muscle physiology 3€¦ · muscle physiology 3 dr. sumera gul associate professor department of...
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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