Neuroanatomy Draw It To Know It

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238 Neuroanatomy: Draw It to Know It


The Ear (Cont.)


Next, return to the vestibule so we can show the otolith
organs. First, draw the saccule, which detects vertical
movement (ie, gravity), and then draw the utricle, which
perceives horizontal (forward/backward) movement.
Th e macula (the neuroepithelial sensory detection
region) of the saccule is principally vertically oriented
and its attached hair cells are horizontally oriented to
detect vertical movement, whereas the macula of the
utricle is principally horizontally oriented and its
attached hair cells are vertically oriented to detect hori-
zontal movement.
Now, let’s label the semicircular canals: the anterior
semicircular canal, horizontal semicircular canal, and pos-
terior semicircular canal. Th ese three semicircular canals
lie perpendicular to one another and detect rotational
acceleration. To understand their directionality, include
within our fi gure an axial view of them. Draw an oval-
shaped cranium and label its anterior and posterior
aspects. Th en, draw the left anterior semicircular canal
facing anterolaterally and the left posterior semicircular
canal perpendicular to it. Th e posterior canal lies along
the axis of the petrous ridge. Next, include the laterally
facing left horizontal canal. In normal, upright head posi-
tion, the horizontal canal is tilted upward about 30 degrees
to the horizontal plane and the anterior and posterior
canals are roughly within the vertical plane. When sitting
upright, if the head is tilted down 30 degrees, the horizon-
tal canals are brought into the earth-horizontal plane.
Now, draw the right-side semicircular canals as mirror
images of the left.
At the utricular end of each canal is a membranous
enlargement, called the ampulla, which contains the sen-
sory cell system of the semicircular canal. Attached to
the ampulla is the cupula, which is a gelatinous mass that
defl ects during angular acceleration. When the cupula
bows, the hair cells are activated and, as a result, the
semicircular canals generate eye movements in their
plane of orientation. Show that the left anterior canal
and the right posterior canal drive the eyes along the


same diagonal, from left anterior to right posterior; then,
show that the right anterior and left posterior canals
drive the eyes along the same diagonal, from right ante-
rior to left posterior; and fi nally, indicate that the hori-
zontal canals drive the eyes laterally.^1
Next, we will divide the directionality of the eye
movements generated by the anterior and posterior
semicircular canals (the vertical canals) based on whether
the canal is anterior or posterior, right or left. Label across
the top of the page that we will show the eye movement
directionality from the observer’s perspective in coronal
view. Also, to avoid confusion, since eye movements are
commonly addressed in regards to the nystagmus they
produce, indicate that we are showing the slow phase of
the nystagmus. Now, indicate that the anterior canals
produce upward movements and that the posterior
canals produce downward movements, and, again, from
the observer’s perspective, indicate that the right-side
canals produce torsional movements in a clockwise direc-
tion whereas the left -side canals produce torsional move-
ments in a counterclockwise direction.^2
In benign paroxysmal positional vertigo, calcium car-
bonate crystals from the utricle fall into one of the semi-
circular canals — most oft en the posterior canal — and
stimulate positional vertigo. If the right posterior canal is
activated, then from the observer’s perspective, the slow
phase of the nystagmus is downward with clockwise
rotation. Th e fast phase, which is how the nystagmus is
actually named, is in the opposite direction: upward and
counterclockwise.^2
To conclude our diagram, let’s draw the vestibuloco-
chlear nerve, which transmits the vestibular and auditory
impulses from the inner ear to the vestibulocochlear
nuclei in the brainstem. First, draw the vestibular seg-
ment of the vestibulocochlear nerve and then the cochlear
segment. Indicate that the vestibulocochlear nerve passes
through the internal acoustic meatus (along with the
facial nerve) and crosses the pontocerebellar cistern to
enter the brainstem at the pontomedullary junction.
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