Neuromuscular Control Flashcards
Where are alpha motor neurons?
Occupy anterior horn - ventral horn of grey matter in spinal cord
What do the lower motor neurons of the brainstem and spinal cord inntervate? What does activation cause?
- extrafusal muscle fibres of the skeletal muscles
2. activation causes muscle contraction
What does the motor neuron pool contain?
all alpha motor neurons innervating a single muscle
What is a motor unit?
a single motor neuron together with all the muscle fibres that it innervates. It is the smallest functional unit with which to produce force
How many motor neurons and skeletal muscle fibres do humans have?
- 420,000 motor neurons
- 250 million skeletal muscle fibres.
How many muscle fibres does each motor neurone supply on average?
600
What does the stimulation of one motor unit cause?
contraction of all the muscle fibres in that unit
What are the slow (S, type I) motor units?
- smallest diameter cell bodies
- small dendritic trees
- thinnest axons
- slowest conduction velocity
- For posture muscles, as slow long time
What are the fast, fatigue resistant (FR, type IIA) motor units?
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
- Calf, high
What are the fast, fatiguable (FF, type IIB) motor units?
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
- Fast twitch muscle in calf
Is one single motor neuron innervate one muscle fibre?
An individual neuron may innervate 10 or 100 of muscle fibres, none of muscle fibres will innervated be motor neuron 1 will be innervated by motor neurons 2 - one neuron always wins (at baby more and then one neuron wins)
Why do you want more motor neurons to innervate muscle fibres?
If one motor neuron innervates thousands of muscle fibres the control over that muscle is not as refined as 10 muscle fibres by one motor neuron - innervation ratio relates to function of muscle - small ratio, fine control muscle
How are the 3 different motor unit types classfied?
- amount of tension generated
- speed of contraction
- fatiguability
- Look at graphs
How does the CNS regulate the force that a single muscle can produce?
- Recruitment
2. Rate coding
How are motor units recruited?
Not random
By what principal are motor units recruited?
- Size principal
- Smaller units recruited first - generally slow twitch units
- As more force required, more units are recruited
- Slow then fast recurited
What does recruitment allow?
-Fine control - e.g. when writing, under which low force levels are required
At what frequencies to motor units fire at?
- Motor unit can fire at range of frequencies
- Slow units fire at lower frequency
What happens as the firing rate increases?
Force produced by the unit increases
When does summation of rate coding occur?
When units fire at frequency too fast to allow the muscle to relax between arriving at action potentials
When will a muscle waste?
Even when have arterial supply, need nerve supply otherwise will waste
What do motor neurons supply?
- Neurotrophic factors
- Prevent neuronal death
- Promote growth of neurons after injury
What are motor unit and fibre characteristic dependent on?
the nerve which innervates them
What happens in cross innervation?
If a fast twitch muscle and a slow muscle are cross innervated, the soleus becomes fast and the FDL becomes slow.
