Basal Ganglia Flashcards
Why is the term “basal nuclei” more accurate than “basal ganglia”?
- basal nuclei is more accurate as it describes a collection of cell bodies within the CNS and the basal ganglia are located within the brain
- ganglia refers to cell bodies of neurones located outside the CNS / in the periphery
Where do signals for movement originate from?
How do the basal ganglia influence these signals?
- signals for movement originate in the cerebral cortex, but are modulated by a variety of subcortical structures
- the subcortical structures (including the basal ganglia) ensure that movements are appropriate (e.g. timing, force, speed)
What is the primary function of the basal ganglia?
What other structure is involved in allowing this to happen?
- the primary function is to provide a feedback mechanism to the cerebral cortex for the initiation, control and cessation of the motor response (movement)
- the basal ganglia is invovled in the modulation of voluntary movement
- it can only provide a feedback mechanism to the cortex via the thalamus
Where does the basal ganglia receive inputs from?
What different types of outputs can it send and what does this depend on?
- the basal ganglia receives a copy of the package of intended movement created by the cortex
- it receives and interprets this information relating to the direction and amplitude of movement
- it then relays the decision to move via the thalamus:
- it can excite the cortex to facilitate wanted movement
- it can dampen down the cortex to inhibit unwanted movement
- the basal ganglia (via the thalamus) can either be excitatory or inhibitory to the cortex depending on what type of movement is appropriate at any given time
What are the 5 structures of the functional basal ganglia?
What do they have in common?
- caudate nucleus
- putamen
- globus pallidus
- substantia nigra
- subthalamic nucleus
- they all share connections and are all involved in the modulation of voluntary movement
What structures are encompassed by the corpus striatum (“striped body”)?
corpus striatum = neostriatum + paleostriatum
(Neo)striatum:
- this is a term used to refer to the caudate nucleus and the putamen
- they act together as one structure
Paleostriatum:
- this refers to the globus pallidus, which can be divided into 2 parts:
- globus pallidus internus (GPi)
- globus pallidus externus (GPe)
- the globus pallidus and the putamen together form the lentiform nucleus
- this is because they have the appearance of the lens / lentil
Where is the substantia nigra found and how can it be divided into 2 parts?
- located within the midbrain
Substantia nigra pars reticulata:
- contains diffuse neurones and appears pale in colour
Substantia nigra pars compacta:
- contains dopaminergic neurones that are clustered close together
- appears dark in cross-section as melanin is produced as a by-product of dopamine production
What parts of the corpus striatum and substantia nigra are functionally the same?
- the substantia nigra pars reticulata and globus pallidus internus are functionally the same
- they are referred to as one structure as they have exactly the same function, even though they are in anatomically different locations
What is the difference between the primary and secondary functions of the basal ganglia?
Primary function:
- involved with the normal initiation, control and cessation of voluntary movement
Secondary functions:
- involved with the cognitive and emotional aspects of movement
- e.g. how someone’s body language reflects the way they are feeling
Describe the neurones and structures involved in the default circuit of the basal ganglia
What needs to be done in order to initiate movement?
Structures involved:
- cortex sends a package of intended movement to the input nuclei
- these are the caudate nucleus + putamen (neostriatum)
- the input nuclei relay this information to the output nuclei
- these are the GPi + SNr
- the output nuclei send connections to the thalamus
The thalamus is the ONLY structure that is capable of communicating with the cortex
Neurones involved:
- the neurones travelling from the thalamus to the cortex are excitatory
- if these were active all the time, we would be moving all the time
- the thalamus is dampened down by inhibitory neurones travelling from the output nuclei
To initiate movement, we need to remove the inhibition of the thalamus by the output nuclei
- the default state is inhibition of the thalamus, which can be removed or enhanced depending on whether movement is required or not
What happens if there is a lesion to the basal ganglia?
What are the 2 different types of lesion?
dyskinesia - abnormal, involuntary movements
Hypokinetic lesion:
- occurs when a lesion causes the outputs from the basal ganglia to be excessive
- this leads to movement being slowed
Hyperkinetic lesion:
- occurs when a lesion causes the outputs from the basal ganglia to be reduced
- this leads to abnormal and involuntary movements that occur during rest
How can the locations of the thalamus and caudate nucleus be described using the ventricular system as a landmark?
- the thalamus forms the lateral border of the IIIrd ventricle
- the caudate nucleus follows the C-shape of the lateral ventricle and is located lateral to the anterior horn and body
- the tail of the caudate nucleus forms the roof of the inferior horn of the lateral ventricle
What structure lies lateral to the caudate nucleus and what lies lateral to this?
- the internal capsule is located lateral to the caudate nucleus
- the lentiform nucleus is located lateral to the internal capsule
- from medial to lateral:*
- lateral ventricle (most medial)
- head of caudate
- internal capsule
- lentiform nucleus (most lateral)
What 3 structures make up the corpus striatum?
How are they connected?
- globus pallidus
- putamen
- caudate nucleus
- the globus pallidus and the putamen together form the lentiform nucleus
- the putamen and the caudate nucleus are separate, but are connected in several locations by cellular bridges
- these structures have the same embryological origin so started as one single mass of grey matter
What are the relationships between the caudate nucleus and the lateral ventricle?
- the caudate nucleus is C-shaped and follows the course of the lateral ventricle
- the head and body of the caudate form the lateral wall of the lateral ventricle (anterior horn and body)
- the tail of the caudate forms the roof of the inferior horn of the lateral ventricle within the temporal lobe
Where is the septum pellucidum found?
What can be seen if this is removed?
- the septum pellucidum spans fron the corpus callosum** to the **columns of the fornix
- if the septum is removed, the lateral ventricle is exposed
- this allows viewing of the caudate nucleus, which is located lateral to the lateral ventricle
- to view the tail of the caudate nucleus, we would need to enter the temporal lobe
What is represented by 4, 5, 13 and 14 in this superior cadaver view?
- 5 is the septum pellucidum which lies between the 2 exposed lateral ventricles
- the body grey matter lateral to the lateral ventricle is the caudate nucleus (4)
- lateral to the caudate nucleus are fibres of the internal capsule (14)
- lateral to the internal capsule is the lentiform nucleus (13)
What is used as a landmark when looking at the corpus striatum in different planes?
How can the putamen and globus pallidus be distinguished from each other?
ventricular system
- the head of the caudate nucleus forms the lateral border of the lateral ventricle (anterior horn / body)
- the tail of the caudate nucleus forms the roof of the inferior horn of the lateral ventricle
- the globus pallidus is medial to the putamen and is smaller in size
it also stains paler than the putamen (globus pallidus means “pale globe”)
Complete the diagram
What is the claustrum, extreme capsule and external capsule?
Name the structures from lateral to medial starting at the insula
- the insula is the grey matter cortex located most laterally
- the extreme capsule is a thin layer of white matter located medial to the insula
- the claustrum is a layer of grey matter located medial to the extreme capsule
- the external capsule is a thin layer of white matter located medial to the claustrum and lateral to the putamen
- From lateral to medial:*
- insula
- extreme capsule
- claustrum
- external capsule
- putamen
- globus pallidus
- internal capsule
- thalamus
Where does the putamen originate and extend to?
What separates the putamen and caudate nucleus?
- putamen originates at the level of the head of the caudate and extends posteriorly to the posterior third of the thalamus
- the anterior limb of the internal capsule separates the head of the caudate (medial) from the putamen (lateral)
- the anterior limb is absent rostrally, so the head of the caudate and the putamen are fused
Where is the globus pallidus located in relation to the internal capsule?
What are the functions of the medullary laminae?
- it is located immediately medial to the putamen and lateral to the internal capsule
- the apex** of the globus pallidus sits against the **genu of the internal capsule
- the posterior limb of the internal capsule separates the globus pallidus from the thalamus
- the lateral medullary lamina separates the globus pallidus from the putamen
- the medial medullary lamina separates the globus pallidus into external and internal segments
Where is the subthalamic nucleus located?
- it is located inferior and slightly lateral to the thalamus
- it is almost in the region of the midbrain, but it is part of the diencephalon
What is the difference in the ways the 2 components of the substantia nigra stain?
Where is the internal capsule located at the level of the midbrain?
Pars compacta:
- this is the most posterior part of the substantia nigra
- it stains dark due to the presence of dopaminergic neurones
- melanin is produced as a by-product of dopamine production
Pars reticulata:
- located anterior to the pars compacta
- the neurones located here are more diffuse and it does not stain as dark
Internal capsule:
- the fibres of the internal capsule are located lateral to the substantia nigra as the cerebral peduncles
- the cerebral peduncles are continuous with the internal capsule
What neurones travel from the cortex to the basal ganglia?
corticostriatal neurones
- they travel from the cerebral cortex to the input nuclei of the basal ganglia (caudate and putamen)
Corticostriatal neurones travel from the cortex to the neostriatum
- the putamen is involved in the pure motor aspect of movement
- the caudate has additional roles in the emotional and cognitive processing of movement
Where do striatal neurones travel to and from?
What is their function?
- striatal neurones originate from the caudate nucleus or putamen and travel to the globus pallidus (internus or externus)
- striatal neurones are INHIBITORY
- they do not do anything by themselves unless they are stimulated
- when they are activated by an afferent neurone, they release inhibitory neurotransmitters to inhibit the neurone that they are targeting
Where do nigrostriatal neurones travel from?
What neurotransmitter do they release and what is the action of this?
- they travel from the substantia nigra pars compacta (SNc) to the striatum
- they release dopamine as a neurotransmitter
Nigrostriatal neurones can be either excitatory or inhibitory depending on the receptor expressed by the striatal neurone they come into contact with
- if they come into contact with a striatal neurone expressing a D1 receptor, they are excitatory
- if they come into contact with a striatal neurone expressing a D2 receptor, they are inhibitory
Where do thalamostriatal neurones originate from and what is their effect?
- they originate from the centromedian nucleus of the thalamus and run to the striatum
- they are always excitatory
Where do pallidosubthalamic and subthalamopallidal fibres travel to and from?
How do they travel there?
Pallidosubthalamic fibres:
- travel from the globus pallidus externus (GPe) to the subthalamic nucleus
- these fibres are always INHIBITORY
Subthalamopallidal fibres:
- travel from the subthalamic nucleus to the globus pallidus internus (GPi)
- these fibres are always EXCITATORY
Neurones travelling to and from the subthalamus do so in the subthalamic fasciculus
Describe the 2 different pathways of the pallidothalamic fibres
Where do they travel to and what is their function?
- pallidothalamic fibres travel from the GPi (and SNr) to the thalamus
- some fibres leave the GPi and pass inferior to the subthalamic nucleus - these travel within the ansa lenticularis
- some fibres leave the GPi and pass superior to the subthalamic nucleus - these travel within the lenticular fasciculus
- just before they reach the thalamus, the ansa lenticularis and the lenticular fasciculus come together to form the thalamic fasciculus
- the thalamic fasciculus projects to the VA, VL & centromedian nuclei of the thalamus
Thalamocortical neurones are always excitatory, but they are always being INHIBITED by pallidothalamic neurones from the GPi & SNr
What is the difference in the activity of (neo)striatal neurones and pallidothalamic neurones?
Striatal neurones:
- these pass from the striatum (putamen & caudate nucleus) to the globus pallidus (GPe or GPi)
- they are inhibitory but are quiescent - they do not exert an effect unless they are activated by something else
Pallidosubthalamic neurones:
- these pass from the GPi & SNr to the thalamus via the ansa lenticularis or lenticular fasciculus
- they are always inhibitory and are always active
- they inhibit excitatory thalamocortical neurones that travel from the thalamus to excite the cortex
- this is the default state that inhibits movement
Describe the neurones involved in the direct pathway
- corticostriatal neurone projects from the cortex to the input nuclei (caudate / putamen)
this neurone is excitatory to the quiescent striatal neurones
- striatal neurones are activated and begin to release inhibitory neurotransmitters
- inhibitory neurotransmitters from the striatal neurones decrease the activity of pallidothalamic neurones
- as pallidothalamic neurones are inhibitory, when their activity decreases, there is less inhibition of thalamocortical neurones
- thalamocortical neurones are able to excite the cortex, which can initiate a movement program via the corticospinal / corticobulbar pathways
This pathway facilitates wanted movement by removing inhibition of the thalamus
Describe the neurones involved in the indirect pathway
- corticostriatal neurones excite a quiescent striatal neurone passing from the input nuclei to the GPe
- the activity of the inhibitory striatal neurone is increased
- the inhibitory striatal neurone inhibits the pallidosubthalamic neurone passing from the GPe to the subthalamic nucleus
- as the activity of the inhibitory pallidosubthalamic neurone is decreased, this removes the inhibition of the excitatory subthalamopallidal neurone
- the subthalamopallidal neurone passes from the subthalalmus to the GPi / SNr
- the subthalamopallidal neurone excites the inhibitory pallidothalamic neurone
- increasing the activity of the pallidothalamic neurone enhances the inhibition of the thalamus and further decreases the activity of the thalamocortical neurone
This pathway inhibits unwanted movement by enhancing the inhibition of the thalamus
How does the effect of dopamine depend on which receptor it comes into contact with?
- dopamine + D1 receptor = excitation
- dopamine + D2 receptor = inhibition
- dopaminergic inputs facilitate the initation of movement via both the direct and indirect pathways
What neurone expresses a D1 receptor?
What happens when dopamine comes into contact with this receptor?
- D1 receptors are expressed by striatal neurones of the direct pathway
- When dopamine comes into contact with a D1 receptor, the effect is to be excitatory to these neurones
- the striatal neurone is being excited to a higher extent, so will exert a greater inhibitory effect on the pallidothalamic neurone
- this leads to enhancement of the excitation of the cortex
What neurone expresses a D2 receptor?
What happens when dopamine comes into contact with this receptor?
- D2 receptors are expressed by striatal neurones of the indirect pathway
- when dopamine comes into contact with a D2 receptor, the effect is to be inhibitory to that neurone
- inhibition of the inhibitory striatal neurone increases the activity of the pallidosubthalamic neurone
- there is greater inhibition of the subthalamopallidal neurone, which can no longer excite the inhibitory pallidothalamic neurone
- the activity of the inhibitory pallidothalamic neurone goes down, which removes the inhibition of the thalamus
- thalamocortical neurones are now able to excite the cortex
How does dopamine influence the direct and indirect pathways?
What happens if there is an injury to the SNc (or it is diminished)?
Excitation of direct pathway:
- dopamine has excitatory effects on D1 receptors of fibres travelling from the striatum to the GPi
Inhibition of indirect pathway:
- dopamine has inhibitory effects on D2 receptors of fibres travelling from the striatum to the GPe
Injury to the SNc leads to a person being unable to initiate movement
What are the 4 main symptoms of Parkinson’s disease and why do they occur?
- bradykinesia / hypokinesia
- resting tremor
- rigidity
- expressionless face
- symptoms result from a dopaminergic deficit
