232 Neuroanatomy: Draw It to Know It
Know-It Points
The Ear
■ Th e external and middle ear canals lie within the
tympanic portion of the temporal bone.
■ Th e middle ear canal contains three ossicles — from
lateral to medial: the malleus, incus, and stapes.
■ Th e stapes abuts the oval window; when sound is
transmitted through the ossicles, the stapes pushes
the oval window into the inner ear.
■ Th e inner ear canal lies within the petrous portion of
the temporal bone.
■ Th e inner ear canal divides into three diff erent parts:
the semicircular canals, which serve vestibular
function; the cochlea, which serves auditory
function; and the vestibule, which is involved in both
auditory and vestibular functions.
■ Th ree ducts form within the cochlea: the cochlear
duct (scala media), the vestibular duct (scala
vestibuli), and the tympanic duct (scala tympani).
■ Reissner’s membrane separates the vestibular
duct from the cochlear duct, and the basilar
membrane separates the cochlear duct from the
tympanic duct.
■ Th e vestibular and tympanic ducts are fi lled with
perilymphatic fl uid, and the cochlear duct is fi lled
with endolymphatic fl uid.
■ Th e three semicircular canals (anterior, horizontal,
and posterior) lie perpendicular to one another and
detect rotational acceleration.
■ From the observer’s perspective, the anterior canals
produce upward eye movement; the posterior canals
produce downward eye movement; the right-side
canals produce clockwise torsional eye movement;
and the left -side canals produce counterclockwise
torsional eye movement.The Vestibular System
■ Th e vestibular nuclear complex excites the
contralateral abducens nucleus and the abducens
nucleus excites the contralateral oculomotor nucleus,
which is ipsilateral to the vestibular nuclear complex
where the impulse originated.
■ Th e vestibular nuclear complex spans from the
upper pons to the inferior fl oor of the fourth
ventricle (in the medulla).
■ Th e medial division of the vestibular nuclear
complex comprises the superior nucleus and medial
nucleus.
■ Th e lateral division of the vestibular nuclear
complex comprises the lateral nucleus and inferior
nucleus.Central Auditory Pathways
■ Th e ventral cochlear nucleus lies at the
pontomedullary junction.
■ Th e dorsal cochlear nucleus lies in the upper medulla.
■ Th e superior olivary complex lies in the mid-pons.
■ Th e inferior colliculi lie in the inferior midbrain.
■ Th e lateral lemniscus tracts project to the inferior
colliculi.
■ Each inferior colliculus projects to the ipsilateral
medial geniculate nucleus via the brachium of the
inferior colliculus.
■ Each medial geniculate nucleus projects to
the ipsilateral transverse temporal g yri (Heschl’s
g yri) — the primary auditory cortex.