5.5 Ear and Hearing
The ear has three main parts main parts; the outer ear, the middle ear and the inner ear.
The ear is designed to convert weak mechanical waves in the air into electrical pulses to be sent to the brain.
It consists of
-a mechanical collection and amplification system (the outer and middle ear)
-transducers to produce electrical pulses in the nerves (the inner ear)
Outer Ear
The pinna the auditory canal and the tympanic membrane (ear drum) make up the outer ear The pinna collects the sound and its structure as well as the positioning of the two ears enables the hearer to determine the direction of the sound The auditory canal channels sound waves towards the tympanic membrane The tympanic membrane vibrates in response to changes in air pressure. Its maximum amplitude is of the order of 〖10〗^(-11) m (less than the diameter of a hydrogen atom)
Middle Ear
The middle ear is an air-filled chamber with three bones, (the ossicles), connecting the tympanic membrane to the oval window. Eustachian tube connects the middle ear to the throat and enables pressures on both sides of the tympanic membrane to equalise. Sudden changes in pressure, as in the landing or taking off of an aeroplane, may cause pain and even rupture the tympanic membrane. The middle ear serves an important function of matching impedances of sound waves in two media: air, in the outer ear and a denser fluid in the inner ear.
The lever action of the ossicles of the leads to an amplification of force. This force is amplified by 1.5 times. This leads to a pressure amplification of 25 times. For a typical ear, the cross-sectional area of the tympanic membrane = 55 〖mm〗^2 and the cross-sectional area of the oval window = 3.2 〖mm〗^2.
Inner Ear
The inner ear is a bony chamber filled with fluid. It houses the semicircular canals and the cochlea. The semicircular canals are responsible for sense of balance and detection of body movement. The cochlea is a spirally coiled tube of three chambers and resembles a snail shell (Figure5.11). One chamber (scala vestibuli) begins at the oval window and another chamber (scala tympanic) stars at the round window. The two chambers join at the tip of the spiral, called the helicotrema. A third chamber (scala media) separates the first two chambers (Figure 5.12).
The organ of Corti is located on one wall of the scala media (basilar membrane) and runs the entire length of the duct. The organ of Corti is made up of more than 15 000 hair cells (transducers). These hair cells (Figure 5.13) have hairs that vary in length and stiffness and resonate at different frequencies. The resonating hairs slide against the tectorial membrane and create neural impulses in the nerve fibres (attached to the hairs). The auditory nerve then transmits the impulses to the brain.
To summarise, sound waves in air enter the auditory canal and cause the auditory canal and cause the tympanic membrane to vibrate. The ossicles attached to the tympanic membrane then transmit the sound waves to the oval window, and at the same time, sound pressure is amplified which is necessary to cause the denser fluid in the cochlea to vibrate. The fluid vibrations, in turn, cause the basilar membrane to vibrate. The sensitive hair cells of the organ of Corti, located on the basilar membrane are stimulated and create neural impulses sent to the brain via the auditory nerve.