The auditory system Flashcards
What is sound?
Sound is the oscillation of molecules or atoms in a compressible medium. The energy of the oscillations is transmitted as a longitudinal wave in which the medium is alternately compressed and rarefied, causing periodic variations in the pressure of the medium.
Define the amplitude of a sound wave. How may it be expressed? State the unit. Differences in amplitude may be perceived as what?
- The amplitude of a sound wave is the total change in pressure that occurs during a single cycle.
- It is expressed in a logarithmic scale as a ratio of a reference pressure (a sound pressure which is at the threshold of human hearing = 2 x 10-5 Pa).
- The unit of sound pressure amplitude is the decibel (dB)
- Differences in sound pressure level are perceived as differences in loudness which varies with frequency in a manner that is determined by the sensitivity of the ear.
How would you define the frequency of a sound?
What is the audible range for humans in terms of frequency?
The frequency of a sound wave, the perceived pitch of the sound, is the reciprocal of the period (no. of cycles per second)
The frequency response of the human ear is from 20 Hz to 20 kHz optimally.
What is the name given to the wing shaped flap skin and cartilage that makes up the outer ear?
Pinna
Describe the shape of the external auditory meatus and its importance.
It is conical – starts off wide and narrows to the tympanic membrane/ear drum
This focuses the noise and increases the pressure on the tympanic membrane
Describe the functioning of the middle ear structures in sound conduction
- Sound waves pass along the external auditory meatus striking the tympanic membrane which resonates faithfully in response.
- The movement of the ear drum is transferred with an overall efficiency of about 30% to the fluid in the inner ear by a lever system, composed of three ear ossicles, lying in the tympanic cavity (air-filled).
- The malleus is fixed at its thin end (the handle) to the tympanic membrane. Its thick end (the head) articulates with the head of the incus via a saddle- shaped joint. The long process of the incus makes a ball and socket joint with the head of the stapes. The base of the stapes is attached by an annular ligament to the oval window of the labyrinth. The malleus vibrates with the tympanic membrane. Inward movement locks the joint between the malleus and the incus, driving the long process of the incus inward, pushing the stapes in the same direction to exert a pressure on the perilymph beyond the oval window.
What is the point of the middle ear? Why isn’t the tympanic membrane continuous with the cochlea?
The cochlea contains fluid, in which you are trying to induce a pressure wave
If the tympanic membrane was continuous with the cochlea, you would go straight from air to fluid and 99% of the energy will bounce back due to impedance
Sound waves require more energy to travel through fluid than air so the increase in pressure of vibration allowed by the ossicles is crucial for this conduction
State the 2 ways by which the ossicles increase the pressure of vibration of the tympanic membrane by the time it reaches the inner ear
Since the area of the oval window is 20 times smaller than the tympanic membrane, the pressure at the oval window is proportionally greater.
The mechanical advantage gained by the lever action of the three ossicles.
They amplifies the sound by about 20-30 dB
State the two middle ear muscles
Tensor Tympani
Stapedius
How do these two middle ear muscles help to prevent damage of the ossicles by excessive vibration due to loud noise?
- When they contract together the handle of the malleus and the tympanic membrane are pulled inwards and the base of the stapes is pulled away from the oval window. This reduces sound transmission by 20 dB, especially for low frequencies.
- Reflex contraction of these muscles in response to loud noise (or self-generated vocalisation) may prevent damage to the inner ear but since the reaction time is 40–60 ms this ‘tympanic reflex’ affords no protection against brief loud sounds.
State the purpose of the auditory canal/Eustachian tube
The auditory canal/Eustachian tube connects the middle ear to the pharynx which allows the air pressure to be equalised when changing altitude.
What is hyperacusis? What can it be caused by?
Painful sensitivity to low intensity sounds – can occur in conditions that lead to flaccid paralysis of the auditory reflex muscles (e.g. Bell’s Palsy)
Describe the anatomy of the auditory part of the inner ear i.e. the cochlea
- The cochlea is a bony canal/tube 3.5 cm long, which spirals around a central pillar, called the modiolus.
- The cochlea is bisected from its basal end almost to its apical end by the cochlear partition (or cochlear duct) which is bounded on three sides by the basilar membrane, the stria vascularis, and Reissner’s or vestibular membrane.
- A distinct channel runs within the cochlear duct called the scala media, which contains endolymph.
- The floor of the scala media is the basilar membrane, on which rests the sensory apparatus = the spiral organ of Corti
- The cochlear duct also supports the tectorial membrane when lies immediately above the organ of Corti
- The compartments on either side of the cochlear duct, called the scala vestibuli and the scala tympani, contain perilymph and are continuous with each other via a small gap known as the helicotrema situated at the apex of the cochlea.
- Both the oval window and round window (regions where bone is absent surrounding the cochlea) are at the basal end of the tube.
Describe the conduction/vibration transmission in the cochlea, and the importance of the round window
- Pressure waves generated at the oval window (due to vibration of the oval window by the footplate of the stapes) are propagated through the perilymph of the scala vestibuli into that of the scala tympani and to the round window where the energy dissipates (by causing a compensatory bulge of the round window)*.
- The pressure wave importantly causes the flexible basilar membrane (and vestibular membrane) to vibrate.
*Note that if it were a completely closed and inflexible system, then the movement of the stapes would be unable to displace the perilymph
What is the Organ of Corti?
A narrow sheet of columnar epithelium running the length of the cochlear duct that contains, as well as other supporting cells, sensory hair cells.
The Organ of Corti includes the basilar and tectorial membranes.
What are the two types of sensory receptor cells in the Organ of Corti?
Inner and outer hair cells
Describe the features and overall function of inner hair cells.
- Single row of cells
- Approx. 3500 per human cochlea
- Stereocilia not in contact with the tectorial membrane
- They are the sensory receptors; 95% of afferent projections (sensory axons that carry signals from the cochlea towards the brain) project from inner hair cells
Describe the features and function of outer hair cells.
- Three rows of cells
- Approx. 12000 per human cochlea
- Steriocilia in contact with the tectorial membrane
- Most of the efferent projections (from the brain to the cochlea) terminate on outer hair cells
- Outer hair cells contract in a voltage-dependent manner. Depolarisation causes them to shorten (=electromotility) and this is due to the reorientation of the protein prestin. They are therefore able to augment the vibrations of the basilar membrane, a process called cochlear amplification.
Describe the innervation of the two hair cell types
Each inner hair cell is innervated by about 10 myelinated axons of large bipolar cells with their cell bodies in the spiral (or cochlear) ganglion of the cochlear nerve (part of CN8) found in the modiolus.
An unmyelinated axon from small bipolar cells in the spiral ganglion synapses with 10 outer hair cells.
What are otoacoustic emissions?
They are (inaudible) sounds of cochlear origin that are caused by the motion of the outer hair cells as they energetically respond to auditory stimulation.
Describe the appearance and organisation of the stereocilia
- The hair cell is a flask-shaped epithelial cell named for the bundle of hair-like processes that protrude from its apical end into the scala media.
- Each stereocilium tapers where it inserts into the apical membrane, forming a hinge about which it pivots.
- The stereocilia are graded in height and are arranged in a bilaterally symmetrical fashion (with the cluster shaped like an ‘arc’ in the case for outer hair cells)
- Fine filamentous structures, known as tip links, run in parallel to the plane of bilateral symmetry, connecting the tips of adjacent stereocilia
Describe and explain the topographical mapping of frequency (tonotopy)
- The basilar membrane is wider and more flexible at the apical end and narrower and stiffer at the basal end.
- A membrane that varies systematically in its width and flexibility vibrates maximally at different positions as a function of the stimulus frequency.
- Points responding to high frequencies are at the base; points responding to low frequency are at the apex.
- This allows for encoding the frequencies of a sound.
Explain how a propagating pressure wave initiates sensory transduction
- Because the basilar membrane and the overlying tectorial membrane are anchored at different positions, the vertical component of the traveling wave is translated into a shearing motion between these two membranes.
- This motion causes the stereocilia in contact with the tectorial membrane to bend first one way and then the other => voltage changes across the hair cell membrane
With respect to the hair cells themselves, describe and explain the process of mechanoelectrical transduction
- Bending of the stereocilia in the direction of the tallest stereocilium stretches the tip links, directly opening cation-selective mechanoelectrical transduction (MET) channels located at the end of the link => depolarising the hair cell (i.e. K+ channels open leading rapid influx of K+ from the endolymph) - the opening of MET channels relaxes the tip link
- Movement in the opposite direction compresses the tip links, closing the channels => hyperpolarising the hair cell
- Depolarisation leads to transmitter/glutamate release (calcium influx mediated exocytosis) from the basal end of the hair cell.
- This triggers action potentials in cranial nerve fibres the follow the up-and-down vibration of the basilar membrane at relatively low frequencies.