1. First a digression .. chemoreception.

• First, let's revisit the topic of intracellular reception and signalling caused by chemicals binding to a membrane. With olfactory reception, we once again see the use of G proteins, this time using cyclic AMP (cAMP) as an intracellular messanger, much as in some hormonal signalling systems: (Eckert, Fig. 7-21a)

2. Hair cells form the basis of most systems that sense sound (pressure waves causing mechanical movement).

• Hair cells contain two types of cilia. Bending of the cilia causes bending of the cell membrane surrounding the cilia and opening or closing of ion channels. Before, we saw chemically and voltage gated channels. The channels on the membranes of cilia appear to be mechanically gated. (Eckert, Fig. 7-24a) (Eckert, Fig. 7-24b) (Eckert, Fig. 7-24c)
• Hair cells are typical receptor cells. They don't produce action potentials but do produce a receptor potential that becomes larger or smaller in proportion to the amount of stimulus. Larger or smaller receptor potentials produce larger or smaller, graded releases of neurotransmitter. On the postsynaptic nerve cells, more neurotransmitter, for a longer time, produces a larger or smaller FREQUENCY of action potentials on that postsynaptic nerve.
• Hair cells are found in the lateral-line sensory system of fish and amphibians. (Eckert, Fig. 7-25)

3. The vertebrate ear.

• The vertebrate ear is the evolutionary result of modification of the anterior end of the lateral line.
• Part of the vertebrate ear is modified to sense position. The three semicircular canals contain hair cells used to sense rotational ACCELERATION in each of three mutually perpendicular planes. Move your head up and down or side to side and your brain will be receiving information about rotational acceleration from the semicircular canals.
• The utriculus and sacculus contain mineralized particles called otoliths, that sense translational ACCELERATION. In practical terms, this means that the utriculus and sacculus let your brain know which way is up and which way is down, due to the effects of gravitational acceleration. The sensory receptor area (maculae) of the utriculus is oriented horizontally and that of the sacculus is oriented vertically. (Eckert, Fig. 7-27)
• The external ear is a coupling device to transmit sound from the air environment to the water environment. Try sometime to talk to someone who is underwater - the sound bounces off the surface of the water! The large external eardrum (tympanic membrane) transmits sound to the malleus, incus, and stapes (hammer, anvil, and stirrup) and then to the oval window, that contacts the fluid inside the cochlea.
• The cochlea is a complex structure fundamentally consisting of a a fluid filled canal that begins with the "oval window", connects to the fluid-filled chamber of the "scala vestibuli", coils and then bends back on itself connecting to the "scala tympani", and ending in the "round window". (Note: you are responsible for the details of figure 7-29.) (Eckert, Fig. 7-29)
• The endolymph in the scala media is unusual in that potassium concentration is high and sodium low. When ion channels in the stereocilia of the hair cells open as the result of bending of the stereocilia's membrane, potassium permeability of the hair cells increases, depolarizing them. (We will discuss how this is different from the hyperpolarizing afterpotential on nerve membranes. But, think about how the potassium Nernst potential is different here than on the nerve axon!) (Eckert, Fig. 7-30a) (Eckert, Fig. 7-30b) (Eckert, Fig. 7-30c)
• Hair cells in the human ear develop as complete hair cells, with both a single kinocilium and stereocilia on each hair cell. However, it seems that most, if not all, kinocilia degenerate, leaving just the stereocilia. The Eckert book is unclear on this point, hence the clarification.
• The outer hair cells are responsible for mechanical amplification. Outer hair cells can rapidly change their length in response to vibrations of their stereocilia. So, there is a positive feedback of sound intensity, causing local amplification of membrane vibrations. The inner hair cells then detect these vibrations and send electrical signals to the brain.
• At the high frequency (narrow) end of the cochlea, the stereocilia are shorter and stiffer, the basilar membrane is narrower. These factors contribute to the high frequency response of the narrow end and the low frequency response of the wider end of the cochlea. Additionally, there is a difference in electrical response between the two ends of the cochlea and a difference in the electromechanical tuning of the outer hair cell amplification.
• Frequency of the sound is encoded as frequency of action potentials on auditory nerves. Intensity of sound is encoded as the number of nerve fibers that are sending signals to the brain.

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2008, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.