Functional Anatomy of Visual, Auditory & Vestibular Pathways PDF

Functional Anatomy of Visual Auditory & Vestibular Pathways Dr. Ashraf Ramzy Professor of Anatomy & Embryology [email protected]  VISUAL PATHWAY ** Is formed of three successive neurons; two in the retina while the 3rd lies in the L...

Functional Anatomy of Visual Auditory & Vestibular Pathways Dr. Ashraf Ramzy Professor of Anatomy & Embryology [email protected]  VISUAL PATHWAY ** Is formed of three successive neurons; two in the retina while the 3rd lies in the LGB. * First order neuron: is formed by the bipolar cells of the retina. Dr Ashraf Ramzy * Second order neuron: is formed by the ganglion cells of the retina: Their axons form the optic nerve fibers. The two optic nerves join together in the optic chiasma. * In the optic chiasma → fibers from the nasal ½ of the retina decussate into the contralateral optic tract whereas the temporal fibers pass uncrossed to the ipsilateral optic tract. * The macular fibers: partially decussate in the chiasma and pass into the optic tracts of both sides. Dr Ashraf Ramzy ** Accordingly → the optic tract carries temporal fibers from the same side, nasal fibers from the opposite side and macular fibers from both sides. * Note: The optic tract "divides into two roots: 1. Medial root → contains supraoptic commissural fibers (commissure of Gudden) which connect the two medial geniculate bodies together. 2. Lateral root → contains visual fibers that terminate in the LGB. Some fibers of the lateral root pass to the superior colliculus of midbrain and the pretectal nucleus. These fibers are concerned with light reflexes. Dr Ashraf Ramzy * Third order neuron: is formed by neurons of the LGB. Their axons form the optic radiation which passes through the retrolentiform part of internal capsule, splitting into two groups: 1. Dorsal group (from the upper quadrant of retina) → in the parietal lobe. 2. Ventral group (from the lower quadrant of retina) → make a loop in the temporal lobe (loop of Meyer). * Both groups join together in the occipital lobe to end in the cortical visual area. Dr Ashraf Ramzy * The visual area (area 17) lies in the occipital lobe, below the precalcarine sulcus, and on both sides of the postcalcarine sulcus, extending to the occipital pole. * It is supplied by the posterior cerebral artery. Dr Ashraf Ramzy AUDITORY PATHWAY 1. First order neuron: * Central processes of the bipolar neurons of the spiral ganglion of the inner ear. * They leave the petrous temporal bone through the internal acoustic meatus and enter the pons. 2. Second order neuron: * The fibers entering the pons divide into 2 branches which synapse with dorsal or ventral cochlear nuclei. * Axons of the cochlear nuclei pass in the pons forming the trapezoid body. * Fibers of the trapezoid body synapse with superior olivary nucleus or trapezoid nucleus of the same or opposite side. Dr Ashraf Ramzy 3. Third order neuron: * Axons of the superior olivary nucleus or the nucleus of the trapezoid body of the same or opposite side ascend as the lateral lemniscus. * Some fibers synapse in the nucleus of lateral lemniscus. * Other fibers ascend and some synapse in the inferior colliculus of midbrain while other fibers pass through the brachium of inferior colliculus to synapse in the medial geniculate body (MGB). Dr Ashraf Ramzy 4. Fourth order neuron: * Axons of cells in the MGB form the auditory radiation which passes through the sublentiform part of the internal capsule to reach the 1ry auditory area of cerebral cortex (AI, 41) in the upper lip of the posterior ramus of lateral sulcus. Dr Ashraf Ramzy  VESTIBULAR PATHWAY * Receptors are present in: Semicircular canals (Ampullae) ‘Crista Ampularis’ and in Saccule and Utricle (maculae). * From these receptors impulses pass to the vestibular ganglion → Central processes of vestibular ganglion form vestibular nerve that passes through internal auditory meatus to enter Pons where they end on the vestibular nuclei in Pons and Medulla * There are four vestibular nuclei: superior, inferior, medial and lateral vestibular nuclei; all located beneath the lateral part of floor of fourth ventricle in Pons and Medulla. Dr Ashraf Ramzy * Connections of the vestibular nuclei: 1. To cerebellum through ICP; vestibulo-cerebellar fibers end in the flocculonodular lobe to affect equilibrium 2. To spinal cord form vestibulo- spinal tracts to influence motor neurons concerned with control of posture and balance. 3. To Medial Longitudinal Bundle (MLB) to connect with nuclei of III, IV, VI for coordination of head and eye movement. 4. To Thalamus; Ventral Posterior Nucleus then to cerebral cortex ‘vestibular area’. Dr Ashraf Ramzy