Physiology: Skeletal Muscle 4

define economy

the ratio of force to the energy needed to maintain that force

• smooth muscle myosin is more economical than skeletal muscle

economy is dictated by what? myosin ATPase activity not by the amount of…

• myosin ATPase activity

  • not by the amount of ATP

• ATP is always saturated in cell

what is the native state of tropomyosin without troponin?

• in the unblocked position

what muscle switch type is this

• fast skeletal muscle

what muscle twitch type is this

• smooth muscle

  • slow twitch is slightly less than this

What happens to skeletal muscle tension if a second action potential arrives before the muscle has fully relaxed from the first?

the second contraction builds directly on top of the remaining tension from the first

• creating a stronger overall contraction

  • increasing calcium saturation

What is complete (fused) tetanus, and how much relaxation occurs during it?

• smooth, sustained, maximum contraction with no relaxation

  • continious AP keep calcium continiously available

    • calcium is saturated maximally

the more calcium made available… the more myosin’s you can activate…

• in series

  • increased speed

• in parallel

  • increased force

all skeletal muscle fibers are electrically ____ from one another

• electrically insulated

do motor units cross-innervate fibers?

• no

  • slow motor units innervate slow fibers

  • fast motor units innervate fast fibers

slow muscles use what kind of ATP source?

• mitochondria

  • can stay contracted without fatigue

whats the cost of mitochondria in slow twitch

• so much mitochondria packed in because although the diameter may be the same…

  • slow fiber has less force

    • less contractile apparatus

do fast-fibers have a lot of blood flow? what are the consequences?

• no

  • they don’t need oxygen because they use glycolysis

    • they have a lot of by-product build up

      • fatigue really quickly

what happens to build up of phosphate in fast fibers

• product inhibition

  • slower to spit out phosphate and produce force

• safety mechanism to NEVER run out of ATP

what’s fatigue at the molecular level

• by-product build up of phosphate

whats fatigue at the neurological level

• brain does not want to send out signals

do you ever run out of ATP

• no!

  • you do build up by-products that inhibits the cycle

simply describe how action potential frequency impacts muscle tension

• more frequent action potential…

  • more force sustained for longer periods

simply describe how fiber length impacts muscle tension

• skeletal muscle is designed to work in optimal force-length relationship

  • too stretched… too little overlap…

    • more passive force… less active force

  • too compressed… too much overlap…

    • less active force

simply describe how fiber diameter impacts muscle tension

• increased force

  • more myosin in parallel

simply describe how fatigue impacts muscle tension

• decreased cycling

  • more product inhibition… must reach equilibrium first

describe how the number of active motor units impacts muscle tension

• increased contraction

  • more myosin activated in parallel

describe how the type of motor units impacts muscle tension

• slow motor unit

  • slow fiber

    • recruited first

• fast motor unit

  • fast fiber

    • recruited later

describe how the geometry of muscle impacts muscle tension

• lever arm geometry increases speed…

  • at the cost of force

describe the relationship between myosin and increasing load

• you must at least have as many myosins bound at a given time to meet and hold the load… (isometric contraction always first)

  • and if there is any extra myosin available…

    • they can shorten the muscle and move the load (isotonic contraction

you have to build enough myosins to ____ before you can ____

you have to build enough myosin’s to meet the load before you can move the load

what are factors that affect your movement

• environment

  • colder… less efficient

• body type.. organism

  • lever arm length…

  • muscle group arrangements

• how you can modulate force of the organ at the cellular level

  • different muscle groups

at what point here do you see 50% of force efficiency

10 hz

what what point here do you see almost 100% of force efficiency

20-30 Hz

describe this image

• the lightest force a participants thinks they generated…

  • recruited 60 motor units at 8-9 hz

    • about 30% of force possible

    • you’re already recruiting at the most efficient contraction

• above 30% of efficieny…

  • the discharge rate increases a lot more

at what point do most motor units get recruited

30% of muscle efficiency

whats the takeaway message?

• we’ve already recruited most motor neurons at 30%

  • beyond that, we increase force by summing up our action potentials

when is muscle recruitment complete

• 30 to 50%

  • this is when we begin to see summation of action potentials to really drive up force

what are the factors that affect speed of contraction

• type of myosin

  • slow vs fast myosin

• number of sarcomeres in series

• load that the muscle is contracting against

• lever arm geometry

what are the things that affect myosin contraction in skeletal muscle

• geomtery of muscle

  • lever arm: sacrifices force for speed

• number of active fibers

  • type of motor unit/myosin type

  • number of motor units

    • number of fibers per motor unit

• tension developed

  • action potential frequency (calcium release)

  • filament overlap

  • fiber diameter (parallel arrangement)

  • fatigue

define motor unit

the motor neuron and its accompanying myofibrils that it innervates

describe the order of motor unit recruitment

• fibers that are more economical are recruited first

  • slow twitch

  • fast twitch

  • super fast twitch

how is contraction strength graded

• motor units are recruited first

• then temporal summation rapidly increases force

List the factors that determine force generation by a skeletal muscle in the body.

• geometry

  • increased length at the cost of force

• motor unit

  • type of motor unit/myosin

  • number of motor units

    • amount of muscle fiber per motor unit

• tension developed

  • action potential frequency (calcium)

  • force length relationship

    • optimal overlap = more force

  • number of sarcomeres

  • fatigue