Basal Ganglia Flashcards
Why is the term basal ganglia a misnomer?
Ganglia are collections of cell bodies outside of the CNS
Should be called basal nuclei (nuclei are collections of cell bodies surrounded by white matter).
What constitutes the lentiform nucleus?
Putamen
GP
In which section of the SN is the pars compacta?
RC
The posterior part is the pars compacta
More ventral is the pars reticularis
The traditional classification of BG?
Lentiform nucleus (putamen, GB)
Claustrum
Caudate nucleus
Amygdaloid body
Clinical classification of BG?
Lentiform nucleus
Caudate nucleus
Subthalamus
SN
Corpus striatum=
Caudate nucleus
Putamen
Globus pallidus
Striatum (neostriatum)
Caudate
Putamen
Paleostriatum=
Globus pallidus
Why are the caudate nucleus and putamen grouped as neostriatum?
As they are functionally and embryologically similar
Components of generating motor movement mot
Prefrontal cortex -> Premotor area and SMA -> BG -> Thalamus -> Primary motor cortex-> Corticospinal fibres
Descending connection of cortex with BG
Descending cortical fibres stimulate (glutamate) neostriatum (caudate and putamen) as corticostriatal fibres.
After stimulation by corticostriatal fibres, where do efferents from the striatum go?
GPi (striatopallidal fibres) inhibitory (GABA + Substance P)
What happens after GPi stimulated by striatum?
Sends inhibitory fibres to the thalamus (pallidothalamic fibres) which then feeds back to the cortex (glutaminergic thalamocortical fibres)
When at rest, what is the predominant input on the motor cortex from the basal ganglia?
GPi is actively firing and inhibiting thalamocortical stimulation
How does the direct pathway result in movement?
Corticostriatal fibres cause inhibition of the inhibitory pallidothalamic fibres releasing thalamocortical fibres and allowing corticospinal outflow.
Draw the direct pathway
What is the purpose of the direct and indirect pathway
If you want to flex fingers, not only should the finger flexors activate (direct pathway) but the antagonistic muscles, in this case the finger extensors, should relax, this is mediated through an inhibitory (indirect) pathway
To which part of the GP do striatopallidal fibres of the indirect pathway project?
The GPe
To where do fibres from the GPe project?
Subthalamus
Where do fibres in the indirect pathway project to from the subthalamus?
To the GPi
What fibre loop constitutes the additional part in the indirect pathway?
GPe-> subthalamus -> GPi
What is the consequence of firing of the indirect loop of the basal ganglia
Inhibition of the striatopallidal inhibition of the pallidosubthalamic inhibition of subthalamopallidal excition of the pallidothalamic inhibition of firing.
Net= increased inhibition of thalamocortical firing.
Draw the indirect pathway
Difference between indirect and direct pathway
Direct pathway shorter
Doesn’t involve the subthalamus
Firing results in disinhibition of thalamocortical fibres leading to them firing.
Indirect pathway longer
Involves subthalamus
Firing causes inhibition of thalamocortical fibres.
Which structures in the basal ganglia are derived from the telencephalon?
Caudate
Lentiform nucleus
What is the role of dopamine in the basal ganglia circuit?
Nigrostriatal fibres from the pars compacta project to the putamen and release dopamine on to both the direct and the indirect pathways.
Facilitate initiation of movement.
Therefore it stimulates direct pathway (D1 receptors) and inhibits (D2) indirect pathway.
D1 receptors
Excitatory
D2 receptors
Inhibitory
Dopaminergic vs glutaminergic action on the indirect pathway?
Antagonistic
As what should the pars reticularis of the SN be considered?
Should be functionally considered as downwards displaced GPi as it has the same connections.
What is the counterbalance to dopaminergic action in the putamen?
Cholinergic neurones
Has an inhibitory effect on the direct pathway
Has an excitatory effect on indirect pathway
What is the aetiology of lead-pipe rigidity in PD?
Normally corticoreticular fibres are inhibiting the reticular formation.
In PD there is reduced activity of corticoreticular fibres, this leads to disinhibition of reticulospinal fibres (which normally determine tone)
Subsequently, there will be increased tone in both flexors and extensors.
What is the aetiology of tremor in PD?
Unopposed cholinergic activity in the lentiform nucleus through overstimulation of reverberating circuits.
Dopamine normally prevents these alternating movements at rest.
How does Hungtington’s disease cause chorea?
Causes degeneration of GABAnergic (enkephalin) neurones in the indirect pathway (particularly in caudate nucleus).
Causing loss of inhibition of cortical firing-> chorea
Different classes of dyskinesia
Chorea
Athetosis
Dystonia
Hemiballismus
Tardive dyskinesia
Wilson’s
Pathology of HD
CAG trinucleotide repeat in Huntingtin gene on chromosome 4
Embryological origin of globus pallidus
Diencephalon
What are the three major sources of input onto striatum
Cortex
Intralaminar nuclei of thalamus
Pars compact of the SN
What are the three major destinatinos for pallidal efferents
Ventral lateral and ventral anterior thalamic nucleiS
STN
Brainstem reticular formation
Ventral lateral and ventral anterior thalamic tract
Major outflow pathway of the basal ganglia
Fibres projected from the globus pallidus to the thalamus are carried in two bundles- the ansa lenticularis and the lenticular fasciculus.
These fibre bundles fuse to reach the thalamus as the thalamic fasciculus
Thalamic nuclear fibres project to prefrontal and premotor cortex
