Thursday, December 30, 2010


Light reflex
When light is shone in one eye, both the pupils
constrict. Constriction of the pupil to which light is
shone is called direct light reflex and that of the other
pupil is called consensual (indirect) light reflex. Light
reflex is initiated by rods and cones.
Pathway of light reflex (Fig. 12.6). The afferent fibres
extend from retina to the pretectal nucleus in the midbrain.
These travel along the optic nerve to the optic
chiasma where fibres from the nasal retina decussate
and travel along the opposite optic tract to terminate
in the contralateral pretectal nucleus. While the fibres
from the temporal retina remain uncrossed and travel
along the optic tract of the same side to terminate in
the ipsilateral pretectal nucleus.
Internuncial fibres connect each pretectal nucleus
with Edinger-Westphal nuclei of both sides. This
connection forms the basis of consensual light reflex.
Efferent pathway consists of the parasympathetic
fibres which arise from the Edinger-Westphal nucleus
in the mid-brain and travel along the third
(oculomotor) cranial nerve. The preganglionic fibres
enter the inferior division of the third nerve and via
the nerve to the inferior oblique reach the ciliary
ganglion to relay. Post-ganglionic fibres travel along
the short ciliary nerves to innervate the sphincter
Near reflex
Near reflex occurs on looking at a near object. It
consists of two components: (a) convergence reflex,
i.e., contraction of pupil on convergence; and
(b) accommodation reflex, i.e., contraction of pupil
associated with accommodation.
Pathway of convergence reflex (Fig. 12.7). Its afferent
pathway is still not elucidated. It is assumed that the
afferents from the medial recti travel centrally via the
third nerve to the mesencephalic nucleus of the fifth
nerve, to a presumptive convergence centre in the
tectal or pretectal region. From this the impulse is
relayed to the Edinger-Westphal nucleus and the
subsequent efferent pathway of near reflex is along
the 3rd nerve. The efferent fibres relay in the
accessory ganglion before reaching the sphincter
Pathway of accommodation reflex (Fig. 12.7). The
afferent impulses extend from the retina to the
parastriate cortex via the optic nerve, chiasma, optic
tract, lateral geniculate body, optic radiations, and
striate cortex. From the parastriate cortex the impulses
are relayed to the Edinger- Westphal nucleus of both
sides via the occipito-mesencephalic tract and the
pontine centre. From the Edinger-Westphal nucleus
the efferent impulses travel along the 3rd nerve and
reach the sphincter pupillae and ciliary muscle after
relaying in the accessory and ciliary ganglions.
Psychosensory reflex
It refers to dilatation of the pupil in response to
sensory and psychic stimuli. It is very complex and
its mechanism is still not elucidated.
1. Amaurotic light reflex. It refers to the absence of
direct light reflex on the affected side (say right eye)
and absence of consensual light reflex on the normal
side (i.e., left eye). This indicates lesions of the optic
nerve or retina on the affected side (i.e., right eye),
leading to complete blindness. In diffuse illumination
both pupils are of equal size.
2. Efferent pathway defect. Absence of both direct
and consensual light reflex on the affected side (say
right eye) and presence of both direct and consensual
light reflex on the normal side (i.e., left eye) indicates
efferent pathway defect (sphincter paralysis). Near
reflex is also absent on the affected side. Its causes
include: effect of parasympatholytic drugs (e.g.,
atropine, homatropine), internal ophthalmoplegia,
and third nerve paralysis.
3. Wernicke’s hemianopic pupil. It indicates lesion
of the optic tract. In this condition light reflex
(ipsilateral direct and contralateral consensual) is
absent when light is thrown on the temporal half of
the retina of the affected side and nasal half of the
opposite side; while it is present when the light is
thrown on the nasal half of the affected side and
temporal half of the opposite side.
4. Marcus Gunn pupil. It is the paradoxical response
of a pupil of light in the presence of a relative afferent
pathway defect (RAPD). It is tested by swinging flash
light test. For details see page 474.
5. Argyll Robertson pupil (ARP) . Here the pupil is
slightly small in size and reaction to near reflex is
present but light reflex is absent, i.e., there is light
near dissociation (to remember, the acronym ARP may
stand for ‘accommodation reflex present’). Both pupils
are involved and dilate poorly with mydriatics. It is
caused by a lesion (usually neurosyphilis) in the
region of tectum.
6. The Adie’s tonic pupil. In this condition reaction
to light is absent and to near reflex is very slow and
tonic. The affected pupil is larger (anisocoria). Its exact
cause is not known. It is usually unilateral, associated
with absent knee jerk and occurs more often in young
women. Adie’s pupil constricts with weak pilocarpine
(0.125%) drops, while normal pupil does not.

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