Lecture 3 Flashcards
(13 cards)
Motor units
- a single alpha moor neuron + all muscle fibres it innervates
- increase operating motor units = increase contractile force
- each alpha motor neuron innervates a wide cluster of muscle fibres (from 20-2000+)
- alpha motor neuron axon divides into as many branches as necessary to innervate all the muscle fibres of a given moor unit
- i.e. if a motor unit innervates 300 muscle fibres, the axon will have 300 branches
Motor unit distribution in muscle
Motor neuron “pool” represents all the alpha motor neurons that innervate one muscle
One muscle may have many motor units of different fibre types
Axon of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibres scattered throughout the muscle
All or none principle of motor units
All muscle fibres of a motor unit act synchronously if stimulus triggers a moorneuron’s action potential
Single motor unit cannot generate strong and weak contractions:
- an impulse either elicits an action or it does not
Once the neuron fires and the impulse reaches the neuromuscular junction, muscle cells always act to their fullest extent
Force of muscle action varies from slight to max in one of two ways:
- increasing number of recruited motor units
- increasing frequency of motor unit discharge
Increase contribution of force
Motor unit characteristics
- Size (i.e. # of muscle fibres innervated)
- Physiological properties of their muscle fibres
- Biochemical properties of their muscle fibres
Motor unit size
The number of muscle fibres divided by the alpha motor axons, will = the innervation ratio
Motor unit contraction time
Action potential hits, and the motor units increase a lot and then slowly decrease
motor unit force vs. Contraction time
Motor units with fastest twitch contraction times develop greatest twitch force; motor units with slower twitch contraction times develop the least twitch force
Fast twitch MU120
Slow twitch MU1
Properties of motor unit types
Fast glycolytic fibres
- more muscle fibres, higher force (fast fatigable FF: type 2x)
Fast oxidative glycolytic fibres
- fast fatigue-resistant (FR: type 2a)
Slow oxidative fibres
- slow (S: type 1)
Muscle fibre type classification
System 1 (preferred) - type 1, type 2a, type 2x
System 2 - slow twitch (ST), fast- twitch a (FTa) [oxidative/glycolytic], fast-twitch x (FTx) [anaerobic/glycolytic]
System 3 - slow oxidative (SO), fast oxidative/glycolytic (FOG), fast glycolytic (FG)
Type 1 - oxidative capacity is high, glycolytic capacity low, contractile speed slow, fatigue resistance high, motor unit strength low
Type 2a - oxidative capacity is moderately high, glycolytic capacity high, contractile speed fast, fatigue resistance moderate, motor unit strength high
Type 2x - oxidative capacity low, glycolytic capacity highest, contractile speed fast, fatigue resistance low, motor unit strength high
Fibre type determinants
Highest oxidative capacity vs. Lowest
- cross country skiing
- long-distance running
- canoeing
- trained nonathletes
- swimming
- trained students
- weight lifting
- alpine skiing
- wrestling
- ice hockey
- running 100-200m
Motor unit recruitment
Method for altering force production
- less force production: fewer or smaller motor units
- more force production: more or larger motor units
Size principle:
- order of recruitment relates directly to size of alpha motor neuron and their excitability
- recuit in same order each time:
Most excitable S -> FR -> FInt -> F units least excitable
1 -> 2a -> 2x -> 2b units
Threshold current needed to depolarize a given motor unit
- curved line (positive) starts and high excitability and curves up to low excitability
1 m/s = 3.6 km/h
Glycogen depletion of muscle fibres
Not active is type 2
Active is type 1
Exercise (75% of VO2 max)
Most excitable 1 -> 2a -> 2x -> 2b units Least excitable
Low force High force
Dynamic muscle contraction
From the standpoint of neural control, the selective recruitment and firing pattern of all types of motor units that control hip, knee, ankle, toe movement and other stabilizing regions, provide the mechanism to produce a desired coordinate response
Coordination of:
- nerve impulses
- calcium release and reuptake
- cross-bridg cycling
- ATP hydrolysis and resynthesis
Down stroke - EMG of Vastus Lateralis during cycling
