Structure and function of the basal ganglia Part 2 Flashcards
What is the function of the basal nuclei?
Initiation and control of voluntary movements
Also
Eye movements
Learning routine behaviours — habits
Emotional and motivational behavioural responses
What are the motor loops of the basal nuclei?
The direct pathway
The indirect pathway
Nigrostriatal pathway
What does the direct pathway of the basal nuclei do
Acts to allows movement to occur
Facilitates wanted movement
What does the indirect pathway of the basal nuclei do
Inhibits unwanted movement
How does the indirect pathway of the basal nuclei carry out its function
activation enhances inhibitory input to thalamus
no excitation of cortex
Describe the direct pathway of the basal nuclei
Motor cortex sends excitatory signals to the striatum via the corticostriatal pathways (excitatory connections)
Striatum projects to the globus pallidus and substantia nigra pars reticula (inhibitory connections)
project to thalamus which is in close proximity (inhibitory connections)
Through the thalamocortical pathway projects back to cerebral cortex (excitatory connections)
What is important to note about the direct pathway of the basal nuclei
The globus pallidus internal segment has tonic inhibitory activity
What is tonic inhibitory activity
always an action potential firing through the GABAergic inhibitory neurons in the globus pallidus internal segment
What happens to the direct pathway of the basal nuclei at rest
Internal segment of the basal nuclei constantly firing which inhibits thalamic activity so no information os being sent from the thalamus to the cortex
What happens to the direct pathway of the basal nuclei at rest when movement occurs
Neurons along the coticostriatal pathway has an action potential.
Cortex excitatory process which initiates striatum causing firing of the striatum which is an inhibitory GABAergic connection.
Inhibitory neurons inhibits GABAergic inhibitory neurons of the internal segment (disinhibition)
so allows firing from the thalmaus which is excitatory and activates the cortex
What is disinhibition
inhibition of the inhibitory signals
How does the indirect pathway of the basal nuclei carry out its function
Same as direct pathway but at striatum project s to external segment of globus pallidus which projects onto the subthalamus or subthalamic nuclei
Ontop of tonic activity of the internal globus pallidus the external segment is also a tonic inhibitory signal
What happens to the indirect pathway of the basal nuclei at rest
Tonic activity of the internal and external globus pallidus inhibits signals of the thalamus so no information to cortex
What happens to the direct pathway of the basal nuclei at rest when movement occurs
excitatory signal to come from the cerebral cortex to the striatum
inhibitory GABAergic neurons fire an inhibitory signal onto the globus pallidus external segment (GPe)
inhibit inhibitory signals from the GPe
so subthalamus can fire
firing exitatory signals from the subthalamic nucleus leads to stimulation of the globus pallidus internal segment
Increase in tonic activity of the inhibitory neurons
so inhibit excitatory signals from the thalamus to the cortex
What is special about the nigrostriatal pathway
Largest dopaminergic pathway in the brain
Where does the nigrostriatal pathway originate in
Substantia nigra pars compacta (SNpc)
What does the nigrostriatal pathway do?
Balance between direct and indirect pathways
Enhances activation of Dl neurons
Supresses activation of D2 neurons
What are the disorders of the basal nuclei
Parkinson’s disease
Huntington disease
How is Parkinsons disease initiated?
Loss of substantia nigra pars compacta dopamine neurons (nigrostriatal)
Excessive inhibition of GPe = increased inhibition of the thalamus
Trouble initiating willed movements
What are symptoms of Parkinson’s disease
Bradykinesia — slowness of movement
Akinesia — difficulty in initiating voluntary movements
Rigidity — increased muscle tone, stiffness
Resting tremors of hand and jaw
Cognitive deficits, depression, sleep disorders
How is Huntington’s disease initiated?
Loss of striatal neurons in the indirect pathway
Decreased inhibition of the thalamus & subsequent loss of cortical neurons
Failure to suppress unwanted movements
