The document discusses the vestibular system, which detects angular and linear acceleration of the head. It has two main parts: the semicircular canals and otolith organs. The semicircular canals contain hair cells that detect rotational movement and signal the brain. The otolith organs contain hair cells and calcium crystals that detect gravity and linear acceleration. The vestibular system provides input to areas of the brainstem, cerebellum and cortex that are important for balance, posture, eye movements and awareness of head position. It discusses the anatomy and function of the vestibular system and several reflexes it controls like the vestibulo-ocular reflex.
3. Vestibular System: Function
3
Detects angular and linear acceleration
Important in maintaining balance, posture, and
vision
Connections with brainstem, cerebellum, and
somatic sensory cortices to provide info about
the motions & position of the head & body
6. Semicircular canals: structure
6
each semicircular canal
contains an ampulla
Contains hair cells
embedded in
sensory epithelium
called crista
ampullaris
Cilia of hair cells
project into
gelatinous cap called
cupula
Enlargement of
ampulla
Crista ampullaris
Semicircular
canals
7. Semicircular canals:
function7
Specialized for responding to
rotational acceleration of the head
Head rotation results in intertial
movement of endolymph in
opposite direction
Bends cupula which bends hair
cells
Same mechanical/electrical
coupling as in auditory hair
cells
Excites/suppresses release of
NTs from hair cells depending on
direction of movement
B&B Figure 13-18
8. Semicircular canals: sensory
transduction8
B&L Figure 8-26
Steriocilia maintain directionality on both sides of the head
Bending towards kinocilium opens mechanically gated cation
channels K+
influx depolarization
Bending away from kinocilium closes channels that are open
during resting state hyperpolarization
9. Semicircular canals: sensory
transduction9
Kandel Figure 40-7
Paired canals work
together to signal head
movement
With turning of the
head, hair cells on
one side of the body
send excitatory
signals to the brain
while hair cells on the
opposite side are
inhibited
10. Otolith Organs: Structure
10
Two otolith organs; utricle and
saccule
Each contains a sensory
epithleium called the
macula
Horizontally oriented in
utricle
Vertically oriented in
saccule
cilia of hair cells embedded in
gelatinous otolithic membrane
Embedded on surface are
calcium carbonate crystals
called the otoliths
otoliths
11. Otolith Organs: Function
11
Specialized to respond to gravity and linear acceleration
Otoliths have a higher density than endolymph
Shift when angle of head changes
Causes otolithic membrane to shift in same direction
Cilia of certain hair cells deflected
Excites/suppresses release of NTs from hair cells
depending on orientation of cilia
12. Otolith Organs: Function
12
kinocilia of each hair cell are oriented in
different directions in relation to striola
Utricle: towards striola
Saccule: away from striola
striola
13. Otolith Organs: Function
13
Same sensory
transduction as
semicircular canals
Bending of cilia towards
kinocilium depolarizes
the hair cell
Kandel Figure 40-3
otoliths
Is this a picture of a
macula from the utricle
or saccule?
14. Vestibular Pathways
14
vestibular afferents synapse on vestibular nuclei located in medulla & pons
Nuclei integrate information from vestibular, visual, and somatic receptors and send
collaterals to
1.cerebellum
Sends corrective adjustments to motor cortex: maintenance of balance and posture
15. Vestibular Pathways
15
2.nuclei of cranial nerves
Control coupled movements of the eyes, maintain focus and visual field
3.nuclei of accessory nerves
Control head movement and assist with equilibrium
16. Vestibular Pathways
16
4.ventral posterior nucleus of thalamus and vestibular area in cerebral cortex
(part of primary somatosensory cortex)
Conscious awareness of the position and movement of head
Areas 1,2,3
17. Vestibular Reflexes
17
Vestibulospinal Reflexes
Senses falling/tipping
contracts limb muscles for postural support
Vestibulocollic Reflexes
acts on the neck musculature to stabilize the head if
body moves
Vestibulo-ocular Reflexes
stabilizes visual image during head movement
causes eyes to move simultaneously in the opposite
direction and in equal magnitude to head movement
18. Vestibulo-Ocular Reflex (VOR)
18
Example: head movement to the
LEFT
1. inertia of endolymph movement to the
right in horizontal vestibular canals
causes:
a ’d firing of left vestibular
afferent
b ’d firing of right vestibular
afferent
2. Excitatory connections with
contralateral abducens nuclei and
inhibitory connections to ispilateral side
3. Excitatory connection to inhibitory
interneuron in contralateral vestibular
nuclei
4. Movement of the eyes to the right
abducens nuclei
occulomotor nuclei
vestibular nuclei
B&L Figure 9-
19. Objectives
After this lecture you should be able to:
Relate the anatomical organization of the semicircular
canals and otolith organs to sensation of
movement/acceleration
Describe the mechanism of sensory transduction in these
structures
Outline the vestibular pathways and projections to
various brain regions
Describe the pathway of the horizontal vestibulocular reflex
19
20. 20
1. What specific part of the vestibular system would
sense
1. Movement in an elevator
2. Abrupt stop of a moving vehicle
3. Shaking your head side to side
2. In the utricle, if hair cells bend away from the striola,
will this cause depolarization or hyperpolarization of
the receptor?
3. Based on what you know about the vestibular system,
if you spin around for 5-10 seconds, why do you feel
dizzy even after you stop?
Test your knowledge
The auditory system is one of the engineering masterpieces of the human body. At the heart of the system is an array of miniature acoustical detectors packed into a space no larger than a pea. These detectors can faithfully transduce vibrations as small as the diameter of an atom, and they can respond a thousand times faster than visual photoreceptors. Such rapid auditory responses to acoustical cues facilitate the initial orientation of the head and body to novel stimuli, especially those that are not initially within the field of view. Although humans are highly visual creatures, much human communication is mediated by the auditory system; indeed, loss of hearing can be more socially debilitating than blindness. From a cultural perspective, the auditory system is essential not only to language, but also to music, one of the most aesthetically sophisticated forms of human expression. For these and other reasons, audition represents a fascinating and especially important aspect of sensation, and more generally of brain function.
permits rapid compensatory movements in response to both self-induced and externally generated forces
key component in both postural reflexes and eye movements
if the system is damaged, balance, control of eye movements when the head is moving, and sense of orientation in space are all adversely affected
manifestations of vestibular damage are especially important in the evaluation of brainstem injury
circuitry of the vestibular system extends through a large part of the brainstem, and simple clinical tests of vestibular function can be performed to determine brainstem involvement, even on comatose patients
canals are surrounded by perilymph and contain endolymph
the ionic composition of the endolymph is high in K+ and low in Na+
canals connect with utricle which is linked to saccule
utricle oriented horizontally; saccule oriented vertically
lateral canals located in a plane tilted about 25 above horizontal when head is level
posterior canal is rotated about 55 posterior to sagittal plane
anterior canal is rotated about 40 anterior to sagittal plane
The utricle and saccule are specialized primarily to respond to linear accelerations of the head and static head position, whereas the semicircular canals are specialized for responding to rotational accelerations of the head.
specialized for responding to rotational accelerations of the head
canals are surrounded by perilymph and contain endolymph
the ionic composition of the endolymph is high in K+ and low in Na+
canals connect with utricle which is linked to saccule
utricle oriented horizontally; saccule oriented vertically
lateral canals located in a plane tilted about 25 above horizontal when head is level
posterior canal is rotated about 55 posterior to sagittal plane
anterior canal is rotated about 40 anterior to sagittal plane
The utricle and saccule are specialized primarily to respond to linear accelerations of the head and static head position, whereas the semicircular canals are specialized for responding to rotational accelerations of the head.
the ampulla is an expanded region which contains sense organs made of hair cells that respond to rotational motion
hair cells innervated by afferent fibers of vestibular nerve are embedded in crista ampullaris
hair cell is made up of columns of stereocilia which are graded in height and single kinocilium (tall cilium)
stereocilia are embedded in cupula (gelatinous structure) which extends to the upper wall of ampulla
stereocilia bending in the opposite direction creates a hyperpolarization by closing those channels that are constantly open, even in the resting state, thus further obstructing K+ flow down the electrochemical gradient.
This view of the horizontal semicircular
canals from above shows how the
paired canals work together to signal head
movement. Because of inertia, rotation of the
head in a counterclockwise direction causes endolymph
to move clockwise with respect to the
canals. This reflects the stereocilia in the left
canal in the excitatory direction, thereby exciting
the afferent fibers on this side. In the right
canal the hair cells are hyperpolarized and afferent
firing there decreases.
canals are surrounded by perilymph and contain endolymph
the ionic composition of the endolymph is high in K+ and low in Na+
canals connect with utricle which is linked to saccule
utricle oriented horizontally; saccule oriented vertically
lateral canals located in a plane tilted about 25 above horizontal when head is level
posterior canal is rotated about 55 posterior to sagittal plane
anterior canal is rotated about 40 anterior to sagittal plane
The utricle and saccule are specialized primarily to respond to linear accelerations of the head and static head position, whereas the semicircular canals are specialized for responding to rotational accelerations of the head.
shift also occurs due to force of gravity when head is tilted because otoliths have higher density than endolymph
, providing sensitivity to acceleration in all directions
kinocilium points toward curved ridge called striola in utricle and away from striola in saccule
macula of utricle oriented in horizontal plane; macula of saccule oriented in vertical plane
Utricle
providing sensitivity to acceleration in all directions
kinocilium points toward curved ridge called striola in utricle and away from striola in saccule
macula of utricle oriented in horizontal plane; macula of saccule oriented in vertical plane
lateral & medial vestibular nuclei give rides to lateral and medial vestibulospinal tracts (influence activity of postural muscles)
vestibular nuclei also project to the cerebellum, reticular formation, and thalamus
superior and medial vestibular nuclei project through medial longitudinal fasiculus to the oculomotor nuclei and are involved in the control of reflex eye movements (e.g. vestibulo-ocular reflex)
lateral & medial vestibular nuclei give rides to lateral and medial vestibulospinal tracts (influence activity of postural muscles)
vestibular nuclei also project to the cerebellum, reticular formation, and thalamus
superior and medial vestibular nuclei project through medial longitudinal fasiculus to the oculomotor nuclei and are involved in the control of reflex eye movements (e.g. vestibulo-ocular reflex)
Areas 1, 2 3 are part of the primary somatosensory cortex
Analogous reflexes occur for vertical movement in the anterior and posterior canals (mainly involved with the superior vestibular nucleus)
Inhibitory neurons project ipsilaterally and excitatory neurons project contralaterally
MLF – medial longitudinal fasciculus
M stands for medial vestibular nucleus
Vestibular nuclei
Abducens and oculomotor nuclei second
Reticular formation third