1. THE VESTIBULAR SYSTEM The vestibular system is highly specialized for maintenance of body equilibrium. It is composed of the vestibular apparatus of the inner ear, The vestibular pathway and The brainstem and cortical centers that are involved in vestibular functions. The vestibular apparatus comprises: The static labyrinth (Maculae of the Utricle and the saccule) The kinetic (Cristaampullaris) membranous labyrinth. The macula is the vestibular peripheral receptor organ located in the Utricle and saccule of the membranous labyrinth. The cristaampullaris is the vestibular peripheral receptor organ located in the semicircular ducts of the membranous labyrinth.

2. The external features of the two organs are illustrated in the diagram below. Note the following features of in the two organs: • Type 1sensory hair cells which are flask-shaped • Type 2 sensory hair cells which are cylindrical in shape. • Supportive cells and • Dendritic terminals of the vestibular ganglion cells, which synapse with the hair cells.

5. THE VESTIBULAR PATHWAY • The vestibular system communicates with the central nervous system through four pathways. These are: • The pathway to the spinal cord. • The pathway to the brainstem. • The pathway to the cerebellum. • The pathway to the cerebrum.

6. The first order neurons of All the four pathways are the vestibular ganglion cells located in the internal acoustic meatus. • The axons of these cells project to the vestibular nuclear complex of the rostral medulla and the caudal pons, which consists of the • lateral, • medial, • inferior and • superior vestibular nuclei. • From this point the projections of the four pathways diverge.

7. The spinal pathway: • This consists of the vestibulospinal tract and • The descending component of the medial longitudinal fasciculus, from the lateral and medial vestibular nuclei (2nd order) respectively. This pathway is utilized in the maintenance of muscle tone. Cerebellarpathway: • This consists of projections from the vestibular nuclear complex viz. Medial, Inferior and Superior nuclei (2nd order) to the vestibulocerebellum. This pathway coordinates the activities of thespinal pathway

8. The Brainstem pathway: This consists of projections from the vestibular nuclear complex via: • The ascending componentof the medial longitudinal fasciculus. • It connects the vestibular nuclei (2nd order) to CN III, IV and VI. This pathway is utilized in the coordination of conjugate eye movements with head movement.

9. The Cortical pathway: • This consists of contralateral and few Ipsilateral projections from the vestibular nuclear complex (2nd order) to the thalamus. • Thalamic neurons (3rd order) associated with the vestibular pathway project to the primary vestibular areas of the parietal and temporal lobes. • This pathway providesfor conscious awareness of the position and movements of the head