The motor control hierarchy Flashcards
What are all movements produced by the skeletal musculature initiated by?
Lower motor neurons
What is Fritsch and Hitzig experiment?
Electrical stimulation of anterior cortex elicits contraction of contralateral body muscles
- Became motor cortex
How is the sensory map presented?
Lower body is represented medially
Upper body is represented laterally
Proportions reflect density of innervation and behavioural significance
What do axial muscles control?
Trunk movement
What do proximal muscles control?
Shoulder
Elbow
Pelvis
Knee movement
What do distal muscles control?
Hands
Feet
Digits
How does the ventral horn innervate striated muscle?
Via the neuromuscular junction
Define a motor unit
Motor neuron and the muscle fibres it innervates
Define a motor neuron pool
All the motor neurons that innervate a single muscle
What does each muscle fibre receive input from?
A single lower (alpha) motor neuron
How are motor pools organised?
Grouped in rod shaped clusters within the spinal cord
- Extending over several vertebral segments
- Somatotopically (both mediolaterally and rostrocaudally)
What technique would you use to trace where motor neurons go back to in the spinal cord?
Retrograde labelling
Where do lower motor neurons receive inputs from?
Upper motor neurons
Local spinal cord inputs
How do upper motor neurons project onto lower motor neurons?
Via descending tracts
Which lateral pathway is in control of voluntary movement?
Corticospinal tract
Where do axons of the CST originate?
Layer 5 of the motor cortex
What are the axons of the corticospinal tract derived from?
Large pyramidal cells
Where are the main outputs for the motor cortex located?
Layers 3, 5 and 6
Where are the main inputs to the cortex located?
Stellate cells in layer 4
Describe how the axons of the corticospinal tract control distal muscles
Cross the midline in the pyramidal decussation
- Project laterally in spinal cord
- Synapse onto laterally located lower motor neuron circuits that control distal muscles
Where do CST outputs to the upper body originate from?
Lateral motor cortex
Where do CST outputs to the lower body originate from?
Medial motor cortex
What are motor cortex upper motor neurons concerned with?
Fine voluntary control of more distal structures
What are brainstem upper motor neurons concerned with?
Postural movement
What do brainstem motor neurons project onto?
Medial motor pools
What are the ventromedial pathways?
Vestibulospinal
Reticulospinal
How do the ventromedial pathways project onto the spinal cord?
Medially onto the spinal cord
- Synapse on medially located lower motor neuron circuits that control axial muscles
What does the vestibulospinal tract control?
Head balance and turning
Where are the vestibulospinal tract inputs from?
The vestibular system
What does the tectospinal tract control?
Orientating response
Where are tectospinal inputs from?
The visual system via superior colliculus
What does the reticulospinal tract control?
Antigravity reflexes
How do the ventromedial pathways project?
Mainly ipsilaterally and medially
How does the body compensate for forces applied?
An anticipatory feedforward mechanism
- Pre-adjusts body posture
What are the 2 areas which indirectly control lower MNs?
Area 6 from movement anticipation
Area 4 for movement initiation
How does movement anticipation project onto axial muscles?
Via reticular formation
How does movement initiation project directly onto the spinal cord?
Via the corticospinal tract
What does activity in the PMA precede?
Movement planning and anticipation
Where is the circuit for anticipation?
Motor cortex to brainstem nuclei
What disease is characterised by muscle atrophy and sclerosis?
Motor neuron disease
What is the typical time course of MND?
3 years
Describe lower motor neuron disease
Muscle paresis or paralysis
Loss of stretch reflexes
Leads to severe muscle atrophy
- Patients usually die from lung dysfunction
Describe upper motor neuron disease
Muscle weakness
Spasticity
Hyperactive reflexes
Loss of fine voluntary movement
- Patients usually die from loss of bulbar muscles via corticobulbar tract
What drug alleviates MND symptoms? What is its mechanism?
Riluzole
- Blocks glutamate release leading to neuronal cell death
- Only works for a few months
A mutation in which gene causes MND?
Superoxide dismutase
- SOD1
