Which of the following is involved in auditory pathway?
Correct Answer: Inferior colliculus
Description: Ans: Inferior colliculusRef: Gray's Anatomy, The Anatomical Basis of Clinical Practice, Susan Standring, 41st edition, 2016, Page 325 and 326The inferior colliculus is part of the ascending auditory pathway and is the principal site of termination of the lateral lemniscus.Inferior Colliculus:StructureIt has a central, ovoid, main nucleus, which is lateral to the periaqueductal grey matter.It is surrounded by a lamina of nerve fibres, many from the lateral lemniscus, which terminate in it.The central nucleus has dorsomedial and ventrolateral zones, which are covered by a dorsal cortex.Cortex has four cytoarchitectonic layers:Layer I contains small neurones with flattened radial dendritic fields.Layer II, medium-sized neurones with ovoid dendritic fields aligned parallel with the collicular surface,Layer III, medium-sized neurones with spherical dendritic fields,Layer IV, large neurones with variably shaped dendritic fields.ConnectionsMost efferent fibres from the inferior colliculus travel via the inferior brachium to the ipsilateral medial geniculate nucleus.Lemniscal fibres relay only in the central nucleus, and some pass without relay to the medial geniculate nucleus.Ventral division of the medial geniculate nucleus receives a topographic projection from the central nucleus.Dorsal division receives a similar projection from the dorsal cortex.Some colliculogeniculate fibres do not relay in the geniculate nucleus but continue, with those that do, via the auditory radiation to the auditory cortex.A descending projection from the auditory cortex reaches the inferior colliculus via the medial geniculate nucleus.This descending path may produce effects at levels from the medial geniculate nucleus downwards, and it probably links with efferent cochlear fibres, through the superior olivary and cochlear nuclei.Superior Colliculus:StructureThe superior colliculus is laminated structure.It is divided into seven layers (Zonal, Superficial grey, Optic, Intermediate grey, Deep grey, Deep white and Periventricular strata).The zonal layer consists chiefly of myelinated and non-myelinated fibres from the occipital cortex (areas 17, 18 and 19) which arrive as the external corticotectal tract.The superficial grey layer (stratum cinereum) forms a crescentic lamina over the deeper layers and contains many small multipolar interneurons.The optic layer consists partly of fibres from the optic tract.This arrangement provides a retinotopic map of the contralateral visual field, in which the fovea is represented anterolaterally. Retinal axons terminate in clusters from specific retinotectal neurons and as collaterals of retinogeniculate fibres. Efferent fibres to the retina are said to start in this layer.The intermediate grey and white layers collectively constitute the main receptive zone.ConnectionsThe main afferent input is the medial corticotectal path from layer Vneurones of the ipsilateral occipital cortex (area 18), and from other neocortical areas that are concerned with ocular movements.Afferent fibres are also received from the contralateral spinal cord (via spinotectal fibres), the inferior colliculus, and the locus coeruleus and raphe nuclei (from noradrenergic and serotoninergic neurones).The deep grey and deep white layers adjacent to the periaqueductal grey matter are collectively called the parabigeminal nucleus. They form collicular efferents.The superior colliculus receives afferents from many sources including the retina, spinal cord, inferior colliculus and occipital and temporal cortices.Collicular efferents pass to the retina, lateral geniculate nucleus, pretectum, parabigeminal nucleus, the inferior, medial and lateral pulvinar, and to numerous sites in the brainstem and spinal cord.Fibres passing from the pulvinar are relayed to primary and secondary visual cortices and form an extrageniculate retinocortical pathway for visual orientation and attention.The tectospinal and tectobulbar tracts start from neurones in the superior colliculi.Inferior collicular projections to the brainstem and spinal cord appear to traverse the superior colliculi before they descend. In this way they connect with the origins of the tectospinal and tectotegmental tracts. These projections are relatively small and probably mediate reflex turning of the head and eyes in response to sounds.
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Anatomy
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