lectures - muscle tissue growth

how is muscle tissue growth reflected

in terms of protein turnover

describe protein turnover

ongoing cycle of muscle protein synth and degradation that operates in equilibrium for normal muscle tissue function

what is the net protein accretion equation

amt of protein synth - amt of protein degraded

for homeostasis net protein accretion =

zero, amt synth = amt degraded

for growth, net protein accretion =

must be positive for ↑ growth and ↑ differential

for atrophy, net protein accretion =

must be negative due to insufficient protein synth or excessive protein degradation

how does + net protein accretion occur

through hyperplasia and hypertrophy

describe hyperplasia

increase in muscle fiber number

mostly prenatal during myogenesis

describe hypertrophy

increase in muscle fiber size by ↑ length/diameter

predominant process for postnatal muscle tissue growth

how exactly does postnatal muscle tissue addition work

adds nuclear DNA to individual muscle fibers to ↑ protein synth via nuclei donation of satellite cells

what are some prenatal factors that affect hyperplasia

individual variation

muscle and body size

genetic selection

nutrition

hormones

describe how individual variation can affect hyperplasia

muscle fiber number ↔ complex trait affected by multiple genes/regions

describe how muscle and body size affect hyperplasia

species effect as larger size due to more fibers, not larger fiber size

describe how genetic selection affect hyperplasia

selection for meat production is indirect selection for ↑ muscle fiber number

better daily average grain and feed efficiency is positively correlated with > muscle fibers

describe how nutrition can affect hyperplasia

critical during 2nd wave of myogenesis when 2° myotubes are being formed

prenatal malnutrition ↓ 2° myotubes ↓ muscle fibers

describe how hormones can affect hyperplasia

> exposure to prenatal androgens ↑ muscle fiber number

sex - in cattle and chickens males have > muscle fiber than females

describe why post natal hyperplasia occurs

when muscle tissue injury results in muscle fiber death

how can muscle fibers be regenerated

activation of satellite cells

what are the steps of satellite cell hyperplasia repair

1. activation

2. proliferation

3. differentiation

describe the activation step of satellite cell hyperplasia repair

satellite cells reanimate and divide sym and asym

sym - 2 satellite cells

asym - 1 satellite + 1 TA myoblast

describe the proliferation step of satellite cell hyperplasia repair

TA myoblasts divide

describe the differentiation step of satellite cell hyperplasia repair

myoblasts diff into myocytes and lose ability to undergo mitosis

if there is a major injury then

new myotubes are formed from satellite myoblasts

if there is a minor injury then

myocytes make a little patch over injury

what are the 2 ways that hypertrophy can occur

increase in fiber diameter

increase in fiber length

describe hypertrophy increase in fiber diameter

work/exercise induced

existing myofibrils within muscle split longitudinally into 2 myofibrils

describe hypertrophy increase in fiber length

stretch induced hypertrophy

occurs in 2 ways - sarcomere addition and sarcomere lengthening

describe sarcomere addition stretch induced hypertrophy

new sarcomeres added to distal ends of muscle

accounts for ~80% of muscle fiber lengthening

describe sarcomere lengthening stretch induced hypertrophy

individual sarcomeres can increase actin and myosin myofilament lengths

accounts for remainder of muscle fiber lengthening

how does hypertrophy occur

satellite cells donate nuclei to give more sources of protein synth, provide ~95% of nucleic material to adult muscle tissue

what percent of adult muscle DNA is postnatally derived

~95%

what are some factors that affect hypertrophy

sex hormones

nutrition

genetic selection

intrafascicularly terminating fibers

age

how do sex hormones affect hypertrophy

testosterone enhances hypertrophy by ↑ protein synth

how does nutrition affect hypertrophy

better nutrition = better fibers

describe how genetic selection can affect hypertrophy

applying heritability of genetic predispositions for and/or mutations that cause larger muscle fiber diameter

describe how intrafascicularly terminating fibers affect hypertrophy

lengthening of these increases girth/belly of muscle

begins to extend into parts of muscle they didnt previously reach and joint/add to fibers already there

describe how age can affect hypertrophy

size typically increases until muscle maturity but satellite ability to engage in recruitment diminishes with age

muscles must be used to maintain work induced hypertrophy

what do bodies have natural limits to

bodies have natural limits to their ability to synth protein in response to testosterone

artificial growth enhancement focuses on

reducing degradation not shutting it down

why does artificial growth enhancement not focus on shutting protein degrad down

knockouts for ability to degrade causes ill effects

what are the ill effects of knockouts of protein degradation ability

abnormal protein buildup

muscle atrophy

premature death