Basal ganglia (wk 5) Flashcards
What are the 4 main basal ganglia functions?
- Strategic aspects of movement
- No direct connections to spinal cord
- Reciprocal connections to almost all areas of cerebral cortex
- Damage results in problems initiating and terminating movement
Describe the basal ganglia connections and the cerebellum connections:
-Basal ganglia connections -> Widespread reciprocal connections to cerebral cortex (via thalamus). Not just involved in movement. Behaviour and emotion.
-Cerebellum connections -> Connections only to sensorimotor areas of cerebral cortex (via thalamus). Receives input from brainstem and spinal cord. Involved almost exclusively with movement.
Describe the 5 nuclei which make up the basal ganglia
- Caudate
- Putamen
- Globus pallidus (internal – Gpi and external – Gpe)
- Subthalamic nucleus (STN)
- Substantia nigra (pars compacta SNc and pars reticulata SNr). Caudate and putamen are part of the striatum.
What are the inputs of the basal ganglia?
-> Striatum (caudate and putamen). Receives direct and indirect (via thalamus) connections from cerebral cortex.
What are the internal connections of the basal ganglia?
-> Striatum projects to: Globus pallidus (striato-pallidal pathway) and Substantia nigra (striato-nigral pathway). STN forms indirect pathway between GPe and SNr. GPi and SNr are the output centre of the basal ganglia and sends inhibitory signals to the thalamus.
What are the outputs of the basal ganglia?
-Outputs -> GPi and SNr. Provide tonic inhibitory output to thalamus.
What are the inputs and outputs of the feedback loop:
-> Basal ganglia receives excitatory input from the cerebral cortex which then sends inhibitory signals to the thalamus, in turn supressing motor cortical activity. Excessive basal ganglia input results in slow movement and reduced input results in enhanced movement.
What are the inputs and outputs of the cortical loop and the 3 stages of the transcortical loop:
-> The cerebral cortex, basal ganglia and thalamus form a transcortical loop, comprising of 3 stages:
1. Striatum (putamen) receives vast array of inputs from the cerebral cortex (motor and sensory)
2. Output from GPi and SNr project to the thalamus (inhibitory signal)
3. Inhibited thalamus projects to motor cortex, thus suppressing movement
Describe the 3 neurotransmitters:
- Excitatory (glutamate, open arrows)
- Inhibitory (GABA, filled arrows)
- Mixed (dopamine, mixed arrows)
-STN is a relay from the striatum (input) to the GPi/SNr (output), forming an indirect motor pathway
Describe the direct and indirect motor pathways
-Direct pathways involves two inhibitory synapses resulting in disinhibition, thereby facilitating movement
-Indirect pathways involves excitation of the inhibition, thereby supressing movement
What are the effects of dopamine on the striatum?
-> Dopamine originates from SNc – effects may be excitatory or inhibitory depending upon the receptor it binds to. Dopamine has opposite actions in the direct and indirect pathways. Excites direct pathway – D1R and Inhibits indirect pathway – D2R. Both effects disinhibit thalamic output, thereby facilitating movement.
What is basal ganglia pathology and what are the 2 main diseases?
-Basal ganglia pathology -> Basal ganglia disorders can be explained by an imbalance between the action of direct versus indirect pathways:
1. Parkinson’s -> Increased activation of indirect pathway (inhibitory) and decreased activation of direct pathway (excitatory). Movement is therefore suppressed.
2. Huntington’s -> Decreased activation of indirect pathway (inhibitory) and increased activation of direct pathway (excitatory). Movement is facilitated.
Describe Parkinson’s disease:
-> Affects ~1% of individuals over 50 years of age. Loss of dopaminergic cells in substantia nigra. Increases tonic inhibitory output of basal ganglia. Symptoms include: Bradykinesia, Tremor and Rigidity. Treatments include: deep brain stimulation – disrupts function of STN, inhibits indirect pathway, thereby inhibiting thalamic inhibition.
Describe the loss of dopamine in Parkinson’s:
Normal dopamine uptake on left. Parkinson’s on right shows reduced dopamine uptake in putamen and again 5 years later when the patient became symptomatic.
Describe Huntington’s disease:
-> Rapid, jerky motions with no clean purpose (choreiform = dance-like). Patients often try to integrate these motions into more purposeful movements. Caused by mutation of the huntingtin gene resulting in selective atrophy of striatum. Decreases activation of indirect pathway (inhibitory) resulting in movement facilitation.
