4. 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
5. 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
6. 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.
9. 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.
10. 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
11.
12.
13.
14. 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.
15. Near reflex
Occures on looking at a near object
Two components:
1- convergence reflex (convergence of visual axes
and pupil constriction).
2-accomodation reflex.
16.
17. 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.
18. 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
19.
20.
21.
22.
23.
24. 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.
25.
26.
27.
28. 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.
29. 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.
30. Other miotics
Histamine : act as a protoplasmic irritant and
affect the sphincter fibres directly.
Morphine: causes myosis by cutting cortical
inhibition of EWN.
31. 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.
32. 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.
33. 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
