Abnormalities of Refraction &sensation of smell 1

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Abnormalities of Refraction : 

Abnormalities of Refraction physiology

Emmetropia/Normal Vision : 

Emmetropia/Normal Vision Eye is considered to be normal if parallel light rays from distant objects are in sharp focus on retina when ciliary muscles are completely relaxed.

Hyperopia/Farsightedness : 

Hyperopia/Farsightedness May be due to, Too short eyeball or too weak lense system. Parallel light rays are not bent sufficiently by the relaxed lens system to come to focus by the time they reach to retina.

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Treatment : Treated by using the convex lense.

Myopia / Nearsightedness : 

Myopia / Nearsightedness Ciliary muscles are completely relaxed, light rays coming from distant objects are focused in front of retina.

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Treatment : Treated by use of concave lens.

Astigmatism : 

Astigmatism When an eye is astigmatic it lacks a uniform surface which results in the inability to have one focal point. Usually astigmatism is the combination of both myopia and hyperopia and therefore both distance and near objects are blurry.

Presbyopia: : 

Presbyopia: Presbyopia is an age-related vision problem that typically begins after age 40 and worsens with increasing age. As you age, your eye’s lens loses the elasticity it needs to focus on close-up objects and often reading glasses are needed.

Smell / Olfaction : 

Smell / Olfaction

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Sense of smell is a chemical sense mediated by chemoreceptor's of olfactory mucosa.

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They are called chemical senses because they detect chemicals in the environment, with the difference being that smell works at dramatically larger distances than that of taste.

Primary sensations of smell : 

Primary sensations of smell Camphoraceous Ethereal Floral Musky Peppermint Pungent Putrid

Olfactory membrane / Mucosa : 

Olfactory membrane / Mucosa Lies in superior part of each nostril. Specialized receptors are contained in the olfactory neuroepithelium( confined to roof of nasal cavity).

Olfactory cells : 

Olfactory cells These are receptor cells for smell sensation. Mucosal end of each olfactory cell forms a knob from which 6-12 olfactory hairs / cilia projects into mucus that coat th inner surface of nasal cavity.

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Cilia reacts to odor in the air & stimulate the olfactory cells. These secrete the mucus. If olfactory neuroepithelium becomes inaccessable to sniffed air( chronic nasal obstruction due to edema of the mucosa) total loss of smell or anosmia takes place.

Olfactory Adaptation : 

Olfactory Adaptation Olfactory adaptations decreases very rapidly with continual exposure to an odorant, called the olfactory adaptation.

Neural Mechanism : 

Neural Mechanism Nerve fibers from olfactory region of brain pass backward along the olfactory tract of granule cells of olfactory bulb which causes the olfactory adaptation.

How the sense of smell works : 

How the sense of smell works The process of smelling goes more or less like this: 1. Vaporized odor molecules (chemicals) floating in the air reach the nostrils and dissolve in the mucus (which is on the roof of each nostril).

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2. Underneath the mucus, in the olfactory epithelium, specialized receptor cells called olfactory receptor neurons detect the odor. These neurons are capable of detecting thousands of different odors.

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3. The olfactory receptor neurons transmit the information to the olfactory bulbs, which are located at the back of the nose.

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4. The olfactory bulbs has sensory receptors that are actually part of the brain which send messages directly to: The most primitive brain centers where they influence emotions and memories (limbic system structures), and “Higher” centers where they modify conscious thought (neo-cortex).

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5. These brain centers perceive odors and access memories to remind us about people, places, or events associated with these olfactory sensations.

Theories for Excitation of Olfactory Cells : 

Theories for Excitation of Olfactory Cells Two theories are there. Physical theory Chemical theory

Physical Theory : 

Physical Theory Physical shape of the odorant molecule determine, which olfactory cell will be stimulated.

Chemical Theory : 

Chemical Theory Odorant molecule bind specifically to the protein receptor in the membrane of olfactory cell. Increase in the permeability of the olfactory ciliary membrane, so receptor potential in olfactory cells is excited & action potential is produced in nerve fiber.

Olfactory Disorders : 

Olfactory Disorders Anosmia: loss of sense of smell is called the anosmia. Hyperosmia Increase in the olfactory sensation is known as hyperosmia.:

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