The basal ganglia Flashcards
What structures comprise the basal ganglia?
Substantia nigra (pars compacta = DA + pars reticulata)
Caudate + Putamen (= striatum)
Globus pallidus (internal and external)
Subthalamic nucleus - only excitatory nucleus
What are the main inputs/outputs of the basal ganglia?
Excitatory input from many accessory cortical areas + subcortical ie superior colliculus = Glu; mostly to striatum
Inhibitory output - SNr and GPi are output nuclei; mostly goes via thalamus, also superior colliculus = GABA
What (motor) disorders is basal ganglia dysfunction implicated in?
Parkinon’s, Huntington’s, Tourette’s, tardive dyskinesia, hemiballismus - all involve variations on tremor, bradykinessia, rigidity, involuntary/uncoordinated actions, difficulty initiating movements etc
Also ADHD, schizophrenia (flight of thought)
What is the action selection problem?
Predisposing conditions (sensory input, cognitive state, homeostasis etc) drive desire for specific action
These multiple command systems that are spatially distributed and processing in parallel are all reliant on one common motor path - bottleneck appears
How do we overcome the selection problem?
Distributed (all-toall connectivity) recurrent reciprocal inhibition = too complicated, too many connections
Centralised selection model (ie the basal ganglia) = exploits advantages of distributed switching at a local level (wut), separates problems of perception and motor control from selection, economy of connection
What are some differences in striatal architecture concerning projection neurons?
One pathway to GPi = mostly D1 DA receptors; also express mainly substance P, dynophin and GABA
One pathway to GPe = mostly D2 receptors; express mainly enkephalin, GABA
What is the disinhibition gating hypothesis?
The idea that there must be a mechanism which the BG can switch off/on selected resources
Tonic inhibition from GPi/STN is very strong - small excitatory input to striatum leads to small inhibitory input from striatum = typically not strong enough to reduce output tonic inhibition to thalamus
If there is significant excitatory input to the striatum, this increases the inhibitory striatal output and reduces the tonic inhibition of the output nuclei to the thalamus
What are cortico-basal ganglia loops?
5 main loops according to primary cortical input areas:
Motor (supplementary motor area)
Occulomotor - frontal eye fields
Prefrontal 1 - dlPFC
Prefrontal 2 - lateral orbitofrontal PFC
Limbic - anterior cingulate area (?high level error detection - cognitive dissonance)
How are discrete actions represented in the basal ganglia?
Motor commands concerning body parts are represented topographically
What subcortical structures are important inputs to the basal ganglia?
Superior colliculus; peduncularpontine nucleus; periaqueductal gray
Input via intra-laminar thalamus to basal ganglia which then project back to subcortical structures (loop) and then on to brainstem motor nuclei
What is salience/urgency and how might it be encoded?
Necessary to prioritise task else there may be a series of partially disinhibited actions allowed to progress - poor
Salience is encoded in the strength of signal inputting to the basal ganglia
How do the basal ganglia extract salience from each request?
Medium spiny neurons (MSNs) in the striatum have 50,000 synapses = necessary to be able to differentiate between levels of input strength - may be responsible for salience extraction
Normally inactive due to hyperpolarisation and require substantial input in order to spike - acts as a filter, only significant action requests are processed
Local GABAergic inhibitory circuits within striatum may help modulate MSN polarisation
Also the remaining basal ganglia and their own recurrent recurrent inhibition will also contribute to processing salience
What have rat foraging experiments shown us about action selection?
Motivations - hungry (food deprived), frightened (open space) (arseholes)
Behaviours - keeps to walls and corners (high salience), collects food from centre (salience temporarily greater than fear) and returns to corner to eat
Has been repeated in robots - embodied computations of basal ganglia function - ‘complex’ behaviour based on simple stimulus/drive/output models
What is the role of the substantia nigra pars compacta?
Dopaminergic input to striatum, synapse near the cortical neuron synapses on medium spiny neurons = modulates cortical input way before action potentials are generated
D1 receptors on MSNs = excitatory, increase cAMP; D2 receptors = inhibitory, decrease cAMP
Related to reward Processing - sacadic latencies towards targets in monkeys can be modulated by D1/D2 receptor antagonists
