Background: According to modern concepts, the implementation
of the accommodation mechanism in humans involves such eye structures as the
iridocyclolenticular complex, the vitreous body and extraocular muscles.
However, a key role in the mechanism of accommodation is played by changes in
the shape of the crystalline lens.
Purpose: Consider the energy-saving of the mechanism of crystalline lens
accommodation and justify the hydraulic component in its implementation.
Materials and Methods: Various theories of the mechanism of accommodation were
analyzed taking into account data from ultrasound biomicroscopy, magnetic
resonance imaging and optical coherence tomography. The mechanism of rapid
decrease in pressure in the anterior and posterior chambers of the eye during
contractions of the meridional portion of the ciliary muscle was considered.
Particular emphasis was placed on the participation of mechanosensitive
aquaporins in the exchange of intraocular fluid and ensuring pressure in the
closed hydrostatic system of the lens, exceeding the level of IOP.
Results: An analysis of various theories of accommodation showed directly
opposite views about the participation of the ciliary muscle and ligamentous
apparatus in the implementation of the mechanism of the crystalline lens
accommodation. In this work, we are the first to consider the crystalline lens
as a unique closed hydrostatic system in which the pressure level is
established due to ultrafiltration and diffusion of intraocular fluid with the
participation of aquaporins. Aquaporins form ion channels in the capsule,
cuboidal epithelial cells, the crystalline lens fibers and are mechanosensitive
receptor proteins. The opening and closing of ion channels regulates the
potassium-sodium pump, directed transport and exchange of intraocular fluid in
the crystalline lens. The hydrostatic balance between the pressure in the
crystalline lens and the anterior and posterior chambers of the eye is ensured
by the lens capsule. The capsular bag of the crystalline lens can be considered
as a curved diaphragm that separates two hydrostatic systems with different
levels of pressure. Due to the hydrostatic buffering effect, the IOP level does
not affect the crystalline lens, but it responds to a rapid decrease. This
decrease in pressure in the anterior and posterior chambers occurs with
contraction of the meridional portion of the ciliary muscle, tension of the
scleral spur and activation of the valve mechanism of the scleral sinus. The
greater the decrease in pressure, the more convex the crystalline lens takes on
and increases its refraction.
Conclusion: The presence of a hydraulic component in the mechanism of
crystalline lens accommodation allows us to understand how the contraction of
the small ciliary muscle can change the shape and refractive power of the large
crystalline lens.
Author(s) Details:
Igor Kornilovskiy
Department of Ophthalmology, Institute for Postgraduate
Medical Education, National Medical and Surgical Center Named after N.I.
Pirogov Ministry of Health of the Russian Federation, Moscow, Russia.
Please see the link here: https://stm.bookpi.org/NVMMS-V6/article/view/14288
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