Thalamocortical Physiology

Question 1

Characteristic EEG pattern of slow wave sleep

Answer 1

• stage IV sleep = “slow wave sleep”
• characterized by delta waves
  
  • pattern of slow wave oscillation
  • frequency = ~3 Hz

Question 2

Characteristic EEG of absense seizure/epilepsy

Answer 2

• absence epilepsy = activity is suddently stopped, few seconds of staring, activity resumed
  
  • common in children
• EEG patter = ~ delta (slow) wave pattern @ 3 Hz

Question 3

Thalamic contribution to EEG

Answer 3

• EEG collects info from activity of large neurons near brain surface
• thalamus is w/in deep brain, thus cannot contribute to EEG directly
• Indirectly contributes: thalamus impacts the activity of cortical neurons through axonal connectivity btwn the thalamus and cortex

Question 4

Thalamic relay neurons fxn (general)

Answer 4

• TR = cells located in thalamus
  
  • receive input from a sensory sytem
  • relay sensory info to cortex via excitatory glutamatergic synapses onto pyramidal cortical neurons w/soma @ later IV of cortex

Question 5

Characteristics of Thalamic relay neuron behavior

Answer 5

• @ awake state:
  
  • little inhibitory input
  • E₀ = -55 mV
  • depolarization ==> fire a series of APs @ high frequency
• @ slow-wave sleep state:
  
  • thalamic reticular neurons inhibit TR neurons (via GABA)
  • E₀ = -85 mV
  • fire bursts of APs riding on top of a Ca²⁺ spike
    
    • Ca²⁺ spike occurs @ freq = ~ 3 Hz

Question 6

Characteristics of Ca²⁺ spikes @ TR neurons

Answer 6

• occur during slow-wave sleep @ frequency = ~ 3 Hz (same as delta waves)
• channel responsible = T-type Ca²⁺ channel
  
  • inactivated by depolarization
  • hyperpolarization (e.g. via thalamic reticular cells) ==> opening of inactivation gate
  • ==> Ca²⁺ spikes @ ~3 Hz
• riding on top of slow Ca²⁺spikes are AP bursts (several fast APs)
• ==> activity @ cortical neurons @ freq = 3 Hz ==> delta waves on EEG

Question 7

Thalamocortical circuit involvement in absence epilepsy

Answer 7

• absence seizures cannot be induced in mice w/genetically remove T type Ca²⁺ channels ==> implicates involvement in absence seizures
• mice w/mutations @ T-type channels ==> altered voltage dependence inactivation ==> TR neurons can fire slow Ca²⁺ spikes @ depolarized resting potentiats
• anticonvuslants effective in absence epilepsy block T-type channels

Question 8

Ascending control of thalamocortical circuits

Answer 8

• regulated by axons ascending from brain stem
  
  • from cholinergic, noradrenergic, and serotonergic neurons
  • exert effects on thalamocortical activity that determines sleep and wakefulness
• @ sleep state: stimulation of cholinergic neurons ==> ACh @ thalamus ==> interruption of slow waves @ EEG + awakening
• release of noradreline (from noradrenergic neurons @ locus ceruleus) takes place during fight or flight behaviors
• serotonergic neurons (from raphe nuclei) ==> serotoning release @ thalamus