GOAL: TO RELATE THE FUNCTION OF THE VIIIth CRANIAL NERVE TO COMMUNICATION.

OBJECTIVES:

1. Students will relate the functions of the two divisions of VII to human communication.

2. Students will distinguish two types of auditory functions served by VIII.

3. Students will identify afferent and efferent functions of VIII.





  1. The VIII is generally said to be entirely sensory.


    1. Recent findings suggest that there is an efferent function (see Katz, 1994)


    2. Most agree that the primary function of the nerve is to convey impulses to the CNS.


    3. The VIII has two branches: Cochlear and Vestibular


      1. Fibers from both divisions enter the rhombencephalon at the cerebellopontine angle


      2. The auditory component conveys impulses form the cochlea to be interpreted as sound.


      3. The vestibular component transmits sensations associated with balance from the semicircular system.


        1. Balance impulses are associated with changes in the position of the head relative to the earth (and the pull of gravity).


        2. Interestingly, the extent to which one should separate the auditory and balance divisions is not clear.


          1. Particularly in the saccule portion of the vestibular apparatus.


          2. Some (see Hamilton) have suggested evidence of the interrelationship of the two functions is manifested by the way some move their heads in response to rhythmical sounds.


  2. The Cochlear nerve serves two gross auditory functions: transmission of neural impulses generated by sound energy and transmission of balance impulses generated by changes of the position of the head in space.


    1. Sound generated (auditory) impulses are of two general types: "Discriminatory" and "Reflex."


      1. Discriminatory impulses occur in the form of sound generated impulses which enable the listener to discriminate among frequencies between 30 and 30KHZ.


        1. Discrimination is a ultimately a function of the cerebral cortex, and has to be learned. Perception of speech depends upon the ability to discriminate complex sounds.


        2. Analysis and perception of discriminatory impulses depends upon psychological factors, and relate to quality and duration of segments.


        3. Sensory fibers originating in the Organ of Corti connect with single higher level neurons for greater discrimination. Most of the higher level terminations are situated in the ventral cochlear nucleus.


      2. Reflex actions occur in response to gross sound.


        1. These impulses are not discriminated, but the brain simply becomes aware of their presence. Reflex actions are part of the basic survival mechanism.


        2. Reflex sensory fibers connect to multiple dendrites from higher level neurons. Most of the terminations for reflex impulses are situated in the dorsal cochlear nucleus


    2. The Vestibular portion of VIII registers movements of the head to allow appropriate postural actions.


      1. They have sensory receptors in the semicircular canals and in the utricle and saccule.


        1. These impulses evoke responses from the extraocular m.m. to keep the eyes focused on objects as the head changes position.


        2. They also evoke movements of the postural m.m. for maintenance of upright (etc.) posture in response to changes in gravity pull.


      2. Most vestibular fibers bifurcate into ascending and descending divisions as they course to their terminal nuclei.


        1. These nuclei are located in the lateral recess of the floor of the fourth ventricle.


        2. Ascending fibers go to three nuclei:


          1. The Medial Vestibular Nucleus


          2. Lateral Vestibular Nucleus (of Deiters)


            1. Fibers of the vestibulospinal tract originate here.


            2. Axons of the Lateral Vestibular Nucleus convey vestibular impulses to the lower motor neurons of the spinal cord.


          3. Superior Vestibular Nucleus (of Bechterew)




        3. Descending fibers form the long descending vestibular nucleus down the medulla as far as the sensory decussation.


        4. Some vestibular n. fibers inter the cerebellum and terminate there without interruption.


      3. After they leave the nuclei, vestibular system fibers go in three directions.


        1. Some form an association tract:


          1. These convey vestibular impulses to the various cranial nerve nuclei.


          2. such impulses may be associated with head/neck responses to labyrinthine stimulation.


        1. Some enter the cerebellum through the inferior cerebellar peduncle.


        2. Those from the lateral vestibular nucleus form the vestibulospinal tracts (see above).


      1. In general, vestibular functions are not conscious.