1. Unit Two: Membrane Physiology, Nerve, and Muscle
Chapter 6: Contraction of Skeletal Muscle
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Physiological Anatomy of Skeletal Muscle
Skeletal Muscle Fiber
Sarcolemma (plasma membrane) is a thin membrane
enclosing a muscle fiber
Myofibrils are composed of actin and myosin
Titin filaments keep the myosin and actin filaments
in place
Sarcoplasm is the intracellular fluid between
myofibrils
Sarcoplamic reticulum is a specialized endoplasmic
reticulum of muscle cells

3. Fig. 6.1

4. General Mechanism of Muscle Contraction
An action potential travels along a motor nerve to the
motor end plate
The nerve secretes acetylcholine
The AcH binds to sarcolemma and opens gated channels
Large amounts of Na+ enter the cell and initiates and AP
AP travels along the sarcolemma the same as in a nerve
cell
AP causes depolarization and triggers release of Ca++ from
the sarcoplasmic reticulum
Ca++ initiate the contraction cycle
After contraction, Ca++ ions are reabsorbed by the
sarcoplasmic reticulum

5. Fig. 6.3 Organization of proteins in a sarcomere

6. Molecular Mechanism of Muscle Contraction
Sliding Filament Mechanism of Muscle Contraction
Fig Relaxed and contracted state of a myofibril

7. Molecular Mechanism (cont.)
Molecular Characteristics of the Contractile Filaments
Myosin filaments are composed of multiple myosin
molecules
Fig. 6.6

8. Molecular Mechanism (cont.)
ATPase activity of the myosin hear
Actin filaments are composed of actin,
tropomyosin and troponin
Fig Actin filament

9. Molecular Mechanism (cont.)
d. Tropomyosin molecules-wrapped spirally around
the sides of the F-actin helix
Troponin and its role in muscle contraction-helps attach the tropomyosin to the actin; strong affinity
for calcium during contraction
Inhibition of the actin filament by the troponin-tropomyosin complex and activation by calcium ions
Interaction between actin and myosin cross
bridges
h. Chemical events in the motion of the myosin heads

10. Molecular Mechanism (cont.)
Fig “Walk along” mechanism for contraction

11. Amount of Actin and Myosin Filament Overlap
Determines Tension Developed by the
Contracting Muscle
Fig Length-tension diagram

12. Effect of Muscle Length on Force of Contraction in Whole Intact Muscle
Fig Relation of Muscle
Length to Tension

13. Relation of Velocity of Contraction to Load-contracts
rapidly when it contracts against no load; velocity
decreases as load increases
Fig Relation of Load to
Velocity of Contraction

14. Energetics of Muscle Contraction
Work Output-when a muscle contracts against a load
it performs work. Energy is transferred from the
muscle to the external load to lift an object
Sources of Energy for Muscle Contraction
Phosphocreatine
Glycolysis (uses stored glycogen as energy source
Oxidative metabolism

15. Characteristics of Whole Muscle Contraction
Muscle Twitch-demonstrated by eliciting single
muscle twitches
Isotonic vs Isometric Contraction
Isometric-muscle does not shorten during contraction
Isotonic-muscle shortens but the tension remains
constant during the contraction

16. Muscle Contraction (cont.)
Characteristics of Isometric Twitches From
Different Muscles
Fig. 6.13

17. Muscle Contraction (cont.)
Fast vs. Slow Muscle Fibers
Slow (Type I, Red Muscle)
Smaller fibers
Innervated by smaller nerve fibers
Extensive blood vessel system for oxygenation
Increased numbers of mitochondria
Large amounts of myoglobin (combines with
oxygen and stores it until needed

18. Muscle Contraction (cont.)
b. Fast (Type II, White Muscle)
Large fibers for great strength of contraction
Extensive sarcoplasmic reticulum for rapid
release of calcium
Large amounts of glycolytic enzymes for
glycolysis
Less extensive blood supply (anaerobic)
Fewer mitochondria
Low levels of myoglobin

19. Mechanics of Skeletal Muscle Contraction
Motor Unit-all the muscle fibers innervated by a single
nerve fiber
Summation-adding together of individual
twitch contractions to increase the intensity
of the overall muscle contraction; can happen
two ways: (1) increasing the number of motor
units, or (2) increasing the frequency of
contraction
Multiple Fiber Summation
Frequency Summation and Tetanus

20. Mechanics (cont.)
Fig Frequency of Summation and Tetanus

21. Maximum Strength of Contraction
The Staircase Effect (Treppe)-when a muscle begins to contract after a long rest, its initial strength may be as little as ½ its strength 10 to 50 muscle twitches later.
Thought to be due to a progressive increase of calcium ions in the sarcoplasm.
Muscle Tone-when muscles are at rest, a certain degree of tautness remains. Due to a low rate of impulses coming from the spinal cord; also involves the muscle spindles (receptors)

22. Muscle Fatigue-increases almost in the direct
proportion to the depletion of muscle glycogen;
also contributing is a loss of ATP, oxygen,
decreased blood flow to the muscle
Lever Systems of the Body-muscles operate by
applying tension to their points of insertion;
analysis of the lever systems depends on
the point of muscle insertion
its distance from the fulcrum of the lever
the length of the lever arm
the position of the lever