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Auditory pathwayDr.Sreeram(M.S) E.N.TSvs Medical CollegeMahaboobnagar.Auditory pathway begins in the cochlear nucleus of the medulla and ends with the auditory radiations which run from the thalamus to auditory cortex. Neural pathway

Basilar membrane vibration stimulates overlying hair cells in the Organ of Corti Stimulation of bipolar neurons in the spiral ganglion of the cochlear division of CN VIII Cochlear nucleus Superior Olivary Nucleus (medulla)Lateral lemniscus Inferior colliculus(mid brain)medial geniculate(thalamus)Sylvian Fissure of Temporal Lobe(Primary auditory cortex).

Ascending (afferent) system: Provides stimulation from one ear to both sides of the brain including the temporal cortex Descending (efferent) system: Provides inhibition to both cochleas from each side of the brain

The classical ascending auditory system is more complex than the ascending pathways of other sensory systems. The auditory nerve extends from the organ of Corti to the cochlear nucleus (CN) where each nerve fiber makes contact with neurons in all three main divisions of the CN. From the cochlear nucleus, fibers cross over to the opposite side in three fiber tracts that connect to the central nucleus of the contralateral inferior colliculus (ICC) . Fibers from the ICC project to the medial geniculate body (MGB). The fibers from the MGB project to the primary auditory cortex (AI), and the posterior auditory field(PAF) There are connections between the two sides of the brain at several levels of the classical ascending auditory pathways. These connections are important for directional hearing. This anatomical organization is the basis for the parallel and hierarchical neural processing that occurs in the ascending auditory pathways.

CochleaAuditory NerveDorsalCochlearNucleusAnteroVentralCochlearNucleusPosteroVentralCochlearNucleusMedialSuperior OliveLateralSuperiorOliveLateralLemniscusInferior ColliculusMedial GeniculateCortexAcousticStria:DorsalIntermediateVentralFUNCTION:Identify and processcomplex soundsPrincipal relay to cortexForm full spatial mapLocate soundsources in spaceStart soundfeatureprocessingAuditory nerveExtends 17 to 19 mm beyond the internal auditory canal Attaches to the brainstem at the cerebellopontine angle (CPA)Auditory nerve fibers terminate in the CNThe AN has two types of fibers known as type I and type II. The nerve fibers of the AN are bipolar cells that have their cell bodies in the spiral ganglion located in the modiolar region of the cochlea.The peripheral portions of the type I fibers of the AN terminate on the inner hair cells and the central portions terminate on the cells of the cochlear nucleus (CN). The type I nerve fibers are known as the radial fibers and they are thought to carry all the auditory information from the organ of Corti to higher centers of the central nervous system.The type II fibers form the outer spiral fibers innervate outer hair cell.The most central portion of the auditory nerve (1 cm) is similar to brain tissue. This has practical implications because it implies that the auditory nerve is at considerable risk of being injured in surgical operations in the cerebello pontine angle.

Cochlear nucleusThe auditory nerve terminates in the cochlear nucleus , which is the first relay nucleus of the ascending auditory pathways. It is located in the lower brainstem, at the junction between the medulla and the pons.The CN has three main divisions, dorsal cochlear nucleus (DCN), posterior ventral cochlear nucleus (PVCN), anterior ventral nucleus(AVCN) . Each auditory nerve fiber connects to all three divisions of the CN. This represents the initiation of the parallel processing that is abundant in the auditory system.The fibers from the CN mainly project to the contralateral inferior colliculus through three fiber tracts:the dorsal stria (stria of Monaco [SM])intermediate stria (stria of Held [SH]); and the ventral stria( trapezoid body [TB]) These three striae, after crossing to the opposite side, form the lateral lemniscus (LL), a fiber tract that projects to the central nucleus of the inferior colliculus.

Some fibers from the AVCN and PVCN do not cross the midline but ascend on the same side to reach the ipsilateral ICC Fibers from the PVCN reach the dorsal nucleus of the lateral lemniscus (DNLL) and from there fibers travel to the ipsilateral ICC. The ventral cochlear nucleus also sends fibers to the facial motor nucleus and the trigeminal motornucleus as part of the acoustic middle ear reflex.Superior Olivary ComplexThe superior olivary complex (SOC) consists of 3 main nuclei: the medial superior olivary nucleus (MSO); the lateral superior olivary (LSO)nucleus .Some of the fibers of the three striae (SM, SH, and TB) give off collaterals to nuclei of the SOC and some fibers are interrupted by synaptic contacts in one of the nuclei of the SOC before forming the LL. Receives input from both the IPSI & CONTRA cochlear nuclei.The SOC is thus the first group of nuclei that integrate information from both ears. The nuclei of the SOC are involved in directional hearing, mainly by comparing arrival time of neural activity from the two ears and intensity differences.Mediates the reflex of activity of tensor tympani and stapedius muscle Of ME Lateral LemniscusMajor pathway for impulses transmission from IPSI lower BSThe LL is the most prominent fiber tract of the classical ascending auditory pathways .The LL is formed by the three striae that emanate from the CN. The LL is composed of fibers from all divisions of the CN. The axons of the LL cross the midline and reach the contralateral ICC. Axons from cells in the nuclei of the SOC contribute to the LL. Since fibers of the LL extend from different sources, the LL contains both second, third, and possibly, fourth order neurons although second order axons dominate.The fibers of the LL have many collaterals some of which terminate on neurons in nuclei of the SOC and some terminate on neurons in the dorsal and ventral nuclei of the lateral lemniscus. The DNLL receives input from both ears and is involved in binaural hearing while the VNLL mainly receives input from the contralateral ear. Inferior ColliculusThe inferior colliculus is located in the midbrain just caudal to the superior colliculus. The IC is the midbrain relay nucleus where all ascending auditory information is channeled. The IC consists of the central nucleus (ICC), the external nucleus (ICX) and dorsal cortex of the IC (DC) .The ICC receives its input from the LL and all the fibers of the LL are interrupted by neurons in the ICC. The ICC on one side connects to the ICC on the other side and these connections are important for directional hearing that is based on the differences in the sound intensity at the two ears. Medial Geniculate BodyThe medial geniculate body is the thalamic auditory relay nucleus.

The MGB has three distinct divisions: ventral; dorsal; and medial

The ventral division of the MGB includes the pars lateralis and the pars ovoidea . The ventral division receives its input from the ICC.The main output of the ICC is the brachium of the inferior colliculus, which terminate in neurons in the ventral part of the MGB. This divergence indicates that considerable signal processing occurs in the auditory nervous system.There are no connections between the two sides of the MGB.Auditory CortexThe auditory cortex is a complex. The primary auditory cortex is located in the transverse temporal gyri, also known as Heschels gyri, located on the superior aspect of the temporal lobe. The auditory cortex is tonotopically organized. High pitch sounds are perceived in the medial parts of the gyri and low pitch sounds are perceived at the lateral aspect of the gyri. The cortex encodes three fundamental aspects of sound: its intensity, its location and its pitch. Sounds that are related to speech are distinguished from other kinds of sounds in the Auditory association cortex, also know as Wernickes area. Wernickes area is located in the posterior on third of the superior temporal gyrus, The primary auditory cortex (AI) and the posterior auditory field (PAF) receive input from the ventral division of the MGB .The AAF receives input from the dorso-medial MGB.A large fiber tract that is a part of the corpus callosum connects the auditory cerebral cortices on the two sides.neurons in the auditory cortical areas are connected through diffuse fiber tracts that travel in the posterior two thirds of the corpus callosum. The connections between the two sides that are present at the midbrain level (ICC) and at the cerebral cortex are the main reason that sounds presented to one ear are represented in the auditory cortices on both sides. The ipsilateral connections from the CN to the IC may also play some role in providing auditory information to both sides cortical areas. DESCENDING PATHWAYSThe descending auditory pathways have often been described as two separate pathways, the corticofugalcortico-cochlear systemsThe most central part of the corticofugal system originates in the auditory cerebral cortex and the cortico-cochlear systemprojects from the auditory cortex to the cochlearThank you