IB Syllabus focus: 'Sliding filament theory describes interactions between myofilaments and molecules responsible for sarcomere or muscle contraction. Calcium, ATP, actin, myosin, troponin and tropomyosin have specific roles.'
Skeletal muscle contraction depends on coordinated molecular events inside each sarcomere. Sliding filament theory explains how protein filaments slide past one another, using calcium and ATP, to shorten muscle and produce force.
Sarcomere structure and myofilaments
Sliding filament theory takes place within the sarcomere, the repeating functional unit of a myofibril. Each sarcomere contains two main types of myofilaments:
Practice Questions
FAQ
Rigor mortis happens because ATP production stops after death.
Without ATP:
myosin heads cannot detach normally from actin
calcium control also fails, so binding sites may remain exposed
This leaves many cross-bridges locked in place, producing stiffness until muscle proteins begin to break down.
Myosin is called a molecular motor because it converts chemical energy from ATP into mechanical movement.
Its head:
binds ATP
hydrolyzes ATP
changes shape
pulls actin during the power stroke
This is motor behavior at the protein level, even though it happens on a microscopic scale.
Force depends on how much overlap exists between actin and myosin before contraction starts.
Too little overlap means fewer cross-bridges can form.
Too much overlap can interfere with effective pulling.
Maximum force is usually produced when overlap is optimal, allowing many myosin heads to bind without the filaments being excessively compressed.
The core idea of actin and myosin sliding also applies to cardiac muscle.
Smooth muscle also uses actin and myosin, but regulation is different. In smooth muscle, calcium mainly acts through other regulatory proteins rather than troponin.
So the basic principle is shared, but the control system is not identical in all muscle types.
Cross-bridge cycling is asynchronous, meaning different myosin heads are at different stages of the cycle.
This is useful because it:
produces smoother force
reduces sudden jerky changes in tension
allows some heads to stay attached while others detach and reset
If every head acted simultaneously, force production would be much less steady.
