Motor Systems Flashcards
Hierarchical organisation
High order areas of hierarchy: involved in more complex tasks
-Programme and decide on movements, coordinate muscle activity
Lower level areas of hierarchy perform lower level tasks (execution of movement)
-Carrying out the movements – not the planning or coordination
Motor system hierarchy
Primary motor cortex
Projects down into the spinal cord
It supplies innervation of muscles in the body via alpha neurones in the ventral horn
Brainstem motor nuclei are responsible for muscles in the face, head and neck
The cerebellum and thalamus are extrapyramidal areas of the brain and fine tune output of the motor cortex
Primary motor cortex location and function
Location: precentral gyrus, anterior to the central sulcus
Function: control fine, discrete, precise voluntary movement
Homunculus
It is inverted
Lower limb near median/longitudinal fissure
Upper limb and torso are lower and face is the lowest
Significance of homunculus in stroke
Lack of blood supply through branches of the middle cerebral artery causes problems in the upper limbs
The middle cerebral artery also supplies subcortical structures (including the basal ganglia)
Therefore middle cerebral artery blockage may result in complete hemiparesis or hemiplegia
Strokes affecting the anterior cerebral artery are likely to have more lower limb consequences
Descending motor pathways
Motor neurones pass from the primary cortex through the subcortical structure- through the pathway called the internal capsule
This then becomes the cerebral peduncle- still ipsilateral
Neurones then pass through the pons- but can’t see the corticospinal tract due to transverse fibres
The tract re-emerges at the medulla as the pyramids. At the base of the medulla, 90-95% of fibres cross over (pyramidal decussation).
The fibres descend in the spinal cord in the lateral corticospinal tract (in the dorsal area of the spinal cord).
At the appropriate level, fibres synapse with alpha motor neurones in the ventral horn of the spinal cord. Axons of these neurones go out through the ventral root -> spinal nerve.
Anterior corticospinal tract- what forms it, where they cross and what they supply
Formed by the 5-10% of fibres that don’t cross over at the medulla
These cross over at the appropriate level in the spinal cord
These supply the axial musculature (muscle of trunk and head)
Corticobulbar pathways
The primary motor cortex to the motor nuclei within the brainstem
Premotor cortex - location and fucntion
Anterior to motor cortex
Involved in planning of movements
Regulates externally cued movements
Supplementary motor area- location and function
Frontal lobe anterior to M1 (motor cortex)
Planning complex movement and programming sequencing of movements
Association cortex
Areas that impact other areas that are involved in motor activity
Posterior parietal cortex- ensures movements are targeted accurately to objects in external space
Prefontal cortex- involved in section of appropriate movement for a particular course of action
Loss of function of upper motor neurone
Paresis- graded weakness of movements
Paralysis- complete loss of muscle activity
Increased abnormal motor function of upper motor neurones due to loss of inhibitory descending inputs
Spasticity: increased muscle tone
Hyperflexia: exaggerated reflexes
Clonus: abnormal oscillatory muscle contraction
Babinski’s sign- stroke foot and toes fan out
Apraxia
Disorder of skilled movement
Usually related to parietal lobe problems
Lower motor neurone disease features
- Weakness
- Hypotonia (reduced muscle tone)
- Hypoflexia (reduced reflexes)
- Muscle atrophy (due to lack of innervation to the musculature)
- Fasciculations: damaged motor units produce spontaneous action potentials, resulting in a visible twitch
- Fibrillations: spontaneous twitching of individual muscle fibres
Motor neurone disease
Progressive neurodegenerative disorder of the motor system
A disease of both upper and lower motor neurones
Upper neurone signs of motor neurone disease
- Increased muscle tone (spasticity of limbs and tongue)
- Brisk limbs and jaw reflexes
- Babinski’s sign
- Loss of dexterity
- Dysarthria
- Dysphagia
Lower neurone signs of motor neurone disease
- Weakness
- Muscle wasting
- Tongue fasciculations and wasting
- Nasal speech
- Dysphagia
Upper vs lower neurone
Upper descends in the spinal cord
Then synapse with lower motor neurones which leave and join the muscle
Basal ganglia contents
Caudate nucleus Lentiform nucleus (pitman+ external globes pallidus) Subthalamic nucleus Substantia nigra Nucleus accumbens
Function of basal ganglia
- Elaborating associated movements (e.g. swinging arms when walking; changing facial expression to match emotions)
- Moderating and coordinating movement (suppressing unwanted movements)
- Performing movements in order
Basal ganglia circuitry and their relation with diseases
For Parkinson’s there is a breakdown in the nigrostriatal connectivity. Dopaminergic cells project to the striatum and are very important in initiation and planning of movement.
Huntington’s disease is affecting neurones within the striatum, and affects local circuitry within the striatum.
Can also get ballistic movements- uncontrolled movement of the arm- due to problems in sub thalamic area
Neuropathology of Parkinson’s
Black substance is nueormelanin- breakdown product of normal dopamine metabolism in dopaminergic neurones
This is pale in people with Parkinson’s as the dopaminergic neurones die and are cleared
Nigro-strial pathway
Dopaminergic neurones project from the substantia nigra in the midbrain to the striatum (specifically to the caudate nucleus and the putamen)
In Parkinson’s you lose this input to the basal ganglia
