Vestibular System Flashcards
Recognize and name the components of the vestibular system.
Three semicircular canals (anterior, posterior, horizontal) detect head rotation or angular acceleration.
Two otolith organs, the utricle and the saccule, detect linear position/acceleration and the direction of gravity.
Note that hair cells transduce vibration into APs in the vestibular system just as they transduce vibration into APs in the auditory system.
The fluid that surrounds the organs in the vestibular system: perilymph. Fluid inside the organs: endolymph. The same potassium gradient that keeps the hair cells depolarizing and hyperpolarizing in the cochlea is responsible for them doing the same thing in the vestibular system.
The nerves that innervate the hair cells exit into two main nerve branches that feed into the vestibular part of the eighth nerve, which joins up with the auditory part of the eighth nerve and together go into the pons along with the facial nerve.
Describe the hair cell and its physiology
- Each hair cell has actin filled stereocilia from apical surface (staircase pattern)
- Hair bundle is bathed in endolymph (high K, low Na and Ca). Base surrounded in perilymph (more regular composition)
- single kinocilium (tallest)
- Stereocilia connected by tip links (attached to ion channel, mechano-transduction; channel opens with movement toward kinocilium, K+ enters hair cell, depolariz, NT release)
- Vestibular afferent nerves fire APs even at rest.
Explain how the vestibular sensors communicate motion information.
Epithelia of utricle and saccule at right angles to each other, thus activated by different signals. Utricular hair bundles project vertically, tilt of head from the horizontal causes shear b/t hair cells and overlying otolitihic memb. So the UTRICLE gives brain info of head with respect to GRAVITY.
- Saccular hair bundles: horizontal projection, so sensitive to vertical displacement of head (elevator); linear acceleration in vertical direction.
- In the utricle, the axes of polarity of hair cells reverse direction along a central region of epithelium called the striola.
- In the saccule, the hair vundles are oriented facing AWAY from the striola.
Discuss the three vestibular motor reflexes
Vestibulo-colic reflex: causes the head to be maintained in balance on the top of the head when you’re moving.
Vestibulo-spinal reflexes: involve all muscles in limbs: keep stable/upright, balance.
Vestibular Ocular reflex:
VOR
short
enables stable visual field during head movement.
Oculomotor nuclei in brainstem
3 neuron arc
Leads to nystagmus
Otolith organs
Utricle (floor of vestibule) and saccule (hangs on lateral wall of vestibule); signal postural changes (“static receptors”)
The sensory epithelia (maculae) of both contain hair cells and supporting cells whose apical surfaces are overlaid with a gelatinous mass (otolithic membrane) which holds the otoconia (calcium carbonate crystals) in place.
The otoconia give additional mass and inertia
What is the vestibular system filled with/surrounded by?
Filled with endolymph, surrounded by perilymph. Located in petrous portion of temporal bone.
Actions of 3 semicircular canals
The two horizontal canals act together to signal horizontal rotation of the head. Pairs of anterior and posterior canals are in approximately equivalent vertical planes and together signal head rotation vertically.
Semicircular ducts project from what?
Utricle
3 pairs function to detect ANGULAR acceleration
What is the ampulla?
A swelling at one end of each canal. Within each ampulla, there is epithelium called “crista” with sensory hair cells. Hair bundles project into portion called the “cupula” (projects across ampulla, connects to non-sensory epithelium).
- Kinocilia in same direction in the crista.
- Cupula movement by endolymph changes hair cell receptor potentials.
- In the horizontal semicircular canals, hair cells are oriented with kinocilia facing toward utricle. In anterior and posterior canals, kinocilia facing away from utricle.
At the beginning of rotation, endolymph will move in a direction that is ____ that of head roation.
What happens when rotation stops?
opposite (movement of labyrinth and endolymph causes cupula to be deflected).
When it stops: endolymph keeps going, cupula is deflected, but in opposite direction than beginning.
In horizontal clockwise rotation: canal fluid in horizontal canals moves counter clockwise and hair cells are of right horizontal ampulla are depolarized. In contrast, left ampulla hair cells are hyperpolarized, and afferent AP freq decreases.
Hair cells in otolith organs respond to ___ accel of head.
Hair cells in ampullae in semicirc canals respond to _____.
otolith (utricle, saccule): linear accel of head
ampullae in semicirc canals: angular acceleration (head rotations)
What motions are sensed by the cupula?
acceleration
deceleration
does NOT sense continued motion (adaptation)
Vestibular hair cells types
Type I (calyx assoc) Type II
Which NT is released by hair cells onto afferent nerve terminals?
glutamate
Efferent fibers from brainstem can excite or inhibit afferent firing.
Vestibular nuclei subdivisions
Afferents have cell bodies in Scarpa’s ganglion, project to vestib nuclei at border of caudal pons and rostral medulla.
Superior
Medial
Lateral
Descending
Sup and med receive fibers mostly from semicircular canals (gaze control). Lateral nucelus (Deiter's nuc) receives input from canals and otoliths and is involved in postural reflexes. Descending nucleus receives mostly otolith input and integrates vestibular and motor signals.
Utricles to lateral, medial and desc/inferior nuc. (Make up descending lateral vestibulospinal tract; activation leads to excitation of extensor muscles of limbs, participate in postural reflexes assoc w/gravity)
Semicirc to medial and sup vestib nuc. Give rise to medial longitudinal fasciculus, carries info to oculomotor nuc. Also medial nuc to cervical motor neurons in neck. “medial vestibulospinal tract” (coord eye and head movements)
Saccule utricle and semicirc to inferior vestib nucleus.
