Sliding Filament Theory

Page 1.  Introduction

When a muscle cell contracts, the thin filaments slide past the thick filaments, and the sarcomere shortens.

Page 2.  Goals

To explore the molecular structure and functional features of the thin and thick filaments.

To understand the sequence of events in a single cross bridge cycle.

To examine the sequence of events in multiple cross bridge cycling.

Page 3.  Molecular Participants

The chemical players in muscle contraction are:

1. myosin (protein)

2. actin (protein)

3. tropomyosin (protein)

4. troponin (protein)

5. ATP (nucleotide)

6. calcium ions

Page 4.  Sarcomere

The next several pages explain how each of these chemicals participate in the contraction of a sarcomere.

Page 5.  Myosin

Myosin is a protein molecule found in the thick filaments.

Page 6.  Myosin Molecule with Hinged Head

Myosin has a tail and two heads (called cross bridges) which will move back and forth, providing the power stroke for muscle contraction.

Page 7.  Myosin Molecule with Hinged Head and Tail

The tail of myosin has a hinge which allows vertical movement so that the cross-bridge can bind to actin.

 

Page 8.  Myosin ATP Binding Site

The cross bridge (head) of myosin has a binding site for  ATP.

Myosin is in its low energy conformation when the cross bridge is in this position:

Page 9.  Energized Cross Bridge

ATP is a molecule with a high chemical energy.  ATP binds to myosin heads when they are tilted back in their low energy position.  When ATP is hydrolyzed into ADP and phosphate, the energy is released and transferred to the myosin head.

Note that the ATP is glowing yellow indicating that it's in a high energy state.  After the ATP has been hydrolyzed to ADP and phosphate, the energy is transferred to the myosin head.  Now the head is glowing to show that it's high energy.  When the myosin head is pointing up, it is in a high energy state.

 

As myosin functions within muscle cells, it undergoes the following four steps:

Page 10.  Actin Binding Site on Myosin

There are two binding sites on each myosin head, one for ATP and one for actin.

Page 11.  Thin Filaments of the Sarcomere

Thin filaments are made of these three protein molecules:

1. actin                                     2. tropomyosin                           3. troponin

Page 12.  Actin

The major component of the thin filament, actin is composed of a double strand of actin subunits each of which contain myosin binding sites.

Page 13.  Tropomyosin

The regulatory protein, tropomyosin, is also part of the thin filament.  Tropomyosin twists around the actin.  When the sarcomere is not shortening, the position of the tropomyosin covers the binding sites on the actin subunits and prevents myosin cross bridge binding.

Page 14.  Troponin

Troponin, which is found periodically along the tropomyosin strand, functions to move the tropomyosin aside, exposing the myosin binding sites.

Page 15.  Calcium Ions

Role of Calcium in Muscle Contraction:

Action Potential Occurs

Calcium Ions are Released from the Terminal Cisternae

Calcium Ions then Bind to Troponin

Tropomyosin Moves Away from the Myosin Binding Sites on Actin

Page 16.  Review of Molecular Participants

Review of participants in the Cross Bridge Cycle:

Participant                                          Will bind to:

1. Myosin                                            ATP, Actin

2. Actin                                               Myosin, Troponin

3. Tropomyosin                                   Troponin

4. Troponin                                         Calcium, Tropomyosin, Actin

5. ATP                                                  Myosin

6. Calcium ions                                  Troponin

Page 17.  Overview: Single Cross Bridge Cycle

This animation shows a single cross bridge cycle.

Page 18.  Six Steps of Cross Bridge Cycling

Cross bridge cycling is broken down into six steps which will be explained in detail during the next six pages.

Page 19.  Step 1: Exposure of Binding Sites on Actin

Detail of steps required to expose the binding sites on actin:

Presence of an action potential in the muscle cell membrane.

Release of calcium ions from the terminal cisternae.

Calcium ions rush into the cytosol and bind to the troponin.

There is a change in the conformation of the troponin-tropomyosin complex.

This tropomyosin slides over, exposing the binding sites on actin.

 

Page 20.  Step 2: Binding of Myosin to Actin

The animation shows the hinge on the tail of the myosin bending and the energized myosin head binding into the actin.

Page 21.  Step 3: Power Stroke of the Cross Bridge

Detail of steps shown in this animation:

The ADP and Pi are released from the actin.

The myosin head (cross bridge) tilts backward.

The power stroke occurs as the thin filament is pulled inward toward the center of the sarcomere.

 

There has been a transfer of energy from the myosin head to the movement of the thin filament.

Page 22.  Step 4: Disconnecting the Cross Bridge

This animation shows ATP binding to the cross bridge, allowing the cross bridge to disconnect from the actin.

Page 23.  Step 5: Re-energizing the Cross Bridge

In this animation, ATP is hydrolyzed into ADP and phosphate.  The energy (yellow glow) is transferred from the ATP to the myosin cross bridge, which points upward.

Page 24.  Step 6 Removal of Calcium Ions

Detail of steps in this animation:

Calcium ions fall off the troponin.

Calcium is taken back up into the sarcoplasmic reticulum.

Tropomyosin covers the binding sites on actin.

Page 25.  Calcium Pumps

This animation shows how calcium ions are pumped back into the sarcoplasmic reticulum.  Two calcium ions enter the pump (transport protein embedded in the membrane of the SR).  ATP binds to the pump and is hydrolyzed into ADP and Pi.  The energy released from ATP hydrolysis is used to change the conformation of the pump, allowing the calcium ions to move into the lumen of the SR.  ADP and Pi fall off the pump, allowing it to return to it's original conformation.

In a relaxed muscle cell, the concentration of calcium ions about 10,000 lower in the cytosol than in the SR.  During a muscle contraction, the concentration of calcium in the cytosol increases, but it is still higher inside the SR.  To move the calcium against the gradient, from the lower concentration in the cytosol to the higher concentration inside the SR, Active transport is needed.

Page 26.  Review: Single Cross Bridge Cycle

This animation reviews the entire process of a cross bridge cycle.

Try to pick out these six steps:

 

                                                           

                         

                  

                    

Page 27.  Multiple Cross Bridge Cycles

During the contraction of a sarcomere about half of the cross bridges are attached to actin and about half are bound at any given time.  If all the cross bridges detached at the same time, then the thin filament would slide back on the thick filament.

Page 28.  Multiple Myofilaments

Many power strokes occur to bring the Z lines of the sarcomere closer together during the contraction of a muscle cell.  During relaxation, the myosin heads detach from the actin and the thin filaments slide back to their resting position.

The width of the H zone decreases during a contraction and increases during relaxation.

The length of the sarcomere shortens during a contraction, but the thin and thick filaments do not shorten, they just slide by each other.

Page 29.  Review of the Role of ATP

Summary of the role that ATP plays in the contraction of muscle:

1. ATP transfers its energy to the myosin cross bridge, which in turn energizes the power stroke.

2. ATP disconnects the myosin cross bridge from the binding site on actin.

3. ATP fuels the pump that actively transports calcium ions back into the sarcoplasmic reticulum.

 

Page 30.  Summary

The sequence of events in a single cross bride cycle includes:

1. In influx of calcium, triggering the exposure of binding sites on actin.

2. The binding of myosin to actin.

3. The power stroke of the cross bridge that causes the sliding of the thin filaments.

4. The binding of ATP to the cross bridge, which results in the cross bridge disconnecting from actin.

5. The hydrolysis of ATP, which leads to the re-energizing and repositioning of the cross bridge.

6. The transport of calcium ions back into the sarcoplasmic reticulum.

Multiple cross bridge cycling is coordinated sequentially to prevent all cross bridges from either being connected or disconnected at the same time.

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