A TIMELINE FROM NEURON TO CELL
Before movement can happen, the applied tension must overcome resistance.
When muscle cells contract, they pull on collagen fibers, this force is called tension.
The term ‘lock and key’ refers to the calcium as the key that unlocks the active sites and starts a contraction
An impulse travels through a motor neuron, releasing ACh innervating a receptor called the synaptic cleft on the muscle fiber/cell.
Electrical impulses given by the sarcolemma trigger a contraction by altering the chemicals inside the muscle fiber
At the opening of each transverse tubule (T-tube) on the muscle fiber surface, the action potential spreads inside the muscle fiber
ACh binds with receptors on the cell membrane of the muscle fiber/cell, opening calcium and sodium channels.
The membrane of the fiber will depolarize as positively charged sodium ions enter it, this triggers action potential
At each point where a T-tubes touch part of the sarcoplasmic reticulum, it causes the release of calcium ions
The rest of the membrane will depolarize which triggers the release of calcium ions stored in the sarcoplasmic reticulum
Expanded chambers of the SR called terminal cisternae are on both sides of the T-tubules; they contain high concentrations of calcium ions
Calcium is released from the cisternae into the muscle fiber.
Open channels allow sodium ions into the cytoplasm of the muscle fiber.
The incoming sodium also sends a message in the muscle fiber to trigger the release of stored calcium ions.
The calcium ions diffuse into the muscle fiber.
The relationship between the chains of proteins within the muscle cells changes
Tropomyosin is a protein that covers links of actin filament and myosin binding sites to prevent actin from binding to myosin.
Tropomyosin binds to troponin to form a troponin-tropomyosin complex.
Troponin also has a binding side for calcium ions.
Myosin heads attach to actin by breaking down ATP to ADP and a phosphate
Tropomyosin has to expose the myosin binding site on an actin filament to allow cross-bridge formation between actin and myosin.
Calcium has to bind with troponin so tropomyosin can slide away from the binds sites, which allows it to form cross bridges on exposed binding sites.
Thin filaments are pulled by the myosin to slide past thick filaments towards the sarcomere.
For thin filaments to
continuously slide past thick filaments, myosin has to pull actin, which slides over the myosin – known as the power Stroke.
Every time ATP is turned into ADP + Phosphate, the myosin cocks into place to form another cross bridge with actin
The release of ADP completes the cross-bridge movement and ATP attaches to myosin, breaking the cross bridge.
Calcium ions remain in the sarcoplasm to bind with troponin, and ATP is available to uphold the cross-bridge cycle and keep myosin pulling actin strands.
When the stimulation of impulses to the muscle fibers stops, the chemical reaction originally caused stops. The chemical process in the muscle fibers is reversed, and the muscle relaxes.
Myofibrils: bundles of protein filaments made of actin and myosinEpimysium: layer of collagen fibers that surrounds the entire muscle
Perimysium: divide the skeletal muscle into compartmentsFascicle: a bundle of muscle fibers
Endomysium: Within a fascicle, it surrounds each skeletal muscle fiber (cell) and ties adjacent muscle fibers together
Tendon: bands of collagen fibers that attach skeletal muscles
Aponeurosis: broad sheet of collagen fibers connect different skeletal muscles
Muscles Cell: larger than most cells, has hundreds of nuclei
Motor Neuron: a nerve cell that controls skeletal muscle fibers
Motor unit: a single motor neuron and all the muscle fibers it controls
Calcium: the key that unlocks active sites and starts a contraction
Thick Filaments: A myosin filament in a skeletal or cardiac muscle
Thin Filaments: An actin filament in a skeletal or cardiac muscle cell
Actin: a type of protein found in thin filaments
Myosin: a type of protein found in thick filaments
Sarcoplasmic Reticulum: specialized form of smooth endoplasmic reticulum that forms a tubular network around each myofibril and is tightly bound to T tubules
T-Tubes: form passageways through muscle fiber like tunnels
Sarcolemma: plasma membrane of a muscle cell that surrounds sarcoplasm (cytoplasm)
Sarcoplasm: cytoplasm of muscle cells
Axon: Nerve fibers
Acetylcholine (ACh): a neurotransmitter; contained in the mitochondria and vesicles of an axon
Receptors: a protein molecule that receives chemical signals from outside a cell
Terminal cisternae: A storage area of calcium
Lock & Key: calcium is the key that unlocks the active sites and starts a contraction
Sarcomere: smallest unit of the muscle fiber that make up myofibril
ATP: the energy source for muscle contraction
Creatine phosphate: high energy compound as a result of ATP transferring energy to creatine
Creatine: a small molecule that muscle cells assemble from fragments of amino acids
Sodium: help your nerve cells send electrical signals that tell your muscles to contract
ADP: result of one phosphate being removed from ATP
Action potential: electrical impulse made by skeletal muscle fibers after being stimulated by neurons
Synaptic Cleft: a narrow space that separates the axon terminal from the sarcolemma; the part of the sarcolemma that it separates contains receptors that bind ACh
Active site: a place where actin interacts with myosinNeuromuscular junction: nervous system and a skeletal muscle fiber communicate at a specialized intercellular connection
By Serena Krueger