422 Neuroanatomy: Draw It to Know It
Pupillary Light Refl ex
Here, we will draw a diagram of the pupillary light refl ex.
First, in axial view, draw the eyes and the midbrain, and,
then, lateral to the posterior midbrain tegmentum, draw
the lateral geniculate nucleus. Th en, within the eyes,
draw the lenses, and within the midbrain, draw the red
nuclei and label the superior colliculi, for reference.
Next, draw a sagittal view of the midbrain and dienceph-
alon and indicate that our axial section lies at the level of
the midbrain–diencephalic junction. Now, draw the fol-
lowing pupillary light refl ex landmarks: the pretectal oli-
vary nucleus of the pretectal area, which lies anterolateral
to the superior pole of the superior colliculus; the poste-
rior commissure, which forms the roof of the cerebral
aqueduct (of Sylvius); the paired Edinger–Westphal
nuclei of the oculomotor complex; the bilateral ciliary
ganglia in the posterior orbit, each of which lies in
between the related optic nerve and lateral rectus muscle;
and the sphincter pupillae of the iris, which are small cir-
cumferential rings of smooth muscle in the pupillary
margin of each eye. Next, to better illustrate the anat-
omy of the sphincter pupillae muscle, draw a coronal
view of the iris and pupil. Show that the parasympathetic-
innervated sphincter pupillae muscle fi bers are circum-
ferentially arranged to constrict pupil size, whereas the
sympathetic-innervated pupillary dilator muscle fi bers
are radially arranged to widen pupil size.
Now, let’s draw the pupillary light refl ex pathway.
First, show that light activates the retinae. Th en, show
that the optic nerve fi bers combine to form the optic
tracts, which pass toward the lateral geniculate nucleus —
we show the left optic tract, only, here, for simplicity.
Next, show that whereas the majority of optic fi bers syn-
apse within the lateral geniculate nucleus, fi bers of the
pupillary light refl ex, instead, synapse in the pretectal
olivary nucleus of the pretectal area. Th en, indicate that
the prectectal olivary nucleus projects directly to the
ipsilateral Edinger–Westphal nucleus and to the contral-
ateral Edinger–Westphal nucleus via the posterior com-
missure. Now, show that each Edinger–Westphal nucleus
projects to the ipsilateral ciliary ganglion. Th en, using
the left side of the diagram, only, show that the ciliary
ganglion sends short ciliary nerves to innervate the
sphincter pupillae muscles to produce pupillary constric-
tion. Th e short ciliary nerves pass initially within the
sclera and then within the suprachoroidal space as they
wrap around the globe to innervate the sphincter pupil-
lae. Note that the majority of short ciliary nerves actu-
ally innervate the ciliary muscle of the ciliary body (see
Drawing 22-3), which assists in the accommodation
refl ex, rather than the sphincter pupillae muscles for the
pupillary light refl ex (drawn here).^5 , 10 , 11
As a clinical corollary, in an Adie’s tonic pupil, pathol-
og y within the ciliary ganglion or the short ciliary nerves
prevents the pupil from constricting in reaction to light.
On evaluation, cholinergic supersensitivity is present: a
dose of pilocarpine (a cholinergic agonist) that would
not aff ect a normal eye is able to produce pupillary con-
striction because the denervated sphincter pupillae in the
Adie’s pupil is supersensitive to acetylcholine. Finally,
consider that unlike saccadic eye movement circuitry,
which involves brainstem projections that pass inferior to
the midbrain to enter the lower pons and upper medulla,
the circuitry for the pupillary light refl ex does not descend
below the upper midbrain. Th us, impaired pupillary con-
striction is extremely important to detect because it can
be an early warning sign of brainstem herniation.