ex phys chapter 8: muscle

how many skeletal muscles are there in the human body?

over 600

how much total body weight do skeletal muscles account for?

40-50%

skeletal muscle functions

force production for locomotion and breathing, postural support, and heat production during cold stress

flexor muscles

decrease joint angle

extensor muscles

increase joint angle

epimysium

covers the entire skeletal muscle

perimysium

surrounds skeletal muscle fascicles (bundles of muscle fibers)

endomysium

surrounds individual muscle fibers

basement membrane

just below endomysium

sarcolemma

muscle cell membrane

myofibrils

contain contractile proteins actin and myosin

actin

thin filaments

myosin

thick filaments

sarcomere

contractile unit of a muscle fiber

what is included within a sarcomere?

Z line, M line, H zone, A band, and I band

sarcoplasmic reticulum

storage sites for calcium; terminal cisternae

cisternae

flattened stacked membrane folds

transverse tubules

extend from sarcolemma to sarcoplasmic reticulum

what do transverse tubules do?

transmit muscle impulses into the cell interior

satellite cells

play key role in muscle growth and repair

how to satellite cells assist with muscle growth and repair?

they increase the number of nuclei in mature muscle fibers

myonuclear domain

volume of cytoplasm surrounding each nucleus, each nucleus can support a limited myonuclear domain

more nuclei allow for...

greater protein synthesis

neuromuscular junction

point of contact between a motor neuron and a skeletal muscle cell

motor unit

A motor neuron and all of the muscle fibers it innervates

motor end plate

pocket formed around motor neuron by sarcolemma

neuromuscular cleft

short gap between neuron and muscle fiber

what does the motor neuron release?

acetylcholine

what does the release of acetylcholine cause?

an end-plate potential (EPP) - depolarization of muscle fiber

end-plate potential

initial depolarization of motor end plate

acetylcholine

neurotransmitter chemical released at the ends of nerve cells

sliding filament theory

the concept that a sarcomere shortens as the thick and thin filaments slide past one another

whats the other name for the sliding filament theory?

swinging lever-arm model

power stroke

The myosin heads bind to actin and pull them toward the sarcomere center, which slides the filaments past each other, shortening the muscle.

what is the result of a power stroke?

shortening in distance between Z-lines of the sacromere

excitation-contraction coupling

depolarization of motor end plate (excitation) is coupled to muscular contraction

steps of excitation-contraction coupling

1. action potential travels down t-tubules

2. release of calcium from sarcoplasmic reticulum

3. calcium binds to troponin and causes position change in tropomyosin

4. myosin binding sites are exposed on actin filaments

5. strong binding state formed between actin and myosin

6. contraction occurs (power stroke)

step one of excitation-contraction coupling

action potential travels down t-tubules

step two on excitation-contraction coupling

release of calcium from SR

step three of excitation-contraction coupling

calcium binds to troponin - tropomyosin position change

step four of excitation-contraction coupling

myosin binding sites exposed

step five of excitation-contraction coupling

strong binding state formed between actin and myosin

last step of excitation-contraction coupling

contraction/power stroke occurs

three stages of excitation-contraction coupling

excitation, contraction, relaxation

excitation phase of excitation-contraction coupling

nerve signal arrives at synaptic knob, acetylcholine released into synaptic cleft and binds to motor end plate, sodium enters muscle, sodium influx causes depolarization conducted down transverse tubules

contraction phase of excitation-contraction coupling

depolarization of t-tubules causes release in calcium from SR, calcium binds to troponin, shift in tropomyosin, myosin binds to actin (cross bridge formed), cross-bridge movement, ATP breaks cross bridge, ATP broken down

relaxation phase of excitation-contraction coupling

motor neuron stimulation ends, muscle fiber repolarizes, calcium pumped back into SR, tropomyosin covers myosin binding sites, muscle relaxation occurs

why is ATP required for muscle contractions?

release of energy from ATP hydrolysis provides energy for power stroke

when is ATP broken down in muscle contraction steps?

as fiber contracts

what enzyme breaks down ATP during muscle contractions?

ATPase

sources of ATP

ATP-PC, glycolysis, oxidation

muscle fatigue

decline in muscle power output/force generation/shortening velocity

when does muscle fatigue occur during high-intensity exercise?

within 60 seconds

what causes fatigue during high-intensity exercise?

accumulation of lactate, H+, ADP, Pi, and free radicals

when does fatigue occur with long-duration exercise?

within 2-4 hours

what causes fatigue during long-duration exercise?

accumulation of free radicals, electrolyte imbalance, and glycogen depletion

muscle cramps

spasmodic, involuntary muscle contractions

does an electrolyte imbalance or dehydration cause muscle cramps?

no

what likely causes muscle cramps?

excessing firing of motor neurons in the spinal cord; increased excitatory activity of muscle spindles and reduced inhibitory activity of golgi tendon organs

what can often be used to relieve muscle cramps?

passive stretching; strong inhibitory stimulus to the spinal cord

what are studies hypothesizing that could help muscle cramps?

ingestion of spices

muscle twitch

contraction as the result of a single impulse/stimulus

which muscle fibers have a faster speed of shortening? why?

fast fibers, SR releases calcium at a faster rate

post-activation potential

Increase in muscle force production that occurs following a bout of non-fatiguing, submaximal muscle contractions (warm-up)

isometric contraction

muscle exerts force without changing in length, pulling against immovable object

isotonic/dynamic contractions

muscle changes in length and moves load

two types of isotonic contractions

concentric and eccentric

concentric contraction

muscle shortens during force production

eccentric contraction

muscle lengthens during force production

muscle biopsy

small piece of muscle removed and stained to determine different fiber types present in sample

what is the staining in a muscle biopsy targeting?

a type of myosin ATPase isoform

muscle fiber types

type I, type IIa, type IIx

type I muscle fiber

slow twitch, fatigue resistant, high capacity for aerobic energy supply, limited potential for rapid force development, low myosin ATPase activity, low anaerobic power, numerous large mitochondria

type IIa muscle fiber

fast twitch with the capacity for both aerobic and anaerobic metabolism

type IIx muscle fiber

fast twitch, primarily anaerobic muscle fibers

what does the type of myosin ATPase isoform a muscle type has determine?

the speed of ATP degradation

specific force production

Force per unit of cross-sectional area

what regulates the speed of a muscle contraction?

myosin ATPase activity

myosin ATPase

enzyme that catalyzes ATP hydrolysis for cross-bridge recycling

max power output

force x shortening velocity

what determines muscle fiber efficiency

lower amounts of ATP are used to generate force

what percentage/ratio of muscle fibers do non-athletes have?

50% slow twitch, 50% fast twitch

power athletes tend to have more...

fast twitch fibers

endurance athletes tend to have more....

slow twitch fibers

what ratio do sprinters tend to have of muscle fiber types?

30% slow twitch, 70% fast twitch

what ratio do distance runners have of muscle fiber types?

70% slow twitch, 30% fast twitch

what ratio of muscle fiber types does tanaka have?

22% slow twitch, 78% fast twitch

force-velocity relationship

speed of movement is greater in muscles with a higher percentage of fast-twitch fibers; max velocity of shortening is greatest at the lowest force

force-power relationship

peak power generated is greater in a muscle with a higher percentage of fast-twitch fibers; peak power increases with velocity

how does aging affect muscle function?

muscle mass loss, loss of fast twitch fibers, gain of slow twitch fibers, diabetes

how to delay effects of aging on muscle function

resistance training

how much muscle loss occurs between the ages of 25 and 50?

10%

how much muscle loss occurs between the ages of 50 and 80?

40%

muscle dystrophy

hereditary defects in muscle protein resulting in loss of muscle fibers and weakness

how does cancer affect muscle function?

cachexia (rapid muscle loss) resulting in weakness