PHO131 - The Auditory System

[@TheAwesomeGingerGuy]

(0:03) Hello. The starting point for any auditory analysis of speech is the study of the anatomy and physiology of the ear and the brain. (0:13) The focus of this e-lecture is the ear - generally referred to as the auditory system. It consists of three central components: the outer ear, which modifies the incoming sound signal and amplifies it at the eardrum; the middle ear, which improves the signal and transfers it to; the inner ear, where the signal is converted from mechanical vibrations into nerve impulses and is transmitted to the brain via the auditory nerve. Let us look at these components in more detail. (0:55) The outer ear consists of the visible part, known as the oracle or pinner, and the interior part. The oracle collects sound waves and is also important in our ability to localize the source of a sound. Furthermore, it can modify incoming sounds - particularly high frequencies. From here, the ear canal, a 2.5cm long tube leads to the eardrum. The main function of the ear canal is to filter out tiny substances that might approach the eardrum. Furthermore, it amplifies certain sound frequencies, especially those between 3000Hz & 4000Hz, and it protects the eardrum (also called [the] tympanic membrane) from changes in temperature as well as from damage. The eardrum is set into vibration by the sound waves. This vibration is picked up by the bones in the next section of the ear: the middle ear. (2:12) The middle ear is a cavity which is filled with air via the eustachian tube which is linked to the back of the nose and the throat. When the eardrum vibrates due to the varying air pressure caused by the sound waves, it causes the three small bones (the so-called ossicles) to move back and forth. The ossicles, here in an enlarged form, are a system of three small bones which are named after their shape: the hammer, or malleus, is attached to the ear drum; the anvil is the link between the hammer and the stirrup (the final little bone); and the stirrup fits into the oval window of the inner ear and transmits the mechanical vibrations to it. Together, the ossicles function as a kind of leverage system, amplifying the vibrations by a factor of over 30dB by the time they reach the inner ear via the membrane covered oval opening of the inner ear. (3:32) Now, the inner ear contains the vestibular organ with the semi-circular canals. They control our sense of balance. And then there is the cochlea - a coiled cavity about 35mm long, resembling a snail’s shell. The cochlea is responsible for converting sounds which enter the ear canal from mechanical vibrations into electrical signals. The mechanical vibrations are transmitted to the oval window of the inner ear via the stirrup. This conversion process, known as transduction, is performed by special sensory cells within the cochlea. And the electrical signals, which code??? the sounds characteristics, are carried to the brain by means of the auditory nerve. (4:35) Let us look at the cochlea in more detail by means of a cross-section. As we can see over here, the cochlea is divided into three chambers by the so-called basilar membrane. The upper chamber is the scala vestibuli and the lower chamber is the scala tympani. They are both filled with a clear, viscous fluid called perilymph. Between these two chambers is the cochlear duct which is filled with endolymph. On the basilar membrane rests the organ of corti which contains a systematic arrangement of hair cells which pick up the pressure movements along the basilar membrane. Here is the basilar membrane: first, coiled like its original form within the cochlea; and then uncoiled for a better understanding of its function. In the basilar membrane different sound frequencies are mapped onto different membrane sites from [the] apex (that is the small end) to its base. The numbers that are given here provide the frequency in Hertz. On the basilar membrane we have hair cells that bend in wave-like actions in the fluid and set off nerve impulses which then pass through the auditory nerve to the hearing centre of the brain. Short hair cells respond to high frequencies, and longer fibres respond to lower frequencies. (6:35) So much… for the three components of the ear: the outer ear, which is filled with air; the middle ear, which is a sort of mechanical amplifier by means of a leverage system; and the inner ear, which picks the impulses of the ossicles and converts them to nerve impulses transmitted to the brain. (7:04) Let us finally see a short video we produced sometime ago. This video summarizes the central functions of the components of the human auditory system. (7:17) “When sound travels through the ear canal it causes the eardrum to vibrate. These vibrations are transmitted through the middle ear by three small bones (the so-called ossicles) to the cochlea (the main organ of the inner ear). When the cochlea is set in motion by an incoming sound, tiny hair cells convert the mechanical movements into neural activity.” (7:57) Well, that's it for now. Thanks for your attention and see you again.

[@Cernoise]

Your pronunciation of 'leverage' is the norm in New Zealand English; I don't think I've even heard the other one (even though I pronounce 'lever' with /li:/)

[@nishatfatima2256]

Thanku so.... Much sir. This video really help me a lot. U explain very well.

[@SeknaTS]

I'm under the impression that /li:və/ and /levɜ:/ are both acceptable pronunciations of 'lever', at least in my dialect. (Australian English) RP could perhaps have different tendencies.

[@AmitKumar-lb7mk]

thanks.........