Muscle Fatigue And Plasticity Flashcards
Colloquial definition of fatigue
Reduction in physical or mental performance
Physiological definition of fatigue
1 failure to maintain the required or expected force
- definition appropriate for isometric contraction)
2 failure to maintain force, power, velocity
- definition for dynamic contraction
What is submaximal fatigue
Some fibres become fatigued and less able to generate force (power) so additional motor units recruited in order to achieve the same task
- increased effort required to maintain submaximal task
How is skeletal muscle controlled by the nervous system
Controlled by CNS via alpha-motor neuron efferents
Where are alpha-motor neurons cell bodies
Cell bodies localized to ventral horn
How does one muscle fibres get innervated
ONLY ONE FIBRE INNERVATED BY ONE ALPHA MOTOR NEURON
What determines exercise duration
Ability of muscle to meet energy needs for exercise
What factors impact fatigue and what factors do not impact fatigue
Do- metabolic byproducts, mental fatigue, pain, rapid failure of fast motor units, accumulation of Pi and lactic acid
Don’t- depleted energy stores = ATP always present
Where does fatigue occur neurally
Any point between CNS or PNS
Central vs peripheral fatigue
Central- associated with CNS (pre motor cortex, motor cortex, descending pathways, motor neuron activation/inhibition)
Peripheral- associated with function of motor units (NMJ, sarcolemma, T-tubules, voltage sensors, RYR1, Cav1.1, Ca availability, actin myosin interaction)
Causes of central fatigue
- reduced excitatory drive from the motor cortex
- presynaptic inhibition may be increased via increased firing of III-IV afferents
- neural factors: 5-HT, DA, NA, play an important role during exercise- heat
- exercise induced increases in neurotransmitters?
How is central fatigue measured
Using twitch interpolation
What is central fatigue hypothesis
Exercise induced increases in these neurotransmitters lead to CNS fatigue
What is the twitch interpolation technique
- subject performs maximum voluntary contraction
- partway through maximal electrical stimulation elicits a twitch
- if force increases during interpolated twitch- shows that muscle was capable of generating extra force
- so poor force during MVC was due to reduced motor drive from CNS
How does peripheral fatigue occur and why
Two phases- rapid fall and slower decline
Rapid- due to fatigue of type II fibres (type I fatigue resistant)
All factors of peripheral fatigue
- type II fibre fatigue (limited tetanic ability)
- depletion of glycogen and CrP
- increased intracellular H+ and lactic acid
- decreased pH alters Ca binding to TnC and actin myosin interactions
- increased Pi exerts effects via Ca release, Ca sensitivity and actin myosin binding
During high impulse activity what ion pools in T-tubules and what does this mean for the muscle
K+
High concentrations of K+ cause a decrease in muscle cell efficiency
What T-tubule events can cause fatigue
- High concentration of K+ causes decrease in muscle cell efficiency (K+ pools)
- sustained depolarization of T-tubule membrane blocks local action potentials (causes decrease in release of Ca from SR- leads to decreased contractility)
What SR event can cause fatigue, explain possible mechanisms
Impaired Ca release from the SR
- high K+ concentration extra cellular reduces voltage sensor activation and AP amplitude reducing RYR1 activity
- use of ATP increases free Mg2+ therefore decreasing RYR1 activity
- increased myoplasmic Pi levels can reduce Ca release by entering SR and chelating Ca
What kind of motor units do fibres innervated with type II motor neurons become
Fast MU
What kind of motor units do fibres innervated with type I motor neurons become
Slow oxidative MU
How do muscles lengthen
Formation of additional sarcomeres (during growth)
What happens to muscles if you become immobile
Shortening of muscle can occur- remove sarcomeres
How do changes in sarcomere length effect force and velocity of shortening
DO NOT effect FORCE
Change velocity of shortening
What changes does hypertrophy cause
Increases in strength and diameter of myofibril
(Strength training- no change velocity)
What does a muscle need to maintain normal growth, what happens if it doesn’t
Muscle must be used and experience a load
Otherwise muscle atrophy- inhibition of protein synthesis, stimulation of protein degradation
What can anabolic steroids abuse cause
Hormone disturbances
Impaired testosterone production
