Muscles let your body move, hold posture, breathe, and protect joints. A muscle contraction begins when the nervous system sends an electrical signal to muscle tissue. In the arm, the biceps contracts to bend the elbow while the triceps relaxes.
This teamwork makes movement smooth, controlled, and safe.
Key Facts
- A motor neuron carries a nerve signal from the spinal cord to muscle fibers.
- At the neuromuscular junction, acetylcholine helps start a muscle fiber action potential.
- Calcium ions allow actin and myosin filaments to interact inside muscle fibers.
- Sliding filament model: myosin pulls actin, so the sarcomere shortens.
- Muscles pull on bones through tendons, but they do not push.
- For elbow bending, the biceps is the agonist and the triceps is the antagonist.
Vocabulary
- Motor neuron
- A nerve cell that carries signals from the brain or spinal cord to a muscle.
- Muscle fiber
- A long muscle cell that can contract when it receives the correct signal.
- Sarcomere
- The repeating unit inside a muscle fiber that shortens during contraction.
- Actin
- A thin protein filament that slides past myosin during muscle contraction.
- Tendon
- A strong band of connective tissue that attaches muscle to bone.
Common Mistakes to Avoid
- Saying muscles push bones is wrong because muscles create movement by pulling through tendons.
- Thinking the biceps and triceps contract at the same full strength during elbow bending is wrong because one muscle usually contracts while the opposing muscle relaxes to allow motion.
- Forgetting the nerve signal is wrong because voluntary muscle contraction begins with signals from the nervous system.
- Saying muscle fibers shorten because the filaments shrink is wrong because actin and myosin slide past each other while the filaments themselves stay nearly the same length.
Practice Questions
- 1 A motor neuron signal travels 1.2 m from the spinal cord to a muscle at 60 m/s. How long does the signal take to arrive?
- 2 A sarcomere shortens from 2.4 micrometers to 2.0 micrometers during contraction. What is the change in length and what percent of the original length is this?
- 3 When you slowly lower a book back to a table, explain how the biceps and triceps work together to control the motion.