Basal Ganglia Flashcards
Label the diagram showing the pathways and centres involved in movement
What are the roles of the various components involved in movement?
- Cerebellum fine-tines and coordinates movement initiated in the cerebrum
- basal ganglia acts as an “on-off” switch that determines if movement initiated in the cortex is appropriate or not
- all information is relayed via the thalamus
Where are movement signals initiated?
Signals originate in the cerebral cortex
They are modulated by a variety of subcortical structures, including the basal nuclei
How do the cerebellum and basal ganglia act as control centres?
Control centres regulate the activity of descending pathways
cerebellum - involved in coordination and synergy
basal ganglia - normal initiation, control and cessation of movement
What is the primary function of the basal ganglia (nuclei)?
To provide a feedback mechanism to the cerebral cortex for initiation, control and cessation of motor response (movement)
it determines whether a movement (from the cortex) is appropriate or not
How does the basal ganglia perform its primary function?
- It receives and interprets information on plan and sequence of movement (from cortex)
- it relays the decision to move, via the thalamus
it can excite the cerebral cortex to facilitate wanted movement
it can dampen the cerebral cortex to inhibit unwanted movement
- it is involved in modulation of voluntary motor activity
What can lesions of the basal ganglia lead to?
Dyskinesia
these are abnormal, involuntary movements
it can be hypokinetic or hyperkinetic
What is the difference between hypokinetic and hyperkinetic?
Hypokinetic:
- Partial or complete loss of muscle movement due to a disruption in the basal ganglia
Hyperkinetic:
- increase in muscular activity that can result in excessive abnormal movements, excessive normal movements or a combination of both
What would be the overall effects on the structures in each of the following situations?
Label the basal ganglia
What are the anatomical basal ganglia?
Masses of grey matter in the forebrain which are closely related anatomically / embryologically
they are found in close proximity and develop from the same embryonic tissue
they are not necessarily functionally related
What are the 5 components of the functional basal ganglia (nuclei)?
- Caudate nucleus
- Putamen
- Globus pallidus
- Substantia nigra
- Subthalamic nucleus
the putamen and globus pallidus make up the lentiform nucleus
What are the 2 components of the corpus striatum?
Where is this found?
The corpus striatum is found within the telencephalon of the forebrain
Neostriatum:
- consists of the caudate nucleus and the putamen
- these function as one unit
Paleostriatum:
- globus pallidus
- this is divided into the internus (GPi) and externals (GPe)
- this forms later in development
What are the 2 components of the globus pallidus?
Globus pallidus internus (GPi)
Globus pallidus externals (GPe)
Where is the substantia nigra found?
What are the 2 components?
It is ‘mesencephalic’ as it is located in the midbrain
Pars reticulata (SNr) and pars compacta (SNc)
The substantia nigra has this name as it stains dark
What is significant about the pars reticulata of the substantia nigra?
The globus pallidus internus (GPi) and pars reticulata (SNr) share functions and act as one unit
SNr is functionally part of the GPi
Complete the diagram
Label the basal ganglia involved in the corpus striatum
What are the components of the striatum?
How are they separated?
The caudate nucleus and the putamen
the internal capsule separates these 2 structures
they are fused in some areas by cellular bridges
How is the internal capsule related to the basal ganglia?
It carries information between:
caudate and putamen
and
globus pallidus and thalamus
What are the 4 divisions of the diencephalon?
- Thalamus
- epithalamus
- hypothalamus
- subthalamus
Label the components of the subthalamic nucleus of the basal ganglia
The subthalamic nucleus is close to the tegmentum of the midbrain
It is located laterally and below the thalamus
Label the components of the substantia nigra (mesencephalic)
Which part stains darker and why?
The SNc (pars compacta) produces dopamine, leading to the production of melanin
melanin leads to this area staining darker
Which area of the basal ganglia is implicated in Parkinson’s disease?
The pars compacta of the substantia nigra
this is due to dysfunction in the neurones which produce dopamine
What is the primary function of the basal ganglia?
What are the other functions?
Primary function:
- normal initiation, control and cessation of movement
Other functions:
- cognitive processes - perception, learning, memory and abstract thought
- emotion
Label the components of the circuitry of the basal ganglia
There are multiple, parallel looping circuits from the cerebral cortex back to the cerebral cortex via the thalamus
these modulate cortical output
input nuclei:
- caudate nucleus and putamen
- together they make up the striatum
output nuclei:
- globus pallidus internus (GPi) and substantia nigra reticulata (SNr)
- these communicate to the thalamus
What is the default circuitry fo the basal ganglia?
Inhibitory pathway
this stops the thalamus from initiating movement
this pathway is spontaneously firing all the time unless the cortex excites input nuclei and inhibition of the thalamus is removed
What are the 2 different afferent connections of the basal ganglia?
Corticostriatal and nigrostriatal
What is the relationship between the input nuclei and afferent connections?
Input nuclei - striatum (caudate and putamen)
these are receiving areas of afferent connections
What are the 2 forms of afferent projections to the striatum?
From the cortex:
- excitatory corticostriatal fibres
- these contain information about intended movement
From the substantia nigra pars compacta:
- nigrostriatal fibres release dopamine
- the effect is excitatory or inhibitory depending on the type of receptor on the postsynaptic neurone
What is shown in this diagram?
Afferent connections
Orange shows the excitatory corticostriatal neurones from the cortex to the striatum
Red shows the nigrostriatal (excitatory or inhibitory) neurones from the substantia nigra to the striatum
What is the efferent connection pathway of the basal ganglia?
The pallidothalamic pathway
What is the relationship between the output nuclei and the efferent connections?
The output nuclei are GPi + SNr
these areas send efferent fibres to supply the thalamus
these are pallidothalamic fibres which are inhibitory
these fibres tonically inhibit the thalamus
What fibres travel from the thalamus to facilitate movement as part of the efferent connections?
The thalamus is the way to talk back to the motor cortex (mainly premotor and supplementary motor areas)
thalamocortical fibres are inherently excitatory to the cortex (facilitate movement)
the motor cortex then influences LMNs via the corticospinal pathway and extrapyramidal pathways
What is shown in this diagram?
The efferent connections
this is the pallidothalamic pathway
it is ALWAYS inhibitory and runs from the GPi to the thalamus
What is the default circuit of the basal ganglia?
The thalamus is under chronic inhibition by pallidothalamic fibres from GPi and SNr neurones
What is the role of the direct pathway of the basal ganglia?
The direct pathway facilitates a specific movement programme
It involves neurones from the striatum that project to the GPi directly
What neurones are involved in the direct pathway?
- Starts from the striatum (caudate and putamen)
- Neurones of the striatum are excited by the cortex
- The neurones running from the striatum to the internal globus pallidus are inhibitory neurones
- The neurones running from the internal globus pallidus to the thalamus are also inhibitory neurones
- From the thalamus, excitatory pathways go to the cortex where they affect the planning of the movement by synapsing with the neurones of the corticospinal and corticobulbar tracts
What is the role of the indirect pathway of the basal ganglia?
To inhibit a specific movement programme, to allow the direct pathway’s programme
Involves neurones from the striatum that project to GPe
from the GPe, neurones project strongly to the subthalamic nucleus
this later projects back to GPi
When flexing the fingers, what are the roles of the direct and indirect pathways?
Contract flexors - direct pathway
relax extensors - indirect pathway
What neurones are involved in the indirect pathway?
- The indirect pathway begins in the striate (putamen and caudate)
- Inhibitory neurones from the striatum enter the globus pallidus externus
- Inhibitory neurones run from the globus pallidus externus to the subthalamic nucleus
- Excitatory fibres run from the subthalamus back to the globus pallidus internus
- Inhibitory fibres run from the globus pallidus internus to the thalamus
- Excitatory fibres run from the thalamus to the cortex
What is the final result of the indirect pathway?
The striatum inhibits the globus pallidus externus, which causes disinhibition (still active) of the subthalamus
the neurones of the subthalamus become more active and excite the globus pallidus internus
this loop inhibits the thalamic nuclei
this leads to decreased activity of the cortical motor neurones and inhibition of movement
How must the basal nuclei “balance” to influence movement?
To influence movement, the basal nuclei function through balancing between inhibition and disinhibition (release from inhibition) of thalamocortical pathways
What are the 2 roles of the basal ganglia?
Through which pathways are these actions acheived?
Facilitate ongoing movements that are required and appropriate:
- acheived through the direct pathway
- leads to excitation of the cortex from the thalamus
inhibit unwanted movements:
- acheived through indirect pathway
- leads to inhibition of the cortex from the thalamus
What is the role of the substantia nigra pars compacta?
Its main function is to initiate movement
it releases dopamine
it has a modulatory role in the direct and indirect pathways
(dopamine provides a burst of excitation to the cortex to initiate movement from rest)
How does the substantia nigra pars compacta initiate movement?
It excites the direct pathway
it will inhibit the indirect pathway
(the indirect pathway inhibits movement, so the substantia nigra pars compacta must remove the inhibition)
what are the potential consequences if the substantia nigra pars compacta is diminished?
Inability to initiate movement
this is seen in Parkinson’s disease
What happens if there is a lesion of the basal ganglia?
Disordered movement (not paralysis) on the contralateral side
There is slow movement (hypokinesia) with poor initiation:
- walking slowly with little steps
- external initiation - someone is needed to push you
- mask face - cannot initiate facial expression and infrequent blinking
also rigidity and tremor
What is the most common disease of the basal ganglia?
It involves disruption of the nigrostriatal input
this leads to Parkinson’s disease
there is degeneration of neurones in the substantia nigra pars compacta
What are other degenerative changes that can affect basal ganglia circuitry?
Other degenerative changes lead to hyperkinesia
this is the occurrence of unwanted movements
What is Hemiballismus as an example of a hyperkinetic disorder?
Degeneration of the subthalamic nucleus (excitatory to globus pallidus)
leads to jerky violent unwanted movements on the contralateral side to the lesion
what is chorea (Huntington’s disease) as an example of a hyperkinetic disorder?
Degeneration of inhibitory fibres from the striatum to the globus pallidus
mainly in the GPe
leads to excessive unwanted movements due to an overactive thalamocortical neurone
