The auditory and vestibular systems

Question 1

Name the three major divisions of the auditory system

Answer 1

Outer ear, middle ear, inner ear

Question 2

What does the outer ear consist of?

Answer 2

The visible portion of the ear consists primarily of cartilage covered by skin, forming a sort of funnel called the pinna. The entrance to the internal ear is called the auditory canal, which extends about 2.5 cm (1 inch) inside the skull before it ends at the tympanic membrane, also known as the eardrum.

Question 3

What implications does the shape of the pinna have?

Answer 3

The pinna helps collect sounds from a wide area. The shape of the pinna makes us more sensitive to sounds coming from ahead than from behind. The convolutions in the pinna play a role in localising sounds.

Question 4

How is the pinna different in other animals than it is in humans?

Answer 4

In humans, the pinna is more or less fixed in position, but animals such as cats and horses have considerable muscular control over the position of their pinna and can orient it toward a source of sound.

Question 5

Describe the structure of the middle ear

Answer 5

Connected to the medial surface of the tympanic membrane is a series of bones called ossicles (from the Latin for “little bones”; the ossicles are indeed the smallest bones in the body).

Question 6

Describe the structure of the inner ear

Answer 6

Located in a small air-filled chamber, the ossicles transfer movements of the tympanic membrane into movements of a second membrane covering a hole in the bone of the skull called the oval window. Behind the oval window is the fluid-filled cochlea, which contains the apparatus for transforming the physical motion of the oval window membrane into a neuronal response.

Question 7

Thus, how do the first stages of the basic auditory pathway look like?

Answer 7

Sound wave moves the tympanic membrane. → Tympanic membrane moves the ossicles. →
Ossicles move the membrane at the oval window. →
Motion at the oval window moves fluid in the cochlea. →
Movement of fluid in the cochlea causes a response in sensory neurons.

Question 8

What happens when a neural response of sound is generated in the inner ear?

Answer 8

The signal is transferred to and processed by a series of nuclei in the brain stem. Output from these nuclei is sent to a relay in the thalamus, the medial geniculate nucleus (MGN). Finally, the MGN projects to primary auditory cortex, or A1, located in the temporal lobe.

Question 9

Why does the auditory pathway appear more complex than the visual cortex? How are these two systems actually similar?

Answer 9

The auditory pathway appears more complex than the visual pathway because there are more nuclei intermediate between the sensory organ and the cortex. Also, in contrast to the visual system, there are many more alternative pathways by which signals can travel from one nucleus to the next.

Nonetheless, the amount of information processing in the two systems is similar when you consider that the cells and synapses of the auditory system in the brain stem are analogous to interactions in the layers of the retina.

Question 10

What do afferents from the spiral ganglion cells enter the brain stem through and where do they innervate?

Answer 10

Afferents from the spiral ganglion enter the brain stem in the auditory– vestibular nerve. At the level of the medulla, the axons innervate the dorsal cochlear nucleus and ventral cochlear nucleus ipsilateral to the cochlea where the axons originated. Each axon branches so that it synapses on neurons in both cochlear nuclei.

Question 11

From this point on, the system gets more complicated, and the connections are less well understood, because there are multiple parallel pathways.

however describe one particularly important pathway from the cochlear nuclei to the midbrain

Answer 11

Cells in the ventral cochlear nucleus send axons that project to the superior olive (also called the superior olivary nucleus) on both sides of the brain stem.

Axons of the olivary neurons ascend in the lateral lemniscus (a lemniscus is a collection of axons) and innervate the inferior colliculus of the midbrain.

Question 12

Many efferents of the dorsal cochlear nucleus follow a route similar to the pathway from the ventral cochlear nucleus, however they differ in what key way?

Answer 12

The dorsal path bypasses the superior olive.

Question 13

What do all ascending auditory pathways have in common regarding their pathway?

Answer 13

Although there are other routes from the cochlear nuclei to the inferior colliculus, with additional intermediate relays, all ascending auditory pathways converge onto the inferior colliculus.

Question 14

Where do the connections go from the inferior colliculus?

Answer 14

The neurons in the inferior colliculus send axons to the medial geniculate nucleus (MGN) of the thalamus, which in turn projects to auditory cortex.

Question 15

Give an example of how projections and brain stem nuclei other than the ones described contribute to the auditory pathways

Answer 15

The inferior colliculus sends axons not only to the MGN but also to the superior colliculus (where the integration of auditory and visual information occurs) and to the cerebellum.

Question 16

Is there feedback in the auditory pathways? If not, why?

Answer 16

There is extensive feedback in the auditory pathways. For instance, brain stem neurons send axons that contact outer hair cells, and auditory cortex sends axons to the MGN and inferior colliculus.

Question 17

Do auditory nuclei in the brainstem receive input from one or both ears?

Can deafness in one ear therefore arise in the brainstem?

Answer 17

Each cochlear nucleus receives input from just the one ear on the ipsilateral side; all other auditory nuclei in the brain stem receive input from both ears.

This explains the clinically important fact that the only way by which brain stem damage can produce deafness in one ear is if a cochlear nucleus (or auditory nerve) on one side is destroyed.

Question 18

Describe the nature of the input from the neurons in the spiral ganglion of the cochlea

Answer 18

Because most spiral ganglion cells receive input from a single inner hair cell at a particular location on the basilar membrane, they fire action potentials only in response to sound within a limited frequency range. Hair cells are excited by deformations of the basilar membrane, and each portion of the membrane is maximally sensitive to a particular range of frequencies.

Question 19

What is meant by a characteristic frequency?

Answer 19

Action potentials were recorded from a single auditory nerve fiber (i.e., the axon of a spiral ganglion cell).The neuron is most responsive to sound at one frequency, called the neuron’s characteristic frequency, and it is less responsive at neighbouring frequencies.

Question 20

Are characteristic frequencies observed in relays beyond the afferents belonging to spiral ganglions?

Answer 20

This type of frequency tuning is seen in many neurons in each of the relays from cochlea to cortex. However as one ascends, the auditory pathway in the brain stem, the response properties of the cells become more diverse and complex, just as in the visual pathway.

For instance, some cells in the cochlear nuclei are especially sensitive to sounds varying in frequency over time (think of the sound of a trombone as it slides from a low note to a high note).

Question 21

Give an example of complex sounds which cells in the MGN respond to

Answer 21

In the MGN, there are cells that respond to fairly complex sounds such as vocalisations, as well as other cells that show simple frequency selectivity, as in the auditory nerve.

Question 22

What is acoustic radiation?

Answer 22

Axons leaving the MGN project to auditory cortex via the internal capsule in an array called the acoustic radiation.

Question 23

Describe the layers of the primary auditory cortex (A1) and the secondary auditory cortexes

Answer 23

The structure of A1 and the secondary auditory areas is in many ways similar to corresponding areas of the visual cortex. Layer I contains few cell bodies, and layers II and III contain mostly small pyramidal cells. Layer IV, where the medial geniculate axons terminate, is composed of densely packed granule cells. Layers V and VI contain mostly pyramidal cells that tend to be larger than those in the superficial layers. Let’s look at how these cortical neurons respond to sound.

Question 24

How are response properties of cells in A1 organised?

Answer 24

A1 are relatively sharply tuned for sound frequency and possess characteristic frequencies covering the audible spectrum of sound. In electrode penetrations made perpendicular to the cortical surface in monkeys, the cells encountered tend to have similar characteristic frequencies, suggesting a columnar organisation on the basis of frequency.

In the tonotopic representation in A1, low frequencies are represented rostrally and later- ally, whereas high frequencies are represented caudally and medially. Roughly speaking, there are isofrequency bands running mediolaterally across A1. In other words, strips of neurons running across A1 contain neurons that have fairly similar characteristic frequencies.

Question 25

In the visual system, it is possible to describe large numbers of cortical neurons as having some variation on a general receptive field that is either simple or complex. Is this reflected in A1

Answer 25

So far, it has not been possible to place the diverse auditory receptive fields into a similarly small number of categories. As they do at earlier stages in the auditory pathway, cortical neurons have different temporal response patterns; some have a transient response to a brief sound, and others have a sustained response.

Question 26

In addition to the frequency tuning that occurs in most cells, how are some cells tuned?

Answer 26

Some neurons are intensity-tuned, giving a peak response to a particular sound intensity.

Question 27

Describe a second organisational principle of these auditory areas

Answer 27

The presence in auditory cortex of columns of cells with similar binaural interaction. As at lower levels in the auditory system, one can distinguish cells that respond more to stimulation of both ears than to either ear separately, as well as cells that are inhibited if both ears are stimulated.

Question 28

What function may interaural neurons with specific sensitivities play?

Answer 28

Neurons sensitive to interaural time delays and interaural intensity differences probably play a role in sound localisation.

Question 29

What often results from bilateral or unilateral damage to the auditory cortex?

Answer 29

Bilateral ablation of auditory cortex leads to deafness, but deafness is more often the consequence of damage to the ears. A surprising degree of normal auditory function is retained after unilateral lesions in auditory cortex. In humans, the primary deficit that results from a unilateral loss of A1 is the inability to localise the source of a sound. Performance on such tasks as frequency or intensity discrimination is near normal.

Question 30

In the visual cortex a unilateral cortical lesion of striate cortex leads to complete blindness in one visual hemifield. Give a possible reason for why this is not reflected in the auditory cortex

Answer 30

both ears send output to cortex in both hemispheres.

Question 31

What is meant by the vestibular system?

Answer 31

Listening to music and balancing on a bicycle both involve sensations that are transduced by hair cells. The vestibular system monitors the position and movement of the head, gives us our sense of balance and equilibrium, and helps coordinate movements of the head and eyes, as well as adjustments to body posture.

Question 32

What is experienced when the vetibular system is disrupted?

Answer 32

The results can include the unpleasant, stomach-turning feelings we usually associate with motion sickness— vertigo and nausea, plus a sense of disequilibrium and uncontrollable eye movements.

Question 33

Common biological structures often have common origins. how is this relevent to the auditory and vestibular systems?

Answer 33

In this case, the organs of mammalian balance and hearing both evolved from the lateral line organs present in aquatic vertebrates, including fish and some amphibians. Lateral line organs are small pits or tubes along an animal’s sides. Each pit contains clusters of hairlike sensory cells whose cilia project into a gelatinous substance that is open to the water in which the animal swims.

The purpose of lateral line organs in many animals is to sense vibrations or pressure changes in the water. In some cases, they are also sensitive to temperature or electrical fields. Lateral line organs were lost as reptiles evolved, but the exquisite mechanical sensitivity of hair cells was adopted and adapted for use in the structures of the inner ear that derived from the lateral line.

Question 34

What is the vestibular labrynth? Name the structures and their functions

Answer 34

In mammals, all hair cells are contained within sets of interconnected chambers called the vestibular labyrinth. The auditory portion of the labyrinth is the spiraling cochlea. The vestibular labyrinth includes two types of structures with different functions: the otolith organs, which detect the force of gravity and tilts of the head, and the semicircular canals, which are sensitive to head rotation.

The ultimate purpose of each structure is to transmit mechanical energy, derived from head movement, to its hair cells. Each is sensitive to different kinds of movement not because their hair cells differ but because of the specialised structures within which the hair cells reside.

Question 35

Name the otolith organs

Answer 35

the saccule and the utricle

Question 36

Where do the hair cells in the vestibular organs synapse

Answer 36

Each hair cell of the vestibular organs makes an excitatory synapse with the end of a sensory axon from the vestibular nerve, a branch of the auditory–vestibular nerve (cranial nerve VIII).

Question 37

What is the function of the central vestibular pathways?

Answer 37

The central vestibular pathways coordinate and integrate information about head and body movement and use it to control the output of motor neurons that adjust head, eye, and body positions.

Question 38

To where do primary vestibular axons from cranial nerve VIII make connections to?

Answer 38

The medial and lateral vestibular nuclei on the same side of the brain stem, as well as to the cerebellum

Question 39

Is this the sole source of input for the vestibular nuclei?

Answer 39

The vestibular nuclei also receive inputs from other parts of the brain, including the cerebellum, and the visual and somatic sensory systems, thereby combining incoming vestibular in- formation with data about the motor system and other sensory modalities.

Question 40

To where do the vestibular nuclei project? Give an example for both the ottolith organs and the semicircular canals

Answer 40

The vestibular nuclei in turn project to a variety of targets above them in the brain stem, and below them into the spinal cord.

For example, axons from the otolith organs project to the lateral vestibular nucleus, which then projects via the vestibulospinal tract to excite spinal motor neurons controlling muscles in the legs that help maintain posture. This pathway helps the body stay upright even on the rolling deck of a boat.

Axons from the semicircular canals project to the medial vestibular nucleus, which sends axons via the medial longitudinal fasciculus to excite motor neurons of trunk and neck muscles that orient the head. This pathway helps the head stay straight even as the body cavorts around below it.

Question 41

Where does the vestibular system connect to before reaching the neocortex?

Answer 41

Similar to the other sensory systems, the vestibular system makes connections to the thalamus and then to the neocortex. The vestibular nuclei send axons into the ventral posterior (VP) nucleus of the thalamus, which projects to regions close to the representation of the face in the primary somatosensory and primary motor areas of cortex

Question 42

What keep your eyes pointed in a particular direction, even while you are dancing like a fool?

Answer 42

The vestibulo-ocular reflex (VOR) of the central vestibular system performs this function.

Question 43

How does the vestibo-ocular reflex work?

Answer 43

Accurate vision requires the image to remain stable on the retinas despite movement of the head. Each eye can be moved by a set of six extraocular muscles. The VOR works by sensing rotations of the head and immediately commanding a compensatory movement of the eyes in the opposite direction. The movement helps keep your line of sight tightly fixed on a visual target.

Question 44

How would the vestibo-ocular reflex work in the dark?

Answer 44

Because the VOR is a reflex triggered by vestibular input rather than visual input, it works amazingly well even in the dark or when your eyes are closed.

Question 45

What does the effectiveness of the VOR depend on?

Answer 45

The effectiveness of the VOR depends on complicated connections from the semicircular canals to the vestibular nucleus to the cranial nerve nuclei that excite the extraocular muscles.

Question 46

Describe what happens when the head turns to the left and the VOR induces both eyes to turn right

Answer 46

Axons from the left horizontal canal innervate the left vestibular nucleus, which sends excitatory axons to the contralateral (right) cranial nerve VI nucleus (abducens nucleus). Motor axons from the abducens nucleus in turn excite the lateral rectus muscle of the right eye. Another excitatory projection from the abducens crosses the midline, back to the left side, and ascends (via the medial longitudinal fasciculus) to excite the left cranial nerve III nucleus (oculomotor nucleus), which excites the medial rectus muscle of the left eye.

Both eyes are turning right. However, to further ensure speedy operation, the left medial rectus muscle also gets excited via a projection from the vestibular nucleus di- rectly to the left oculomotor nucleus. Speed is also maximized by activating inhibitory connections to the muscles that oppose this movement (the left eye’s lateral rectus and right eye’s medial rectus, in this case).