Summary
Highlights
Muscle contraction occurs when an action potential signals the sarcoplasmic reticulum to release calcium. Calcium binds to troponin, displacing tropomyosin, which frees myosin binding sites on actin, allowing myosin heads to form cross-bridges and pull actin filaments. Relaxation happens when calcium channels close, calcium returns to the sarcoplasmic reticulum, and tropomyosin re-blocks myosin binding sites.
The length of the sarcomere significantly affects muscle tension. Optimal tension is achieved when sarcomeres are at an ideal length, allowing for maximum overlap between myosin and actin. If sarcomeres are too stretched or too compressed, the muscle produces minimal tension during contraction because the myosin heads cannot effectively bind to actin.
The neuromuscular junction is the site where a somatic motor neuron transmits a signal to a skeletal muscle fiber. An action potential in the neuron causes the release of acetylcholine (ACh) into the synaptic cleft. ACh then binds to receptors on the muscle fiber membrane, which are also sodium channels. This binding opens the channels, allowing sodium to rush into the muscle cell, initiating an action potential and muscle contraction. Acetylcholine is subsequently broken down by acetylcholinesterase to allow muscle relaxation.
Muscles generate ATP using three main pathways: stored ATP (used in the first 6 seconds), creatine phosphate (provides ATP for another 10 seconds), and anaerobic glycolysis (produces 2 ATP per glucose, generating lactic acid). For prolonged activity, aerobic cellular respiration is used, producing a much larger amount of ATP (36-38 ATP per glucose) in the mitochondria, utilizing glucose, fatty acids, and amino acids with oxygen.
After exercise, the body enters a period of oxygen debt, where it replenishes oxygen stores, creatine phosphate, and glycogen. Lactic acid accumulated during anaerobic glycolysis is transported to the liver, where it can be converted back into glucose or pyruvate for energy. Cardiac muscle metabolizes very efficiently, primarily using aerobic respiration and having a constant oxygen supply.
A motor unit consists of a single neuron and all the muscle fibers it innervates. High-precision motor units have fewer muscle fibers per neuron (e.g., eye muscles), while low-precision units have many (e.g., thigh muscles). Motor units follow an 'all-or-none' principle, meaning all fibers in the unit contract simultaneously if the stimulus is strong enough. Muscle contractions can be observed in a myogram, showing phases like latent period, contraction, and relaxation, and demonstrating phenomena like wave summation and tetanus.
Isotonic contractions involve the muscle changing length while maintaining constant tension, leading to movement. This includes concentric (muscle shortening, e.g., lifting a weight) and eccentric (muscle lengthening, e.g., lowering a weight) movements. Isometric contractions involve muscle tension increasing without a change in length, resulting in no movement (e.g., holding a weight steady). Both types expend energy.
Skeletal muscle fibers are classified based on appearance (red vs. white meat, referring to myoglobin, mitochondria, and blood supply) and metabolic characteristics (ATP turnover rate). Three main types are slow oxidative fibers (small, least powerful, slow contraction, high endurance), fast oxidative-glycolytic fibers (intermediate size, moderate fatigue resistance, faster contraction), and fast glycolytic fibers (largest, most powerful, quick but low endurance, rely on glycolysis).
Regular exercise, including warm-ups and strength training, improves muscle size (hypertrophy), strength, endurance, and bone health, while also increasing resting metabolic rate. Anabolic steroids, similar to testosterone, can increase muscle size but carry significant health risks. Muscles regenerate to a limited extent, mainly through hypertrophy (increase in cell size) rather than hyperplasia (increase in cell number). As we age, muscle mass and strength progressively decline, emphasizing the importance of lifelong physical activity.