420 Neuroanatomy: Draw It to Know It
Smooth Pursuit
Here, we will draw a model for smooth pursuit eye move-
ments. Smooth pursuit eye movements allow us to keep
a moving target in our fovea and clearly visualize it. To
begin our diagram, show a target object move from left
to right. Each hemisphere is responsible for ipsilateral
smooth pursuit eye movements, meaning the right
hemisphere detects and tracks images as they move to
the right and the left hemisphere detects and tracks
images as they move to the left.^6 – 8 Within the circuitry
for smooth pursuit, the contralateral cerebellum and
medulla and the ipsilateral pons are incorporated via a
double decussating pathway.
Now, divide the remainder of the page into left and
right sides, making the right side larger than the left.
Next, indicate that M retinal ganglion cells receive rod
photoreceptor detection of the target’s movement.
Note that the visual system is generally divided into the
pathway for detection of movement (the magnocellular
[or M] pathway, which receives rod photoreceptor
stimulation) and the pathway for detection of color (the
parvocellar [or P] pathway, which receives cone photo-
receptor stimulation). Show that visual detection of the
target’s movement from left to right is projected from
the retinae to the right lateral geniculate nucleus and
then to the right primary visual cortex (V1). Next, let’s
show the notable cortical visual processing steps for
motion detection. Show that the primary visual cortex
projects to visual area V5 (the human homologue to the
macaque middle temporal [MT] area), which then proj-
ects to visual area V5a (the human homologue to the
macaque medial superior temporal [MST] area) — in
humans, both V5 and V5a lie at the temporo-occipito-
parietal junction. Th en, indicate that visual area V5a
projects to the posterior parietal cortex, which projects to
the frontal eye fi elds. Note that although we have shown
this projection pathway as being sequential and unidirec-
tional, many non- sequential, bidirectional connections
exist. For instance, visual area V5 also projects directly to
the frontal eye fi elds without projecting through any of
the intervening connections; also, reciprocal connec-
tions between visual areas exist — for instance, the poste-
rior parietal cortex both receives projections from and
sends projections to visual area V5a.
Now, show that the frontal eye fi elds (and other corti-
cal visual areas, as well) project to the ipsilateral dorso-
lateral pontine nuclei (DLPN) in the high pons and the
ipsilateral nucleus reticularis tegmenti pontis (NRTP)
in the upper pons. Next, show that the right DLPN
and right NRTP project across midline to the left side of
the cerebellum, specifi cally to the fl occulus and parafl oc-
culus of the vestibulocerebellum and also to the dorsal
vermis. Th en, show that the vestibulocerebellum and
dorsal vermis project to the ipsilateral medial vestibular
nucleus in the medulla. Finally, show that the left medial
vestibular nucleus projects to the contralateral abducens
nucleus in the right mid- to low pons, which completes
the double decussation. Th e right abducens nucleus then
initiates horizontal pursuit eye movements to the right.
Note that y-group vestibular nuclear connections to the
oculomotor and trochlear nuclei also exist, which are
involved in vertical pursuit movements.^9
Lastly, let’s use axial pontine sections to draw the top-
ographic anatomy of the DLPN, NRTP, and abducens
nucleus. First, draw the posterior–anterior and right–
left planes of orientation for an anatomic, axial view of
the pons. Th en, draw an outline of the pons and remind
ourselves of the territories of the pontine basis and teg-
mentum. In each section of the following pontine dia-
grams, we will include the anatomy of the fourth ventricle
for reference. Now, show that the abducens nucleus of
cranial nerve 6 lies near midline within the posterior
tegmentum of the mid- to low pons. Next, show that
the NRTP lies in the anterior, midline tegmentum of the
upper pons. Lastly, show that the DLPN lies within the
posterior lateral aspect of the high pontine basis.^3