anatomy vestibular anatomy Ear nose throat

ENT

1. Anatomy of Vestibular Canal D S Varsha PHASE III MBBS NMCRI

2. The Vestibular system: Main function is Balance It lies in the otic capsule in the petrous portion of the temporal bone. It consists of 5 distinct end organs: 3 semicircular canals that are sensitive to angular accelerations (head rotations) 2 otolith organs "utricle & saccule" that are sensitive t o linear (or straight- line) accelerations.

3. Peripheral receptors: Cristae and Macule The sensory organs of the utricle and saccule are the maculae . Each macula consists of hair cells and supporting cells. The ciliary bundles of the hair cells project into the overlying gelatinous matrix known as the otolith membrane . Otoliths are mineral and protein particles embedded in the otolith membrane (Calcium Carbonate crystals).

4. Macula It consists of supporting cells and hair cells It detect and respond to the position of the head with respect to linear acceleration and pull of gravity . Each macula contains thousands of hair cells that synapse with sensory endings of vestibular nerve Each hair cell has 60- 80 small cilia called stereocilia plus one large cilium called kinocilium. The kinocilium is always located in one side, and the stereocilia gradually become shorter. Minute filaments connect the Tip of each sterocilium to the Next longer cilia and finally to kinocilium .

5. Macula When stereocilia bend to the direction of kinocilium , it results in opening of K+ channels at the tip of the stereocilia, allowing K+ ions to enter and depolarize the hair cell. In respone to depolarization voltage- gated Ca+2 channels are activated allowing for Ca+2 influx and the subsequent liberation of transmitters to produce an action potential.

6. Macula Conversely, bending of stereocilia in the opposite direction (backward to the kinocilium) reduces the tension on attachments and this closes the ion channels causing receptor hyperpolarization and inhibition of the cell.

7. Macula When the head is upright, the hairs project upward into the gelatinous material. When the head bends forward, backward, or to one side, the hair cells are stimulated as the gelatinous material of the maculae sag in response to gravity causing the hair to bend. Stimulated hair cells signal nerve fibers resulting in impulses traveling to the CNS on the vestibular branch of the vestibulocochlear nerve and informing the brain of the head’s new position. Brain responds by sending motor impulses to skeletal muscles to contract/relax to maintain balance.

8. Macula In the utricle , macula lies in the horizontal plane of the inferior surface of utricle so it determines the orientation of the head in upright position, senses motion in the horizontal plane(eg, forward-backward movement, left-right movement, combination ) In the saccule , macula lies in the vertical plane and senses motion in the sagittal plane ( eg, up- down movement)

9. The semicircular canals: Lateral or horizontal, Anterior or superior and Posterior. The semicircular canals are connected to the utricle at their bases. Oriented at right angles to one another. At the end of each canal is an enlarged chamber, the ampulla . The ampulla contains a sensory receptor called crista ampullaris .

10. The semicircular canals: The sensory organ of the semicircular duct is crista ampullaris. The crista consists of a gelatinous mass, the cupula . Embedded in the cupula are the cilia of hair cells . The basal membranes of the hair cells synapse on the sensory neurons of the vestibular nerve.

11. Crista ampullaris: Detect and respond to angular acceleration & deceleration of the head. Rapid turns of the head or body stimulate the hair cells of the crista ampullaris. Appropriate rotation of the head in one direction bends cilia in the opposite , depolarizing the cells. Nerve fibers send impulses to the brain – cerebellum Analysis of information allows the brain to predict the consequences of the rapid body movements and signal appropriate skeletal muscle to maintain balance.

12. Crista ampullaris: Each semicircular canal works in concert with a partner located on the other side of the head, which has its hair cells aligned oppositely. There are three such pairs: the two pairs of horizontal canals, and the superior canal on each side working with the posterior canal on the other side. Head rotation deforms the cupula in opposing directions for the two partners, resulting in opposite changes in their firing rates.

13. Crista ampullaris: For example, the orientation of the horizontal canals makes them selectively sensitive to rotation in the horizontal plane. More specifically, the hair cells in the canal towards which the head is turning are depolarized, while those on the other side are hyperpolarized. when the head turns to the left, the cupula is pushed toward the kinocilium in the left horizontal canal, and the firing rate of the relevant axons in the left vestibular nerve increases. In contrast, the cupula in the right horizontal canal is pushed away from the kinocilium, with a concomitant decrease in the firing rate of the related neurons. If the head movement is to the right, the result is just the opposite.