point is called range of accommodation.

Since, the focal length of eye is different in near vision and far vision, the refractive power of eye is also

altered. The refractive power during far vision is called static refraction (R) and that during near vision is called dynamic refraction (P). The difference between these two refractive powers (P – R) is called amplitude of accommodation, which is expressed in diopter. The refractive power is reciprocal of focal length and the unit for focal length is 1 meter or 100 cm. The refractory power is expressed as diopter (D).

For example, in a normal eye, if the near point is 10 cm, the dynamic refraction is:

1 meter 100 cm

P = = = 10 D

10 cm 10 cm

In emmetropic (normal) eye, since the far point is at infinite distance, the static refraction is taken as zero.

Now,

Amplitude of accommodation = P – R

= 10 – 0

= 10 D

Amplitude of Accommodation at Different Ages

Amplitude of accommodation varies with age. Amplitude of accommodation at different age groups is:

10 years = 11.0 D

20 years = 9.5 D

30 years = 7.5 D

40 years = 5.5 D

50 years = 2.0 D

60 years = 1.2 D

70 years = 1.0 D