Vestibular physiology 2017
Utricle A flat epithelium: horizontal in the upright head
Utricle Hair cells: no axons hair cells
Utricle Hair cells synapse onto 8th nerve afferents. 8th nerve afferents
Hair bundles at apical end. Utricle hair bundles
Utricle Gelatinous structure overlying hair cells. otolithic membrane
Utricle Gelatinous structure overlying hair cells. Otoconia (calcium carbonate crystals)
Utricle Endolymph: special high K +, positively charged fluid. endolymph
Utricle Perilymph: normal high Na + fluid. perilymph
Effect of tilting head Weight of otoconia causes distortion of otolithic membrane.
Linear acceleration The inertia of the heavy crystals causes the crystals to accelerate more slowly than the hair cell bodies, so the hair bundles are bent.
Hair cell structure There is one kinocilium (true cilium). A kinocilium is present on all vestibular hair cells. 1 kinocilium ~60 stereocilia n. VIII
Hair cell structure About 60 stereocilia Arranged in a step-like manner 1 kinocilium ~60 stereocilia n. VIII
Fine fibers link the tips of adjacent stereocilia. Hair cell structure tip links 1 kinocilium ~60 stereocilia n. VIII
Activation Fas K + Ca2+
Endolymph The main cation that enters the stereocilia is potassium. Endolymph, the extracellular fluid of the inner ear, has a very high potassium concentration.
Endolymph disorders Inflammation of the inner ear can disrupt normal production of endolymph. vertigo (dizziness with a feeling of spinning) vomiting hearing impairment
Meniere s disease Meniere s disease, affects about 1 in 500 people. During an attack, the person experiences dizziness and vertigo (sensation that the world is spinning) and often nausea and some hearing loss or tinnitus. Attacks tend to come on with no warning and to last for 20 min 4 hours.
Meniere s disease Possible causes of Meniere s disease Rise in endolymph volume Rupture of membranes separating endolymph from normal fluids of the cochlea Improper endolymph drainage Head trauma Virus???
orthogonal tilt: no voltage change hair cell receptor potential (mv) -40-50 -60 8th nerve firing rate
The utricle can detect head tilt in any direction away from vertical because it contains hair cells in many different orientations. A given tilt of the head will depolarize some hair cells, hyperpolarize others, and have no effect on the rest. Direction coding anterior lateral
Saccule (sacculus) The sacculus is very similar in structure to utricle, but it is oriented vertically in the upright head
Saccule (sacculus) The sacculus is very similar in structure to utricle, but it is oriented vertically in the upright head It senses linear acceleration and head tilt, particularly when the head is on its side.
Semicircular canals They respond to head rotation. There are three canals on each side. One is horizontal and is stimulated by turning the head to the left or right. The anterior and posterior ducts are upright, at 90 to each other.
ampulla
ampulla
ampulla ampullary crest
ampulla ampullary crest hair cell
ampullary crest ampulla hair cell cupula
ampulla ampullary crest endolymph hair cell cupula perilymph
How does head rotation stimulate the hair cells?
not moving rotating to right
A turn to the right depolarizes all the hair cells in the right horizontal canal
A turn to the right depolarizes all the hair cells in the right horizontal canal
turn to right turn to left left horizontal canal right horizontal canal DEPOLARIZATION
The horizontal canal on the left side of the head is the mirror image of the right canal
The horizontal canal on the left side of the head is the mirror image of the right canal
turn to right turn to left left horizontal canal HYPERPOLARIZATION DEPOLARIZATION right horizontal canal DEPOLARIZATION HYPERPOLARIZATION The 8th nerve afferents will increase or decrease their firing rate depending upon the direction of motion & rate of motion (faster acceleration produces bigger changes from resting rate).
Vestibular compensation The redundancy of information from the two sides of the vestibular system is a factor permitting vestibular compensation. The immediate effect of damage to the canals, otoliths, and/or vestibular nerve on one side of the head is severe problems with balance, posture, gait, and eye movements.
Vestibular compensation Gradual recovery with time: vestibular compensation Different symptoms recover at different rates. Vestibular compensation is also important when there are more subtle changes in vestibular input such as loss of a few hair cells. 44
Vestibular compensation The vestibular nuclei and cerebellar are likely sites where compensation occurs. Possible mechanisms: Regulation of efficacy of different transmitters. Changes in gene expression 45
Falls in the elderly: failure of vestibular compensation?? Many hair cells die as we age. One out of three adults age 65 and older falls each year. Among those age 65 and older, falls are the leading cause of injury death. Lateral semicircular canal utricle
Falls in the elderly: failure of vestibular compensation?? Does decreased ability to compensate for this cell loss lead to decreased vestibular capability? One out of three adults age 65 and older falls each year. Among those age 65 and older, falls are the leading cause of injury death.
Falls in the elderly: failure of vestibular compensation?? Many people who fall, even if they are not injured, develop a fear of falling. This fear may cause them to limit their activities, leading to reduced mobility and loss of physical fitness, which in turn increases their actual risk of falling.
Vestibular physiology of the thalamus Vestibular information is widely distributed in the thalamus. In some regions, vestibular influences converge with visual and proprioceptive input: Vestibular information about head rotation Visual responses to optokinetic stimuli (largefield, moving stimuli) Proprioceptive responses to neck muscle movement
Vestibular physiology of the cortex Somatosensory cortex Some cells respond to head rotation. Some cells respond to head tilt. Lesions in this area lead to the sensation that vertical objects are tilted toward the side of the lesion.
Cortical vestibular responsive areas A major vestibular area is in temporo-parietal and parieto-insular cortex. Some cells have purely vestibular responses. Others are multimodal (visual, proprioceptive, vestibular).
Some effects of temporal cortical lesions Posterosuperior temporal lesions can impair eye movements that are responses to vestibular signals. Insulotemporal infarctions may lead to a tilted perception of visual vertical and to rotational vertigo in stroke patients.