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Physiology 735- lab 4. Recording Gross potentials. Orientation. 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 opening. 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) joint. 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 frequencies. Figure 7. The middle-ear bones of the chinchilla. S: stapes; B: bone to which the stapedius muscle is attached.