L22 - Control of movement and action

Question 1

What is the neuromuscular junction?

Answer 1

• Motor neurons release acetylcholine into neuromuscular junction.
• Acetylcholine binds to nicotinic receptors on motor endplate.
• Triggers influx of Ca++ that causes contraction.
• Number of fibres innervated by each motor axon determines precision of movement.
  
  • E.g., in thigh muscle each motor unit has ~1,000 fibres, whereas extra-ocular muscles have ~10 fibres per motor unit (more control in your eye, make smaller contractions)
  • Terminal of motor axon binding onto the muscle itself

Question 2

What are some disorders that affect neuromuscular junctions?

Answer 2

• Myasthenia gravis:
  
  • autoimmune attack on acetylcholine receptors
  • symptoms include weakness and fast fatigue
  • Less able to cause contractions
• Botulinum toxin (Botox)
  
  • irreversibly binds to acetylcholine vesicles in axon terminal after being taken up, preventing release of acetylcholine into synapse
  • But the terminals themselves are capable of regrowing - so over weeks can start to recover movement and previous terminals are disconnected

Question 3

How does motor output occur?

Answer 3

• All motor signal to muscles go via spinal cord (or medulla)
• Spinal cord serves as relay from brain
• But also contributes to motor-control for very fast responses or adjustments - spinal reflexes (e.g. pain withdrawal response or stretch reflex) - so also generated from the spinal cord itself than just the brain
• Some ongoing orchestrated movements (e.g. breathing and walking) driven by central pattern generators - clusters of neurons whose circuity can allow them to generate certain packages of activity that are sent to the muscles

Question 4

How do spinal reflexes occur?

Single Synapse Reflex

Answer 4

Single synapse reflexes (e.g. stretch reflex)

• Allows us to keep upright when we’re walking or running

Question 5

How do spinal reflexes occur?

Two synapse reflex

Answer 5

• Two synapse reflex: Golgi tendon reflex
  
  • Protects tendon from excess force
  • When it picks up the excessive force - activates reflex to supress motor output so stops the muscle from contracting to prevent tendon being torn

Question 6

What are the three different pathways coming down from the brain to the spinal cord?

Answer 6

• Brainstem
  
  • Control muscles of trunk, neck and proximal limbs (upper arms and legs)
  • Posture, correcting balance & also involved in walking etc - for coordinated activity
• Red nucleus
  
  • Controls arms and legs
  • Relate to motor neurons in spinal cord
  • Limb movements but independent of trunk
• Motor cortex (cortical spinal tract)
  
  • Controls muscles in arms, hands and fingers
    
    • Fine manual movements (e.g. writing, picking up objects)
  • Particularly important in humans

Question 7

What is the internal capsule?

Answer 7

• Ribbons of fibres from cortex to thalamus, basal ganglia, brainstem and spinal cord
• If stroke happens in this area can cause hemiparesis (rigidity in one side of the body but there is still muscle tone - loss of voluntary control) and hemianaesthesia (loss of sensation)
• • Can see evidence of rigidity in one side (below)
    
    • Only lost conscious movement (from motor cortex), not automatic movement from the brain stem

Question 8

What are the three areas in the motor cortex that influence the control of movement?

Answer 8

• Neocortex
  
  • Frontal lobe contains 3 areas of motor cortex - separate areas as they contain their own separate maps
    
    • Primary motor cortex (M1)
      
      • Strip of cortex in front of central sulcus
      • Contains topographic map of body: muscle groups represented by discrete patches of cortex
      • Electrical stimulation evokes movements in corresponding limb or muscle group
      • Involved in final execution of movement: output to spinal cord via corticospinal (”pyramidal”) tracts, and via red nucleus
    • Supplementary motor area (SMA) & Pre-motor area (PMA)
      
      • Active before the primary motor area becomes active
      • Involved in planning of movements
        
        • Neurons most active shortly before performing a movement or even more an aborted movement as need to stop it getting to primary motor cortex
        • Present in the imagined rehearsal of a movement
      • Neurons in PMA and SMA code for anticipated direction of destination of movement
        
        • Monkeys moved a joy stick towards a light: Some cells only fired when the monkey was about to move stick in a specific direction
        • These direction cells maintained firing during a delay interval (if monkey had to wait several seconds after light went off before moving stick)