Sound waves impinge on the auricle (pinna) of the outer ear and travel through the external ear canal (external auditory meatus) to the tympanic membrane (eardrum). The eardrum vibrates according to the frequency of the sound e.g. 1000 cycles per second if it is a 1000 Hz signal.
 
Sound travels through the three ossicles (bones), the malleus (hammer), incus (anvil), and stapes (stirrup) of the middle ear. The eustachian tube, which opens into the middle ear, is responsible for equalizing pressure between the air outside the ear to that within the middle ear. The stapes attaches to the oval window, which connects the middle ear to the inner ear.
 
Two important structures are located within the inner ear: the vestibular organ of balance, and the organ of hearing, the cochlea (with the organ of corti). The cochlea is a spiral-shaped structure with a base and an apex. It lies on the basilar membrane within the scala media of the cochlear duct. Outer and inner hair cells (OHC and IHC) are found on the basilar membrane and are coupled to two types of nerve fibers. Efferent fibers and the IHCs predominantly effect the OHCs by afferent nerve fibers.
 
The nerve fibers form the auditory section of the eighth nerve lead from the brainstem to terminate in the auditory cortex (cochlear nucleus, superior olivary complex, inferior olivary complex, medial geniculate body and AI and AII of the cortex).
 

Outer Ear
 
                     
The pinna (27) collects sound and directs it into the ear canal (29). In adults, the ear canal is about 25 mm long and 7 mm wide. The canal narrows near the middle and then widens again at the ear drum (31). The first part of the ear canal is cartilaginous and can be changed in shape by tugging on the pinna.
 
Cerumen (wax) is produced by glands in the lateral half of the ear canal. Wax keeps the ear canal moist, inhibits the formation of bacteria, prevents insects from migrating towards the eardrum, and has a bitter taste. The wax also serves as a cleaning mechanism for the ear, which is why doctors normally discourage any manual cleaning (for example, with cotton swabs) because this may push the wax too far into the ear.
 

Middle Ear
                     
The middle ear cavity is filled with air and contains several small bones then transfer vibrations from the ear drum to the cochlea of the inner ear. The malleus is the largest bone in the middle ear, with the stapes being the smallest bone in the human body (the stapes is also fully developed at birth). The eustachian tube connects the middle ear to the pharynx and respiratory system.
 

Inner Ear
 
 
The vestibular is made up of three semicircular canals and is responsible for our sense of balance. The vestibular and cochlea share the same fluid system and have similar hair cells (see below).
   
 
The Organ of Corti is shown here in cross-section. The outer and inner hair cells (OHC and IHC respectively) lie on the basilar membrane (BM). The OHCs are affected by efferent fibers (EFF) and the IHCs by afferent nerve fibers (AFF).
 

Brain Stem
 
The auditory nervous system (ANS) is a complex array of nerves originating in the cochlea and terminating in the auditory cortex. The role of the ANS includes analysis of simple sounds (pure tones) and complex sounds (speech) and localization of sounds through the input of both ears. This is accomplished via resolution of frequency, intensity, timing, and spatial information. Nerves from each ear connect to both the left and right auditory cortex, with 80% crossing to the opposite side and 20% ending on the same side as the source ear.

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