Thalamus - Anatomy & Function PDF

**[Thalamus:]** [Where is the thalamus?] - In contact with the wall of the third ventricle - Lies in the diencephalon of the forebrain. - The diencephalon also incorporates: - Epithalamus - Subthalamus - Metathalamus - Hypothalamus - The thalamus can be thoug...

**[Thalamus:]** [Where is the thalamus?] - In contact with the wall of the third ventricle - Lies in the diencephalon of the forebrain. - The diencephalon also incorporates: - Epithalamus - Subthalamus - Metathalamus - Hypothalamus - The thalamus can be thought of as the "gatekeeper" for the cortex and modulator of its function A collage of images of the brain Description automatically generated - Thalamic adhesion = fibre tract that passes through the thalamus + connects the two hemispheres. [The thalamus is made up of many individual nuclei:]  [Thalamic nuclei have discrete roles in regulating the access of information to the cortex:] - Thalamic nuclei can be categorized as: - Relay nuclei - Association nuclei - "other" nuclei - Relay nuclei (motor, sensory and limbic relay nuclei) receive information from the periphery and forward it to the cortex - Association nuclei connect different areas of the cortex with each other (nuclei involved with association = pulvinar and dorsomedial nucleus) - Cortical-cortical connections via the thalamus - "Other" nuclei include: - intralaminar nuclei - interact with projecting pathways of the brainstem and the basal ganglia (involved in drive and consciousness) - reticular nucleus -- master regulator of information flow to the cortex - synchronizes the activity of the thalamus with cortical activity allowing a conscious appreciation of stimuli and events - All thalamic nuclei have bidirectional/reciprocal communication with the cortex - \> 75% of thalamic neurons are projection neurons (mainly to the cortex) and the rest are inhibitory interneurons [Input to thalamic nuclei:] A diagram of a brain Description automatically generated - Inputs to thalamic nuclei can be divided into two categories: - Specific inputs (drivers) contain information that needs to be forwarded via the thalamus to the cortex - Regulatory inputs (modulators) modulate the driver inputs and determine whether it is sent to the cortex  - Cortex -- all connections between the cortex + thalamic nuclei are reciprocal - Examples of selective attention - Blocking of much sensory input to the cortex during sleep - Filtering out extraneous noise when listening to a conversation in a noisy environment - Ignoring distracting visual and auditory stimuli when concentrating on reading a book or playing an instrument [Function of the thalamus:] - Control of sensory information entering primary sensory cortex (all sensory modalities except olfaction) - Control of selective attention - descending input from the cortex commands what information needs to reach the cortex and this is executed by the thalamus -- avoids information overload and confusion. - Emotional and subjective response to sensation - achieved via connections with the limbic system and prefrontal cortex - e.g. sensations agreeable, not agreeable, effects on mood etc. (limbic system has links to the dopaminergic brainstem reward system) - Awareness of nociceptive input (from tissue damage) as pain - nociceptive input from tissue damage is not perceived as "pain" until it reaches the thalamus and is relayed to the cortex -- the perception of pain due to trauma can be delayed in extreme situations to promote survival. - Motor integration through links with the basal ganglia, cerebellum and motor cortex - the thalamus forms an essential component of the extrapyramidal motor system - Aspects of memory - e.g. the thalamus has links to the hippocampus and factual-declarative and geographic memory is relayed to the cortex via the thalamus - Control of cortical arousal and consciousness by linking the brainstem Ascending Reticular Activating System to the cortex [Special senses and the thalamus:]\ A diagram of the brain Description automatically generated - Special senses = hearing + vision. - Hearing: - Sound is transduced into cavity of the spiral ganglion neurones innervating the cochlea. - Neurones project into cochlear nuclei. - Information is relayed to inferior colliculus - Connection is made to the MGN in the thalamus - Information is passed to the primary auditory + association cortices - Vision: - Visual information is carried by the axons of retinal ganglion cells in the optic tract. - Neurones project to the LGN in the thalamus - Information is relayed to the primary visual and visual association cortices - Connections between nuclei and their target region are reciprocal -- allows information to the cortex by controlled by the cortex and other thalamic nuclei. - Another example of control of selective attention [Somatosensation and the thalamus:]  - Sensory innervation of the head (face) is provided by the trigeminal ganglion which uses the trigeminothalamic tract (trigeminal lemniscus) to relay information to the VPM in the thalamus. - Information is relayed to the primary somatosensory cortex - Sensory information of the rest of the body is provide by dorsal root ganglion neurones to VPL via the spinothalamic tract and dorsal column -- medial lemniscus pathway. - The VPL relays the information to all areas of somatic sensory cortex minus the primary. [Thalamus roles in motor integration:] - mainly talking about the extrapyramidal pathways. A diagram of a brain Description automatically generated - Other sources such as other parts of the cortex as well as subcortical structures - Loop of motor control. - Reciprocal connection between motor cortices + thalamus [Limbic relay thalamic nuclei (anterior, lateral dorsal and dorsomedial nuclei):]  - Mammillary bodies (hypothalamus) give driver input to the anterior nucleus - Axons are projected to the cingulate cortex - Allows for the consolidation of memories + selective attention. - Entorhinal cortex gives provides input to the lateral dorsal nucleus - Axons are projected into the cingulate cortex + parietal cortex - Allows for motivation + selective attention - Many parts of the limbic system send inputs to the dorsomedial nucleus - Axons project to the cingulate, prefrontal cortex + orbitofrontal cortex. - Allows for inhibition of inappropriate behaviour + mediates executive function. [Thalamic association nuclei (pulvinar and dorsomedial nuclei):] A diagram of a structure Description automatically generated - Receive input from association cortices of both hemispheres - After integration of information they project back to selected association cortices + particular areas within those selected cortices -- transthalamic connections between the cortices. - The flow of information between these connection is regulated by the thalamic association nuceli. - Direct corticocortical connections [Dorsomedial (DM) thalamic nucleus:] - DM = relay + association nucleus but has other functions. - Main driver input = prefrontal cortex. - Main role = executive control  [The pulvinar nucleus plays a role in high-order **visual** processing:] A close-up of a paper Description automatically generated [The pulvinar nucleus plays a role in high-order **auditory** processing:]  - Strong connections with the superior temporal gyrus -- part of the auditory association cortex. - Important region for the understanding of language and auditory short term memory. - \+ language tasks [Intralaminar thalamic nuclei - (centromedian (CM) and parafascicular (PF) nuclei)] - CM + PF are often termed the centromedian-parafasciular complex - Involved in consciousness + drive. A diagram of a diagram Description automatically generated [The thalamic reticular nucleus (TRN):]  - This nucleus is the main controller of information flow to the cortex.