physio

Muscles act as a lever system in the body to facilitate movement by pulling against a load.
To achieve movement, muscles must shorten or contract.

Epimysium: The outer layer of connective tissue surrounding an entire muscle.
Muscle Fascicle: A bundle of muscle fibers (cells) within the muscle.
Muscle Fiber (Myocyte): An individual muscle cell.
Perimysium: The connective tissue surrounding each fascicle.
Endomysium: The innermost layer of connective tissue surrounding each muscle fiber.
Satellite Cell: A type of stem cell involved in muscle repair and growth.
Sarcolemma: The cell membrane of a muscle fiber.
Myofibril Protein: Proteins that make up the myofibrils which are essential for muscle contraction.

Nucleus: Muscle fibers are multinucleated cells.
Light I Band: Region containing only thin (actin) filaments, appears lighter under a microscope.
Dark A Band: Region containing thick (myosin) filaments, overlaps with thin filaments, appears darker.
Sarcomere: The basic contractile unit of muscle, defined as the segment between two Z discs.
Sarcoplasmic Reticulum: The organelle that stores calcium ions needed for muscle contraction.
Thin Filament: Composed mainly of actin; involved in muscle contraction.
Z Disc: The boundary that delineates one sarcomere from the next and anchors the thin filaments.
H Zone: The central region of the A band where there is no overlap of thin and thick filaments.
M Line: Holds thick filaments in place at the center of the A band.
Mitochondrion: Provides ATP through aerobic respiration for muscle contraction.

Relaxed Sarcomere: A state where the muscle is not contracted, displaying the lengthened configuration of filaments.
H Zone: Area within the A band that contains only thick filaments.
Z Disc: Anchors thin filaments and serves to organize the contractile proteins within the sarcomere.

Tropomyosin: A regulatory protein that blocks myosin binding sites on actin when the muscle is relaxed.
Troponin: A complex of three proteins that regulate muscle contraction by binding calcium ions, facilitating the movement of tropomyosin.
Actin Subunits: The individual molecules that compose the thin filament and hold binding sites for myosin.

Myosin Molecule: Composed of a long tail and a flexible head which interacts with actin.
ATP-Binding Site: Located on the myosin head, necessary for energy transfer in the contraction process.
Actin-Binding Sites: Specific sites on myosin that attach to actin during contraction.

Motor Unit: The motor neuron and all the muscle fibers it innervates.
Somatic Motor Neuron: The nerve cell responsible for inducing movement in skeletal muscles by releasing neurotransmitters.
Neuromuscular Junction: The synapse between a motor neuron and a muscle fiber, where ACh (acetylcholine) is released to induce contraction.

Action Potential Initiation: When an action potential travels down a motor neuron, it causes the release of ACh from synaptic vesicles into the synaptic cleft.
Receptor Binding: ACh binds to nicotinic receptors on the motor end plate, leading to sodium (Na+) influx and depolarization of the muscle membrane (sarcolemma).
EPP Development: The end-plate potential (EPP) is generated as Na+ influx changes the membrane potential.
Action Potential Propagation: The EPP leads to the opening of voltage-gated sodium channels, initiating an action potential that travels along the sarcolemma and into the T-tubules.
Calcium Release from SR: The action potential triggers calcium channels in the sarcoplasmic reticulum to open, releasing Ca2+ into the sarcoplasm.

Calcium Binding: Ca2+ ion binds to troponin, causing a conformational change that moves tropomyosin away from myosin binding sites on actin.
Crossbridge Formation: Myosin heads bind to exposed binding sites on actin, forming a crossbridge.
Power Stroke: Myosin head pivots and pulls actin filament towards the M line, releasing ADP and Pi.
Release and Re-Cocking: ATP binds to myosin, causing it to detach from actin, followed by hydrolysis of ATP which re-cocks the myosin head for another cycle.

Cease Nervous Stimulation: Neural input ends, halting the action potential.
Calcium Channels Close: Calcium levels decrease as channels close.
Calcium Reuptake: Calcium is pumped back into the sarcoplasmic reticulum using active transport.
Myosin Detachment: Myosin heads detach from actin, resulting in relaxation of the muscle.

A single muscle fiber can contract steadily and maximally when adequately stimulated before completing relaxation, ensuring all myosin heads engage in crossbridge cycling.
Some key concepts in excitation-contraction coupling are:
- Absolute Refractory Period: The time following an action potential during which another action potential cannot be generated, affecting muscle fiber responses to stimuli.
- Treppe Phenomenon: The gradual increase in muscle tension with repetitive stimulation.

Isotonic Contraction: Muscle length changes.
- Concentric: Muscle shortens (force > load).
- Eccentric: Muscle lengthens (force < load).
Isometric Contraction: Muscle length does not change (force = load).

Resting Muscle: Uses creatine phosphate to rapidly regenerate ATP.
- Creatine phosphate donates a phosphate group to ADP to form ATP during short burst activities.
Active Muscle: Primarily relies on glycogen breakdown through glycolysis and oxidative phosphorylation for ATP generation.
- Glycolysis: An anaerobic process that converts glucose into ATP, producing lactic acid.
- Aerobic Respiration: Utilizes oxygen and fatty acids for ATP production, often seen in prolonged activities.
Creatine and Creatine Phosphate: Function as an immediate energy source but deplete quickly (6-15 seconds).
Muscle Fatigue Factors: Includes a reduction in force production, depletion of ATP, and the accumulation of ADP causing slower muscle shortening. Central fatigue also affects performance as the brain reduces stimulation to lower motor neurons.

Fast-Twitch Fibers (Type II): Characterized by:
- Rapid ATP production using anaerobic processes.
- High glycolytic capacity and fatigue quickly.
Slow-Twitch Fibers (Type I): Characterized by:
- Slower ATP production via aerobic respiration.
- Enhanced endurance due to high myoglobin content and more mitochondria.

Aerobic Threshold: The exercise intensity at which the body begins to accumulate lactate.
Lactate Threshold: Indicates the efficiency of oxygen usage and when muscles switch from aerobic to anaerobic metabolism.
VO2 Max: Maximum oxygen consumption, correlating with fitness levels and endurance capabilities.

Resistance Training: Leads to muscle hypertrophy and increased glycogen storage, particularly in fast-twitch fibers.
Endurance Training: Augments myoglobin levels, capillary density, and overall mitochondria count in slow-twitch fibers, improving aerobic capacity and reducing fatigue.
Exercise can also enhance overall metabolism and decrease inflammation, contributing to improved cardiovascular health and better muscle function with age.