Control of Muscle Activation

Question 1

How is an action potential propagated?

What role do sodium channels play in the propagation of action potentials?

Answer 1

• The action potential does not move, positive ions diffuse outwards, causing a membrane potential further down leading ot another action potential
  
  • Period of inactivity of sodium channels where the action potential has already occurred
  • The more sodium channels there are the more likely a rush of sodium ions is to stimulate the action potential

Question 2

Describe stochastic resonance:

Answer 2

• occurs in a threshold measurement when an appropriate measure of info transfer is maximized in the presence of a non-zero level of input noise thereby lowering the response threshold”
  ○ Something that occurs when a threshold is reached
  ○ Transmission of information
  ○ Maximised when there is some noise (minimal level of noise in the system, causes it to take less real signal to reach the threshold)

Question 3

Define:
orthodromic transmission
antidromic transmission

Answer 3

• Orthodromic transmission - most neurons transmitting action potentials from the body down the axon
• Antidromic transmission - when transmission is in the other direction (towards body)

Question 4

What is a motor unit?

roughly how many muscle fibres can occupy a single motor unit:
- in the eye & hand
- lower leg

Answer 4

• The smallest functional unit of the neuromotor system - smallest system that we can actively control
• “the motoneuron and all the muscle fibres it innervates”
• α motoneuron is the motoneuron that actually innervates muscle fibre
• Hand & eye have <100 muscles fibres occupy a motor unit
  Lower leg motor fibre can contain >1000muscle fibres

Question 5

What is:
twitch contraction
latent period

Answer 5

Twitch contraction - increase in force which then dies down again, 0.01 seconds
Latent period - the excitation of the stimulus creates a delayed response in the action potential

Question 6

What happens when all motor units have been maximally stimulated?

Answer 6

Tetanic contractions
- creates max force

Question 7

Describe the difference between these motor units:
- slow (MU3)
- FF (MU1)
- FFR (MU2)

Where do motor units get their energy from?

Answer 7

• The slower the motor unit the less fatigued it is
• Fast twitch fatigable
• Fast twitch fatigue resistant
• Energy provided from ATP in myofibrils

Question 8

Describe the size principle/Heneman’s principle:

Answer 8

“the recruitment of motor units within a muscle proceeds from small motor units to large ones”
- The first activated motor unit is usually the smallest & least forceful –> as recruitment continues the motor units become bigger and more forceful
- Weak inputs to the motor neuron pool produce action potentials in small motor neurons, as the strength of the input grows the larger motor neurons fire.
- Slow motor units are typically smaller - smaller motoneurons, has a lower threshold - then derecruited for larger motor units
- Fast motor units are typically bigger - bigger motoneurons, has a higher threshold
- Known as innervation ratio

Question 9

What advantages does the size principle have?

Answer 9

• recruitment can stop once desired force reached
• large forces aren’t produced when not required
• orderly recruitment reduces complexity

Question 10

How can tension vary at different muscle lengths?

Answer 10

Inverted U reflects the strength of different actin-myosin cross-bridges formed at different muscle lengths
- When the muscle is short, there is too much overlap among filaments = more tension
- When muscle is stretched, there is too little overlap among filaments = very little tension

Question 11

Describe postsynaptic inhibition:

Answer 11

Postsynaptic inhibition - the following neuron inhibits the previous neuron
Excitatory is empty circle, inhibition is a full circle(taking the action potential away from the threshold)
- makes neuron less (in)sensitive to any incoming excitatory signal
○ Moves the membrane away from its threshold so it is less likely to generate an Action potential

Question 12

How does a Renshaw cell control the motoneuron pool?

Answer 12

all α motoneurons for 1 muscle
- 1 Renshaw cell can excite/inhibit the whole muscle

Question 13

What is a Renshaw cell?

What type of feedback do these use?

Answer 13

The same α motoneuron excites the renshaw cell within the motoneuron, which can inhibit the muscle.
CNS can excite/inhibit the Renshaw cell(increase/decrease cell sensitivity) to increase/decrease the muscle sensitivity
- Recurrent inhibition - when axons of Renshaw cells project back to the bodies of α motoneurons and inhibitory synapses

• negative feedback - Recruitment inhibition

Question 14

Describe the elements of a muscle spindle:

Answer 14

• Extrafusal muscle fibre are the actual muscle fibres
• Spindle is covered in a capsule of connective tissue
• Intra & extrafusal fibres connected by tendinous ligaments
• Muscle spindle is an intrafusal muscle fibre, running parallel within the muscle
  And detect changes in length and velocity by:
  
  • Consists of chain fibre(sensitive to changes in length)
  • bag fibre(sensitive to changes in velocity)

Question 15

Define:
Primary spindle endings
Secondary spindle endings
Ganglion

Answer 15

• Primary: most intrafusal fibres - velocity sensitive(rapid firing)
  
  • Secondary: seen on dynamic bag fibres and are common in chain
    And static bag fibres - length sensitive(slower speed firing)
• End of a spindle is a spinal ganglion(proprioceptive neuron)

Question 16

what is the role of γ motoneuron?

Answer 16

Γ motoneruon - changes the length of the muscle spindle
- Changes sensitivity to more appropriate
- e.g.: when muscle contracts γ motoneuron changes length
Of muscle spindle so that is doesn’t become slack
- Conduct action potentials at lower velocities

Question 17

Define:
- afferent neuron
- Ia
- IIa
- efferent neuron

Answer 17

• Afferent neurons - take sensory info from sensory organs to CNS e.g.: goes through the back(dorsal root) - Ia afferent
  Ia - goes from muscle spindle to CNS
  IIa - is outside of muscle spindle, less sensitive
• Efferent neurons - take motor information from the CNS to the muscle e.g.: goes through the front(ventral root) α motoneuron

Question 18

Describe Ia interneurons:

Answer 18

• Receives afferent input in the spinal cord from Ia afferent forming a feedback loop to prevent unwanted contractions
• Inhibits the stretch through antagonistic muscles(reciprocal)
  Ia afferent –> Ia interneuron –> α motoneuron
• Heterogenic - contraction of muscle through other muscles
• Autogenic - projection of afferents controlling the same muscle

Question 19

What is disinhibition?

Answer 19

• Disinhibition - when Renshaw cells inhibit the Ia interneurons & decrease the interneurons
  Inhibitory effects on antagonistic α motoneurons
• the Renshaw cell inhibits the alpha motoneuron that excited it

Question 20

What is the role of the Golgi tendon?

Where is it found?

Answer 20

Golgi tendon measures force/tension to provide protection for the muscle
Can reach magnitudes of 80m/s

• is found alongside the muscle

Question 21

Describe presynaptic inhibition:

Answer 21

• Presynaptic inhibition - subtle & selective, making only specific inputs(synapses) into the
  Neurons less effective
  ○ Act on the presynaptic axonal membrane near synaptic cleft (uses GABA mediator)
  ○ GABA leads to long-lasting depolarisation of presynaptic membrane

Question 22

What is the role of Ib interneurons?

What type of feedback do they use?

Answer 22

Left line - Force inhibits the triceps and inhibits the inhibitor of the biceps(turn biceps on)
Trisynaptic - 3 synapses

• Negative feedback

Question 23

Name some cutaneous receptors:

Answer 23

Cutaneous receptors:
- Found in the skin
- Thermoreceptors: Sensitive to temperature
- Mechanoreceptors: important for human movement