The auditory system

The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system

A stimulus is detected by the ear and then passed to the brain

Sound is caused by vibrations in the air. These vibrations make the eardrum vibrate, and the vibrations are passed through three small bones, called the ossicles, to the cochlea. Signals are passed from the cochlea to the brain, and the brain interprets these signals as sound.

Dimensions of sounds:

• Loudness = Amplitude. It describes the intensity of sound
• Pitch = Frequency (Hz). Whether the sound is high or low
• Timbre = Complexity. It regards the nature of the sound

The outer ear is the external portion of the ear, which consists of the pinna and the ear canal

• The pinna is the visible part of the ear. It is composed of a thin plate of elastic cartilage, and connected to the surrounding parts by ligaments and muscles
• Sound waves from the pinna move into the ear canal, which is a simple tube running through to the middle ear. This tube conducts the vibrations to the tympanic cavity (or eardrum) and amplifies frequencies in the range 3kHz to 12 kHz. Most human speech sounds are distributed in the bandwidth around 3kHz.

The middle ear is the portion of the ear internal to the eardrum. It contains three ossicles (tiny bones) known as the malleus, incus, and stapes (or hammer, anvil, and stirrup) which transfer the vibrations of the eardrum into waves in the fluid and membranes of the inner ear. The primary function of the middle ear is to efficiently transfer acoustic energy from compression waves in air to fluid-membrane waves within the cochlea

• The ossicles directly couple sound energy from the ear drum to the oval window of the cochlea.
• The eardrum is merged to the malleus, which connects to the incus, which in turn connects to the stapes
• Vibrations of the stapes footplate introduce pressure waves in the inner ear
• The auditory ossicles can also reduce sound pressure by uncoupling each other through particular muscles

The inner ear is the innermost part of the ear. It consists of the "bony labyrinth", a hollow cavity in the temporal bone of the skull with a system of passages comprising 2 main functional parts: the cochlea and the vestibular system.

• The cochlea is a spiral-shaped (sometimes compared to a snail-shell) cavity in the bony labyrinth
• It contains the basilar membrane (tissue that runs throughout the cochlea), tectorial membrane, and the organ of corti, which contains hair cells (cilia). Sound causes the basiliar and tectorial membranes to move up and down. Hair cells are bent by this movement and fire sending signal to the auditory nerve
• The cochlea is filled with a watery liquid which moves in response to the vibrations coming from the middle ear via the oval window. As the fluid moves, the cochlear partition moves; thousands of hair cells sense the motion and conver that motion to electrical signals that are communicated via neurotransmitters to many thousands of nerve cells
• These primary auditory neurons transform the signals into electrochemical impulses known as action potentials, which travel along the auditory nerve to structure in the brainstem for further processing

• The cochlear nucleus is the first site of the neuronal processing of the data from the inner ear.
• The superior olivary complex is located in the pons, and receives projections predominately from the ventral cochlear nucleus. It detects interaural level differences and distinguishes interaural time difference
• The inferior colliculi are located just below the visual processing centers known as the superior colliculi. It most likely acts to integrate information from the superior olivary complex and dorsal cochlear nucleus before sending it to the thalamus and cortex
• The medial geniculate nucleus is part of the auditory thalamus. It's thought that the MGB influences the direction and maintenance of attention
• The primary auditory cortex is the part of the temporal lobe that processes auditory information. It's located bilaterally, roughly at the upper sides of the temporal lobes. The auditory cortex plays an important role in hearing, but the specifics of what it actually does are unclear