Lecture 8 - Motor system (contd) and electromyography

Question 1

Distribution of muscle Fibers of a motor unit

Answer 1

Three types

1. S - slow oxidated (SO) muscle fibers forming S type motor units
2. FR - fatigue resistant motor uni forming fast oxidized glycolytic (FOG) muscle fibers
3. FF - fast fatigue motor units forming fast glycolytic (FG) muscle fibers

The bigger the motor unit the faster. Single motorneruron and all the fibers it innervates

Question 2

Twitch contraction

Answer 2

Slow does not fatigue (60min), Fatigue resistant ones do eventually but takes longer 8-20 min), fast is around two minutes

Question 3

Orderly motor unit recruitment

Answer 3

Given the same synaptic input, smaller motor units are more excitable then larger ones (takes more to get them fired up, more neurons). larger=more leaky. Motor units begin firing AP at different levels of force output. Recruited in one order and derecruited in opposite

Alpha motor neuron in ventral spinal cord

Larger motor units needs more sodium to fire for AP

Question 4

Hennemans Size Principle

Answer 4

Larger MUs innervated more fibers

Lager motor units turn off faster

He measured MU fibre diameter and related it to the number of fibers of fibers it innervated

Larger more powerful motor units get recruited when the desired movement requires more force

Question 5

Surface electromyography

Answer 5

Measure of muscle activity

Measures electro difference between muscle 1 and 2

As AP in muscle fibrepasses below the electrode, we register the above traces

Standard EMG placements across the human body

Electrodes placed over thickets part of muscle ‘belly’ such that the pads are aligned with the direction of muscle fibers

No AP in tendons

Know about cleaning and placement: skin to be cleaned and shaved to remove dead skin cells to improve signal quality