Week 13: Basal Ganglia/ Nuclei Flashcards
What is the primary function of the basal ganglia?
- to provide a feedback mechanism to the cerebral cortex for normal initiation, control and cessation of movement
- receives and interprets information on plan and sequence of movement from cortex
- relays decision to move through thalamus
- modulation of voluntary motor activity
How can the basal ganglia modulate voluntary motor activity?
- Excite cerebral cortex: facilitate wanted movement
2. Dampen cerebral cortex: inhibit unwanted movements
What will lesions to the basal ganglia result in?
dyskinesia - abnormal, involuntary movements
What is meant by ‘enhanced inhibition’
an excitatory neurone exciting an inhibitory neurone
What is meant bu disinhibition?
inhibitory neurone inhibits an inhibitory neurone, so the neurone stays active
What structures are considered to be in the ANATOMICAL basal ganglia?
- corpus striatum
- amygdala
- neostriatum
- paleostriatum
- caudate nucleus
- putamen + globus pallidus = lentiform nucleus
Explain the make up of the functional basal ganglia
- Corpus striatum = neostriatum + paleostriatum
Neostriatum = caudate nucleus + putamen
Paleostriatum = globus pallidus
- Globus pallidus = internus pars reticulata (Gpi) + externus (Gpe)
- Substantia nigra = internus pars reticulata + pars compacta
- Subthalamic nucleus
What are the two portions of the globus pallidus?
Internus (Gpi) and externus (Gpe)
What are the two portions of the substantia nigra?
Pars reticulata and Pars compacta
What makes up the corpus striatum?
- head of caudate nucleus
- putamen
- globus pallidus internus & externus
Where does the subthalamic nucleus originate from?
diencephalon
Where does the substantia nigra originate from?
mesencephalon
What is the difference between the pars reticulata and pars compacta in the substantia nigra?
pars compacta = region that contains many tiny black stains because it contains dopamine-releasing neurones as melanin is a by product of dopamine which is why it stains black
pars reticulata = anterior to pars compacta, does not stain black
Explain the functions of the basal ganglia
primary function: normal initiation, control and cessation of movement
other functions:
- cognitive processes - perception, learning, memory, abstract thought
- emotion
Explain the circuitry of the basal ganglia
- Cortex sends afferents to the input nuclei of the basal ganglia (caudate + putamen) - excitatory
- Input nuclei send connections to output nuclei (Gpi + pars reticulata) - inhibitory
- Output nuclei (always inhibitory) communicate through thalamus back to the cortex
What is the ‘default’ pathway?
output nuclei are always inhibiting the thalamus as the thalamus is always trying to excite the cortex
How is the caudate nucleus anatomically related to the lateral ventricle?
caudate nucleus forms lateral wall of the lateral ventricle
Which artery is the main supply to the basal ganglia?
middle cerebral artery
What are the two types of afferents that the putamen and caudate receive when we want to initiate/ modulate movement?
- Corticostriatal
- excitatory
- comes from the cortex and contains information about intended movement - Nigrostriatal
- comes from the substantia nigra pars compacta
- release dopamine
- effect is excitatory or inhibitory depending on the type of receptor on the post synaptic neurone
What are the two routes that afferents can use to influence movement?
- Direct route
the neurone coming from the striatum goes directly to the GPi of the globus pallidus - The neurone coming from the striatum goes through the GPe of the globus pallidus
Explain how efferent fibres are sent to the thalamus?
- Efferent fibres leave the GPi and SNr complex (output nuclei)
- these areas send efferent fibres to supply the thalamus
- Pallidothalamic fibres inhibit the thalamus
What is the default state of efferent connections?
the thalamus is under chronic inhibition by pallidothalamic fibres from GPi + SNr neurones
Are thalamocortical fibres excitatory or inhibitory?
inherently excitatory to cortex i.e they facilitate movement
Explain broad control of movement between the cortex, basal ganglia and thalamus
- Corticostriatal afferent fibres from the cortex to the striatum. These neurones are always excitatory
- At this point, we have a direct route from the striatum to the globus pallidus internus. This neurone is always inhibitory
- Or we have an indirect route from the striatum to the globus pallidus externus. This neurone is always inhibitory
- From the globus pallidus internus, connections go to the thalamus. These are always inhibitory
- From the thalamus, we have thalamocortical fibres that are always excitatory
Explain how the direct pathway facilitates movement
when we want to facilitate an intended movement:
- DP neurones are excited
- so activity of the neurone from the putamen to the GPi will go up
- the activity of the next neurone will go down as we’ve enhanced this inhibitory neurone
- therefore we decrease inhibition of the thalamocortical neurone so it remains active
- so we facilitate movement
Explain how the indirect pathway inhibits unwanted movements
- excitatory neurone excites the activity of the neurone going from the putamen to the GPe
- from the GPe, we send an inhibitory neuron to the subthalamic nucleus so the activity of the next neurone goes down
- neurone goes from subthalamic nucleus to GPi is always excitatory
- so the neurone from the GPi to the thalamus is going to inhibit the neurone going from the thalamus to the cortex
- so we can inhibit unwanted movements
Summarise the direct and indirect pathway
Direct pathway: facilitate movement where neurones from the striatum project to the GPi directly
-leads to excitation of the cortex from the thalamus
Indirect pathway: inhibit a specific movement
- neurones from the striatum project to the GPe
- from the GPe, neurones project strongly to the subthalamic nucleus which later projects to the GPi
- leads to inhibition of the cortex from the thalamus
Explain the role of substantia nigra pars compacta
- structure in the midbrain
- releases dopamine to excite the cortex and initiate movement
- dopamine facilitates movement initiation via nigrostriatal neurones through the direct and indirect pathway
Why do patients with parkinsons have problems initiating movement?
dopamine releasing neurones degenerate
What are the signs of lesions to the basal ganglia?
- disordered movement (not paralysis) on the contralateral side
- hypokinesia (slow movement)
- walk slowly, little steps
- external initiation
- cannot initiate facial expression (‘mask face’)
- rigidity
- tremor
What is the characteristic feature of huntington’s disease?
chorea
What is hemiballismus?
- degeneration of the subthalamic nucleus
- hyperkinetic disorder - increased unwanted movements occur
