NEURANAT 3 - Thalamus and Somatosensory cortex PDF

NEURANAT 3 - Thalamus and Somatosensory cortex: beam me up, Scotty Information arising from the basal nuclei, cerebellum, limbic system, and sensory systems is relayed to the thalamus, a subcortical relay station. Here, this information is processed and integrated and is then transmitted to specifi...

NEURANAT 3 - Thalamus and Somatosensory cortex: beam me up, Scotty Information arising from the basal nuclei, cerebellum, limbic system, and sensory systems is relayed to the thalamus, a subcortical relay station. Here, this information is processed and integrated and is then transmitted to specific areas of the ipsilateral cerebral cortex. All the information to the telencephalon, apart from olfaction, passes through the thalamus to be integrated. These cortical areas are in turn connected via reciprocal feedback projections to the thalamic nuclei (subnuclei), making the cerebral cortex the most prominent input source to the thalamus. The thalamus therefore functions in the processing and of sensory and motor information, The thalamus is interconnected not only with the neocortex of all four lobes, but also with the phylogenetically older cortical areas including the paleocortex (piriform cortex) and archicortex (hippocampal formation). 1. Relationships and borders Its boundaries are: - Anteriorly, the interventricular foramen (of Monro) - Posteriorly, the posterior extent of the pulvinar - Medially, the third ventricle - Laterally, the posterior limb of the internal capsule - Dorsally, its free surface, which contributes to the floor of the lateral ventricle - Ventrally, the hypothalamic sulcus on the lateral wall of the third ven tricle, separating it from the hypothalamus  2. Thalamic nuclei The thalamus is made up of two symmetrical structures formed from the diencephalon. The thalami are made up of grey matter that is partitioned by a “Y” shaped white matter structure known as the internal medullary lamina. As a result of the location of the internal medullary lamina, each thalamus is divided into roughly three main parts: the anterior, medial and lateral thalamus. The anterior part lies between the short limbs of the internal medullary lamina, while the medial and lateral parts lie on the respective side of the main stem of the “Y”. The left thalamus communicates with the right thalamus by way of the interthalamic adhesion. There are two protuberances on the posteroventral surface. These are the medial and lateral geniculate bodies, which are responsible for the processing of auditory and visual sensory inputs, respectively. Furthermore, the thalami are each surrounded two layers of white matter. Dorsally, it is covered by a layer known as the stratum zonale. Laterally, it is covered by the external medullary lamina, which separates the lateral and ventral thalamus from the thalamic reticular nucleus and the subthalamus. Thalamic nuclei are grouped based on their function: specific and non-specific - Specific nuclei direct their fibers to a precise destination in the telencephalon. - Non-specific nuclei are not related to circuits that are connected to a peculiar area of the cortex Nuclei of the thalamus Functional group Anterior (A) - Anterior ventral (AV) Specific relay - Anterior medial (AM) Specific relay - Anterior dorsal (AD) Specific relay Medial (M) - Dorsomedial (DM) Association (multimodel) - Midline (median, Non-specific periventricular) Lateral Dorsal tier - Lateral dorsal (LD) Association (multimodel) - Lateral posterior (LP) Association (multimodel) - Pulvinar (P) Association (multimodel) Ventral tier - Ventral anterior (VA) Motor relay Motor relay - Ventral lateral (VL) - Ventral posterior (VP) Sensory relay Ventral posterior medial (VPM) Sensory relay Ventral posterior lateral (VPL) Ventral posterior inferior (VPI) Metathalamic nuclei Sensory relay - Medial geniculate body Sensory relay - Lateral geniculate body Intralaminar - Centromedian (CM) Non-specific - Parafascicular (PF) Non-specific Reticular Non-specific  a. Specific nuclei Metathalamus - Geniculate bodies They are dedicated to special sensitivity: - Medial geniculate body: o It receives ascending, auditory info along the lateral lemniscus, coming from each acoustic apparatus. The fibers are mainly crossed but some ipsilateral exist. o The fibers go through a relay station ALO the inferior colliculus of the tectum (pons) o The fibers reach the medial geniculate body o From the thalamus, fibers travel though the sublentiform limb (inferior brachium) of the internal capsule to reach the acoustic areas in the temporal lobe (Brodmann areas 41 and associative 42) - Lateral geniculate body o It receives optic info mainly from the opposite field of vision and some from the ipsilateral field, travelling through the optic tract. o Part of the fibers are deviated to the superior colliculus of the cerebellum o Majority reaches the LGB. The LGB is organised in 5 segregated laminae o From the thalamus, fibers travel through the retrolentiform limb of the internal capsule and reach the optic areas of the occipital lobe (Brodmann area 17) Nuclei of the Input Output Function thalamus Anterior (AV, Mammillary body via Cingulate gyrus Expression of emotions AM, AD) the mammillothalamic Parahippocampal Learning, memory tract gyrus Hippocampal formation via fornix Dorsomedial Prefrontal Prefrontal association Integration of sensory information (DM) association cortex, cortex, periorbital Amygdala, areas, hypothalamus Orbitofrontal cortex, Olfactory system, Hypothalamus, Intralaminar, midline, and lateral posterior thalamic nuclei Ventral From reticular Premotor cortex, Control of eye, head, neck and limb movement anterior (VA) formation and basal supplementary motor ganglia area, basal ganglia, reticular formation Ventral From the correlation Premotor cortex, Control of movement lateral (VL) system (the supplementary motor cerebellum) cortex Ventral posterior (VP) Orofacial region via 1° somatosensory It processes taste, touch, pressure, pain, and temp - VPM trigeminothalamic cortex in post central sensation, and proprioception tracts (trigeminal and gyrus gustatory fibers) Processes pain, temperature and nondiscriminat Anterior and lateral sensation from the body - VPL/ spinothalamic tracts 1° somatosensory Processes discriminatory touch, pressure, joint m VPI cortex in post central and vibratory sense from the body Medial lemnisci gyrus 1° somatosensory cortex in post central gyrus Dorsal: - LD Mammillary body via Cingulate gyrus Expression of emotions mammillothalamic Parahippocampal tract gyrus - LP Sensory integration  Superior parietal Superior parietal lobule, Precuneus, lobule Precuneus - P Superior colliculus, Integration of visual, auditory, and somatosenso Pretectal area, information Occipital lobe Association areas of parietal, temporal, Association areas of and occipital lobes parietal, temporal, and occipital lobes, Retina, lateral geniculate body, medial geniculate body, cerebrellum Single specific 🡪 projections (anterior, medial, posterolateral+ methathalamus); Associative specific (dorsal nuclei)🡪integration of information Anterior nuclear: The anterior nuclear group, a relay nucleus of the Papez circuit of emotion, has a significant number of connections with the limbic system. It is the primary target of the afferent fibers relaying information from the mammillary body of the hypothalamus via the mammillothalamic tract, and the hippocampal formation via the fornix. It in turn projects to the cingulate gyrus of the limbic association cortex and the parahippocampal gyrus. Due to its limbic connections, the anterior nuclear group functions in the expression of emotions, and due to its considerable connections with the hippocampal formation, this group of nuclei is associated with learning and memory processes For more of this see book p. 471 b. Non-specific nuclei These groups of nuclei are connected to the reticular formation and to all the specific nuclei, they are important to activate the cerebral cortex in a diffuse way. This happens in order to create a sort of preparatory activity to respond properly to the stimuli: arousal and consciousness of the cortex. Thalamic nuclei (non Description Projections specific) Reticular thalamic Thin cells layer sandwiched between the posterior Other thalamic nuclei (inhibition) nucleus limb of the internal capsule and the external medullary lamina It receives afferents from the reticular formation Nuclei of the midline Adjacent to the inter-thalamic connection Hypothalamus, limbic lobe, ventral (paraventricularis and reuniens) striatus Intralaminar nuclei Nuclei within the internal medullary lamina Prefrontal areas, sensory and motor (parafascicular, centrolateral, centromedial, areas, dorsal striatus paracentralis, centromedian) RECAP You can notice that:  - The lateral dorsal and lateral posterior both reach the parietal lobe, but the lateral dorsal projects to a more medial portion of it - Superimposition between the projection of the lateral dorsal and the anterior nuclei - Pulvinar and lateral geniculate are very close to each other for the integration of visual information in the understanding of language. 3. Somatosensory cortex Somatosensory cortex: area where the VPL and VPM thalamic nuclei project after accepting fibers from medial spinal lemniscus, and trigeminal lemniscus The primary sensory areas are involved in distinguishing and integrating sensory input relayed there primarily by the thalamic nuclei. a. 1° somatosensory cortex The primary somatosensory cortex is located in the post central gyrus on the lateral surface of the parietal lobe, and corresponds to Brodmann’s areas 3a, 3b, 1, and 2. This cortical region is the site of termination of the thalamocortical fibers arising from the VPL and VPM nuclei of the thalamus. These fibers relay sensory information from the respective contralateral half of the body and head apart from the pharynx, larynx, perineum (bilateral) and from oral region (ipsilateral). These exceptions are due to the trigeminal lemniscus pathway. The information from the medial lemniscus are located in 3a and 3b areas mainly. Spinal lemniscus information projects both into 3a and 2 areas. There is a distinctive somatotopism: - cranio-caudal - latero-medial the sensory and motor homunculus are similar but not equal! what is peculiar of the human is to have very developed areas for the finger and for the tongue. The primary somatosensory cortex projects to - the primary motor cortex and Brodamann’s area 5 of the same hemisphere via association fibers - the contralateral corresponding sensory cortex via commissural fibers - Some of the cells residing in the primary somatosensory cortex give rise to projection fibers that descend to terminate in sensory relay nuclei of the brainstem and spinal cord gray matter. There they influence motor activity, not by synapsing with motoneurons, but instead by modulating the transmission of sensory input from peripheral structures into the brainstem (dorsal column nuclei) and spinal cord (dorsal horn). b. Secondary somesthetic area: Information that is involved into a processing step. They are inside of Broadmann area 2a, receiving info from primary somatosensory cortex (S1) and from the brainstem. c. Somesthetic associative areas A somesthetic association area (parietal association area) is located in the cortex of the superior parietal lobule and the inferior parietal lobule. - The superior parietal lobule includes Brodmann’s areas 5 and 7 which process somatic sensory information. - The inferior parietal lobule is involved in sensory perception, and language, consists of the supra marginal gyrus (area 40) and the angular gyrus Meaning to be connected to the primary somatosensory areas to connect with other lobes integrating the inputs, helping in the understanding and interpretation of the sensory signal= creation of complex functions.