Vestibular System Flashcards
Vestibular Function
Generates compensatory responses to head motion
- postural responses
- ocular-motor responses
Informs the brain about the position and motion of the head in space
Helps to maintain equilibrium or balance by detecting the static tilt, and linear or rotary acceleration of the head
Helps to keep eyes fixated on a relevant target when the orientation of the body changes (Vestibulo-Occular Reflex, VOR)
Structure
The peripheral vestibular system is located in the inner ear, within the bony labyrinth.
The bony labyrinth is a system of tunnels and caverns in the temporal bone
- consists of the membranous labyrinth (epithelium associated with sensory receptor organs, endolymph and perilymph)
What are the two receptor systems in the inner ear
Two receptor systems are found in the inner ear:
- Hearing (cochlea)
- Equilibrioception
(detecting balance + spatial orientation)
= vestibular system;
What are the two receptor systems in the inner ear
Two receptor systems are found in the inner ear:
- Hearing (cochlea)
- Equilibrioception
(detecting balance + spatial orientation)
= vestibular system;
What does the peripheral vestibular apparatus contain
Three semicircular ducts, found on each side, that are ~perpendicular to one another
A utricle, and a saccule (= otolith organs)
Osseous Labyrinth
Tubes and chambers in the petrous part of the temporal bone that contain perilymph fluid and house the membranous labyrinth.
What are the three osseous components
- Cochlea — a spiral chamber that is related to hearing
- Vestibule — a large chamber adjacent to the middle ear
- Semicircular Canals — three semicircular channels in bone, each semicircular canal is orthogonal to the other two
Membranous Labyrinth
Consists of interconnected tubes and sacs that are filled with endolymph, a fluid high in potassium.
(Fluid outside the membranous labyrinth is perilymph, which is low in potassium and high in sodium like typical extracellular fluids.)
Contains the sense organ receptor cells
What do the ampulla of each semicircular duct contain
A crista ampullaris (receptor cell organ) and detects rotational acceleration/deceleration
What are maculae
Maculae are the sensory organs in utricle and saccule and detect, linear acceleration, deceleration and static tilt of the head with respect to gravity
What does the membranous labyrinth, which contains the sense organ receptor cells, consist of
1) Cochlear Duct — related to hearing
2) Utricle — larger of two sacs located in the vestibule
3) Saccule — smaller of two sacs located in the vestibule
4) 3 Semicircular Ducts — each duct is located within one of the semicircular canals. Each duct has a terminal enlargement called an ampulla which contains a crista ampullaris, a small crest bearing sensory receptor cells.
Vestibular apparatus
Is a collective term for sensory areas within the membranous labyrinth responsible for detecting linear acceleration (e.g., gravity) and angular acceleration of the head.
The vestibular apparatus consists of
1) macula of the utricle — the sensory area (spot) located in the wall of the utricle; it is horizontally oriented and detects linear acceleration in the horizontal plane (side to side).
2) macula of the saccule — the sensory spot in the wall of the saccule; it detects linear acceleration in the vertical plane (up and down).
3) crista ampullaris — one per semicircular duct ampulla; each detects angular acceleration directed along the plane of the duct.
What do all components of the vestibular apparatus have
Have the same kind of sensory epithelium, composed of supporting cells and receptor (hair) cells (ciliated epithelium). From the apical surface of each hair cell, stereocilia protrude into an overlaying membranes
Cilia
(Stereocilia or stereovilli) are arranged by size, and project into a gelatinous mass (called cupula for crista)
What are the two different types of fluids bath the hair cells
Endolymph on the luminal side,
Perilymph on the basolateral side.
Perilymph
The perilymph is similar in composition to the CSF; has close to 0 voltage, and like that of most ECFs (relatively low [K+], high [Na+]). The basolateral resting potential of the hair cells is -40 mV.
Endolymph
The endolymph {with very high [K+] (150 mM) $ a low [Na+] (1 mM)} is more like ICF than ECF. It also has a relatively high bicarbonate (30 mM). Its voltage is ∼0 mV relative to perilymph.
However, the electrical gradient for K across the apical membrane is large, ∼40 mV. Thus, a substantial force tends to drive K+ into the vestibular hair cell
Kinocilium
Largest cilium
Otoliths
Signal transmission
The hair cell is stimulated by the bending of its hairs, but not just any deflection will do.
Bending of the hair bundle toward the longer stereovilli excites the cell and causes a depolarizing receptor potential;
Bending leads to K influx; the mechanically (K) induced hair cell depolarization opens Ca channels; Ca enters hair cells and leads to release of neurotransmitters that generate action potential in sensory nerve fibers (Fig).
Bending away from kinocilium inhibits (hyperpolarizes) the cell
Angular Acceleration
The movement of the head is followed by movement of the gelatinous mass (endolymph) in opposite direction. The fluid then deflects the ciliaThe direc
A= Crista when the head is at rest.
B, Crista during rotational acceleration of the head in the indicated direction. The relative inertia of the endolymphatic fluid displaces the cupula, and thus the hair cell cilia, in the opposite direction
tion of deflection of cilia determines the type of action potential
Polarization of Vestibular Hair cells
A cross section of hair cells shows that the kinocilia of a group of hair cells are all located on the same side of the hair cell. The arrow indicates the direction of deflection that depolarizes the hair cell. Thus, when the cupula moves in the appropriate direction, the entire population of hair cells is depolarized and activity in all of the innervating axons increases. When the cupula moves in the opposite direction, the population is hyperpolarized and neuronal activity decreases
when the head turns to the left, the cupula is pushed toward the kinocilium in the left horizontal canal, and the firing rate of the relevant axons in the left vestibular nerve increases. In contrast, the cupula in the right horizontal canal is pushed away from the kinocilium, with a concomitant decrease in the firing rate of the related neurons. If the head movement is to the right, the result is just the opposite. When you tip your head forward and to the left ear, the left anterior canal is maximally activated and the right posterior maximally decreased
Linear acceleration and static tilt
Utricle’s macula is horizontally oriented. When the head is stationary and upright, there is little or no bending of cilia.
When the head tilts and remains tilted, the heavy otolith layer “falls over,” producing a drag and bending cilia, via endolymph, in the direction of the tilt
During acceleration in a straight line, the hair cells accelerate in the same direction, but the otolith lags behind, producing a drag in the opposite direction; bends cilia backwards.
Hair cells whose cilia are bent directly toward the largest cilium (green) will be depolarized and produce greatest AP. Conversely, hair cells whose cilia are bent directly away from the largest cilium (red) will be hyperpolarized the most and least AP frequency in their associated sensory neurons.
Stimulation occurs only during rotary acceleration or deceleration, not during constant velocity when the endolymph will catch-up and cilia will not be bent
- E.g plane take off/landing feelings; none during flight.
- Elevator rides (feeling at the beginning and last)
Vestibulo-Ocular reflex
- Detection of rotation
- excitation of extraocular muscles on the other side
- Inhibition of extraocular muscles on one side
- Compensating eye movement
A rotation of the head triggers an inhibitory signal to the extra ocular muscles of on one side and an excitatory signal to the muscles on the other side. The result is a compensatory movement of the eyes to one side (multiple rotation leads to physiological nystagmus).
The important function of the VOR is to stabilize the retinal image during rotations of the head.
For example, when the head rotates with a certain speed and direction, the eyes must rotate with the same speed but in the opposite direction.
head rotation to the right produces increased AP frequency in the right vestibular
nerve and decreased frequency in the left. Vestibular nuclei on the right side dominate activity in
the left abducens nucleus & right oculomotor nucleus, causing the eyes to move to the left.
The vestibular system effects on neck and limbs
Vestibular nuclei influence extensor muscles in the limbs;
Extensor muscles are contracted on the side toward which the head is accelerating (to preclude falling).
Vestibular Pathways
The vestibular system provides what type of information
The Vestibular system provides sensory information for reflexes involving spinal motor neurons, the cerebellum, and extraocular muscles of the eye
CNS connections of the vestibular system
Vestibular nerve fibers (axons from neuron cell bodies of the vestibular ganglion) travel from the inner ear to the brain. They synapse in vestibular nuclei of the brainstem and in the nodulus or flocculus of the cerebellum.
What happens when the vestibular pathway is damaged
Damage anywhere along the vestibular pathway can lead to disturbances in balance/position
General signs of vestibular disease: falling, rolling, head tilting, circling, nystagmus, positional strabismus (deviation of one in a certain position of the head), asymmetric ataxia
In unilateral peripheral vestibular disease, animals usually move/tilt their head toward the side of the lesion
