2.4.9 The sliding filament theory

Learning Objectives
 To understand the sliding filament model
Success Criteria
 Outline the role of ATP in muscle contraction
(Grade E - D)
 Outline how the supply of ATP is maintained in the muscle
(Grade C –B)
 Explain using diagrams the sliding filament model of muscular contraction
(Grade B – A)

1.
2.
Blood vessels such as arterioles contain circular smooth muscle. Contraction of this muscle constricts the vessel.
Why do blood vessels not need longitudinal muscle to act against the circular muscle in order to cause dilation?
Suggest the advantage of the electrical activity of the heart being able to pass from atria walls to ventricle walls only at the AV node

Sarcomere – the smallest contractile unit of a muscle

Microscopic Structure of Skeletal Muscle
Myofibrils are made up of actin and myosin
Myofibrils appear striped due to alternating I-bands and
A-bands

 Light bands are isotropic bands (I-
bands) only actin is found in these bands
 Dark bands are anisotropic bands (A-
bands) actin and myosin overlap in these bands

 In the middle of each
A-band is a lighter part called the H-zone
 In the centre of each Iband is the Z-line, where the actin filaments join
 The section of muscle between Z-lines is called a sarcomere

Figure 1 shows a diagram of part of a muscle myofibril.
(a) Name the protein present in the filaments labelled
W and X.
( 1 mark )
10

 Actin and myosin slide past one another when the muscle contracts
Evidence for this:
 Sarcomere gets shorter
 More overlap
 Z-lines get closer together
 I-band gets narrower
 H-zone gets narrower

Understanding the sarcomere’s bands
12 of 36 © Boardworks Ltd 2009

The sliding filament theory
13 of 36 © Boardworks Ltd 2009

14 of 36 © Boardworks Ltd 2009

How does the sarcomere change?
15 of 36 © Boardworks Ltd 2009

The structure of myosin
The myosin filament is formed from a number of myosin proteins wound together. Each ends in a myosin head, which contains an ATPase . actin binding site myosin filament
ATP binding site myosin head
16 of 36
ATPase head myosin neck
© Boardworks Ltd 2009

The structure of actin
The actin filament is formed from a helix of actin sub-units. Each contains a binding site for the myosin heads. troponin tropomyosin actin sub-unit myosin head binding site
Two other proteins are attached to the actin fibre:

 tropomyosin is wound around the actin troponin molecules are bound to tropomyosin and contain calcium ion binding sites.
17 of 36 © Boardworks Ltd 2009

 Heads of myosin form cross-bridges with the actin filaments (attach to binding sites)
 Myosin heads flex together and pull the actin along the myosin
 They detach
 Return to original angle and re-attach (uses ATP)
 Repeats 100 times a second

What controls the sliding filaments?
20 of 36 © Boardworks Ltd 2009

 Using the play doh demonstrate muscle contraction
 Four sections
1.
2.
Arrival of an action potential – Ca2+, troponin, tropomyosin, actin-myosin crossbridge
Movement of the actin filament – ATPase, ATP, power stroke
3.
4.
Breaking of the cross bridge – ATP, myosin head
Return to resting state – troponin, Ca2+, sarcoplasmic reticulum
Use keywords – write down a flowchart explaining each stage

2. Anaerobic respiration in muscle cell sarcoplasm
Produces lactate and can lead to fatigue/cramp.
1. Aerobic respiration in muscle cells mitochondria
Needs a supply of respiratory substrate and oxygen
3. Creatinine phosphate
– another chemical present in muscle cells can donate its phosphate to recharge ADP back to
ATP (supports a further
2-4 seconds
© Boardworks Ltd 2009 23 of 36

Plenary
24 of 36 © Boardworks Ltd 2009

Sliding filament theory
• When a nerve impulse arrives at a neuromuscular junction, calcium ions are released from the sarcoplasmic reticulum.
• The calcium ions diffuse through the sarcoplasm.
• This initiates the movement of the protein filaments as follows:
• Calcium ions attach to the troponin molecules causing them to move.
• As a result, the tropomyosin on the actin filament shifts position, exposing myosin binding sites on the actin filaments.
• Myosin heads bind with myosin binding sites on the actin filament, forming cross bridges.
• When the myosin head binds to the actin, ADP and Pi on the myosin head are released.
• The myosin changes shape, causing the myosin head to nod forward. This movement results in the relative movement of the filaments. The attached actin moves over the myosin.
• An ATP molecule binds to the myosin head. This causes the myosin head to detach.
• An ATPase on the myosin head hydrolyses the ATP forming ADP and Pi.
• This hydrolysis causes a change in the shape of the myosin head. It returns to its upright position. This enables the cycle to start again.
© Boardworks Ltd 2009 25 of 36

Learning Objectives Success Criteria
• To understand the sliding filament model
• Outline the role of ATP in muscle contraction
(Grade E - D)
• Outline how the supply of ATP is maintained in the muscle
(Grade C –B)
• Explain using diagrams the sliding filament model of muscular contraction
(Grade B – A)
© Boardworks Ltd 2009 26 of 36