Next, I want to guide you through the pupillary light reflex. So in this visual, we have
two eyeballs. This happens to be the one on the left. We also have an eyeball here.
That would be the right eyeball of the person looking at you. We are now going to shine light
in the left eye. So we’ll shine the light in through here. The light will travel through the
structures, components of the left eyeball, hit the retina. Then ultimately, ganglion retinal
cells will be stimulated. Then the action potentials will travel through the optic nerve
to the pretectal region of the brain. It’s going to be on the left side here of that area.
Then this will activate nerve cells that will communicate bilaterally to the Edinger-Westphal
nuclei. So we see in this simple illustration, we have a nerve cell sending an axon here to
activate the left Edinger-Westphal nucleus. Then the contralateral or the right one is also
going to be stimulated in response to this pupillary light reflex. From the Edinger–Westphal
nuclei, what will happen here is there will be parasympathetic fibers that will leave
this nucleus. They’re going to travel to the ciliary ganglia on both sides. So here’s the
left one to the left eye, ciliary ganglion then to the right eye. These parasympathetic fibers
are going to travel in the oculomotor nerve as we see identified here. From the ciliary ganglia,
nerve fibers are going to innervate the sphincter pupillae muscle of each eye and cause it
to contract. So what we have here would be direct and consensual light reflexes.
The direct reflex is that the light shined in the left eye will cause constriction. The consensual
reciprocal response is that the right eye will do the same in response to that light stimulus.
Accommodation is another mechanism that’s associated with the eye. Accommodation
is necessary to provide focus upon objects that we are viewing close to us. For near vision,
the structures that we have here would be the Edinger-Westphal nucleus, which we saw
as a principal player in the pupillary light reflex. We also have our oculomotor nerve again
and our ciliary ganglia. So what we’ll have is preganglionic parasympathetic fibers will travel
from the Edinger-Westphal nucleus via the oculomotor nerve. They will then synapse
within the ciliary ganglion. Then from here, postganglionic parasympathetic fibers
will innervate the ciliary muscle, which is shown in this area here and on the opposite side here.
The lens is in the center here. As a result of that innervation, the ciliary muscle will be
prompted to contract. This will decrease the tension on the suspensory ligaments
that are maintaining the thinness of the lens at this point. As the suspensory ligaments lose
their tension, the lens will increase its thickness or convexity, as we see here. That will more
sharply bend the light to focus it on the fovea centralis when we’re looking at objects that are
close to us. This would be a bilateral or reciprocal reflex, so it would be occurring in both eyes.