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Lecture 10 1 2 ◦ By far largest division ◦ Involved in thought, perception, higher functions ... ◦ Entirely covered by sheets of gray matter – cerebral cortex ◦ Gyri (bulges) & sulci (grooves) (deep sulci – fissures) 3 Cerebellum Brainstem ◦ Co-ordinates muscle activity, processes information from vestibular system, … ◦ Continuous with top of spinal cord ◦ Divided into midbrain, pons, medulla ◦ Contains tegmentum (part of the mid brain)– central core (continuous within all three divisions) ◦ ... and non-tegmental portions – various different structures attached to tegmentum ◦ Central attachment of most cranial nerves Nuclei involved in initial stages of processing sensory input, many reflex arcs NVIII attaches in area where cerebellum, pons & medulla meet – cerebello-pountine angle 4 NVIII passes from cranial cavity to cochlea through canal in temporal bone – internal auditory meatus The internal auditory meatus is a canal in the petrous part of the temporal bone of the skull, on each side, and serves as the passageway for the cranial nerves, namely cranial nerve VII and cranial nerve VIII, and for the labyrinthine artery, between the middle and inner ear. 5 Fibres form cochlear division of NVIII Cell bodies form spiral ganglion within modiolus (Rosenthal’s canal) Terminate centrally in cochlear nucleus in brainstem (ipsilaterally only) Approximately 30,000 in man (in each ear) 6 90-95% innervate IHCs (‘true receptor cells’) – ‘Type I’ auditory neurons ◦ ◦ ◦ ◦ ◦ ◦ ◦ Relatively large diameter Bipolar sensory neuron Myelinated (both processes as well as cell body) ‘Inner radial fibres’ within organ of Corti Each IHC connects with ~ 20 fibres But each fibre innervates only one IHC Innervates closest to point of entry 7 A bipolar cell is a type of neuron which has two extensions. Bipolar cells are specialized sensory neurons for the transmission of special senses. As such, they are part of the sensory pathways for smell, sight, taste, hearing and vestibular functions. 8 Remaining 5-10% innervate OHCs– ‘Type II’ neurons ◦ ◦ ◦ ◦ ◦ ◦ ◦ Relatively small diameter Monopolar (‘pseudomonopolar’) (Fig 5) Not myelinated ‘Outer spiral fibres’ within organ of Corti Each OHC connects with ~ 6 fibres Each fibre innervates ~ 10 OHCs Crosses tunnel of Corti, runs basally for ~ 0.6 mm before innervating OHCs 9 All reported data are from Type I cells Individual neurons exhibit spontaneous firing (action potentials) – spontaneous rate Presentation of stimulus – increases firing rate (See Fig 7) ◦ Primary afferents always excitatory (on the whole) 10 The afferent innervations to the organ of corti. Notice that the type I auditory neurons in the spiral ganglion continue in the organ of Corti as inner radial fibers to inner hair cells. Type II auditory neurons continue as outer spiral fibers to outer hair cells . 11 A) afferent and efferent innervation of the organ of Corti. Efferent fibers are shown in black. B) arrangement of type I and type II afferent auditory nerve fibers to inner and outer hair cells. 12 Brainstem nuclei ◦ Cochlear nucleus ◦ Superior olivary complex Binaural processing ◦ Inferior colliculus Medial geniculate body 13 14 Located in _______________ ◦ Spans junction of pons and medulla First ‘relay station’ in auditory nervous system Receives ipsilateral afferent input only Three distinct anatomical divisions ◦ Antero-Ventral Cochlear Nucleus (AVCN) ◦ Postero-Ventral CN (PVCN) ◦ Dorsal CN (DCN) 15 Each primary (‘first order’) afferent bifurcates twice at CN ◦ One branch from first bifurcation to AVCN ◦ Other branch bifurcates again to innervate both PVCN and DCN ◦ Thus each primary fibre innervates all three divisions of CN ◦ Each branch may synapse with several CN (‘second order’) neurons ◦ Each CN neuron may receive information from one or more primary neurons 16 Each CN division exhibits cochleotopic organisation Various anatomical cell types described – e.g. bushy (spherical and globular), octopus, multipolar, stellate cells Distribution of each varies in different divisions 17 Predominant cell type – bushy cells (1 or 2 profusely branching dendrites) ◦ Primary afferent terminates in large nerve ending that envelopes cell body – endbulb of Held Allows for ‘one-to-one’ transmission of action potentials ◦ Neural responses (firing patterns) of these cells are very much like Type I primary afferents ◦ Cells used to be regarded as simple ‘relays’, no real processing of afferent input 18 AVCN also contains multipolar cells ◦ Several profusely branching dendrites, irregularly shaped cell bodies ◦ More complex firing patterns than bushy cells ◦ Sensitive to changes in acoustic stimuli ◦ In particular, onset and offset of sounds, as well as changes in intensity, frequency Axons of both cell types leave AVCN as large tract – ventral acoustic stria (becomes trapezoid body further along) 19 Predominant cell type – octopus cells (C in Fig 1) ◦ Large cell bodies with thick dendrites extending from one side of cell body only ◦ Cells receive input from several adjacent Type I primary afferents ◦ Cells therefore sensitive to bands of frequencies ◦ Axons leave PVCN as intermediate acoustic stria (also called stria of Held) Also contains multipolar cells, similar to those in AVCN described above 20 Contains stellate cells exclusively (in human) ◦ Many dendrites, in star-shaped arrangement Function unknown Axons leave DCN as dorsal acoustic stria or stria of Monakow 21 Group of nuclei in pons Receives input from cochlear nuclear complexes (primarily AVCN) on both sides ◦ First stage in auditory pathway to receive binaural input Three major nuclei, surrounded by smaller, more diffuse nuclei 22 Largest component of SOC (in humans) Each neuron receives input from left and right AVCN (from low-frequency fibres) Axons project to higher centres via ipsilateral lateral lemniscus tract 23 MSO neurons are sensitive to difference in arrival time of sound at each ear (ITD) Mechanism is thought to be along lines of on ‘coincidence detection’ model first proposed by Jeffress 24 Jeffress’ ‘coincidence detection’ model of lowfrequency sound localisation Delay lines Coincidence detectors 25 0 ITD 26 Response 0 ITD 27 Response Smallest component of SOC Neurons receive input from contralateral AVCN only (high-frequency fibres) Axons terminate in ipsilateral LSO 28 Each neuron receives input from ipsilateral AVCN (high-frequency fibres) and ipsilateral MNTB ◦ Thus – binaural, high-frequency input Axons contribute to lateral lemnisci on both sides 29 LSO neurons are sensitive to interaural _______ differences (Neurons also code for horizontal sound localisation, but for high frequencies) 30 Smaller groups of nuclei surrounding the three main nuclei Include cell bodies of olivocochlear bundle (OCB) – provide efferent innervation of cochlea Medial and lateral groups (in vicinity of MSO and LSO respectively) 31 Axons from both MSO and LSO also project bilaterally to nuclei of N VII (________ nerve), also within pons Synapse with neurons that innervate (ipsilateral) stapedius muscle SOC therefore important stage in stapedius reflex loop ◦ Note existence of an additional pathway between CN and (ipsilateral) facial nucleus 32 LL is the major tract of the auditory brainstem The lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the contralateral inferior colliculus of the midbrain. Three distinct, primarily inhibitory, cellular groups are located interspersed within these fibers, and are thus named the nuclei of the lateral lemniscus 33 Group of nuclei located in midbrain Major nucleus is central nucleus of inferior colliculus (CNIC) ◦ Largest brainstem auditory nucleus Also contains pericentral nucleus and external nucleus 34 Vast majority of axons forming lateral lemniscus terminate in ipsilateral CNIC ◦ Some via additional synapses at the interstitial nuclei within tract A few lateral lemniscus axons terminate in contralateral CNIC (via commissure of Probst) 35 Most axons of CNIC cells form brachium of IC, which leaves brainstem to travel to ipsilateral thalamus A few CNIC axons cross midline (commissure of IC), then either ◦ Synapse on cells within contralateral CNIC; or ◦ Pass through contralateral CNIC to join (contralateral) brachium of IC 36 All the major ascending pathways (crossed and uncrossed) converge here (see Fig 4) ◦ (Pathways then diverge again – somewhat unique organisation amongst sensory systems) Lower centres extract different features of acoustic signal ◦ e.g. frequencies, frequency bands, onsets, offsets, changes in intensity, localisation ... 37 Information needs to be integrated as different aspects of same acoustic signal Integration (‘synthesis’) thought to commence in CNIC 38 CNIC also processes binaural information independently of lower stages ◦ Appears to ◦ Enhance sensitivity to ITD, IID demonstrated by SOC neurons ◦ Extract more sophisticated information from binaural input e.g. change in ITD, possibly encoding motion (rather than location) of a sound source 39 Auditory portion of diencephalon Complex group of nuclei within thalamus Receives input from IC, processes and relays to cerebral cortex (via internal capsule) Three divisions – ventral, dorsal, medial ◦ Ventral largely specific to auditory information ◦ Dorsal and medial divisions less so Receive information from non-auditory pathways (as well as CNIC) 40 Most fibres of brachium of IC terminate here Receives & further processes detailed auditory information, e.g. on location, onset/offset, frequency, intensity, … Axons project to primary auditory cortex Ventral MGB also receives considerable input from primary auditory cortex 41 Axons project mainly to association auditory cortices Proposed function in directing and maintaining attention 42 Also receives input from vestibular, somesthetic, visual systems ◦ Regarded as a ‘multimodal’ nucleus, rather than purely auditory Axons project to both auditory & nonauditory cortices Also receives axons from widespread areas of cerebral cortex Proposed function – multi-sensory arousal/attention system 43