Muscle contraction

Physiology  ›  Muscles  ›  Muscle Contraction
Physiology • Muscles

MUSCLE CONTRACTION

Changes taking place during muscular contraction, electrical, physical, molecular, chemical and thermal changes.

1. CHANGES TAKING PLACE DURING MUSCULAR CONTRACTION

Muscle contracts when it is stimulated.

Contraction of the muscle is a physical or mechanical event.

In addition, several other changes occur in the muscle when it is stimulated.

Changes During Muscular Contraction

1. Electrical Changes

2. Physical Changes

3. Histological or Molecular Changes

4. Chemical Changes

5. Thermal Changes

2. ELECTRICAL CHANGES DURING MUSCULAR CONTRACTION

Electrical events take place in the muscle during:

  • Resting condition
  • Active conditions

Electrical potential in the muscle during resting condition is called resting membrane potential (RMP).

Electrical changes that occur when the muscle is stimulated are together called action potential.

Electrical potentials in a muscle or any living tissue are measured by using a cathode ray oscilloscope or computerized polygraph.

Resting Membrane Potential (RMP)

Resting membrane potential is the electrical potential difference (voltage) across cell membrane between inside and outside of the cell under resting condition.

It is also called:

  • Membrane potential
  • Transmembrane potential
  • Transmembrane potential difference
  • Transmembrane potential gradient

When both electrodes (microelectrodes) of a voltmeter are placed over the surface of a muscle fiber, there is no potential difference and the voltmeter shows only zero potential difference.

If one of the electrodes is inserted into the interior of muscle fiber, potential difference develops across the cell membrane (sarcolemma) with negative potential inside and positive potential outside the muscle fiber.

This potential difference is called polarized state.

In human skeletal muscle, resting membrane potential is about −90 mV.

RMP in Different Tissues

Tissue Resting Membrane Potential
Neuron −70 mV
Skeletal muscle fiber −90 mV
Cardiac muscle fiber −85 to −90 mV
Smooth muscle cells −50 to −95 mV
Erythrocytes −12 mV

Ionic Basis of Resting Membrane Potential

In a muscle fiber or a neuron, resting membrane potential is developed and maintained by movement of ions, which produces ionic imbalance across the cell membrane.

This results in development of more positivity outside and more negativity inside the cell.

Ionic imbalance is produced by two factors:

1

Sodium-Potassium Pump

  • Sodium and potassium ions are actively transported in opposite directions across the cell membrane by means of an electrogenic pump called sodium-potassium pump.
  • It moves three sodium ions outside the cell and two potassium ions inside the cell by using energy from ATP.
  • Since more positive ions (cations) are pumped outside, a net deficit of positive ions occurs inside the cell.
  • This leads to negativity inside and positivity outside the cell.
2

Selective Permeability of Cell Membrane

  • Permeability of cell membrane depends largely on transport channels.
  • Transport channels are selective for movement of some specific ions.
  • Most of the channels are gated channels.
  • Specific ions can move across the membrane only when these gated channels are opened.

Channels for Major Anions

Channels for some negatively charged large substances such as:

  • Proteins
  • Organic phosphate
  • Sulfate compounds

are absent or closed.

These substances remain inside the cell and cause development and maintenance of negativity inside the cell (resting membrane potential).

Channels for Important Ions

Channels for three important ions are important in maintaining resting membrane potential:

  • Sodium
  • Chloride
  • Potassium

3. ACTION POTENTIAL

Action potential is defined as a series of electrical changes that take place when a muscle or nerve is stimulated.

Action potential occurs in two phases:

  1. Depolarization
  2. Repolarization

Depolarization

Depolarization is the initial phase of action potential during which inside of the muscle becomes positive and outside becomes negative.

Thus, the polarized state (resting membrane potential) is abolished during depolarization.

Repolarization

Repolarization is the second phase of action potential during which potential inside the muscle reverses back to resting membrane potential.

Within a short time after depolarization:

  • Interior of muscle becomes negative.
  • Outside becomes positive.

Thus, the polarized state of muscle is re-established.

Action Potential and Graded Potential

Action Potential Graded Potential
Propagative Nonpropagative
Long-distance signal Short-distance signal
Consists of both depolarization and repolarization Consists of either depolarization or hyperpolarization
Obeys all-or-none law Does not obey all-or-none law
Summation is not possible Summation is possible
Has refractory period Has no refractory period

Action Potential Curve

Action potential curve is the graphical registration of electrical activity that takes place in an excitable tissue after stimulation.

Action potential curve has three major segments:

  1. Latent period
  2. Depolarization
  3. Repolarization

Resting membrane potential in skeletal muscle is −90 mV and is recorded as a straight baseline.

1. Latent Period

Latent period is the period during which no change occurs in electrical potential immediately after applying the stimulus.

It is a very short period with duration of 0.5 to 1 millisecond.

Stimulus Artifact

  • Resting membrane potential is recorded as a straight baseline at −90 mV.
  • When a stimulus is applied, there is a slight irregular deflection of baseline for a very short period.
  • This is called stimulus artifact.
  • This artifact is due to leakage of current from stimulating electrode to the recording electrode.
  • Stimulus artifact is followed by latent period.

2. Depolarization

  • Depolarization starts after the latent period.
  • Initially, it is very slow and the muscle is depolarized for about 15 mV.

Firing Level

After the initial slow depolarization for about 15 mV (up to −75 mV), rate of depolarization increases suddenly.

This point, at which depolarization increases suddenly, is called firing level.

Overshoot

From firing level, the curve reaches the isoelectric potential (zero potential) rapidly.

Then the curve shoots up beyond zero potential (isoelectric base) to +55 mV.

It is called overshoot.

3. Repolarization

When depolarization is completed (+55 mV), repolarization starts.

Initially, repolarization occurs rapidly and then it becomes slow.

Spike Potential

Rapid rise in depolarization and rapid fall in repolarization are together called spike potential.

It lasts for 0.4 millisecond.

Afterdepolarization or Negative Afterpotential

Rapid fall in repolarization is followed by a slow repolarization.

It is called negative afterpotential.

Its duration is 2 to 4 milliseconds.

Afterhyperpolarization or Positive Afterpotential

  • After reaching the resting level (−90 mV), it becomes more negative beyond resting level leading to hyperpolarization.
  • It is followed by slow rise in the curve towards resting level.
  • This is called afterhyperpolarization or positive afterpotential.
  • This lasts for more than 50 milliseconds.
  • After this, normal RMP is restored slowly.

Ionic Basis of Action Potential

Voltage-gated sodium channels and voltage-gated potassium channels play an important role in development of action potential.

Ionic Basis of Depolarization

  • During onset of depolarization, voltage-gated sodium channels open.
  • There is slow influx of sodium.
  • When depolarization reaches 7 to 10 mV, voltage-gated sodium channels start opening at a faster rate.
  • It is called sodium channel activation.
  • When firing level is reached, influx of sodium is very great and leads to depolarization.

Ionic Basis of Repolarization

  • Sodium transport is short lived.
  • This is because of rapid inactivation of sodium channels.
  • Thus, sodium channels open and close quickly.
  • At the same time, potassium channels start opening.
  • This leads to efflux of potassium out of the cell, causing repolarization.
  • Unlike sodium channels, potassium channels remain open for longer duration.
  • Potassium channels remain opened for few milliseconds after completion of repolarization.
  • This causes efflux of greater number of potassium ions, producing more negativity inside, i.e. hyperpolarization.

4. TYPES OF ACTION POTENTIAL

1. Monophasic Action Potential

  • Series of electrical changes taking place in a single phase when a muscle or nerve is stimulated.
  • Characterized by either positive deflection or negative deflection.
  • Recorded by placing one electrode on its surface and the other inside.
  • Action potential in the muscle discussed above belongs to this category.

2. Biphasic Action Potential

  • Biphasic or diphasic action potential is the series of electrical changes taking place in two phases when a muscle or a nerve is stimulated.
  • Characterized by both positive and negative deflections.
  • Recorded by placing both recording electrodes on surface of muscle or nerve fiber.

3. Compound Action Potential

  • Compound action potential is the algebraic summation of all action potentials produced by all nerve fibers.
  • Each nerve is made up of thousands of nerve fibers (axons).
  • While stimulating the whole nerve, all nerve fibers are activated and produce action potential.
  • Compound action potential is obtained by recording the action potentials in all nerve fibers simultaneously.

5. GRADED POTENTIAL

Graded potential is a mild local change in the membrane potential that develops in:

  • Receptors
  • Synapse
  • Neuromuscular junction

when stimulated.

It is also called:

  • Graded membrane potential
  • Graded depolarization
  • Graded potential is distinct from action potential.
  • Properties of both potentials are given in the comparison above.
  • In most cases, graded potential is responsible for generation of action potential.
  • In some cases, graded potential hyperpolarizes the membrane potential (makes it more negative than resting membrane potential).

Different Graded Potentials

  1. Endplate potential in neuromuscular junction.
  2. Electrotonic potential in nerve fibers.
  3. Receptor potential.
  4. Excitatory postsynaptic potential.
  5. Inhibitory postsynaptic potential.

6. PHYSICAL CHANGES DURING MUSCULAR CONTRACTION

Physical change which takes place during muscular contraction is the change in length or tension.

Depending upon this, muscular contraction is of two types:

1. Isotonic Contraction

2. Isometric Contraction

7. HISTOLOGICAL OR MOLECULAR CHANGES DURING MUSCULAR CONTRACTION

Actomyosin Complex

Actomyosin complex is defined as a complex protein in skeletal muscle formed by actin and myosin filaments.

During relaxed state of muscle:

  • Actin filaments from opposite ends of sarcomere are away from each other.
  • This leaves a broad H zone.

During contraction of muscle:

  • Actin (thin) filaments glide over myosin (thick) filaments.
  • They form actomyosin complex.

Molecular Basis of Muscular Contraction

Molecular mechanism is responsible for formation of actomyosin complex resulting in muscular contraction.

Molecular mechanism includes three stages:

  1. Excitation-contraction coupling
  2. Role of troponin and tropomyosin
  3. Sliding mechanism

1. Excitation-Contraction Coupling

Excitation-contraction coupling is a process that occurs in between excitation and contraction of the muscle.

This process involves a series of activities which are responsible for contraction of the excited muscle.

Sequence of Excitation-Contraction Coupling

1. Impulse passes through a motor neuron and reaches the neuromuscular junction.
↓
2. Acetylcholine is released from motor endplate of neuromuscular junction.
↓
3. Acetylcholine causes opening of ligand-gated sodium channels.
↓
4. Sodium ions enter the neuromuscular junction.
↓
5. This leads to development of endplate potential.
↓
6. Endplate potential causes generation of action potential in the muscle fiber.
↓
7. Action potential spreads over sarcolemma and into the muscle fiber through T-tubules.
↓
8. T-tubules are responsible for rapid spread of action potential into the muscle fiber.
↓
9. When action potential reaches the cisternae of L-tubules, these cisternae are excited.
↓
10. Calcium ions from the cisternae are released into the sarcoplasm.
↓
11. Calcium ions move towards actin filaments.
↓
12. This produces muscular contraction.

Role of Calcium Ions

  • Calcium ion forms the link or coupling material between excitation and contraction of muscle.
  • Calcium ions are said to form the basis of excitation-contraction coupling.

2. Role of Troponin and Tropomyosin

  • Normally, head of myosin molecules has a strong tendency to get attached with the active site of F-actin.
  • During relaxed condition, the active site of F-actin is covered by tropomyosin.
  • Therefore, myosin head cannot combine with actin molecule.
  • Large number of calcium ions released from L-tubules during excitation of the muscle bind with troponin C.
  • Loading of troponin with calcium ions produces some changes in the position of troponin molecule.
  • Troponin molecule, in turn, pulls tropomyosin molecule away from F-actin.
  • Due to movement of tropomyosin, active site of F-actin is uncovered.
  • Immediately, head of myosin gets attached to actin.

3. Sliding Mechanism and Formation of Actomyosin Complex

Sliding Theory

Sliding theory explains how actin filaments slide over myosin filaments and form the actomyosin complex during muscular contraction.

It is also called:

  • Ratchet theory
  • Walk-along theory

Cross Bridge

Each cross bridge from the myosin filaments has three components:

1. Hinge

2. Arm

3. Head

Power Stroke

After binding with active site of F-actin, myosin head is tilted towards the arm.

Thus, the actin filament is dragged along with it.

This tilting of head is called power stroke.

Formation of Actomyosin Complex

  • After tilting, the head immediately breaks away from the active site and returns to the original position.
  • Now, it combines with a new active site on the actin molecule.
  • Tilting movement occurs again.
  • Thus, the head of cross bridge bends back and forth and pulls the actin filament towards center of sarcomere.
  • In this way, all the actin filaments of both ends of sarcomere are pulled.
  • The actin filaments of opposite sides overlap and form actomyosin complex.
  • Formation of actomyosin complex results in contraction of the muscle.

Changes in Sarcomere During Muscular Contraction

  • When the muscle shortens further, actin filaments from opposite ends of sarcomere approach each other.
  • The H zone becomes narrow.
  • The two Z lines come closer with reduction in length of sarcomere.
  • Length of A band is not altered.
  • Length of I band decreases.

Changes in Sarcomere

  1. Length of all sarcomeres decreases, as Z lines come close to each other.
  2. Length of I band decreases, since actin filaments from opposite sides overlap.

8. ENERGY FOR MUSCULAR CONTRACTION

Energy for movement of myosin head (power stroke) is obtained by breakdown of adenosine triphosphate (ATP) into:

  • Adenosine diphosphate (ADP)
  • Inorganic phosphate (Pi)
  • Head of myosin has a site for ATP.
  • Actually, head itself can act as the enzyme ATPase and catalyze the breakdown of ATP.
  • Before onset of contraction, an ATP molecule binds with myosin head.
  • When tropomyosin moves to expose the active sites, head is attached to active site.
  • Now ATPase cleaves ATP into ADP and Pi, which remains in head itself.
  • Energy released during this process is utilized for contraction.
  • When head is tilted, ADP and Pi are released and a new ATP molecule binds with the head.
  • This process is repeated until muscular contraction is completed.

Resynthesis of ATP

Adenosine diphosphate, which is formed during ATP breakdown, is immediately utilized for resynthesis of ATP.

But, for resynthesis of ATP, ADP cannot combine with Pi.

It should combine with a high-energy phosphate radical.

There are two sources from which the high-energy phosphate is obtained:

1. Creatine Phosphate

2. Carbohydrate Metabolism

9. RELAXATION OF MUSCLE

Calcium ions are pumped back into the L-tubules.
↓
Calcium content in sarcoplasm decreases.
↓
Calcium ions are released from the troponin.
↓
Myosin detaches from actin filaments.
↓
Relaxation of the muscle occurs.
  • Detachment of myosin from actin obtains energy from breakdown of ATP.
  • Therefore, the chemical process of muscular relaxation is an active process, although the physical process is passive.

10. CHEMICAL CHANGES DURING MUSCULAR CONTRACTION

Liberation of Energy

Energy necessary for muscular contraction is liberated during the process of breakdown and resynthesis of ATP.

Breakdown of ATP

During muscular contraction, energy is supplied from breakdown of ATP.

ATP → ADP + Pi + Energy

Energy Liberated by Breakdown of ATP

The energy is responsible for:

  1. Spread of action potential into muscle.
  2. Liberation of calcium ions from cisternae of L-tubules into sarcoplasm.
  3. Movements of myosin head.
  4. Sliding mechanism.

Changes in Reaction and pH During Muscular Contraction

Reaction and pH of muscle are altered in different stages of muscular contraction.

In Resting Condition

During resting condition, the muscle is alkaline with a pH of 7.3.

During Onset of Contraction

At the beginning of muscular contraction, the reaction becomes acidic.

The acidity is due to dephosphorylation of ATP into ADP and Pi.

During Later Part of Contraction

During the latter part of contraction, the muscle becomes alkaline.

It is due to resynthesis of ATP from CP.

At the End of Contraction

At the end of contraction, the muscle becomes once again acidic.

This acidity is due to formation of:

  • Pyruvic acid
  • Lactic acid

11. THERMAL CHANGES DURING MUSCULAR CONTRACTION

During muscular contraction, heat is produced.

Not all the heat is liberated at a time.

Stages of Heat Production

Heat is released in three stages:

1. Resting Heat

2. Initial Heat

3. Recovery Heat

1. Resting Heat

Heat produced in the muscle at rest is called resting heat.

It is due to basal metabolic process in the muscle.

2. Initial Heat

During muscular activity, heat production occurs in three stages:

1

Heat of Activation or Maintenance Heat

  • Heat is produced before actual shortening of the muscle fibers.
  • Most of this heat is produced during release of calcium ions from L-tubules.
2

Heat of Shortening

Heat is produced during contraction of muscle due to structural changes in muscle fiber like:

  • Movements of cross-bridges
  • Myosin heads
  • Breakdown of glycogen
3

Heat of Relaxation

  • Heat is released during relaxation of the muscle.
  • In fact, it is the heat produced during contraction of muscle due to breakdown of ATP molecule.
  • It is released when the muscle lengthens during relaxation.

3. Recovery Heat

Recovery heat is the heat produced in muscle after the end of activities.

After the end of muscular activities, some amount of heat is produced due to chemical processes involved in resynthesis of chemical substances broken down during contraction.

MUSCLE CONTRACTION
Physiology • Muscles