Neuromuscular Response to Exercise (year 2) Flashcards
what is a motor unit?
- Functional unit of a skeletal muscle.
* A single motor neuron and all the muscle fibres innervated by it.
motor neuron: Motor neurons stimulate muscle fibres to contract:
•Cell body of neurons located in spinal cord or brain
- Axons extend to the skeletal muscle
- Branch out to innervate every muscle fibre
•Connection between nervous and muscular systems occur through the neuromuscular junction
what are the first 3 parts of the function of the neuromuscular junction?
- The action potential reaches the nerve terminal in the presynaptic region.
- The action potentials activate calcium channels.
- The increase in intracellular calcium concentration triggers the fusion of the synaptic vesicles with the nerve terminal membrane.
what are the 4th to 7th parts of the function of the neuromuscular junction?
- Once fused, the vesicle releases Acetylcholine (Ach) into the synaptic cleft.
- ACh binds to its receptors on post-synaptic membrane and opens ligand-gated Na+/K+ channels, producing an end plate potential (EPP).
- The EPP opens voltage-gated Na+ channels in the postsynaptic region, resulting in an action potential that triggers muscle fibre contraction.
- Acetylcholinesterase degrades ACh so that it can be recycled to produce new ACh molecules and terminates synaptic transmission
All-or-None Principle?
• The principle that the strength by which a
muscle fibre responds to a stimulus is independent of the strength of the stimulus.
- If that stimulus exceeds the threshold potential, the nerve or muscle fiber will give a complete response.
- Otherwise, there is no response
Muscle twitch and summation (frequency)
- Twitch - A single action potential from a motor neuron produces one muscle fibre contraction
- Higher frequency stimulation leads to Treppe
- Single twitch responses with time for complete recovery but the tension generated progressively increases
Motor unit Innervation Ratio?
• In large muscles, one motor neuron innervates
many muscle fibres
• Results in high power but low dexterity
• In small muscles, one motor neuron innervates
only a few muscle fibres
• Results in low power but fine control
fibre type recruitment:
• Recruitment of motor units is by force required:
- Slow twitch (Type I) fibres provide most of the force in low intensity exercise
- As intensity increases, type IIa fibres are recruited
- During maximal intensity exercise all muscle fibres become active
- Aka Henneman’s Size Principle
Variation in Proportion of Muscle Fibre Types: men vs woman fibre composition
- Mean proportion of type I fiber was lower in males than in females; mean CSA of all fiber types was smaller in females
- Higher glycolytic enzyme markers in males than females
- Large interindividual variability and gender differences in the most common characteristics of the human skeletal muscle
muscle fibre distribution:
type 1: long distance running (coated in alkaline solution reducing the effect of increased acidity)
type 2A: middle distance running
Type 2B: Sprint (fatigue due more to increase in acidity and lack of alkaline solution)
can fibre types change?
type 2A and Type 2B can be converted
After a 7 day training programme they found an increase in capillarisation of Type 2B highlighting the shift from Type 2B to Type 2A as well as increases o2 uptake.
Effect of Training on Muscle Fibres: type 1 to type 2
1 hour per day, 4 days a week for 5 months at 85-90% VO2max,8 weeks of jump squats at 30 or 80% 1 RM, 3 sets of 3 second sprints 4 days per week, 2 or 3 days a week for 6 or 9 weeks resistance training
• Percentage type I and type II unchanged
Intensity Important to Determine Effect:
Suggests that high velocity movements may
stimulate type I and II interconversions.
Neural Response to Training: Structural adaptations to neuromuscular junction:
- Less intense, prolonged exercise → increased NMJ area
* Intense exercise → greater dispersion of synapses
Neural Response to Training: Increased efficiency of motor unit recruitment
- Improved co-ordination
* Increased activation
Hypertrophic Response to Training: leads to what?
• Detectable after a few weeks but slower than neurological adaptations
• Mechanical stress triggers signalling proteins which activate genes responsible for translation of messenger
RNA
- Results in protein synthesis in excess of protein breakdown
- Provided adequate amino acids available → hypertrophy
- Enlarges individual muscle fibres
what intensity is needed for low oxidative fibres:
high intensity for hypertrophy or alteration of mitochondrial components
what intensity is needed for high oxidative fibres:
any intensity for hypertrophy or alteration of mitochondrial components
Inactivity and atrophy, what happens when you become inactive?
- Decreased oxidative enzyme activity → decreased ATP synthesis
- Glycolytic enzyme activity – unsystematic change
- Fibre type distribution initially unchanged but after ~8 weeks oxidative fibre percentage may decrease in endurance and increase in strength-trained athletes
- Muscle cross-sectional area rapidly declines in strength and sprint athletes
- Muscle memory – myonuclei retained
Muscle memory:
untrained–>(first training route) fusion of satellite cells–>(growth) Hypertrophy–> (De-training)–> Previously trained–>(Re-training)–>Hypertrophy
Type 1 Fibres characteristics:
Contraction time: slow
size of motor neuron: small
Resistance of fatigue: high
activity used in: aerobic
Maximum duration of use: hours
Force production: low
Mitochondrial density: high
Capillary density: high
Oxidative capacity: high
Glycolytic capacity: low
Major storage fuel: Triacylglyceral
Type 2A Fibres characteristics:
Contraction time: moderately fast
size of motor neuron: medium
Resistance of fatigue: fairly high
activity used in: long distance anaerobic
Maximum duration of use: <30 mins
Force production: medium
Mitochondrial density: high
Capillary density: intermediate
Oxidative capacity: high
Glycolytic capacity: high
Major storage fuel: Creatine phosphate, glycogen
Type 2X Fibres characteristics:
Contraction time: fast
size of motor neuron: large
Resistance of fatigue: intermediate
activity used in: short distance anaerobic
Maximum duration of use: <5 mins
Force production: high
Mitochondrial density: medium
Capillary density: low
Oxidative capacity: intermediate
Glycolytic capacity: high
Major storage fuel: Creatine phosphate, glycogen
Type 2B Fibres characteristics:
Contraction time: very fast
size of motor neuron: very large
Resistance of fatigue: low
activity used in: short distance anaerobic
Maximum duration of use: <1 mins
Force production: very high
Mitochondrial density: low
Capillary density: low
Oxidative capacity: low
Glycolytic capacity: high
Major storage fuel: Creatine phosphate, glycogen
