A. AFFERENT PATHWAY (ear to brain)
1. OUTER EAR
The PINNA scoops up sound (pressure) waves and directs them internally —> sound travels down the AUDITORY CANAL, which ends with the tympanic membrane (EARDRUM) The auditory canal functions as RESONATOR (amplifying the incoming signal).
2. MIDDLE EAR
Three ossicles (malleus, incus, stapes) function as amplifier, linking the tympanic membrane with the oval window of the cochlea. The middle ear preserves sound information as it travels from air to fluid (which is of a higher density than air; more energy is required for the sound to propagate in fluid (high impedance) than air (low impedance)) The total energy of the sound wave is maintained as the ossicles increase AMPLITUDE.
3. INNER EAR
COCHLEA = TRANSDUCER Unrolled, the cochlea measures approx. 3.5 cm.
The ORGAN OF CORTI is laid out on the surface of the basilar membrane in the MEDIAN canal. It is comprised of inner and outer bands of sensitive hair cells (two rows).
TRANSDUCTION: Any process in which one type of signal is converted into another type. Here, transduction involves the conversion of sound wave energy into electro-chemical energy (neural impulses).
COCHLEAR ENCODING The movement of fluid in the cochlea displaces the basilar membrane. Hair cells push against the dense and positively charged endolymph (fluid) that fills the median canal. The hair cells bend, opening at the base to allow positively charged potassium ions to flow inward, which alters the difference between the inner and outer regions of the hair cell. Sound energy is transduced into nerve impulses (only the inner hair cells are involved in transmission).
Sound frequencies are mapped onto the cochlea TONOTOPICALLY.
Nerve fibers (the cochlear branch of CRANIAL NERVE VIII) connect signals from the cochlea and transmit them to a pair of crossconnected auditory cortices.