Vestibular System Flashcards
It receives information from
Where we are moving to (linear + angular a of the head)
Orientation of our head in space
Main functions
Eye movement control
Balance and posture
How does it achieve its functions?
By integrating visual, somatosensory and cerebellar information
Cerebellum provides
Proprioception
Components
Temporal bone (petrous portion)
Bony and membranous labyrinths
Endolymph (K+) and Perilymph (Na+)
5 components (bilateral and coplanar)
- 3 semicircular canals: 2 vertical, 1 horizontal
- Utricle and Saccule (join with cochlea)
Vestibulum
Membranous organ found inside the petrous portion of the temporal bone
Bony labyrinth
Cortical bone underneath vestibulum
Inside the membranous labyrinth, we find
Endolymph
Semicircular canals perceive
Rotational info (body, head)
Utricle and saccule perceive
Linear acceleration
Semicircular canals - elements
Ampulla: enlargements at the end of semicircular canals
Cupulae: membrane inside ampulla
Cupula: collagenous struct that divides ampullary region and creates compartments -> inside and outside cupula
Endolymph: fills cupula and space surrounding it
Crysta: neuroepithelial elevation inside the cupula where we find Hair Cells
Hair cells - types, function, elements
Type I and type II.
Perceive movement in vestibular system.
Stereocilia on apical surface.
Kinocillium (longer hair) on extreme side of apical surface
Utricle and saccule - elements
Macula: homologous structure to crysta, contains hair cells
Otolith memb: gelatinous compartment formed by macula
Endolymph: surrounds macula + otolith membrane
Otoconia: Ca crystals on top of otolith memb (makes it + difficult to move)
Striola: indentation along the midline of the otolithic memb, divides HC type I in centre and HC type II in peryph.
How does endolymph move within the membranous cupula and macula?
When we move our head one way, endolymph moves the other way.
Translation to neurological signal
Position of stereocilia and Kinocilium changes depending on the movement of endolymph.
DEPOLARIZATION: movement towards kinocilium
(K channels open on apical side = K+ influx into HC - depolariz - glutamate release - AP transmission to neurons)
HYPERPOLARIZAT.: movement away from kinocilium
(K channels open on basolateral side = K+ efflux - hyperpolariz = “silencing”, no AP)
Mechanism of horizontal rotation
Everytime we move our head to one side:
ipsilateral side = depolarized
contralateral = hyperpolarized
(endolymph displacement)
Same happens w/ vertical SC canals —> 3D spectrum of movement
Head inclination
Depending on which sense we incline our head to, cells on one side of striola will depolariz. while others will hyperpol
(Saccule: kinocilium away from striola // Utricle: kinocilium towards striola)
Utricle and saccule in each person —> encoded map for every movement you’ ve made in your life.
VIII CN formation
From the basal portion of HC, axons of vestibular branch are formed + join cochlear nerve
Depolarization
K channels open on apical side (contact w/ K+ rich med)
K enters the cells -> depolarization -> Ca2+ channels open + Ca entrance -> exocytosis: Glutamate released to neuron terminals -> fire at higher frequencies (basal act)
Hyperpolarization
K channels open on basolat side (contact with Na+ rich, K+ poor medium)
K exits the cells into the perilymph = hyperpolarize
No glutamate release = rate of firing is decreased
Vestibular nerve pathway
Neuronal bodies in Scarpa’s ganglion -> brainstem -> 4 vestibular nuclei per side (in Pons and Medulla Oblongata)
Vestibular nuclei - projections that receives are
Topographically organized
Sup and Med vestibular nuclei - afferences, projection
Afferences from SC canals (info about angular movement)
Project to oculomotor nuclei or spinal cord.
Lat, Med and Inf vestibular nuclei - afferences, projections
Afferences from Utricle and Saccule (linear acceleration info).
Project to cerebellum, reticular formation and spinal cord.
Medial vestibular nucleus - additionally…
Receives inputs from the retina
Juxtarestiform body
Projections which go directly from labyrinth to cerebellum, w/out passing through vestibular nuclei
Comparator units
Reach all the nuclei bilaterally (don’t only get to their destination but everywhere else)
Way of comparison.
Vestibular networks - formed by
Other afferents that reach, providing information
Vestibular networks
Vestibulo-ocular network
Vestibulo-spinal network
Vestibulo-cerebellar network
Vestibulo-thalamo-cortical network
Vestibulo-ocular reflex - function
Allows us to keep our eyes fixed on something as we move
(Keep the image focused on the fovea as we move)
Vestibulo-ocular reflex - characteristics
Compensatory (= magnitude of movement, opposite direction)
Habituate and eventually cease if prolonged
Can be controlled and suppressed
Vestibulo-ocular reflex - types
Rotational reflexes
Translational reflex
Counter-rolling reflex
Rotational reflexes takes place when
We turn our head but want to keep looking in = direction
Rotational reflexes are
Simple
Rotational reflexes are the origin of
Oculocephalic reflex (physiological Nystagmus)
Pathological Nystagmus occurs when
Semicircular canals are stimulated while the head is stationary
Optokinetic reflex is mediated by
input received from ganglionar cells in the retina.
Input to optokinetic reflex …
conveys info about movement collected through vision and carries it to oculomotor nucleus
Optokinetic reflex result
Move extrinsic eye muscles to maintain the image focused on the fovea during movement and recover a clear image.
Optokinetic reflex characteristics
Slower
Opposed the vestibulo-ocular
Doesn’t habituate and stop working
Rotational reflexes pathway (say turn the head to left)
Endolymph moves to R: L HSC depolarize & R HSC hyperpolarize —> L vestibular nerve reach sup & med vestibular nuclei + cerebellum through juxtarestiform body
- Excitatory signal —> ipsilateral oculomotor nucleus (L) = contract L med rectus —> turn eye to R
- Excitatory signal —> contralateral abducens nucleus (R) = contract R lat rectus —> turn eye to L
- Fibers —> ipsilateral (L) abducens nucleus = inhibit it (L lat rectus not to contract = med rectus act)
Translational reflexes
Needed to keep looking a certain object as we move forward
+ complex than the rotational one
Influenced by our distance of the fixed object
Counter-rolling reflex
Torsion eye movement
Vestibulo-spinal reflexes involve
muscle tone
Vestibulo-spinal reflexes generate
Postural adjustments of head and body
Vestibulo-spinal reflexes generate main pathways
Lateral vestibulospinal tract
Medial vestibulospinal tract
Lateral vestibulospinal tract is formed by
Fibres descending from lat & inf vestibular nuclei ipsilaterally (ONLY excites ipsilaterally)
Lateral vestibulospinal tract organization
Topographically organized:
- Rostroventral region —> cervical levels of spine
- Dorsocaudal region —> lumbosacral levels
Lateral vestibulospinal tract projections excite
Motor alpha & gamma neurons from laminae VII-IX (ant horn grey matter)
Lateral vestibulospinal tract function
Stabilizing the body (gives corrective movements)
Medial vestibulospinal tract function
stabilizing head and neck
Medial vestibulospinal tract is formed by
Fibers descending from sup & med vestibular nuclei bilaterally. Ipsilateral and contralateral projections
Vestibulo-cerebellar reflexes
Only which provides DIRECT info to cerebellum, fibers from ear to cerebellum
Vestibulo-cerebellar reflexes connections
Primary, direct, through the juxtarestiform body (w/out passing through vestibular nuclei)
Secondary, indirect, from vestibular nuclei to different deep cerebellar nuclei
Vestibulo-thalamo-cortical network function
Perception of how we are moving in space = conscious.
Perception of vertical orientation and rotation
Vestibulo-thalamo-cortical network nuclei will project to the thalamus
Bilaterally
Vestibulo-thalamo-cortical network nuclei will reach (once in the thalamus)
Ventroposterolat (VPL) & posteroinf (PI) nuclei —> 1ª somatosensory cortex, area 3a (posture control)
Post nuclear group (centromedian & parafascicular nuclei) —> 1ª somatosensory cortex, area 2V.
