Neuromuscular system Learning objectives



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  • Learning objectives
  • To describe the characteristics of slow twitch and fast twitch muscle fibres.
  • To identify the role of the nervous system in muscle contraction.
  • To understand the role of proprioceptors in the PNF stretching process.
  • To be able to explain motor unit recruitment and muscle innervation.
  • What is the difference between fast twitch and slow twitch muscle fibres?
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  • Training and Muscle Fibre Types
  • It is possible to increase the size of muscle fibres through training. This increase in size (hypertrophy) is caused by an increase in the number and size of myofibrils per fibre.
  • There are 3 main types of muscle fibre in the body:
  • Type 1 (slow oxidative or SO)
  • Type 2a (fast oxidative glycolytic or FOG)
  • Type 2x (fast glycolytic or FG)
  • Our skeletal muscles contain a mixture of all three types of fibre but not in equal proportions. The mix is mainly genetically determined but training can influence this too.
  • Muscle fibre types
  • Muscle fibre types
  • Characteristics of muscles fibre types:
  • Characteristic
  • Type I
  • Type 11a
  • Type IIx
  • Contraction speed
  • Slow
  • Fast
  • Fast
  • Force produced
  • Low
  • High
  • High
  • Fatigue levels
  • Low
  • Medium
  • High
  • Myoglobin levels
  • High
  • Medium
  • Low
  • Glycogen stores
  • Low
  • High
  • High
  • Triglyceride (fat) stores
  • High
  • Medium
  • Low
  • Capillary density
  • High
  • Medium
  • Low
  • High
  • Medium
  • Low
  • Anaerobic capacity
  • Low
  • Medium
  • High
  • Muscle fibre types
  • The relationship between muscle fibre type and force production over time is shown below:
  • Muscle fibre types
  • Small motor neurones stimulate a small number of fibres and creates slow amount of force but for a sustained period. Larger neurones produce high force but fatigue rapidly.
  • Slow oxidative muscle fibres are recruited and recover very quickly, within 90 seconds.
  • Muscle fibre types
  • Aerobic training should therefore have limited rest periods.
  • i.e. 3 x 800m set with 90 seconds rest.
  • Fast oxidative muscle fibres only provide 2-20 seconds of contraction. Both FO and FOG types take much longer to fully recover. Training should reflect this, i.e. 2-6 repetitions with 3-4 minutes rest.
  • Muscle fibre types
  • The relative proportion of each fibre type varies in the same muscles of different people.
  • e.g. an elite endurance athlete will have a greater proportion of slow twitch fibres in the leg muscles, while an elite sprinter will have a greater proportion of fast twitch fibres.
  • Muscle fibre types
  • The Parasympathetic Nervous System and Sympathetic Nervous System are part of the Autonomic Nervous System which is responsible for the involuntary functions of the human body.
  • Nervous system
  • PNF is an advanced stretching technique. It is one of the most effective forms of flexibility training for increasing range of motion.
  • Proprioceptive neuromuscular facilitation is where the muscle is contracted isometrically for a period of at least 10 seconds. It then relaxes and is stretched again, usually going further the second time.
  • The role of proprioceptors
  • PNF stretches cause the proprioceptors within the muscle and tendon to be stimulated. One of these proprioceptors, the Golgi Tendon Organ is sensitive to increased tension within a muscle and causes muscular relaxation in the opposite muscle.
  • The cell body processes the information and sends an impulse down the axon. The motor neuron and its muscle fibres are called a motor unit.
  • To protect the signal, an insulator surrounds the axon. This is called the myelin sheath and is made up of fatty material.
  • The myelin sheath has breaks along the axon. These breaks are called nodes of Ranvier. The impulse travels from one node of Ranvier to the next. This results in it travelling quicker to the muscle. The thicker the myelin sheath the faster the impulse is conducted.
  • The Motor Unit
  • The motor unit must carry nerve impulses form the brain and spinal cord to the muscle fibres.
  • The nerve impulse travelling to the muscle fibre is an electrochemical process which requires action potential. This wave of electrical charge moves down the axon to the motor end plate.
  • The Motor Unit
  • As the impulse reaches the end of the axon, it triggers the release of acetycholine (a neurotransmitter) at the neuromuscular junction. This neurotransmitter is secreted into the synaptic cleft to assist the never impulse to cross the gap. If enough neurotransmitter is present muscle action potential is created and a wave of contraction occurs.
  • One motor neurone cannot stimulate the whole muscle.
  • Instead, a motor neurone will stimulate a number of fibres within that muscle.
  • The Motor Unit
  • The all-or-none law The motor units exhibit an all-or-none response.
  • Think. Pair. Share – What is meant by the ‘all-or-none’ law? Link this to muscle recruitment.
  • Characteristics of a Motor Unit
  • A minimum amount of stimulation is required to start a muscle contraction.
  • If an impulse is strong enough then all the muscle fibres in a motor unit will contract. However, if the impulse is less than the threshold required then no muscle action will occur.
  • Gradation of contraction
  • This refers to the strength or force exerted by a muscle and is dependent on the following:
  • Recruitment
  • 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.
  • Characteristics of a Motor Unit
  • Frequency
  • The greater the frequency of stimuli, the greater the tension developed by the muscle. If the stimuli occur very infrequently, the calcium concentration in the sarcomere returns to resting levels before the arrival of the next stimulus.
  • Tension
  • Stimuli
  • (1) TWITCH
  • Characteristics of a Motor Unit
  • When the stimuli occur frequently, not all the calcium released in response to the first stimulus is taken back into the sarcoplasmic reticulum. As a result, summation occurs.
  • This is also called wave summation. Repeated activation of a motor neurone stimulating a muscle fibre results in summation.
  • Tension
  • Stimuli
  • (1) TWITCH
  • Characteristics of a Motor Unit
  • Timing
  • If all the motor units are stimulated at exactly the same time, then maximum force can be applied. This is referred to as spatial summation or synchronisation.
  • Tension
  • Stimuli
  • Characteristics of a Motor Unit
  • Tectanic
  • A tetanic contraction happens after several stimuli cause a muscle to contract in rapid succession.
  • Characteristics of a Motor Unit
  • Apply it!
  • What has stuck with you?
  • Exam questions
  • Explain how wave summation allows a gymnast to gain the required height in a floor routine. [3 marks]
  • Practice it!
  • Exam questions
  • 2. Fast twitch glycolytic muscle fibres (type IIx) are used to produce powerful contractions. Identify two characteristics of fast twitch glycolytic muscle fibres (type IIx). [2 marks]
  • 3. Explain how the characteristics of fast twitch glycolytic muscle fibres (type IIx) you identified in question 04.1 are suited to producing ATP anaerobically during powerful contractions.
  • [2 marks]
  • Practice it!
  • Marks Scheme:
  • Wave summation will allow the gymnast to produce a more powerful contraction (1) because the muscle is stimulated again before it is relaxed (1) therefore the gymnast will be able to apply greater force to adjust the height achieved to match the requirements of the technique performed (1). Accept other appropriate explanations of how wave summation allows a gymnast to gain the required height. Answers must relate to a floor routine.
  • Practice it!
  • Marks Scheme:
  • 2. • Fast motor neurone conduction velocity (1).
  • • Large muscle fibre diameter (1).
  • • More sarcoplasmic reticulum development (1).
  • • High PC stores (1).
  • • High glycogen stores (1).
  • • High myosin ATPase/glycolytic enzyme activity (1).
  • Practice it!
  • Marks Scheme:
  • 3. High PC stores – increased energy source for ATP production via the ATP-PC system (1).
  • High glycogen stores – increased energy source for ATP production via the lactate anaerobic system (1).
  • High myosin ATPase activity – increased enzyme activity for ATP production within the ATP-PC system (1).
  • High glycolytic enzyme activity – increased enzyme activity or ATP production within the lactate anaerobic system (1).
  • Practice it!


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