Disorders of the motor system Flashcards
Review Parkinson's flow diagram
Outline the anatomy of the basal ganglia
- Substantia nigra pars compacta
- Striatum (consists of caudate nucleus and putamen)
- Globus pallidus (internal and external segment)
- Lentiform nucleus (consists of putamen and globus pallidus)
- Subthalamic nucleus
How does the basal ganglia communicate with the motor cortex?
- Via the thalamus
- Increased thalamic activity causes increased cortical activity and vice versa
What is the normal function of the basal ganglia
- Unclear
- Probable role in reinforcing appropriate movements and removing inappropriate movements
What reinforces appropriate movements?
- Direct pathways
- Excitatory to motor cortex
What edits out appropriate movements?
- Indirect pathways
- Inhibitory to motor cortex
What role does dopamine play in movement?
- Facilitates movement by exciting the motor cortex
How does dopamine excite the motor cortex?
- Stimulates excitatory D1 receptors on striatal neurones of direct pathway
- Inhibits indirect pathway by activating D2 receptors on striatal neurones
Which motor cortex is regulated by the basal ganglia?
- Basal ganglia regulate ipsilateral motor cortex
- So if SNc is affected unilaterally there are contralateral signs
- Due to decussation of the corticospinal tract
Outline the indirect pathway of the basal ganglia
- Subthalamic nucleus is inhibited
- This then allows glutamate to excite the globus pallidus internal segment
- Because globus pallidus internus is excited, it is able to inhibit the thalamus
- This allows inhibition of movement
Outline the direct pathway of the basal ganglia
- Putamen sends inhibitory signals to Globus Pallidus internal segment
- Because Globus Pallidus is inhibited, it can no longer inhibit the thalamus
- So the thalamus is able to excite the cortex and cause movement
What is the inhibitory neurotransmitter in the basal ganglia?
- GABA
What causes Parkinson’s disease?
- Degeneration of dopaminergic neurones in SNc (substantia nigra pars compacta)
- So patients lose the dopamine-driven facilitation of movement via both pathways
What are the symptoms and signs of Parkinson’s disease?
- Tremor
- Rigidity
- Bradykinesia
- Hypophonia (quiet speech)
- Decreased facial movement
- Micrographia
- Dementia
- Depression
Why does Parkinson’s cause tremor?
- Unclear mechanism
- May be related to dysfunction of indirect pathway which would normally suppress unwanted movements
Why does Parkinson’s cause rigidity?
- Unknown mechanism
- May be related to lack of coordination between agonists and antagonists
Why does Parkinson’s cause bradykinesia?
- Best understood mechanism
- Slow movements due to loss of cortical excitation
Why does Parkinson’s cause hypophonia?
- Bradykinesia of larynx and tongue
Why does Parkinson’s cause decreased facial movement?
- Bradykinesia of face
Why does Parkinson’s cause micrographia?
- Bradykinesia of hands
Why does Parkinson’s cause dementia?
- Possible progression of currently unknown causative agents e.g. protein aggregates
Why does Parkinson’s cause depression?
- Basal ganglia also have a role in cognition and mood
What is Huntington’s chorea?
- Autosomal dominant progressive disorder
- Early onset of around 30-50 years old
What are the early stages of Huntington’s chorea associated with?
- Loss of inhibitory projections from striatum to the external segment of the globus pallidus
- This leads to hyperkinetic features (brakes have been taken off the thalamus)
What are the features of Huntington’s chorea?
- Chorea (dance-like movements)
- Dystonia
- Loss of co-ordination
- Cognitive decline and behavioural disturbances
Why does Huntington’s chorea lead to chorea?
- Increased motor cortex activation
Why does Huntington’s chorea lead to dystonia?
- Uncomfortable contractions of agonists and antagonists simultaneously
- Leads to odd postures caused by over activity in agonist/antagonist muscle circuits and loss of co-ordination between these
What is Hemiballismus?
- Rare disorder
- Can be caused by damage to subthalamic nucleus which normally inhibits the thalamus via GPi
What causes hemiballismus?
- Subcortical stroke (lacunar infarct)
What does hemiballismus cause?
- Unilateral explosive (ballistic) movements
Outline the anatomy of the cerebellum
- Midline vermis and two laterally placed hemispheres
- Communicates with the rest of the CNS via the cerebellar peduncles
- Sits above fourth ventricle
What does the cerebellum deal with?
- Trunk,
- Hemispheres deal with ipsilateral side of the body
What do the different peduncles connect to?
- Superior cerebellar peduncle connects to midbrain
- Middle cerebellar peduncle connects to pons
- Inferior cerebellar peduncle connects to medulla
Why is the anatomical position of the cerebellum clinically important?
- Cerebellar lesions (e.g. tumours) can cause hydrocephalus
What are the normal functions of the cerebellum?
- Obscure
- Has a clear role in the sequencing and coordination of movements
- Uses sensory information to decide upon the most appropriate sequence of movements to perform an action
How does the cerebellum work with the basal ganglia?
- Basal ganglia decides what the most appropriate movements are
- Cerebellum then sequences movement
Where does the cerebellum receive sensory input from?
- Has profuse sensory inputs from proprioceptive neurones and sensory cortices
- Receives sensory input from ipsilateral spinal cord and contralateral sensory cortices
Where does the cerebellum send its outputs to?
- Contralateral motor cortex
- Signs of cerebellar damage are ipsilateral due to decussation of corticospinal pathway
What are some signs of cerebellar dysfunction?
- Dysdiadochokinesia
- Ataxia
- Nystagmus
- Intention tremor
- Slurred speech
- Hypotonia
What is dysdiadochokinesia?
- Difficulty with rapidly alternating movements
What is ataxia?
- Unsteady gait as a result of difficulty sequencing lower limb muscle contractions as well as loss of unconscious proprioception from lower limbs
What is nystagmus?
- Flickering eye movements due to malcoordination of extraocular muscles
What is an intention tremor?
- A tremor that worsens as a target is approached
What causes dysarthria?
- Malcoordination of laryngeal and tongue musculature
