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Physiology 735- lab 4.
Recording Gross potentials.
Chinchillas weigh in at 250 – 800 gm.
They possess a very large bulla for their size. The bulla is contains a large volume of air
that is sealed to the environment by the tympanic membrane. The middle-ear bones and
the cochlea are also contained in it. Figure 1 below illustrates the location of the ear
canal. The tympanic membrane is not visible in this view as it is located beneath the
earcanal opening and is covered by bone. Cutting the bone away permits the tympanic
membrane to be visualized (Figure 2).
Figure 1. View of earcanal.
Figure 2. Location of the tympanic
membrane relative to the earcanal
A higher power view of the tympanic membrne is shown is Figure 3. The first of the
middle-ear bones, the malleus (or hammer), is located in the center of the tympanic
membrane. The tympanic membrane is not flat but conical in shape.
Figure 3. High power view of the
tympanic membrane.
Figure 4. Portion of the chinchilla cochlea that
can be viewed when the bulla is opened. RW =
round window.
The bulla has to be opened in order to visualize the middle-ear and its contents and to
place a recording electrode on the rim(edge) of the round window. Cochlear
microphonics(CMs) and compound action potentials (CAPs) can be recorded with a gross
electrode (usually a small silver ball electrode). The cochlea consists of three fluid filled
chambers, the scala vestibuli receives the direct input of acoustical energy from the
movement of the stapes. There is a communication between the scala vestibuli and the
scala tympani at the apical end of the cochlea called the helicotrema. The pressure relief
for this communication is the round window which terminates the scala trympani at the
basal end of the cochlea. Figure 4 illustrates that the round window (RW) of the cochlea
is partially obscured by a bony ledge.
When the bony ledge is removed the entire round window can be seen (Figure 5). The
round window is a thin, partially transparent membrane through which the cochlear
partition can be seen. The membrane has been removed to better expose the transparent
cochlear partition. The basilar membrane delimits the scala tympani portion of the scala
media, the third fluid filled chamber. This chamber contains the organ of Corti including
the all important hair cells that are responsible for transducing acoustical energy into
neural activity. The entire organ of Corti is pretty transparent with the use of special
optics or tissue dyes.
Figure 5. View of through the round
window of the basilar membrane.
Figure 6. View of the incudo (I)-stapedial (S)
The middle-ear bones of the chinchilla are shown in Figure 7. Consider the size:
remember that the stapes is the smallest bone in the body. There is still some tympanic
membrane attached to the malleus. Note that the incus is “fused” to the malleus. That is,
there are really only two middle-ear bone in the chinchilla. There is also a small amount
of bone below the stapes that represents the region of the middle ear where the stapedius
muscle is attached. Contraction of the stapedius muscle is one of the mechanisms
whereby the sound entering the ear can be attenuated with the greatest effect on low
Figure 7. The middle-ear bones of the
chinchilla. S: stapes; B: bone to which
the stapedius muscle is attached.