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🫀 Science · Human Anatomy and Physiology · Lesson 3 of 12 · 5 min

Muscular system: muscle types, sliding filaments and muscle pairs

💪 Every heartbeat, breath and step depends on proteins sliding past each other. The TEAS wants you to know the three muscle types and how a contraction actually happens.

💡 Key idea

Calcium exposes binding sites on actin, and ATP-powered myosin heads pull actin so the sarcomere shortens.

🧠 The idea

There are three MUSCLE TYPES. Skeletal muscle is striated, multinucleated and voluntary; it moves bones. Cardiac muscle is striated and involuntary, with branched cells joined by intercalated discs. Smooth muscle is non-striated and involuntary, lining hollow organs and vessels. A skeletal muscle fiber is packed with myofibrils made of repeating SARCOMERES, the contractile units. In the SLIDING FILAMENT model, a nerve impulse at the neuromuscular junction releases acetylcholine; the fiber releases calcium from the sarcoplasmic reticulum; calcium binds troponin, shifting tropomyosin off the binding sites on actin; myosin heads bind actin and pull the thin filaments toward the center using ATP. The sarcomere shortens but the filaments themselves do not. ATP is also needed for myosin to detach. Tendons attach muscle to bone. Muscles work in pairs: the AGONIST (prime mover) contracts while the antagonist relaxes, as the biceps and triceps do at the elbow. The origin is the fixed attachment; the insertion is the one that moves.

✏️ Worked example

You bend your elbow to lift a cup. The biceps brachii is the agonist and contracts; the triceps brachii is the antagonist and relaxes. Inside the biceps: acetylcholine triggers calcium release, calcium binds troponin, myosin grabs actin, and each sarcomere shortens. To straighten the arm, the roles flip: the triceps becomes the agonist. If ATP ran out, myosin could not let go of actin, which is why muscles stiffen after death (rigor mortis).

📌 Takeaways

  • Skeletal = striated, voluntary; cardiac = striated, involuntary; smooth = non-striated, involuntary.
  • Sarcomere shortens as actin slides over myosin; calcium and ATP are required.
  • Agonist contracts, antagonist relaxes; tendons connect muscle to bone.

✅ Check yourself

Question 1 of 3

Which muscle type is striated and involuntary?

  1. Smooth
  2. Visceral
  3. Cardiac
  4. Skeletal
Show the answer and rationale

Answer: C

Cardiac muscle has striations like skeletal muscle but is not under conscious control. Skeletal muscle is voluntary, and smooth (visceral) muscle has no striations.

Question 2 of 3

During contraction, calcium ions bind to which protein?

  1. Tropomyosin
  2. Actin
  3. Myosin
  4. Troponin
Show the answer and rationale

Answer: D

Calcium binds troponin, which moves tropomyosin and exposes binding sites on actin. Myosin then binds actin; calcium does not bind myosin, actin or tropomyosin directly, since tropomyosin is the strand that troponin shifts aside.

Question 3 of 3

When the knee extends, the quadriceps contract. What is the hamstring group doing?

  1. Contracting as a synergist
  2. Holding as a fixator
  3. Contracting as the prime mover
  4. Relaxing as the antagonist
Show the answer and rationale

Answer: D

The quadriceps are the agonist (prime mover) for knee extension, so the hamstrings, which flex the knee, relax as the antagonist. A synergist contracts to assist the prime mover and a fixator steadies a bone, but contracting hamstrings would oppose extension.

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