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Physiology of the pupil.pptx

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Physiology of the pupil.pptx

  1. 1. Prepared by Dr. Gracy P. Matarweh Physiology of the pupil
  2. 2. Goals: Introduction about the pupil Physiology of the pupil Pharmacology Abnormalities
  3. 3. The Pupil  It is an aperture present in the centre of the iris.  It limits the amount of light reaching the retina and controls the amount of chromatic and spherical aberrations.  Normally one pupil, rarely more than one(polycoria)  Normaly central (slightly inferonasal)  Circular in shape  Size 3-4mm  Varies with age (small at birth, largest during adolescence)  Pupil tends to dilate during emotional stress and
  4. 4. The pupil  Usually Equal in size but anisocoria is present in 20% of normal population  Pupillary unrest :constant fluctuation in pupillary diameter under normal environmental conditions.  Hippus: exaggeration of pupillary unrest, usually pathological but can be seen in normal individuals
  5. 5. Muscles controlling the Pupil •Sphincter papillae muscle : It forms 1mm broad circular muscular band in the papillary part of the iris. Derived from the ectoderm. Supplied by parasympathetic fibres through the third nerve.it constricts the pupil. •Dilator pupillae muscle: It lies in the posterior part of the stroma of ciliary zone of the iris . Its myofilaments are located in the outer part of the cells of anterior pigment epithelial layer. It is supplied by the cervical sympathetics and dilates the pupil.
  6. 6. Physiology of the pupil
  7. 7. Light reflex  Light reflex is that when light is shone in one eye, both pupils constrict.  Direct light reflex and consensual light reflex: Identical in time, course and magnitude.  If both pupils are illuminated, the response summates.  Amount of constriction depends upon the state of adaptation of the retina, the individuals emotional state, alertness and other factors.  It is initiated by rods and cones. With light intensity up to 9 log units above threshold strength. Above that the pupillary response plateaus off.  Latent period is 0.2-0.5s  The pupil is not capable of responding to stimuli with frequency greater than 5Hz.
  8. 8. Pathway of light reflex  The afferent fibres: extend from the retina to the pretectal nucleus in the midbrain.  Internuncial fibres: connect each pretectal nucleus with Edenger-Westphal nucleus on both sides.  Efferent pathway: parasympathetic fibres arise from the Edinger-Westphat nucleus and travel along occulomotor nerve, preganglionic fibres enter the inferior devision of the occulomotor nerve to inferior oblique and relay in the ciliary ganglion. Postgaglionic fibres travel along the short ciliary nerves to the sphincter papillae muscle.  Normally there is a tonic inhibitory input from
  9. 9. Functions of light reflex 1. Protects against excessive bleaching of the visual pigments by reducing the amount of light entering the eye. 2. It helps in light and dark adaptations; thus maximizing visual acuity at different light levels.
  10. 10. Near reflex  Occures on looking at a near object  Two components: 1- convergence reflex (convergence of visual axes and pupil constriction). 2-accomodation reflex.
  11. 11. Pathway of accommodation reflex  Afferent fibres from retina to parastriate cortex. (optic n., chiasma, optic tract, LGB, optic radiations and striate cortex).  Internuncial fibres from cortex to EWN of both sides via occipitomesencephalic tract and pontine centre.  Also fibres to frontal cortex(motor nerve to medial rectus)  Efferent fibres. From EWN, efferent impulses travel along the the 3rd nerve and reach the sphincter papillae and ciliary muscle and ciliary muscle after relay in the accessory and ciliary ganglia.
  12. 12. Pathway of the convergence reflex  Afferent pathway: from medial recti travel centrally via third nerve to mesencephalic nucleus of the fifth nerve, to a presumptive convergence centre in the tectal or pretectal region.  Internuncial pathway:from convergence center to EWN.  Efferent pathway along the third nerve, efferent fibres relay in the accessory ganglion then reaching the sphincter pupillae
  13. 13. Darkness reflex  It is that the pupils dialate when a person goes from a lighted environment to darkness.  Two causes: 1. Abolition of light reflex with consequent relaxation of the sphincter pupillae. 2. Contraction of the dialator pupillae supplied by sympathetic nervous system.
  14. 14. PHARMACOLOGY OF THE PUPIL  Miotics:  Parasympathomimetic drugs(cholinergics): 1. Direct acting or agonists e.g. pilocarpine Similar to acetylcholine and directly act on the muscarinic receptors. 2. Indirect acting parasympathomimetics or cholinesterase inhibitors: I. reversible cholinesterase inhibitors (physostigmine) II. Irreversible cholinesterase inhibitors (echothiophate iodide, demecarium and diisopropyl fluorophosphate) 3. Dual action parasympathomimetics.
  15. 15.  Sympatholytic drugs: 1. Alpha adrenergic blocker drugs : Such as thymoxamine, phenoxybenzamine,dibenamine and tolazoline. They produce miosis by preventing dilator contraction. 2. Guanethidine: disrupt noreepinephrine release from nerve ending and deplete norepinephrine stores.
  16. 16. Other miotics  Histamine : act as a protoplasmic irritant and affect the sphincter fibres directly.  Morphine: causes myosis by cutting cortical inhibition of EWN.
  17. 17. Mydriatics  Sympathomimetic mydriatics:  Increase the dilator activity by any of the three ways: 1. Increase norepinephrine release. 2. Preventing its uptake by presynaptic vesicle. 3. Directly stimulating the dilator fibres.
  18. 18.  Adrenaline (epinephrine) which acts directly on the alpha-receptors on the dilator pupillae.  Phenylephrine is a synthetic analog of epinephrine.  Hydroxyamphetamine and ephedrine; cause norepinephrine release from periphral nerve terminals. (short duration mydriasis)  Cocaine: act as a local anaesthatic, prevents the reuptake of norepinephrine at presynaptic terminal.  When the sympathetic nerve is paralyzed only adrenaline is effective.
  19. 19.  Parasympatholytic mydriatics:  They compete with acetylcholine at the myoneural junction and thus cause mydriasis by blocking sphincter activity.  Atropine is the strongest mydriatic which completely paralyse the sphincter pupillae and ciliary muscle.  Homatropine  Cyclopentolate  Tropicamide
  20. 20. Thank you for your kind attention

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