The Vestibular System Vestibular and Auditory Sensory Organs Bill Yates, Ph.D. Depts. Otolaryngology & Neuroscience University of Pittsburgh Organization of Sensory Epithelium Displacement of Stereocilia Alters Vestibular Afferent Firing High K+ Displacement of Stereocilia Opens Channels Near Tips Transduction by Vestibular Hair Cells 1
Type 1 vs. Type 2 Hair Cells Calyz Afferent Terminations Completely Surround Type 1 Hair Cells Small Bouton Endings Contact a Large Number of Type 2 Hair Cells Regularly-firing vestibular afferents mainly innervate type II hair cells and have a low sensitivity. Irregular vestibular afferents have a higher sensitivity, and mainly innervate type I hair cells. Semicircular Canals: Sensory Epithelium (Crista Ampullaris) Relative Locations of the Canals Ampulla Cupula Utricle Lumen of Semicircular Canal 2
Relative Locations of the Canals Sensory Transduction by Semicircular Canals Factors involved in establishing the discharge properties of semicircular canal afferents during angular head rotations: Effect of Direction of Rotation on Semicircular Canal Afferent Firing The inertial reaction of the endolymph (proportional to head angular velocity) The damping of endolymph movement by viscous forces of the fluid within the small canal lumen The springlike restoring force opposing deflections of the cupula In the crista of the horizontal canals, the kinocilia face towards the utricle (away from the major part of the canal). In the crista of the vertical canals, the kinocilia face away from the utricle (towards the major part of the canal). This explains the relative effects of left and right movements on the vertical and horizontal canals. What Information Isn't Provided by the Semicircular Canals? Information concerning the static position of the head in space (location with respect to gravity). Otolith Organs: Sensory Epithelium (Macula) Otolithic Membrane (Comprised of Otoconia) Information regarding linear accelerations placed on the head, as occur during falling or when accelerating/stopping in a car. 3
Organization of the Utricular and Saccular Maculae Striola Vestibular Afferent Responses to Prolonged Tilts Vestibular Afferent Responses to Sinusoidal Tilts Terminations of Vestibular Afferents in the Vestibular Nuclei Projections of the Vestibular Nuclei and their Function The vestibular nuclei on the two sides have extensive interconnections. The vestibular nuclei have extensive connections with the cerebellum. Cerebellar inputs to the vestibular nuclei come from the medial cerebellum (cortex + fastigial nucleus) and from the vestibulocerebellum (flocculus). The vestibular nuclei provide inputs to cranial nerve nuclei III, IV, and VI, to stabilize images on the retina as the head changes position. 4
Projections of the Vestibular Nuclei and their Function, Cont. The vestibular nuclei have extensive effects on spinal motoneurons The vestibular nuclei also have effects on neurons in cerebral cortex. Although no one area of cortex exclusively processes vestibular signals, several regions contain cells that receive convergent vestibular and other inputs. The cortical areas receiving vestibular inputs include somatosensory cortex (both SI and SII). The inputs to somatosensory cortex involve a relay through the VPL thalamic nucleus. Projections of the Vestibular Nuclei and their Function, Cont. Recent experiments show that the neural network that mediates navigation, including place cells and head direction cells, receives extensive vestibular influences The vestibular nuclei also make direct and indirect connections with brainstem monoaminergic neurons The vestibular system has influences on regions of the brainstem that control respiration and circulation. Vestibular effects on the respiratory and circulatory systems are important in making adjustments in breathing and circulation that are necessary during movement and changes in posture. Integration of Nonlabyrinthine Inputs by Vestibular Nucleus Neurons Horizontal Vestibulo- Postural Eye View of Autonomic Movement Control Vestibular Region Nuclei Many vestibular nucleus neurons (although not all) integrate inputs from neck receptors and the inner ear. Example of Neck + Vestibular Integration by Vestibular Nucleus Neuron Important Point: The Vestibular Nuclei Integrate Signals from Many Sources! Multiple Vestibular Receptors (including both semicircular canals and otolith organs) Somatosensory receptors The Cerebellum Cerebral Cortex Other CNS regions Thus, the responses of vestibular nucleus neurons during body movements can be complex! 5
Example of Caudal Vestibular Nucleus Neuronal Responses Example of Vestibular Nucleus Neuronal Responses in Animal Lacking Vestibular Inputs 6