Basal Ganglia wk8

Question 1

what is the BG, and what does it assert influence on?

Answer 1

old system predating the neo-cortex, that exerts influence on motor and prefrontal areas via the Thalamus and also on limbic circuits

Question 2

what does BG play a major role in?

Answer 2

in action selection, reinforcement learning, and cognition and decision-making – filters actions to make a selection, ready for execution

Question 3

what does the BG consist of?

Answer 3

of a collection of nuclei that sit above the brainstem, but below the cerebral cortex:

Question 4

How does the BG work in brief terms

Answer 4

• Cerebral cortex sends signal to the BG , forming a functional circuitry loop
• The cortex sends signals of a number of candidate actions and the BG filters these to make a selection of which action to perform

Question 5

structure of the BG

Answer 5

• striatum = input structure that the cortex projects information through to enter the BG. conists of different regions:
• caudate, putamen, and nucleus accumbens each receive input from a different area of the cortex within the straitum
• globus pallidus
  o interna (GPi)
  o externa (GPe)
• subthalamic nucleus sitting below the thalamus (STN)
• substantia nigra with multiple subdivisions and the pars compacta (SNc)

Question 6

what does the cortex project to?

Answer 6

the striatum via an excitatory connection, simultaneously activating two following pathways - direct and indirect

Question 7

direct pathway of cortex-striatum projection

Answer 7

striatum inhibits the internal Globus Palidus which inhibits the thalamus .

this double inhibition results in an increase in Thalamic activity via disinhibition (meaning when its released from inhibition= higher lvl of firing then default)

Net excitatory pathway-> exciting back to motor cortex to selecct motor plan

Question 8

indirect pathway of cortex-striatum projection

Answer 8

striatum inhibits the external globus palidus, which inhibits the Subthalamic nucleus , before exciting the internal globus palidus .

as this becomes more active, it increases inhibiton of the Thalamus (gateway back to the cortex)

Net inhibatory = a balance of exi/inhib , suppressed actions go through this

Question 9

how is the balance between BG pathways regulated?

Answer 9

regulated by the substantia nigra, which innervates the striatum with dopamine.

this has one excitatory and one inhibitory connection

burst of dopamine facilitates movement, activating the direct whilst inhibiting the indirect

Question 10

dopaminergic connections

Answer 10

D1 receptors are excited by dopamine and innervate the direct pathway.

D2 receptors are inhibited by dopamine and directly innervate the indirect pathway.

Question 11

what are the domain consequences for neuronal loss in the SNc?

Answer 11

on movement, cognitive control, and decision-making

Question 12

where can consequences for neuronal loss in the SNc be seen?

Answer 12

this can be seen in PET scans where (ParkinsonsD) patients have an asymmetric loss of uptake on the tracer vs. normal patients when BG binds to D receptors

as cells die, black staining therefore vanishes from brain

Question 13

weak direct pathways -> akinesia

Answer 13

the loss of dopaminergic innervation to the striatum causes severe imbalances in components of the BG circuit – no longer drives the direct pathway nor inhibits the indirect pathway

we lose excitatory input to the system

Question 14

how does akinesia affect action plans?

Answer 14

little faccilitation of motor plans sent to cortex

unable to be selected efficiently, and those selected are slow and unrefined due to excessive inhibition from the indirect pathway – precisely what happens in PD

Question 15

cardinal symptoms of PD

Answer 15

1. Absence/slowness of movement (akinesia/bradykinesia)
2. Stiffness or rigidity
3. Tremor at “rest” (much reduced with active movement)

Question 16

explanations of PD

Answer 16

• degeneration of dopaminergic neurons in the substantia nigra
• neurological disease caused by altercation in a single neurochemical- dopamine
• disordered motor signals are sent to the SMA
• imbalanced limbic system causes motivation and emotional disturbances as BG involved in various domains of functioning

Question 17

Treatments for Parkinson’s disease

Answer 17

• Levodopa (L-dopa)
• Transplants (bone marrow, stem cells)
• Neuro-surgery to rebalance connections between striatum and SMA (deep brain stimulation)

Question 18

what does L-DOPA do?

Answer 18

attempts to reinstate the balance between the pathways, by flooding the striatum with dopamine

though side effects are severe (too much dopamine)

Question 19

why has neurosurgery been suggested to treat PD?

Answer 19

parts of the thalamus and globus Polus are taken out

suggested to reduce excess inhibition in the indirect pathway

Question 20

strong direct and weak indirect pathways -> hyperkinesia

Answer 20

too much dopamine can push the balance of BG circuits too far towards the direct pathway, as this excitation can cause premature selection of action or uncontrolled selection of inappropriate actions as suppression is difficult

Question 21

Causes of hyperkinesia

Answer 21

Long term L-DOPA outcome

Loos of striatal neurons or of STN (Huntingdons, Hemiballism, Tourettes)

Question 22

examples of other basal/ cortical-based ganglia loops

Answer 22

• motor
• oculomotor
• executive/associative
• emotion/motivation
  loops

Question 23

what does BG act as during eye movements? (the occular motor loop)

Answer 23

a “filter” to add contextual information onto reflexive control of eye movements, through inhibiting the superior colliculus except for the target of interest

Prevention of reflexive eye movements also occur through the BG

Question 24

input and output of BG

Answer 24

receives diverse cortical information from different systems (sensory-motor,limbic and cognitive)

output modulates activity in the Thalamus to shape selection and self-initiation of appropriate behaviours (modulation of thalamus + cortical state)

Question 25

different learning systems

Answer 25

cortex = learning relationships between stimulus conditions and responses BG = reinforcement learning cerebellum = learning through error correction

Question 26

what does reinforcement learning rely on?

Answer 26

reward and punishment, rather than error reinforcement signal does not directly provide information about how to improve performance – uses reward-based learning to maximise future rewards and yield intelligence behaviour (requires time to remember associations)

Question 27

What is a exploration exploitation behaviour

Answer 27

This is when we have a tension between using a policy that we already know will yield a reward or exploring the environment to try out different policies if they yield superior rewards

Question 28

Rewards and dopamine

Answer 28

All types of rewards release dopamine -dopamine drives plasticity in the basil ganglia , fine tuning the strength of synaptic connections This predisposes the selection of action over others

Question 29

what does intracortical electrical stimulation (ICS) or reward cause?

Answer 29

Stimulating the substantia nigra causes a release of dopamine from SNc to the striatum.

Question 30

different pathways of dopamine release

Answer 30

Mesolimbic pathway – VTA to ventral striatum (NAcc) Mesocortical pathway – VTA to frontal cortex Nigrostriatal pathway – SNc to striatum

Question 31

what is ICS/ dopamine seen as?

Answer 31

as a reward signal, which potentiates the glutamate transmission in the cortex and leads to strengthening the connections between the cortex and striatum for whichever motor plan is active

Question 32

what does repeated application of dopamine rewards cause?

Answer 32

leads to LTP of their synapses, to reinforce behaviour and lead to habit forming

Question 33

reward and punishment in parkinsosn disease

Answer 33

levels of dopamine has a direct impact on the extent we use reward or punishing information to guide our learning overflow of dopamine= more driven by reward parkinsons= driven by punishment

Question 34

dorsolateral prefrontal cortex

Answer 34

recently evolved part of the brain undergoes prolonged periods of maturation lasting until adulthood and is heavily connected to the BG and dopamine. is active when considering cost-benefit of decision-making

Question 35

what does the wisconsin card sort test do?

Answer 35

assesses rule changes and what it does to the DLPFC observes the executive control of behaviour – DLPFC lesions experience: perseverance errors (unsuccessful strategy of prev strategy) and capricious errors (abandon successful rule when still appropriate )

Question 36

two models of action selection

Answer 36

1. the central executive (traditional view) 2. affordance competition

Question 37

what is the central executive view?

Answer 37

we respond to our environment by first using sensory input to perceive the world around us, then make a central executive decision and implement this through the motor system strict order

Question 38

separate modules in the central executive view

Answer 38

Perception, cognition, and action (sense, think, act) are three separate modules within the serial model of action selection

Question 39

what is the motor system in the central executive view?

Answer 39

the motor system is simply the output stage

Question 40

what is the affordance competition model?

Answer 40

simultaneous parallel systems are used as actions compete within sensorimotor maps, which are influenced by various factors such as rewards

Question 41

how are decisions made in the affordance competition model, through what?

Answer 41

through a ‘distrributed consensus,’ meaning motor control is integral to decision making.

Question 42

what is an affordance

Answer 42

the properties of objects that define its possible use (e.g. door handle with knob) if an object has a clear affordance, we will have pre-encoded motor plans before selection, , rather than the central exec where we would need a model of the motor control

Question 43

affordance competition and executive control

Answer 43

executive control biases the competitive processing of action selection in particular directions depending on its affrodance. so each subsystem bias a possible choice Multiple candidate actions are considered, and competition narrows down choices as information accumulates

Question 44

BG and the affordance competition model

Answer 44

decision-making involves consensus among various brain regions, with the BG influencing behavioural biasing to each region until a specific action is determined

Question 45

Affordance model study

Answer 45

activation for both possible motor programmes but then strong activity in neurons long before motor ,movement/ programmed implemented under central exec= not be able to plan the action until we know which action we should plan however in reality we plan for both, directly contributing to the process of decision making

Question 46

affordance model brain imaging evidence

Answer 46

comparing activity in the motor cortex with pre motor indicates processing of go and no go cues is performed in both areas all occurring around 150 milliseconds - Doesn’t happen later

Question 47

Implications of the affordance model

Answer 47

motor plans are already encoded in the motor system before decision to act is made consensus tells our consciousness a decision has been made before us thinking--> lack of free will

Question 48

Free will- affordance model findings

Answer 48

There is brain activity that rep the movement that you will choose before consciously encoding the decision Same can be seen even with imagining movement --> implications for rehibilitation/ neural prosthetic device for paralysed patients