muscle physiology
Myofibril: structures that fill the sarcoplasm of the muscle fiber
Myofilaments: basic components of the muscle fiber, including thick and thin filaments. During shortening, they do not change in length, but their overlap changes as sarcomeres shorten.
Sarcomere: individual unit of muscle shortening. Muscle cells shorten because individual sarcomeres shorten which pulls the z-discs closer together
Actin: Actin is a contractile protein that, in muscles, forms part of the thin filaments that slide past myosin filaments to create muscle contraction. It works with myosin to convert the chemical energy from ATP into the mechanical energy of movement.
Myosin: protein that, in muscle cells, converts chemical energy (from ATP) into mechanical energy to drive muscle contraction by sliding along actin filaments.
Troponin: calcium-binding molecule which is stuck to each tropomyosin molecule. It is a regulatory protein. When calcium binds to it the complex changes shape. This complex essentially acts like a switch, starting and stopping shortening of muscle cells.
Tropomyosin: regulatory protein which is held in the groove of the thin filament. Along with tropomyosin it acts like a switch which starts and stops shortening.
Acetylcholine (ACh): a neurotransmitter which is stored in vesicles within the synaptic knob. It binds to receptors on the motor end plate, which opens channels that start an electrical signal, which then stimulates the muscle cell to contract.
Acetylcholinesterase: an enzyme on the motor end plate which removes ACh from the receptors, causing nerve stimulation to cease during muscle relaxation.
Motor end plate: a component of the synapse which has ACh receptors and acetylcholinesterase. The voltage change in this region is called the end-plate potential (EPP).
Ligand-gated channels: ACh receptors act as ligand gates which open when ACh binds. This allows Na+ (sodium ions) to rush into the cell.
Voltage gated channels: these channels are stimulated by the local potential (EPP). These stimulate each other, resulting in the production of the action potential. In the T tubules, they open, causing calcium gates to open in the sarcoplasmic reticulum.
Na+: sodium which rushes into cells when ACh binds to ligand gates, resulting in the local potential (EPP).
K+ (potassium): channels for this ion open upon ACh binding. Accumulation of extracellular K+ lowers the membrane potential and excitability, contributing to fatigue.
Ca+ (calcium): stored in the sarcoplasmic reticulum and released from SR during excitation-contraction coupling. It binds to troponin. Active transport pumps calcium from the sarcoplasm back into the SR during relaxation. In smooth muscle, calcium comes from extracellular fluid.
Primary active transport: described as active transport pumps which return calcium from the sarcoplasm back into the SR during relaxation.
Triads: this structure contains one tubule and two terminal cisternae, and lies in the sarcoplasmic reticulum. It is related to sarcolemma which has t tubules that penetrate the cell and sarcoplasmic reticulum for calcium storage. The action potentials reach the t tubules causing calcium gates to open in the SR.
ATP: all muscle contraction depends on ATP. It converts chemical energy into mechanical energy. It is hydrolyzed by myosin to activate the myosin head. It is also required for the myosin head to release the thin filament and is necessary for muscle relaxation. Declining synthesis of ATP causes fatigue.
Phophogen system: meets most of the ATP demand during short, intense exercise such as sprinting. It works by transferring Pi from other molecules. This system is quickly exhausted and is replenished during EPOC.
Myoglobin: supplies the oxygen needed during shirt, intense exercise, it is one of the oxygen reserves that must be replaced after strenuous exercise (EPOC). Slow-twitch fibers contain more myoglobin.
Treppe: (staircase phenomenon) occurs during moderate frequency stimulation. Each twitch develops more tension than the one before, even though there is time to revolver because calcium was not completely put back into the sarcoplasmic reticulum.
Tetanus: a state achieved through high-frequency. When the frequency is maximum, twitches fuse into a smooth, prolonged contraction known as complete tetanus. Incomplete tetanus generates sustained fluttering contractions.
Isometric: a type of muscle contraction that develops tension without changing the length of the muscle.
Isotonic: a type of muscle contraction characterized by tension development while shortening
Concentric: the phase of isotonic muscle contraction that involves tension development while the muscle is shortening.
Eccentric: type of muscle contraction that occurs when a muscle lengthens under tension. Eg: lowering a weight during a bicep curl.
Motor unit: consists of a single motor neuron and all the muscle fibres it innervates. These muscle fibers are dispersed throughout the muscle. This arrangement allows for sustained long-term contraction, such as postural control, as different motor units take turns resting.
Slow vs fast twitch: also known as glycolytic vs aerobic
Slow oxidative (slow twitch) fibers are adapted for aerobic respiration and are resistant to fatigue. They contain mitochondria, myoglobin, and capillaries.
Fast glycolytic (fast twitch) fibers are rich in enzymes for the phosphogen and glycogen-lactic acid system. Their sarcoplasmic reticulum releases calcium quickly, resulting in faster contractions.
Hypertrophy: (growth) is stimulated by resistance training (weight lifting), which causes cell enlargement due to the synthesis of more myofilaments.
Atrophy: decrease in muscle fiber size due to protein degradation exceeding synthesis, which is often a result of disuse, aging, or disease which leads to muscle weakness.
Effects of resistance training: stimulates cell enlargement due to synthesis of more myofilaments.
Effects of endurance training: aerobic exercise produces an increase in mitochondria, glycogen and the density of capillaries.
Characteristics of smooth muscle: cells are fusiform with only one nucleus and lack visible striations, sarcomeres, or z-discs. Contraction is involuntary and can be triggered without nerve stimulation by factors like hormones, stretch, or O2 deficiency. The sarcoplasmic reticulum is scanty, and there are no t-tubules; calcium for contraction comes from extracellular fluid. Contraction and relaxation are very slow compared to skeletal muscle.