Otolith organs: utricle and saccule
Semicircular canals/ducts: anterior, posterior, horizontal
Part of bony labyrinth
Perilymph is fluid between bone and duct, and has low K+ and high Na+ (similar to CSF)
Membranous tube within semicircular canal
Endolymph is fluid within the ducts and has high K+ and low Na+; made in dark cells in vestibular organs and by marginal cells in the cochlea
What does each hair cell contain?
Kinocilium lies on one side of stereocilia, which line up in height
Stereocilia and kinocilium are linked by "tip links" which are linked to K+ channels
Note: stereocilia and kinocilium are poking into endolymph (within duct) and rest of the cell is in perilymph (between bone/canal and duct)
How do you depolarize a hair cell?
1) Hairs all move toward kinocilium
2) Tip links are stretched, which causes K+ channels to open and let K+ in (endolymph has a high concentration of K+)
3) Voltage-gated Ca2+ channels activated and let Ca2+ in
4) Cell is depolarized
5) NT (glutamate and aspartate) released onto afferent nerve ending (to CN VIII)
How do you hyperpolarize a hair cell?
1) Hairs move away from kinocilium
2) Channels close
3) Cell hyperpolarizes
What does movement orthogonal to the stereocilia-kinocilium axis do?
Movement orthogonal to stereocilia-kinocilium axis does not change the membrane potential
Because tip links are not stretched so no K+ channels open
How are the semicircular canals arranged?
3 semicircular canals in each inner ear
Orthogonal to each other
Each have a swelling (ampulla) close to where they intersect with the utricle, and the hair cells sit in the ampulla
Internal organization of the semicircular canals
Hair cells within ampulla sit among supporting cells in the crista
Kinocilia and stereocilia protrude into a gelatinous mass (cupula)
Hair cells all lined up with same polarity: kinocilium closest to utricle in horizontal canals and farthest from utricle in anterior and posterior canals
So if cupula pushed towards or away from the utricle, all hair cells in one ampulla will respond the same way
What induces cupula movement?
Angular acceleration of the head in a direction parallel to the axis of the semicircular canal
When head starts to move, endolymph initially stays in place due to inertia, so cupula pushed by it and will either depolarize or hyperpolarize hair cells
When head rotating constantly, endolymph moves at same speed as head so no pressure on cupula and hair cells have same response as if head was stationary
When head stops rotating, endolymph briefly continues to rotate due to inertia, so cupula pushed in opposite direction
Because bony labyrinths are mirror images of each other, each canal has reverse polarity from partner
Angular rotation that increases activity from left canal will reduce activity from paired right canal
Advantage of push-pull response is that small changes in firing rate are more easily discernable because CNS takes difference in activity!
Why does the room spin when you've drank too much alcohol?
Alcohol changes the relative density of the cupula (alcohol makes the very vascular cupula less dense) so that it floats in the endolymph, pulling the hair of the hair cells with it no matter what position you're in
Hair cells are telling you that there's constant angular acceleration!
How are the utricle and saccule aligned?
Utricle and saccule are orthogonal to one another
Utricle points toward eyes, but 30 degrees up (because natural to walk looking down to avoid tripping)
Saccule points down toward chin
Layer of sensory epithelium within small section of both the utricle and saccule
General area of lots of receptors
Hair cells of the utricle and saccule
Hair cells sit among supporting cells in macula and hairs poke into gelatinous membrane (otolithic membrane) which has otoconia on top
Hair cells do not line up same way and instead there is an organized arrangement in which hair cells on either side of a groove (striola) in otolithic membrane face in opposite directions (different from semicircular canals!)
Because utricular macula and striola macula are orthogonal to each other and because striola are not straight, all potential directions of linear acceleration are represented
How does linear acceleration work in the utricle and saccule?
Otoconia are calcium carbonate crystals embedded in otolithic membrane that are more dense (heavier) than endolymph
Movement causes heavy otoconia to initially stay behind because of inertia and this moves the otolithic membrane --> membrane movement changes K+ influx in hair cells (toward kinocilium means depolarization, etc like in semicircular canals)
When body stops moving, heavy otoconia take a little longer to stop due to inertia so again changes in K+ influx occur in the hair cells
Does gravity affect the otoconia?
Yes, gravity pulls the otoconia down because remember utricle angled up 30 degrees
Why don't we confuse tilt with linear acceleration?
Visual input has strong influence on perception
Semicircular canals provide confirmation
Efference copy provides additional confirmation
Efference copy (corollary discharge)
Copy of a motor command that is sent to other regions of the brain so that the CNS has an internal representation of what movements it makes
"I pulled control stick up so should be accelerating now..."
Where to afferent nerves have their cell bodies?
Vestibular (Scarpa's) ganglion
Central processes of these bipolar neurons enter the brainstem via CN VIII at pontomedullary junction
Where do central processes of bipolar neurons (afferents) project to?
Vestibular nuclei: inferior, superior, medial, lateral
Cerebellum (via juxtarestiform body in inferior cerebellar peduncle)
Where do second order neurons (cell bodies in vestibular nuclei) go?
Lateral vestibular nuclei
Medial vestibular nuclei
Thalamus and cortex
Eye movement control nuclei
Contralateral vestibular nuclei
Lateral vestibulospinal tract (LVST)
Projections from lateral vestibular nuclei (second order neurons now) descend ipsilaterally in lateral vestibulospinal tract (LVST)
Project to ventral horn of spinal cord and synapse on motor neurons controlling limbs and trunk and are used to mediate balance and posture
Medial vestibulospinal tract (MVST)
Projections from medial vestibular nuclei (second order neurons now) descend bilaterally in medial longitudinal fasciculus (MLF) as the medial vestibulospinal tract (MVST)
Project to ventral horn of upper cervical spine and used to mediate head position
What are projections to the reticular formation thought to mediate?
Feeling of nausea that is linked to vestibular system
Second order then third order neurons to thalamus and cortex
Vestibular nuclei (second order neurons) project bilaterally to ventral posterolateral nucleus (VPL) and the posterior nuclear group within the thalamus
Third order neurons in thalamus project to parietal cortex (areas 3a (primary somatosensory), 2v, posterior parietal areas 5, 7)
Projections from vestibular nuclei to oculomotor nuclei are responsible for what?
Projections are bi-lateral but sign of activity is different (excitatory vs. inhibitory!)
Important for maintenance of gaze
Do neurons in vestibular nuclei project to their reciprocal vestibular nuclei?
Yes, neurons in vestibular nuclei project contralaterally to reciprocal vestibular nuclei to enable push-pull analysis of signals from functional pairs of canals on either side of the head
What organs convey information about linear vs. rotational acceleration?
Otolithic organs (utricle and saccule) convey information about linear acceleration
Semicircular canals convey information about rotational acceleration
How are vestibular signals brought into the CNS and what do they do from there?
1) Vestibular signals brought into CNS via CN VIII and terminate in cerebellum and 4 vestibular nuclei
2) 4 vestibular nuclei are connected to 2 descending tracts, cerebellum, reticular formation, thalamus, and ocular motor nuclei
3) Info from vestibular nuclei is used to maintain gaze, balance and posture, and is used in cortex to track position in space