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Additional Notes on Muscle
twitch contraction (from today’s lecture)
Twitch contraction occurs when a single action potential in motor neuron causes all
muscle fibers in a motor unit to contract
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The recording is a myogram = record of a muscle contraction
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9
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because, when a muscle fiber is stimulated enough to contract, it then
loses excitability and cannot respond during refractory period (about 5
msec for skeletal muscle, 300 msec for cardiac)
frequency of stimulation (see Fig
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17 on page 298)
After refractory period, second of two stimuli will elicit contraction
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—Figure 9
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—Figure 9
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17c shows fused tetanus = sustained maximal contraction
when stimuli come at rate of 80 – 100 per second
Why? Because more Ca++ is released into the sarcoplasm with each stimulus and adds
to Ca++ already there
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Filaments are already in

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a partly contractec state
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motor unit recruitment
Recruitment is the process in which the number of active motor units increases
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Some motor units contract while others relax
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Weakest motor units are recruited first
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This produces smooth muscle movements, no jerks
aerobic training versus strength training
Aerobic training: regular repeated activities such as running, dancing
over time will increase supply of oxygen-rich blood to skeletal muscle
for aerobic respiration
builds endurance for prolonged activities
Anaerobic training: weight lifting
relies on anaerobic production of ATP through glycolysis
stimulates synthesis of muscle proteins and hypertrophy
builds muscle strength for short term feats
Interval training incorporates both aerobic and anaerobic activties
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Neurons in brain or spinal cord establish muscle tone
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Muscle tone keeps muscle firm but does not produce movement
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It’s also important in smooth muscle of gut; and in blood vessels to maintain BP
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In isometric contraction, tension increases greatly without change in muscle
length
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Types of skeletal muscle fibers
Muscle fibers vary in content of myoglobin
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Low myoglobin white muscle fibers
Fibers vary in rates of contraction/relaxation
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slow oxidative fibers SO = slow twitch (type I)
small diameter least powerful
dark red, high myoglobin and blood capillaries, mitochondria
generate ATP mainly by aerobic cellular respiration, hence oxidative
contraction cycle is slower, as ATPase in myosin heads hydrolyzes ATP relatively slowly
twitch 100 – 200 msec, take longer to reach peak tension
very resistant to fatigue
adapted for maintaining posture, endurance type activities (marathon)
fast oxidative-glycolytic FOG = fast twitch (type IIa)
intermediate diameter
large amount myoglobin and blood capillaries
generate ATP aerobically and by anaerobic activity using glycogen stores
fast ATPase in myosin heads hydrolyzing ATP 3 – 5 x faster than SO
twitches reach peak tension more quickly and are brief in duration
adapted for walking and sprinting
fast glycolytic FG = fast twitch (type IIb)
largest diameter, most myofibrils
most powerful contractions
lower myoglobin, mitochondria, blood capillaries
large amount glycogen, generate ATP by glycolysis
strong, quick contractions
adapted for intense anaerobic movements
hypertrophy in weight lifter, 50% larger than in sedentary person
distribution and recruitment of different fibers
skeletal muscles are a mix of SO, FOG, FG but each motor unit is all one kind
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People with more FG fibers excel in periods of intense activity = weight lifting, sprinting
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Induce changes from FG to FOG by doing lots of endurance type activity
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anabolic steroids
illegal
similar to testosterone
increase muscle size and strength
Side effects: liver cancer
kidney damage
risk of heart disease
stunted growth
mood swings
increased irritability and aggression
masculinization of female
diminished testosterone and testicular atrophy in male
Cardiac Muscle Tissue
involuntary
branched, cylindrical, one centrally located nucleus
striated with sarcomeres
Intercalated discs are cell junctions
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Gap junctions allow muscle action potentials to spread from fiber to fiber
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—long refractory period
Ca++ enters from sarcoplasmic reticulum AND from interstitial fluid
—contracts in response to own autorhythmic fibers about 75 times per minute
requires constant supply of oxygen
has large numerous mitochondria
—largely depends on aerobic cellular respiration to generate ATP
—can also use lactic acid produced during exercise by skeletal muscle to make ATP
Contraction is regulated by both ACh and NE from autonomic nervous system
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visceral single-unit in sheets that wrap around walls of arteries, veins, hollow organs
autorhythmic: one fiber stimulated and action potential spreads via gap junctions
multiunit individual fibers have own motor neuron terminals, rare gap junctions
—in walls of large arteries, airways, arrector pili, iris muscles, ciliary body that
adjusts focus of lens
Regeneration
Skeletal muscle fibers don’t divide
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Satellite (stem) cells divide and fuse with existing cells at a slow rate
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Cardiac muscle hypertrophies in well-conditioned athletes
It can be replaced to some extent by stem cells, but not rapidly enough
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Pericytes — stem cells— give rise to new cells as well
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