Download LEARNING OBJECTIVES Muscle Metabolism At the end of this

LEARNING OBJECTIVES
Muscle Metabolism
At the end of this session the students should be able to

Know the energy sources of the muscles

How the energy is utilized during muscle contraction

Describe the various mechanisms of energy provision

Describe the types of muscle on the basis of their metabolic properties
LECTURE OUTLINE
Muscle Metabolism

Muscles, consume fuel in the form of energy yielding biochemical compounds,

Muscle cells, like all others, use ATP as their energy currency

But some muscle cells must exhibit activity levels in which they cannot make
ATP as fast as it is consumed

Muscle cells have several mechanisms to provide the ATP they need

Contraction of muscle requires energy but surprisingly little ATP available inside
muscle, just enough to power contraction of a few seconds

Skeletal muscles usually function in a ‘stop and go’ manner, unlike most cells of
the body and they switch between virtual inaction & great activity

If strenuous exercise is to continue (> than a few seconds), additional ATP must
be produced

On demand the muscles metabolic machinery makes arrangements to set up
ATP production
Phosphagen System

Muscles have a unique molecule known as creatine phosphate (CrPPhosphocreatine)

It has the ability to transfer its high energy phosphate group to ADP (adenosine
diphosphate) to form ATP (adenosine triphosphate) & creatine

this is the use of immediately available ATP

This is not from stored ATP itself

Muscle cells can store only very limited amounts.

It is from energy stored as the related molecule CP, creatine phosphate

Creatine phosphate can be stored and is made from ATP during periods of rest.

During periods of high activity CP is broken down quickly and its energy
converted to ATP.

But this source of ATP can only supply a cell for 8 to 10 seconds during the most
strenuous exercise.

Creatine released during muscle activity shows up in the urine as creatinine, a
combination of two creatine molecules.

Training can increase the amount of creatine phosphate stored but this alone
does not increase the strength of a muscle, just the length of time before it runs
out of CP, and that by only a few seconds

CrP is about 3-5 times more plentiful than ATP

Together, CrP & ATP constitute phosphagen system

This provide enough energy for muscle to contract maximally for about 15
seconds

It is used for a maximal short burst of energy, to run a 100m meter dash
Glycogen-Lactic acid System

Continuity of muscle activity depletes supply of creatine phosphate, so glucose is
catabolized to generate ATP

Glucose reaches easily into contracting muscles from blood (by facilitated
diffusion) & also is produced by breakdown of glycogen within the muscles

A series of ten reactions known as glycolysis quickly splits each glucose
molecule into 2 molecules of pyruvic acid & forms 2 molecules of ATP

Since glycolysis does not require oxygen it is said to be anerobic process

Glycolysis is the initial way of utilizing glucose in all cells & used exclusively by
certain cells to provide ATP when insufficient oxygen is available for aerobic
metabolism

Glycolysis doesn't produce much ATP in comparison to aerobic metabolism
but it has the advantage that it doesn't require oxygen

In addition, glycolysis occurs in the cytoplasm, not the mitochondria so used by
cells which are responsible for quick bursts of speed or strength

Like most chemical reactions, glycolysis slows down as its product, pyruvic acid,
builds up

In order to extend glycolysis the pyruvic acid is converted to lactic acid in a
process known as fermentation.

Lactic acid itself eventually builds up slowing metabolism and contributing to
muscle fatigue.

Ultimately the lactic acid must be reconverted to pyruvic acid and metabolized
aerobically either in the muscle cell itself, or in the liver.

The oxygen which is "borrowed" by anaerobic glycolysis is called oxygen debt
and must be paid back

Oxygen debt is partly oxygen reserves in the lungs, tissues, and myoglobin in the
lungs (lactic acid oxygen debt)

But mostly it is the amount of oxygen which will be required to metabolize the
lactic acid produced

Strength training increases the myofilaments in muscle cells and therefore the
number of crossbridge attachments which can form.

Training does not increase the number of muscle cells in any real way
(Sometimes a cell will tear and split resulting in two cells when healed).

Lactic acid removal by the cardiovascular system improves with training which
increases the anaerobic capacity

Even so, the glycolysis-lactic acid system can produce ATP for active muscle
cells for only about a minute and a half

Ordinarily, the pyruvic acid formed by glycolysis enters mitochondria, where its
oxidation produces a large amount of ATP from ADP

During some activities, there is not enough oxygen to complete breakdown
pyruvic acid

When this happens most of the puruvic acid is converted into lactic acid about
80% of which diffuses from the skeletal muscle into the blood

Lactic acid can be used to produce ATP by tissues like Heart, kidney cells & liver
cells

Liver cells can convert some of the lactic acid back to glucose, this conversion
has two benefits; providing new glucose molecules & reducing acidity

Some lactic acid accumulates in blood & muscle tissue

This glycogen-lactic acid system can provide enough energy for about 30-40
seconds for maximal muscle activity (e.g. to run a 300 meter race)

Eventually muscle glycogen must also be restored, accomplished mainly through
eating carbohydrates rich foods & may take several days depending upon the
intensity of exercise
Aerobic System

Molecular activity ( >½ minute) depends increasingly on aerobic process
(reactions requiring oxygen)

If sufficient oxygen present mitochondrial enzymes can completely oxidize
puruvic acid to CO2, water ATP and heat – process called cellular respiration or
biological oxidation

While slower than glycolysis, it yields more energy 36 molecules of ATP from
each glucose molecule

Muscle tissues has two sources of oxygen; oxygen that diffuses into muscles
from blood and oxygen that is released by myoglobin inside muscles

Both myoglobin and hemoglobin (in RBC) are red colored, oxygen-binding
proteins; bind oxygen when it is plenty and release it in times of scarcity

The aerobic system will provide activity as long as there is availability of sufficient
oxygen and nutrients fatty acids, from breakdown of TG’s in adipose cells &
amino acids from breakdown of proteins, in addition to glucose

Activities lasting more than 10 minutes get from the aerobic system more than
90% of the needed ATP

During a long term event (marathon race) almost 100% of the ATP needed is
produced aerobically

The maximum rate of oxygen consumption during the aerobic catabolism of
pyruvic acid is called maximal oxygen uptake production it is influenced by
gender (higher in males), age (highest at about age 20) and size (increases with
body size)

Highly trained athletes can have maximal oxygen uptake that are twice those of
untrained people, owing to a combination of both heredity and training. As a
result they are capable of greater muscular feats than untrained people
Oxygen Consumption After Exercise

During muscular exercise blood vessels in muscles dilate, blood flow increases,
and oxygen delivery increases

The available oxygen is sufficient to meet the energy needs of the contracting
muscles up to a point, when muscular exertion is very great oxygen can not be
supplied to muscles fast enough & cellular respiration can not produce enough
ATP

After exercise has stopped heavy breathing continues for a period of time &
oxygen consumption is above the resting level

Depending upon the intensity of exercise the recovery period may be just a few
minutes or several hours termed Oxygen debt (AV Hill), for the added oxygen
that is taken into the body after exercise, over and above the resting oxygen
consumption

He proposed that this extra oxygen was all used to ‘pay back’ or restore
metabolic conditions to the resting level


In this view the oxygen is used to

Convert lactic acid back into puruvic acid

Reestablish the glycogen stores

Re-synthesize creatine phosphate and ATP

Replace the oxgen removed from the myoglobin
The metabolic change that occurred during the exercise, account for only some
of the extra oxygen used after exercise

Post exercise oxygen use also is boosted by ongoing changes

Body temperature is elevated after strenuous exercise

This increases the pace for chemical reactions throughout the body as faster
reactions use ATP more rapidly and more oxygen is needed to produce ATP

Heart and muscles used in the breathing are still working harder than they were
at rest & so consume more ATP

Tissue repair processes are occurring at an increasing pace

For these reasons a better term than oxygen debt for the elevated oxygen use
after exercise of recovery oxygen consumption
Muscle fatigue

If a skeletal muscle or a group of muscles is over stimulated the strength of
contraction becomes progressively weaker until the muscle no longer responds

The inability of a muscle to maintain its strength of contraction or tension is called
muscle fatigue

It occurs when a muscle can not produce enough ATP to meet its needs

Several factors appear to contribute to muscle fatigue including

insufficient oxygen,

depletion of glycogen,

build up of lactic acid,

failure of action potential in the motor neuron to release enough
acetylcholine &

unexplained fatigue mechanism in CNS
Types of muscle cells

Different types of cells perform the differing functions of endurance activities and
speed- strength activities.

There are three types
1. red,
2. white, and
3. Intermediate.

The main differences can be exemplified by looking at red and white fibers and
remembering that intermediate fibers have properties of the other two.
Classification of skeletal muscles based on biochemical and functional
properties
White Fibers Fast twitch

Large diameter, used for speed and strength.

Depends on the phosphagen system and on glycolysis-lactic acid.

Stores glycogen for conversion to glucose.

Fewer blood vessels.

Little or no myoglobin
Red Fibers Slow twitch

Small diameter,

Used for endurance.

Depends on aerobic metabolism.

Utilize fats as well as glucose.

Little glycogen storage.

Many blood vessels and much myoglobin

give this muscle its reddish appearance
Intermediate Fibers:

Sometimes called "fast twitch red",

these fibers have faster action

but rely more on aerobic metabolism

and have more endurance.

Most muscles are mixtures of the different types.

Muscle fiber types and their relative abundance cannot be varied by training,

although there is some evidence that prior to maturation of the muscular system
the emphasis on certain activities can influence their development