Notes
Muscle structure
- Muscle tissue has excitability, contractility, extensibility and elasticity. Skeletal muscle fibers contain myofibrils made of sarcomeres, with thick (myosin) and thin (actin) filaments.
- Connective tissue wraps: endomysium, perimysium (around fascicles), epimysium. A motor unit is one motor neuron and its fibers.
Contraction
- Sliding filament model: ACh at the neuromuscular junction triggers an action potential, the SR releases calcium, calcium binds troponin, myosin heads bind actin and pull thin filaments inward using ATP. Filaments slide; they do not shorten.
- ATP sources: creatine phosphate, anaerobic glycolysis (lactic acid) and aerobic respiration. Fatigue and oxygen debt follow intense activity.
- Isotonic contractions shorten the muscle; isometric ones do not. Smooth muscle is slow and involuntary; cardiac muscle is autorhythmic.
Movement
- Origin is the stable attachment and insertion is the movable one. Agonists produce movement, antagonists oppose it. Know the common movements (flexion, extension, abduction, adduction) and major muscles.
Topics in this unit
Each topic has its own short notes and practice.
Practice
The SR releases calcium to trigger contraction and takes it back up to end it.
The sliding filament model says the filaments slide past each other and keep their lengths.
In an eccentric contraction the muscle lengthens while resisting a load.
Without ACh, the sarcolemma is not stimulated and the fiber cannot contract.
The hamstrings flex the knee, the opposite of knee extension by the quadriceps.
Extra oxygen intake restores energy stores after anaerobic activity.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A student lifts a heavy textbook by bending the elbow, holds it still for 10 seconds, then lowers it slowly.
(a) Name the prime mover and antagonist for lifting the book, and name the type of joint movement.
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The biceps brachii is the prime mover and the triceps brachii is the antagonist. The movement is flexion of the elbow.
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(b) Describe the type of muscle contraction while holding the book still and while lowering it.
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Holding still is an isometric contraction (tension without shortening). Lowering slowly is an eccentric contraction (the muscle lengthens while still producing tension).
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(c) Using the sliding filament model, explain how calcium and ATP allow a muscle fiber to contract and relax.
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Calcium released from the SR binds troponin, exposing binding sites on actin so myosin heads form cross-bridges and pull the thin filaments. ATP lets the heads detach and re-cock. When stimulation stops, calcium returns to the SR and the muscle relaxes.
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