2.4.9 The sliding filament theory

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)

Starter1. 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?

2. 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

Myofibrils are made up of actin and myosinMyofibrils appear striped due to alternating I-bands and A-bands

Microscopic Structure of Skeletal Muscle

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

Microscopic Structure of Skeletal Muscle

In the middle of each A-band is a lighter part called the H-zone

In the centre of each I-band is the Z-line, where the actin filaments join

The section of muscle between Z-lines is called a sarcomere

Microscopic Structure of Skeletal Muscle

10

(a) Name the protein present in the filaments labelled W and X.

(1 mark)

Figure 1 shows a diagram of part of a muscle myofibril.

The Sliding Filament MechanismActin and myosin slide

past one another when the muscle contracts

Evidence for this:Sarcomere gets shorter More overlapZ-lines get closer togetherI-band gets narrowerH-zone gets narrower

12 of 36 © Boardworks Ltd 2009

Understanding the sarcomere’s bands

13 of 36 © Boardworks Ltd 2009

The sliding filament theory

14 of 36 © Boardworks Ltd 2009

15 of 36 © Boardworks Ltd 2009

How does the sarcomere change?

16 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.

myosin filament

myosin head

actin binding site

ATP binding site

ATPase head myosin neck

17 of 36 © 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.

Two other proteins are attached to the actin fibre:

troponin

tropomyosinmyosin head binding site

actin sub-unit

tropomyosin is wound around the actin troponin molecules are bound to tropomyosin and contain

calcium ion binding sites.

Muscle Contraction – Sliding Filament MechanismHeads of myosin form cross-bridges with the actin

filaments (attach to binding sites)Myosin heads flex together and pull the actin along the

myosinThey detachReturn to original angle and re-attach (uses ATP)Repeats 100 times a second

Muscle Contraction – Sliding Filament Mechanism

20 of 36 © Boardworks Ltd 2009

What controls the sliding filaments?

TaskUsing the play doh demonstrate muscle contraction

Four sections1.Arrival of an action potential – Ca2+, troponin, tropomyosin,

actin-myosin crossbridge2.Movement of the actin filament – ATPase, ATP, power stroke3.Breaking of the cross bridge – ATP, myosin head4.Return to resting state – troponin, Ca2+, sarcoplasmic

reticulumUse keywords – write down a flowchart explaining each stage

23 of 36 © Boardworks Ltd 2009

Maintaining ATP supply

1. Aerobic respiration in muscle cells mitochondria

Needs a supply of respiratory substrate and oxygen

2. Anaerobic respiration in muscle cell sarcoplasm

Produces lactate and can lead to fatigue/cramp.

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

24 of 36 © Boardworks Ltd 2009

Plenary

• Complete cloze sheet – summary sliding filament theory

25 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.

26 of 36 © Boardworks Ltd 2009

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)