Muscle Contraction

 

Muscle Contraction

Introduction

Muscle contraction is the process by which muscle fibers generate force and shorten to produce movement. It is one of the most important physiological processes in the human body and forms the basis of locomotion, posture maintenance, respiration, circulation, and many other vital functions.

Muscle contraction occurs due to the sliding of thin actin filaments over thick myosin filaments, known as the Sliding Filament Theory. This theory was proposed independently by Andrew Huxley and Rolf Niedergerke and by Hugh Huxley and Jean Hanson in 1954.

Muscle Tissue

Muscles are specialized tissues capable of:

1. Excitability
2. Contractility
3. Extensibility
4. Elasticity

There are three types of muscles:

Type	Location	Control
Skeletal Muscle	Attached to bones	Voluntary
Smooth Muscle	Walls of visceral organs	Involuntary
Cardiac Muscle	Heart	Involuntary

Sliding filament theory is best explained in skeletal muscles.

Structure of a Skeletal Muscle

Hierarchical Organization

Muscle

↓

Muscle Fascicle

↓

Muscle Fiber (Muscle Cell)

↓

Myofibril

↓

Sarcomere

↓

Actin & Myosin Filaments

Muscle Fiber

A skeletal muscle fiber is:

• Long
• Cylindrical
• Multinucleated
• Striated

Components

Sarcolemma

Cell membrane of muscle fiber.

Sarcoplasm

Cytoplasm of muscle fiber.

Sarcoplasmic Reticulum (SR)

Specialized endoplasmic reticulum storing calcium ions.

T-tubules

Invaginations of sarcolemma that transmit impulses deep into muscle fiber.

Myofibrils

Contractile elements arranged parallel to each other.

Myofibril Structure

Myofibrils show alternate:

Dark Bands (A Bands)

• Anisotropic
• Contain thick myosin filaments
• Remain constant during contraction

Light Bands (I Bands)

• Isotropic
• Contain only actin filaments
• Shorten during contraction

Sarcomere

Definition

Sarcomere is the structural and functional unit of muscle contraction.

Boundaries

Between two successive Z-lines.

Diagrammatic Representation

Z     I     A      H      A      I     Z

|-----|-----|------|------|------|-----|

 

     Actin ←→ Myosin ←→ Actin

Parts of Sarcomere

Z-Line

• Boundary of sarcomere
• Anchors actin filaments

I-Band

Contains only actin filaments.

A-Band

Contains entire length of myosin filament.

H-Zone

Contains only myosin.

M-Line

Middle of H-zone.

Anchors myosin filaments.

Contractile Proteins

1. Actin (Thin Filament)

Major protein of thin filament.

Components

• F-actin
• G-actin
• Troponin
• Tropomyosin

G-actin

Globular protein molecule.

F-actin

Double helical polymer of G-actin.

Tropomyosin

• Fibrous protein
• Lies in groove of actin filament
• Covers myosin binding sites during relaxation

Troponin Complex

Three subunits:

Subunit	Function
Troponin T	Binds Tropomyosin
Troponin I	Inhibitory
Troponin C	Binds Calcium

Calcium binds to Troponin-C.

2. Myosin (Thick Filament)

Major protein of thick filament.

Structure

Each myosin molecule consists of:

• Two heavy chains
• Four light chains

Parts

Head

• Cross-bridge formation
• ATPase activity

Neck

Connects head and tail.

Tail

Forms shaft of thick filament.

Types of Muscle Contraction

1. Isometric Contraction

• Muscle length unchanged
• Tension increases

Example: Pushing a wall.

2. Isotonic Contraction

Length changes.

Concentric- Muscle shortens.

Example: Lifting dumbbell.

Eccentric- Muscle lengthens under load.

Example: Lowering dumbbell.

Sliding Filament Theory

Principle

During contraction:

• Actin slides over myosin
• Filaments themselves do NOT shorten
• Sarcomere shortens
• Muscle shortens

This is known as the Sliding Filament Theory.

Events of Muscle Contraction

Step 1: Nerve Impulse Arrives

Motor neuron carries impulse to neuromuscular junction.

Step 2: Release of Acetylcholine

Acetylcholine (ACh) is released into synaptic cleft. ACh binds receptors on sarcolemma. This generates an action potential.

Step 3: Spread of Action Potential

Action potential spreads through:

• Sarcolemma
• T-tubules

Step 4: Calcium Release

Sarcoplasmic reticulum releases calcium ions into sarcoplasm. Calcium concentration rises rapidly.

Step 5: Exposure of Active Sites

Calcium binds to Troponin-C.

Result:

• Troponin changes shape
• Tropomyosin shifts away
• Active sites on actin become exposed

Cross-Bridge Cycle

This is the heart of muscle contraction.

Stage 1: Cross-Bridge Formation

Myosin head attaches to exposed actin site.

Actin + Myosin

      ↓

Cross Bridge

Stage 2: Power Stroke

ADP and Pi leave myosin head. Myosin bends. Actin filament is pulled toward center. This is called the Power Stroke.

Stage 3: Detachment

New ATP binds myosin. Myosin detaches from actin.

Stage 4: Reactivation

ATP hydrolysis occurs:

ATP → ADP + Pi + Energy

Energy cocks myosin head. Head becomes ready for next cycle.

Role of ATP

ATP is essential for:

1. Detachment of Myosin from Actin

Without ATP, myosin remains attached.

2. Re-cocking of Myosin Head

ATP hydrolysis energizes myosin.

3. Calcium Pumping

ATP pumps calcium back into SR.

Changes In Sarcomere During Contraction

Structure	Change
Sarcomere Length	Decreases
Z-Lines	Move closer
I-Band	Shortens
H-Zone	Shortens/disappears
A-Band	Unchanged

Rigor Mortis

Definition

Stiffening of muscles after death.

Cause

No ATP production after death. Myosin cannot detach from actin. Cross-bridges remain locked.

Result: Muscle rigidity. Rigor mortis occurs due to ATP depletion.

Memory Table

Structure	During Contraction
Actin Length	No change
Myosin Length	No change
A-Band	No change
I-Band	Decreases
H-Zone	Decreases
Sarcomere	Decreases

Muscle Relaxation

Contraction stops when nerve impulse ceases.

Steps

1. Acetylcholine Breakdown

By acetylcholinesterase.

2. Calcium Reuptake

Calcium pumped back into SR.

3. Troponin Loses Calcium

Calcium dissociates.

4. Tropomyosin Covers Active Sites

Cross-bridge formation stops.

5. Muscle Returns to Resting Length

Muscle relaxes.

Excitation-Contraction Coupling

Definition

Process linking electrical excitation with mechanical contraction.

Sequence

Nerve Impulse

↓

Acetylcholine Release

↓

Action Potential

↓

Calcium Release

↓

Cross-Bridge Formation

↓

Muscle Contraction

Role Of Calcium

Calcium is the key regulator of contraction. Without calcium, contraction cannot occur.

Functions:

1. Binds Troponin-C
2. Removes Tropomyosin blockade
3. Initiates cross-bridge formation
4. Maintains contraction

Energy Sources for Muscle Contraction

Immediate Source

ATP- Stored ATP lasts only 2–3 seconds.

Creatine Phosphate

Creatine-P + ADP

        ↓

Creatine + ATP

Provides energy for 10–15 seconds.

Anaerobic Glycolysis

Produces ATP quickly.

End product: Lactic acid

Aerobic Respiration

Major ATP source during prolonged exercise.

Muscle Fatigue

Definition

Temporary inability of muscle to contract effectively.

Causes

• ATP depletion
• Lactic acid accumulation
• Ionic imbalance
• Reduced calcium availability

Important Points

Most abundant protein in muscle: Myosin

Functional unit of contraction: Sarcomere

Calcium binds: Troponin-C

ATPase activity present in: Myosin head

Energy currency: ATP

H-zone during contraction: Decreases

I-band during contraction: Decreases

A-band during contraction: Remains unchanged

Theory explaining contraction: Sliding Filament Theory

Rigor mortis due to: ATP deficiency

Highlights

• Contraction occurs due to sliding of actin over myosin.
• Cross-bridges are formed by myosin heads.
• Calcium binds troponin.
• Tropomyosin masks active sites during relaxation.
• ATP is required for muscle contraction and relaxation.
• A-band remains unchanged during contraction.

Flowchart For Quick Revision

Motor Neuron Stimulated

↓

Acetylcholine Released

↓

Action Potential

↓

T-Tubule Activation

↓

Ca²⁺ Released from SR

↓

Ca²⁺ + Troponin-C

↓

Tropomyosin Shifts

↓

Active Sites Exposed

↓

Actin-Myosin Cross Bridge

↓

Power Stroke

↓

Sarcomere Shortens

↓

Muscle Contraction

↓

Ca²⁺ Reuptake into SR

↓

Relaxation

High-Yield One-Liners

1. Sarcomere is the contractile unit of muscle.
2. Actin forms thin filaments; myosin forms thick filaments.
3. Calcium binds Troponin-C.
4. Myosin head possesses ATPase activity.
5. Cross-bridge formation is essential for contraction.
6. ATP is required for both contraction and relaxation.
7. A-band remains constant during contraction.
8. I-band and H-zone shorten during contraction.
9. Sliding filament theory was proposed in 1954.
10. Rigor mortis occurs due to lack of ATP.
11. Calcium is stored in the sarcoplasmic reticulum.
12. Excitation-contraction coupling links electrical and mechanical events.