Define key terms: Motor unit, summation, all or none law Generating Force Objectives: Define key terms: Motor unit, summation, all or none law Describe the various ways in which the size of a force generated by a muscle can be changed.

1. Action potential reaches the motor end plate and causes depolarisation 2. This causes calcium ions to be released 3. The calcium ions attach to troponin 4. This moves/changes the tropomyosin complex 5. This leaves the binding sites on the actin free 6. Myosin heads attach to the actin binding site to form cross bridges 7. ATP required to form cross bridge 8. Myosin head moves towards the centre/power stroke occurs 9. Actin slides over the myosin/actin moves towards the centre of the sarcomere 10. Cross bridge then broken but can be recreated if calcium ions are still present 11. ATP is also required to allow the myosin to break the cross bridge

How do you think a muscle exerts more force? Or less force?

All of the muscle fibres in one motor unit will be of the same type The Motor Unit Motor Unit… … One motor neuron and all the muscle fibers it innervates (stimulates). Depending on the degree of control needed, different muscles have different numbers of muscle fibres per motor unit. Each muscle will contain motor units made up of each of the 3 muscle fibre types. All of the muscle fibres in one motor unit will be of the same type

Muscle Fibre Innervation The point at which the motor neuron meets the muscle fibres is called the motor end plate, and it forms the neuromuscular junction. A nerve impulse arriving at the end plate causes acetylcholine (ACH, a neurotransmitter) to be released across the junction. The ACH excites the muscle sarcolemma and if enough ACH is released, the impulse will be transmitted into the muscle fibre, creating a wave of depolarisation to spread to the sarcoplasmic reticulum. The SR releases Ca2+ which makes………

The ALL or NONE Law If the impulse that arrives at the neuromuscular junction is strong enough to produce an action potential in the muscle then all the fibres in that motor unit contract at the same time, to the same extent. If the impulse is not strong enough (if the ACH threshold is not reached) then none of the fibres in that unit contract.

IMPULSE TRANSMISSION at a NEUROMUSCULAR JUNCTION ACH = Acetylcholine

Changing the Force of Muscle Contractions The strength of a muscle contraction depends upon three things: 1. The number of motor units recruited Number of Motor Units Recruited by the Brain The greater the number of units recruited by the brain, the greater the force produced by the muscle.

2. The type of motor unit recruited RECRUITMENT based on intensity of exercise at low intensity ST motor units recruited first at higher intensity FOG IIa motor units recruited at greatest intensity FG IIb motor units recruited to produce powerful fast muscle contractions all available fibres are recruited for highest power activities

S S S S SINGLE FIBRE TWITCH There are 3 ways in which we can vary the excitation of individual motor units to vary the strength of contraction. 3.Frequency of nerve impulses: WAVE SUMMATION the same fibre group fired repeatedly will build the force exerted by wave summation S S S S S = stimulus

*READ PG 42 WAVE SUMMATION* Tetanic Contraction (tetanus): Rapid firing of stimuli occurs, giving muscles little or no relaxation - tetanic contraction takes place, maximising force but causing rapid fatigue *READ PG 42 WAVE SUMMATION*

MULTIPLE FIBRE TWITCHES 3. Frequency of nerve impulses: SPATIAL SUMMATION different fibre groups are fired in succession to control a movement the total force across the space of a muscle is the sum of the effect of different fibre groups

Homework: Long Answer Question: A Tennis player needs to use their muscles to exert different amounts of force throughout a match. Use your knowledge of muscle contraction to explain how this is possible. (12 marks)