biol 223 muscles 2

tension “pulling strength”

sliding of actin and myosin filaments causes sarcomere shortening

in a muscle cell, all sarcomeres shorten causing the muscle cell to shorted

tension in a muscle depends on:

• tension that develops in individual muscle cells during contraction

• number of muscle cells that contract

amount of shortening depends on tension and resistance

fiber shortening

as sarcomeres shorten, muscle cell shortens, producing tension

pulls on connective tissue and bone to which it is attached

tension produced in individual muscle fibers

can vary due to:

length-tension relationship (how stretch/compressing is for a muscle)

frequency of stimulation by motor neuron

tension produced by entire muscle

can vary due to:

number of muscle cells receiving nerve stimulation, commanding them to contract

• muscle cells are grouped in motor units

  • group of muscle cells all told to contract simultaneously

length-tension relationships

amount of tension depends on number of cross bridges formed

• depends on degree of overlap of actin and myosin filaments

skeletal muscle contracts most forcefully over a narrow range of resting lengths

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twitch

cycle of contraction, relaxation produced by a single action potential in a muscle cell

not typical of most normal skeletal muscle activity

twitch latent phase

action potential occurs

no contraction until Ca+2 is released from SR

twitch contraction phase

tension rises to peak

Ca+2 moves tropomyosin off actin active sites

myosin cross bridges form, actin is pulled

twitch relaxation phase

tension falls to resting levels

Ca+2 is pumped back into SR

actin sites covered by tropomyosin

no cross bridges remain

frequency of stimulation

most muscular activities involve sustained muscular contractions

• produced by high frequency of action potentials in muscle cell

• produced in response to high frequency of action potentials in motor neuron (high frequency of stimulation)

• summation of tension produces greater tension due to more available calcium

twitch

summation

repeated stimulation produced before relaxation phase has been completed

• summation of tension caused by build up of calcium ions in sarcoplasm

complete tetanus

sustained contraction

maximum tension production in a muscle cell - maximum cross bridge formation

tetanus disease

caused by clostridium tetani bacteria

bacterial toxin causes high frequency of action potentials in motor neurons

treppe

an increase in peak tension with each successive stimulus delivered shortly after the completion of the relaxation phase of the preceding twitch

the fiber’s maximum potential tension is not reached until tetanus

wave summation

occurs when successive stimuli arrive before the relaxation phase has been completed

action potentials are happening faster, nerve still hasn’t been released

incomplete tetanus

occurs if the stimulus frequency increases further. tension production rises to a peak and the periods of relaxation are very brief

sustaining tension but not to the maximum

complete tetanus

during complete tetanus, the stimulus frequency is so high that the relaxation phase is eliminated. tension plateaus at a maximum level

tension

produced in individuals muscle fibers (cells) can vary due to

• length-tension relationship

• frequency of stimulation

produced by entire muscle can vary even more widely due to

• number of muscle cells receiving nerve stimulation, commanding them to contract

  • muscle cells are grouped in motor units

motor units

all the muscle fibers innervated by one motor neuron

amount of tension produced in a muscle determined by number of motor units activated

asynchronous motor unit summation for sustained contractions

differences in number and size of motor units in different muscles determines precision of control and movements

small motor unit

precise control

one motor neuron innervates a small number of muscle fibers

large motor unit

gross movement control

one motor neuron innervates a large number of muscle fibers

muscle tone

resting tension in a skeletal muscle

in any muscle, some motor units are always active; tense and firm the muscle

which motor units are active is constantly changing, muscle tone is not produced by a specific subset of motor units

stabilizes bones and joints

greater resting muscle tone causes higher resting rate of metabolism

isotonic

tension rises, length of muscle changes

concentric and eccentric

concentric

muscle tension exceeds resistance and muscle shortens

eccentric

peak tension developed is less than the resistance, muscle elongates

isotonic concentric contraction

a musle is attached to a weight that is ½ its maximum potential tension. when stimulated, it develops enough tension to lift the weight. the tension remains constant, but the muscle shortens

isotonic eccentric contraction

the tension remains constant, but the muscle lengthens

when the eccentric contraction ends, the unopposed load stretches the muscle until either the muscle tears, a tendon breaks, or the elastic recoil of the skeletal muscle is sufficient to oppose the load

isometric (iso- same, metric- measurement/length)

tension rises, length of muscle remains constant

tension produced never exceeds resistance

muscle as a whole does not shorten but individual muscle fibers shorten until internal connective tissues and tendons are taut

cannot shorten further because tension does not exceed resistance

isometric contraction

when stimulated, the tension rises, but the muscle length stays the same

lengthening a muscle

no active mechanism for muscle fiber elongation

• a muscle cell does not cause itself to lengthen after contraction process ends

returns to resting length due to

• recoil in elastic components in connective tissue

• contraction of opposing muscle groups

• gravity

  • when you stop producing tension, the muscle will rebound

energy use and muscle contraction

muscle contraction requires large amounts of ATP

muscle cells stores only enough high energy molecules to sustain contraction until additional ATP can be generated

• ATP and creatine phosphate (CP) reserves last ~ 15 seconds once contraction begins

muscle cell must generate ATP at approx. the same rate as it is used for remainder of contraction

creatine phosphate reserves

ATP not used for long term storage of energy

at rest, muscle cell makes more ATP than needed; extra ATP transfers high energy phosphate to creatine for storage

CP reserves released stored energy to convert ADP to ATP when ATP is needed at start of contraction

ATP generation

aerobic cellular respiration: most ATP needed for resting muscle and for moderate levels of muscle activity

aerobic AND anaerobic glycolysis pathways needed to generate additional ATP for PEAK PERFORMANCE (will produce lactic acid)

aerobic metabolism

aerobic cellular respiration

• uses O2 - releases CO2

occurs in mitochondria

• citric acid cycle

  • CO2 is produced

• electron transport chain

  • ATP synthesis

  • O2 is used

muscle cell ATP generation

resting muscle fiber surely on aerobic metabolism of fatty acids to generate ATP

• FA absorbed from circulation

• broken down to 2-carbon units of acetyl CoA which enter DIRECTLY into the Citric Acid Cycle

  • excess ATP used to store glucose into glycogen, create creatine phosphate

muscle metabolism in a resting muscle fiber

the demand for ATP is low and sufficient oxygen is available for mitochondria to meet that demand

fatty acids are absorbed and broken down in the mitochondria creating surplus of ATP

some mitochondrial ATP is used to convert absorbed glucose to glycogen

mitochondrial ATP is also used to convert creatine to creatine phosphate (CP)

this results in the buildup of energy reserves (glycogen and CP) in the muscle

muscle cell ATP generation

contracting muscle fibers rely on aerobic AND anaerobic metabolism of glucose

• amount of aerobic vs. anaerobic metabolism depends on intensity of muscle contraction

  • moderate vs peak

• glucose comes from circulation and breakdown of glycogen reserves within muscle cell

muscle metabolism during moderate activity

the demand for ATP increases

there is still enough oxygen for the mitochondria to meet the increased demand, but no excess ATP is produced

ATP is generated primarily by aerobic metabolism of glucose from stored glycogen

if the glycogen reserves are low, the muscle fiber can also break down other substrates, such as fatty acids

all of the ATP being generated is used to power muscle contraction

pyruvate metabolism

anaerobic

if oxygen supply to cells is too slow to allow all of pyruvate to be metabolized aerobically by cellular respiration, rest of pyruvate converted to lactic acid

conversion of pyruvate to lactic acid recycles cofactors needed by glycolysis enzymes

muscle metabolism during peak activity

the demand for ATP is enormous. oxygen cannot diffuse into the fiber fast enough for the mitochondria to meet that demand. only a third of the cell’s ATP needs can be met by the mitochondria (not shown)

the rest of the ATP comes from glycolysis, and when this produces pyruvate faster than the mitochondria can utilize it, it pyruvate builds up in the cytosol

the pyruvate is converted to lactate. hydrogen ions from ATP hydrolysis are not absorbed by the mitochondria

the buildup of hydrogen ions increases cytosol acidity, which inhibits muscle contraction, leading to rapid fatigue

aerobic and anaerobic at peak activity, 2/3rds of activity done by glycolysis

anaerobic metabolism

produces ATP rapidly

• allows muscle cell to generate additional ATP when mitochondrial cellular respiration pathway is unable to meet cell’s energy demands

anaerobic metabolism disadvantages

inefficient use of glucose

lactic acid lowers intracellular pH

recovery period

begins immediately after activity ends

oxygen debt (excess post-exercise oxygen consumption)

• amount of oxygen required during resting period to make enough ATP to restore muscle to normal conditions

rebuilds ATP and creatine phosphate levels

recycle lactic acid to make pyruvate

rebuild glycogen reserves

muscle fatigue

a muscle that can no longer perform at required level of activity

possible causes of fatigue

• exhaustion of energy resources

• build up of lactic acid and lowering of pH

  • psychological fatigue