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Muscle Metabolism PhysiologyZone Muscular Series with English subtitles  

hey this is Jack from physiology zone

and in this fourth part on skeletal

muscle we're going to be looking at

muscle metabolism and the ways in which

muscles source their ATP so this is

going to be a basic introduction to aid

on your understanding and hopefully this

is going to provide enough clarity that

if you're required to study this in

depth this will help as a starting point

so as we've seen in the previous

episodes ATP supplies the energy for

muscle contraction and without it no

muscle contraction could take place but

ATP also is required in the calcium

active transport pumps on the

sarcoplasmic reticulum which we've seen

previously and you'll find that ATP is

used throughout a variety of receptors

within our body but the amount of ATP

within muscle fibers for muscle

contraction is only sufficient enough to

power it for a few seconds so therefore

the muscle fibers require other ways to

source their ATP and to do this there's

three methods which are utilized and

we're going to go through these in turn

they use creatine phosphate an aerobic

cellular respiration aerobic cellular

respiration and then finally in the end

of this tutorial

debt is and how we repay it so creatine

phosphate is a molecule which can store

energy within its phosphate bonds so

whilst the muscle is relaxed they

produce more ATP than is required for

their resting metabolism and this excess

of ATP transfers a phosphate group to

creatine producing adp adenosine

diphosphate and creatine phosphate this

acts as a very quick energy reserve for

creating ATP so when a muscle starts to

contract and requires that energy

creatine phosphate transfers its

phosphate group back to adp in a

reaction which is catalyzed by the

enzyme creatine kinase to form creatine

and adenosine triphosphate or ATP so the

ATP

can then be used to create muscle

contraction so this type of energy

source is extremely quick but it

unfortunately only lasts for

approximately 15 seconds due to finite

amounts of creatine so this system would

power something like a hundred meters

sprint and on the side note creatine

supplementation is widely used as a

performance-enhancing supplement so it

makes sense that if you increase the

amount of creatine within the sarcoplasm

you're therefore going to increase the

amount of readily available creatine

phosphate that can then allow the

production of more ATP so this of course

is best for exercises which require

large amounts of strength and power

output There is obviously a limit to

this and people differ in terms of their

response to creatine supplementation and

there are also some concerns regarding

its safety due to large intakes and

prolonged usage of this supplement

so as the ATP produced by creatine

phosphate is depleted the muscle turns

to glycolysis as a method by which it

can source its ATP so glycolysis is an

anaerobic process by which glucose is

broken down to produce ATP in a series

of 10 steps which are beyond the scope

of this tutorial

however glycolysis cannot generate ATP

as quickly as creatine phosphate so ATP

is provided to the muscle slower the

glucose used in glycolysis can be gained

by either glucose which diffuses

straight into the muscle from the blood

or glycogen which is within the muscle

tissue

now glycogen is a polysaccharide so it

just has tons of glucose molecules stuck

together like as in up to 20 or 30,000

of them and glycogen must first undergo

glycogenolysis to make glucose available

for glycolysis so the input point is one

glucose molecule and that glucose

molecule undergoes the anaerobic process

which is called glycolysis and that is a

series of

actions that ultimately produces two ATP

molecules and two pyruvates as a quick

side note your text might talk about

pyruvate or pyruvic acid and so long as

you're not studying biochemistry they

are functionally the same thing so in

terms of physiology they will be used uh

turley interchangeably so pyruvate can

then be used for one of two things if we

have no oxygen available its then broken

down from pyruvate into lactic acid and

this is an important process which we're

just going to touch on a little bit more

now so pyruvate with the reduced form of

something called nicotinamide adenine

dinucleotide which is the nad and the h

on the end just means that it's the

reduced form of that molecule with a

hydron is catalyzed by the enzyme

lactate dehydrogenase and the job of

this enzyme is to take the hydride group

from one molecule and put it onto

another so this reaction by which

pyruvate is converted to lactate is

called a redox reaction now these are

very important physiologically and occur

throughout the body and redox just

refers to one half of the reaction being

reduction and the other half being

oxidation so pyruvate gains electrons

and becomes lactate and that part is

reduction and the other half is

oxidation so our reduced form of

nicotinamide adenine dinucleotide the

NADH is oxidized and loses electrons to

form nicotinamide adenine dinucleotide

in its oxidized form the importance of

that molecule is that it allows

glycolysis to keep going and it's the

only thing that can allow glycolysis to

keep going ultimately glycolysis comes

to an end because it can't be sustained

for very long because if it's

inefficiency it makes two ATP molecules

per

one glucose molecule so after about one

minute the anaerobic respiration process

is completely depleted so it's not much

good for anything more than say a 400

meter race what we have to use is

aerobic respiration so this is where if

during the glycolysis process in the

production of pyruvate we have oxygen

available this pyruvate then enters

aerobic respiration which is what we're

going to talk about now aerobic

respiration is the breakdown of glucose

or other nutrients in the presence of

oxygen to produce carbon dioxide water

and ATP there are three important inputs

for aerobic respiration

which are the pyruvate or pyruvic acid

fatty acids and amino acids so firstly

the glucose which is broken down by

glycolysis produces our pyruvate and

that's what's being used first in our

aerobic respiration but when we are low

on glucose and its immediate

polysaccharide glycogen then the body

turns to fat to make fatty acids and

glycerol and these can then enter

aerobic respiration and finally when

protein is high or if glucose and fat

stores are very low then via a series of

reactions they can be broken down into

amino acids which can then be used in

aerobic respiration so this process all

occurs within the mitochondria so the

powerhouse of our muscular fiber and

it's in the presence of oxygen from

either hemoglobin so oxygen that we find

in the blood or from oxygen which is

attached to myoglobin the protein within

the muscle fibers that binds up oxygen

our pyruvate fatty acids or amino acids

can enter aerobic respiration which

occurs in a two-step process within this

mitochondria first they enter the Krebs

cycle this is also known as the TCA or

citric acid cycle so that's three names

for the same thing

they then following this enter the

electron transport system again the

details of this are beyond the scope

of this tutorial the outcome is

ultimately to yield carbon dioxide water

and our ATP and this is much more

efficient than our glycolysis because

you get 36 ATP molecules per glucose

molecule and a hundred per fatty acid

now this is obviously much more

efficient in terms of yield but

unfortunately it requires a steady

stream of oxygen so it's much slower but

if you're doing moderate activity like

walking or a very steady jog then this

system is primarily used to provide our

ATP for muscle contraction and

interestingly after exercise you'll

probably notice that despite having

stopped you continue to breathe quickly

and deeply and the reason for this is

due to what is called the oxygen debt

this is the oxygen that's required to

compensate for the ATP which we have had

to produce without oxygen during our

exercise so you are literally paying

back the oxygen and the way that this is

done is in three ways firstly it's used

to convert lactic acid into glucose or

glycogen stores which can then be put

into the liver to resynthesize creatine

phosphate from creatine and to replace

the oxygen that's been removed from our

myoglobin in our muscles to make oxy

myoglobin you can breathe like this

after exercise for a few minutes or if

it's really strenuous exercise then this

can last several hours and as a final

extra note it's interesting to think

what causes your muscles to fatigue so

you'll notice that at some point during

continuous use of our muscles the speed

and the force of their contraction slows

down and this is because they are no

longer contracting effectively in

response to signals from the nervous

system it would appear logical to think

that this is because we've run out of

ATP but in fact multiple studies have

shown that the ATP within our muscles at

rest and fatigue are relatively

unchanged and we have correlated other

things that we think are involved in

fatigue including lactic acid buildup

which makes

the intracellular pH more acidic and

stops the enzymes working correctly

inadequate release or reuptake or some

sort of leaking of calcium out of the

sarcoplasm which is affecting

contractivity or failure of action

potentials to release enough

acetylcholine at the neuromuscular

junction and that's just naming a few

but ultimately the truth is we don't

really know yet

but the purpose of the fatigue is

obviously to prevent significant injury

to our muscles so that's everything we

wanted to go through so just to recap

what we've been through we looked at

creatine phosphate and we saw how ATP is

in abundance in the resting muscle n is

combined with creatine to form creatine

phosphate which can then be used to give

up its phosphate group to adp and make

that ATP as its initial energy source

but remember this is very quick and it

only lasts for about 10 seconds we have

an aerobic cellular respiration which is

where we use glucose from the blood or

in the muscle and that undergoes

glycolysis which is that anaerobic

10-step process that produces ATP two

molecules of that and two molecules of

pyruvate now the pyruvate can be broken

down to lactate or lactic acid if we

don't have any oxygen but if we do have

oxygen then it may well go on into

aerobic respiration so aerobic cellular

respiration uses our pyruvate and fatty

acids and amino acids and they undergo

the Krebs cycle or the TCA or citric

acid cycle within the mitochondria to

produce 36 plus molecules of ATP after

exercise we have to repay our oxygen

debt and that oxygen is used to

reproduce glucose or glycogen which can

be stored creatine phosphate and oxy

myoglobin so I hope you found this

tutorial useful and we'll see you in the

next one which is the final episode on

skeletal muscle

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