Motor systems

Question 1

What is the force of a contraction determined by?

Answer 1

• The increasing spike frequency of motorneurons

- The recruiting of more motorneurons

Question 2

Where do most motorneurons originate from?

Answer 2

Spine, exit from the ventral root

Question 3

What are the properties of alpha motorneurons?

Answer 3

• Large in diameter
• Fast conduction
• Innervate normal skeletal muscle
• Vary in size of motor units

Question 4

What is Manneman’s size principle?

Answer 4

A small motor unit: small soma, thin axon, slow conduction, low threshold, weak conduction
Large motor unit: large soma, thick axon, fast conduction, high threshold, strong contraction

Question 5

What are the corticospinal tracts?

Answer 5

• Direct lines from the motor cortex to the spinal cord which usually cross pathways
• Sometimes called ‘upper’ motorneurons

Question 6

What are the basic characteristics of a reflex?

Answer 6

• Does not require brain activity although brain may modulate response
• Can be monosynaptic or polysynaptic

Question 7

What is a myotactic reflex and give an example

Answer 7

• Resistance reflex which helps maintain posture by resisting external forces
• Cannot be overidden by the brain
  e. g knee jerk

Question 8

What is the purpose of muscle spindles and where are they found?

Answer 8

• Important stretch receptors for proprioception. Act as range-setting control, retaining sensitivity over a wide dynamic input
• Embedded in parallel to main contractile fibres

Question 9

What is the structure of muscle spindles

Answer 9

Have a stretch-sensitive central region with contractile distile regions

Question 10

What is a polysynaptic reflex? Give an example

Answer 10

• Where interneurons activate the alpha motorneurons of several muscles to produc a coordinated reflex
• Brain is told what is happening and can overrise
  e. g withdrawing from painful stimulus

Question 11

What are central pattern generators?

Answer 11

The neural mechanism for generating basic locomotion which exists within the spinal cord (although brain needed for start, stop and speed)

Question 12

How is the labour of locomotion divided within the brain and spinal cord?

Answer 12

Brain - sends non-rhythmic excitation down the spine
- intergarets visual and balance input to send corrections down to the spine
Spine- Generates rhythmic activity, coordinates limb rhythms and integrates sensory input from stretch reflexes

Question 13

What is the ‘endogenous burster’ model?

Answer 13

That each neuron is rhythmically active without any external influence or interactions with other neurons