Basal Ganglia

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 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

structure of the BG

Answer 4

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

Question 5

what does the cortex project to?

Answer 5

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

Question 6

direct pathway of cortex-striatum projection

Answer 6

striatum inhibits the GPi, which inhibits the STN.

this double inhibition releases the Thalamus from inhibition, resulting in an increase in Thalamic activity via disinhibition

(disinhibits the thalamus)

Question 7

indirect pathway of cortex-striatum projection

Answer 7

striatum inhibits the GPe, which inhibits the STN, before exciting the GPi.

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

(increases inhibition of the thalamus)

Question 8

how is the balance between BG pathways regulated?

Answer 8

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

this has one excitatory and one inhibitory connection

Question 9

dopaminergic connections

Answer 9

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

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

Question 10

what are the consequences for neuronal loss in the SNc?

Answer 10

on movement, cognitive control, and decision-making

Question 11

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

Answer 11

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

Question 12

weak direct pathways -> akinesia

Answer 12

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

Question 13

how does akinesia affect action plans?

Answer 13

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 14

cardinal symptoms of PD

Answer 14

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

Question 15

explanations of PD

Answer 15

• degeneration of dopaminergic neurons in the substantia nigra
• disordered motor signals are sent to the SMA
• imbalanced limbic system causes motivation and emotional disturbances

Question 16

what does L-DOPA do?

Answer 16

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

Question 17

why has neurosurgery been suggested to treat PD?

Answer 17

suggested to reduce excess inhibition in the indirect pathway

Question 18

strong direct and weak indirect pathways -> hyperkinesia

Answer 18

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

Question 19

examples of other cortical-based ganglia loops

Answer 19

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

Question 20

what does BG act as during eye movements?

Answer 20

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

Question 21

input and output of BG

Answer 21

receives diverse cortical information from different systems

output modulates activity in the Thalamus to shape selection and self-initiation of appropriate behaviours

Question 22

different learning systems

Answer 22

cortex = learning relationships between stimulus conditions and responses

BG = reinforcement learning

cerebellum = learning through error correction

Question 23

what does reinforcement learning rely on?

Answer 23

reward and punishment, rather than error.

reinforcement signal does not directly provide information about how to improve performance – must use reward-based learning to maximise future rewards and yield intelligence behaviour

Question 24

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

Answer 24

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

Question 25

different pathways of dopamine release

Answer 25

Mesolimbic pathway – VTA to ventral striatum (NAcc)

Mesocortical pathway – VTA to frontal cortex

Nigrostriatal pathway – SNc to striatum

Question 26

what is dopamine seen as?

Answer 26

as a reward signal, which potentiates the glutamate transmission in the cortex and leads to strengthening the connections between the cortex and striatum

Question 27

what does repeated application of dopamine rewards cause?

Answer 27

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

Question 28

dorsolateral prefrontal cortex

Answer 28

undergoes prolonged periods of maturation and is heavily connected to the BG and dopamine.

is active when considering cost-benefit of decision-making

Question 29

what does the wisconsin card sort test do?

Answer 29

observes the executive control of behaviour – DLPFC lesions experience:

perseverance errors (unsuccessful strategy) and capricious errors (abandon successful rule)

Question 30

two models of action selection

Answer 30

1. the central executive
2. affordance competition

Question 31

what is the central executive view?

Answer 31

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

Question 32

separate modules in the central executive view

Answer 32

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

Question 33

what is the motor system in the central executive view?

Answer 33

the motor system is simply the output stage

Question 34

what is the affordance competition model?

Answer 34

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

Question 35

how are decisions made in the affordance competition model?

Answer 35

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

Question 36

affordance competition and executive control

Answer 36

executive control biases the competitive processing of action selection in particular directions.

Multiple actions are considered, and competition narrows down choices as information accumulates

Question 37

BG and the affordance competition model

Answer 37

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