Applied Sciences - Muscle Physiology Part 1

Applied Sciences - Muscle Physiology Part 1

• Course code: PT4050 and PT7001
• Lecturer: Chris Chamberlin and Dr Ben Hunter
• Email: [email protected]

Learning Outcomes

• Neuromuscular junction
• Muscle structure
• Contraction types
• Cross-bridge formation
• Motor units

Structure

• Sarcolemma: The plasma membrane of a muscle cell
• Sarcoplasm: The cytoplasm of a muscle cell
• Myofibrils: Contractile proteins within muscle cells
• Mitochondria: Cellular energy producers
• Sarcoplasmic reticulum: Smooth ER, stores and releases calcium
• Transverse tubule (T tubule): Network that carries action potentials

Neuromuscular Junction (NMJ)

• Action potential travels down a motor neuron axon terminal
• Calcium channels open, releasing acetylcholine (ACh)
• ACh binds to receptors on the motor end plate
• Ligand-gated sodium channels open
• Action potential generated, following “all-or-nothing” principle
• Action potential propagates along sarcolemma

Excitation-Contraction Coupling

• ACh released at NMJ, receptor, AP
• Action potential down T tubules
• AP triggers Ca²⁺ release from Sarcoplasmic Reticulum (SR)
• Ca²⁺ binds to troponin, exposing myosin-binding sites
• Cross-bridge cycling occurs
• Ca²⁺ re-uptake actively
• Myosin binding sites blocked

Striations

• Skeletal muscles exhibit striations when viewed longitudinally.
• Highly organized intracellular structures called myofibrils are responsible for striations.

Muscle Structure - Levels of Organisation

• Epimysium: Surrounds the entire muscle.
• Perimysium: Surrounds fascicles (bundles of muscle fibers).
• Endomysium: Surrounds individual muscle fibers.

Structure of a Myofibril / Sarcomere

• Myofibril: Elongated protein structures within muscle fibers.
• Sarcomere: The basic contractile unit of a myofibril; it's the region between two Z lines
• Z lines: Define the borders of a sarcomere.
• M lines: Located in the center of the sarcomere.
• A band: Contains the entire length of the thick filaments.
• I band: Contains only thin filaments.
• H zone: Center of a sarcomere, and only thick filaments.
• Thin filaments: Primarily made of actin.
• Thick filaments: Primarily made of myosin.
• Cross-bridges: Myosin heads that extend and interact with actin.

Myofilaments

• Actin: A contractile protein.
• Troponin: A regulatory protein.
• Tropomyosin: A regulatory protein

Myosin

• Myosin molecules have heads and tails.
• Two myosin molecules bound at their tails—forming cross-bridges

The Crossbridge Cycle

• Binding of myosin to actin
• Power stroke (low energy state)
• Rigor (low energy state) Unbinding of myosin and actin
• Cocking of myosin head (high energy state)

Sliding Filaments

• Myosin/actin filaments do not shorten
• A-band remains the same length
• I-band shortens
• H-band shortens

Length-Tension Relationship

• Optimal length for maximum force generation.
• Relevance in physiotherapy

Twitch Contraction

• Contraction produced in response to a single action potential
• All or nothing for a given muscle fibre
• 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

• Isometric: Muscle does not shorten, load > tension
• Isotonic: Constant force, work is done, load < tension. Types: concentric (muscle shortens), eccentric (muscle lengthens)

Frequency and Summation

• Action potential: 2 ms.
• Muscle contraction: 10-200 ms. The tension force depends on [Ca²⁺]
• When the system is saturated, tetanus (long lasting contraction): freq. of stim.↑, Ca²⁺ release > uptake, [Ca²⁺] ↑

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".

Recruitment

• MU recruitment occurs in a specific order according to Henneman's size principle.
• Smallest MUs recruited first (type I), largest MUs recruited last (type II).

This quiz focuses on the fundamentals of muscle physiology, covering key concepts such as the neuromuscular junction, muscle structure, and types of contractions. It also delves into the components of muscle cells, including sarcolemma, myofibrils, and mitochondria. Test your knowledge on these essential topics!