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Biology LibreTexts

Part 3: Hearing

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  • Hearing involves the signal transduction of mechanical waves into neural signals in the cochlea, within the inner ear.

    Anatomy: The auditory receptors for the ear (shown below) include the:

    • outer ear - from pinna (or auricle) to tympanum (or tympaninc membrane, or ear drum)
    • middle ear - contains 3 ossicles, anchored between tympanum & oval window. The Eustachian (or auditory) tube connects the middle ear cavity to the pharynx (it is an evolutionary descendant of pharyngeal pouches).
    • Inner ear – Cochlea, which contains the hair cells (receptors) within the Organ of Corti.

    Figure \(\PageIndex{1}\)


    Balance & Movement (The Vestibular System)

    Anatomy: Within the inner ear, the 3 semicircular canals are arranged at right angles to each other, and they contain hair cells and fluid similar to the cochlea. In addition, two separate clusters of hair cells – the saccule and utricle – are oriented to detect vertical and horizontal movements. Each of the hair cell clusters has a small collection of dense connective tissue attached to the hair cell membrane extensions (stereocilia) to add mass to the system.

    Physiology: Movements of the head cause dislocations of the fluid in the chambers around the hair cells. Movements generate electrical signals in hair cells, which signal sensory neurons with released chemical neurotransmitters. Patterns of signals are integrated in the cerebellum and parietal cortex.

    3A: Identifying structures

    Using the model of the ear find the following structures:

    • ear canal
    • tympanum (tympanic membrane)
    • ossicles (malleus, incus, stapes in order moving inward into the ear)
    • cochlea
    • semicircular canals.

    3B: Sound localization

    Using a tuning fork, have a subject sit with their eyes closed. Strike the fork so it makes a sound and move it to front, back, side and top of head at a constant distance, holding it to allow the subject to point out the location. Note the accuracy at each position of their pointing, and determine the most and least accurate positions for localization. Can you explain any differences? List the positions for localization from most to least accurate.

    3C: Romberg testing involves maintaining balance

    1. Have the subject stand with their back to the white board. The board should be marked at approximately shoulder height with centimeter units covering ~1 meter.
    2. Note the shoulder positions of the subject.
    3. Have the subject stand and stare straight ahead for 2 minutes, and note the range of movement.
    4. Repeat with eyes closed.
    5. Repeat while standing with your right or left side closest to the board, and note front-to-back swaying, First with eyes open and then with eyes closed.
    Data: Eyes open Eyes closed
    side to side (cm)
    front to back (cm)

    Describe any differences in relation to the sensory input required to maintain balance.