Motor Unit, contraction and fibres

• The skeletal muscle can only contract when stimulated by an electrical impulse sent from CNS
• Motor neurones: specialised cells which transmit nerve impses rapidly to a group of muscle cell fibres.
• Motor unit: the motor neuron and the muscle fibres stimulated by the axon.
• 1. nerve impulse initiated in the motor neuron
• 2. nerve impulse travels down axon by positive sodium moving in and out creating action potential
• 3. neurotransmitter called acetylcholine is secreted into synaptic cleft to allow impulse across gap
• 4. if electrical charge above threshold the muscle fibres will contract
• 5. happen due to the “all or none law”
• If -55mV is reached all the muscle fibres will contract
• Neuromuscular junction: end of the axon meets the muscle fibre
• Neurotransmitter = acetylcholine
• The inside of the axon is negative (-70mV)
• Outside of the axon is positive
• Action potential - a positive electrical charge inside the nerve which conducts the nerve impulse into muscle fibres
• Type 1 = Slow oxidative
• Type 2a = Fast oxidative glycolytic
• Type 2b = fast glycolytic
• Type 1: moderate force, slow contraction, high fatigue resistance and more suited to aerobic work.
• Type 2a: fast contraction speed, high contraction force and muscles fatigue relatively quickly
• Type 2b: fastest contraction speed, largest force, only anaerobic and quickest to fatigue
• Small motor neurones: stimulate small fibres, creates small and slow amount of force over a long period of time, resisting fatigue well.
• Large motor neurones: stimulate many large muscle fibres, creates large amount of force rapidly but fatigued quickly.
• Slow oxidative provide energy for sub-maximul aerobic work, low force of contraction, fibre will be available for recruitment in just 90 seconds.
• Fast glycolytic muscle fibres only recruited in the last 2-20 seconds of contraction when maximal efforts are required quickly.