Muscle Physiology 3Dr. Sumera Gul

Associate Professor

Department of Physiology

Learning Objectives:

At the end of the lecture the students should be able to:

• Explain the different energy systems of the skeletal muscle contraction

• Explain the concept of isometric and isotonic contractions

• Describe the features of types of muscle fibers

• Describe mechanics of skeletal muscle contraction

• Explain the remodeling of muscle to match function

Review of previous lecture

Review

• Functions and features of actin and myosin and other proteins in skeletal muscle

• Steps of skeletal muscle contraction

• Sliding filament theory

• Walk along theory

• Rigor mortis

Energetics of Muscle Contraction

• When a muscle contracts against a load, it performs work

• To work we require energy

• Where does that energy come from?

Sources of energy for muscle contraction?

Sources of energy for muscle contraction

• We require energy for

• Walking of myosin heads on actin during contraction

• Pumping of calcium back into sarcoplasmic reticulum

• Pumping of sodium and potassium

Sources of energy for muscle contraction

• ATP is adequate for contraction for 1-2 seconds

• ATP is dephosphorylated and produces energy

• ADP has to be re-phosphorylated to be used again

Sources of energy for muscle contraction

• Phosphocreatine--- 5-8 seconds

• Glycolysis - -1 minute and is rapid

• Oxidative metabolism– longer periods

Characteristics of whole muscle contractions

a

b

c

1.5 2.0 2.5 3.0 3.5

a b

c

100

75

50

25

0

sarcomere length

[mm]te

nsio

n g

en

era

ted

Force-Length Relationship

• Isometric

• Vs

• Isotonic contractions

There are two types of muscle contraction.

1. Isotonic

ii. Isometric

Types of Muscle Fibers

• Slow Red fibers

• Fast White fibers

Type of Fibers

Slow, Type I, Red Muscles• Smaller diameter

• Innervated by smaller nerve fibers.

• More extensive blood vessel system and more capillaries to supply extra amounts of oxygen.

• Greatly increased numbers of mitochondria to support high levels of oxidative metabolism.

• Large amounts of myoglobin, an iron-containing protein similar to hemoglobin in red blood cells.

• The myoglobin gives the slow muscle a reddish appearance and hence the name red muscle.

Fast, Type II, White Muscles• Larger diameter

• An extensive sarcoplasmic reticulum is present for rapid release of calcium ions to initiate contraction.

• Large amounts of glycolytic enzymes are present for rapid release of energy by the glycolytic process.

• Less extensive blood supply .

• Fewer mitochondria than do slow fibers, also because oxidative metabolism is secondary.

• A deficit of red myoglobin in fast muscle gives it the name white muscle.

Type of Fibers

Slow, Type I, Red Muscles

• Resistant to fatigue

• Extensors and flexors

• Glycogen low

Fast, Type II, White Muscles

Fatiguable

Ocular muscles

Glycogen high

Mechanics of Skeletal Muscle Contraction

Muscle Twitch

• Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers

• Phases• Lag or latent

• Contraction

• Relaxation

Summation

Summation

• The adding together of the individual twitch contractions to increase the intensity of overall muscle contraction

• Multiple Fiber summation

• Frequency summation

Multiple Motor Unit Summation

Multiple Fiber Summation

Multiple Fiber Summation

• As the strength of signal increases, larger and larger motor units begin to contract

• Size Principle

The size principle

• motor units are recruited in order of their size.

• Small motor units have small motor neurons and they are recruited first.

• Large motor units have large motor neurons and they are recruited last.

Multiple-Wave Summation or Frequency Summation

• As frequency of action potentials increase, frequency of contraction increases

• Action potentials come close enough together so that the muscle does not have time to completely relax between contractions.

Incomplete tetanization

INCOMPLETE TETANIZATION

• Stimulation of muscle fiber by increasing frequency, that every next stimulus

acts during the preceding one i.e. relaxation period

• Every contraction starts before the end of previous one

• Partial relaxation between the subsequent contractions

• Calcium ions increase the strength of contraction

Complete tetanization

Tetanization

COMPLETE TETANIZATION

• Stimulation of muscle fiber by increasing frequency, that every next stimulus acts

during the preceding one i.e. contraction period

• Every contraction merges with the previous one

• No relaxation between the contractions

• Smooth sustained contraction

• Early fatigue

Treppe or Staircase effect

• Graded response

• Occurs in muscle rested for prolonged period

• Each subsequent contraction is stronger than

previous until all equal after few stimuli

Muscle Remodeling

Muscle Remodelling

• Atrophy

• Hypertrophy

• Hyperplasia

• Adjustment of Muscle length

Summary of Mechanics

1 Muscles pull. The sarcomere shorten

2 Muscle force can be graded by recruitment of motor units

3 Small motor units are activated first: the size principle

4 Muscle force can be increased by repetitive stimulation

5 Muscle velocity is inversely related to muscle force

6 Muscles fatigue: they drop force on continued use

7 Muscle are in certain amount of tautness even at rest

8 Muscle remodels to match the functions

Skeletal Muscle Action Potential