10/25 The Thalamus Flashcards
• Compare and contrast first-order versus second-order thalamic nuclei, and identify which nuclei belong to each category.
o First order: VPL, VPM, VL, LGN, MGN. peripheral afferent input. sensory system. sending excitatory projections to layer 4. constrained cortical target (VPL, VPM) primary somatosensory nucleus.
o Second order: anterior n, dorsomedial n, centromedian n, pulvinar, n reticularis. higher order dont have a single direct cortical target. They project widely throughout many association cortical areas. Input from lots of different structures massive cortical input from layer 5 and 6
• Describe properties of second order thalamic n.
o There is a high proportion in the RE which projects both to pfc and hpc. Can help synchronize.
• Describe how functional connectivity changes between waking behaviors and light sleep for the following connections: thalamus and cortex, the inter-hemispheric thalamic connections between the same nuclei, different thalamic nuclei in the same hemisphere.
Connectivity increases very slightly in nearly all regions except TEM with increase depth of sleep.
o Between hemispheres: there is a gradual increase in connectivity during sleep.
o Between different nuclei: same gradual increase in each connection.
• Describe what is meant by the term ‘limbic thalamus’ and identify the nuclei that are part of the limbic thalamus.
o Anterior and dorsomedial thalamus together are limbic thalamus.
• Identify the thalamic nuclei that are part of the Papez circuit and describe their primary afferent/efferent connections. Identify the cognitive deficits associated with damage to these nuclei.
o There is a loop between amygdala, hpc, and rhinal cortices. HPC goes to mammillary through fornix then to anterior thalamus and pfc through mammillothalamic tract. HPC receives input from anterior and dorsomedial thalamus. Rhinal cortices receive input from pfc via cingulum and project to dorsomedial thalamus. If there is a lesion in either thalamus, there is amnesia similar to hpc or mammillary body damage.
• Describe the behavioral correlates of neural activity in the anterior nucleus, dorsomedial nucleus, and pulvinar.
o Anterior: cognitive map. Firing rate of cells depends on head direction
o Dorsomedial: receives inputs from olfactory cells through the amygdala. The mediodorsal n then sends those signals to endopiriform n and orbitofrontal cortex. Also involved in short term memory (especially MD cells). Ability to hold information in your head for a minute or so before use.
o Pulvinar: attention. Activity peaks during initial stimulus and is mildly raised during delay when there is an expectation that the cue will come back. Then there is another spike with response.
• Identify the neuroanatomical explanation for “blindsight.”
o If you have damage to the visual cortex, you can still navigate space and not understand how because there is direct input to lower visual areas for action and planning. Pulvinar can support visual processing independent of conscious awareness.
• Describe the role of the nucleus reticularis in thalamocortical interactions and the 3 modes by which activity in the nucleus can gate thalamocortical interactions.
o TRN contains a huge number of fast spiking gaba-ergic cells. Efferent/afferent axons of the thalamus pass through nucleus reticularis and make contact with dendrites. Disruptions are related to pain disorder, schizophrenia, etc.
o 1: excitatory efferent signals from thalamus (LGN) activate NRt neurons as they leave which inhiit neighoring neurons in thalamus
o 2: ascending pathways to thalamus (medial lemniscus) have fibers that directly inhibit NRt promoting transmission of ascending excitatory to cortex
o 3: pfc input acting as dorsomedial and anterior n. can have variable effects.
