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8.9 - Hearing and Equilibrium

The ear has a dual function as a sensory organ: it allows us to hear the mechanical vibrations of sound, and maintains our equilibrium both while moving and while staying still. Mechanoreceptors of the ear detect sound waves, which are pressure waves that disturb fluid in the ear. Larger movements of the head disturb the mechanoreceptors controlling equilibrium (balance).

Hearing

Hearing is a complex process that is used to convert the mechanical movements of a sound wave into action potentials that can be processed by the brain. Sound waves travel from the outer ear to the inner ear, then sound information is sent to the auditory cortex of the brain.

The following lists the steps involved in hearing:

• The pinna (earlobe) focuses sound waves so they enter the external ear.
• Sound pressure wave waves travel into the ear through the auditory canal, also called the external acoustic meatus.
• The sound wave hits the tympanic membrane (eardrum) at the end of the auditory canal, causing it to vibrate.
• The auditory ossicles (malleus, incus and stapes) vibrate with the tympanic membrane and amplify the vibrations. The malleus is attached to the tympanic membrane.
• The last auditory ossicle (stapes) transmits the sound vibrations to the perilymph (fluid) of the inner ear through the oval window. The perilymph fills the scala vestibuli of the spiral-shaped cochlea, which is the hearing part of the inner ear.
• As the sound travels through the perilymph, it moves the endolymph in the cochlear duct. The endolymph is separate from the perilymph.
• Movements of the endolymph move the tectorial membrane, which is attached to the sensory hairs of hair cells in the organ of Corti. The hair cells are attached to the basilar membrane, which also moves with the sound vibrations.
• The sound bends the sensory hairs providing information about the frequency and amplitude of the bending is converted into action potentials
• The information is sent in the form of action potentials through the vestibulocochlear nerve (cochlear branch) to the auditory cortex of the cerebrum.
• At the end of the scala vestibuli, any remaining pressure from the sound wave is released through the round (circular) window, so pressure does not build up in the perilymph.

Anatomy of the ear

By User:Dan Pickard [Public domain], via Wikimedia Commons

Figure 8.9.1 - Anatomy of the ear

Organ of Corti cross-section

By Madhero88 (Own work) [CC-BY-SA-3.0 (www.creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Figure 8.9.2 - Cross-section of the Organ of Corti; the hearing structure of the cochlea.

Equilibrium

Our ability to maintain an upright position is extremely important in our everyday lives. The inner ear detects sound in the cochlea, but it can also regulate equilibrium using the vestibule and semicircular canals. The cerebellum combines information from the inner ear, retina and proprioceptors to help us keep our equilibrium.

Static equilibrium: Otoliths

Static equilibrium is the sensation that lets us know where our head is positioned relative to gravity. In other words, it tells us which way our head is tilted.

• The utricle (horizontal) and the saccule (vertical) are two sensory chambers in the vestibule of the inner ear.
• Hair cells (surrounded by supporting cells) project sensory hairs into a gel-like otolithic membrane. At rest, the hairs are oriented to a midpoint called the striola.
• Otolith crystals (otoconia) are embedded in the otolithic membrane, positioned above the cilia.
• When the head moves relative to gravity (either to tilt or to accelerate), the otoliths are pulled in the direction of gravity or opposite the direction of movement.
• This movement pulls the gelatinous membrane, which in turn bends the hairs of the hair cells.
• Hair cells transmit the information about position to the sensory neurons
• The sensory neurons then send action potentials through cranial nerve VIII (vestibular branch) to the cerebellum.

Static equilibrium

By The Anome at en.wikipedia [Public domain], from Wikimedia Commons

Figure 8.9.3 - Structures used to sense static equilibrium in the vestibule.

Dynamic equilibrium: Semicircular canals

Our sense of dynamic equilibrium tells us which way our head is moving in three-dimensional space, including giving us information about rotation. This information is detected in the semicircular canals, which are attached to the vestibule.

There are three semicircular canals, one in the direction of each body plane (frontal, sagittal and transverse). Tufts of hairs from hair cells are embedded in cupulas, gel-like structures that project into the endolymph, the fluid inside the semicircular canals.

• Endolymph fluid fills the semicircular canals, and begins to flow when the head moves
• The cupula bends in the direction of fluid flow and the hairs of hair cells bend with it
• The hair cells transmit information about the direction of bending to the sensory neurons of the vestibulocochlear nerve (vestibular branch), which sends movement information to the cerebellum

Cupula

By NASA[see page for license], via Wikimedia Commons

Figure 8.9.4 - Structures used to sense dynamic equilibrium in the semicircular canals.

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