15.2 – Anatomy & Physiology of the Sensory System
Audition (Hearing), Equilibrium (Balance), and Somatosensation (Touch)
Watch Hearing & Balance: Crash Course Anatomy & Physiology #17 (11 min) on YouTube
Media 15.2: CrashCourse. (2015, May 4). Hearing & balance: Crash Course anatomy & Physiology #17 [Video]. YouTube. https://youtu.be/Ie2j7GpC4JU
Audition (Hearing)
Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear (see Figure 15.1).
- The external ear consists of the auricle, sometimes referred to as the pinna, ear canal, and tympanic membrane.
The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, which vibrates after it is struck by sound waves. - The middle ear consists of the ossicles, oval window, and tympanic membrane.
The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. Vibrations of the ossicles travel through the oval window, moving fluid in a wave-like motion. The frequency of the fluid waves match the frequencies of the sound waves. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning.
- The inner ear is often described as a bony labyrinth, as it is composed of a series of canals embedded within the temporal bone.
It consists of the cochlea which is responsible for hearing and the vestibule that is responsible for balance. The neural signals from these two regions are relayed to the brain stem through separate fiber bundles. However, these two distinct bundles travel together from the inner ear to the brain stem as the vestibulocochlear nerve. Sound is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the spiral ganglia. These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window.
The image below is a cross-sectional view of the cochlea that shows the scala vestibuli and scala tympani run along both sides of the cochlear duct (see Figure 15.2). The cochlear duct contains several organs of Corti, which tranduce the wave motion of the two scala into neural signals. The organs of Corti lie on top of the basilar membrane, which is the side of the cochlear duct located between the organs of Corti and the scala tympani. As the fluid waves move thro