Physiology 735

Lab 5 & 6- addendum

Fall, 2004


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).



















Figure 1: Right side of chinchilla brain. Left side has been partially dissected.

AC: auditory cortex

Floc: floculus











In contrast, the cat cortex has deep sulci

and prominent gyri.



Text Box: 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.


















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 5, 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.







In lab 5 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