Vestibular System

caloric stimulation test

*Warm water:* Sets up convection current in endolymph which causes the cupula to distort and excite the hair cells. *Cold Water:* Sets up convection current in endolymph which causes the cupula to distort in opposite direction and inhibit the hair cells.

boney labyrinth

Filled with perilymph Low K+; high Na+, Ca2+ (like CSF) Produced from CSF and/or blood CSF can exchange with perilymph via the perilymphatic duct

caloric nystagmus

Occurs when the fluid motion in the canals of the vestibular system is stimulated by temperature, as by putting warm water in one ear and cold in the other.

Planes of semicircular canals

Right anterior canal plane aligned w/ right posterior canal plane and the left posterior canal plane aligned w/ the left anterior canal plane Bilateral canals work in a push and pull manner. Afferents on excited side increase rate while afferents from paired canal on opposite side decrease rate. Left anterior canal works w/ right posterior canal (when max stem of LAC, max inhib of RPC) Right anterior canal works w/ left posterior canal (max stem RAC, max inhibition LPC) Rotation down excites both anteriors and inhibits both posteriors

Which does linear motion in vertical plane?

Saccule - linear motion in the vertical plane and detects gravitational pull on the body Polarized away from striola (sacculus separate) Otoliths

Cupula

Sense angular acceleration (not linear)

vestibule-ocular reflex (VOR)

The brainstem mechanism that maintains gaze on a visual object despite movements of the head Primary vestibular afferents into brain synapse on second order vestibular nucleus neurons (primarily horizontal reflex in medial vestibular nucleus). Two population: 1. Excitatory second order vestibular neurons excited by horizontal semicircular canal inputs. 2. Inhibitory 1. Excitatory - Axons cross the midline and innervate the contralateral VI nucleus and synapse on contralateral LR. Contraction lateral rectus muscle and abduct contralateral eye. collaterals synapse on VI internucleuar motor neurons which instead of innervating muscle, cross the midline, and ascend in the MLF. Then innervates MR subdivision of the CNIII nucleus. Innervate the ipsilateral MR neuron 2. Inhibitory - ascend to the VI nuclei where they inhibit the LR on the ipsilateral side and inhibit neurons that ascend on contralateral side and innervate MR on right side. Third pathway— commisural pathway that inhibits the same circuit on the other side, reinforcing actions described

degrees of freedom

The three semicircular ducts within the semicircular canals contain receptors that detect rotational movements of the head, including pitch, roll, and yaw. *The superior/anterior semicircular canal is responsible for pitch*, which is detected when the head moves up and down, as when we nod to say yes. *The lateral/horizontal semicircular canal is responsible for yaw,* which is detected when the head moves from side to side, as when we shake our head to say no. *The posterior semicircular canal is responsible for roll*, which is detected when we lean our head toward our shoulder or when doing a cartwheel

*Vestibular nuclei* Lateral vestibular nucleus

To lateral vestibulospinal tract for VSR Found all levels and provides input from limb motor neurons to otoliths and facilities extensor motor neuron activity and inhibits flexor motor neuron activity (upper and lower). Important for vestibulospinal reflexes

*Vestibular nuclei* Medial and inferior nuclei

To vestibulospinal tract; bilaterally via the medial longitudinal fascicles for VCR Important for vestibulocollic reflexes (tip body forward & keep head still like owl)

Which does linear motion in horizontal plane?

Urticle - linear motion in the horizontal plane and detects forward or backward acceleration Polarized toward striola (urticle unite) Otholiths

Hair Cells general

base of hair cell - perilymph. Motion of stereocilium toward kinocilium open mechanosensing channels in tips of stereocilia to allow current to come in and depolarize the hair cells and produce a potential. Motion in the opposite direction produces a hyper polarization. Move perpendicular, no change in membrane potential Has baseline tonic firing rate and this rate can increase or decrease

Scarpa's Ganglion

contains cell bodies of the vestibular branch of the vestibulocochlear nerve scarpa's ganglion - bipolar cells w/ afferent terminal in contact w/ sensory neuron (hair cells) within sensory epithelium

membranous labyrinth

filled with endolymph High K+; Low Na+, Ca2+ (like cytoplasm) Produced in stria vascularis Drains via endolymphatic sac into dural space Increased endolymphatic pressure (eg impaired drainage) is associated w/ Meniere's disease