Sound is transmitted by waves that travel through the air. The auditory system receives these signals and relays them to the brain. Structures in this sensory system take these waves and convert them into electrical signals. These signals are then sent to the auditory parts of the brain.
The auditory system begins at the outer ear, the part of the ear that is visible. Sound waves pass through the outer ear and enter the auditory canal. In the middle ear, these sound waves vibrate the eardrum, which in turn transfers the energy to three small and delicate bones, the malleus, incus, and stapes, sometimes known as the hammer, anvil, and stirrup. This structure of bones serves to amplify and transfer sound waves.
The auditory system transforms vibrations into electrical signals at the inner ear. The inner ear consists of the fluid-filled cochlea, which contains the organ of Corti. The organ of Corti consists of hair cells, cylindrical cells that have thin cilia strands at the top. When a sound wave travels through the cochlea, the cilia strands at the top of the hair cells move back and forth. The inner hair cells transform this energy into electrical signals.
The auditory nerve carries signals from the organ of Corti to the brainstem, as part of cranial nerve eight, the vestibulocochlear nerve. In the brainstem, the auditory information is processed by the cochlear nuclei and superior olivary complex before it travels up into the midbrain. The three nuclei in this structure, the medial superior olive, the lateral superior olive, and the nucleus of the trapezoid body, are involved in localization of sound. They do this using cues such as differences in the amount of time a sound takes to reach each ear, or the relative intensity of sounds.
The auditory system continues into the midbrain, where the inferior colliculus does higher level processing and integration of auditory information from the previous structures. It also is involved in some localization of sound. From the midbrain, the electrical signals travel to the thalamus, which relays them to the auditory cortex of the brain, located in the temporal lobe.
The auditory system ends at the auditory cortex of the brain. The primary auditory cortex is found on the superior temporal gyrus, which is above the ear on either side of the brain. This cortex can be mapped by the frequency of sound each region processes. Lower frequencies occur closer to the frontal lobe, and higher frequencies occur farther back.
