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
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