Motor Pathways: Basal Ganglia and Cerebellum Flashcards
What is the collective term given to the basal ganglia and cerebellum?
Extrapyramidal system
Broadly speaking, what is the role of this system?
(basal ganglia and cerebellum
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)
Contributes to the smoothness of complex action
Involved in moderating and coordinating movements-(movemens done in right order)
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
ie no prjection to STN
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
ie projection to STN
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 correctmotor 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 (exicitatory)
The basal ganglia inhibit the competing motor programmes via the indirect pathway
In summary, 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 hen 80% of the dopamine cells in the substantia nigra have died
What are the consequences of this loss of dopamineric neurones in SNc 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
Tremor
Rigidity – increase in muscle tone
Parkinsonian gait.
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)
disrupts fine balance as motor cortex gets excessive excitatory input–>continuous involuntary commands to muscles
What are the consequences of this (huntingtons) 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 on the patients will develop cognitive decline and dementia.
also affected: speech, dysphagia, unsteady gait
State the 3 lobes of the cerebellum.
Anterior Posterior Flocculonodular
The cerebellum is divided sagitally into 3 zones. What are these zones?
Vermis (midline) Intermediate hemisphere (closest to vermis) Lateral hemisphere
State the 3 layers of the cerebellar cortex.
Granular layer
Pirifor layer (contains purkinje cells)
Molecular layer
What are the 2 sources of input into the cerebellum and what do they connect with?
Mossy Fibres – from the cortex and pons (corticopontine fibres) and from the spinocerebellar tract
Climbing Fibres – from the inferior olive
Functionally, the cerebellum can be divided in three. What are these three divisions?
Vestibulocerbellum Spinocerebellum Cerebrocerebellum
