Vestibular system

Vestibular system is clearly explained

1. Physiology of vestibular system - DEV

2. Inner ear structures Cochlea Vestibular apparatus : 1.semi-circular canal 2.vestibule – a)Utricle b)Saccule

3. Semi-circular canal Bony labyrinth – semi circular canal , contains Perilymph .( na+ and k+) inside Membranous labyrinth – semi circular duct , contains endolymph . inside ( na+ & k+) sensory epithelium – Crista Ampullaris There are three semi circular canals Anterior or superior semi circular canal Lateral or horizontal semi circular canal Posterior semi circular canal The semi circular canals join the vestibule, at the junction there is a dilated part called ampulla where the Crista Ampullaris is located.

4. Position of the semi circular canals 1. Anterior canal – 41 degree from the sagittal plane. 2. Posterior canal – 56 degree from the sagittal plane. 3. Lateral canal – 30 degree from the naso-occipital plane. This position allows the semi circular canals to sense the angular acceleration from all direction , Also in this position they are perpendicular to each other . Their main function is to maintain the Dynamic equilibrium .

5. Crista Ampullaris It is the sensory epithelium of the semi circular canal. It has a gelatinous structure called the cupula , which is attached to the hair cells. The hair cells give out the fibers for the vestibular nerve .

6. How does it work ? Consider a case of rotating head to left Due to the endolymph inertia the cupula will shift to right. This causes the hair cells of the left semi circular canal to excite and release action potentials but in the right semi circular canal it will cause inhibition. Afferent fibres vestibular ganglion/Scarpa’s ganglion vestibular nuclei right abducens nuclei left oculomotor nuclei right lateral rectus Left medial rectus This helps in fixing our gaze

7. Saccadic movement Whenever you rotate to the left the eye moves to the right very fast Afterwards it has to compensate and go back to left to maintain the gaze to the left. This is done by the frontal eye field . Frontal eye field Paramedian pontine reticular formation Left abducens right abducens Left Lateral rectus right oculomotor right medial rectus This causes the eye to move to left, this movement is called saccadic movement. The beating to the right and to the left is called Vestibular Nystagmus. (physiological)

8. Vestibule Outer bony part – vestibule - perilymph inside Inner membranous – utricle and saccule - endolymph inside Sensory epithelium – macula or otolith organ

9. Utricle In here the macula is present horizontally on the floor. The stereocilia is facing upwards. It responds to linear acceleration in horizontal plane and head tilt. Saccule Macula is on vertical plane . The stereocilia facing outwards. It responds to vertical acceleration. Both utricle and saccule maintain the static equilibrium.

10. Macula It is the sensory epithelium of the vestibule. It responds to linear acceleration in vertical and horizontal plane. The otolith membrane attached to the hair cells move in the opposite direction of the acceleration due to the inertia of the otoconia present on the top of it. This causes depolarisation of the hair cells.

11. There are two types of vestibular hair cells Type I and Type II. There are stereocilia on them, the longest one is called kinocilia . The K+ channel on the stereocilia are linked to each other by Tip link. When the tip link is stretched the K+ channel opens and K+ influx occurs which also opens the voltage gated Ca channel and causes depolarization. Excitatory neurotransmitters like glutamate and aspartate are released. Vestibular hair cells

12. Pathway Vestibular branch of CN VIII Vestibular nucleus Vestibulospinal fastigial nucleus abducens & thalamus tract oculomotor Spinal cord cerebellum eye cerebral cortex Maintains posture posture, coordinate eye higher and balance muscle tone, movements perception balance

13. THANKS FOR LISTENING ref: Diseases of ENT by PL Dhingra, H uman physiology by Guyton, Ninja nerd YouTube channel(pic), c super boys