Relaxation
• Ca2+ moves back into sarcoplasmic
reticulum by active transport. Requires
energy
• Ca2+ moves away from troponintropomyosin complex
• Complex re-establishes its position and
blocks binding sites.
9-1
Muscle Twitch
• Muscle contraction in
response to a stimulus that
causes action potential in
one or more muscle fibers
• Muscle contraction
measures as force, also
called tension. Requires
up to a second to occur.
• Phases
– Lag or latent
(neuromuscular junction & step
#1 of cross-bridge movement)
– Contraction (step #2 - #6
of cross-bridge movement)
– Relaxation (powerpoint
slide # 28)
9-2
9-3
Stimulus Strength and Muscle
Contraction
• All-or-none law for muscle fibers
– Contraction of equal force in response to
each action potential
• Sub-threshold stimulus: no
action potential; no
contraction
• Threshold stimulus: action
potential; contraction
• Stronger than threshold; action
potential; contraction equal to
that with threshold stimulus
• Motor units: a single motor neuron
and all muscle fibers innervated by it
9-4
Contraction of the Whole Muscle
• Whole muscles exhibit characteristics that are more complex than
those of individual muscle fibers or motor units. Instead of
responding in an all-or-none fashion, whole muscles respond to
stimuli in a graded fashion, which means that the strength of the
contractions can range from weak to strong.
• Remember: There are many muscle fibers in one fasciculi and many fasciculi in
one whole muscle.
• Strength of contraction in whole muscle is graded: ranges from weak
to strong depending on stimulus strength
• Multiple motor unit summation: the force in which a whole muscle
contracts depends on the number of motor units stimulated to contract.
(force of contraction increases as more & more motor units are stimulated). A
muscle has many motor units
– Submaximal stimuli
– Maximal stimulus
– Supramaximal stimuli
9-5
Contraction of the Whole Muscle
9-6
Stimulus Frequency and Muscle Contraction
• Relaxation of a muscle fiber is not required before a second action potential
can stimulate a second contraction.
• As the frequency of action potentials increase, the frequency of contraction
increases
– Incomplete tetanus: muscle fibers partially relax between contraction
– Complete tetanus: no relaxation between contractions
– Multiple-wave summation: muscle tension increases as contraction
frequencies increase
9-7
Types of Muscle Contractions
• Isometric: no change in length of muscle but
tension increases during contraction
– Postural muscles of body ex: muscles hold spine erect while
person is sitting or standing
• Isotonic: change in length but tension constant
ex: waving using computer keyboard
– Concentric: tension is so great it overcomes opposing
resistance and muscle shortens
ex: raising of a weight during a bicep curl.
– Eccentric: tension maintained but muscle lengthens
ex: person slowly lowers a heavy weight
• Muscle tone: constant tension by muscles for long
periods of time
9-8
Fatigue
• Decreased capacity to work and reduced efficiency of
performance
• Types
– Psychological: depends on emotional state of
individual ex: burst of activity in tired athlete in response to
encouragement from spectators shows how psychological fatigue
can be overcome
– Muscular: results from ATP depletion ex: fatigue in lower
limbs of marathon runners or in upper & lower limbs of swimmers
– Synaptic: occurs in NMJ due to lack of acetylcholine
ex: rare-----only under extreme exertion
9-9
Physiological Contracture and
Rigor Mortis
• Physiological contracture: state of extreme
fatigue (extreme exercise) where due to lack of
ATP neither contraction nor relaxation can occur
• Rigor mortis: development of rigid muscles
several hours after death. Ca2+ leaks into
sarcoplasm and attaches to myosin heads and
crossbridges form but no ATP available to bind to
myosin---------so the cross-bridges are unable to
release. Rigor ends as tissues start to deteriorate.
9-10
Energy Sources
• ATP provides immediate energy for muscle contractions.
Produced from three sources
– Creatine phosphate
• During resting conditions stores energy to synthesize ATP
• ADP + Creatine phosphate------------------ Creatine + 1ATP
(Creatine Kinase)
– Anaerobic respiration
• Occurs in absence of oxygen and results in breakdown of
glucose to yield ATP and lactic acid
– Aerobic respiration
• Requires oxygen and breaks down glucose to produce ATP,
carbon dioxide and water
• More efficient than anaerobic
9-11
Slow and Fast Fibers
• Slow-twitch oxidative
– Contract more slowly, smaller in diameter, better blood supply, more mitochondria
(also called oxidative because carry out aerobic respiration), more fatigue-resistant than
fast-twitch, large amount of myoglobin (dark pigment which binds oxygen & acts as a
muscle reservoir for oxygen when blood does not supply adequate amount) .
– Postural muscles, more in lower than upper limbs. Dark meat of chicken.
– Functions: Maintenance of posture & performance in endurance activities.
• Fast-twitch
– Respond rapidly to nervous stimulation, contain myosin that can break down ATP
more rapidly than that in Type I, less blood supply, fewer and smaller mitochondria
than slow-twitch (adapted to perform anaerobic respiration)
– Lower limbs in sprinter, upper limbs of most people. White meat in chicken.
– Comes in oxidative and glycolytic forms
– Functions: Rapid, intense movements of short duration
• Distribution of fast-twitch and slow-twitch
•
– Most muscles have both but varies for each muscle
Exercise: weight lifting enlarges fast-twitch & aerobic training enlarges slow-twitch
• Effects of exercise: change in size of muscle fibers
– Hypertrophy: increase in muscle size
• Increase in myofibrils
• Increase in nuclei due to fusion of satellite cells
• Increase in strength
– Atrophy: decrease in muscle size
• Reverse except in severe situations where cells die
9-12
9-13
Smooth Muscle
•
•
•
•
•
Not striated, fibers smaller than those in skeletal muscle
Spindle-shaped; single, central nucleus
More actin than myosin
Caveolae: indentations in sarcolemma; may act like T tubules
Dense bodies instead of Z disks as in skeletal muscle; have noncontractile
intermediate filaments
• Ca2+ required to initiate contractions; binds to calmodulin (protein). Calmodulin
molecules with Ca++ bound to them activate an enzyme called myosin kinase,
which transfers a phosphate group from ATP to heads of myosin molecules.
Cross-bridging occurs
• Relaxation: caused by enzyme myosin phosphatase
9-14
9-15
Electrical Properties of Smooth
Muscle
• Slow waves of
depolarization and
repolarization transferred
from cell to cell
• Depolarization caused by
spontaneous diffusion of
Na+ and Ca2+ into cell
• Does not follow all-ornone law
• Contraction regulated by
nervous system and by
hormones (ex: epinephrine)
9-16
Regulation of Smooth Muscle
• Innervated by autonomic nervous system
(composed of nerve fibers that send impulses from CNS to smooth
muscle, cardiac muscle, glands)
• Neurotransmitters are acetylcholine and
norepinephrine (increases cardiac output, blood glucose
levels)
• Hormones important as epinephrine and
oxytocin
• Receptors present on plasma membrane;
which neurotransmitters or hormones bind
determines response
9-17
Cardiac Muscle
•
•
•
•
•
•
•
Found only in heart
Striated
Each cell usually has one nucleus
Has intercalated disks and gap junctions
Autorhythmic cells
Action potentials of longer duration
The depolarization of cardiac muscle results from
influx of Na+ and Ca2+ across the plasma
membrane
9-18
Effects of Aging on Skeletal
Muscle
• Reduced muscle mass
• Increased time for muscle to contract in
response to nervous stimuli
• Reduced stamina
• Increased recovery time
• Loss of muscle fibers
9-19