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Physiology 735
Lab 5- addendum
Below are some images of both a cat and chinchilla brain intended to orient one for the next
series of experiments. The animals were perfused and the brains removed for viewing. The
cortex of the chinchilla is smooth (lisencephalic).
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Figure 1: Right side of chinchilla brain. Left
side has been partially dissected.
AC: auditory cortex
Floc: floculus
Figure 2. left side of brain after removal of
cerebellum and visual cortex on left side.
IC: inferior colliculus
Sc: superior colliculus
Lgb: lateral geniculate
CN: cochlear nucleus
OB olfactory bulb
In contrast, the cat cortex has deep sulci
and prominent gyri.
Figure 3. Partially
dissected brain of a cat.
Ac: spinal cord.
IV: fourth ventricle
CN: cochlear nucleus
A close –up view of the chinchilla brain that includes the cochlear nucleus is shown in Fig. 4
and Fig. 5.
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Figure 4. Chinchilla brainstem with
the cochlear nucleus exposed (CN).
SC: spinal cord.
Figure 5. Close-up view of the cochlea nucleus
and the medial wall of the temporal bone.
Figure 6. A higher power view of the insertion
of the auditory nerve (AN) and vestibular
nerve (VN) into the temporal bone.
The cochlear nucleus is located on the dorso-lateral aspect of the brainstem. It touches the
skull laterally. In fact it partially inserts into the niche where the AN enters the modiolus of
the cochlea. In order to visualize the AN small cotton pellets are inserted between the wall of
the skull and the brain stem. This pushes the brainstem medially and exposes the eight nerve.
The nerve consists of two branches, the caudal branch is the auditory nerve and the rostral
branch is the vestibular nerve. The diameter of the nerve is ~1mm.
The figure below from Kiang (Peripheral neural processing of auditory information, in
Handbook of Physiology, The Nervous System III, Chapter 15, 1984) reviews the anatomy of
the organ of Corti. In lab 2, recordings are to be made from the output of the type I ganglion
cells which are the auditory nerve fibers. Remember that the type I fibers synapse with the
inner hair cells. There are 10-20 ganglion cells connected to each inner hair cell.
The following figure shows a cross section of the auditory nerve of the cat. Auditory nerve
fibers have a 2-4 micron diameter. They are myelinated. Recordings are made with glass
micropipettes that have a tip diameter on the order of 0.1 - 0.2 microns.
a micropipette will be inserted into the nerve to record the response of individual fibers in
response to a variety of acoustic stimuli. A small amount of agar is placed on the nerve to
stabilize it mechanically.
The figure above is also from Kiang’s article. It illustrates that one can consider the
discharges of the auditory nerve as a three dimensional plot. The x-axis is stimulus frequency
and the y-axis is intensity. The z-axis is the discharge rate of the fiber in spikes/second. A
plane parallel to any of the three sets of axis will result in one of the three 2-D plots that are
shown.
Another useful reference for the auditory periphery:
Ruggero, M.A. (1992) Physiology and coding of sound in the auditory nerve. In: The
Mammalian Auditory Pathway: Neurophysiology. Springer-Verlag, New York. Pp. 34-93.
In lab 6 the electrode will be located in the cochlear nucleus which is just medial to the
auditory nerve.
Figure 7. View of the chinchilla brain with
the cerebellum removed. This exposes the
inferior colliculus and the forth ventricle.
CN: cochlear nucleus.
RB: restiform body
CA: cerebral aqueduct