EAR ANATOMY HANDOUT
The primary auditory receptive area is the anterior temporal transverse gyrus also
known as Heschl’s gyri.
The main function of the auditory system is to transform sound waves into electrical
impulses that can be processed by the nervous system.
BAEPs are good for showing posterior fossa demyelination, but MRIs are good for
supratentorial plaques (in white matter or internal capsule).
Retrocochlear lesions that have absent BAEP waves are located in the distal portion of
the acoustic nerve, near the cochlea.
Normal BAEPs are rarely seen when the pons is involved.
The auditory system can be separated into the peripheral which is contains the ear, and
the central which is made up of the brainstem and cortex (brain).
PERIPHERAL NERVOUS SYSTEM
The ear is divided into three parts, 1) external ear, 2) middle ear and 3) the inner ear.
(Picture on page 6 of this handout.)
EXTERNAL EAR
External ear- also called the pinna and the auricle is the part of the ear that we can see
from the outside. (1) This is where sound impulses start. It’s distinctive contour assists
in funneling sound inward, it thins to form the external auditory meatus (the opening of
the ear) stopping at the eardrum or tympanic membrane (which is in the middle ear).
External auditory meatus- (2) is the opening of the ear, which leads inward toward the
middle ear to the tympanic membrane.
MIDDLE EAR
Middle ear- is a chamber filled with air made up of the tympanic membrane and the
three ossicles. (3) Sound waves hit the huge surface of the tympanic membrane (also
called ear drum), causing a vibration that sets off the ossicles.
Ossicles- three small bones in the middle ear called 1) malleus or hammer, 2) incus or
anvil and 3) stapes or stirrup. Movement of the ossicles is restricted to avoid harm from
loud noises. The middle ear has two tiny muscles that are attached to the ossicles that
relax and stiffen to control the ossicles. (4) The ossicles transfer the sound waves onto the
footplate of the stapes.
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Footplate of the stapes- (5) forces fluid of the cochlea backwards and forwards with
every new sound wave.
INNER EAR
Inner ear- includes the cochlea, scali, basilar membrane and the organ of corti
Oval window- is the entrance to the scala vestibuli, which contains the footplate of the
stapes (1 of the ossicles). (6) The movement of the stapes on the oval window creates a
wave in the fluid of the upper 2 scalae. As the wave gets stronger, the basilar membrane
starts to vibrate, shifting the wave to the scala tympani.
Semicircular canals- contain hair cells that generate impulses. The canals are oriented
at 90 degrees to each other, providing a mechanism that is sensitive to rotation along any
axis. They are involved with the vestibular portion of the acoustic nerve, and are not a
part of the BAEP recording. They orient the head to the environment. The semicircular
canals and the cochlea make up the labyrinth.
The utricle and the saccule are both apart of the inner ear and are also involved with the
vestibular portion of the acoustic nerve. They orient the body to the environment. They
too are not a part of the BAEP recording.
Cochlea- (7) a hollow, coiled tube filled with fluid in the inner ear, if uncoiled it is
comprised of 3 smaller fluid filled tubes known as 1) scala vestibuli, 2) scala media and
3) scala tympani. The basilar membrane divides the upper scalae (vestibuli & media)
from the lower tympani. (media is in middle) The Reissner’s membrane separates the
scala vestibule and the scala media. The entrance to the scala vestibuli is the oval
window. The round window is at the end of the scala tympani. The two windows
separate the scala vestibule and the scala tympani the two windows also separate the
middle ear from the inner ear. The cochlear portion of the VIIIth cranial nerve takes the
impulse into the central portion of the auditory system.
Middle Ear
Oval Window
Scala Vestibuli
Reissner’s Membrane
Scala Media
Basilar Membrane
Scala Tympani
Round Window
Inner Ear
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Basilar membrane- (8) is very thin and firm at the bottom of the cochlea, it gets broader
and more flexible toward the cochlea’s apex. The thin firm end, recognizes high
frequencies and the broader more flexible end, recognizes low frequencies. This is how
the basilar membrane helps the ear recognize different sounds that are created.
Organ of Corti- (9) is above the basilar membrane in the scala media, it is in charge of
changing the electrical impulses created by the vibrations of the hair cells into nerve
impulses that move through the cochlear nerve. The cochlear microphonic potential is
formed from these impulses. Corti is the doctor who discovered it.
Wave I- (10) is generated in the distal or extracranial (outside the brain) portion of the
auditory nerve (VIIIth cranial nerve) (peripheral).
CENTRAL NERVOUS SYSTEM
(11) Wave II is generated as the auditory nerve (VIIIth cranial nerve) enters the cranium
(skull), at the medullary pontine junction and may be a proximal or intracranial portion of
the auditory nerve in the cochlear nucleus.
(12) The auditory nerve (VIIIth cranial nerve) enters the brainstem between the medulla
and pons and ends in the ipsilateral dorsal and ventral cochlear nuclei. This is where
wave III is generated. It may correspond to action of the superior olivary
nucleus/complex and trapezoid body.
(13) Almost all of the neurons decussate (cross) to the opposite side of the brainstem,
going through the trapezoid body to the superior nucleus in the upper pons and then
climbs up to the lateral lemniscus. This is where wave IV is generated. Wave IV is the
first to get contributions from both ears.
(14) The next part of the trail generates wave V as the signal climbs the lateral lemniscus
in the mid to upper pons and ends in the inferior colliculus in the midbrain.
(15) From the midbrain the signal spreads to the medial geniculate bodies
(metathalamus) of the thalamus. The geniculocortical (goes from the geniculate fibers to
the cortex) fibers, also called the acoustic radiations, climb to the primary auditory
receptive areas in the anterior transverse temporal gyrus also know as Heschl’s
gyrus. This is where wave VI is generated. A lesion in this area could cause an auditory
hallucination which may or may not be epileptic in nature.
Acoustic or auditory radiations- (also called the geniculocortical fibers) are higher in
the brain, between the thalamus and the auditory cortex. The BAEPs we record are short
latency. We would have to record middle or long latency BAEPs to record from the
acoustic radiations. If we had enough time on our screen, we could record wave VII from
the acoustic radiations.
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GENERATOR SITES FOR THE BAEP
These are the best guestimates for the generator sites of the BAEP. There is ongoing
controversy regarding the actual generators for these waves. Waves III on, may have
several sources that when combined, generate them. The distance between the auditory
nerve’s entry point into the brainstem and the inferior colliculus is 3 to 4 centimeters.
A Confused Surfer Landed In My Auditorium.
I
It comes from the distal or extracranial (outside the brain) portion of the
auditory nerve. (spiral ganglion) It is a nearfield negative wave recorded
from Ai (ipsi). In our montage, the ear is negative, so the pen goes up.
II
It comes from the entrance of the auditory nerve at the medullary pontine
junction and may be a proximal or intracranial portion of the auditory
nerve in the cochlear nucleus. It is a farfield positive wave recorded from
Cz. In our montage, Cz is positive, so the pen goes up.
III
It comes from the lower mid pontine level where the auditory nerve enters
the brainstem between the medulla and pons. It may correspond to action
of the superior olivary nucleus/complex and the trapezoid body. It is a
farfield positive wave recorded from Cz (positive), so the pen goes up.
IV
It is mostly from the mid to upper pontine level, contributions from the
cochlear nucleus and nucleus of the lateral lemniscus are probable as well.
The majority of neurons decussate (cross) to the opposite side of the
brainstem, traveling through the trapezoid body to the superior olivary
nucleus in the upper pons. It is a farfield positive wave recorded from Cz.
V
It comes from fibers of the lateral lemniscus ending in the inferior
colliculus. It is generated in the upper pontine level or by the inferior
colliculus at the lower mid brain level. It is a farfield positive wave
recorded from Cz (positive), so the pen goes up. (On the ABRET exam, it
may just have the “brainstem” as the generator for wave I.)
VI
It is generated in the medial geniculate body. It is possibly due to
continuous firing of the inferior colliculus. It is a farfield positive wave.
VII
It is generated in the auditory radiations. It is also possibly due to
continuous firing of the inferior colliculus. It is farfield and positive.
From here the signal goes to the primary auditory receptive area also
known as the anterior transverse temporal gyrus or Heschl’s gyri.
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