CN8 Vestibulocochlear Nerve PDF

Vestibulocochlear Nerve VIII & System External, middle, and inner ear Vestibulocochlear nerve (CNVIII) • special somatic afferent (SSA) – cochlear and vestibular branches • Nuclei at lateral pontomedullary junction – Cochlear and vestibular nuclei – Vestibular nuclei have connections to the spina...

Vestibulocochlear Nerve VIII & System External, middle, and inner ear Vestibulocochlear nerve (CNVIII) • special somatic afferent (SSA) – cochlear and vestibular branches • Nuclei at lateral pontomedullary junction – Cochlear and vestibular nuclei – Vestibular nuclei have connections to the spinal cord, nuclei of CN III, IV & IV, reticular formation and cerebellum. – Cochlear nuclei house 2nd order neurons of the acoustic pathway. Vestibular ganglia/Scarpas ganglia (vestibular N.) and Spiral ganglia/Cochlear ganglia (cochlear N) are sensory ganglia, cell bodies of bipolar neurons Inner Ear • Bony labyrinth – Cochlea - Vestibule - Semicircular canals • Membranous labyrinth – Cochlear duct - Utricle/Saccule - Semicircular ducts • Region of specialized epithelium (includes ‘hair’[stereocilia] cells – Organ of Corti - Macula - Crista ampullaris • Membranes – Tectorial - Otolithic - Cupula Organ of Corti • Sound waves cause the basilar membrane to vibrate which in turn causes the “hairs” embedded in the tectorial membrane to bend. • Hair cells release neurotransmitters and initiate action potentials in the sensory nerve endings of the bipolar neurons of the cochlear division of CN VIII. • Nerve fibers transmit the action potentials to the brain stem. (spiral) Lateral lemniscus (cochlear nucleus) and then suddenly turns into slime – SLIMA Auditory Pathway • In general the pathway is: – Cochlear nuclei (rostral medulla), 2nd order neurons • Trapezoid body/acoustic stria (axons decussating) to synapse in – Superior olivary nucleus – Lateral lemniscus (major tract) – Inferior colliculus • Brachium of inf colliculus (axons leaving) to synapse in – Medial geniculate body/nucleus (thalamocortical fibers) – Primary auditory cortex (transverse gyri of Heschl) • This pathway ascends bilaterally!! Vestibular division Senses head position/orientation; is responsible for generating reflexes that stabilize the visual image on the retina and adjust posture during head movement • Static labyrinth detects head position with respect to gravity and responds to linear acceleration and deceleration – Utricle (horizontal movements) and Saccule (vertical movements) • “hairs” of macula embedded in otolithic membrane bend as a result of head movement • action potentials travel along bipolar neurons of vestibular division of CN VIII to brain stem Macula • Kinetic labyrinth – detects angular acceleration and deceleration of head – Semicircular ducts • “hairs” of crista ampullaris embedded in cupula bend as a result of head movement • action potentials travel along bipolar neurons of vestibular division of CN VIII to brain stem Crista ampullaris Comparison of Semicircular Canals and Vestibule Canals • Rotary/angular acceleration and deceleration • Crista ampullaris- sensory organ • Cupula • Haircells Utricle & Saccule • Linear/vertical/horizontal • Maculae- sensory organ • Otoliths (otolithic membrane) • Haircells Vestibular Nuclei • • • At the pontine-medullary junction, posterior-lateral 4 nuclei on each side of the brainstem Special somatic afferent SSA Vestibular Pathways • Vestibular nuclei receive the majority of CN VIII fibers (some course directly to the cerebellum) & project to: – Cerebellum - Flocculonodular lobe – Spinal cord • Vestibulospinal tract • descending medial longitudinal fasciculus (MLF) – Brain stem via MLF (ascending) • conjugate eye movement coordinated with head movement to maintain visual fixation – Cortex – via numerous thalamic nuclei • Has a role in spatial cognition • Numerous cortical areas but the main vestibular cortex (humans) is in the temporo-parietal junction (i.e., the superior temporal gyrus, posterior insula, inferior parietal lobule) aka parieto-insular vestibular cortex (PIVC) Vestibular Nuclei Projections 1. 2. Cerebellum – control posture and coordinate movements, especially eye Cortex- Spatial awareness 3. Spinal cord – 2 pathways to cord from vestibular nuclei VESTIBULOSPINAL Neck Muscles & All Muscles for posture Descending Medial longitudinal Fasciculus aka: Medial Vestibulospinal Tract this descends to the level of the neck to control head and neck movements Vestibulospinal Tract aka: Lateral Vestibulospinal Tract this descends entire level of cord to control the extensors for posture ipsilateral 4. Motor nuclei of extraocular eye muscles in brainstem MLF Muscles that move the eyeballs Cranial Nerves Oculomotor, Trochlear, Abducens Path is the Ascending Medial Longitudinal Fasciculus Vestibulo-ocular Reflex, VOR – coordinate eye muscles so that image stays focused on retina/fovea even when head is rotating (fixation/foveation) **Medial Longitudinal Fasciculus – ascends and descends, main function to coordinate head and neck with vision Motor Nuclei of Extraocular Muscles Cranial Nerves 3, 4, 6 The Medial Longitudinal Fasciculus (MLF) ascending tract carries fibers from the vestibular nuclei to the lower motor neurons/ Cranial nerve nuclei 3, 4, 6 Vestibulo-ocular Reflex, VOR – coordinate eye muscles so that image stays focused on retina/fovea even when head is moving (fixation/foveation) Head rotating to right, eyes move to left paired eye movement – conjugate gaze Tract – MLF is both ipsilateral and contralateral A lesion in the vestibular system may cause vertigo, ataxia, and/or nystagmus