Properties of skeletal muscle

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Physiology • Muscles

PROPERTIES OF SKELETAL MUSCLE

Excitability, contractility, simple muscle contraction, muscle fibers, factors affecting force of contraction, refractory period and muscle tone.

1. EXCITABILITY

Definition

Excitability

  • Excitability is a physicochemical change.
  • It is defined as the reaction or response of a tissue to irritation or stimulation.

Stimulus

  • Stimulus is the change in environment.
  • It is defined as an agent or influence or act which causes the response in an excitable tissue.

Types of Stimuli

Stimulus which can excite a living tissue is of 4 types:

1. Mechanical Stimulus

Example → Pinching

2. Electrical Stimulus

Example → Electric shock

3. Thermal Stimulus

Example → Heated glass rod or ice piece

4. Chemical Stimulus

Example → Chemical substances like acids

2. COMMONLY USED STIMULUS

Electrical stimulus is commonly used for experimental purposes because:

  1. It can be handled easily.
  2. Intensity (strength) of stimulus can be easily adjusted.
  3. Duration of stimulus can be easily adjusted.
  4. Stimulus can be applied to a limited (small) area on the tissues.
  5. Damage caused to tissues is nil or least.

Qualities of Stimulus

To excite a tissue, stimulus must possess two qualities:

  1. Intensity or Strength of Stimulus
  2. Duration of Stimulus

1. Intensity or Strength of Stimulus

Depending upon intensity or strength, stimuli are categorized into 5 types:

1. Subminimal Stimulus

  • Has a strength which is less than that of minimal stimulus.
  • Does not produce any response in the muscle if applied.

2. Minimal / Threshold / Liminal Stimulus

  • Has sufficient strength to excite the muscle.
  • Produces muscular contraction with minimum force.

3. Submaximal Stimulus

  • Has strength which is more than that of minimal stimulus and less than that of maximal stimulus.
  • Produces contraction with more force than minimal stimulus.

4. Maximal Stimulus

  • Produces almost the maximum force of contraction.

5. Supramaximal Stimulus

  • Produces muscular contraction with maximum force.
  • Beyond this strength, force of contraction cannot be increased.

2. Duration of Stimulus

  • Whatever may be the strength, a stimulus must be applied for a minimum duration to excite the tissue.
  • Duration of a stimulus depends upon the strength of the stimulus.
  • For a weak stimulus, duration is longer.
  • For a stronger stimulus, duration is shorter.
  • Relationship between strength and duration of stimulus is demonstrated by excitability curve or strength-duration curve.

3. EXCITABILITY CURVE

Excitability curve or strength-duration curve is a graph demonstrating the exact relationship between:

  • Strength of stimulus
  • Duration of stimulus

It is also called strength-duration curve.

In this curve:

  • Strength of stimulus is plotted on Y-axis in volts.
  • Duration is plotted horizontally on X-axis in milliseconds.

Characteristic Features of the Curve

1. Rheobase

  • Rheobase is the minimum strength (voltage) of stimulus which can excite the tissue, whatever may be the duration of stimulus.
  • Voltage below this cannot excite the tissue.
  • Rheobasic strength is also called threshold strength.

2. Utilization Time

  • Utilization time is the minimum time required for a threshold strength of stimulus to excite the tissue.

3. Chronaxie

  • Chronaxie is the minimum time required for a stimulus with double the rheobasic strength (voltage) to excite the tissue.

Importance of Chronaxie

  • Measurement of chronaxie determines the excitability of tissues.
  • It is used to compare excitability in different tissues.
  • Longer the chronaxie → lesser is the excitability.
  • In human skeletal muscles → 0.08 to 0.32 msec.
  • In frog skeletal muscle → 3 msec.

4. CONTRACTILITY

  • Contractility is the response of muscle to a stimulus.
  • It is defined as internal events of muscle with change in either length or tension of the muscle fibers.

Types of Contraction

Muscular contraction is classified into 2 types based on change in length of muscle fibers or tension of muscle:

1. Isotonic Contraction

  • Tension remains the same.
  • Length of muscle fiber is altered.
  • Iso → same
  • Tonic → tension

Example: During sudden flexion of arm, shortening of muscle fibers occurs but tension does not change.

2. Isometric Contraction

  • Length of muscle fibers remains the same.
  • Tension is increased.

Example: Pulling any heavy object when muscles become stiff and strained with increased tension but the length does not change.

5. SIMPLE MUSCLE CONTRACTION

  • Contractile property of muscle is studied by using gastrocnemius-sciatic preparation from frog.
  • It is also called muscle-nerve preparation.

When a threshold stimulus is applied:

  • Muscle contracts and then relaxes.
  • These activities are recorded graphically by using suitable instruments.
  • Contraction is recorded as upward deflection from the base line.
  • Relaxation is recorded as downward deflection back to the base line.
  • Simple contraction of the muscle is called simple muscle twitch.
  • Graphical recording of this is called simple muscle curve.

Important Points in Simple Muscle Curve

Four points are observed in simple muscle curve:

1. Point of Stimulus (PS)

Time when the stimulus is applied.

2. Point of Contraction (PC)

Time when the muscle begins to contract.

3. Point of Maximum Contraction (PMC)

  • Point up to which the muscle contracts.
  • It also indicates beginning of relaxation.

4. Point of Maximum Relaxation (PMR)

Point when muscle relaxes completely.

Periods of Simple Muscle Curve

The above 4 points divide the entire simple muscle curve into 3 periods:

1

Latent Period

Time interval between:

  • Point of stimulus
  • Point of contraction

Muscle does not show any mechanical activity during this period.

Causes of Latent Period

  1. Latent period is the time taken by impulse to travel along the nerve from place of stimulation to muscle.
  2. It is the time taken for onset of initial chemical changes in muscle.
  3. It is due to delay in conduction of impulse at neuromuscular junction.
  4. It is due to resistance offered by viscosity of muscle.
  5. It is also due to inertia of recording instrument.

Variations in Latent Period

  • Latent period is not constant.
  • It varies even in physiological conditions.
  • It increases:
    • In high temperature
    • In low temperature
    • During fatigue
    • With increase in weight
2

Contraction Period

Interval between:

  • Point of contraction
  • Point of maximum contraction

Muscle contracts during this period.

3

Relaxation Period

Interval between:

  • Point of maximum contraction
  • Point of maximum relaxation

Muscle relaxes during this period.

Duration of Different Periods in a Typical Simple Muscle Curve

Period Duration
Latent period 0.01 sec
Contraction period 0.04 sec
Relaxation period 0.05 sec
Total twitch period 0.10 sec
  • Contraction period is always shorter than relaxation period.
  • It is because contraction is an active process and relaxation is a passive process.

6. CONTRACTION TIME: RED AND PALE MUSCLES

  • Contraction time or total twitch period varies from species to species.
  • It is less in homeothermic animals than in poikilothermic animals.
  • In the same animal, it varies in different groups of muscles.

Based on contraction time, skeletal muscles are classified into:

  1. Red muscles
  2. Pale muscles

Similarly, based on contraction time and myosin ATPase activity, muscle fibers are divided into:

  1. Type I fibers or slow fibers or slow twitch fibers → small diameter
  2. Type II fibers or fast fibers or fast twitch fibers → large diameter
Most skeletal muscles in human beings contain both types of fibers.

Red or Slow Muscles

  • Red muscles contain a large quantity of myoglobin and a large number of Type I fibers.
  • Contraction is longer in this type of muscle.

Examples:

  • Back muscles
  • Gastrocnemius muscles

Pale or White or Fast Muscles

  • Pale muscles contain less quantity of myoglobin and a large number of Type II fibers.
  • Contraction time is shorter in this type of muscle.

Examples:

  • Hand muscles
  • Ocular muscles

Features of Red and Pale Muscles

Feature Red (slow) muscle Pale (fast) muscle
Type I / Type II fibers Type I fibers are more Type II fibers are more
Myoglobin Myoglobin content is high → red Myoglobin content is less → pale
Sarcoplasmic reticulum Less extensive More extensive
Blood vessels More extensive Less extensive
Mitochondria More in number Less in number
Response Slow with long-latent period Rapid with short-latent period
Contraction Less powerful More powerful
Activity Sustained contraction by prolonged and continued activity Relaxes immediately and does not show prolonged and continued activity
Fatigue Occurs slowly Occurs quickly
ATP production Depends on cellular respiration Depends on glycolysis

7. FACTORS AFFECTING FORCE OF CONTRACTION

Force of contraction of skeletal muscle is affected by:

A. Strength of stimulus

B. Number of stimulus

C. Temperature

D. Load

A. Effect of Strength of Stimulus

  • When a muscle is stimulated by stimuli with different strength (voltage of current), the force of contraction also differs.
  • Five types of stimuli with their strength are given in the table above.

B. Effect of Number of Stimulus

  • Contractility of the muscle varies depending upon the number of stimuli.
  • If a single stimulus is applied → muscle contracts once (simple muscle twitch).
  • Two or more than two (multiple) stimuli produce two different effects.

Effects of Two Successive Stimuli

When two stimuli are applied successively to a muscle, three different effects are noticed depending upon the time interval between the two stimuli.

i

Beneficial Effect

  • When two successive stimuli are applied to a muscle such a way that the second stimulus falls after relaxation period of first curve:
  • Two separate curves are obtained.
  • Force of second contraction is greater than that of first one.
  • This effect is called beneficial effect.

Cause for Beneficial Effect

  • Temperature in the muscle increases during first contraction.
  • It decreases the viscosity of muscle.
  • Therefore, force of second contraction is increased.
ii

Superposition

  • When applying two successive stimuli, if the second stimulus falls during relaxation period of first twitch, two curves are obtained.
  • However, first curve is superimposed by the second curve.
  • This is called superposition or incomplete summation.
  • Here also, second curve is bigger than first curve because of beneficial effect.
iii

Summation

  • If second stimulus is applied during contraction period or during second half of latent period:
  • Two contractions are summed up.
  • A single curve is obtained.
  • This is called summation curve or complete summation curve.

Effects of Multiple Stimuli

In a muscle-nerve preparation, multiple stimuli cause two types of effects depending upon frequency of stimulus:

1. Fatigue

2. Tetanus

8. FATIGUE

Fatigue is defined as the decrease in muscular activity due to repeated stimuli.

  • When stimuli are applied continuously, after some time muscle does not show any response to the stimulus.
  • This condition is called fatigue.

Causes for Fatigue

  1. Exhaustion of acetylcholine in motor endplate.
  2. Accumulation of metabolites such as:
    • Lactic acid
    • Phosphoric acid
  3. Lack of nutrients like glycogen.
  4. Lack of oxygen.

Site (Seat) of Fatigue

In intact body, sites of fatigue are in the following order:

  1. Betz cells (pyramidal cells) in cerebral cortex.
  2. Anterior gray horn cells (motor neurons) of spinal cord.
  3. Neuromuscular junction.
  4. Muscle.

Recovery of Muscle After Fatigue

  • Fatigue is a reversible phenomenon.
  • Fatigued muscle recovers if given:
    • Rest
    • Nutrition

9. TETANUS

Tetanus is defined as the sustained contraction of muscle due to repeated stimuli with high frequency.

  • When multiple stimuli are applied at a high frequency in such a way that the successive stimuli fall during contraction period of previous twitch:
  • Muscle remains in a state of tetanus.
  • All contractions are fused.
  • Muscle relaxes only after stoppage of stimulus or when muscle is fatigued.
  • If frequency of stimuli is less:
    • Partial fusion of contractions takes place → incomplete tetanus or clonus.

Frequency of Stimuli Necessary to Cause Tetanus and Clonus

Condition Frequency in gastrocnemius muscle of human being
Tetanus 60/sec
Clonus 55/sec

Pathological Tetanus

  • Sustained contraction of muscle due to repeated stimuli of high frequency is usually called physiological tetanus.
  • It is distinct from pathological tetanus.
  • Pathological tetanus refers to spastic contraction of different muscle groups in pathological conditions.
  • This disease is caused by bacillus Clostridium tetani found in:
    • Soil
    • Dust
    • Manure
  • This bacillus enters the body through:
    • Cut
    • Wound
    • Puncture caused by objects like:
      • Metal pieces
      • Metal nails
      • Wood splinters
      • Etc.
  • Pathological tetanus affects:
    • Nervous system
    • Muscular system
  • Common features are muscle spasm and paralysis.
  • First appearing symptom is spasm of jaw muscles resulting in locking of jaw.
  • Therefore, tetanus is also called lockjaw disease.
  • Manifestations of tetanus are due to a toxin secreted by the bacteria.
  • If timely treatment is not provided:
    • Condition becomes serious.
    • It may even lead to death.

10. EFFECT OF VARIATIONS IN TEMPERATURE

If temperature of muscle is altered, the force of contraction is also affected.

Effect of Warm Temperature

At warm temperature of about 40°C, force of contraction increases because:

  1. Excitability of muscle increases.
  2. Chemical processes involved in muscular contraction are accelerated.
  3. Viscosity of muscle decreases.

Effect of Cold Temperature

At cold temperature of about 10°C, force of contraction decreases because:

  1. Excitability of muscle decreases.
  2. Chemical processes are slowed or delayed.
  3. Viscosity of muscle increases.

Effect of High or Hot Temperature: Heat Rigor

  • At high temperatures, heat rigor occurs in the muscle.
  • Rigor refers to shortening and stiffening of muscle fibers.
  • Heat rigor is the rigor that occurs when temperature is increased above 60°C.
  • Cause of heat rigor is coagulation of muscle proteins actin and myosin.
  • It is an irreversible phenomenon.

Other Types of Rigors

1. Cold Rigor

  • Occurs due to exposure to severe cold.
  • It is a reversible phenomenon.

2. Calcium Rigor

  • Due to increased calcium content.
  • It is also reversible.

3. Rigor Mortis

Develops after death.

11. RIGOR MORTIS

Rigor mortis is the after-death condition of body characterized by:

  • Stiffness of muscles
  • Joints

Latin word, rigor → stiff

  • It occurs due to stoppage of aerobic respiration, which causes changes in muscles.
  • Soon after death, cell membrane becomes highly permeable to calcium.
  • A large number of calcium ions enter the muscle fibers.
  • This promotes formation of actomyosin complex, resulting in contraction of muscles.
  • Few hours after death, all muscles of body undergo severe contraction and become rigid.
  • Joints also become stiff and locked.

Cause of Rigor Mortis

  • Normally for relaxation, muscle needs to drive out calcium, which requires ATP.
  • During continuous muscle contraction after death:
    • ATP molecules are completely exhausted.
    • New ATP molecules cannot be produced because of lack of oxygen.
  • Therefore, in the absence of ATP:
    • Muscles remain in contracted state until the onset of decomposition.

Medicolegal Importance of Rigor Mortis

  • Rigor mortis is useful in determining the time of death.
  • Onset of stiffness starts between 10 minutes and 3 hours after death, depending upon:
    • Condition of body
    • Environmental temperature at the time of death.
  • If the body is active or environmental temperature is high at the time of death, stiffness sets in quickly.
  • Stiffness develops first in facial muscles and then spreads to other muscles.
  • Maximum stiffness occurs around 12 to 24 hours after death.
  • Stiffness of muscles and joints continues for 1 to 3 days.
  • Afterwards, decomposition of general tissues starts.

Relief of Rigor Mortis

  • Lysosomal intracellular hydrolytic enzymes like:
    • Cathepsins
    • Calpains
  • These enzymes hydrolyze muscle proteins, actin and myosin, resulting in breakdown of actomyosin complex.
  • This relieves the stiffness of muscles.

12. EFFECT OF LOAD

Load acting on muscle is of two types:

1. After Load

2. Free Load

After Load

  • After load is the load that acts on the muscle after beginning of muscular contraction.

Example: Lifting any object from the ground.

Load acts on muscles of arm only after lifting the object from the ground, i.e. after beginning of muscular contraction.

Free Load

  • Free load is the load which acts on muscle freely, even before onset of contraction of the muscle.
  • It is otherwise called free load.

Example: Filling water from a tap by holding the bucket in hand.

Free Load vs After Load

Free load is more beneficial (advantageous) since:

  • Force of contraction and work done by muscles are greater in free-loaded condition than in after-loaded condition.
  • This is because, in free-loaded condition:
    • Muscle fibers are stretched.
    • Initial length of muscle fiber is increased.
    • It facilitates the force of contraction.
  • This is in accordance with Frank-Starling law.

Frank-Starling Law

Frank-Starling law states that the force of contraction is directly proportional to initial length of muscle fibers within physiological limits.

13. REFRACTORY PERIOD

Refractory period is the period at which the muscle does not show any response to a stimulus.

  • It is because already one action potential is in progress and the muscle is in depolarized state during this period.
  • Muscle is unexcitable to further stimulus until it is repolarized.

Types of Refractory Period

1. Absolute Refractory Period

Absolute refractory period is the period during which muscle does not show any response at all, whatever may be the strength of stimulus.

2. Relative Refractory Period

Relative refractory period is the period during which muscle shows some response if the strength of stimulus is increased to maximum.

14. MUSCLE TONE

Definition

Muscle tone is defined as continuous and partial contraction of the muscles with certain degree of vigor and tension.

Maintenance of Muscle Tone

In Skeletal Muscle

  • Maintenance of tone in skeletal muscle is neurogenic.
  • It is due to continuous discharge of impulses from gamma motor neurons in anterior gray horn of spinal cord.
  • Gamma motor neurons in spinal cord are controlled by higher centers in brain.

In Cardiac Muscle

  • In cardiac muscle, maintenance of tone is purely myogenic.
  • Muscles themselves control the tone.
  • Tone is not under nervous control in cardiac muscle.

In Smooth Muscle

  • In smooth muscle, tone is myogenic.
  • It depends upon:
    • Calcium level
    • Number of cross bridges.

15. APPLIED PHYSIOLOGY: DISEASES INVOLVING MUSCLE TONE

Abnormalities of muscle tone lead to:

1. Hypertonia

2. Hypotonia

3. Myotonia

1. HYPERTONIA

Hypertonia or hypertonicity is a muscular disease characterized by:

  • Increased muscle tone
  • Inability of muscle to stretch.

Causes for Hypertonia

  • Hypertonia occurs in upper motor neuron lesion.
  • During lesion of upper motor neuron, inhibition of lower motor neurons (gamma motor neurons in spinal cord) is lost.
  • It causes exaggeration of lower motor neuron activity, resulting in hypertonia.

In Children

  • Hypertonia is associated with cerebral palsy.
  • Cerebral palsy is a permanent disorder characterized by muscular impairment.
  • It is caused by damage of cerebral cortex, which occurs at or before birth.
  • Here also, motor pathway is affected.
  • Such children usually have:
    • Speech delays
    • Language delays
    • Lack of communication skills.

Hypertonia and Spasticity

  • Hypertonia may be related to spasticity, but it is present with or without spasticity.
  • Spasticity is a motor disorder characterized by stiffness of certain muscles due to continuous contraction.
  • Hypertonicity is one of the major symptoms of spasticity.
  • Paralysis (complete loss of function) of the muscle due to hypertonicity is called spastic paralysis.

2. HYPOTONIA

  • Hypotonia is the muscular disease characterized by decreased muscle tone.
  • Tone of the muscle is decreased or lost.
  • Muscle offers very little resistance to stretch.
  • Muscle becomes flaccid (lack of firmness).
  • This condition is called flaccidity.

Causes for Hypotonia

  • Major cause for hypotonia is lower motor neuron lesion.
  • Paralysis of muscle with hypotonicity is called flaccid paralysis.
  • It results in muscle wastage.
  • Hypotonia may also occur because of:
    • Central nervous system dysfunction
    • Genetic disorders
    • Muscular disorders

Important Clinical Conditions Associated with Hypotonia

1. Down Syndrome

Chromosomal disorder characterized by physical disabilities and mental retardation.

2. Myasthenia Gravis

Autoimmune disease of neuromuscular junction caused by antibodies to cholinergic receptors.

3. Kernicterus

Brain damage caused by jaundice in infants.

4. Cerebellar Ataxia

Lack of coordination of movements.

5. Muscular Dystrophy

Muscular disease characterized by progressive degeneration of muscle fibers.

3. MYOTONIA

Myotonia is a congenital disease characterized by:

  • Continuous contraction of muscle
  • Slow relaxation even after cessation of voluntary act.
  • Main feature of this disease is muscle stiffness, which is sometimes referred to as cramps.
  • Muscle relaxation is delayed.
  • This type of muscular stiffness with delayed relaxation causes discomfort during simple actions like:
    • Walking
    • Grasping
    • Chewing
  • Muscles are enlarged (hypertrophy) because of continuous contraction.

Cause for Myotonia

  • Myotonia is caused by mutation in the genes of channel proteins in sarcolemma.
  • Such disorders are called channelopathies.
PROPERTIES OF SKELETAL MUSCLE
Physiology • Muscles