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Essentials of Anatomy & Physiology, 4th Edition
Martini / Bartholomew
7C
The Muscular
System
PowerPoint® Lecture Outlines
prepared by Alan Magid, Duke University
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Mechanics
Some Basic Muscle Definitions
• Muscle tension—The pulling force on the
tendons that muscle cells generate when
contracting
• Muscle twitch—A brief contraction-relaxation
response to a single action potential
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Mechanics
The Twitch and Development of Tension
Figure 7-6
Muscle Mechanics
The Frequency of Muscle Fiber Stimulation
• Summation—Addition of twitch tension when
a stimulus is applied before tension has
completely relaxed
• Incomplete tetanus—Tension peaks and
falls repeatedly and builds up beyond twitch
tension
• Complete tetanus—Tension is steady (no
relaxation phase) and largest if stimuli arrive
at very high rates
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Mechanics
The Effects of Repeated Stimulations
Figure 7-7
Muscle Mechanics
Motor Units
• Motor Unit —A motor neuron and all
the muscle cells it controls
• Recruitment—To increase muscle
tension by activating more motor units
• Small motor units provide finer control
• Motor units are intermixed in the
muscle to pull evenly on the tendon
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Mechanics
Motor Units
Figure 7-8
Muscle Mechanics
Key Note
All voluntary (intentional) movements
involve the sustained, sub-tetanic
contractions of skeletal muscle fibers
organized into distinct motor units. The
force generated can be increased by
increasing the frequency of action
potentials (=impulses) or by recruiting
additional motor units.
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Mechanics
• Muscle tone—Tension in a
“resting” muscle produced by a low
level of spontaneous motor neuron
activity. Distinct from resting
tension produced by passive
stretching.
• Function of muscle tone
• Stabilizes bones, joints
• Prevents atrophy (muscle wasting )
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Mechanics
Types of Contractions
• Isotonic contraction
The tension (load) on a muscle stays
constant (iso = same, tonic = tension)
during a movement. (Example: lifting a
baby)
• Isometric contraction
The length of a muscle stays constant
(iso = same, metric = length) during a
“contraction” (Example: holding a baby
at arms length)
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Mechanics
Muscle Elongation
• Muscle contracts actively
• Muscles can only pull
• Muscles never push
• Muscle elongates passively
• Elastic forces
• Contraction of opposing
muscles
• Effects of gravity
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Energetics of Muscle Contraction
ATP and Creatine Phosphate Reserves
• Muscle contraction consumes much ATP
• ATP transfers energy directly to cycling
cross-bridges and calcium pumping
• CP stores energy and regenerates ATP
• CP transfers its energy to ADP
• Creatine phosphokinase (CPK) catalyzes
• ADP (2 “P”s) becomes ATP(3 “P”s)
• CP levels greatly exceed ATP levels
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Energetics of Muscle Contraction
ATP Generation
• Light activity
• Aerobic metabolism of fatty acids
• Storage of glucose as glycogen
• Moderate activity
• Breakdown of glycogen to glucose
• Glycolysis of glucose
• Peak activity
• Anerobic breakdown of glucose
• Production of lactic acid
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Energetics of Muscle Contraction
Muscle Metabolism
Figure 7-9(a)
Energetics of Muscle Contraction
Muscle Metabolism
Figure 7-9(b)
Energetics of Muscle Contraction
Muscle Metabolism
Figure 7-9(c)
Energetics of Muscle Contraction
Muscle Fatigue—When a muscle
loses ability to contract due to a low
pH (lactic acid buildup), low ATP
levels, or other problems
Recovery Period—Time after muscle
activity that it takes to restore preexertion conditions
Oxygen Debt—Amount of excess
oxygen used during the recovery
period
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Energetics of Muscle Contraction
Key Note
Skeletal muscles at rest metabolize
fatty acids and store glycogen. During
light activity, muscles can generate
ATP through the aerobic breakdown of
carbohydrates, lipids, or amino acids.
At peak levels of activity, most of the
energy is provided by anaerobic
reactions that generate lactic acid.
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Performance
Two Types of Skeletal Muscle Fibers
• Fast fibers
Large diameter, abundant myofibrils, ample
glycogen, scant mitochondria. Produce
powerful, brief contractions
• Slow fibers
Smaller diameter, rich capillary supply, many
mitochondria, much myoglobin. Produce
slow, steady contractions
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Performance
Physical Conditioning
• Anaerobic endurance
Time over which a muscle can contract
effectively under anerobic conditions.
• Hypertrophy
Increase in muscle bulk. Can result from
anerobic training.
• Aerobic endurance
Time over which a muscle can contract
supported by mitochondria.
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Performance
Key Note
What you don’t use, you lose. When
motor units are inactive for days or
weeks, muscle fibers break down their
contractile proteins and grow smaller
and weaker. If inactive for long periods,
muscle fibers may be replaced by
fibrous tissue.
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings