How are we able to Hear? Explaining the Human Auditory Pathway

The auditory pathway converts sound waves from the external environment into perception of sound and patterns of neural activity in the brain. Pathways involved can be hindered by disease, obstruction, physical damage, leading to hearing impairments. Several tests can be used clinically to determine the type of impairment and whether an individual is suffering from any form of hearing loss.There are three important physical sections of the Human ear that lead to stimulation/depolarization of the auditory nerve leading to the eventual auditory cortex and the perception of sound. These are the external, middle, and inner ear, all of which are important in the conversion of sound waves into neural activity.

External Ear: The first stage of the auditory pathway involves an external output of sound waves and the external ear to collect these sound waves heard. Externally the ear consists of three prominent structures: the auricle (pinna), external auditory canal, and the tympanic membrane. The auricle which is the external cartilaginous flap and the external auditory canal help guide the sound waves to the tympanic membrane, or the ear drum. The auditory canal helps increase sound pressure of frequencies around 3 kHz leading to sensitivity of frequencies in the 2-5 kHz range. Another important function of the external ear is to provide cues to a sound source by filtering sound frequencies.

Middle Ear: It is an air filled region divided by the tympanic membrane. Sound wave vibrations from the tympanic membrane travel to the small ossicles of the middle ear. These small bones, the malleus, incus, and stapes are connected by ligaments, transferring sound energy from the external environment to the inner ear for neural interpretation. The middle ear primarily functions as a travelling bridge of sound energy between an air medium of the external environment to the fluid medium of the inner ear. It accomplishes this by drastically increasing (by approximately 200 fold) pressure from the tympanic membrane to when the sound reaches the oval window, connecting to the inner ear. A major pressure increase is derived from the force of the large tympanic membrane onto the small oval window by the lever action of the ossicle bones.

Inner Ear: Sound energy that has been amplified by the external and middle ear sections reaches the cochlea of the inner ear by means of the oval window. The cochlea is a fluid filled coiled structure which has a cochlea partition with fluid filled hollows on either side. The partition contains two membranes; the basilar membrane important for conduction of sound to neural activity, and the tectorial membrane. The travelling wave of a specific frequency moves from the base to the apex of the basilar membrane until a point of maximal displacement of a specific frequency reached. The area along the basilar membrane where maximal displacement is reached will change, dependent on the frequency received. Vibrations occur along the area of the basilar membrane which matches the input frequency and is sensed by the outer and inner hair cells above the basilar membrane. These hair cells convert the vibrations at a specific area along the basilar membrane to an electrical signal for neural activity. Hair cells are connected at their tips by tip links which convert hair bundle movement into an auditory signal. When a hair cell bundle is pushed towards the tallest hair cell, K+ ion channels open at the tips of the cells, leading to a cell depolarisation and Ca2+ influx. Neurotransmitter released from hair cells propagates an electrical response of a postsynaptic afferent auditory nerve.

Brain Pathway: The auditory nerve has its axons extended to the brainstem location of the lateral superior olive where neurons locate the source of a sound. Auditory pathways from the lateral superior olive and others directly from the cochlea nucleus reach the inferior colliculus in the midbrain. This is an important area where sound frequencies are processed and sound localization occurs. The inferior colliculus responds to sounds of biological importance, including sounds from predators or signs of danger, but also from human speech. The auditory pathway finally leads to the auditory cortex which is divided into a primary area, peripheral, and secondary areas. The primary auditory cortex is found on the superior temporal gyrus and is mapped out with areas corresponding to specific frequencies that will be received from the cochlea.