Lecture #23: The Physiology of Hearing, Taste, and Smell

I. HEARING

1. Anatomy of the ear: stress oval window, vestibular canal, tympanic canal, round window, basilar membrane. (Figs 49.17)

2. Basilar membrane bears organ of Corti, which contains the receptor cells (hair cells)

3. A vibrating basilar membrane pushes hairs against the tectorial membrane.

4. The process of hearing: (compare basilar membrane to a harp) (Fig 49.18)

a. Frequency at which a given segment of basilar membrane vibrates depends on length and tension.

b. When fluid in canals oscillate, appropriate basilar fiber(s) begin to vibrate.

c. Human basilar membrane can vibrate from 20-20,000 Hz.

d. Site of vibration depends on "resonant frequency" (like every pure solid) (Mrx).

II. BALANCE AND EQUILIBRIUM (lateral line of fish)

1. Anatomically, the organ of equilibrium consists of the utricle and saccule. (Fig 49.19)

a. utricle canal has three semicircular canals branching off of it.

b. the ampullae of these canals contain the cupula and hair cells (mechanoreceptor cells)

2. Collectively, these structures generate action potentials and nerve signals that facilitate equilibrium by:

a. assessing the body's fixed position in space

b. detecting changes in the rate of rotation or angular movements.

III. TASTE AND SMELL

1. Taste depends on gustatory cells that respond to specific molecular shapes & charges.

a. ring structure of glucose is important for sweet.

b. positively charged sodium ions are for salty.

c. acidity of H⁺ ions is necessary for sour taste.

2. Smell: Much of what we think of as taste is actually due to olfaction.

3. Cilia of olfactory chemoreceptor cell dendrites extend down into nasal turbinates. (Fig. 49.24)