Central Pathways: Monoaural and Binaural pathways: 1. Monoaural Pathway: from one ear about freq and intensity of sound; also presence of sound. a. Synapse in cochlear nucleus ? second order neuron decussates and ascends in lateral lemniscus to the inferior colliculous in midbrain. 2. Binaural pathways: information about location of sound. a. The cochlear nucleus is connected to the superior olivary nucleus bilaterally by 2nd order neuron. i. Superior olivary nucleus 1. Note: the superior olivary nucleus (SOC) consists of the medial superior olive (MSO) and the lateral superior olive (LSO). 2. Additionally, the SOC contains many neurons, so 3rd, 4th, or 5th order neurons exiting. 3. Bilateral projections ascend to the inferior colliculus. b. Bilateral projections ascend to the inferior colliculus as 3rd, 4th, 5th order neurons. c. From the IC, there are projections to the thalamic nuclei, specifically the medial geniculate bodies (MGN). d. MGN sends auditory radiations to the auditory cortex in the transverse temporal gyrus. Localization of sound: Sound takes longer to travel to further ear, and it is also less intense. The MSO (medial superior olive) sends dendrites out laterally, and receives excitatory input from the cochlear nucleus on both sides. Information MUST arrive at the MSO at the same time for the neuron to fire. 3. MSO ipsilateral to sound (figure A): ? auditory information reaches MSO at separate times so DOES NOT FIRE. a. sound to contralateral ear i. takes longer to get there, path to ipsilateral MSO is longer b. sound to ipsilateral ear i. sound gets there faster, and has a shorter path to MSO 4. MSO contralateral to sound (figure B): ? auditory information will get to the MSO at the same time so IT FIRES. a. sound to contralateral ear i. takes more time to get there, path to contralateral MSO is shorter b. sound to ipsilateral ear i. sound takes less time to get there, but path to MSO is longer * MSO ONLY fires when excited by contralateral sound. Inter-aural time delay allows localization. Intensity differentiation of binaural pathway (sign switch pathway): 1. Lateral Superior Olive (LSO): receives input from both cochlear nuclei. a. Input from contralateral CN enters the MNTB in a stimulatory fashion first, and then the MNTB projects to the LSO, inhibiting it. Projections to the IC are inhibited (glycine). b. Input from the ipsilateral side from CN to the LSO is stimulatory. Projections to the IC are stimulatory (glutamate). c. The LSO is either stimulated or inhibited based on which signal is the strongest. The brainstem takes inter aural time (from MSO) and intensity (from LSO) information and localizes it in time and space. This information travels to the inferior colliculus ? MGN ? auditory cortex. Descending pathways: 1. SOC to: a. outer hair cells, mediating motility b. dendrites by bases of inner hair cells, mediating connection with auditory nerve Reflex for loud noises: 1. Loud noise travels from cochlear nucleus ? SOC a. SOC sends projections to 5th and 7th cranial nerves. i. CN V (Trigeminal) ? tensor tympani ii. CN VII (Facial) ? stapedius muscle b. loud noises cause these muscles to contract and dampens transfer Anatomy of the Vestibular System Central Vestibular Pathways: Lateral Vestibular Reflex (reflex allows you to stand, and oppose gravity): Saccule and Utricle are the receptors. (Activate extensors; inhibit flexors) There are 4 vestibular nuclei on each side: 1) lateral vestibular nuclei 2) inferior vestibular nucleus 3) medial vestibular nucleus 4) superior vestibular nucleus 1. Macula ? activates the vestibular nerve. 2. 1st order neuron (vestibular nerve) ? synapses in the lateral vestibular nucleus a. there is also a minor contribution to the inferior vestibular nucleus 3. Projections from these nuclei descend in the lateral vestibulospinal tract a. This 2nd order neuron synapses on many internuerons to act on lower motor neurons which: i. excite extensors (EE) ii. inhibit flexors (IF) Pathway from Horizontal Canal: (leads to lateral and medial recti of the eyes) Maintain eye position in eye opposite to direction of motion. 3 motor neuron reflex 1. 1st order neuron: cell body in vestibular ganglion (originates in ampulla), ? synapses in the medial vestibular nucleus 2. 2nd order neuron: MVN crosses in the medial longitudinal fasciculus to synapse in motor nucleus of the abducens nerve 3. 3rd order neuron: (lower motor nucleus) projects from abducens nucleus to the lateral rectus muscle. Move opposite eye in same direction (cn reflexes are bilateral). 4 motor neuron reflex 1. A projection from the motor nucleus of the abducens nerve to the contralateral oculomotor nucleus that ascends in the medial longitudinal fasciculus. a. A 4th order neuron (lower motor neuron) projects to the medial rectus muscle. Inhibitory input is necessary to ensure movement in ONE direction. 1. Inhibition is also from the vestibular nerve, but it synapses in the superior vestibular nucleus. 2. Inhibitory pathway also ascends in the MLF to the a. motor nucleus of the abducens nerve on the same side of the stimulus i. to inhibit the lateral rectus on the same side b. oculomotor nucleus on the opposite side of the stimulus i. to inhibit the medial rectus on the opposite side END RESULT: Eye movement away from the ear that is stimulated to maintain eye position (opposite of the direction of motion). This occurs through stimulation of the lateral and medial rectus. Pathway from the anterior and posterior canals: Sensory input from the anterior and posterior canals 1. 1st order neuron synapses in the MVN a. excitatory neuron ascends in medial lemniscal fasciculus b. MLF decusses and ascends contralaterally until it sends out projections to both oculomotor nuclei and both trochlear nuclei. 2. The vestibular nerve sends projections to the SVN a. inhibitory information ascends in the ipsilateral MLF to both oculomotor nuclei and both trochlear nuclei. Vestibulospinal reflex: works to stabilize the head 1. Receptors located (crista) in the semicircular canal 2. Vestibular nerve synapses in the MVN and gives rise to the medial vestibulospinal tract a. This tract descends and enters interneurons of the head and neck, acting on muscles to stabilize the head. The Vestibular System receives other input: 1. Vestibular apparatus: sends information to the cerebellum a. cerebellum sends info to and from vestibular nuclei 2. Reticular system: vestibulospinal tracts run parallel to the reticulospinal tract and these run closely together. a. vestibular nuclei excite the reticular formation while giving rise to the vestibulospinal tract 3. Ascending spinovestibular tracts give somatosensory information to the vestibular nuclei 4. A commissure connects two sets of vestibular nuclei on both sides of the brain. 5. Connections to the accessory visual system and the superior colliculus. a. visual information can compensate for loss of vestibular function (if can see) b. can confuse if get different sensations from vestibular system and sight i. happen on plane with shade drawn ii. Nausea results from opposition of information from these two systems.
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