Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009
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Physiology |
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long ciliary nerve (Thus, avoiding ciliary ganglion). Entering the eyeball by piercing sclera on either side of the optic nerve run along with the long ciliary arteries between the choroid and sclera, enter the ciliary body and thus, reach the iris.
Pupillary Reflexes
Reflexes that Constrict the Pupil
Light reflex: Whenever the intensity of illumination increases above a threshold value within a certain minimum period of time, the pupil constricts. The constriction of the pupil when light enters the eye is called the direct reflex. The constriction of opposite side pupil is called the consensual light reflex. This is an involuntary reflex, it has a threshold value in both time and intensity.
Direct Light Reflex
a)The chief factor that determines what effect a given change in illumination will have on the pupil is state of adaptation of the retina.
b)The second factor is the portion of the retina stimulated. Not all parts of the retina are of equal value in producing light reflex. The effectiveness of various portions of the retina in causing contraction of the pupil parallels the sensitivity to light. In the dark adapted eye the perimacular area is the
most sensitive: in the light adapted, the fovea produces greatest pupillary response. This shows that the receptors for pupillo – motion are the same as or similar to light receptors.
The latent period of the light reflex is 0.2sec, with moderately strong stimulation. The response of the pupil depends upon the duration of the stimulus as well as on the intensity. Following maximum contraction the pupil dilates slightly. The contraction of the pupil to light may be attenuated by repeated stimuli, due to the adaptation of the retinal receptors. The rate of constriction shows considerable individual variation. In response to bright light, the average pupil reaches its maximum contraction in less than 5 seconds.
Consensual Light Reflex
The stimulation of one retina by light produces a contraction of the pupil in the opposite eye in all animals in which there is partial decussation of the optic nerve fibres in the chiasma. The latent period in mean is the same as that of direct reflex. If the light is thrown simultaneously into both eyes, the response is greater than that which results from stimulation of one eye, for the direct and consensual responses summate.
Afferent Pathway
The outer limbs of the rods and cones are the receptor cells and the fibres follow the course of visual fibres, decussate in the chiasma; travel in the optic tract near the lateral geniculate body where they leave the optic tract to enter the anterior colliculi and proceed to the pretectal region. Here the fibres synapse
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with cells of the pretectal region whose axons connect the cells of the Edinger Westpal nucleus. The intercalated neurons from each side of the pretectum are distributed equally to the 3rd nerve nuclei of both sides. The intercalated fibres destined for the opposite 3rd nerve nucleus cross dorsal to the central aqueduct in the posterior commissure, then in company with axons arising on that side, arch ventrally to reach the 3rd nerve nucleus.
Efferent Pathway
The 3rd nerve nucleus consists of 3 parts:
1.Large celled nucleus typical somatic motor cells, subdivided into dorsal, ventral and scattered groups.
2.Peritia’s nucleus, typical efferent cells lying between the right and left large celled masses.
3.Edinger Westpal nucleus cells of visceral motor type.
Accommodation–Convergence Reaction–Near Reflex
When a person is requested to direct his eyes to an object held close to the face, his pupils contract. It is not a true reflex but an associated movement or synkinesis. The three components, the sphincter pupillae, the ciliary muscle and the medial recti are dependent on supranuclear connections.
Trigeminal Reactions (Occulo Pupillary Reflex)
Constriction of the pupil is initiated in any way e.g., any marked irritation of the cornea or conjunctiva results in constriction of the pupil. Both the pupils may be constricted, but the pupil of the affected side is narrowed to a greater extent.
Reflexes that Dilate Pupil
1.Withdrawal of Light: The dilatation of the pupil in the dark is due to relaxation of the sphincter pupillae and constriction of dilator pupillae.
2.Stimulation of Sensory Nerve: causes dilatation of the pupil especially, if it is a painful stimulus due to simultaneous sympathetic activity and parasympathetic activity.
3.Vestibular Stimulation: Rotatory or caloric stimulation causes dilatation of the pupil during and for a short time after cessation of the stimulus, as some fibres of the sympathetic nerves connected with the iris muscle probably run through the middle ear.
4.Psychic Stimulation: Emotional states, such as fear and anxiety cause dilatation of the pupil caused by the stimulation of the sympathetic nerves and the inhibition of the Edinger Westpal nucleus, blood borne epinephrine may enhance the reaction.
33
Microbiology
INTRODUCTION
Microbiology is the science of living organisms that are directly visible to the naked eye, but only under the microscope. Medical microbiology deals with the causative agents of infection, diseases, the ways in which they produce disease in humans body and essential information for diagnosis and treatment.
The credit for observation and description of bacteria goes to a Biologist Antony Von Leeuwenhook of Holland (1674). His microscopes consisted of a simple biconvex lens that magnified the object * 200.
Scientific development of microbiology was ushered by Louis Pasteur. He has done studies on fermentation, pasterurisation and vaccination. He was called as the “Father of Modern Microbiology”.
Joseph Lister introduced antiseptic technique in surgery (1867) for killing bacteria in wounds and in the air with carbolic acid. He is known as the “Father of Antiseptic Surgery”.
Robert Koch, a general practitioner perfected the bacteriological technique and introduced methods for isolation of pure strains of bacteria. The important discoveries of Robert Koch are Anthrax bacilli, Tubercle bacilli and Cholera vibrios. He is known as the "Father of Medical Microbiology”.
Other Scientists and their discoveries:-
1. |
Malaria |
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Ronald Ross |
2. |
Antitoxin |
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Von Behring |
3. |
Theories of Immunology |
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Paul Ehrlich |
4. |
Anaphylaxis |
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Charles Ritchet |
5. |
Blood groups |
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Carl Landsteiner |
6. |
Sulphonamide |
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Gerhardt and Domagk |
7. |
Penicillin |
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Alexander Fleming |
8. |
DNA |
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Watson and Crick |
9. |
Australia antigen |
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Blumberg |
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(HbsAg) |
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GENERAL MICROBIAL CHARACTERS OF BACTERIA
Morphology
Living material is organized in unit known as cell. Microorganisms are living forms of microscopical size and usually unicellular in structure. Originally microorganisms were classified under plant and animal kingdoms as they were found to contain combination of both animal and plant properties. This classification being unsatisfactory, a third kingdom " Protista" was proposed by Hachel in 1866.
Protista: They are mostly unicellular or coenocytic (many nuclei per cell ) and each cell is capable of maintaining independent life. Protista is subdivided into 2 groups based on cellular organization and biochemical properties.
Classification of Protista:
A.Higher Protist (Eukaryocyte )
1.Algae
2.Protozoa
3.Fungi
B.Lower Protist (Prokaryocyte)
1.Bacteria
Structures |
Eukaryocyte |
ProKaryocyte |
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Nucleolus |
Present |
Absent |
Nuclear Division |
Mitosis |
Binary fission |
Chromosome |
Many |
One |
Mitochondria |
Present |
Absent |
Lysosome |
Present |
Absent |
Muramic acid |
Absent |
Present |
Sterols |
Present |
Absent |
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Bacteria are prokaryotic unicellular microorganisms which are divided by binary fission and do not possess chlorophyll and true branching except in the so called higher bacteria (Actinomycetes). Their size varies from 0.5 - 15 microns.
Microscopy
Optical or Light microscope: Bacteria can be studied by light microscope which generally uses 100* power objective (Oil immersion lens) along with 10* power ocular lens and magnifies the specimen 1000 times (Fig. 33.1).
Phase contrast microscope: Phase contrast microscope is used to study the internal structures of unstained living tissue, bacteria and protozoa.
Dark ground microscope (dark field illumination): DGI is used to visualise spirochetes. The bacteria looks luminous against a dark background.
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Fig. 33.1: The microscope
Fluorescent microscope: In this UV light is used. Bacteria stained with fluorescence dye (e.g. Auramine) becomes visible as bright objects against a dark background.
Immunofluorescence combines serology with fluorescence microscopy by using antibody labeled with fluorescent dyes (fluoroscein-isothiocyanate, Lissamine and Rhodamine) for detection of specific antigen and bacteria.
Electron microscope: A beam of electrons is focused by a circular electromagnet. When the electron beam passes through an object, the electrons get scattered producing an image in the built-in fluorescent viewing screen.
Adjustments to be made when viewing a stained smear (Gram's or ZeihlNeelsen)
1.Oil immersion lens (100* objective) to be chosen.
2.Condenser should be raised fully.
3.Diaphragm should be completely opened.
4.Plane mirror should be used (if there is no self-illumination)
5.A drop of cedar wood oil should be placed on the smear.
6.10* or 5* Eyepiece can be used for viewing.
Adjustments to be made when viewing a wet mount (saline, KOH or Lactophenol staining).
1.10* (Low power) or 40 * (High power ) objective to be chosen.
2.Condenser should be lowered.
3.Diaphragm should be partially opened.
4.Concave mirror should be used to focus the light.
5.The specimen either a drop of KOH or saline amount) should be covered by cover slip avoiding air-bubbles.
6.10* or 5* Eyepieces can be used for viewing.
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Gram's Stain Procedure
It is the most widely used stain in Bacteriology, The stain was originally devised by Christian Gram (1884).
Method
1.Heat fixed smear (from clinical material or culture ) on a glass slide is flooded with a primary stain solution, e.g. Aniline dyes such as methyl violet, gention violet or crystal violet for one minute.
2.Pour the stain and add a solution of dilute Iodine (Gram's iodine) - Wait for a minute.
3.Wash with water.
4.Decolourise with a few drops of Acetone (10 seconds).
5.Wash with water
6.Counterstain with dilute carbol fuschin, safranin or neutral red for 30 seconds.
Gram positive bacteria will retain the primary stain and appear violet (Fig. 33.2).
Fig. 33.2: Bacteria as seen with the oil-immersion lens
1.Staphylococcal in pus
2.Gonococci in pus
3.Tubercle bacilli in sputum
4.Pneumococci stained to show capsule
5.Cl. tetani stained to show spores
6.B. authrosis stained to show spores
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Gram negative bacteria will take the counterstain and appear pink.
The exact mechanism is not understood. Cell wall permeability and pH of the protoplasm of the bacteria are believed to be responsible for these staining reactions.
Zeihl-Neelsen (Acid fast stain)
1.Heat fixed stain is stained by hot carbol fuschin (strong) for 10 minutes and washed in tap water.
2.The smear is decolourised by 20% sulphuric acid in 95% absolute alcohol for a minute
3.Then the smear is counterstained with 2% methylene blue or malachite green for 2 minutes and washed in water.
Mycobacterium will appear as pink colour whereas the background will appear as blue (Fig. 33.3).
Principle:Acid fastness has been attributed to the high content of lipids, fatty acids and higher alcohol and mycolic acid.
Fig. 33.3: Acid fast bacilli (pink against blue background (Zeihl-Neelsen)
Albert's Stain
Some bacteria, e.g. Corynebacterium diphtheriae possessing metachromatic granules stain better with Albert's stain.
Procedure
Heat fixed smear of throat/nasal swab is covered with Albert's stain I (a mixture of toludine blue and malachite green) and allowed to stand for 4-5 minutes. Pour off Albert's stain I and cover the smear with Gram's Iodine (Albert's II ) for 1 minute.
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Observation: The body of the bacilli will appear green and the metachromatic granules blue black.
The other stains which are less commonly used in microbiology are Giemsa, Giminez, Machivallo (for Chlamydiae), Silver Impregnation (Flagella), India ink (Capsule)and Leishman (peripheral blood) staining.
Classification of Bacteria: Bacteria can be classified into several varieties depending on their shape:
1.Cocci: e.g. Staphylococci and Streptococci
2.Bacilli: e.g. Klebsiella, E. coli
3.Vibrios: e.g. Vibrio cholerae
4.Spirochaetes: e.g. Treponema pallidum, Leptospira
5.Actinomycetes
6.Mycoplasma
7.Chlamydiae
Bacterial cell: The outer layer or cell envelope of a bacterial cell consists of two components. (a) a rigid cell wall proper and (b) underlying cytoplasmic membrane or plasma membrane. The cell wall encloses the protoplasm comprising of cytoplasm (Ribosomes, inclusion granules, mesosomes) and a single circular chromosome of DNA. Some bacteria may possess a capsule (Pneumococcus), flagella (organ of locomotion) and fimbriae (pili).
Structure of cell wall: The chemical structure of cell wall of gram-positive and gram-negative bacteria differ considerably. It is comparatively simpler in gram positive bacteria than that of gram-negative bacteria. The rigid part of the cell wall is a peptidoglycan and is the principal structural component of the cell wall. It is present in the cell wall of both gram-positive and gramnegative bacteria although it is abundant in gram-positive bacteria.
Fig. 33.4: Scanning electron microscopic picture of a bacilli with flagella