L25 Nervous System Vestibular system

Overview

The vestibular apparatus and the cochlea are two parts of the same inner ear labyrinth and use the same sensory cell, the hair cell, to convert mechanical deflection into a change in afferent firing. The lecture first covers the components and functions of the external, middle and inner ear, then the vestibular apparatus (semicircular canals for angular acceleration, otolith organs for gravity and linear acceleration) and its central pathways, then auditory physiology: the properties of sound, how air movement is conducted to the basilar membrane, transduction in the Organ of Corti, how pitch, loudness, duration and direction are coded, the central auditory pathways, and the forms of hearing loss.

Ear anatomy and the conduction pathway

Route taken by sound from outside to nerve:

1. Pinna (auricle).
2. External auditory canal.
3. Tympanic membrane.
4. Middle ear cavity, containing the ossicles malleus, incus and stapes, with the stapes seated in the oval window. The auditory (eustachian) tube runs from the middle ear cavity to the pharynx.
5. Scala vestibuli, the upper fluid-filled canal of the cochlea.
6. Around the helicotrema at the cochlear apex.
7. Scala tympani, the lower fluid-filled canal.
8. Round window.

The temporal bone region houses both the semicircular canals and the cochlea. The cochlear (auditory) nerve leaves the cochlea; the cochlear duct connects the vestibular apparatus to the coiled cochlea.

The membranous labyrinth and vestibular apparatus

Vestibular function is balance and equilibrium. Components:

• Semicircular canals (3), each ending in an ampulla containing a cupula.
• Otolith organs: utricle and saccule. The ampullae open into the utricle, which sits beside the saccule.
• Membranous labyrinth containing the above.
• Hair cells as the receptors throughout.

The vestibular nerve arises from the utricle, saccule and ampullary region. The cochlear duct leads from this vestibular apparatus into the coiled cochlea, from which the auditory nerve arises.

Hair cell transduction, common to both systems

Hair cells carry a bundle of stereocilia arranged as a staircase in rows of graduated height, alongside a kinocilium. They have a resting (tonic) discharge rate that can be raised or lowered, so the direction of bending is encoded as an increase or decrease from baseline rather than as firing versus silence.

Bending toward the kinocilium / tallest row (excitation):

1. Tip links connecting adjacent stereocilia stretch.
2. Mechanically gated K+ channels open and K+ enters the stereocilia.
3. The resulting depolarisation spreads to the cell body.
4. Voltage-gated Ca2+ channels open at the base of the hair cell.
5. Ca2+ influx triggers fusion of neurotransmitter-containing vesicles with the membrane.
6. Transmitter is released onto the afferent neuron and afferent discharge increases.

Bending away from the kinocilium (inhibition): tip links go slack, the K+ channels close, the cell hyperpolarises, Ca2+ entry at the base falls, vesicle release onto the afferent neuron falls, and discharge decreases below baseline.

Semicircular canals and the detection of rotation

Key points:

• They sense changes in angular motion (“turning”) of the head, and not turning at a constant rate.
• Three canals lie perpendicular to each other: horizontal, anterior and posterior.
• Opposite pairs function together.
• Structures involved: ampulla, crista ampullaris, gelatinous cupula, hair cells with stereocilia.

Structure of the crista ampullaris: the gelatinous cupula sits within the ampulla surrounded by endolymph and enclosed by the canal wall. Hair cells sit at the base of the cupula on support cells, with their stereocilia projecting up into the cupula body, and afferent nerve fibres connect to the base of the hair cells and carry the signal away.

Mechanism of detecting rotation:

1. The head and skull begin to turn.
2. Inertia of the endolymph makes it lag behind the moving skull.
3. The lagging endolymph exerts pressure on the cupula and deflects it to one side.
4. The enclosed hair cells bend and a signal is generated; the response depends on the direction of rotation.
5. During steady rotation the skull and endolymph move together, so the cupula is not distorted.
6. On deceleration the effect reverses.
7. The signal travels by cranial nerve VIII to the ganglion and then the brainstem, with efferent traffic also possible (+/- efferent).

Because opposite canals work as a pair, the two sides are deflected in opposite senses, and this drives slow eye movements:

• Prior to rotation: equal baseline vestibular nerve firing on both sides, no eye movement.
• Beginning of rotation: relative endolymph motion deflects the two cupulae in opposite senses, firing increases on one side and decreases on the other, producing a slow eye movement.
• Late in rotation at constant velocity: endolymph has caught up with the skull, the cupulae return to neutral, firing returns to equal baseline, no eye movement.
• Just after rotation stops: endolymph continues moving from inertia and deflects the cupulae in the direction opposite to that at onset, giving asymmetric firing again and a slow eye movement opposite in direction to that at the beginning of rotation.

Warning

The slide carrying this four-panel rotation and eye-movement figure has no visible figure or source citation, and rendered at low resolution, although its panel labels and content were legible.

Otolith organs

Key points:

• The saccule and utricle.
• The sensory organ is the macula.
• Hair cells are embedded in the otolithic membrane.
• The membrane contains otoliths (otoconia, dense calcium-carbonate crystals covering its upper surface), which are heavy and move with gravity.
• They detect the gravitational field and falling.

Structure and mechanism: in the macula, hair cells sit in an epithelium alongside supporting cells with afferent fibres leaving their base, and their stereocilia project upward into the otolith membrane. The added mass of the otoconia makes the membrane lag behind head movement under gravity or linear acceleration, which shears and bends the stereocilia. Changing head position relative to gravity, for example standing upright versus bending forward at the waist, tilts and displaces the otolith epithelium, altering the shear force on the stereocilia and therefore the vestibular nerve output.

Central vestibular pathways

Vestibular nerve → vestibular nuclei in the brainstem (lateral, superior, medial and inferior). From the vestibular nuclei there are three onward projections:

1. Descending, via the medial and lateral vestibulospinal tracts to the spinal cord.
2. To the cerebellum.
3. Ascending, via the medial longitudinal fasciculus to the abducens, trochlear and oculomotor nuclei, which control eye movements, and onward to the superior and inferior colliculus and the thalamus.

Why vestibular function matters, and its disorders

Functions listed: reflexes for posture and balance; the vestibulo-ocular reflex (VOR) for visual fixation; nystagmus; caloric testing. [slide does not elaborate beyond these labels]

Disorders listed: vertigo, nausea, nystagmus. [slide does not elaborate]

Sound and the properties of hearing

• Sound is vibration of air molecules travelling as waves; detectable frequencies are 20 Hz to 15,000 to 17,000 Hz.
• Frequency is perceived as pitch; amplitude is perceived as loudness.
• The ear is most sensitive between 1000 and 3000 Hz.
• Loudness is measured in decibels (dB) on a log scale: a 10 dB increase is roughly a 3-fold increase in loudness.
• Pain occurs at 120 dB, and anything above roughly 80 dB is harmful.
• Protective reflexes exist. [slide does not elaborate]

Threshold of hearing curve (threshold in dB against log frequency) is U-shaped: threshold is high at around 65 to 70 dB at low frequencies near 50 Hz, falls steeply as frequency rises, reaches its lowest point near 0 dB (most sensitive) around 1000 to 3000 Hz with a small secondary bump near 2 to 3 kHz, then rises steeply again above about 5000 Hz.

Localising a sound source

Recognition of direction depends on frequency:

• Low frequency, below 1.5 kHz: long wavelength, the wave passes over the head at matched amplitude on both sides, so localisation uses the relative time of arrival at the two ears.
• High frequency, above 1.5 kHz: the head blocks and attenuates the sound so amplitude is markedly reduced on the far side, and localisation uses relative loudness between the two ears.
• Complex sounds: both mechanisms are used.

Structure of the cochlea

• The cochlea is a spiral membranous tube, encased in bone, with a central bony core (modiolus) housing the spiral ganglion of the cochlear nerve. Cochlear nerve fibre bundles emerge from the base.
• It has three fluid-filled compartments: scala vestibuli, scala media (cochlear duct) and scala tympani. The scala vestibuli and scala tympani form a continuous fluid-filled loop joined at the helicotrema, with the cochlear duct lying between them.
• In cross-section: the scala vestibuli is separated from the cochlear duct by Reissner’s membrane; the cochlear duct contains the inner and outer hair cells sitting on the basilar membrane, with the tectorial membrane overlying the hair cells and the stria vascularis on the lateral wall of the duct; the scala tympani lies below the basilar membrane.

The Organ of Corti and auditory transduction

The basilar membrane is the important structure: vibrations (“sound”) are transmitted to it, and the Organ of Corti sitting on it transduces those vibrations into neural signals (action potentials).

Mechanism:

1. The basilar membrane vibrates.
2. The hair cells move with it, but their cilia cannot, because they are embedded in the fixed tectorial membrane.
3. The cilia therefore bend.
4. The hair cells depolarise.
5. Neurotransmitter is released.
6. Action potentials are generated in the associated nerves.

Division of labour: inner hair cells transduce sound into action potentials; outer hair cells increase sensitivity. Nerve fibres run from beneath the hair cells along the basilar membrane, carrying the signal away.

Recognising pitch and the neural code

Place theory: different frequencies activate different areas of the basilar membrane.

• Near the oval window the membrane is narrow and stiff and responds to higher frequencies.
• Proceeding along the membrane toward the helicotrema, it becomes responsive to lower frequencies.
• Hair cells in each region are somewhat frequency specific, and outer hair cells help.

Neural correlates of the percept:

• Pitch: which hair cells, and which part of the basilar membrane, are stimulated.
• Loudness: frequency of action potentials.
• Duration: duration of firing.
• Direction: differences in phase and/or intensity, depending on frequency.

Central auditory pathways

Ear → cochlear nerve, which is tonotopically organised → cochlear nucleus → superior olivary nucleus, bilaterally, which handles localisation → via the lateral lemniscus and reticular formation → inferior colliculus → medial geniculate nucleus of the thalamus → primary auditory cortex in the temporal lobe. Input from both ears reaches both cortices, and the primary auditory cortex has bilateral representation, a columnar organisation and a tonotopic map. Other branches and interpretation are also listed. [slide does not elaborate on these two]

Hearing loss

Conduction deafness: a problem in the canal, drum, middle ear or bones.

Sensorineural deafness, with these causes:

• Loss of hair cells, with inner hair cell loss affecting frequency and outer hair cell loss affecting resolution; caused by noise, drugs and age.
• Damage to cranial nerve VIII.
• Tumour.
• Meningitis, malaria, syphilis.

Self-test

1. List the components of the vestibular apparatus named in the lecture, and state its overall function.
2. Describe, in order, the structures air vibrations pass through from the pinna to the round window.
3. Describe the steps by which deflection of stereocilia toward the kinocilium leads to increased afferent firing.
4. Explain what happens at each step of that chain when the stereocilia are deflected in the opposite direction, and what happens to afferent discharge.
5. Explain why the resting discharge rate of vestibular hair cells matters for what the vestibular nerve can signal.
6. Describe the arrangement of the three semicircular canals and how they work as pairs.
7. Describe the steps by which the semicircular canals detect the onset of head rotation.
8. Predict what happens to cupula position, vestibular nerve firing and eye movement during sustained rotation at constant velocity, and explain why.
9. Predict what happens just after rotation stops, and how it differs from the beginning of rotation.
10. Explain how the otolith organs detect a change in head position relative to gravity, naming the structures involved.
11. Distinguish the semicircular canals from the otolith organs in terms of what each detects.
12. List the three onward projections from the vestibular nuclei and what each targets.
13. What is the frequency range of human hearing, and in which range is the ear most sensitive?
14. What is the decibel level for pain, above what level is sound harmful, and how much louder is a 10 dB increase?
15. Distinguish the mechanism used to localise sounds below 1.5 kHz from that used above 1.5 kHz, and explain why they differ.
16. List the three fluid-filled compartments of the cochlea and name the membranes that separate them.
17. Explain why vibration of the basilar membrane bends the cilia of the Organ of Corti hair cells.
18. Distinguish the function of inner hair cells from that of outer hair cells.
19. Explain place theory, including which end of the basilar membrane responds to high frequencies and why.
20. For each of pitch, loudness, duration and direction, state its neural correlate.
21. Describe the central auditory pathway from cochlear nerve to primary auditory cortex.
22. Distinguish conduction deafness from sensorineural deafness, and list the causes of each given in the lecture.
23. Integrative: a patient has lost cochlear hair cells and also reports vertigo and nystagmus. Using the transduction mechanism shared by the two organs, explain why one cell type can produce both auditory and balance symptoms, and what differs between the two systems.

Answers

Reveal answers

1. Semicircular canals (3), the otolith organs (utricle and saccule), the membranous labyrinth, and hair cells. Its function is balance and equilibrium.
2. Pinna (auricle), external auditory canal, tympanic membrane, middle ear cavity with malleus, incus and stapes, stapes at the oval window, scala vestibuli, around the helicotrema at the cochlear apex, scala tympani, round window.
3. Tip links between adjacent stereocilia stretch; mechanically gated K+ channels open and K+ enters the stereocilia; depolarisation spreads to the cell body; voltage-gated Ca2+ channels open at the base; Ca2+ influx triggers vesicle fusion with the membrane; neurotransmitter is released onto the afferent neuron and discharge increases.
4. Tip links go slack, the K+ channels close, the cell hyperpolarises, Ca2+ entry at the base falls, vesicle release falls, and afferent discharge decreases below baseline.
5. Because there is tonic baseline firing, the cell can signal direction: bending one way raises firing above baseline and bending the other way lowers it below baseline, rather than only signalling by being on or off.
6. Three canals set perpendicular to one another, horizontal, anterior and posterior, with opposite pairs functioning together.
7. The skull turns; inertia makes the endolymph lag behind it; the lagging endolymph pushes on the cupula and deflects it; the enclosed hair cells bend and generate a signal that depends on the direction of rotation; the signal passes via cranial nerve VIII to the ganglion and brainstem, with possible efferent traffic.
8. The endolymph has caught up with the skull so the two move together, the cupula is undistorted and returns to neutral, firing returns to equal baseline on both sides, and there is no eye movement. This is why turning at a constant rate is not sensed.
9. The endolymph keeps moving from inertia and deflects the cupulae in the direction opposite to that at onset, so the asymmetry of firing between the two sides reverses and the slow eye movement is opposite in direction to the one at the beginning of rotation; deceleration gives the reverse effect.
10. Hair cells in the macula project their stereocilia into the otolithic membrane, whose upper surface carries dense otoconia (calcium-carbonate crystals). The added mass makes the membrane lag behind head movement under gravity or linear acceleration, shearing and bending the stereocilia and altering vestibular nerve output; tilting the head, for example bending forward at the waist, displaces the otolith epithelium and changes that shear force.
11. The semicircular canals sense changes in angular motion (turning) of the head but not turning at a constant rate; the otolith organs detect the gravitational field and falling, responding to gravity and linear acceleration and hence to head position.
12. A descending projection via the medial and lateral vestibulospinal tracts to the spinal cord; a projection to the cerebellum; an ascending projection via the medial longitudinal fasciculus to the abducens, trochlear and oculomotor nuclei for eye movement, and onward to the superior and inferior colliculus and the thalamus.
13. Roughly 20 Hz to 15,000 to 17,000 Hz, with greatest sensitivity between 1000 and 3000 Hz, where threshold is near 0 dB compared with 65 to 70 dB around 50 Hz.
14. Pain at 120 dB; above roughly 80 dB is harmful; a 10 dB increase is about a 3-fold increase in loudness, because decibels are a log scale.
15. Below 1.5 kHz the wavelength is long and the wave reaches both ears at matched amplitude, so relative time of arrival at the two ears is used. Above 1.5 kHz the head blocks and attenuates the sound, so the far ear receives markedly reduced amplitude and relative loudness is used. Complex sounds use both.
16. Scala vestibuli, scala media (cochlear duct) and scala tympani. Reissner’s membrane separates the scala vestibuli from the cochlear duct, and the basilar membrane separates the cochlear duct from the scala tympani.
17. The hair cells sit on the basilar membrane and move with it, but their cilia are embedded in the tectorial membrane, which is fixed, so the relative movement bends the cilia, depolarising the hair cells, releasing neurotransmitter and generating action potentials.
18. Inner hair cells transduce sound into action potentials; outer hair cells increase sensitivity and help with the frequency specificity of each region. Correspondingly, in sensorineural loss inner hair cell loss affects frequency and outer hair cell loss affects resolution.
19. Different frequencies activate different areas of the basilar membrane. Near the oval window the membrane is narrow and stiff, so it responds to higher frequencies; moving along it toward the helicotrema it becomes responsive to lower frequencies. Hair cells in each region are somewhat frequency specific, with outer hair cells helping.
20. Pitch: which hair cells and which part of the basilar membrane are stimulated. Loudness: frequency of action potentials. Duration: duration of firing. Direction: differences in phase and/or intensity, depending on frequency.
21. Cochlear nerve, tonotopically organised, to the cochlear nucleus, to the superior olivary nucleus bilaterally for localisation, via the lateral lemniscus and reticular formation to the inferior colliculus, to the medial geniculate nucleus of the thalamus, to the primary auditory cortex in the temporal lobe, which has bilateral representation, columns and a tonotopic map; input from both ears reaches both cortices.
22. Conduction deafness arises in the canal, drum, middle ear or bones. Sensorineural deafness arises from loss of hair cells (from noise, drugs or age, with inner hair cell loss affecting frequency and outer hair cell loss affecting resolution), damage to cranial nerve VIII, tumour, or meningitis, malaria and syphilis.
23. Both the cochlea and the vestibular apparatus use hair cells in which deflection of the stereocilia stretches tip links, opens K+ channels, depolarises the cell, opens basal Ca2+ channels and releases transmitter onto an afferent fibre, so an insult to hair cells can degrade both. What differs is what deflects the stereocilia and therefore what is encoded: in the cochlea the basilar membrane vibrates against the fixed tectorial membrane and frequency is mapped by place along the membrane; in the semicircular canals lagging endolymph deflects the cupula to signal changes in angular motion; in the otolith organs the otoconia-loaded membrane shears to signal gravity and falling. Vestibular hair cells also sit on a resting discharge that can be raised or lowered, so disturbed balance input shows up as vertigo, nausea and nystagmus rather than as altered pitch.