1
Q
What is the primary definition and purpose of the Vestibular System?
A
It is a sensory system that provides a sense of balance and spatial sensation (orientation). Its purpose is to coordinate movements, often involuntary reflex movements, to control eye, head, and body positions.
2
Q
What specific physical stimulus does the vestibular system detect?
A
It detects acceleration (specifically of the head).
3
Q
: How is Acceleration defined (including the formulas from the slide)?
A
Acceleration is the change of velocity with time. Formula: a = dv/dt
4
Q
Does the vestibular system detect velocity?
A
No. It does not detect velocity itself. It only detects if an object is increasing or decreasing its velocity.
5
Q
What specific examples illustrate the inability to detect constant velocity?
A
If driving in a car with a constant velocity of 60 km/h or flying in a plane with a constant velocity of 900 km/h (deprived of visual input), we would be unable to tell that we are moving at all.
6
Q
According to physics principles, when does acceleration ensue?
A
Acceleration ensues when a force acts on a mass. Formula: F = m times a.
7
Q
Why is it necessary for the vestibular system to detect forces acting on the body?
A
To coordinate movements that compensate for these forces.
8
Q
What are the two kinds of acceleration detected by the vestibular system?
A
Linear acceleration Angular (or rotational) acceleration .
9
Q
What are examples of Linear Acceleration?
A
Vertical axis: Moving in an elevator. Horizontal axis: Riding in an accelerating car.
10
Q
What are examples of Angular Acceleration?
A
A figure skater during a spin. Simply turning our head.
11
Q
What structure senses both linear and angular accelerations?
A
The Vestibular Labyrinth. Together with the cochlea, it forms the inner ear.
12
Q
What are the main components of the Vestibular Labyrinth?
A
Three Semicircular Canals Two Otolith Organs (the Utricle and the Saccule)
13
Q
What is the function of the Semicircular Canals?
A
They detect angular (or rotational) acceleration.
14
Q
What are the two Otolith Organs and their specific functions?
A
Utricle: Detects horizontal acceleration. Saccule: Detects vertical acceleration.
15
Q
What is the Membranous Labyrinth?
A
The collective term for the Semicircular canals, Utricle, Saccule, and the Cochlear Duct. It is enclosed within the bony labyrinth.
16
Q
What fluid is inside the Membranous Labyrinth?
A
Endolymph.
17
Q
What is the origin and connection of the Endolymph?
A
It is generated by the Stria Vascularis (lining the cochlea’s Scala Media) and is in direct contact with the endolymph of the cochlea.
18
Q
What is the Bony Labyrinth?
A
A series of bony cavities within the petrous part of the temporal bone that encloses and protects the delicate membranous labyrinth.
19
Q
What fluid is between the Bony Labyrinth and the Membranous Labyrinth?
A
Perilymph.
20
Q
What nerve innervates the vestibular labyrinth?
A
The Vestibular Nerve. It joins with the cochlear nerve to form the 8th cranial nerve (Vestibulocochlear Nerve).
21
Q
Where are the cell bodies of the vestibular neurons located?
A
In Scarpa’s Ganglion (also called the vestibular nerve ganglion), which is located close to the inner ear.
22
Q
What type of neurons are the vestibular neurons?
A
They are Bipolar Neurons (similar to the cochlear spiral ganglion neurons).
23
Q
Describe the two processes of a vestibular bipolar neuron.
A
Peripheral process: Extends to the vestibular labyrinth to make synaptic contact with sensory receptor cells (hair cells). Central process: Relays information to the vestibular nuclei in the brainstem.
24
Q
How are the three semicircular canals oriented, and what is the purpose of this arrangement?
A
They lie in three planes that are orthogonal (at right angles) to each other. This arrangement allows the detection of angular accelerations of the head around all possible axes.
25
How are the canals in the left and right ears paired?
Each canal in one ear has a corresponding canal in the other ear that lies in the exact same plane of rotation and responds to angular accelerations in that plane.
26
What is the Ampulla?
A swelling (or bulge) located at one end of each semicircular canal.
27
What sensory structure is contained within the Ampulla?
The Crista, which is the sensory epithelium.
28
What cells does the Crista contain?
It contains the Hair Cells, which are the sensory receptor cells for angular acceleration.
29
How do the hair cells connect to the nervous system?
On their basal side, hair cells form synaptic connections with the afferents of the vestibular neurons (vestibular portion of the 8th cranial nerve).
30
What is the Cupula?
A gelatinous cap located inside the ampulla.
31
What is the relationship between the hair cells, stereocilia, and the Cupula?
The stereocilia, which are on the apical side of the hair cells, protrude into the gelatinous cap of the Cupula.
32
What is the relationship between the Endolymph and the Cupula?
The Cupula is surrounded by Endolymph and is "soaked in it like a sponge." This means the stereocilia (embedded in the cupula) are in direct contact with the endolymph.
33
What happens to the labyrinth walls, cupula, and endolymph during a head rotation?
The walls and cupula move quickly in the direction of the rotation. The endolymph is more inert and slower to move.
34
How does the inertia of the endolymph affect the cupula?
The endolymph exerts a force on the cupula, displacing it against (opposite to) the direction of the head movement.
35
What is the state of the mechanically gated ion channels when the hair cells are at rest?
They are connected by tip links and are open just a little bit.
36
What happens when stereocilia are bent towards the largest stereocilium (kinocilium)?
The tip links are stretched. The mechanically gated ion channels open maximally.
37
What drives the ion flow when channels open?
The Endolymph has a very high Potassium (K+) concentration (and low Sodium), creating a high potassium equilibrium potential. This causes K+ to diffuse into the hair cells.
38
What is the chain reaction inside the hair cell following K+ influx?
Depolarization of the membrane. Depolarization propagates to the basal end. Opening of voltage-gated Calcium (Ca2+) channels. Calcium influx occurs.
39
How does the hair cell communicate with the vestibular neuron?
The Calcium influx leads to an increase in the release of the neurotransmitter Glutamate at the synapse.
40
What is the effect of Glutamate on the vestibular neuron?
It acts on ionotropic glutamate receptors, leading to membrane depolarization and an increase in the action potential firing rate.
41
What happens when stereocilia are bent away from the largest stereocilium (kinocilium)?
The ion channels close. The membrane hyperpolarizes. Less glutamate is released. The action potential firing frequency becomes lower.
42
How does this transduction mechanism compare to the auditory system?
It is exactly the same transduction mechanism as found in the cochlear hair cells.
43
What happens mechanically and molecularly when a head rotation causes stereocilia to bend away from the largest stereocilium?
The endolymph pushes the Cupula to the other side. Stereocilia bend away from the kinocilium. This causes the closure of mechanically gated ion channels. The membrane hyperpolarizes. There is reduced Calcium influx at the basal end of the hair cell. There is reduced release of the neurotransmitter Glutamate.
44
How do vestibular neurons respond to this hyperpolarization?
They are stimulated less (hyperpolarized) and fire fewer action potentials (frequency decreases).
45
What is the firing rate of vestibular neurons when the head is not moving (in the absence of head movements)?
They fire action potentials at a rather constant "background" rate (baseline firing).
46
Describe the "Push-Pull" bilateral arrangement of the semicircular canals.
Each canal in one ear has a corresponding (complementary) canal in the other ear located in the exact same plane. They respond in exactly the opposite way.
47
If a head rotation causes an increase in firing rate in the right ear, what happens in the left ear?
There will be a decrease in the firing rate of the vestibular neurons innervating the corresponding canal in the left ear.
48
Do vestibular neurons detect constant velocity (e.g., spinning at a constant speed)?
No. Vestibular neurons only respond to angular acceleration (changes in velocity). During constant velocity, the firing rate returns to the background (baseline) level.
49
Using the Merry-Go-Round example, what happens during acceleration (starting the spin)?
The semicircular canals sense the rotation (acceleration), and the firing frequency of the innervating neurons increases (in the stimulated ear).
50
Using the Merry-Go-Round example, what happens during constant speed?
The semicircular canals do not sense the rotation anymore. The firing frequency returns to the constant background rate, and you wouldn't notice the motion if your eyes were closed.
51
Using the Merry-Go-Round example, what happens during deceleration (sudden stop)?
The vestibular neurons respond again due to the angular deceleration. The neurons that were previously excited now decrease their firing, and you feel like your body is spinning in the opposite direction.
52
Looking at the graphs on Slide 8, how does the firing rate change over time during a "step" of rotation speed?
Start of rotation (Acceleration): Firing rate spikes rapidly (up or down depending on the ear). Constant rotation: Firing rate decays back to the baseline level. End of rotation (Deceleration): Firing rate spikes in the opposite direction (e.g., if it spiked up at the start, it spikes down at the stop) before returning to baseline.
53
What are the two Otolith Organs?
The Utricle and the Saccule.
54
What is the Macula?
The sensory epithelium (patch of sensory cells) located within both the Utricle and the Saccule.
55
What two types of cells does the Macula contain?
Hair Cells (the sensory receptors) Support Cells
56
What is the Otolithic Membrane?
A gelatinous layer (gelatinous cap) that lies over the macula. The stereocilia of the hair cells project into this membrane.
57
What are Otoconia (or Otoliths)?
Tiny crystals of calcium carbonate that are encrusted on the surface of the otolithic membrane.
58
What is the approximate size of the Otoconia?
About 1 to 5 microns in size.
59
What is the key physical property of Otoconia?
They have a higher density than the endolymph that surrounds them.
60
Why is the high density of Otoconia functionally important?
It is the key to the otolith organs' sensitivity to linear acceleration (in the plane of the macula).
61
What physical events initiate transduction in the otolith organs?
A linear acceleration or a static head tilt creates a force on the otoconia.
62
How does the force on the otoconia affect the internal structures?
The otolithic membrane moves slightly with the otoconia (in the direction of the acceleration or tilt), causing the cilia of the hair cells to bend.
63
What specific structural feature defines the "preferred direction" of a hair cell?
The presence of one especially tall stereocilium (the kinocilium).
64
What happens when cilia bend towards the tallest stereocilium?
The tip links are stretched. Mechanically gated ion channels open. The hair cell depolarizes.
65
What happens when cilia bend away from the tallest stereocilium?
The tip links relax. The mechanically gated ion channels close. The hair cell hyperpolarizes.
66
How does the change in membrane potential affect synaptic transmission?
Depolarization: Elicits an increase in the release of the neurotransmitter glutamate. Hyperpolarization: Elicits a decrease in glutamate release.
67
How do the vestibular neurons respond to the glutamate release?
The glutamate acts on ionotropic glutamate receptors on the postsynaptic membrane. Increased glutamate -> Increased firing frequency. Decreased glutamate -> Decreased firing frequency.
68
Does the firing rate only increase from zero?
No. Vestibular neurons have a basal firing frequency that is increased or decreased by the stimulus
69
How sensitive are the hair cells in the otolith organs?
They are very sensitive. A movement of less than 0.5 micrometers (about the diameter of one cilium) elicits a maximal response.
70
What is Direction-Selectivity in these hair cells?
The hair cells only respond to bending in specific directions. Bending of cilia perpendicular to the preferred direction does not cause any response (no change in potential) because it does not affect the opening of the ion channels.
71
How does the orientation of hair cells in the otolith maculae differ from those in the semicircular canals (cristae)?
In the semicircular canals, the hair cells are all oriented in the same direction. In the otolith maculae, the hair cells are not all oriented in the same direction; their direction selectivity varies.
72
How are the two otolith maculae (Utricle and Saccule) oriented relative to each other?
They are orthogonal (at right angles) to each other.
73
What is the orientation of the Macula of the Utricle?
It lies in the horizontal plane. It is most sensitive to horizontal acceleration.
74
What is the orientation of the Macula of the Saccule?
It lies in the vertical plane. It is most sensitive to vertical acceleration.
75
How is direction-selectivity organized within a single macula (e.g., the Utricle)?
The direction-selectivity of the hair cells varies in a systematic way, covering all possible directions within that plane (e.g., all 360° of the horizontal plane).
76
How are the maculae on the left and right sides of the head related?
The saccular and utricular maculae on one side of the head are mirror images of those on the other side.
77
What is the "push-pull" effect of this mirror-image organization?
A linear acceleration will have opposite effects on the firing of vestibular neurons in either ear. (If it causes an increase in firing rate on one side, it will cause a decrease in firing rate on the other).
78
What is the overall result of a single head tilt or acceleration on the entire population of otolith hair cells?
It will excite some hair cells, inhibit others, and have no effect on the rest.
79
How does the Central Nervous System (CNS) interpret this complex signal?
The CNS receives information from the entire population of otolith cells and can unambiguously interpret all possible linear movements or head tilts in all directions.
80
Where are the cell bodies of the primary vestibular neurons located, and where do they project?
Their cell bodies are located in Scarpa's ganglion. They project to the Vestibular Nuclei in the brainstem, as well as directly to the Cerebellum.
81
What types of inputs converge at the level of the Vestibular Nuclei?
Vestibular input: A large amount of convergence from both Otolith organs and Semicircular canals. Other sensory input: For instance, Visual and Proprioceptive input. Motor input: From the Cerebellum (providing information about ongoing movements).
82
What does this multimodal input allow neurons in the vestibular nuclei to do?
It allows them to analyze complex head movements (involving linear and angular components) and interpret the vestibular input in the context of other sensory and motor information.
83
What are two specific examples of discriminations that vestibular nuclei neurons can make due to this processing?
They can discriminate head tilt from purely translational movements. They can discriminate passive movements from self-generated movements.
84
What are the two main functions accomplished by the information processed in the vestibular nuclei?
Generating a sense of spatial orientation and self-motion. Controlling rapid reflexes of head, eyes, limb, and trunk muscles.
85
What is the "dual role" of the vestibular system, as reflected in the projections from the vestibular nuclei?
It generates a sense of spatial orientation and self-motion (via ascending sensory projections). It controls rapid reflexes (via premotor neuron projections).
86
What two major functional types of projections arise from the vestibular nuclei?
Ascending sensory projections (to the thalamus/cortex). Premotor neuron projections for short-latency sensory-motor reflex arcs.
87
What are the four primary targets of projections from the vestibular nuclei?
Extraocular motor neurons (Cranial Nerves III, IV, VI) Spinal motor neurons (in the spinal cord, for limb and neck) Cerebellum Thalamus (VP nucleus) (which then projects to the cortex)
88
Describe the ascending sensory pathway from the vestibular nuclei and its purpose.
Projections target nuclei in the Thalamus (VP nucleus), which in turn relay vestibular information to the Cortex. Purpose: To provide a sense of self-motion and spatial orientation.
89
What is the purpose of the projections from the vestibular nuclei to the Cerebellum?
To provide vestibular information for the coordination and adjustment of ongoing movements.
90
What is the purpose of the projections to the Extraocular motor neurons?
They control the Vestibulo-Ocular Reflex (VOR).
91
What is the purpose of the projections to the Spinal motor neurons (and interneurons)?
They drive postural reflexes (e.g., in the limbs and neck).
92
Besides the primary vestibular neurons, what other inputs do the vestibular nuclei receive?
They also receive proprioceptive and cerebellar input.
93
What is the purpose of the Vestibulo-Ocular Reflex (VOR)?
It is a mechanism that produces eye movements that counter head movements, which permits our gaze to remain fixed on a particular point.
94
During a horizontal head turn to the LEFT, what is the initial response in the semicircular canals?
Hair cells in the LEFT horizontal canal are DEPOLARIZED. Hair cells in the RIGHT horizontal canal are HYPERPOLARIZED.
95
How do the vestibular neurons and medial vestibular nucleus (MVN) respond to this LEFT head turn?
Vestibular neurons on the LEFT (turning) side INCREASE their firing rate. These neurons excite the (left) Medial Vestibular Nucleus (MVN).
96
Describe the main EXCITATORY pathway from the Left MVN that causes the eyes to move RIGHT.
Left MVN excites the CONTRALATERAL (RIGHT) Abducens Nucleus. The Right Abducens Nucleus (in the pons) then does two things... (see next flashcards).
97
What is the first output from the Right Abducens Nucleus (CN VI)?
A motor pathway (Abducens Nerve) that contracts the IPSILATERAL (RIGHT) Lateral Rectus muscle, pulling the right eye OUTWARD (away from the midline).
98
What is the second output from the Right Abducens Nucleus?
An interneuron pathway that ascends and crosses to excite the CONTRALATERAL (LEFT) Oculomotor Nucleus (CN III).
99
What is the final step in this pathway, originating from the Left Oculomotor Nucleus (CN III)?
Its motor axons (Oculomotor Nerve) contract the IPSILATERAL (LEFT) Medial Rectus muscle, pulling the left eye INWARD (towards the midline).
100
When the head turns LEFT, which two specific muscles contract to move the eyes RIGHT?
The RIGHT Lateral Rectus (pulls right eye out). The LEFT Medial Rectus (pulls left eye in).
101
What is the (unstated) INHIBITORY pathway that ensures this compensatory movement?
The Left MVN sends inhibitory projections to the IPSILATERAL (LEFT) Abducens Nucleus. This prevents the Left Lateral Rectus and (indirectly) the Right Medial Rectus from contracting.
102
What is Physiological Nystagmus?
The continuous change between a slow compensatory eye movement and a fast reset, which occurs during continuous head rotation (e.g., spinning in a chair).
103
What are the two components of physiological nystagmus?
Slow eye movement: The VOR-driven movement that counters the head rotation. Fast saccade: A quick eye movement that resets the eyes to their original position when they reach their farthest point.
104
What is the clinical utility of the Vestibulo-Ocular Reflex (VOR)?
It is useful to assess the integrity of the vestibular, abducens, and oculomotor nerves, as well as the connections between their nuclei (especially in unconscious patients).
105
What is Caloric Testing?
A diagnostic test where each ear is individually irrigated with either cold or warm water.
106
What is the specific advantage of Caloric Testing regarding the circuitry?
It allows clinicians to separately assess the circuitry on either side and to distinguish between brainstem and midbrain lesions.
107
How does irrigating the ear with Cold Water affect the vestibular system?
Convection currents within the canals and direct cooling of the nerve mimic rotational movements of the head AWAY from the cold/irrigated ear.
108
If the circuit is intact (No Lesion), what is the response to Cold Water irrigation?
It elicits eye movements TOWARDS the irrigated ear (Unchanged VOR). Note: If the patient is unconscious, there is no saccade (fast reset).
109
What is the result of Caloric Testing in a patient with a Lower Brainstem Lesion?
Absent VOR. The eyes do not move at all.
110
What specific dysfunctions cause the "Absent VOR" seen in Lower Brainstem lesions?
Dysfunction of the vestibular system or the abducens nerve.
111
What is the result of Caloric Testing in a patient with a Midbrain Lesion?
Asymmetric eye movements. Specifically, there is lateral movement of the eye only on the less active side.
112
What specific dysfunctions cause the asymmetric movements seen in Midbrain lesions?
Dysfunction of the oculomotor nerve or the connections between the abducens and oculomotor nucleus (specifically the Medial Longitudinal Fasciculus).
113
What are the two major pathways (vestibulospinal projections) from the vestibular nuclei to the spinal cord?
The Medial Vestibulospinal Tract The Lateral Vestibulospinal Tract
114
What is the origin of the Medial Vestibulospinal Tract (MVST)?
It is formed from projections from neurons in the Medial Vestibular Nucleus. (The MVN receives input from the Semicircular Canals).
115
Describe the path and destination of the Medial Vestibulospinal Tract.
It projects bilaterally (to both sides) down to the cervical spinal cord, where it synapses on motor neurons that control neck muscles.
116
What reflex is the Medial Vestibulospinal Tract responsible for?
The Vestibulocervical Reflex (VCR).
117
What is the function and an example of the Vestibulocervical Reflex (VCR)
Its function is to elicit neck movements that counteract passive head movements (postural adjustments of the head). Example: During a downward pitch of the body (like tripping), the vestibular system activates neck muscles to pull the head up.
118
What is the origin of the Lateral Vestibulospinal Tract (LVST)?
It is a larger set of projections, mainly from neurons located in the Lateral Vestibular Nucleus. (The LVN receives input from the Otolith organs).
119
Describe the path and destination of the Lateral Vestibulospinal Tract.
It descends ipsilaterally (on the same side) along the total length of the spinal cord. It synapses directly onto motor neurons (and interneurons).
120
What reflex is the Lateral Vestibulospinal Tract responsible for?
The Vestibulospinal Reflex (VSR).
121
What stimulus primarily initiates the Vestibulospinal Reflex (VSR)?
Activation of the Otolith Organs (e.g., from a body tilt).
122
What is the function and an example of the Vestibulospinal Reflex (VSR)?
It maintains upright posture by causing limb extension to prevent falling. Example: If your body suddenly tilts to one side, otoliths activate the lateral vestibular nucleus, which excites motor neurons to the extensor muscles of the limb on that same side (the lateral side of the tilt).
123
What is the relationship between the Vestibular Nuclei and the Cerebellum (specifically the Vestibulocerebellum)?
They have reciprocal connections. The cerebellum is a major target of ascending vestibular projections, and the cerebellum provides input back to the vestibular nuclei in return.
124
What is the first major function of these reciprocal vestibulocerebellar connections?
To integrate vestibular signals with information about voluntary movements. This allows the brain to distinguish: Passive movements from self-generated movements. Head tilts from purely translational movements.
125
What is the second major function of these connections?
Plasticity of vestibular output and sensorimotor learning. This allows the system to adapt vestibular reflexes to changing sensory inputs.
126
What is an example of a situation that requires this plasticity?
Altered vestibular input (e.g., from an inner ear infection). Mismatched visual and vestibular inputs.
127
Describe the "Prism Glasses" example of mismatched input. What happens initially?
If you wear prism glasses that invert the image of the world, there is a mismatch between vision and vestibular input. Initially, the Vestibulo-Ocular Reflex (VOR) will fail to achieve gaze fixation.
128
In the "Prism Glasses" example, what happens after several days?
The vestibular system will learn to compensate for this altered visual input and will again provide appropriate gaze fixation. This adaptation is called sensorimotor learning.
129
What part of the brain is required for this sensorimotor learning (e.g., VOR gain changes) to occur?
This learning requires the vestibular connections to the Cerebellum.
130
What is the primary thalamic target for ascending vestibular projections?
The Ventral Posterior (VP) Nucleus of the thalamus (this is the same nucleus used by the somatosensory system).
131
Where do the neurons from the Ventral Posterior (VP) thalamic nucleus project?
They send projections to several cortical areas
132
Is there a single, dedicated "primary vestibular cortex" like there is for vision (V1) or hearing (A1)?
No. None of the cortical areas that receive vestibular input are exclusively dedicated to it.
133
How would you describe the nature and function of the cortical regions that receive vestibular information?
They are multisensory regions. They integrate vestibular information with other inputs, such as from the visual and somatosensory (proprioceptive) systems.
134
What is the Parieto-Insular Vestibular Cortex (PIBVC/IVC)?
A key multisensory cortical area that is a "case in point" for vestibular integration. It receives vestibular, proprioceptive, and visual input.
135
What is the primary functional importance of the PIBVC?
It appears to be especially important for our sense of self-motion and orientation in space (spatial sensation).
136
What evidence from electrical stimulation studies supports the role of the PIBVC?
Electrical stimulation of the PIBVC in humans has been reported to elicit strong vestibular sensations.
137
What evidence from stroke patients supports the role of the PIBVC?
Patients with ischemic strokes affecting the PIBVC frequently have an aberrant (incorrect) perception of upright body posture.
138
Which is more common: vestibular dysfunction caused by central processing errors or disorders of the vestibular labyrinth?
Disorders of the vestibular labyrinth are far more common. Aberrant central processing is relatively rare.
139
What are the five main causes of Acute Unilateral Vestibular Dysfunction mentioned?
Labyrinthitis Trauma (to inner ear or vestibular nerve) Acoustic Neuroma Benign Paroxysmal Positional Vertigo (BPPV) Meniere's Disease
140
What is Labyrinthitis?
Inflammation of the inner ear, which can be due to viral or bacterial infections, a reaction to medication, or an allergy.
141
What is an Acoustic Neuroma?
A non-cancerous, slow-growing tumor arising from Schwann cells (the glial cells that ensheath and myelinate peripheral nerves), which affects the functioning of the vestibular nerve.
142
What causes Benign Paroxysmal Positional Vertigo (BPPV)?
Otoconia become dislodged from the otolithic membrane in the Utricle or Saccule and move around loose in the Semicircular Canals, creating aberrant sensations.
143
How is BPPV often treated?
By the Epley Maneuver: a series of movements designed to relocate the dislodged otoconia.
144
What is Meniere's Disease?
A disorder caused by a fluid buildup (and increased hydrostatic pressure) in the labyrinth.
145
Why does Meniere's Disease often involve tinnitus or hearing impairment in addition to vestibular symptoms?
Because the cochlea and the vestibular labyrinth are physically connected, so the fluid buildup affects both systems equally.
146
What is the likely cause of the fluid buildup in Meniere's Disease?
It is unclear, but a genetic predisposition likely plays a role (about 10% of cases run in families).
147
What are the three hallmark symptoms of Acute Unilateral Vestibular Dysfunction?
Vertigo (dizziness) Nausea Spontaneous Nystagmus (involuntary eye movements).
148
What is the physiological mechanism behind Spontaneous Nystagmus in unilateral dysfunction?
Neurons on the affected side stop firing (no excitation from hair cells). This creates a sustained firing imbalance between the two ears (one side firing, one silent). The brain interprets this as a sustained head turn. This leads to sustained activation of the Vestibulo-Ocular Reflex (VOR), moving the eyes constantly in the absence of head movement.
149
What causes the sensations of Dizziness and Nausea?
A mismatch in sensory inputs: The vestibular information (signaling a head turn due to the firing imbalance) no longer matches the visual input and proprioceptive information (which indicate being stationary).
150
How do the symptoms of Bilateral Dysfunction (both labyrinths) compare to unilateral dysfunction?
Symptoms are usually much milder.
151
Why doesn't Bilateral Dysfunction cause vertigo or spontaneous nystagmus?
Because there is no constant mismatch/imbalance in the firing of vestibular neurons if the head is immobile. (Both sides are equally dysfunctional).
152
What is the primary deficit in Bilateral Dysfunction?
Head turns and tilts might no longer be accurately detected, but this can usually be compensated for by proprioceptive or visual input (except in specific situations lacking those inputs).
153
What explains the remarkable recovery seen in persistent unilateral dysfunction over days to weeks?
Plasticity. The vestibular system (via connections with the cerebellum) alters the gain or threshold required to activate vestibular nuclei neurons, compensating for the firing mismatch even if the peripheral input remains aberrant.
