Cataracts

“Cataracts” are an especially common eye abnormality

that occurs mainly in older people.A cataract is a cloudy

or opaque area in the lens. In the early stage of

cataract formation, the proteins in some of the lens

fibers become denatured. Later, these same proteins

coagulate to form opaque areas in place of the normal

transparent protein fibers.

When a cataract has obscured light transmission so

greatly that it seriously impairs vision, the condition can

be corrected by surgical removal of the lens.When this

is done, the eye loses a large portion of its refractive

power, which must be replaced by a powerful convex

lens in front of the eye; usually, however, an artificial

plastic lens is implanted in the eye in place of the

removed lens.

A cataract is a cloudy or opaque area in the lens of the eye. Cataracts usually develop as a person gets older and may run in families. Other environmental factors such as smoking or exposure to toxic substances can also accelerate the development of a cataract. Cataracts can cause visual problems such as difficulty seeing at night, seeing halos around lights, and sensitivity to glare.

Astigmatism

Astigmatism is a refractive error of the eye that causes the visual image in one plane to focus at a different distance from that of the plane at right angles. This most often results from too great a curvature of the cornea in one plane of the eye.

Astigmatism is the most common refractive problem responsible for blurry vision. You may also hear astimatism called a refractive error.Most of the eyeball's focusing power occurs along the front surface of the eye, involving the tear film and cornea (the clear 'window' along the front of the eyeball). The ideal cornea has a round surface. Anything other than round contributes to abnormal corneal curvature -- this is astigmatism. Here's a good way to demonstrate the effects of astigmatism. Look at your reflection in the curved surface of a round soup spoon and compare it with your reflection in an oval teaspoon.The cornea is the transparent window over the colored part of the eye. It bends (refracts) light rays and helps focus the light onto the retina in the back of the eye so people can see. When the cornea is oblong shaped, it causes light rays to focus on two different points on the retina, instead of just one. As a result, people with significant astigmatism may have distorted or blurry vision.

Signs and symptoms of astigmatism may include:

• Distortion in portions of your visual field
• Blurred vision
• Eyestrain
• Headaches

Function of the Cerebellum in Overall Motor Control

The nervous system uses the cerebellum to coordinate motor control functions at three levels, as follows:

1. The vestibulocerebellum. This consists principally of the small flocculonodular cerebellar lobes (that lie under the posterior cerebellum) and adjacent portions of the vermis. It provides neural circuits for most of the body’s equilibrium movements.

2. The spinocerebellum. This consists of most of the vermis of the posterior and anterior cerebellum plus the adjacent intermediate zones on both sides of the vermis. It provides the circuitry for coordinating mainly movements of the distal portions of the limbs, especially the hands and fingers.

3. The cerebrocerebellum. This consists of the large lateral zones of the cerebellar hemispheres, lateral to the intermediate zones. It receives virtually all its input from the cerebral motor cortex and adjacent premotor and somatosensory cortices of the cerebrum. It transmits its output information in the upward direction back to the brain, functioning in a feedback manner with the cerebral cortical sensorimotor system to plan sequential voluntary body and limb movements, planning these as much as tenths of a second in advance of the actual movements. This is called development of “motor imagery” of movements to be performed.

Coordination and control of voluntary movement.

Associated Signs and Symptoms:-

• Tremors.
• Nystagmus (Involuntary movement of the eye).
• Ataxia, lack of coordination.

Vestibulocerebellum—Its Function in Association with the Brain Stem and Spinal Cord to Control Equilibrium and Postural Movements

The vestibulocerebellum originated phylogenetically at about the same time that the vestibular apparatus in the inner ear developed. loss of the flocculonodular lobes and adjacent portions of the vermis of the cerebellum, which constitute the vestibulocerebellum, causes extreme disturbance of equilibrium and postural movements. We still must ask the question, what role does the vestibulocerebellum play in equilibrium that cannot be provided by other neuronal machinery of the brain stem? A clue is the fact that in people with vestibulocerebellar

dysfunction, equilibrium is far more disturbed during performance of rapid motions than

during stasis, especially so when these movements involve changes in direction of movement and stimulate the semicircular ducts. This suggests that the vestibulocerebellum is especially important in controlling balance between agonist and antagonist muscle contractions of the spine, hips, and shoulders during rapid changes in body positions as required by the vestibular apparatus. One of the major problems in controlling balance is the amount of time required to transmit position signals and velocity of movement signals from the different

parts of the body to the brain. Even when the most rapidly conducting sensory pathways are used, up to 120 m/sec in the spinocerebellar afferent tracts, the delay for transmission from the feet to the brain is still 15 to 20 milliseconds. The feet of a person running rapidly can move as much as 10 inches during that time. Therefore, it is never possible for return signals

from the peripheral parts of the body to reach the brain at the same time that the movements actually occur. How, then, is it possible for the brain to know when to stop a movement and to perform the next sequential act, especially when the movements are performed rapidly? The answer is that the signals from the periphery tell the brain how rapidly and in which

directions the body parts are moving. It is then the function of the vestibulocerebellum to calculate in advance from these rates and directions where the different parts will be during the next few milliseconds. The results of these calculations are the key to the

brain’s progression to the next sequential movement. Thus, during control of equilibrium, it is presumed that information from both the body periphery and the vestibular apparatus is used in a typical feedback control circuit to provide anticipatory correction of postural motor signals necessary for maintaining equilibrium even during extremely rapid motion, including

rapidly changing directions of motion.

Cerebellum and Its Motor Functions

The cerebellum, has long been called a silent area of the brain, principally because electrical excitation of the cerebellum does not cause any conscious sensation and rarely causes any motor movement. Removal of the cerebellum, however, does cause body movements to

become highly abnormal. The cerebellum is especially vital during rapid muscular

activities such as running, typing, playing the piano, and even talking. Loss

of this area of the brain can cause almost total incoordination of these activities

even though its loss causes paralysis of no muscles.

But how is it that the cerebellum can be so important when it has no direct

ability to cause muscle contraction? The answer is that it helps to sequence

the motor activities and also monitors and makes corrective adjustments in the

body’s motor activities while they are being executed so that they will conform to

the motor signals directed by the cerebral motor cortex and other parts of the

brain.

The cerebellum receives continuously updated information about the desired

sequence of muscle contractions from the brain motor control areas; it also

receives continuous sensory information from the peripheral parts of the body,

giving sequential changes in the status of each part of the body—its position,

rate of movement, forces acting on it, and so forth. The cerebellum then compares

the actual movements as depicted by the peripheral sensory feedback

information with the movements intended by the motor system. If the two do

not compare favorably, then instantaneous subconscious corrective signals are

transmitted back into the motor system to increase or decrease the levels of

activation of specific muscles.

The cerebellum also aids the cerebral cortex in planning the next sequential

movement a fraction of a second in advance while the current movement is still

being executed, thus helping the person to progress smoothly from one movement to the next. Also, it learns by its mistakes—that is, if a movement does not occur exactly as intended, the cerebellar circuit learns to make a stronger or weaker movement the next time. To do this, changes occur in the excitability of appropriate cerebellar neurons, thus bringing subsequent

muscle contractions into better correspondence with the intended movements.