8. Motor systems Flashcards
What tasks are high order and lower level areas involved in?
- High order - more complex e.g. programme and decided on movements, coordinate muscle activity
- Lower level - execution of movement
The primary cortex supplies innervation to the muscles of the body via which neurones?
α neurones in the ventral horn
Which nuclei are responsible for the muscles of the face, head and neck?
Brainstem motor nuclei
The cerebellum and basal ganglia are part of which system involved in involuntary movements (and fine tuning output)?
Extrapyramidal system
What is the location and function of the primary motor cortex?
• Location - precentral gyrus, anterior to the central sulcus
• Function:
- control fine, discrete, precises voluntary movement
- low in the hierarchy
What is the neocortex?
- A 6 layer part of the primary motor cortex (layer 5)
- High-order function - sensory perception, cognition, generation of motor commands
- Large pyramidal cells - support a long axon
What is the pathway of motor neurones through subcortical structures known as?
- Internal capsule
* Bisects the basal ganglia
What does the internal capsule become further down?
Cerebral peduncle
Why can’t you see the corticospinal tract externally at the level of the pons?
Transverse fibres in the pons
Where do 90-95% of the motor fibres decussate?
- The pyramids as the tract re-emerges - base of the medulla
* Pyramidal decussation
In which part of the spinal cord do most of the motor fibres descend?
Lateral corticospinal tract (dorsal area of the spinal cord)
In which part of the spinal cord do 5-10% of the motor fibres descend and what do they supply?
- Those that don’t decussate in the brainstem
- Anterior corticospinal tract
- Decussate in the spinal cord at the appropriate level
- Supply the axial musculature
What is the corticobulbar pathway?
Pathway from the primary motor cortex to the motor nuclei - within the brainstem
Where is the hypoglossal nucleus?
Near the midline of the medulla
What is the location and function of the premotor cortex?
• Location - anterior to the motor cortex (M1)
• Function:
- planning of movements
- regulates externally cued movements
How do you describe moving a body part relative to another body part, and movement of the body in the environment?
Intra-personal and extra-personal space
What is the location and function of the supplementary motor area?
• Location - frontal love anterior to M1, medially
• Function:
- planning complex movements
- programming sequencing of movements
- regulates internally driven movements (plan speech and carry out) (Wernicke’s and Broca’s area)
What is the posterior parietal cortex important for?
Ensures movements are targeted accurately to objects in external space
What is the prefrontal cortex for?
Involved in selection of appropriate movements for a particular course of action
What are the signs of a loss of function of upper motor neurones?
- Neurones located in the primary motor cortex
- Paresis - graded weakness of movements
- Paralysis
How can a loss of inhibitory descending inputs affect upper motor neurones?
• Increased abnormal motor function
- spasticity: increased muscle tone
- hyperflexia: exaggerated reflexes
- clonus: abnormal oscillatory muscle contraction
- Babinski’s sign
What is apraxia?
- Disorder of skilled movement
- Usually related to parietal lobe problems
- Any disease of the inferior parietal lobe and the frontal lobe can cause apraxia - stroke and dementia are the most common
What are the effects of lower motor neurone disease?
- Weakness
- Hypotonia
- Hypoflexia
- Muscle atrophy
- Fasciculations - visible twitch
- Fibrillations - spontaneous twitching - recorded during needle electromyography examination
What is motor neurone disease?
- Progressive neurodegenerative disorder of the motor system
- Multiple genetic causes
- Spectrum of disorders with similar clinical presentations
- Affects upper and lower motor neurones
- aka Amyotrophic Lateral Sclerosis (ALS)
What are the upper motor neurone signs of ALS?
- Increased muscle tone
- Brisk limbs and jaw reflexes
- Babinski’s sign
- Loss of dexterity
- Dysarthria
- Dysphagia
What are the lower motor neurone signs of ALS?
- Weakness
- Muscle wasting
- Tongue fasciculations and wasting
- Nasal speech
- Dysphagia
Problems in the basal ganglia underlie which two important motor disorders?
- Parkinson’s
* Huntington’s
What structures does the basal ganglia comprise?
• Caudate nucleus
• Lentiform nucleus (putamen + external globus pallidus)
(Striatum = caudate + lentiform nucleus)
• Subthalamic nucleus - where surgeons put stimulations in Parkinson’s patients to turn off tremor
• Substantia nigra
• Ventral pallidum
• Claustrum
• Nucleus accumbens - involved in limbic reward
• Nucleus basalis of Meynert - all cholinergic innervation to the cortex (memory)
What is the lateral wall of the anterior horn of the lateral ventricle?
Caudate nucleus
Where is the putamen and nucleus accumbens located with reference to the Caudate nucleus?
- Putamen is more lateral and slightly inferior
* Nucleus accumbens is inferior
What are the functions of the basal ganglia?
- Elaborating associated movements e.g. swinging arms when moving, changing facial expressions to match emotion
- Moderating and coordinating movement
- Performing movements in order
Why is the circuitry of the basal ganglia significant in different disorders?
- Interruption at different components give different clinical presentations
- Parkinson’s - breakdown in nigrostriatal connectivity (dopaminergic cells project to the striatum)
- Huntington’s - local circuitry within the striatum
What are ballistic movements?
Uncontrolled arm movements - generally down to problems in the subthalamic area
How do patients with Parkinson’s disease present?
- Slightly stooped in posture
- Slightly broad based gait - shuffling movement
- Bradykinesia - slowness of small movements
- Hypomimic face - expressionless
- Akinesia - difficulty in initiation of movements
- Rigidity - muscle tone increase
- Tremor at rest
- No external pathological change in brain
- Lewy Body pathology at microscopic level
What does a midbrain look like in a Parkinson’s patient?
- No substantia nigra (black line)
* Dopaminergic neurones that normally project up to the striatum have died
Why do younger people have a pale substantia nigra?
- Black substance is neuromelanin
- Breakdown product of normal dopamine metabolism in dopaminergic neurones
- Builds up during lifetime, therefore there is less when younger
What causes Hungtington’s disease?
- Genetic disease - gene on chromosome 4 (Huntingtin protein), autosomal dominant
- Triple repeat - CAG repeat (>35 repeats almost certain to get disease)
- Degeneration of GABAergic inhibitory neurones in striatum, caudate, then putamen
What are the motor signs of Huntington’s disease
- Choreic movements - rapid jerky involuntary movements, hands and face affected first, then legs and rest of body
- Speech impairement
- Difficulty swallowing
- Unsteady gait
- Cognitive decline and dementia at later stages
What keeps the cerebellum in place?
- Sits in the posterior cranial fossa
* Covered by the tentorium cerebelli (fold of the dura)
How many pathways are there to the cerebellum and what are they?
- 3
- Inferior cerebellar peduncle - from spinal cord to spino-cerebellar tracts
- Middle cerebellar peduncle - transverse fibres connecting the 2 halves of the cerebellym
- Superior cerebellar peduncle - main output pathway to the basal ganglia and thalamus
Describe the laminar structure of the cerebellum?
- Outer-molecular layer - lots of glial cells
- Piriform layer - lots of Purkinje cells
- Granular layer - granule cells (neuronal as well)
Where does the inferior olive (nucleus in the rostral medulla) project to in the cerebellum?
• Purkinje cells
• via climbing fibres
- terminate in the molecular layer where the dendritic tree of the purkinje is found
Where do all inputs apart from the climbing fibres go in the cerebellum?
• Granule cells via mossy fibres
• Onwards via parallel fibres
- innervate purkinje cells
• All output from the purkinje cells is via deep nuclei
What are the 3 sections of the cerebellum?
- Vestibulocerebellum
- Spinocerebellum
- Cerebrocerebellum
Describe the function of the vestibulocerebellum
- Regulation of gait, posture and equilibrium
- Coordination of head movements with eye movements
- Connections with the superior colliculus
(small transverse section)
What is vestibulocerebellar syndrome?
- Damage (tumour) causes syndrome similar to disease
* Gait ataxia and tendency to fall (even when sitting and eyes open)
Describe the function of the spinocerebellum
- Coordination of speech
- Adjustment of muscle tone
- Coordination of limb movements
- Fine control of the main corticospinal output
- Modulates and refines output to the musculature
(medial vertical section)
What is spinocerebellar syndrome?
- Damage (degeneration and atrophy associated with chronic alcoholism) mainly affects legs
- Abnormal gait and stance (wide-based)
Describe the function of the cerebrocerebellum
- Coordination of skilled movements (motor learning)
- Cognitive function, attention, processing of language
- Emotional control
(lateral section)
What is cerebrocerebellar/lateral cerebellar syndrome?
- Damage mainly affects arms/skilled coordinated movements (tremor) and speech
- Results in skilled movement loss
What are the main motor signs of cerebellar problems?
Deficits only apparent upon movement
• Ataxia - impairements in movement coordination and accuracy
• Dysmetria - inappropriate force and distance for target-directed movements
• Intention tremor - increasingly oscillatory trajectory of a limb in a target-directed movement
• Dysdiadochokinesia - inability to perform rapidly alternating movements
• Scanning speech - staccato, due to impaired coordination of speech muscles
