1. Vestibulocochlear Nerve (CN VIII)
MADE BY : DANI MAMO

2. Cochlear Component and the Organ of Hearing
The organs of balance and hearing are derived from a single embryological precursor in the petrous portion of the temporal bone: the utriculus gives rise to the vestibular system with its three semicircular canals, while the sacculus gives rise to the inner ear with its snaillike cochlea

3. Auditory perception
Sound waves are vibrations in the air produced by a wide variety of mechanisms (tones, speech, song, instrumental music, natural sounds, environmental noise, etc.). These vibrations are transmitted along the external auditory canal to the eardrum (tympanum or tympanic membrane), which separates the external from middle ear

4. middle ear
The middle ear contains air and is connected to the nasopharyngeal space (and thus to the outsideworld) through the auditory tube, also called the Eustachian tube. The middle ear consists of a bony cavity (the vestibulum) whose walls are covered with a mucous membrane. Its medial wall contains two orifices closed up with collagenous tissue, which are called the oval window or foramen ovale (alternatively, fenestra vestibuli) and the round window or foramen rotundum (fenestra cochleae).

5. These two windows separate the tympanic cavity from the inner ear, which is filled with perilymph. Incoming sound waves set the tympanic membrane in vibration. The three ossicles (malleus, incus, and stapes) then transmit the oscillations of the tympanic membrane to the oval window, setting it in vibration as well and producing oscillation of the perilymph. The tympanic cavity also contains two small muscles, the tensor tympani muscle (CN V) and the stapedius muscle (CN VII). By contracting and relaxing, these muscles alter the motility of the auditory ossicles in response to the intensity of incoming sound, so that the organ of Corti is protected against damage from very loud stimuli.

7. Inner ear
The auditory portion of the inner ear has a bony component and a membranous component .The bony cochlea forms a spiral with two-and-a-half revolutions, resembling a common garden snail. The cochlea contains an antechamber (vestibule) and a bony tube, lined with epithelium that winds around the modiolus, a tapering bony structure containing the spiral ganglion. A cross section of the cochlear duct reveals three membranous compartments: the scala vestibuli, the scala tympani, and the scala media (or cochlear duct), which contains the organ of Corti.

8. The scala vestibuli and scala tympani are filled with perilymph, while the cochlear duct is filled with endolymph, a fluid produced by the stria vascularis. The cochlear duct terminates blindly at each end (in the cecum vestibulare at its base and in the cecum cupulare at its apex). The upper wall of the cochlear duct is formed by the very thin Reissner’s membrane, which divides the endolymph from the perilymph of the scala vestibuli, freely transmitting the pressure waves of the scala vestibuli to the cochlear duct so that the basilar membrane is set in vibration. The pressure waves of the perilymph begin at the oval window and travel through the scala vestibuli along the entire length of the cochlea up to its apex, where they enter the scala tympani through a small opening called the helicotrema; the waves then travel the length of the cochlea in the scala tympani, finally arriving at the round window, where a thin membrane seals off the inner ear from the middle ear.

9. The organ of Corti
The organ of Corti (spiral organ) rests on the basilar membrane along its entire length, from the vestibulum to the apex. It is composed of hair cells and supporting cells. The hair cells are the receptors of the organ of hearing, in which the mechanical energy of sound waves is transduced into electrochemical potentials. There are about 3500 inner hair cells, arranged in a single row, and outer hair cells, arranged in three or more rows. Each hair cell has about 100 stereocilia, some of which extend into the tectorial membrane. When the basilar membrane oscillates, the stereocilia

10. are bent where they come into contact with the nonoscillating tectorial membrane; this is presumed to be the mechanical stimulus that excites the auditory receptor cells. In addition to the sensory cells (hair cells), the organ of Corti also contains several kinds of supporting cells, such as the Deiters cells, as well as empty spaces (tunnels). Movement of the footplate of the stapes into the foramen ovale creates a traveling wave along the strands of the basilar membrane, which are oriented transversely to the direction of movement of the wave. An applied pure tone of a given frequency is associated with a particular site on the basilar membrane at which it produces the maximal membrane deviation (i.e., an amplitude maximum). The basilar membrane thus possesses a tonotopic organization, in which higher frequencies are registered in the more basal portions of the membrane, and lower frequencies in more apical portions. This may be compared to a piano keyboard, on which the frequency becomes higher from left to right. The basilar membrane is wider at the basilar end than at the apical end .

14. The spiral ganglion contains about bipolar and 5000 unipolar neurons, which have central and peripheral processes. The peripheral processes receive input from the inner hair cells, and the central processes come together to form the cochlear nerve.

16. Cochlear nerve and auditory pathway
The cochlear nerve, formed by the central processes of the spiral ganglion cells, passes along the internal auditory canal together with the vestibular nerve, traverses the subarachnoid space in the cerebellopontine angle, and then enters the brainstem just behind the inferior cerebellar peduncle. In the ventral cochlear nucleus, the fibers of the cochlear nerve split into two branches (like a “T”); each branch then proceeds

17. to the site of the next relay (second neuron of the auditory pathway) in the ventral or dorsal cochlear nucleus. The second neuron projects impulses centrally along a number of different pathways, some of which contain further synaptic relays

18. Neurites (axons) derived from the ventral cochlear nucleus cross the midline within the trapezoid body. Some of these neurites form a synapse with a further neuron in the trapezoid body itself, while the rest proceed to other relay stations—the superior olivary nucleus, the nucleus of the lateral lemniscus, or the reticular formation. Ascending auditory impulses then travel byway of the lateral lemniscus to the inferior colliculi (though some fibers probably bypass the colliculi and go directly to the medial geniculate bodies).

19. Neurites arising in the dorsal cochlear nucleus cross the midline behind the inferior cerebellar peduncle, some of them in the striae medullares and others through the reticular formation, and then ascend in the lateral lemniscus to the inferior colliculi, together with the neurites from the ventral cochlear nucleus.

20. The inferior colliculi contain a further synaptic relay onto the next neurons in the pathway, which, in turn, project to the medial geniculate bodies of the thalamus. From here, auditory impulses travel in the auditory radiation, which

21. is located in the posterior limb of the internal capsule , to the primary auditory cortex in the transverse temporal gyri (area 41 of Brodmann), which are also called the transverse gyri of Heschl. A tonotopic representation of auditory frequencies is preserved throughout the auditory pathway from the organ of Corti all the way to the auditory cortex , in an analogous fashion to the somatotopic (retinotopic) organization of the visual pathway.

25. Bilateral projection of auditory impulses
Not all auditory fibers cross the midline within the brainstem: part of the pathway remains ipsilateral, with the result that injury to a single lateral lemniscus does not cause total unilateral deafness, but rather only partial deafness on the opposite side, as well as an impaired perception of the direction of sound.

26. Auditory association areas
Adjacent to the primary auditory areas of the cerebral cortex, there are secondary auditory areas on the external surface of the temporal lobe (areas 42 and 22), in which the auditory stimuli are analyzed, identified, and compared with auditory memories laid down earlier, and also classified as to whether they represent noise, tones, melodies, or words and sentences, i.e., speech. If these cortical areas are damaged, the patient may lose the ability to identify sounds or to understand speech (sensory aphasia).

27. Integration of auditory processing in various reflex arcs
The pathway from the organ of Corti to the primary auditory cortex is 46 neurons long; at each of the relay stations in this pathway (superior olivary nucleus, reticular formation, nucleus of the lateral lemniscus, and inferior colliculi), collateral fibers arise that participate in a number of reflex arcs.

28. * Some impulses travel to the cerebellum, while others pass in the medial longitudinal fasciculus to the nuclei innervating the extraocular muscles and bring about conjugate eye movements in the direction of a sound. * Some impulses pass through the inferior and superior colliculi to the pretectal area and then, by way of the tectobulbar tract, to various brainstem nuclei, including the nucleus of the facial nerve (stapedius muscle), or by way of the tectospinal tract to motor anterior horn cells in the cervical spinal cord. The impulses that descend to the cervical spinal cord bring about a repositioning of the head toward or away from the origin of a sound.

29. *Other impulses travel in the ascending reticular activating system to the reticular formation (arousal reaction). *Yet others descend in the lateral lemniscus and, via interneurons, exert a regulating influence on the tension of the basilar lamina. Some of these descending impulses are thought to have an inhibitory effect; their function is presumably to improve the perception of certain frequencies by suppressing other, neighboring frequencies.

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31. Vestibulocochlear Nerve (CN VIII) Vestibular Component and Vestibular System

32. Three different systems participate in the regulation of balance (equilibrium):
the vestibular system,
the proprioceptive system (i.e., perception of the position of muscles and joints),
and the visual system.
The vestibular system is composed of the
labyrinth,
the vestibular portion of the eighth cranial nerve (i.e., the vestibular nerve, a portion of the vestibulocochlear nerve),
and the vestibular nuclei of the brainstem, with their central connections.

33. labyrinth
The labyrinth lies within the petrous portion of the temporal bone and consists of the utricle, the saccule, and the three semicircular canals The membranous labyrinth is separated from the bony labyrinth by a small space filled with perilymph; the membranous organ itself is filled with endolymph. The utricle, the saccule, and the widened portions (ampullae) of the semicircular canals contain receptor organs whose function is to maintain balance.

35. three semicircular canals
The three semicircular canals lie in different planes. The lateral semicircular canal lies in the horizontal plane, and the two other semicircular canals are perpendicular to it and to each other. The posterior semicircular canal is aligned with the axis of the petrous bone, while the anterior semicircular canal is oriented transversely to it. Since the axis of the petrous bone lies at a 45° angle to the midline, it follows that the anterior semicircular canal of one ear is parallel to the posterior semicircular canal of the opposite ear, and vice versa. The two lateral semicircular canals lie in the same plane (the horizontal plane).

36. Each of the three semicircular canals communicates with the utricle
Each of the three semicircular canals communicates with the utricle. Each semicircular canal is widened at one end to form an ampulla, in which the receptor organ of the vestibular system, the crista ampullaris, is located. The sensory hairs of the crista are embedded in one end of an elongated gelatinous mass called the cupula, which contains no otoliths . Movement of endolymph in the semicircular canals stimulates the sensory hairs of the cristae, which are thus kinetic receptors (movement receptors).

38. The utricle and saccule contain further receptor organs, the utricular and saccular macules. The utricular macule lies in the floor of the utricle parallel to the base of the skull, and the saccular macule lies vertically in the medial wall of the saccule. The hair cells of the macule are embedded in a gelatinous membrane containing calcium carbonate crystals, called statoliths. They are flanked by supporting cells.

40. These receptors transmit static impulses, indicating the position of the head in space, to the brainstem. They also exert an influence on muscle tone. Impulses arising in the receptors of the labyrinth form the afferent limb of reflex arcs that serve to coordinate the extraocular, nuchal, and body muscles so that balance is maintained with every position and every type of movement of the head.

41. Vestibulocochlear nerve
The next station for impulse transmission in the vestibular system is the vestibulocochlear nerve. The vestibular ganglion is located in the internal auditory canal; it contains bipolar cells whose peripheral processes receive input from the receptor cells in the vestibular organ, and whose central processes form the vestibular nerve. This nerve joins the cochlear nerve, with which it traverses the internal auditory canal, crosses the subarachnoid space at the cerebellopontine angle, and enters the brainstem at the pontomedullary junction. Its fibers then proceed to the vestibular nuclei, which lie in the floor of the fourth ventricle.

42. The vestibular nuclear complex is made up of: - The superior vestibular nucleus (of Bekhterev) - The lateral vestibular nucleus (of Deiters) - The medial vestibular nucleus (of Schwalbe) - The inferior vestibular nucleus (of Roller) The fibers of the vestibular nerve split into branches before entering the individual cell groups of the vestibular nucleus complex, in which they form a synaptic relay with a second neuron

44. Afferent and efferent connections of the vestibular nuclei
Some fibers derived from the vestibular nerve convey impulses directly to the flocculonodular lobe of the cerebellum (archicerebellum) by way of the juxtarestiform tract, which is adjacent to the inferior cerebellar peduncle. The flocculonodular lobe projects, in turn, to the fastigial nucleus and, by way of the uncinate fasciculus (of Russell), back to the vestibular nuclei; some fibers return via the vestibular nerve to the hair cells of the labyrinth, where they exert a mainly inhibitory regulating effect. Moreover, the archicerebellum contains second-order fibers from the superior, medial, and inferior vestibular nuclei and sends efferent fibers directly back to the vestibular nuclear complex, as well as to spinal motor neurons, via cerebelloreticular and reticulospinal pathways.

45. The important lateral vestibulospinal tract originates in the lateral vestibular nucleus (of Deiters) and descends ipsilaterally in the anterior fasciculus to the γ and α motor neurons of the spinal cord, down to sacral levels. The impulses conveyed in the lateral vestibulospinal tract serve to facilitate the extensor reflexes and to maintain a level of muscle tone throughout the body that is necessary for balance.

46. Fibers of the medial vestibular nucleus enter the medial longitudinal fasciculus bilaterally and descend in it to the anterior horn cells of the cervical spinal cord, or as the medial vestibulospinal tract to the upper thoracic spinal cord. These fibers descend in the anterior portion of the cervical spinal cord, adjacent to the anterior median fissure, as the sulcomarginal fasciculus, and distribute themselves to the anterior horn cells at cervical and upper thoracic levels. They affect nuchal muscle tone in response to the position of the head and probably also participate in reflexes that maintain equilibrium with balancing movements of the arms.

47. All of the vestibular nuclei project to the nuclei innervating the extraocular muscles by way of themedial longitudinal fasciculus. Anatomists have been able to follow some vestibular fibers to the nuclear groups of Cajal (interstitial nucleus) and Darkschewitsch and further on into the thalamus. The complex of structures consisting of the vestibular nuclei and the flocculonodular lobe of the cerebellum plays an important role in the maintenance of equilibrium and muscle tone.

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