Download Bio_257_Unit 2_12muscle phases

Muscle Physiology
-Stimulus Frequency and muscle contraction
-Energy requirements for muscle
-Types of muscle fibers
Muscle twitch
• Contraction of a whole muscle in response to a
stimulus that causes an action potential in one or
more muscle fibers.
– The normal function of muscle is more complex,
however, but an understanding of the muscle twitch
makes the function of muscles in living organisms
easier to comprehend.
• Is usually the contraction of all muscle fibers in a
a motor unit in response to a stimulus
• Represented by 3 phases lag phase (latent
period), contraction, relaxation
Muscle twitch
Lag (latent) phase
Is the time period
b/w application of
the stimulus to the
motor neuron and
the beginning of
contraction.
Contraction phase
Is the time during
which contraction
occurs
Relaxation phase
Is the time during which
relaxation occurs
Events during each phase of a muscle twitch in
response to a single action potential (AP) in the
motor neuron
• Lag phase or latent period
– AP propagated to presynaptic terminal, synaptic vesicles fuse and
release ACh into cleft, ACh diffuses & binds to ACh receptors on
sarcolemma (postsynaptic membrane).
– ACh is rapidly degraded in the synaptic cleft, thus limiting the
length of time ACh is bound to its receptor. The result is that one
presynaptic action potential produces one postsynaptic action
potential in the muscle fiber.
– The AP spreads along the sarcolemma of the muscle fiber and into
the T-tubules. The electrical change that occurs in the T-tubule in
response to the AP make the membrane of the SR very permeable
to Ca ++. Ca++ diffuses from the SR into the sarcoplasm.
– Ca++ binds to troponin and the troponin/tropomyosin complex
changes its position and exposes active sites on the actin
myofilaments.
Events during each phase of a muscle twitch
in response to a single action potential (AP)
in the motor neuron
• Contraction phase
– Cross bridges b/w actin molecules and
myosin molecules form, move, release,
and reform many times, causing the
sarcomere to shorten.
– ATP must be bound to the myosin
molecule for crossbridge formation and
after crossbridge movement is complete,
another ATP must bind to myosin to allow
cross bridge release.
Events during each phase of a muscle twitch
in response to a single action potential (AP)
in the motor neuron
• Relaxation phase
– Ca++ are actively transported into the SR.
– The troponin-tropomysin complex inhibit
cross bridge formation.
– The muscle fibers lengthen passively
Muscle twitch and multiple wave summation
due to stimuli of increased frequency
Complete
tetany (5)
Complete
relaxation
b/w stimuli (1)
Incomplete tetany
(2-4)
Stimuli of increasing frequency. Stimuli 1-4 allow some degree
of relaxation b/w stimulus application. Stimulus 5 leads to tetanus where
there is sustained contraction in the face of repetitive stimuli that is so
rapid that there is no relaxation
Strength of muscle contraction
•
The increased force of contraction produced in
summation and tetanus occurs b/c
1. there is not enough time b/w contractions for muscle
fibers to completely relax and there is a buildup of
Ca++ in myofibrils, which promotes cross-bridge
formation and cycling.
2. the rapid production of AP in the muscle fibers
causes Ca++ to be released from the SR faster than
they are actively transported back into the SR.
Strength of muscle contraction
•
The increased force of contraction produced in
recruitment occurs b/c
1. of increased numbers of muscle fibers contracting
due to increasing the number of motor units
stimulated
2. as the number of motor units stimulated increases,
more muscle fibers are stimulated to contract and
the force of contraction increases. Maximum force
of contraction is produced in a given muscle when
all the motor units of that muscle are stimulated
(recruited).
Stimulus Strength and Muscle
Contraction
• All or None principle:
– States that once a stimulus is applied to an
individual muscle fiber it will contract to its
greatest extent or not at all. Any further increase
in the degree of stimulation (after the threshold
stimulus is reached) will not cause a
corresponding increase in muscle contraction.
– The weakest stimulus capable of causing a
contraction in the muscle fiber is called a
threshold stimulus
– A stimulus not capable of inducing a contraction
is called a subthreshold stimulus
Muscle Metabolism
• The chemical energy
released by the hydrolysis
of ATP is necessary for
both muscle contraction
and muscle relaxation.
• Muscles typically store
limited amounts of ATP –
enough to power 4-6s of
activity.
– So resting muscles must
have energy stored in other
ways.
Resting Muscle
and the Krebs
Cycle
• Resting muscle fibers typically
take up fatty acids from the blood
stream.
– How might they enter the cell?
– Inside the muscle fiber, the FA’s are oxidized to several
molecules of a compound called Acetyl-CoA. This
oxidation will also produce several molecules of NADH
and FADH2.
– Acetyl-CoA will then enter a cyclical series of reactions
known as the Krebs cycle or Tricarboxylic Acid
cycle.
– In the Krebs cycle, acetyl-CoA combines with the
compound oxaloacetate and then enters a series of
rxns. The end product of these rxns is CO2, ATP,
NADH, FADH2, and oxaloacetate (thus we call it a
cycle)
Krebs Cycle
Products
Oxaloacetate will simply
combine with another
molecule of acetyl-CoA and
reenter the cycle.
NADH and FADH2 will enter another series of rxns known as
the Electron Transport Chain (ETS). These rxns occur along the
inner membrane of the mitochondrion. They basically consist of
the passing of electrons from compound to compound with
energy being released each time and used to drive the synthesis
of ATP. The final electron acceptor is oxygen when it combines
with 2 hydrogen atoms to yield water.
Krebs Cycle Products
• CO2 will diffuse out of the mitochondria,
out of the muscle fiber, and into to the
blood stream, which will take it to the
lungs.
• The ATP made in the Krebs cycle plus the
ATP made during the ETC will be used in
many ways.
– See if you can list at least 5!
ATP Use in the Resting Muscle
Cell
• ATP is necessary for cellular housekeeping
duties.
• ATP powers the combination of glucose
monomers (which have been taken up from the
blood stream) into the storage polymer
glycogen.
• ATP is used to create another energy storage
compound called creatine phosphate or
phosphocreatine:
ATP + Creatine  ADP + Creatine-Phosphate
this rxn is catalyzed by the enzyme creatine
kinase
Energy Requirements for Muscle
Contraction
• Muscles can’t store/stockpile ATP in
preparation for periods of activity, but can
store the high-energy moleculecreatine
phosphate.
• Creatine phosphate (CP) provides a means
of storing energy that can be used rapidly
to help maintain an adequate amount of
ATP in a contracting muscle fiber.
Energy Requirements for Muscle
Contraction
• ATP is produced continually in
mitochondria located in the sarcoplasm
b/w myofibrils in both resting and active
muscles.
• Excess ATP in an inactive muscle is used
to synthesize CP.
• In an active muscle, the small pool of ATP
is preferentially used then the energy from
CP helps to restore the ATP pool
Fate of ATP in resting and active
muscle
Working Muscle
• With the onset of exercise, almost immediately our
stored ATP is depleted.
• For the next 15 seconds or so, we turn to the
phosphagen system, a.k.a., the energy stored in
creatine-phosphate.
Creatine-P + ADP
Creatine Kinase
Creatine + ATP
– The ATP is then available to power contraction and
relaxation: myosin ATPase, Ca2+ ATPase in the SR
membrane, and Na+/K+ ATPase in the sarcolemma.
– The phosphagen system dominates in events such as
the 100m dash or lifting weights.
Working Muscle
• After the phosphagen system is depleted, the
muscles must find another ATP source.
• The process of anaerobic metabolism can
maintain ATP supply for about 45-60s.
• Anaerobic means “without air,” and it is the
breakdown of glucose without the presence of
oxygen.
– It usually takes a little time for the respiratory and
cardiovascular systems to catch up with the muscles
and supply O2 for aerobic metabolism.
Anaerobic Metabolism
• Glucose is supplied by the breakdown of glycogen
or via uptake from the bloodstream.
• Glucose is broken down into 2 molecules of
pyruvic acid, with the concomitant production of 2
ATP and the conversion of 2 molecules of NAD+
into NADH. This process is known as glycolysis
and it occurs in the sarcoplasm.
– Unfortunately, w/o O2, we cannot use the NADH in the
ETC.
– In order for more glycolysis to proceed, the muscle cell
must regenerate the NAD+. It does this by coupling the
conversion of pyruvic acid into lactic acid with the
conversion of NADH into NAD+
Anaerobic
Metabolism
• Lactic acid typically diffuses out of
muscles into the blood stream and
is taken to the liver, kidneys, or
heart, which can use it as an energy
source.
• Anaerobic metabolism is inefficient.
Large amounts of glucose are used
for very small ATP returns. Plus,
lactic acid is a toxic end product
whose presence contributes to
muscle fatigue.
• Anaerobic metabolism dominates in
sports that requires bursts of speed
and activity, e.g., basketball.
Aerobic Metabolism
• Occurs when the respiratory and cardiovascular
systems have “caught up with” the working muscles.
– Prior to this, some aerobic respiration will occur thanks to
the muscle protein, myoglobin, which binds and stores
oxygen.
• During rest and light to moderate exercise, aerobic
metabolism contributes 95% of the necessary ATP.
• Compounds which can be aerobically metabolized
include:
– Pyruvic acid (made via glycolysis), fatty acids, and amino
acids.
Aerobic
Metabolism
• It occurs in the mitochondria.
• Pyruvic acid from glycolysis
is the primary substrate, but
the cell may utilizes fatty
acids and amino acids.
• Aerobic respiration typically
yields 36 ATP per molecule
of glucose. Compare this to
anaerobic metabolism.
Immediate source of energy (ATP)
for muscle contracion
• Anaerobic respiration-occurs in the absence of oxygen
and results in the breakdown of glucose to yield ATP
and lactic acid.
– Less efficient but is faster, especially when O2 availability
limits aerobic respiration
– May utilize many glucose molecules to produce many ATPs
for a short period of time
• Aerobic respiration-requires oxygen and breaks down
glucose to produce ATP, CO2 and water.
– More efficient than anaerobic respiration (18 times more
ATP from 1 glucose molecule theoretically)
– Utilizes a greater variety of molecules as energy sources (eg
fatty acids and amino acids can be used to generate ATP)
Muscle Fatigue
• Physiological inability to contract
• Results primarily from a relative deficit
of ATP.
• Other contributing factors include the
decrease in sarcoplasmic pH (what
causes this?), increased sarcoplasmic
[ADP], and ionic imbalances.
Oxygen Debt
• Refers to the fact that post-exercise breathing
rate >>> resting breathing rate
• This excess oxygen intake serves many tasks:
– Replenish the oxygen stored by myoglobin and
hemoglobin
– Converts remaining lactic acid back into glucose
– Used for aerobic metabolism to make ATP which is
used to:
• Power the Na+/K+ pump so as to restore resting ionic
conditions within the cell.
• Replenishes creatine phosphate and glucose from lactic acid
– Replenish the phosphagen system
– Replenish the glycogen stores
Oxygen Debt
• Occurs as a result of intense exercise
– The rate of aerobic metabolism will remain elevated
for a period of time following anaerobic exercise to
“pay back” or rejuvenate depleted energy sources.
• Magnitude of debt depends on intensity,
length of time sustained and physical
condition of individual (i.e. the capacity to
perform oxidative metabolism is not as
great as in a well-trained athlete)
Slow/Fast fibers
2 types:
• Fast twitch- contract quickly and fatigue quickly
• Slow twitch-contract more slowly and are more
resistant to fatigue
• Note: Not all skeletal muscles have identical
functional capabilities. The proportion of muscle
fiber types differs within individual muscles.
Slow-Twitch muscle fibers
type I
• Contract more slowly, are smaller in diameter,
have a better developed blood supply, have
more mitochondria are more fatigue resistant
than fast twitch fibers
• Aerobic metabolism is primary source for ATP
and their capacity to use this pathway is
enhanced by plentiful blood supply, the
presence of numerous mitochondria and large
amounts of myoglobin
Fast-Twitch muscle fibers
type IIa and type IIb
• Contain myosin molecules that breakdown ATP more rapidly
than slow-twitch fibers
– Increase rate of crossbridge formation, release, and reformation
(cycling)
– Type IIb contract 10 x faster than type I fibers
– Type IIa contract at an intermediate speed and are more fatigue
resistant than type IIb fibers.
• Associated with a less well-developed blood supply, lower
levels of myoglobin and fewer and smaller mitochondria
• Possess large deposits of glycogen and are well adapted to
perform anaerobic metabolism.
– Contract rapidly for a shorter time and fatigue relatively quickly and are
not adapted for supplying large amounts of energy for a prolonged
period
Distribution of fiber types
• humans exhibit no clear separation of
slow-twitch and fast twitch fibers in
individual muscles (e.g. in a chicken white
meat is found in the breast and dark meat
is found in the legs)
• In humans, most muscle have both types
of fibers, although the number of each
varies in a given muscle.
– Larger postural muscles contain more slow twitch, whereas
muscles of the arms contain more fast twitch fibers.
Distribution of fiber types
• Distribution is established developmentally and
does not change much for each individual
– Sprinters-greater % of fast-twitch fibers in legs
– Long distance runners-higher % of slow-twitch
fibers in legs
Note: One type of fiber can be converted to another type, with specific
training. With wt training some type IIb can be replaced by IIa myosin
myofilaments. Vigorous exercise programs can also cause a limited
number of type I myofilaments to be replaced by type IIa myofilaments.
Training can also increase the capacity to perform more efficiently (e.g.
aerobic metabolism can convert fast twitch muscle fibers that fatigue
readily to fast-twitch fibers that resist fatigue by increasing # of
mitochondria and increasing the blood supply or slow twitch muscle fibers
can be increased in size or the muscle vascularity may be increased)
Effects of exercise
•
Muscles increase in size (hypertrophies), strength and endurance in response
to exercise…Why?
– Fibers increase in size# of myofibrils and sarcomeres increase within
each muscle fiber
• (muscle cell # does not change appreciably during a person’s life)
– Blood vessels, connective tissue and mitochondria increase
– Nervous systems ability to activate greater numbers of motor units
simultaneously and improvements in neuromuscular coordination
strengthen muscle
– Trained muscle is less restricted by excess fat and metabolic enzymes
increase in hypertrophied muscle fibers and lead to greater capacity for
ATP production
– Improved metabolism, increased circulation to exercising muscle and
increased numbers of capillaries, more efficient respiration, and a greater
capacity for the heart to pump blood in part improves endurance