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October, 2006
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Cochlea: The part of the inner ear (Labyrinth) that is concerned with hearing. It forms the anterior part of the labyrinth, as a snail-like structure that is situated almost horizontally anterior to the Vestibular labyrinth.

The Cochlea

JoVE 10855

The cochlea is a coiled structure in the inner ear that contains hair cells—the sensory receptors of the auditory system. Sound waves are transmitted to the cochlea by small bones attached to the eardrum called the ossicles, which vibrate the oval window that leads to the inner ear. This causes fluid in the chambers of the cochlea to move, vibrating the basilar membrane.

The basilar membrane extends from the basal end of the cochlea near the oval window to the apical end at its tip. Although the cochlea itself narrows towards the apical end, the basilar membrane has the opposite geometry—becoming wider and more flexible towards the apical end. Primarily because of these physical characteristics, the apical end of the basilar membrane maximally vibrates when exposed to low-frequency sounds, while the narrower, stiffer basal end maximally vibrates when exposed to high frequencies. This gradient of frequency response creates tonotopy—a topographic map of pitch—in the cochlea. The hair cells are stimulated by the shearing force created by the vibration of the basilar membrane below them, relative to the stiffer tectorial membrane above them. Because of the tonotopy of the basilar membrane, hair cells are maximally stimulated by different frequencies depending on where they are in the cochlea. Those at the basal end respond be

 Core: Biology


JoVE 10853

When we hear a sound, our nervous system is detecting sound waves—pressure waves of mechanical energy traveling through a medium. The frequency of the wave is perceived as pitch, while the amplitude is perceived as loudness.

Sound waves are collected by the external ear and amplified as they travel through the ear canal. When sounds reach the junction between the outer and middle ear, they vibrate the tympanic membrane—the eardrum. The resulting mechanical energy causes the attached ossicles—a set of small bones in the middle ear—to move. The ossicles vibrate the oval window, the outermost part of the inner ear. In the labyrinth of the inner ear, the sound wave energy is transferred to the cochlea—a coiled structure in the inner ear—causing the fluid within it to move. The cochlea contains receptors that transduce mechanical sound waves into electrical signals that can be interpreted by the brain. Sounds within the hearing range vibrate the basilar membrane in the cochlea and are detected by hair cells on the organ of Corti, the site of transduction. Along the primary auditory pathway, the signals are sent through the auditory nerve to the cochlear nuclei in the brainstem. From here, they travel to the inferior colliculus of the midbrain and up to the thalamus, and then to the primary auditory cortex. Along this pat

 Core: Biology

Hair Cells

JoVE 10854

Hair cells are the sensory receptors of the auditory system—they transduce mechanical sound waves into electrical energy that the nervous system can understand. Hair cells are located in the organ of Corti within the cochlea of the inner ear, between the basilar and tectorial membranes. The actual sensory receptors are called inner hair cells. The outer hair cells serve other functions, such as sound amplification in the cochlea, and are not discussed in detail here. Hair cells are named after the hair-like stereocilia that protrude from their tops and touch the tectorial membrane. The stereocilia are arranged by height and are attached by thin filaments called tip links. The tip links are connected to stretch-activated cation channels on the tips of the stereocilia. When a sound wave vibrates the basilar membrane, it creates a shearing force between the basilar and tectorial membranes that moves the hair cell stereocilia from side to side. When the cilia are displaced towards the tallest cilium, the tip links stretch, opening the cation channels. Potassium (K+) then flows into the cell, because there is a very high concentration of K+ in the fluid outside of the stereocilia. This large voltage difference creates an electrochemical gradient that causes an influx of K+ once the channels are opened. This influx o

 Core: Biology

The Vestibular System

JoVE 10856

The vestibular system is a set of inner ear structures that provide a sense of balance and spatial orientation. This system is comprised of structures within the labyrinth of the inner ear, including the cochlea and two otolith organs—the utricle and saccule. The labyrinth also contains three semicircular canals—superior, posterior, and horizontal—that are oriented on different planes. All of these structures contain vestibular hair cells—the sensory receptors of the vestibular system. In the otolith organs, the hair cells sit beneath a gelatinous layer called the otolithic membrane, which contains otoconia—calcium carbonate crystals—making it relatively heavy. When the head is tilted, the otolithic membrane shifts, bending the stereocilia on the hair cells. In the semicircular canals, the cilia of the hair cells are contained within a gelatinous cupula, which is surrounded by endolymph fluid. When the head experiences movements, such as rotational acceleration and deceleration, the fluid moves, bending the cupula and the cilia within it. Similar to the auditory hair cells, displacement towards the tallest cilium causes mechanically-gated ion channels to open, depolarizing the cell and increasing neurotransmitter release. Displacement towards the shortest cilium hyperpolarizes the cell and decreases neurotr

 Core: Biology

Ear Exam

JoVE 10148

Source: Richard Glickman-Simon, MD, Assistant Professor, Department of Public Health and Community Medicine, Tufts University School of Medicine, MA

This video describes the examination of the ear, beginning with a review of its surface and interior anatomy (Figure 1). The cartilaginous auricle consists of the helix,…

 Physical Examinations II

A Comparative Study of Drug Delivery Methods Targeted to the Mouse Inner Ear: Bullostomy Versus Transtympanic Injection

1Instituto de Investigaciones Biomédicas (IIBm) Alberto Sols CSIC-UAM, 2Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), Instituto de Salud Carlos III (ISCIII), 3Instituto de Investigación Sanitaria La Paz (IdiPAZ), 4Facultad de Veterinaria, Universidad Complutense de Madrid, 5Departmento de Otorrino laringología, Hospital Universitario La Paz

JoVE 54951

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