☰ خطة العام · الأحياء (بالإنجليزية)

Muscles and how they contract

الأسبوع 3 · الفصل الأول (٣): الحركة في الإنسان: الجهاز العضلي وتركيب العضلة الهيكلية، وانتقال السيال العصبي إلى العضلة، ونظرية الخيوط المنزلقة، ودور ATP وإجهاد العضلة · 3 دروس
3

ATP, muscle fatigue and spasm

Biology Sec 3 Term 1, Chapter 1: pp. 15–20
GoalExplain why muscle needs ATP both to contract and to relax, and describe how fatigue builds up and how rest reverses it.
New words
ATP · the molecule that carries energy for the cell’s work, including the work of muscleglycogen · animal starch, a store of glucose in the liver and in musclelactic acid · the acid made when glucose is broken down without oxygen; its build-up goes with muscle fatigueaerobic respiration · breaking down glucose with oxygen to release a large amount of ATP
Explanation

Muscle needs ATP at two points in every contraction: ATP gives the cross bridges the energy to attach to actin and pull, and fresh ATP is needed to detach them again. If there is not enough ATP, the cross bridges cannot let go, so the muscle stays contracted and cannot relax. The book links this with a painful muscle spasm. Fatigue builds up in a different way. When a muscle contracts rapidly again and again, the blood cannot bring oxygen fast enough. The muscle then turns its glycogen into glucose and breaks the glucose down without oxygen, which gives some energy but also makes lactic acid. As lactic acid builds up, the muscle becomes tired. At rest, enough oxygen reaches the muscle again. Aerobic respiration then makes plenty of ATP, the cross bridges are released, and the muscle goes on contracting and relaxing in turn. A very strong spasm can tear a muscle and cause bleeding. For muscle pain that lasts after exercise, the advice is to rest, and to ask a doctor if the pain does not go away; no one should guess the cause or treat it alone.

Enough oxygenOxygen short
How glucose is broken downaerobic respirationwithout oxygen
ATP madeplentymuch less
Waste product that builds upnone that tires the musclelactic acid
Resultthe muscle keeps workingfatigue
Worked examples
Two jobs for ATP in one pull. First ATP gives a cross bridge the energy to grab the actin and pull. Then ATP lets the bridge let go so that it can reach for the next position. Without the second use, the muscle stays locked, like a hand that cannot open after gripping a rope.
Fast against slow, with invented numbers. A muscle that contracts 3 times a second for 20 seconds contracts 3 × 20 = 60 times. A muscle that contracts once every 4 seconds for 20 seconds contracts 20 ÷ 4 = 5 times. The fast, repeated work outruns the oxygen supply and tires the muscle first.
What rest changes. During a pause the blood brings oxygen again, so the muscle can use aerobic respiration, make ATP and return to normal.
Class activity

Feel fatigue, with a grown-up. Squeeze a soft sponge or ball once every second for 30 seconds and write how the hand feels at the start and at the end. Rest for one minute, then squeeze once every 5 seconds for the same time and compare. Stop at once if there is any pain. Write which pattern tired the muscle more, and which substance and which gas the lesson links to your answer.

Practice
1
Give the two jobs of ATP in a contracting muscle.
2
Why does rapid, repeated contraction tire a muscle?
3
Which gas returns to the muscle at rest, and what kind of respiration does it allow?
4
A person has a muscle pain that lasts. What is the sensible advice, and why not guess the cause?