2. Function of Vestibular System
Static head position
Linear acceleration
Rotational
acceleration
Otolith
organs
Semicircular
canals
3. Vestibular Labyrinth
Embryology:
arises from the
otic placode
together with
cochlea
Morphology:
represents set of
interconnected
channels
Histology: sensory hair cells able to transduce motion signals into neural impulses
Anatomy: two classes of sensory
structures 1) the otolith organs:
the utricle and sacculus; 2)
three semicircilar canals.
Canals are filled with endolymph (high K+ / low Na+) and surrounded by perilymph (low K+ / high Na+)
4. Sense of the right to left head tilting and
rolling action in the horizontal plane.
Utricle
horizontal
arrangement of the
hair cells
Sacculus
vertical arrangement
of the hair cells
Sense of flexion and extension of the head,
elevation and depression (as in an elevator).
Otolith
organs
The hair cells are
innervated by the
vestibular division of
cranial nerve VIII
(cell bodies reside in
Scarpa’s ganglia)
7. Otolith Organ
Otolithic membrane –
gelatinous layer upon the hair
cells in otolith organ.
Otoconia – crystals of calcium
carbonate upon the otolithic
membrane
Gravity causes the
gelatinous membrane to
shift
Tonic depolarization and
hyperpolarization of hair
cells.
8. Organization of Semicircular Canals
Only one axis of depolarization
Horizontal
Inferior
(posterior)
Superior
(anterior)
9. Functional
pairs of SC
• left horizontal and right horizontal
• left anterior (superior) and right
posterior
• left posterior and right anterior
(superior)
10. Central Vestibular Processing
Ganglion cells
2.“Vestibulocerebellum”
flocculonodular lobe of
the cerebellum
3. Reticular formation
of the brainstem
1. Vestibular nuclei (S, M,
L) in the rostral medulla
and caudal pons
11. Histology of Vestibular Nuclei (VN)
• Level of mid-pons • Medulla-pons junction
Lat VN
Sup VN
Med VN
Med VN
Spinal VN
Spinal
trigeminal tract
+ nucleus
13. Vestibulo-Ocular Reflex
(eye movements opposite to the direction of rotation)
Impulses from the left
semicircular canal
Activation of right
lateral rectus and
left medial rectus
(right CN VI and
left CN III)
Reduction of tone of
the left lateral rectus
and right medial rectus
(left CN VI, and right
CN III)
15. Vestibular Nystagmus
= rhythmic form of reflexive eye
movements composed of slow component
in one direction interrupted repeatedly by
fast saccadic-like movements in opposite
direction.
It is normally driven by persistent
rotation of the head.
The slow component is driven by the
vestibulo-ocular reflex.
The fast component is the eye
reposition in the orbit – result of
sensory-motor integration reflecting
proprioceptors from eye muscles.
16. Lat.
vestibulo-
spinal truct
Regulation of Posture
cerebellum
LVn
MVn
Spinal moto-
neurons
smn
mm. of neck
and upper
limbs
mm. of
lower limbs
(ipsilateral)
Med.
vestibulo-
spinal truct
Vestibulo-
cervical reflex
(bilateral)
17. Higher vestibular
processing
• Parietal cortex (proprioceptive
representation ‘face zone’= sense of
spacial orientation of the head;
• Reticular formation (coordination of
viscero-motor activity = sickness and
vomiting)
Editor's Notes
: processing sensory information underlying motor responses to (and perceptions of) self-motion, head position, and spatial orientation relative to gravity, helping to stabilize gaze, head, and posture.
otolith organs
macula: sensory epithelium of the otolith organs
(i) hair cells in the maculae of the utricle and sacculus are arranged into two populations with opposing orientations along an axis of mirror symmetry
(ii) motion in one direction will depolarize one subpopulation of hair cells and hyperpolarize the other
deformation of stereocilia toward the largest stereocilium leads to depolarization and increased release of neurotransmitter on peripheral endings of afferent fibers
deformation away from the largest stereocilium leads to hyperpolarization and decreased release of neurotransmitter
deformation of stereocilia toward the largest stereocilium leads to depolarization and increased release of neurotransmitter on peripheral endings of afferent fibers
deformation away from the largest stereocilium leads to hyperpolarization and decreased release of neurotransmitter
mechanism of otolith organ function:
consists of a hair cells and supporting cells, and an overlying gelatinous layer (the otolithic membrane) upon which are embedded crystals of calcium carbonate, called otoconia.
the hair cells protrude into this gelatinous layer, which is heavier than the surrounding epithelium and fluids.
When the head is tilted, gravity causes the gelatinous membrane to shift relative to the underlying epithelium; this displaces the hair cell stereocilium and leads to tonic depolarization and hyperpolarization.
Ampulla – is a bulbous expansion at the base of the semicircular canals that contains the sensory epithelium (called the crista) and an overlying gelatinous mass (called the cupula) into which the stereocilium of the hair cells protrude.
The cupula creates a barrier for the flow of endolymph around the semicircular canal.
When the head is rotated in the plane of the semicircular canal, the inertia of the endolymph produces a transient force that distends the cupulla away from the direction of rotation.
Distension of the cupulla deflects the stereocilia of the hairs cells, which leads to depolarization or hyperpolarization of the hair cells within any given crista.
semicircular canals are paired on the two sides of the head:
left horizontal and right horizontal
left anterior (superior) and right posterior
left posterior and right anterior (superior)
central processes of ganglion cells project via the vestibular division of the eighth cranial nerve to vestibular nuclei in the rostral medulla and caudal pons, and directly to the “vestibulocerebellum” (flocculonodular lobe of the cerebellum)
one major function of the vestibular nuclei is to coordinate movements of the eyes and head to allow for stable visual fixation during head or whole body movements
Stroke of upper region of medulla (posterior inferior cerebellar artery) = complex of symptoms (trigeminal system /via spinal trigeminal nucleus and tract/+ spinocerebellar system/inferior cerebellar peduncle/ + vestibular system / vestibular nuclei.
Afferent:
Efferent: A few Purkinje cell axons pass directly out of the cerebellum to the lateral vestibular nucleus.
vestibulo-ocular reflex
rotational movements of the head induce eye movements opposite to the direction of rotation, thus allowing maintained visual fixation of both eyes pathway
input from the horizontal canals reaches the vestibular nuclei on the two sides of the medulla/pons
each vestibular nucleus in turn sends a excitatory projections to the contralateral abducens nucleus
the abducens nucleus directly innervates the ipsilateral lateral rectus muscle, which pulls that eye toward the lateral side (i.e., abduction)
other cells in the abducens nucleus cross the midline again and project to the opposite oculomotor nucleus, which innervates the ipsilateral medial rectus causing that eye to turn toward the midline (i.e., adduction)
to facilitate this excitatory action, there are also inhibitory projections from the vestibular nuclei to the ipsilateral abducens nucleus
this inhibitory projection turns off the excitatory output of the cranial nerve nuclei (opposite abducens and oculomotor nuclei) that drive the antagonistic muscles
The nystagmus is an indication of balanced activity of the vestibular system. The hypofunction of vestibular apparatus on one side could lead to the appearance of this sigh (symptom) without actual head movement. =pathological alteration in the balance of activity b/w the two sides
The direction of the eye movement during slow component is toward the side with hypofunction of semicircular canals.
If the superior-inferior semicircular canals are misbalanced the patient will have vertical nystagmus.
Another major function of the vestibular nuclei is to make reflexive adjustments of posture that compensate for movements of the head
descending projections from the vestibular nuclei reach the medial aspect of the ventral horn of the spinal cord via medial and lateral vestibulospinal pathways
medial vestibulospinal projection to the upper cervical cord that regulates head and neck position
“lateral” vestibulospinal projection to motor neurons in the medial ventral horn that excite extensor muscles in the trunk and limbs; this pathway mediates balance and maintenance of an upright posture
