Vestibular System

  • Overview: 

    • Composed of the otolith organs (saccule and utricle) and semicircular canals – is responsible for sensing motion, postural stability, and image stabilization during head movement 

    • The vestibular receptors are located in the labyrinth of each inner ear and provide the brain with information about motion of the head and about the orientation of the head relative to gravity 

    • These functions go largely unnoticed except when experiencing unusual conditions

    • Also assists in coordinating body position with other sensory information 

  • Vestibular Hair Cells

    • Similar to auditory system, motion is detected by the vestibular sensory organs through reflections of hairs cells (kinocilium and stereocilia) 

    • Hair cells hyperpolarize or depolarize depending on the direction of deflection of the stereocilia 

      • Depolarization is caused by stereocilia movement TOWARDS the kinocilium

      • Hyperpolarization is caused by stereocilia movement AWAY from the kinocilium

      • Different polarization affects the firing rate of the primary vestibular afferents to the brain stem 

    • Depol/Hyp depends respectively on the K+ rich endolymph and the K+ poor perilymph on the basal and lateral portions of hair cells 

    • Deflection of the stereocilia towards the kinocilium causes K+ channels in the apical portions of the stereocilia and kinocilia to open

      • K+ flows into the cell from endolymph, depolarization

    • Depol causes voltage gated Ca2+ channels at the base to open, allowing Ca2+ to enter the cell, causing synaptic vesicles to release their transmitter 

  • Semicircular Canals

    • Six total semicircular canals (three on each side)

    • Canals are approximately oriented at right angles (orthogonal to each other) 

    • Each canal response to angular head acceleration during rotation of the head in their plane

    • Each canal consists of a membranous canal (filled with endolymph) within a bony canal

    • The space between the membranous canal and bony canal is filled with perilymph 

    • Each canal has an ampulla: a region with hair cells (crista ampulalaris) and a gelatinous structure that connects the hair cell region to the opposite side (cupula) 

    • Angular head motion leads to inertially-driven endolymph motion

    • Endolymph motion causes cupula deflection which causes stereocilia movement towards or away the kinocilium 

    • Motion leads to differences in transmitter release from hair cells and increased and decreased discharge rates of the afferent nerve proportional to the angular head velocity 

  • Otolith Organs: Utricle and Saccule

    • Utricle lies horizontally, and saccule oriented vertically

    • Organs provide information about linear acceleration (back and forth, up and down) and changes in head position relative to forces of gravity  

    • Each organ has a sheet of hair cells (macula) whose cilia are embedded within gelatinous mass with a clump of small calcium carbonate crystals called otoliths (ear stones)

    • Each crystal is about 50 micrometers long, in humans, they are 3-30 micrometers long 

    • LInear acceleration, either due to gravity or translation motion, causes inertially-driven motion of the crystals which applies a shear force to the stereocilia of the hair cells

    • Similar to the semicircular canals, motion towards kinocilium results in excitation and way results in inhibition