Basal Ganglia Flashcards
What are the three major input nuclei of the basal ganglia?
Striatum.
Subthalamic nucleus.
Substantia nigra pars compacta.
The striatum is a collective term for what?
Caudate nucleus and putamen.
What is the largest nucleus of the basal ganglia?
Striatum.
Where does the striatum receive direct input from?
Cerebral cortex.
Brainstem.
Substantia nigra pars compacta.
Midbrain raphe.
Pedunculopontine nucleus.
The basal ganglia receives input about which four classes of processes?
Motivational.
Emotional.
Cognitive.
Sensorimotor.
Where does the subthalamic nucleus receive input from?
Frontal cortex.
Thalamus.
Brainstem.
What is unique about the subthalamic nucleus?
It is the only component of the basal ganglia that has excitatory (glutamatergic) output connections.
Where does the subthalamic nucleus project to?
Output nuclei and intrinsic external globus pallidus.
The substantia nigra pars compacta contains what population of neurons?
An important population of dopaminergic neurons.
What do the dopaminergic neurons in the substantia nigra pars compacta give rise to?
Nigrostriatal dopamine projections.
Where does the substantia nigra pars compacta receive input from?
Other basal ganglia nuclei.
Brainstem structures.
Frontal cortex.
Amygdala.
What does the pattern of connectivity in the substantia nigra pars compacta suggest?
That the most important direct influence over the dopaminergic neurons arises from evolutionary ancient parts of the brain.
What is the role of the internal globus pallidus?
It is one of the two principal output nuclei.
Where does the internal globus pallidus receive input from?
Other basal ganglia nuclei.
Where does the internal globus pallidus project to?
External targets in the thalamus and brainstem.
Describe the neurons of the internal globus pallidus.
GABAergic with high levels of tonic activity.
What is the role of the substantia nigra pars reticulata?
It is the second principal output nucleus.
Where does the substantia nigra pars reticulata receive inhibitory (GABAergic) inputs from?
Striatum.
Globus pallidus.
Where does the substantia nigra pars reticulata receive excitatory input from?
Subthalamus.
What type of neurons are in the substantia nigra pars reticulata?
GABAergic.
Where do the substantia nigra pars reticulata neurons provide output to?
Thalamus.
Brainstem.
Midbrain.
Medullary reticular formation.
In the traditional model of the basal ganglia, where are signals originating in the cerebral cortex distributed to?
Two populations of medium spiny output neurons in the striatum.
In the traditional model of the basal ganglia, what is the proposed direct pathway?
Neurons containing a preponderance of D1 dopamine receptors make direct inhibitory contact with the basal ganglia output nuclei.
In the traditional model of the basal ganglia, what is the proposed indirect pathway?
Striatal neurons containing mainly D2 dopamine receptors make excitatory contact with the output nuclei via relays in the globus pallidus and subthalamus.
Summarise the traditional model of the basal ganglia.
Basal ganglia output was thought to reflect a cortically determined balance between inhibitory and excitatory projections terminating on the two output structures (internal globus pallidus and substantia nigra pars reticulata).
List three recent findings that have shown that the internal circuitry of the basal ganglia is more complex that the traditional model implies.
Medium spiny neurons of the direct pathway also provide collateral input to the globus pallidus.
Globus pallidus neurons also make direct contact with the output nuclei in addition to the traditional indirect connections to the subthalamus.
The globus pallidus also projects back to the striatum and to the structures outside the basal ganglia.
The connections between the cerebral cortex and basal ganglia can be viewed as a series of what?
Re-entrant parallel projecting, partly segregated, cortico-striato-nigro-thalamo-cortical loops or channels.
What implies that the re-entrant loop architecture through the basal ganglia must have predated the evolutionary expansion of the cerebral cortex?
The concept of parallel projecting re-entrant loops through the basal ganglia has been extended to their connections with sensorimotor and motivational structures in the brainstem.
Name some sensorimotor and motivational structures in the brainstem.
Superior colliculus.
Periaqueductal grey.
Pedunculopontine nucleus.
Parabrachial nucleus.
For the cortical loops, where is the thalamic relay located?
On the output side of the loop.
For the subcortical loops, where is the thalamic relay located?
On the input side of the loop.
What is the fundamental selection problem?
Determining which functional system should be allowed to direct behavioural output at any point in time, e.g. choosing between talking and drinking because it is impossible to do both at the same time.
What is the purpose of selective attention?
The sensory systems are continually faced with multiple external stimuli, one of which may drive a movement incompatible with one specified by others. It is important to select a stimulus that will become the focus of attention and direct movement.
Describe the input filter of the basal ganglia.
Cortical and some thalamic inputs make comparatively few contacts with individual striatal neurons, so a large population of sufficiently synchronised excitatory inputs is required to depolarise the membrane of a medium spiny neuron enough to fire an action potential.
How does the input filter of the basal ganglia promote selection?
It excludes weak or less biologically significant competitors.
What causes highly activated striatal elements to suppress activity in adjacent more weakly activated channels?
Local GABAergic inhibitory collaterals between striatal spiny neurons and longer-range inhibitory effects of interneurons.
What is the effect of the combination of focused inhibition from the striatum with the more diffuse excitation from the subthalamus?
Decrease activity in selected channels and increase activity in non-selected channels in basal ganglia output nuclei.
What is the effect of local inhibitory collaterals between output nuclei neurons?
Further sharpen the difference between inhibited and non-inhibited channels.
What is considered the principal function of the basal ganglia?
To select between competing and incompatible behavioural options.
What is considered the basic substrate for selection?
The parallel loops originating from and returning to diverse cortical and subcortical functional systems.
Explain the concept of normal selection within the basal ganglia.
A reduction in inhibition of selected channels below a proposed selection threshold (central channel) while maintaining or increasing inhibition of non-selected channels (left and right channels).
What might account for akinesia in Parkinson’s disease?
Insufficient reduction in tonic inhibition of all channels, meaning no target structure would be sufficiently disinhibited.
What might account for the distractibility and disordered train of thought in schizophrenic and ADHD patients?
A failure to adequately disinhibit the selected channel or suppress disinhibitory activity in competing channels would cause current selections to be vulnerable to interruption.
What might account for the hard selections seen in OCD and addictive behaviours?
One channel may become pathologically dominant either through abnormal disinhibition of the selected channel or excessive tonic inhibition of competing channels. This would make the relevant option easy to select and highly resistant to interruption.
What are the three main symptoms of Parkinson’s disease?
Akinesia.
Bradykinesia.
Rigidity.
Define akinesia.
Difficulties in initiating movement.
Define bradykinesia.
Initiated movements are slow.
Define rigidity.
Stiffness and resistance to passive movement.
What is thought to be the principal neurological deficit responsible for the motor symptoms of Parkinson’s disease?
Progressive degeneration of dopaminergic neurotransmission in the basal ganglia.
Describe how Parkinson’s disease impairs the normal selective function of the basal ganglia.
Since the output of the basal ganglia is GABAergic, in Parkinson’s disease, targeted structures receive high and uneven levels of inhibitory input.
In the early stages of Huntington’s disease, where is neuronal degeneration most evident?
In the striatal medium spiny neurons.
In Huntington’s disease, what is chorea?
