Hearing and Balance 1
Slide 3 Sound is produced by vibration of an object which produces alternating waves of pressure and rarefaction, for example this tuning fork. Slide 4 Two characteristics of sound that we encode in our auditory system: frequency (pitch) and amplitude (intensity, loudness). 2
Slide 5 Measurement of sound intensity with some examples. Those shown in red would have the capacity to cause permanent damage during exposures of a few hours; in the most extreme case, a single gun shot is enough to cause permanent damage. A rock concert is around 110-120 db. Slide 6 Structure of the ear, as an outline of what we will deal with in this lecture. Note the three parts: outer ear (the outside part and the tube); middle ear (the bones) and inner ear (the cochlea). The outer ear won t be dealt with in detail, but you should understand that it alters the sound coming into our ears so that we can tell the difference between a sound from in front and from behind. Slide 7 The outer ear allows us to distinguish the direction from which sound comes (in front or behind us). To demonstrate its function, listen to the recording on the YouTube link shown and also the Virtual Haircut recording on the website. 3
Slide 8 This shows how the binaural recordings were made: if microphones are placed in an artificial head, exactly where the eardrums would be, they pick up all the reflections from the outer ear and give a striking perception of sound coming from in front or behind. Slide 9 The middle ear amplifies sound by exerting force over a reduced area, thus increasing pressure. The eardrum has a 17-fold larger surface area than the oval window of the cochlea, on which the stapes presses. That means that a given force generates a much greater pressure because it is concentrated on a smaller area. Slide 10 The first of two examples of audiograms of hearing loss. The scale on the vertical axis is hearing loss in db, so normal hearing would be a straight line on 0dB. In the example shown here, when the sound stimulus is applied through the bone, hearing is nearly normal. This means that the inner ear is not damaged. However, when the stimulus is given through headphones, i.e. through the air, there is a hearing loss of 20-40dB over a wide range of frequencies. Ths means that the middle ear is not functioning correctly, because the middle ear is involved in conduction of sound coming through the air but not of sound coming through the bone. 4
Slide 11 To compare the size of the bones with something easily recognisable, the bones of the middle ear are laid on a coin in this illustration from a U.S. textbook. The coin is a dime, with diameter 17.91mm. This is larger than a 1 cent coin (16.25mm) and slightly smaller than a 2 cent coin (18.75mm). You can work out from this that the bones are 5-9mm long, the smallest bones in our bodies. Slides 12-13 Position and structure of the cochlea. Imagine cutting through it (red circle in slide 13). This would give you the cross-section shown in the lower right part of this slide. There are 3 fluidfilled cavities running along the length of the cochlea. The basilar membrane is the most important structure in terms of hearing: its vibration due to sound is the direct stimulus that allows us to hear. Reissner s membrane has no mechanical function in hearing and simply separates two fluid spaces; we will not deal with it further. The part of the cochlea that actually detects sound is the organ of Corti. This sits on the basilar membrane as shown. You ve got to imagine it as a long narrow strip of cells - about 3cm long, because that s the length of the cochlea if unrolled. 5
Slide 14 The organ of Corti in close-up. Note what is circled here - the three rows of outer hair cells and one row of inner hair cells. The tectorial membrane sits on top of the hair cells. The inner hair cells are the ones with the main sensory role the outer hair cells function is to contract when stimulated (see later). Slide 15 Transduction (conversion of sound into action potentials) depends on the movement of the hairs (the stereocilia) of the hair calls. When bent in one direction (towards the longest one) the cell is depolarised and the neurotransmitter glutamate is released. When bent in the other direction, the cell is hyperpolarised, and no glutamate is released. Slide 16 This slide shows the consequences of the above for auditory nerve activity. Action potentials only happen when the stereocilia are bent in one direction. 6
Slide 17 This slide explains how we can detect small differences in the pitch of a note. It depends on different parts of the basilar membrane vibrating at different frequencies: there is a stiff end, vibrating more at high frequencies, and a floppy end, vibrating more at low frequencies. Slide 18 Contraction of the outer hair cells increases sensitivity of hearing. When they are stimulated, they contract. This of course will stimulate the inner hair cells at the same point along the basilar membrane. So contraction of the outer hair cells acts as an amplifier that can increase the activation of the inner hair cells. Critically, this will happen only at the frequency that stimulates the outer hair cells. This provides a way of amplifying selectively one particular narrow band of frequencies. Slide 19 When we allow excessively loud noise into our cochlea, we risk damaging the outer hair cells. This damage is irreversible. Here we see the result of exposing the cochlea to very loud noise. Typically about 120dB (the level of noise at a rock concert) for 1 hour would cause damage like this. Note the damage to the hair cell on the right, compared to the normal hair cell shown on the left. When outer hair cell function is lost, we lose the active amplifiction that they would normally give us and thus we lose sensitivity of hearing. 7
Slide 20 Here, both air and bone conduction are equally affected. This means first of all that the middle ear is normal. However, there is hearing loss and this is very pronounced at high frequencies. This is the kind of hearing loss that is typical of old age. It results in difficulty in distinguishing consonant sounds in speech, which involve high frequencies. Slide 22 We have 5 balance organs on each side, in the inner ear. These detect gravity or head tilt (the macula or otolith organs) and head turning (the semicircular canals). 8
Slide 24 Detecting the direction of gravity is a fundamental necessity for all organisms. Plants are sensitive to gravitation and will always grow roots downwards and stems upwards, regardless of what way up the seed is put into the ground. In humans, multiple small stones made of chalk sit in a gel mounted on the cilia of multiple sensory cells. The macula organs are also known as otolith organs, referring to the stones, and the stones themselves are known as otoconia (both from Greek words: see the translations on the slide). Slide 25 The human macula/otolith organs are in the inner ear in the positions shown. There are two in each ear: the utricle, with horizontal orientation, and the saccula, with vertical orientation. Slide 26 Stimuli that excite the otolith organs: relations between linear acceleration, gravity and head tilt 9
Slide 27 The utricle is stimulated by head tilting, where gravity pulls the otoconia in one direction or another, and also by acceleration in the horizontal plane (e.g. starting and stopping a car). Remember what acceleration is: simply, a change in velocity. So stopping is a form of acceleration. Slide 29 Angular acceleration (head turning) is also detected in the inner ear, adjacent to the otolith organs, in the semicircular canals (3 each side). Slide 30 The semicircular canals also contain hair cells, but in one specific place, called the cupula. The cupula blocks off the fluid-filled semicircular canal. When we turn in the plane of one of the canals, we have to accelerate this fluid, and the force of the acceleration is exerted on the cupula. This bends the cupula and thus the hair cells. Hair cels are bent in one direction or the other, corresponding to excitation or inhibition. Hair cells within each cupula have only one preferred orientation, so that a particular movement either excites or inhibits all of them. 10
Slide 31 This means that when we turn continuously, as in the period marked Constant velocity, the hair cells are stimulated when we start to turn (during the acceleration phase), but as we continue turning the stimulus fades away. But when we stop, the hair cells are stimulated in the opposite direction as the fluid in the semicircular canals is decelerated, giving the familiar sensation that the room is spinning when we have been turning and then stop suddenly. Slide 32 It is clear from the slide above that there is a resting level of activity in the vestibular nerve, which can be increased by a stimulus in one direction or decreased by a stimulus in the other direction. This has an important consequence: if nerve activity is lost (e.g. conduction is blocked by a tumour pressing on the nerve), the result is not a loss of sensation, but a sensation of continuous turning (such as would cause strong hyperpolarisation). Slide 33 This helps to illustrate what happens if we lose nerve activity from a semicircular canal. During a normal turn to the left, the activity in the right horizontal semicircular canal is reduced; if it were to become zero owing to conduction block, the person would feel a continuous sensation of turning to the left. The same would happen if conduction from one or more of the vertical semicircular canals were to be lost, but it would be more alarming. 11
Slide 34 As well as block of the vestibular nerve, disturbances of balance can also be caused by abnormalities in the fluids in the inner ear. The composition of the fluids (endolymph and perilymph) can be affected by some medications and diseases. Medications and diseases that affect balance also often affect hearing, and this is because the endolymph and perilymph are continuous between the cochlea and the vestibular system. 12