Hearing

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# 3.8 Revision questions

## Question 1

Discuss the two ways in which the middle ear increases the effectiveness with which sound is transmitted from the external ear to the inner ear.

The first way in which the middle ear enhances the efficiency of sound transfer is to do with the relative sizes of the tympanic membrane and the stapes footplate (which is connected to the oval window). Measurements have shown that the area of the tympanic membrane that vibrates in response to high intensity sound is 55 mm2.

The stapes footplate which makes contact with the oval window has an area of about 3.2 mm2, which is considerably less than the effective area of the tympanic membrane. So, if all the force exerted on the tympanic membrane is transferred to the stapes footplate, then the force per unit area must be greater at the footplate because it is smaller than the tympanic membrane. Put simply, if the same force is applied to a large area and to a small area, the force applied to the smaller area will result in a bigger pressure change. You know that if you hit a wall with a hammer you make a small dent but if you hit a nail with a hammer swung with the same force, all the force is concentrated on a small point and the nail is driven into the wall. In fact the tympanic membrane and the footplate differ in size by a factor of 17 (55 mm2/3.2mm2=17) so pressure at the footplate (force per unit area) is 17 times greater than at the tympanic membrane and therefore the air pressure can stimulate the fluid filled inner ear.

The second way in which the middle ear enhances the efficiency of sound transfer is through the lever action of the ossicles. Figure 2 shows how a lever system can increase the force of an incoming signal. In the figure, the lever is pivoting around a fulcrum at point C. The distance D1 between the fulcrum and the point of the applied force is larger than the distance D2, between the fulcrum and the position of the resultant force. The increase in force due to lever action is given by the formula:

The second way in which the middle ear enhances the efficiency of sound transfer is through the lever action of the ossicles. Figure 2 shows how a lever system can increase the force of an incoming signal. In the figure, the lever is pivoting around a fulcrum at point C. The distance D1 between the fulcrum and the point of the applied force is larger than the distance D2, between the fulcrum and the position of the resultant force. The increase in force due to lever action is given by the formula:

Fresultant = Fapplied × (D1/ D2)

Therefore the closer the fulcrum is to the point of the resultant force, the larger this force will be. The ossicles of the middle ear are arranged so that they act like a lever. The length of the malleus corresponds to D1 (the distance between the applied force and the fulcrum), while the incus acts as the lever portion between the fulcrum and the resultant signal (D2). Measurements of the length of these two bones indicate that the lever system of the ossicles increases the force at the tympanic membrane by a factor of 1.2 at the stapes. In addition, the tympanic membrane tends to buckle as it moves causing the malleus to move with about twice the force. So, overall the increase in pressure at the stapes footplate is in the region of 17 × 1.2 × 2 = 40.8. The reduction in sound level caused by the fluid/air interface is estimated to be about 30 dB. Therefore the middle ear counteracts this reduction.

## Question 2

Draw a flow-diagram to illustrate the route a sound (pressure) wave takes from the time it enters the external ear to the point at which it reaches the round window.

pinna → external auditory canal → tympanic membrane → malleus → incus → stapes → oval window → scala vestibuli → helicotrema → scala tympani → round window

## Question 3

Describe the basic structure of the cochlea and discuss how the different structures contribute to the reception of sound.

The cochlea has a spiral shape resembling the shell of a snail. Unravelled, the cochlea's hollow tube is about 32 mm long and 2 mm in diameter. The tube of the cochlea is divided into three chambers: the scala vestibuli, the scala media (or cochlear duct) and the scala tympani. The three scalae wrap around inside the cochlea like a spiral staircase. The scala vestibuli forms the upper chamber and at the base of this chamber is the oval window. The lowermost of the three chambers is the scala tympani. It too has a basal aperture, the round window, which is closed by an elastic membrane. The scala media or cochlear duct separates the other two chambers along most of their length. The start of the cochlea, where the oval and round windows are located is known as the basal end, while the other end, the inner tip, is known as the apical end. The scala vestibuli and the scala tympani communicate with one another via the helicotrema, an opening in the cochlear duct at the apex. Both scala vestibuli and scala tympani are filled with the same fluid known as perilymph while the scala media is filled with endolymph.

Between the scala vestibuli and the scala media is a membrane called Reissner's membrane and between the scala tympani and the scala media is the basilar membrane. Lying on top of the basilar membrane within the cochlear duct is the organ of Corti, and hanging over the organ of Corti is the tectorial membrane. In response to sound entering the cochlea, the fluid within the cochlea vibrates. The key factor in the response of the inner ear is the mechanical response of the basilar membrane and organ of Corti. These two structures translate the mechanical vibrations of the inner ear fluids into neural responses in the auditory nerve. The vibration of the fluids causes the basilar membrane to move which creates a shearing motion between the basilar membrane and the overlying tectorial membrane. This in turn causes the cilia of the hair cells contained within the organ of Corti to bend. The bending of the cilia results in the nerve fibre at the base of the hair cell initiating a neural potential that is sent along the auditory nerve in the form of action potentials. Thus the hair cells in conjuction with the basilar membrane translate mechanical information into neural information.

## Question 4

What is a travelling wave in the context of the response of the basilar membrane to an incoming sound signal?

It is the motion set up on the basilar membrane in response to movement of the cochlea fluids. The wave propagates from the base of the membrane towards the apex. The point of maximal displacement of the wave is determined by the frequency of the incoming sound.

## Question 5

What are the different properties of the fluids found in the main compartments of the cochlea? How do they contribute to the transduction of a neural signal?

Endolymph is found in the scala media and perilymph is found in the scala vestibuli and scala tympani. Endolymph has an ionic concentration similar to that of intracellular fluid, high K+ and low Na+ (even though it is extracellular). Perilymph has an ionic content similar to that of cerebrospinal fluid, low K+ and high Na+. Because of the ionic concentration differences, the endolymph has an electrical potential that is about 80 mV more positive than the perilymph. The stereocilia of the hair cells are bathed in endolymph while the base of the hair cells and their afferent dendrites are bathed in perilymph. When the stereocilia are bent by movement of the basilar membrane, they either depolarise or hyperpolarise, depending on the direction in which they are bent. The receptor potential, which is either above or below the resting potential of the hair cell, results from opening or closing potassium channels in the tips of the stereocilia. When the cell depolarises, K+ channels open and more K+ enters the cell. When the cell hyperpolarises, K+ channels, which are normally partially open, close and inward movement of K+ is prevented.