Lecture 15: Thalamus/Cortex 2

Question 1

What are the two functions of the thalamus?

Answer 1

i. “Relay” function: relay refers to the transfer of information
ii. “Gate” function: control of the functional state of the brain

Question 2

What types of relay functions are there?

Answer 2

• sensory inputs from periphery to sensory cortex
• motor info from cerebellum and basal ganglia to motor cortex
• associative information between distant cortical sites
• info from limbic system to cortex

Question 3

What types of gate functions are there?

Answer 3

-transition between waking and sleeping states. Mediated by changes in neuromodulation

Question 4

What is the relationship between thalamus and cortex in relay operations?

Answer 4

All relay operations are under heavy control of cortical feedback

Question 5

What are the two functions of the cortex?

Answer 5

i. Generate sensory and motor representations of external and internal worlds using a combination of sensory driven activity, internally generated activity and memory storage
ii. Generate consciousness. Only a small portion of the activity in corticothalamic networks is accessible to consciousness

Question 6

What is the relationship between thalamus and cortex in relay operations?

Answer 6

Generation of representation and consciousness depends on recurrent loops between cortex and thalamus

Question 7

How EEG can be used to study brain function?

Answer 7

Used to diagnose epilepsy and assist in the accurate classification of seizures and specific epileptic syndromes
-used to study patients with seizures, study sleep, and monitor brain state during anesthesia

Question 8

Evoked potentials (Eps):

Answer 8

generally much lower voltage than spontaneous cortical electric activity
-the electric signals sent due to sensory stimuli
Dysfunction of Eps = dysfunction of sensory pathways

Question 9

Rhythmic TC cells

Answer 9

Sleep state; whole thalamus is rhythmic together

Question 10

What type of thalamic firing occurs when animal is awake?

Answer 10

Single spike mode (tonic): the mode of thalamic firing when animal is waking (“W” in the EEG)

Question 11

What type of thalamic firing occurs when animal is asleep?

Answer 11

Burst of spikes (bursting): the mode of thalamic firing when animal is asleep (“S” in the EEG)

Question 12

What finding in the EEG is characteristic of a transition from waking to sleep?

Answer 12

Waking: high frequency low amplitude
Sleep: low frequency (less than 15 Hz) to high amplitude
-Single spike mode (tonic) to bursting mode

Question 13

What are the membrane properties of the thalamocortical neuron during waking state?

Answer 13

Membrane potential = -55mV
Ca2+ channels are inactivated
-reaction to stimuli represented by a single spike

Question 14

What is the mechanism underlying the change in firing mode?

Answer 14

When cell membrane is -70mV, the voltage gated Ca2+ channels are active and depolarization causes them to open and then inactivate and close
-Ca influx causes slow depolarizing waves sufficient to initiate a brief burst of action potentials
Simply by hyperpolarizing the cell, you lead to a “burst” of eletro activity
-thus when you are sleeping, your thalamic neurons are at a hyperpolarized state (generating the bursts)
-when you are awake, thalamic neurons are at normal membrane potential
-the burst or unexpected hump is actually caused by the calcium spike threshold

Question 15

What are T-type calcium channels in Thalamus?

Answer 15

The channel responsible for the bursting action potential property during sleep

Question 16

So why doesn’t the calcium burst exists during normal depolarization phase?

Answer 16

Because calcium channels can be in three stages

i. closed (hyperpolarization) what happens when you hyperpolarize
ii. open (depolarization)
iii. INACTIVATED depolarization  you need to hyperpolarize “t-channel” to reactivate and prime calcium channels to open…that’s why it is important to hyperpolarize the membrane potential

Question 17

Low threshold spike (LTS)

Answer 17

the spike generated by calcium current…phenomenon in which the threshold is lowered

Question 18

What are the 3 functional consequences of the change in firing mode?

Answer 18

The combination of UNRELIABLE RESPONSES, generation of RHYTMIC BURSTING, and LARGE SCALE SYNCHRONY blocks sensory input from the periphery to the cerebral cortex during sleep

i. unreliable responses
- it means that when you stimulate the thalamic cells during sleep, you don’t have a reliable single spike or you don’t have a reliable output…in other words, you don’t respond to stimuli
- it is dominated not by stimuli but by intrinsic input! That’s why you don’t perceive shit when you sleep
- refractory period is the second fundamental properties (1. All-or-none nature and 2. Refractory period) that makes it so that thalamic cell is not responsive to stimuli during sleep

ii. oscillations (rhythmic bursting): in burst firing mode, thalamocortical cells generate rhythmicity due to the presence of the same ionic currents present in the heart

iii. Large scale synchrony: rhythmic bursting in thalamus leads to global synchrony in thalamacortical networks
- a type of activity completely unrelated and disconnected with the outside world
- rhythmic synchronized bursting is incompatible with consciousness

Question 19

unreliable responses, rhythmic bursting, and large scale synchrony

Answer 19

The three characteristics of TC cells that leads to sleep

Question 20

What are the two fundamentals of bursting?

Answer 20

1. all-or-none nature

2. refractory period: making bursting an unreliable response and incompatible with coding of information

Question 21

Neuromodulation

Answer 21

Use of neurotransmitters to modulate wake/sleep cycle

Question 22

How do you get woken up (change in state of vigilance)?

Answer 22

Neuromodulation from brainstem and basal forebrain
Acetylcholine is released
-naturally there is a cycle
Example: getting woken up by a loud scream

Question 23

How do neuromodulatory systems control waking/sleeping state?

Answer 23

• presence of neuromodulatory system maintains depolarization during waking state
  
  • during sleeping state, the neuromodulatory systems SHUT DOWN and allow resting potassium currents to bring Vm to hyperpolarized levels…so you are no longer able to maintain the depolarization necessary for wake
  • this means that shit gets depolarized and you get the activation of t-channel Ca2+

Question 24

What are the types of neuromodulatory systems?

Answer 24

i. brainstem
ii. basal forebrain
Releases acetylcholine and noradrenaline and others