Basal Ganglia Flashcards
What is the collective term given to the basal ganglia and cerebellum?
Extrapyramidal system
Broadly speaking, what is the role of this system?
It checks that the movement selected by the motor cortex is correct Both parts of the extrapyramidal system has functions aside from modulating motor control
Where is the basal ganglion located?
White matter in the middle of the brain
What are the different parts of the basal ganglia?
Striatum = caudate + putamen
Globus Pallidus = external segment (GPext) + internal segment (GPint)
Substantia Nigra = pars compacta (SNc) + pars reticulata (SNr)
Subthalamic Nucleus
What is the function of the basal ganglia?
It is involved in planning and coordinating movement It elaborates associated movements (e.g. swinging arms when walking) Contribute to the smoothness of complex action Involved in moderating and coordinating movements
Describe the structure of the direct pathway in the basal ganglia.
Fibres initially come from the motor cortex to the striatum (caudate and putamen) Direct Pathway: putamen –> globus pallidus internal segment + substantia nigra pars reticulata
Describe the structure of the indirect pathway in the basal ganglia.
Fibres initially come from the motor cortex to the striatum (caudate and putamen) Indirect Pathway: putamen –> globus pallidus external segment –> subthalamic nucleus –> globus pallidus internal segment
What is the difference in the function of the direct and indirect pathways?
Direct Pathway = excitatory on the motor cortex Indirect Pathway = inhibitory on the motor cortex
What extra component modulates the function of the direct and indirect pathways?
Nigro-striatal pathway
Where do the projections go after leaving the basal ganglion structures?
They go to the thalamus From the thalamus they go to the cortex (supplementary motor area and primary motor area = two regions involved in movement preparation and planning)
Describe how the basal ganglia are involved in choosing correct motor programmes to carry out particular functions.
The basal ganglia and cortex form a processing loop. The basal ganglia enable proper motor programmes (stored in the cortex) via the direct pathway (excitatory) The basal ganglia inhibit the competing motor programmes via the indirect pathway
I.E the basal ganglia and its direct and indirect pathways make sure that appropriate motor commands get transmitted down the hierarchy.
Connections with which parts of the brain allow the basal ganglia to have a role in enabling various cognitive, executive and emotional programmes?
Prefrontal association cortex Limbic cortex
What causes Parkinson’s disease?
Parkinson’s disease is the neuronal degeneration of dopaminergic neurones in the substantia nigra pars compacta It is caused by the progressive depletion of dopaminergic neurones
NOTE: Symptoms only appear when 80% of the dopamine cells in the substantia nigra have died
What are the consequences of this with regards to the circuitry of the basal ganglia?
The loss of nigro-striatal dopaminergic axons in the caudate and putamen mean that the connection between the striatum (caudate and putamen) and the substantia nigra pars compacta is lost. This means that the direct pathway is reduced and so the excitation of the motor cortex is reduced. The lack of excitatory input interferes with the ability of the motor cortex to generate commands for voluntary movement, resulting in poverty of movement.
State the main signs of Parkinson’s disease.
Bradykinesia – slow movement Akinesia – difficulty in the initiation of movements Hypomimic face – expressionless face ‘Pill-Rolling’ Tremor – 4-7 Hz Rigidity – increase in muscle tone
Describe the Parkinsonian gait.
Walking slowly, small steps, shuffling feet, reduced arm swing Stooped posture with head and body bent forwards and downwards
What is Huntington’s disease caused by?
Abnormality on chromosome 4 (autosomal dominant) Caused by the degeneration of GABAergic neurones in the striatum (firstly the caudate and then the putamen)
What are the consequences of Huntington’s disease with regards to the circuitry of the basal ganglia?
The inhibitory effect of the indirect pathway no longer keeps the direct pathway under control so the cortex will be hyperexcitable. Patients will continuously have abnormal movements because the cortex is continuously sending involuntary commands for movements.
What are the main signs of Huntington’s disease and how does the disease progress?
Choreic movements – rapid, jerky, involuntary movements of the body the hands and face are affected first
Chorea gradually increases over time until the patients are totally incapacitated by it
Later, the patients develop cognitive decline and dementia.
