skeletal muscle physiology (2)

what makes the whole muscle fiber shorten

shortening of sarcomeres

what does fiber shortening produce on tendons

tension

what is muscle tone

the baseline, continuous amount of tension

two main factors that set tension of a single fiber

1. fiber length at stimulation (overlap of thick/thin)

2. frequency of stimulation

what does the amount of tension depend on

degree of overlap between thick and thin filaments

when is maximum tension produced

when overlap is large but thin filaments don’t cross the center of the sarcomere

what happens if the sarcomere is too short (thick filaments hit Z lines)

myosin heads can’t pivot; tension drops

what happens if the sarcomere is too long (overlap reduced/zero)

few/no cross-bridges; no active tension

normal/optimarl resting length range

about 1.6-2.6 μm (body keeps sarcomeres ~75-130% of optimal length)

what is a twitch

one contraction-relaxation event from a single stimulus

what is a latent period

delay after stimulus/action potential and before contraction starts

what is contraction phase

cross-bridge formation and filament movement; tension rises

what is relaxation phase

Ca2+ removed, active sites covered; tension falls

does contraction last longer than the stimulus

yes

what is always present even at rest

tone (continuous low-level tension)

what is temporal summation

added force when a motor unit gets new signals before the prior twitch ends

what causes the bigger force in summation

more Ca2+ in the cytosol and more cross-bridges before full relaxation

what is incomplete tetanus (partially fused)

rapid stimuli → twitches start to blend, small relaxations remain

what is tetanus

very frequent signals → twitches fuse with no relaxation; force plateaus high

what variable of stimulation drives summation and tetanus

frequency of neural signals (APs)

what is a motor unit

one motor neuron and all the muscle fibers it innervates

how many motor neurons innervate a single muscle fiber, and where

one neuron, at one location on the fiber

can one motor neuron innervate multiple fibers

yes, it can branch to many fibers

typical motor unit size in the eye

~10 fibers per motor neuron (fine control)

typical motor unit size in the quadriceps

~150 fibers per motor neuron (power)

how do you increase force in a whole muscle

recruit more motor units; sum the contractions of many fibers

are fibers from different motor units mixed together

yes, this helps balance force on the tendon

why does tendon tension look smooth even though units contract/relax

asynchronous recruitment - different units take turns

what is spatial summation

added force from multiple motor units being active at the same time

how is spatial different from temporal summation

spatial: recruit more units

temporal: faster firing of the same unit

what happens to total force as more units are recruited 

forces add → overall muscle tension increases

what does isotonic mean

equal tension - muscles changes length while keeping tension relatively steady

concentric contraction definition + effect on length

muscle tension exceeds load → muscle shortens (lifting)

eccentric contraction definition + effect on length

load exceeds muscle tension → muscle lengthens while active (lowering)

which type of contraction often produces higher peak tension and more soreness/injury risk

eccentric

what returns the muscle toward resting length after isotonic work

elastic recoil (and titin), once the load is supported or removed

what does isometric mean

equal length (muscle length does not change)

what happens to tension in an isometric contraction

tension rises to meet the load, then falls as the muscle relaxes

skeletal muscle phenotype

2 major muscle fiber types, classified by contractile and metabolic characteristics

are most muscles pure red or white

most are heterogeneous (~50/50 mix)

new nomenclature

type I = red

type II = white

force level produced by type I

low contractile force

metabolism used (type I)

efficient aerobic metabolism

blood supply and pigment (type I)

highly vascularized; high myoglobin (gives the red color)

primary fuel handling and fatigue (type I)

oxidize fat; fatigue resistant

organelle abundance (type I)

many mitochondria

type II fibers

fast-twitch (fast glycolytic)

order of fiber types from slowest to fastest (type II)

I → IIa → llx

which fibers reach peak force fastest and highest

type II fibers

which fibers have intermediate speed and force

intermediate/type lla fibers

which fibers are slowest with lowest peak force but longer duration

type I fibers

energy systems involving skeletal muscle

1. immediately available ATP

2. ATP-CP (creatine phosphate) system

3. anaerobic system (glucose/glycogen)

4. aerobic system (glycogen/triglycerides)

which energy system is available instantly

immediate ATP already in the fiber

which system provides very short-term backup

ATP-CP (creatine phosphate

what does the immediate ATP system allow

near instant actin myosin interaction to start contraction

how long does this initial ATP support movement

about 2-5 seconds

why is the immediate ATP system useful

it lets contraction begin while other pathways start making more ATP

typical duration the ATP-CP system can power contraction

about 10-15 seconds

enzyme that transfers a phosphate from CP to ADP to make ATP

CPK (creatine phosphokinase)

where is CPK shown relative to the thick filaments

near the M line

simple cycle described for myosin heads

myosin ATPase uses ATP → ADP + Pi during cross-bridge cycling; CPK rapidly reforms ATP from ADP + CP

source molecules for regenerating CP over time

ATP + creatine (ATP often produced in mitochondria)

during moderate activity, can mitochondria meet ATP demand

yes, enough O2 is present; mitochondria meet the need

approximate duration supplied by glycolysis vs aerobic ATP

glycolysis: ~2 min

aerobic: ~40 min of contraction

what fuels can be used at moderate levels 

glucose/glycogen and fatty acids (aerobic)

during peak activity, is O2 delivery sufficient

No, O2 can’t diffuse in fast enough

at peak, about how much ATP can mitochondria provide

only ~1/3; the rest comes from glycolysis

what builds up when glycolysis outpaces mitochondria

pyruvate → lactate (anaerobic), plus H+

effect of H+ buildup on contraction

increases acidity, inhibits contraction, leads to rapid fatigue

what does H+ (from lactic acid) do to the cell environment

makes it more acidic

which key glycolysis enzyme is inhibited by acidity

phosphofructokinase (PFK)

what happens to glycolysis when PFK is inhibited

slows down, so ATP production drops

where does lactate eventually go

into the cori cycle

where is lactate sent from working muscle (cori cycle)

to the liver via the blood

what does the liver turn lactate into (cori cycle)

pyruvate → glucose (and glycogen storage)

how does the glucose help the muscle again

returns as blood glucose to fuel muscle (re-enters glycolysis or stores as glycogen)

definition of hypertrophy

increase in size of skeletal muscle

can muscle fibers divide to grow

no, fibers don’t divide and new fibers are not formed

what is nuclear domain

each nucleus regulates only a fixed area of the fiber

do nuclei divide inside the fiber

no, so adding nuclei requires another source

what state are satellite cells in until needed

G0

what triggers satellite cell division

muscle damage/overstimulation releases signals

how do satellite cells make fibers bigger

one daughter cell fuses to the existing fiber → adds myonuclei

results of fusing a daughter cell to existing fiber

more myofibrils, more mitochondria, increased glycogen reserves, more glycolytic enzymes