Muscle Physiology PSK 4U1

Structure of Skeletal Muscle: Connective Tissue Coverings Epimysium Surrounds entire muscle Perimysium Surrounds bundles of muscle fibers Endomysium Surrounds individual muscle fibers

Structure of Skeletal Muscle: Sarcolemma Muscle cell membrane Myofibrils Threadlike strands within muscle fibers Actin (thin filament) Troponin Tropomyosin Myosin (thick filament)

The Sarcomere The sarcomere is a functional unit, of skeletal muscle.

The ‘Sliding Filament’ Theory of Muscular Contractions Whenever we recruit muscles to cause movement, the following sequence of events occurs: An electrical signal is sent along nerves from the central nervous system (CNS) to a neuromuscular junction at the target muscle.

The Neuromuscular Junction Site where motor neuron meets the muscle fiber

Electrical nerve impulses become chemical 2) When an electrical nerve impulse reaches the axon, a neurotransmitter called Acetylcholine (Ach) is released.

Depolarization 3)Acetylcholine (Ach) triggers the muscle fiber to become permeable to sodium ions, which flow into the muscle from interstitial fluid, and cause the muscle to ‘depolarize’.

The Release of Calcium 4) A depolarized muscle fiber leads to the release of calcium ions (Ca++) from the sarcoplasmic reticulum, a.k.a. ‘Z-lines’.

Troponin & Tropomyosin 5)The thin filaments (actin), in a myofibril, have binding sites for the globular protein heads of the thick filaments (myosin). In a relaxed muscle, these binding sites are covered by the proteins, troponin and tropomyosin. Calcium ions cause the troponin and tropomyosin to unwind, exposing the myosin binding sites on actin filaments.

Actin/Myosin Crossbridging 6)When the myosin binding sites are exposed, actin and myosin filaments bind, and the globular protein heads ‘cock’, pulling the actin filaments along.

Cross-Bridge Formation in Muscle Contraction

Muscular Contraction 7) Muscle shortening occurs due to the movement of the actin filament over the myosin filament because of a reduction in the distance between Z-lines in the sarcomere.

Energy for Muscle Contraction (ATP) ATP is required for muscle contraction It causes the protein heads on myosin to ‘cock’. It is required to break the actomyosin bond. Energy from the hydrolysis of ATP is used for active transport of calcium ions back into the sarcoplasmic reticulum.

Sources of ATP for Muscle Contraction

Motor Units A motorunit is a single motorneuron & all the muscle fibers it innervates. Eye muscles – 1:1 muscle/nerve ratio Hamstrings – 300:1 muscle/nerve ratio

Clinical Physiology Rigor mortis Electrocution Parkinson’s Disease Tetanus

The Sliding Filament Theory https://www. youtube. com/watch

The ‘All or None’ Principle

Optimal Muscle Length for a Maximal Strength Contraction

Muscle Fiber Types Fast Twitch fibers Slow Twitch fibers Type IIb fibers Fast-twitch fibers Fast-glycolytic fibers Type IIa fibers Intermediate fibers Fast-oxidative glycolytic fibers Slow Twitch fibers Type I fibers Slow-twitch fibers Slow-oxidative fibers

Comparison of Maximal Shortening Velocities Between Fiber Types

Fiber Types and Performance Power athletes Sprinters Possess high percentage of fast fibers Endurance athletes Distance runners Have high percentage of slow fibers Others Weight lifters and nonathletes Have about 50% slow and 50% fast fibers

Alteration of Fiber Type by Training Endurance and resistance training Cannot change fast fibers to slow fibers Can result in shift from Type IIb to IIa fibers Toward more oxidative properties

Training-Induced Changes in Muscle Fiber Type

Getting Stronger Hypertrophy and Hyperplasia Increase in size of muscle Increase in number of motor units

Coordination Intermuscular The ability to recruit the optimal balance of agonists, antagonists, synergists, and stabilizers to produce smooth, purposeful, and efficient movements Intramuscular An untrained athlete can only recruit up to 60% of the motor units in a given muscle during maximal contraction, wheras a trained athlete can recruit up to 90%

Force Regulation in Muscle Types and number of motor units recruited More motor units = greater force Fast motor units = greater force Initial muscle length “Ideal” length for force generation The right balance of agonists, antagonists etc.

Proprioceptors 3) Pacinian corpuscles 1) Muscle spindles Detect dynamic and static changes in muscle length Stretch reflex Stretch on muscle causes protective contraction 2) Golgi tendon organs (GTO) Monitor tension developed in muscle Prevents damage during excessive force generation Stimulation results in protective relaxation of muscle 3) Pacinian corpuscles Initiate protective contraction in response to excessive pressure and changes in temperature.

Muscle Spindle

Golgi Tendon Organ

Pacinian Corpuscles

Proprioceptive Neuromuscular Facilitation