Unit Two: Membrane Physiology, Nerve, and

Muscle

Chapter 6: Contraction of Skeletal Muscle

Guyton and Hall, Textbook of Medical Physiology, 12th edition

Physiological Anatomy of Skeletal Muscle

• Skeletal Muscle Fiber

a. Sarcolemma (plasma membrane) is a thin membrane enclosing a muscle fiber

b. Myofibrils are composed of actin and myosinc. Titin filaments keep the myosin and actin filaments

in placed. Sarcoplasm is the intracellular fluid between

myofibrilse. Sarcoplamic reticulum is a specialized endoplasmic

reticulum of muscle cells

Fig. 6.1

General Mechanism of Muscle Contraction

1. An action potential travels along a motor nerve to the motor end plate

2. The nerve secretes acetylcholine3. The AcH binds to sarcolemma and opens gated channels4. Large amounts of Na+ enter the cell and initiates and AP5. AP travels along the sarcolemma the same as in a nerve

cell6. AP causes depolarization and triggers release of Ca++ from

the sarcoplasmic reticulum7. Ca++ initiate the contraction cycle8. After contraction, Ca++ ions are reabsorbed by the

sarcoplasmic reticulum

Fig. 6.3 Organization of proteins in a sarcomere

Molecular Mechanism of Muscle Contraction

• Sliding Filament Mechanism of Muscle Contraction

Fig. 6.5 Relaxed and contracted state of a myofibril

Molecular Mechanism (cont.)

• Molecular Characteristics of the Contractile Filaments

a. Myosin filaments are composed of multiple myosinmolecules

Fig. 6.6

Molecular Mechanism (cont.)

b. ATPase activity of the myosin hearc. Actin filaments are composed of actin,

tropomyosin and troponin

Fig. 6.7 Actin filament

Molecular Mechanism (cont.)

d. Tropomyosin molecules-wrapped spirally around the sides of the F-actin helixe. Troponin and its role in muscle contraction-

helps attach the tropomyosin to the actin; strong affinityfor calcium during contraction

f. Inhibition of the actin filament by the troponin-tropomyosin complex and activation by calcium ions

g. Interaction between actin and myosin crossbridges

h. Chemical events in the motion of the myosin heads

Molecular Mechanism (cont.)

Fig. 6.8 “Walk along” mechanism for contraction

• Amount of Actin and Myosin Filament Overlap Determines Tension Developed by the Contracting Muscle

Fig. 6.9 Length-tension diagram

• Effect of Muscle Length on Force of Contraction in Whole Intact Muscle

Fig. 6.10 Relation of Muscle Length to Tension

• Relation of Velocity of Contraction to Load-contractsrapidly when it contracts against no load; velocity decreases as load increases

Fig. 6.11 Relation of Load to Velocity of Contraction

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

a. Phosphocreatineb. Glycolysis (uses stored glycogen as

energy sourcec. Oxidative metabolism

Characteristics of Whole Muscle Contraction

• Muscle Twitch-demonstrated by eliciting single muscle twitches

• Isotonic vs Isometric Contraction

a. Isometric-muscle does not shorten during contractionb. Isotonic-muscle shortens but the tension remains

constant during the contraction

• Characteristics of Isometric Twitches FromDifferent Muscles

Muscle Contraction (cont.)

Fig. 6.13

Muscle Contraction (cont.)

• Fast vs. Slow Muscle Fibers

a. Slow (Type I, Red Muscle)1. Smaller fibers2. Innervated by smaller nerve fibers3. Extensive blood vessel system for oxygenation4. Increased numbers of mitochondria5. Large amounts of myoglobin (combines with

oxygen and stores it until needed

Muscle Contraction (cont.)

b. Fast (Type II, White Muscle)1. Large fibers for great strength of contraction2. Extensive sarcoplasmic reticulum for rapid

release of calcium3. Large amounts of glycolytic enzymes for

glycolysis4. Less extensive blood supply (anaerobic)5. Fewer mitochondria6. Low levels of myoglobin

Mechanics of Skeletal Muscle Contraction

•Motor Unit-all the muscle fibers innervated by a singlenerve fiber

a. Summation-adding together of individual twitch contractions to increase the intensityof the overall muscle contraction; can happentwo ways: (1) increasing the number of motorunits, or (2) increasing the frequency of contraction

b. Multiple Fiber Summationc. Frequency Summation and Tetanus

Mechanics (cont.)

Fig. 6.14 Frequency of Summation and Tetanus

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)

• 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

a. the point of muscle insertionb. its distance from the fulcrum of the leverc. the length of the lever armd. the position of the lever