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Muscle Physiology II
Cardiac Muscle
• Same mechanism as skeletal
• Less calcium stored but longer T-tubules
& more released with a single impulse
• Impulses travel rapidly from cell to cell so
it is self-stimulating
Smooth Muscle
• Myofibrils are very thin and randomly
organized
• Use a different protein than troponin
• Can be triggered by hormones or even
muscle stretching
Stimulus Response
• An individual fiber’s response to stimulus
is called a twitch – when myosin heads
link with actin
• The delay between stimulation (ACh
release) and twitch is called the latent
period
• This is followed by the contraction & the
relaxation periods
• myogram
Stimulus Response
• For a brief moment after stimulus, a fiber
is unable to respond to further stimulation
–Refractory period
• If the next stimulus comes after the
refractory period but before relaxation the
twitches combine leading to a sustained
contraction
Fiber Types
• 3 types: slow-twitch, fast-twitch, &
intermediate
• Most muscles have a mixture of all 3
–Postural: mostly slow-twitch
–Arms/shoulders: mostly fast-twitch
–Legs: even mix of slow and fast-twitch
• Strength is directly proportional to
diameter
Slow-twitch fibers
• Thinnest in diameter
• Utilize aerobic respiration (oxidative)
• Red due to high concentration of
myoglobin
• Slowest contraction cycle
–Prolonged sustained contractions
–Resistance to fatigue
Fast-twitch Fibers
• Thickest in diameter
• Utilize anaerobic respiration (Glycolytic)
• White due to lack of myoglobin and poor
blood supply
• Most powerful and rapid contractions
–Intense movement but for short
duration
–Fatigue easily
Intermediate Fibers
• Like a combination of slow and fasttwitch
• Red due to myoglobin
• Both oxidative and glycolytic
• A middle ground for resistance to fatigue,
speed, and power
Respiration
Aerobic (Oxidative)
• Mitochondria  ATP
• Molecule called myoglobin able to store O2
in the fiber
Anaerobic (Glycolytic)
• Take over when O2 supply becomes
insufficient
• Lactic acid produced as a byproduct
–Carried to liver for breakdown
Lactic Acid Buildup
• Lactic acid lowers pH which interferes with
the muscles’ ability to respond to stimuli
–fatigue
• Does not directly cause cramps
–Involuntary, sustained contraction
–Could be electrolyte imbalance
–Likely caused by use during fatigue 
damage
Muscle Fatigue
• With prolonged use, a muscle can lose its
ability to contract
–Decreased blood flow
–Ion imbalance
–Lactic acid accumulation
Exercise
• The ratio of slow to fast-twitch fibers
has roots in genetics
• More fast-twitch fibers is well-suited for
weightlifting and sprinting
–Short duration & intense
• More slow-twitch fibers is well-suited
for running and swimming
–Endurance is key
Heat Production
• Respiration creates heat as a by-product
• Dispersed throughout the body through
bloodstream for release via homeostatic
mechanisms
Building Muscle Mass
Generally, the number of fibers does not
change only their characteristics
• Aerobic Training – converts fast-twitch to
intermediate; more mitochondria,
increased blood supply
• Strength Training – increases size and
strength of fast twitch
–Synthesis of actin & myosin
Atrophy
• Lack of use will cause muscles to deteriorate
• Happens naturally with age
–Starts around age 30
–Slow, progressive loss of mass
–Replaced by connective tissue
–Decrease function
• Strength
• Flexibility
• Reflex
Muscles after Death
• At death, calcium ions are released
causing a partial contraction
• No new ATP so the muscles can’t relax
• Joints become fixed
creating the condition
known as rigor mortis
Muscle Memory
• With repeated motion, the nervous
system circuitry becomes more efficient
at stimulating the muscles
• Strength of muscle is first dependent on
ability to stimulate, not mass
Muscle Problems
• Muscles are connected to other muscles
by connective tissue called aponeuroses
• Muscles are connected to bones by
tendons
–Tendinitis occurs when the tendons
swell causing pain and disrupts the
joint function
Strains
• Overstretching or tear of tendon or
muscle
–Insufficient warm-up
–Poor technique
–Untreated minor injuries
• Classified like sprains
Involuntary, Abnormal Contractions
• Spasm: sudden contraction in a single
muscle
• Tic: twitching in muscles usually under
voluntary control
• Tremor: rhythmic, purposeless shaking
Myasthenia Gravis
• Autoimmune
–Antibodies block Ach receptors
• Damages neuromuscular junction
• Muscles weaken and fatigue easily
–Can cease to function
Muscular Dystrophy
• Progressive degeneration of muscle
fibers
–Protein that reinforces sarcolemma not
produced
• Duchenne MD caused by mutation on
the X
• Onset around age 2-5; can’t walk by age
12; cardiac failure at age 20-30
Performance Enhancers
Most performance enhancers help build
muscle mass by stimulating protein
synthesis
• Steroids
–Mood swings, aggression, baldness,
disrupted sexual function, infertility
• HGH
–Overgrowth of hands, feet, head;
enlarged internal organs esp. heart
Power
• In addition to stored ATP, muscle fibers
use creatine for ATP production as well.
• This, however, is still not enough for
more than a quick burst so cellular
respiration must occur
Sex Differences
• Anatomy is basically the same
–Men tend to have more fast-twitch
fibers than women
• Mass differs
–Testosterone  hypertrophy
–So men have larger fibers and thus
more mass and strength