BISC 421 1st Edition Lecture 23 Outline of Current LectureI.Vestibular System Current LectureVestibular System•From Sascha du Lac •Disfunctions impair gaze stability and postural stability •It is very very important for proprioceptionTwo: Otolith organs (the utricle and the sacculus)Three: Semicircular canals, the superior (or anterior), posterio r, and the horizontal (or lateral) •The vestibular system in the labyrinth which has 5 organs (on the slide)•Fibers from the vestibular organ exit through the 8th nerveThe Labyrinth: Holes in Our HeadsThe internal auditory meatus (IAM) is open to the posterior cranial fossa and contains the superior division (sd) of the eighth cranial nerve. The utricular macula (mu), one of two gravity receptors, is visible in the center,along with cuts through the non- sensory portions of the lateral (lsc), posterior (psc) and superior (ssc) semicircular canals.•What the labyrinth looks likeArrangement of Sensory Cells in the Canals and Otolith OrgansPolarization maps of organs, morphological polarization of hair cellsAmpullae (bulge in semicircular canals) Within a single crista, all hair cells have the same polaritySacculus and UtriculeHair cell polarity varies with position in the macula to sample all directions of motion.•Where the transduction machinery lies•This is IMPORTANT•In the ampula there are hair cells•All the hair cells point in the same direction•All hair cells have the same polarity within a single crista•Striola is dividing line down the middle•In the sacculus in the hair cells point away from each other•Otolith organs are in the sacculus and utricleOtolith Organs take their name from otoconia (ear stones)KinociliumOtoconiaThe size of the otoconia vary regionally from 3 to 30 cm in both the utricle and sacculus.Hair cell from the bullfrog sacculus•Deposits of calcium compounds•Collectively called the otoconiaUtricle SacculusIn mammals, the utricle (A) is heart-shaped while the sacculus (B) is hook-shaped. The plane of the utricular macula is mostly horizontal.The plane of the saccular macula is roughly vertical.The macula is divided into two areas ( pars interna and pars externa) by the striola, which curves through the center, parallel with the edge of the tissue (dotted line in figure). At the striola, sensory cells are more densely packed and shorter than elsewhereOtolith Organs detect linear acceleration & head position with respect to gravityThe utricle is sensitive to the horizontal and the sacculus to the vertical.•The otolith•Detect linear acceleration and head position with respect to gravity•Polarity of hair cells change•Hair cells in the otolith organ are embedded in a jelly like goo•This will lag movement and bend hair cellsDisplacement of the Otolithic Membrane of the Utricular Macula•When he moves backwards the hair cells depolarize and when he moves forward theyhyperpolarize•Sent monkeys up before people•A lot of what we know about the vestibular system comes from these space expeditionsExperimental rig used to study the vestibular system, the chair tilts and spinsCell in the Otolith Organs Signaling Constant TiltTilt in the opposite direction•Record from an awake monkey•Leap in the firing rate maintained during tilt•Called a tonic response-‐ maintained over time-Semicircular canals signal information about rotational motion (angular velocity).The Ampulla of the Posterior Semicircular Canal; note that all the hair cells have the same polarity and are embedded in a jellylike cupula.•This is an illusion – involves the semicircular canalsSemicircular canals work in pairsSemicircular canals work in pairs, as above, with the polarity of the hairs cells of the partner canals oppositely aligned.The hairs cells in the canal towards which the head is turning depolarize whilethose in the partner canal hyperpolarize (push-pull)•All the hair cells have the same polarity in the semicircular canalsFluid Displaces the Cupula During Rotational MovementMy head moves towards the left, which is the direction of the of the uppermost arrow.My hair cells in the horizontal canal on my left depolarize as endolymph pushes the cupula in the direction of the kinocilia (equivalent to the tallest stereocilia in the cochlear hair cells).•Summarizes the animation on blackboard•Fluid moves in opposite direction•Motion of the endolymph displaces the hair bundlesCells in the Semicircular Canals Signal Acceleration (not Constant Velocity)Response of an axon relaying information from a hair cell in an ampulla of a semicircular canal to angular acceleration, note that these responses are transient– they adapt over time.•Hair cells only tell you whats happening during accelerationPathways for the VOR*Connections underlying the vestibulo-ocular reflex. Example, head moves left and eyes move right. Loss of the VOR leads to the inability to see clearly and fixate targets whilemoving, oscillopsia.•How this is wired up•Horizontal direction-‐ oculomotor nucleus, cross over at midline through the medial longitudinal fasiculus•Inputs from medial vestibular nucleus send signals – synapses on the ipsilateral pathway (which sends signals through the medial longitudinal nucleusVestibular ReflexesOutput from the medial vestibular nucleus is bilateral and adjusts the muscles controlling head position (vestibulo-cervical reflex), while output from the lateral vestibular nucleus adjusts the extensor (antigravity) muscles in the ipsilateral trunk and limbs (vestibulo-spinal reflex).•Otolith organs feed into the lateral vestibular nucleusVestibular Input to the Cortex via the ThalamusCells in the highlighted cortical areas also receive proprioceptive and visual input and are activated by moving visual stimuli as well as rotation of the body even in darkness. They seem to be involved in negotiating the