Muscle Contraction and Energy Sources

Study Notes

The cycle of myosin attaching to actin and pulling it towards the center of the sarcomere is known as the ratchet mechanism, which shortens the sarcomere and thus the muscle.
Myofibrils are the contractile units of a muscle fiber.
Both ATP and calcium ions are essential for muscle contraction.
ATP is required for the myosin heads to detach from actin and reattach further along the filament.
Calcium ions bind to troponin, which moves tropomyosin away from myosin-binding sites on actin, allowing contraction to occur.

Muscles obtain energy from four sources: glycogen and triglycerides stored within the muscle, and glucose and fatty acids from the blood.
The source of energy used depends on the intensity and duration of exercise.
As exercise time increases, the use of muscle energy stores decreases, while the use of energy sources from the blood increases.

Muscle cells have limited ATP stores and produce more ATP through three pathways: the creatine phosphate pathway, fermentation, and cellular respiration.
The creatine phosphate pathway is the simplest and fastest way to produce ATP, using creatine phosphate stored in the muscle during rest. This pathway provides energy for the first few seconds of muscle contraction.
Fermentation is an anaerobic process that produces ATP by breaking down glucose to lactate. It is fast-acting but leads to lactate buildup, causing short-term muscle aches and fatigue.
Cellular respiration is the slowest but most efficient pathway for ATP production. It occurs in the mitochondria and uses oxygen, glucose, and fatty acids as energy sources.

Motor neurons stimulate muscle fibers to contract.
A motor neuron's axon branches out to stimulate multiple muscle fibers.
The neuromuscular junction is the point of contact between an axon terminal and the sarcolemma (muscle cell membrane).
Acetylcholine (ACh) is a neurotransmitter released from synaptic vesicles at the axon terminal.
ACh binds to receptors on the sarcolemma, triggering electrical signals that spread down the T tubules and cause calcium to be released from the sarcoplasmic reticulum.
Calcium ions bind to troponin, which moves tropomyosin away from myosin-binding sites on actin, allowing myosin to bind and initiate the power stroke.
ATP is required for myosin to detach from actin and reattach further along the filament, continuing the contraction cycle.
The binding of ATP to myosin heads breaks the cross-bridges between myosin and actin.