7- Neurophysiology (Vestibular System)- Pt 1.docx

- **Vestibular System** - The vestibular system can be divided into peripheral and central. - The **Peripheral** vestibular system is located outside of the brain and is composed of the vestibular apparatus in the membranous labyrinth...

- **Vestibular System** - The vestibular system can be divided into peripheral and central. - The **Peripheral** vestibular system is located outside of the brain and is composed of the vestibular apparatus in the membranous labyrinth - The peripheral vestibular system also consists of the vestibular portion of the vestibulocochlear nerve (Cranial nerve 8) - The **Central** vestibular system is located in the cerebellum and brainstem and contains 4 pairs of vestibular nuclei on each side in the brainstem. - [Functions] of the vestibular system include: maintaining balance, and coordination of eye-head-body movements. - The membranous labyrinth is a series of fluid-filled structures: - 3 **semicircular canals**: kinetic labyrinth - 2 **otolith organs**: utricle and saccule - Static labyrinth - The **utricle and saccule** [function] to detect linear acceleration, deceleration (inertial force), and static tilt of the head (gravitational force). - There are hair cells located in the **macula** of the otolith organs. - The **vestibular apparatus** is a bilateral receptor system located in the membranous labyrinth: semicircular canals and otolith organs. - The semicircular canals is composed of the: anterior (superior) canal, lateral (horizontal) canal, and the posterior canal. - Each canal is named based on its position in space. - The semicircular canal is filled with endolymph and is surrounded by perilymph. - The reason people feel dizzy after spinning around and suddenly stopping is because the endolymph in the ears are still moving due to inertia, causing the body to feel like it is still moving despite being still. - **Vestibular Hair Cells** - Vestibular hair cells have a large kinocilium, and they are **tonically active cells** at the resting stage. - These cells have a constant firing rate when they are not stimulated. - If the stereocilia are bent towards one side, the action potential frequency increases. - The opposite makes the frequency decrease. - The change in frequency of action potentials for the vestibular hair cells is how the brain knows which side the body is moving. - The **vestibular hair cell canals** are located in the **crista ampullaris** of the semicircular canals and are covered by **cupula**, which is a gelatinous mass. - During angular acceleration of the head, the cupula is displaced (due to inertial force), causing excitation or inhibition of the hair cells. - As the head moves, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend. - All 6 canals activation can give a very precise indication of head movement in 3 dimensions. - **Vestibular Hair Cells: Otoliths** - Otoliths are located in the macula of the otolith organ, where an otolith mass overlies the vestibular hair cells. - The maculae have hair cells oriented in different directions to get information about linear (and static) movement in different angles. - The macula of utricle is horizontally oriented. - This is why if someone was blindfolded in a car and the car stopped, they can feel it. - The macula of saccule is vertically oriented. - This is why if someone was inside an elevator, they can feel themselves going up or down. - The **maculae** are specialized for sensing linear acceleration, such as when gravity acts on the tilting head or if the head starts moving in a straight line. - Gravitational or inertial forces over the otolith causes the stereocilia to bend in the direction of that linear acceleration. - When the head is tilted, gravitational forces act on the otolith mass, moving it across the vestibular hair cells - During linear acceleration, inertial forces act on the otolith mass, also moving it across the vestibular hair cells. - The otolith vestibular hair cells are either activated or inhibited to alert about a change in the position of the head. - **Vestibular Pathway** - **Sensory vestibular fibers** arrive to the 4 vestibular nuclei and the cerebellum. - The cerebellum - fastigial nucleus and flocculonodular lobe via caudal cerebellar peduncle - **vestibular nuclei** projects to the: - Spinal cord - Lateral and medial vestibulospinal tracts - Brain stem - Motor nuclei of cranial nerves 3, 4, and 6 - Reticular formation leading to the vomiting center - Cerebellum - Fastigial nucleus and flocculonodular lobe via caudal cerebellar peduncle - Thalamus - Relay info to cortex for conscious vestibular information