Muscle Physiology 1 Pre-lecture Notes PDF

Applied Sciences PT4050 and PT7001 Muscle Physiology Part 1 Chris Chamberlin and Dr Ben Hunter [email protected] Learning Outcomes Neuromuscular junction Muscle structure Contraction types Cross-bridge formation Motor units Structure · Sarcolemm...

Applied Sciences PT4050 and PT7001 Muscle Physiology Part 1 Chris Chamberlin and Dr Ben Hunter [email protected] Learning Outcomes Neuromuscular junction Muscle structure Contraction types Cross-bridge formation Motor units Structure · Sarcolemma: plasma membrane · Sarcoplasm: cytoplasm · Myofibrils · Mitochondria · Sarcoplasmic reticulum: smooth ER · Transverse tubule: T tubule) Neuromuscular junction (NMJ) 1. Action potential to axon terminal of motor neuron 2. Ca2+ channels opens 3. Neurotransmitter acetylcholine (ACh) releases 4. Ligand-gated (Na+) channels open when ACh binds to the receptors on motor end plate 5. Action potential is generated when threshold is reached – All or Nothing Principle 6. Propagation of action potential on sarcolemma Neuromuscular junction (NMJ) Excitation-contraction coupling 1. ACh released at N -M J, receptor, AP 2. Action potential down T tubules 3. AP trigger Ca2+ release from SR 4. Ca2+ binds to troponin exposing myosin binding site 5. Crossbridge cycling occurs 6. Ca2+ re-uptake actively 7. Myosin binding sites blocked Striations · When viewed longitudinally, skeletal muscles have striations · Highly organised intracellular structures called myofibrils cause the striations Structure Levels of organisation Epimysium (entire muscle) Perimysium (approx. 150 fibres) Endomysium (single fibre) Muscle Muscle Fibre Myofibril Myofilament Striations Structure of a myofibril / sarcomere Myofilaments · Actin - contractile protein · Troponin – regulatory protein · Tropomyosin – regulatory protein Myosin The crossbridge cycle 1. Binding of myosin to actin 2. Power stroke 3. Rigor (low energy state) 4. Unbinding of myosin and actin 5. Cocking of myosin head (high energy state) The crossbridge cycle Sliding Filaments Sliding Filaments  Myosin/actin filaments do not shorten  A-band remains the same length  I-band shortens  H-band shortens  Length-Tension Relationship Is there a position where maximum force can be generated? What’s the relevance in physiotherapy? Length-Tension Relationship Twitch contraction Contraction produced in response to a single action potential All or nothing for a given muscle fibre Can be induced for a muscle fibre, a motor unit or a whole muscle Latency: time delay between stimulation (nerve AP or direct electrical) – E-C coupling Contraction phase – cross-bridge cycling Relaxation phase – Ca2+ uptake Types of muscle contraction Easy to cause muscle damage Twitch contraction Frequency and Summation Action potential: 2 ms Contraction: 10–200 ms, possible to summate Tension (force) depends on [Ca2+]i Freq. of stim., Ca2+ release>uptake, [Ca2+]i When the system is saturated (all troponin bind with Ca2+ and cross-bridge cycling maxed out) – tetanus (long lasting contraction) Frequency and Summation Types of muscle contraction 1. Isometric contraction  Muscle does not shorten  No work is done  Load > tension 2. Isotonic contraction  Constant force  Work is being done  Load < tension Types of muscle contraction Concentric vs. Eccentric contraction · Both involve changes in muscle length · Concentric contraction: · The muscle contracts and shortens (does work) · Eccentric contraction: · The muscle generates force as it lengthens absorb energy - puts the body in position to contract concentrically · Quadriceps in squat: · eccentric – isometric – concentric Measurement Methods 1. Isometric Muscle Strength Testing 2. Isokinetic Muscle Testing 3. Handheld Dynamometry 4. Manual Muscle Testing (MMT) 5. Electromyography (EMG) 6. Muscle Biopsy 7. Ultrasound Imaging 8. Force Plate Analysis The Motor Unit  One motor neuron innervates many fibres  One fibre innervated by only one neuron  Same type of fibres within the motor unit  All the muscle fibres in a motor unit will simultaneously contract when it “fires” A motor unit is a motoneuron axon and all the muscle fibres it innervates Recruitment  MU recruitment occurs in a specific order according to Henneman’s size principle  Once one MU is activated, all fibres within the MU will contract maximally  Smallest MUs recruited first (type I)  Largest MUs recruited last (type II) Just to get this right… Summary See learning outcomes.