This lecture includes Anatomy and physiology of lens and accommodation. Kindly like it and share it with your colleagues. Thank You.

1. Kanwal Perveen
M.Phil Vision Sciences

2.  The lens is a transparent, biconvex structure situated behind
the iris and the pupil and in front of the vitreous body.
 It is composed of fibers that come from epithelial
cells.
 In fact, the cytoplasm of these cells makes up the
transparent substance of the lens.
 The crystalline lens is composed of 4 layers, from the
surface to the center:
 capsule,
 subcapsular epithelium,
 cortex, and
 nucleus.

3.  The center points on
the anterior and
posterior surface are
referred as anterior
and posterior poles
respectively.
 A line joining the
poles forms the axis
of lens.
 The circumference of
the lens is called the
equator.

4.  The lens capsule is a clear, membrane-like structure
that is quite elastic, a quality that keeps it under
constant tension.
 As a result, the lens naturally tends towards a
rounder or more globular configuration, a shape it
must assume for the eye to focus at a near distance.
 The dioptric power of the entire eye is about 58
diopter, with the cornea is responsible for most of this
refractive ability. The lens contributes only about 15
diopters to the total power.
 In the adults the lens measure approximately 10mm
in diameter and 4 mm thick. The lens is kept in
position by suspensory ligament.

5.  Suspensory Ligaments:
The lens is held in position by a series of delicate
radially arranged fibers collectively known as the
suspensory ligament of the lens, or zonule. The
zonular fibers arise from the epithelium of the
cilliary process and run toward the equator of the
lens.

7. The lens is made of three parts
An elastic capsule
The capsule of the lens is an elastic basement
membrane that envelops the entire lens and is
divided in to an anterior capsule and posterior
capsule.
That part of the lens capsule which covers the
anterior surface of the lens is called anterior
capsule and the one which covers the posterior
part is called posterior capsule.

9.  Anterior capsule is thicker than posterior capsule.
 It is produced by the lens epithelium anteriorly,
through out life zonules fibers that hold the lens in
its position are attached to the lens capsule at its
equator.
 The other end of these fibers is attached to the
cilliary body.

10. Lens epithelium:
It is a single layer of cuboidal cells that cover the
anterior surface of the lens.
There is no epithelium on the posterior surface.
It lies beneath the capsule. These cells are
capable of division.
The central cells are involved in transport of
metabolic substances needed by the lens from
the aqueous and the utilization of such
substances.
The epithelial cells at the equator divide and
transform in to elongated cells called lens fibers.

11.  It forms the main mass of the lens
 They originated from the lens epithelium. They
are arranged like the layer of an onion.
 Lens fibers are continuously produced throughout
life.
 The oldest fibers get collected in the center of the
lens as a single mass called the nucleus the
outer layer consist of newer fibers and constitute
the cortex.
 Each lens fiber is an elongated cell with a length
of 10mm.

12.  If cut across the cell has got the shape of
hexagonal.
 The central fibers in the nucleus do not have
nucleus while new fibers in the cortex retain their
nuclei.
 Lens fibers have many gaps in their membranes
because of these substances can easily move
from one fiber to another.

13.  Main function of the lens is to keep transparent
so that light can pass through it.
 Lens is not like a glass window which can only
transmit light but it can make that light focus on
the retina (The layer of eye that actually sees
light). This ability to focus light is the power of
the lens and it is about 15diopters.

14.  Main role in focusing light on the retina is played
by the cornea because the focusing ability of the
cornea is much more than lens. Total power of
the eye is 58 diopter.
 The human lens is capable of increasing the
focusing power for near and far vision. This
ability to increase the focusing power of eye is
called accommodation.

15.  Cilliary body is the main part involved in
accommodation, but it acts on the lens to make it
happen.
 Bear work constraction of cilliary body muscles.
 Relaxation / loosening of zonules increase
thickness of lens increase refractive power of eye.

16.  Main factors that keep the lens transparent are
 Regular arrangement of lens fibers
 No blood vessels in the lens
 no nerves no lymphatic

17.  The importance of the lens is that it can change its
dioptric power, allowing distance and near objects
to be focused on the retina. The range of dioptric
power is reduced with age.
 The lens grows through out life. It also increases
in thickness.

18. Nourishment:
The lens is metabolically active and requires nourishment in
order to maintain its growth and transparency.
Compared to other tissues in the eye, however, the lens has
considerably lower energy demands.
By nine weeks into human development, the lens is
surrounded and nourished by a net of vessels, the tunica
vasculosa lentis, which is derived from the hyaloid artery.
Beginning in the fourth month of development, the hyaloid
artery and its related vasculature begin to atrophy and
completely disappear by birth.
In the postnatal eye, Cloquet's canal marks the former
location of the hyaloid artery.
The human lens is 2/3 water and 1/3 solid.
Lens protein called crystalline make up most of the solids
of the lens.

19.  After regression of the hyaloid artery, the lens receives all its
nourishment from the aqueous humor.
 Nutrients diffuse in and waste diffuses out through a constant
flow of fluid.
 Lens needs glucose as a fuel.
 It can work in low Oxygen State but not in conditions of
decreased glucose. It gets its glucose from the aqueous is
used for different functions of the lens.
 Glucose is the primary energy source for the lens. As mature
lens fibers do not have mitochondria, approximately 80% of
the glucose is metabolized via anaerobic metabolism. The
remaining fraction of glucose is shunted primarily down
the pentose phosphate pathway.
 The lack of aerobic respiration means that the lens consumes
very little oxygen as well.

20. Accommodation
The ciliary body is circular, and the ciliary muscle within it is a
sphincter muscle, shaped like a tiny doughnut.
The inside diameter of the muscle gets smaller when it contracts and
larger when it relaxes.
When the eye is viewing an object at a far distance (such that parallel
rays of light are entering the eye), the ciliary muscle within the ciliary
body relaxes. The ciliary processes pull on the suspensory ligaments (or
zonules), which in turn pull on the lens capsule around its equator.
This causes the entire lens to flatten or to become less convex,
enabling the lens to focus light from the far-away object.

21.  Conversely, when the eye views an object at a near, an
“accommodative demand” is created.
 As a result, the ciliary muscle works or contracts, causing
tension to be released on the suspensory ligaments and,
subsequently, on the lens capsule.
 This causes both (front and back) lens surfaces to
become more convex and the eye to be able to focus at
near.
 This adjustment in lens shape, to focus at various
distances, is referred to as “accommodation” or the
“accommodative process” and is associated with a
concurrent constriction (decrease in size) of the pupil.

22.  The change in stance of the ciliary body,
crystalline lens, and pupil as the eye looks back
and forth between far and near.
 The “amplitude of accommodation” of an eye
is the maximum amount that the eye’s crystalline
lens can accommodate (change shape),
in diopters (D).
 This amount is very high when a person is young
and decreases with age.
 The amplitude of accommodation is equivalent to
the inverse (reciprocal) of the distance (“near point
of accommodation”) at which the emmetropic eye
can focus clearly.

23. Thank You