( 3f ) Types of Skeletal Muscle Fibers

NOT all skeletal muscle fibers are the SAME

meaning

There are different types of skeletal muscle fibers!

Each motor unit has

ONLY ONE type of muscle cell ( fiber )

NOT all skeletal muscle fibers are the SAME

meaning

There are different types of skeletal muscle fibers!

Each motor unit has

ONLY ONE type of muscle cell ( fiber )

but different motor units

( like for different areas )

might have different kinds of muscle cells

example:

All the purple units are the SAME TYPE of purple muscle cell

All the orange units are the SAME TYPE of orange muscle cell

What are the extreme types of skeletal muscle fibers?

( will go into further detail )

1. Slow-twitch ( oxidative ) fibers

2. Fast-twitch ( glycolytic ) fibers

1. Slow-twitch ( oxidative ) fibers

Remember: Twitch is

the cycle of contracting and relaxing in ONE muscle cell

These are called SLOW twitch because

→ Slow rate of contraction

= it’s rate of contraction is in Slo-motion

slow twitches

GRADUALLY build up tension

and GRADUALLY relaxes

the whole process before the power stroke is slowed down!

Why is it so slow?

It actually has to do with

Myosin!

What’s something special about myosin that hasn’t been mentioned before?

there’s DIFFERENT TYPES of MYOSIN

Which part do they differ in?

Their Myosin ATPase

enzyme ACTIVITY

( where ATP binds → enzyme cuts it = energizes head for powerstroke )

In Slow-twitch ( oxidative ) fibers

there’s

LOW myosin ATPase activity

= LOW ENZYME ACTIVITY

ATP binds on normally to Myosin ATPase site

BUT

the ENZYME is slow at cutting it

= Slow at getting head energized for powerstroke

So since everything related to ATP is so slow here

how does this relate to contraction?

It slows the rate of contraction

( ex. enzyme slow to cut ATP → slower to get powerstroke )

What COLOR is the Slow-twitch ( oxidative ) fibers?

Dark Red Color

Why is it Dark Red?

It contains A LOT of

Myoglobin

( a protein )

review:

Why are red blood cells red?

Hemoglobin

Just like how

Hemoglobin makes red blood cells red

Myoglobin makes slow twitch fibers red

Hemoglobin’s main job was to → carry oxygen

What’s Myoglobin’s job?

to carry oxygen !!

( similar traits! )

It’s really important for Myoglobin to Carry Oxygen because

These slow twitch OXIDATIVE fibers are designed for Aerobic Metabolism !!

That’s why Slow-twitch ( oxidative ) fibers

include the word “Oxidative” because

It’s using Oxidative Phosphorylation

In Aerobic Metabolism:

ALL steps of glucose metabolism takes place

( because oxygen is present to be a final electron acceptor !! )

= Make LOTS Of ATP

Since this cell’s using glucose efficiently

→ they’ll be slow to fatigue

because they’re not using up glucose ( fuel source ) as quickly

→ A LOT of ATP gets produced!!

★ Slow-Twitch ( oxidative ) fibers ★

Can sustain contractions and not fatigue as quickly

• Slow rate of contraction

  → SLOW enzyme ( myosin ATPase ) activity!!

• Dark red color = myoglobin

• Aerobic metabolism = LOTS of ATP

• Slow to fatigue

  • not gonna run out of ATP as quickly

  • not gonna run out of glucose as quickly

Slow-Twitch ( Oxidative ) Fibers

Are Specialized For

Endurance

Endurance is the reason why

• we can stand all day

• posture ( back holds upright )

BECAUSE OF

Slow Twitch ( Oxidative ) Fibers!

→ That can sustain contractions and not fatigue as quickly

if I TRAIN

these Slow-Twitch ( oxidative ) cells/fibers specifically

what happens?

They get BETTER at being slow-twitch oxidative fibers

→ functions improve

With training ( like exercise )

they’ll undergo

Adaptations

What are these 3 adaptations?

• Increased myoglobin

• Increased mitochondria

• Increased number of capillaries going into muscle

Increased myoglobin

means

These cells will MAKE more myoglobin!

( remember : myoglobin collects + carries oxygen )

So if there’s MORE myoglobin to collect MORE oxygen for the cell

what can the cell do now?

do even MORE Aerobic Metabolism = create even MORE ATP!!

( improves the cell’s functions )

Increased mitochondria in cell

means

There’s now MORE equipment for oxidative phosphorylation ( aerobic metabolism )

Remember: most steps happen in the mitochondria!

So having more mitochondria

= more efficient use of fuel

= MORE ATP created!

→ LESS FATIGUE

Increased number of Capillaries going into muscle tissue

remember → capillaries are tiny blood vessels that bring blood supply INTO the muscle tissue

Why is that important?

Blood supply carries in

• oxygen !!

• more fuel sources !!

  • like glucose

What can cell do with this?

Have MORE aerobic metabolism

• not fatigue as quick

2. Fast-twitch ( glycolytic ) fibers

What’s it’s rate of contraction ( speed )?

Fast rate of contraction

Notice How:

Quickly fast-twitch generates tension but then relaxes

in contrast to

Gradual slow-twitch generating tension and relaxing

Why does it have a FAST rate of contraction?

it depends on

the type of MYOSIN in the cell !!

What’s the activity level of Myosin’s ATPase

in Fast-twitch ( glycolytic ) fibers?

HIGH activity

So it’s enzyme is working in

FASTER motion

the moment ATP binds → immediately cuts → immediate powerstoke

= faster rate of contraction

What COLOR is the Fast-twitch ( glycolytic ) fibers?

White ( pale ) color

Why is it so colorless?

low myoglobin

( myoglobin is what made the slow-twitch fibers red )

Why don’t they have lots of myoglobin?

they DON’T use a lot of oxygen

So what type of metabolism do Fast-twitch ( glycolytic ) fibers use?

Anaerobic Metabolism

( ONLY glycolysis - no oxygen present )

= only 2 ATP

( not rly enough for cross bridge cycling )

so these cells are gonna need A LOT of GLUCOSE

→ because it’s not getting used efficiently

What happens to this cell’s fuel source supply?

it will burn out VERY QUICKLY

run out of fuel → run out of ATP

How will this effect this cell’s fatigue rate ?

Fatigue Easily ( Quickly )

Fast-twitch ( glycolytic ) fibers are

specialized for

High Intensity , Short Duration Activities

( because they’ll fatigue relatively quickly )

example:

My bicep has a lot of Fast-twitch ( glycolytic ) fibers in it

• So I can lift weights

  • BUT I can’t hold these weights forever

example:

• I can’t walk on my HANDS all day

  • BUT I can walk on my LEGS all day

how?

Different muscles have different types of muscle fibers ( cells ) !!

★ Fast-Twitch ( glycolytic ) fibers ★

Specialized for high intensity, short duration activities

• Fast rate of contraction

  → FAST enzyme ( myosin ATPase ) activity!!

• White Color = low myoglobin

• Anaerobic metabolism = uses A LOT of glucose

• Fatigues easily

  • uses up glucose quickly

  • runs out of ATP faster

With Training:

( if I work on Fast-twitch ( glycolytic ) fibers specifically )

These cells are going to

get BETTER at being Fast-twitch ( glycolytic ) fibers

→ it’s functions get improved!

What changes in cell with training?

Training will cause:

• Increased Protein Synthesis

  • Increased Myofibrils

    • Increased Diameter of Muscle Fibers

• Increased Glycogen Stores

Increased Protein Synthesis

means

cell will make MORE proteins

What proteins are being created?

• Actin

• Myosin

So what bigger thing is it actually making with these?

makes MORE MYOFIBRILS !!

What process increases with MORE actin and myosin present?

MORE Crossbridge Cycling !!

= generates MORE tension ( force )

So Extra Protein Synthesis will likely cause an increase in

STRENGTH ( more tension gets generated )

what happens to the cell’s SHAPE with increased protein synthesis?

cell gets PUFFED UP ( BIGGER )

INCREASE in DIAMETER ( size )

if we see someone who’s been weight training

→ they’re muscles look bulky

What’s the physiology behind Bulking?

★ Fast-Twitch ( glycolytic ) cells ★

Are increasing protein production

→ cell gets filled with more proteins

→ gets bigger in diameter!

= muscle tissue looks bulky!

Why do marathon runners have NO bulky legs?

They train their leg’s slow twitch ( oxidative ) fibers

→ are doing other things adapt and get better at being slow twitch fibers

= doesn’t effect size

Increased Glycogen Stores

remember: glycogen is

thousands of glucose molecules chained together

Why do these cells want to STORE MORE GLUCOSE?

They are 100% dependent on plowing through a ton of glucose

= More immediate sources of ATP

for their high intensity short duration activities

If you STOP training ( working out on ) one of these particular cell types:

• slow-twitch ( oxidative ) fibers

• fast-twitch ( glycolytic ) fibers

What happens to the cells?

The cell will GET RID of all it’s extra improvements

If you stopped training fast-twitch ( glycolytic ) fibers:

What happens to the extra proteins?

Cell will gradually BREAK DOWN those proteins

= Gradually no more bulky!

example

Slow-twitch losses it’s EXTRA mitochondria

→ will get RID of it

BOTH

• slow-twitch ( oxidative ) fibers

• fast-twitch ( glycolytic ) fibers

would GO BACK to how they were before!!

Microscope picture of Muscle Tissue

why is the “slow-oxidative fiber” so dark?

they have A LOT of myoglobin

Why are the “fast-glycolytic fiber” pale white?

they DON’T have a lot of myoglobin

Note: There’s 3 types!

teacher skips Fast ( Oxidative ) Fiber

→ we’re only focusing on the two EXTREMES

★ ANY muscle is a COMBINATION of BOTH fast twitch AND slow twitch muscle fibers !! ★

example:

My biceps are a combo of fast twitch + slow twitch fibers

My legs are a combo of fast twitch + slow twitch fibers

HOWEVER

★ depending on the FUNCTION of that muscle

the PROPORTION of those fibers could be different !! ★

The leg muscles will have higher proportion of slow-twitch fibers compared to fast-twitch

While my bicep muscles will have higher proportion of fast-twitch fibers compared to slow-twitch

Overall

Any muscle has BOTH fiber ( cell ) types

but what’s DIFFERENT is

the PROPORTION

of those in the tissue

The proportion has Evolved because of

what that muscle is USED ( designed ) for

( ex. fast twitch fibers will be more present for muscles doing heavy lifting like arms )

What’s another factor of Muscle Fiber Proportion?

Genetics !!

→ genetically determined

This Genetic Component means that we are

BORN with whatever muscle fiber proportion we have

Can you change a slow twitch fiber into a fast twitch fiber. ( or the other way around) ?

NO

→ can only TRAIN present cells to better their functions

So if an aspiring athlete doesn’t have the ideal proportions for their dream olympic sport

They might never get in

• EVEN WITH TRAINING

→ someone else was born with more ideal proportions

= that’s why a lot of athletes have athlete parents

Why do turkeys have dark meat AND white meat?

The dark meat is predominantly

Slow Twitch Fibers

→ like in the legs = turkey walks around on it’s legs all day

The white meat is predominantly

Fast Twitch Fibers

( sidenote reminder: it’s what bulking consists of )