The metathalamus, geniculate nuclei and the optic radiation


Today we’re going to discuss the metathalamus and related sensory pathways concerned with hearing and vision. These elements of the brain are closely linked and so it’s useful to discuss them together. So what do we mean by the metathalamus? Well, the metathalamus refers to nuclei that are related to the thalamic nuclei but are not strictly part of the thalamus. These are the medial and lateral geniculate nuclei. They’re found beneath the pulvinar of the thalamus and are essentially relay stations in the auditory and visual pathways, respectively I will start with the medial geniculate nucleus. This nucleus receives input from the ipsilateral inferior colliculus on the dorsal surface of the brainstem and projects to the ipsilateral primary auditory cortex on the superior temporal gyrus via the auditory radiation. The inferior colliculus in turn receives its input from the superior olivary nucleus. Fibers of the auditory pathway have a clear tonotopic organization. That is neurons are arranged in groups that respond to characteristic frequencies. This means that each narrow range of frequencies that are detected in the ear has a separate path to the auditory cortex, enabling those different sounds to be distinguished from each other. We should note here that the input from each ear projects to both ipsilateral and contralateral superior olives, meaning that the information that reaches each auditory cortex comes from both ears. Connectivity between the left and right auditory cortices is via the commissural connections of the corpus callosum. We now come to the lateral geniculate nucleus. This is the major nucleus in the visual pathway. Its inputs are complex and I will give only a very simplified version of them here. The lateral geniculate nucleus receives input from the retina from the contralateral visual field via the optic tract. Efferent fibers from the temporal and nasal halves of each retina project along the optic nerve to the optic chiasma where fibers from the nasal retina decussate and pass to the contralateral side while fibers from the temporal retina project to the ipsilateral side. Now, we have to remember that the image on the retina is upside down and back to front. So this means that the input to the right lateral geniculate nucleus comes from the left visual field while input to the left lateral geniculate nucleus comes from the right visual field. And if that wasn’t complicated enough, inputs from the right and left eyes project to alternate layers of neurons within the
lateral geniculate nucleus. Neurons of the lateral geniculate nucleus project to the primary visual cortex via the optic radiation. This is a large bundle of projection fibers that passes through the internal capsule between the lentiform nucleus and the inferior horn of the lateral ventricle. Fibres pass forward towards the temporal pole to form Myer’s temporal loop. Fibres then track back, lateral to the forceps major and synapses on either side of the calcerine sulcus in the primary visual cortex Thus, like each part of the auditory pathway is a mix of inputs from both ears, each region of the visual cortex receives information from both eyes, but from only half the visual field. Inputs from the left and right visual fields are combined via the commissural connection between the occipital lobes … namely the forceps major, that forms the splenium of the corpus callosum. So hopefully you can say that in order for the brain systems to function, information needs to pass from one part to another using the major white matter tracts, to enable communication and so aid that complex processing that underlies perception

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