Twenty Eight Flashcards
Where are hair cells located? What do they transduce?
Mechanical transduction is similar for Audition and Vestibular Sensations. The auditory receptor cells are the hair cells of the neuroepithelium of the cochlea. The neuroepithelia for both auditory and vestibular sensations are along the surface of the membranous labyrinth within the temporal bones. These neuroepithelia all have hair cells at an interface between the different fluids: the endolymph and perilymph. These hair cells all transduce a mechanical sheering force that is tangential to the hair cell’s surface. [The transduction mechanism will be discussed in the lecture on vestibular physiology].
What is the basic sequence of vibrations from the external ear to the inner ear? How is the cavity of the middle ear continuous with the pharyngeal cavity? Is it normally open or shut? How can it become open or shut? What happens when its open or shut? What are two muscles present in the middle ear? Where do they connect? What is their function and purpose?
Basic Sequence of Vibrations from the Middle to the Inner Ear. Sounds are funneled by the auricle into the external auditory meatus, which guides the air vibrations to the taut
tympanic membrane. These air vibrations cause mechanical vibrations of the tympanic
membrane at the same frequency that is transferred to the little bones (ossicles) of the
middle ear (malleus, incus and stapes), which in turn vibrate the oval window of the
cochlea. The cavity of the middle ear is continuous with the pharyngeal cavity via the
Eustachian tube, which is normally closed. However it can be pulled wide open by the
tensor palati muscle, thereby equalizing the pressure of the middle ear and the
atmospheric pressure. Two other muscles are present in the middle ear cavity: the tensor
tympani and the stapedius muscles. They are attached to the tympanic membrane and the
stapes bone respectively. They function to pull the tympanic membrane medially and the
stapes bone laterally, so that the ossicles can be tensed or relaxed, controlling the
amplitude of vibrations transmitted by the middle ear.
Explain the acoustic reflex? Which muscles are responsible? How long before it starts? How long can it last? What causes it?
The Acoustic Reflex. The tensor tympani and stapedius muscles function to stabilize the
ossicular chain in the middle ear. That is, when they contract they form a more rigid
bridge between the tympanic membrane and the oval window. Sudden very loud sounds
would shake this ossicular chain apart and the transduction process would be lost. The
acoustic reflex occurs when a sudden noise is applied to the ear. These muscles contract
to protect the ossicular chain. The latency between the sound and the time of muscle
contraction is 40-80 milliseconds. For very long-lasting sounds, the muscles can remain
contracted for many minutes. This reflex is bilateral. More recently, it has been found
that these muscles contract in response to yawning, swallowing, speaking and even
during movements of the body. Thus, these muscles are actively damping and
modulating the tension in the ossicular chain.
What is the purpose of the eustachain tube? When is it opened? What can plug it up? What happens if its plugged up? What happens during changes in atmospheric pressure?
The Eustachian Tube provides a means of equalizing the air pressure on either side of the
tympanic membrane. During swallowing, yawning, sneezing and singing, the tensor
palati muscle opens a slit in the pharynx that leads to the middle ear via the Eustachian
tube. Middle ear pressure also equilibrates with the atmospheric pressure via the
Eustachian tube during changes in atmospheric pressure. In rapid increases in altitude (a
drop in atmospheric pressure), the relative pressure in the middle ear rises but has an
“escape route” through the Eustachian tube to the mouth cavity. When descending in
altitude, the atmospheric pressure rises relative to the pressure in the middle ear. For air
to enter the middle ear, the tensor palati muscle has to open up the Eustachian tube.
Unfortunately, the Eustachian tube is also an easy pathway for infection from the buccal
cavity, which is never without pathogenic organisms. Otitis media is a common
infection, especially in children.
What is the pattern of innervation of inner and outer hair cells? Where do the afferents come from and go? Efferents? What do the efferents do at hair cells? What is the purpose?
Pattern of innervation of inner and outer hair cells. Both afferent and efferent axons
contact the hair cells Fig. 4: Afferents: 95% of afferents synapse with inner hair cells.
5% of afferents synapse with outer hair cells. The central afferent axons form the
auditory nerve. Afferents from the inner hair cells have a large diameter and are
myelinated for rapid conduction of information into the CNS. This indicates that they
serve as the main channel for relaying sound information into the brain. Efferent
neurons: The efferent innervation (olivocochlear, OC) originates in the superior olivary
nucleus and sends axons to the cochlea. There are two types of OC axons, (1) Lateral OC
axons which innervate afferent dendrites close to the inner hair cells. Little is known
about these because their very thin axons are difficult to study, and (2). Medial OC axons
innervate outer hair cells. They release acetylcholine that acts on a nicotinic receptor onthe outer hair cell and allows influx of Ca++ which in turn opens Ca-activated K+
channels, allowing K+ efflux that hyperpolarizes the cells. This reduces the response of
outer hair cells, which in turn (a) decreases motility of basilar membrane, (b) protects
cochlear from damage by intense sounds. It has been shown that the OHC can alter its
shape as a function of transmembrane voltage. When an isolated OHC is electrically
stimulated, it responds by altering its length: depolarization induces contraction;
hyperpolarization induces elongation.
How is sound localized?
Sound Localization. The only peripheral factors in localizing sound in your extra-
personal space are the ‘shadowing’ by the head and the shape of the auricle. All other localization cues are detected centrally by comparing the responses from the two ears and interpreting sound echoes. Many auditory brainstem structures are dedicated to this analysis.
What are some ways in which frequency is detected? Where are high frequency waves detected? Low frequency waves?
The “place theory” and “traveling wave” hypotheses of frequency detection. How do we achieve such a wide range (10 1/2 octaves) of hearing? Herman von Helmholtz (1850’s), a surgical resident in the Prussian Army, proposed that the width and tension of the basilar membrane creates a series of resonators that responded selectively to frequency, thus activating different hair cells (the resonance theory). The basilar membrane can thereby be “mapped” for its sound frequency response (Fig. 9). In the 1930’s, Georg von Bekesy proposed that sound does not lead to the resonance of only a narrow segment of the basilar membrane, but initiates a traveling wave along the cochlea’s length from the oval window to the helicotrema (traveling wave theory, Fig. 8). However, Von Bekesy’s measurements were made in cadavers and were later shown not to be an accurate representation of the type of movement occurring in the living cochlea. Recent, accurate measurements show that the normal basilar membrane movement more sharply tuned and larger in amplitude than predicted by a traveling wave. Also, the basilar membrane vibration was non-linear, suggesting an active process in the organ of Corti. Video microscopy now shows electromotile responses in isolated outer hair cells that correlated with membrane potential. In fact, an integral membrane protein Prestin has been cloned in hair cell membranes that is gated by intracellular anions to control contraction. These findings suggest active frequency tuning by a local neural mechanism.
Basically, width, tension of the basilar membrane and the neuronal control of hair cells. High frequency at base where it’s skinny and tight. Low frequency at apex where its thicker and looser.
