Outer earMain article: Outer earThe outer ear includes the  HYPERLINK “https://en.wikipedia.org/wiki/Pinna_(anatomy)” o “Pinna (anatomy)” pinna, the visible part of the ear, as well as the ear canal which terminates at the eardrum, also called the tympanic membrane. The pinna serves to focus sound waves through the ear canal toward the eardrum. Because of the asymmetrical character of the outer ear of most mammals, sound is filtered differently on its way into the ear depending on what vertical location it is coming from. This gives these animals the ability to localize sound vertically. The eardrum is an airtight membrane, and when sound waves arrive there, they cause it to vibrate following the waveform of the sound.

Middle ear
Main article: Middle earThe middle ear consists of a small air-filled chamber that is located medial to the eardrum. Within this chamber are the three smallest bones in the body, known collectively as the  HYPERLINK “https://en.wikipedia.org/wiki/Ossicles” o “Ossicles” ossicles which include the malleus, incus, and stapes (also known as the hammer, anvil, and stirrup, respectively). They aid in the transmission of the vibrations from the eardrum into the inner ear, the cochlea. The purpose of the middle ear ossicles is to overcome the impedance mismatch between air waves and cochlear waves, by providing impedance matching.

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Also located in the middle ear are the  HYPERLINK “https://en.wikipedia.org/wiki/Stapedius_muscle” o “Stapedius muscle” stapedius muscle and tensor tympani muscle, which protect the hearing mechanism through a stiffening reflex. The stapes transmits sound waves to the inner ear through the oval window, a flexible membrane separating the air-filled middle ear from the fluid-filled inner ear. The round window, another flexible membrane, allows for the smooth displacement of the inner ear fluid caused by the entering sound waves.

. Inner earThe inner ear is a small but very complex organ.

Main article: Inner earThe inner ear consists of the cochlea, which is a spiral-shaped, fluid-filled tube. It is divided lengthwise by the organ of Corti, which is the main organ of mechanical to neural transduction. Inside the organ of Corti is the basilar membrane, a structure that vibrates when waves from the middle ear propagate through the cochlear fluid –  HYPERLINK “https://en.wikipedia.org/wiki/Endolymph” o “Endolymph” endolymph. The basilar membrane is  HYPERLINK “https://en.wikipedia.org/wiki/Tonotopy” o “Tonotopy” tonotopic, so that each frequency has a characteristic place of resonance along it. Characteristic frequencies are high at the basal entrance to the cochlea, and low at the apex. Basilar membrane motion causes depolarization of the hair cells, specialized auditory receptors located within the organ of Corti.5 While the hair cells do not produce action potentialsthemselves, they release neurotransmitter at synapses with the fibers of the auditory nerve, which does produce action potentials. In this way, the patterns of oscillations on the basilar membrane are converted to spatiotemporal patterns of firings which transmit information about the sound to the brainstem.6NeuronalThe lateral lemnisci (red) connects lower brainstem auditory nuclei to the inferior colliculus in the midbrain.

Main article: Neuronal encoding of soundThe sound information from the cochlea travels via the auditory nerve to the cochlear nucleus in the brainstem. From there, the signals are projected to the inferior colliculus in the midbrain  HYPERLINK “https://en.wikipedia.org/wiki/Tectum” o “” tectum. The inferior colliculus integrates auditory input with limited input from other parts of the brain and is involved in subconscious reflexes such as the auditory startle response.

The inferior colliculus in turn projects to the medial geniculate nucleus, a part of the thalamus where sound information is relayed to the primary auditory cortex in the temporal lobe. Sound is believed to first become consciously experienced at the primary auditory cortex. Around the primary auditory cortex lies  HYPERLINK “https://en.wikipedia.org/wiki/Wernickes_area” o “Wernickes area” Wernickes area, a cortical area involved in interpreting sounds that is necessary to understand spoken words.

Disturbances (such as stroke or trauma) at any of these levels can cause hearing problems, especially if the disturbance is bilateral. In some instances it can also lead to auditory hallucinations or more complex difficulties in perceiving sound.


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