☰ Year plan · Biology

Muscles and how they contract

Week 3 · Chapter 1 (C): movement in man: the muscular system and the structure of skeletal muscle, the nerve impulse reaching the muscle, the sliding-filament theory, ATP, muscle fatigue and spasm · 3 lessons
1

Three systems, one movement: muscles and their structure

Biology Sec 3 Term 1, Chapter 1: pp. 15–20
GoalName the three systems that work together in movement, and describe skeletal muscle from the whole muscle down to its filaments.
New words
muscle fibre · a long muscle cell with many nuclei; it holds 1000 to 2000 myofibrilsmyofibril · a thread inside a muscle fibre, made of thin actin and thick myosin filamentssarcomere · the stretch of a myofibril between two Z-lines; the unit that shortensstriated muscle · muscle with a pattern of dark and light bands: skeletal and cardiac muscle
Explanation

Movement in the human body needs three systems working together. The skeletal system gives muscles a firm place to attach and supports the moving limbs, with joints playing a big part. The nervous system sends the orders as nerve impulses. The muscular system does the moving. The muscular system is all the muscles of the body, about 620 or more. A muscle can contract and relax. Contraction moves the body from place to place, and it keeps blood moving in the vessels through smooth muscle in their walls. Most muscles are under the control of the will, the voluntary muscles (skeletal, striated); the others are involuntary. A skeletal muscle is built in levels. Its muscle fibres, which are long cells with many nuclei, lie together in bundles wrapped in a membrane called the perimysium. Each fibre has sarcoplasm (its cytoplasm) inside a membrane called the sarcolemma, and it holds 1000 to 2000 myofibrils running along its length. In a myofibril, the light I-band is made of thin actin filaments and is cut in the middle by a dark Z-line; the dark A-band is made of thick myosin filaments and has a lighter H-zone in its middle. The distance between two Z-lines is a sarcomere. Skeletal and cardiac muscle show this pattern of bands and are called striated; smooth muscle does not and is non-striated.

Thin actinfilamentsThick myosinfilamentsMyofibrilsMuscle fibres(cells)Muscle bundlesSkeletal muscle
Worked examples
MuscleStriated?ControlWhere
Skeletalyesvoluntaryattached to bones
Cardiacyesinvoluntarythe heart
Smoothnoinvoluntarywalls of blood vessels and the gut
Three kinds of muscle compared.
Counting inside a muscle, with invented numbers inside the book’s range. If one fibre holds 1500 myofibrils, a small bundle of 20 fibres holds 20 × 1500 = 30 000 myofibrils, and each myofibril is a long chain of sarcomeres.
BandLooksMade ofMiddle
I-bandlightthin actin filamentsa dark Z-line
A-banddarkthick myosin filamentsa lighter H-zone
Reading the bands. The picture shows what each band is made of and what it shows in the middle.
Class activity

Voluntary or not? With a grown-up, list ten things your body does in one hour, for example lifting a cup, your heart beating, food moving along your gut, walking, your blood vessels carrying blood, writing. Sort each one into voluntary muscle (skeletal) or involuntary muscle (cardiac or smooth) and write beside it which of the three systems, skeletal, nervous or muscular, plays a part. Then make a fist and feel the muscles of your forearm tighten.

Practice
1
Name the three systems that work together in movement and say what each one does.
2
What are the cell membrane and the cytoplasm of a muscle fibre called?
3
Which protein forms the thin filaments of a myofibril and which forms the thick ones?
4
Name a muscle that is striated but not controlled by the will.