Neuromuscular system

• Axon - long slender fibre of a motor neurone. Conducts electrical impulses away from the neutron's cell body to the muscle fibre.
• Motor neurons - connected to an axon that connects to skeletal muscle
• CNS - consists of the brain and spinal cord. Sends signals called an action potential to a motor unit and travels through a motor neuron
• Neuromuscular junction - site where the axon (axon terminal) of the motor neuron and motor end plate of the muscle fibre meet
• Sarcoplasm - cytoplasm of a muscle fibre that contains a network of membranous channels surrounding the myofibrils. It is a water solution containing ATP and phosphagens and contains mitochondria
• Sarcoplasmic reticulum - the membranous channels that are storage sites for calcium ions. Play important role in muscle contractions.
• Myofibrils - muscle fibres are made up of many myofibrils bundled together. Numerous thread-like structures containing contractile proteins
• Myosin - a thick, contractile protein filament with protrusions known as 'myosin heads' that bind to form cross bridges.
• Sarcomere - basis contractile unit of muscle myofibril distinguish by Z lines. Each sarcomere is composed of 2 main protein filaments: actin and myosin
• Actin - a thin contractile protein filament containing 'binding' sites on the two globular proteins: troponin and tropomyosin
• Troponin - plays an important role during excitation - contraction, where Ca2+ bind to troponin, then it interacts with tropomyosin to unblock the myosin head binding sites - allows for a cross bridge to start contraction process
• Tropomyosin - a 'thread-like' globular protein that blocks myosin head binding sites on actin filament, preventing cross-bridge formation. This prevents contraction in a muscle without nervous innervation and the binding of Ca2+on troponin
• Resting phase - no muscle action potential stimulated by the motor neurone so the muscle is at rest
• Excitation phase - An impulse travels along the motor neurone to the neuromuscular junction. If threshold is met, acetylcholine is secreted across the synapse, depolarising the motor end plate, creating a muscle action potential. This muscle action potential causes calcium to be released from the sarcoplasmic reticulum, down the t-tubules into the sarcoplasm
• Contraction phase - calcium binds to troponin changing the shape of the tropomyosin to expose the active site on the actin filament. The myosin head attaches to the active site creating a cross bridge. ATP which is attached to the myosin head, is broke down, releasing energy, pulling the actin and myosin filaments towards each other, known as a power stroke
• Recharge phase - The myosin detaches and then reattaches to a new active site further along the actin. ADP detaches from the myosin head and a new ATP molecule attaches and repeats the process, known as the ratchet mechanism.

  This continues until no impulse is received or there is depletion of ATP and calcium.
• Relaxation phase - The muscle relaxes as it returns to its resting state
• All or none law - minimum amount of stimulation is required to start muscle contraction. If an impulse is strong enough then all the muscle fibres in a motor unit will contract. However, is the impulse is less than the threshold then none of the muscle fibres will contract.
• Recruitment - number of motor units stimulated. If only a few motor units within the muscle are stimulated, the strength of the contraction will be weak. The greater the number of motor units that are recruited, the greater the number of muscle fibres that will contract. This increases the force that can be produced.
• Wave summation - frequency of stimuli. For a motor unit to maintain a contraction, it must receive continuous impulses. Usually a frequency of 80-100 stimuli/sec
• Synchronisation - if all motor units stimulated at exactly the same time, maximum force can be applied
• Tetanic contraction - occurs after several stimuli cause a muscle to contract in rapid succession
• Slow twitch - long distance, endurance activities. Contract slowly over a prolonged period, generating a low level of force and have high levels of resistance to fatigue.
• Slow twitch characteristics:

  • red in colour due to high O2 supply
  • small fibre size
  • high mitochondrial density
  • large capillarisation
  • high myoglobin content
  • low PC stores
  • low glycogen stores
  • high triglyceride stores
  • slow speed of contraction
  • low force of contraction
  • high resistance to fatigue
• Fast twitch - needed in sprint, power and strength activities. Divided into fast oxidative glycolytic (iia) and glycolytic (iix). They contract quickly over a relatively short period of time generating a high level of force and have low level of resistance to fatigue
• Fast twitch characteristics:

  • large and largest fibre size
  • low and lowest mitochondrial density
  • moderate and small capillarisation
  • moderate and low myoglobin content
  • high pc stores
  • high glycogen stores
  • moderate and low triglyceride stores
  • fast and fastest speed of contraction
  • fast and fastest force of contraction
  • low and lowest resistance to fatigue
• Responses from short-term activity:

  • Increased number of muscle fibres recruited, leading to increased strength of associated muscles
  • Increased force production, as additional fast twitch muscle fibres are recruited
  • Increased rate of fibre recruitment - speeds up sports related movements
  • Type 1 muscle fibres recruited first as they have best threshold
  • Increased enzyme activity, leading to more efficient breakdown and subsequent re-synthesis of ATP
• Adaptations of endurance training:

  • Results in type iix muscle fibres being converted into type iia
  • Increased aerobic capacity of slow twitch muscle fibres due to an increase in mitochondrial density and efficiency of aerobic enzymes
  • Efficiency of the blood supply to the muscles improve slow twitch
  • Increased storage of fat and glycogen
  • Slow twitch muscle activation is more effective
• Adaptations of power training:

  • Muscular hypertrophy in fast twitch muscle fibres - greater force produced
  • Hyperplasia - increased number of type iix fibres
  • Overall increased muscle fibre recruitment to increase force produced
  • Increased storage of ATP and PC leads to increased strength and efficiency of type 11a and 11x fibres
  • Increased neural firing rates, timing and co-ordination of recruitment, increase rate of force production, speed and agility