9. Light Reflex
The afferent pupillary pathway originate in the retina , pass
through the optic nerve, Decussate in the optic chiasma
and pass through optic tract to the midbrain.
The pupillary fibers do not synapse with the visual fibers in
the lateral geniculate body(LGB)but pass to the pretectal
nuclei (PTN) at the level of the superior colliculus .
Intercalated fibers that pass as the efferent pupillary
pathway to the Edinger Westphal nucleus (EWN)of the
oculomotor nerve (CN III)on each side (both sides).
Preganglionic parasympathetic fibers run in the oculomotor
nerve as it leaves the brain stem.
These fibers synapse in the ciliary ganglion and give rise to
postganglionic parasympathetic myelinated short ciliary
nerves, which stimulate the constrictor pupillae thus cause
pupillary constriction.
12. Consensual Light Reflex:
Basis of consensual light reflex:
When light is thrown in to one eye, the sensation of
light is carried through optic nerve. A few of the
resulting impulses pass from optic nerve to
pretectal nuclei. The pretectal nuclei excite both
right and left sides of Eddinger Westphal nucleus
even though the light was perceived by just one eye.
The right and left sides of Edinger Westphal nucleus
generate action potentials through the right and left
oculomotor nerves causing both pupil to constrict.
13. The function of the light reflex :
Help the eye to adapt extremely rapidly to changing light
conditions.
The limits of pupillary diameter are about 1.5 mm - 8 mm.
Pupillary constriction in bright light prevents damage to
delicate layers of retina.
Decreases chromatic aberration and spherical aberration.
14. Argyll Robertson pupil :
A pupil that fails to respond to light but does respond to
accommodation and is also very small.
Central nervous system syphilis, alcoholism, encephalitis , midbrain
tumors and diabetes damage nerve transmission of visual signals from
the pretectal region of the brain stem to the Edinger-Westphal
nucleus, thus blocking the pupillary reflexes.
The site of lesion is in the pretectal nuclei of midbrain.
pupils remains mostly constricted, and fails to respond to light.
Yet the pupils can constrict a little more if the Edinger-Westphal
nucleus is stimulated through some other pathway.
The signals that cause accommodation also cause a mild degree of
pupillary constriction at the same time.
20. AUTONOMIC CONTROL OF ACCOMMODATION
AND PUPILLARY APERTURE
The eye is innervated by both parasympathetic and sympathetic nerves
The Parasympathetic Control:
preganglionic fibers arise in the Edinger- Westphal nucleus (the
visceral nucleus portion of the third cranial nerve).
then pass in the third nerve to the ciliary ganglion.
In ciliary ganglion the preganglionic fibers synapse with
postganglionic parasympathetic neurons, which in turn send fibers
through ciliary nerves into the eyeball.
These nerves excite:
(1) the ciliary muscle that controls focusing of the eye lens
(2) the sphincter of the iris that constricts the pupil.
21. The sympathetic control:
Originates in the intermediolateral horn cells of the
first thoracic segment of the spinal cord.
Sympathetic fibers enter the sympathetic chain and pass
upward to the superior cervical ganglion.
In superior cervical ganglion they synapse with
postganglionic neurons.
Postganglionic sympathetic fibers from these then spread
along the surfaces of the carotid artery and successively
smaller arteries until they reach the eye.
The sympathetic fibers innervate the radial fibers of the
iris (which dilate the pupil), and several extraocular muscles
of the eye.
22. 1-CONTROL OF ACCOMMODATION
Accommodation of the lens is regulated by a negative
feedback mechanism that automatically adjusts the
refractive power of the lens to achieve the highest degree
of visual acuity.
Different types of clues help to change the lens strength in
the proper direction:
1. Chromatic aberration:
That is, red light rays focus slightly posteriorly to blue light
rays because the lens bends blue rays more than red rays.
The eyes appear to be able to detect which of these two
types of rays is in better focus, and this clue relays
information to the accommodation mechanism with
regard to whether to make the lens stronger or weaker.
23. 2. When the eyes fixate on a near object, the eyes must
converge. The neural mechanisms for convergence
cause a simultaneous signal to strengthen the lens of
the eye.
3. Because the fovea lies in a hollowed-out depression
that is slightly deeper than the remainder of the
retina, the clarity of focus in the depth of the fovea
is different from the clarity of focus on the edges. This
difference may also give clues about which way the
strength of the lens needs to be changed.
24. 4. The degree of accommodation of the lens oscillates slightly all
the time at a frequency up to twice per second. The visual image
becomes clearer when the
oscillation of the lens strength is changing in the appropriate
direction and becomes poorer when the lens strength is changing
in the wrong direction.
This could give a rapid clue as to which way the strength of the
lens needs to change to provide appropriate focus.
The brain cortical areas that control accommodation closely
parallel those that control fixation movements of the eyes.
Analysis of the visual signals in Brodmann’s cortical areas 18 and
19 and transmission of motor signals to the ciliary muscle occur
through the pretectal area in the brain stem, then through the
Edinger-Westphal nucleus, and finally by way of parasympathetic
nerve fibers to the eyes.
25. 2-CONTROL OF PUPILLARY DIAMETER
Miosis: Stimulation of the parasympathetic nerves excites the
pupillary sphincter muscle, thereby decreasing the pupillary aperture
Mydriasis: stimulation of the sympathetic nerves excites the radial
fibers of the iris and causes pupillary dilation.
Pupillary Light Reflex. When light is shone into the eyes, the pupils
constrict; pupillary light reflex.
When light impinges on the retina, a few of the resulting impulses
pass from the optic nerves to the pretectal nuclei. From here,
secondary impulses pass to the Edinger-Westphal nucleus and,
finally, back through parasympathetic nerves to constrict the
sphincter of the iris.
Conversely, in darkness, the reflex becomes inhibited, which results in
dilation of the pupil.
26. Horner’s Syndrome.
The sympathetic nerves to the eye are occasionally interrupted in the
cervical sympathetic chain, which causes Horner’s syndrome :
1) The pupil remains persistently constricted to a smaller diameter
than the pupil of the opposite eye.
2) The superior eyelid droops (smooth muscle fibers embedded in the
superior eyelid are innervated by the sympathetics. Therefore,
destruction of the sympathetic nerves makes it impossible to open
the superior eyelid as widely as normally.
3) The blood vessels on the corresponding side of the face and head
become persistently dilated.
4) Sweating (which requires sympathetic nerve signals) cannot occur
on the side of the face and head affected by Horner’s syndrome.
Editor's Notes
Pretectal area is midbrain structure composed of seven nuclei
because light brightness on the retina increases with the square of pupillary diameter, the range of light and dark adaptation is about 30 to 1—that is, up to as much as 30 times change in the amount of light entering the eye.
The final nerve fibers in the pathway through the pretectal area to the Edinger-Westphal nucleus are of the inhibitory type.
When their inhibitory effect is lost, the nucleus becomes chronically
active, causing the pupils to remain mostly constricted, and fails to
respond to light.
Pretectal area is midbrain structure composed of seven nuclei
