THE AUDITORY AND VESTIBULAR

SYSTEMS

Ahmed A. E. Eljack 1

Objectives:

By the end of this session you should be able to:

■ Describe the structure of the ear.

■ Discuss the auditory transduction and auditory pathways.

■ Describe the anatomy and function of the auditory cortex.

■ Describe the vestibular organ.

■ Discuss the vestibular pathways.

■ Mention important clinical consideration about the auditory

and vestibular systems.

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Structure of the Ear:

■ The three parts: inner, middle, and

external.

■ The cochlea contains three ducts:

– Scala vestibuli.

– Scala tympani.

– Scala media.

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Auditory Transduction:

■ From sound waves into electrical impulses.

■ Movement of the tympanic membrane

causes movement of the ossicles pushing

the stapes into the oval window and

vibration of the organ of Corti.

■ Bending of the cilia and change in

potassium ion conductance.

■ The role of calcium and glutamate.

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Auditory Pathways:

■ From the spiral ganglion to the dorsal and ventral nuclei

(upper medulla).

■ From the dorsal and ventral nuclei to the superior olivary

nucleus.

■ Form the superior olivary nucleus through the lateral

lemniscus to the inferior colliculus then to the medial

geniculate nucleus.

■ From the medial geniculate nucleus to the auditory cortex

through the auditory radiation.

■ Important fibers to the reticular activating system and the

cerebellum.

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Auditory Cortex:

■ Superior temporal gyrus.

■ Divided into primary auditory cortex

and auditory association cortex.

■ Auditory association cortex receives

impulses from the primary association

area and some projections from the

thalamic association area.

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The Vestibular Organ

■ The vestibular system responsible for

balance and equilibrium (detecting both

linear and angular movements of the head).

■ The vestibular organ lies in the bony

labyrinth within the membranous labyrinth

(temporal bone).

■ Three perpendicular semicircular canals

and utricle and saccule.

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Vestibular Pathways:

■ From the vestibular organ to the vestibular nuclei through the vestibular nerve.

■ Some fibers go directly to the reticular nuclei and the cerebellum.

■ Second order neurons from the vestibular nuclei send fibers to:

– The cerebellum.

– The vestibulospinal tract.

– The medial longitudinal fasciculus.

– Areas in the brainstem (especially reticular nuclei).

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Clinical Considerations:

■ Deafness:

– Conductive deafness.

– Sensorineural deafness.

– The two types are differentiated by:

■ Weber’s test.

■ Rinne’s test.

■ Vertigo.

■ Nystagmus.

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Further Reading:

■ Barret, K.E, Barman, S.M, Boitano, S, Brooks, H.L. Ganong's

Review of Medical Physiology. (24th ed.). : McGraw-Hill; 2012.

■ Costanzo, L.S. Physiology. (5th

ed.). Philadelphia: Saunders; 2014

■ Hall, J.E. Guyton and Hall Textbook of Medical

Physiology. (12th ed.). Philadelphia: Saunders; 2011.

■ Eljack, A. A. E. Eljack’s Lecture Notes in Neuroscience.

Khartoum; 2015

■ Siegel, A, Sapru, H.N. Essential Neuroscience. (3rd

ed.). Philadelphia: Lippincott Williams & Wilkins; 2015.

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