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CLASS: Fundamentals I
DATE: August 16, 2011 10:00 – 11:00
PROFESSOR: DETLOFF
I.
II.
III.
IV.
V.
MUSCLE PROTEINS
Scribe: Nicola Gough
Proof: Frannie Davis
Page 1 of 6
MUSCLE PROTEINS [S1]
a. Translating chemical energy into motion and muscles.
i. The way muscles work is they contract.
1. If you want to move your arm out, you use your triceps.
2. If you want to move your arm in, you use your biceps.
ii. You’re contracting the muscle that pulls on a bone.
b. When you look at a muscle, it’s a bunch of fiber bundles. Let’s separate it out.
c. There are different kinds of muscles:
i. Striated muscles (Skeletal type muscles) - ex: biceps, triceps
ii. Smooth muscles (present inside other organs, such as the intestines)
1. Smooth muscles act in a slightly different way.
2. Smooth muscles have the same basic mechanism of contraction, but they are regulated in a different
way. (We will go over that later, but we’re going to start with skeletal or striatal muscles).
SKELETAL MUSCLE ANATOMY [S2]
a. A fiber bundle contains hundreds of myofibrils.
i. Most of those myofibrils run the length of a fiber.
ii. Each myofibril is a linear array of a sarcomere.
1. A sarcomere is the unit that carries out the contraction.
2. Sarcomeres are covered with a membranous sac that contains calcium.
3. Calcium is the signal. Once calcium is released from the sac into the muscle fibers, the contraction
will occur.
16.1 - WHAT IS A MOLECULAR MOTOR? [S3]
a. What’s shown here is what I just described
b. The blue is a lipid bilayer membrane called a sarcolemma.
c. The sarcoplasmic reticulum (shown in yellow) is like the endoplasmic reticulum; it’s a membrane that contains
all the calcium.
d. This is a single muscle cell.
i. Muscle cells are multinucleated: they start out as individual cells during development, and then once they
have formed the structures that they need to, the cell membranes can break down and fuse together so
that there are many different nuclei in the singe cell.
ii. So it really is a single muscle cell, but it’s multinucleated.
iii. These are nuclei here.
e. The fibers and filaments inside here is what really carry out the work.
i. One sac of those, that has this sarcoplasmic reticulum around it, is called the myofibril.
ii. There are a lot of mitochondria in there. You need a lot of energy for this to take place, and mitochondria
produce energy.
f. When we look at a sarcomere (an independent structure) many, many of these are laid down in one myofibril.
i. They are laid down head to tail, or head to head (they are symmetrical, so either is correct).
IMAGE: ELECTRON MICROGRAPH OF SKELETAL MUSCLE MYOFIBRIL [S4]
a. When you look at an Electron Micrograph (EM) of a single sarcomere, the sarcomere ends at the Z line
(heavy staining, electron dense area).
b. The thin fibers overlap with thicker fibers (thicker fibers from M disk), and if you took a cross-sectional area,
you get what looks like the crisscross area demonstrated in the 3rd cross-section on your slide, which takes
place at the very middle of the EM image.
i. The thick filament (purple dots / gray area of the EM) and the thin filaments (blue specs / white area of
the EM) interdigitate.
ii. You have a fiber coming from the Z line (the 2 thick outer lines), a thicker fiber coming from the M disk
(the center line), and then they interdigitate (these are protein structures).
iii. The thin filament area is actin.
iv. The thick filament area is myosin.
v. The overlap area is where they look like the 4th cross-section, and the thick dark lines (next to the white
sections). This is where they interdigitate.
vi. The interaction between the actin and myosin fibers is what can shorten the sarcomere.
FIGURE: 3D STRUCTURE OF ACTIN MONOMER [S5]
a. This is the structure of an actin monomer.
b. Actin is one of these subunit filaments – it’s a repeating subunit, a subunit of itself, repeating over and over
and over to make a long quaternary structure (this F-actin filament).
CLASS: Fundamentals I
Scribe: Nicola Gough
DATE: August 16, 2011 10:00 – 11:00
Proof: Frannie Davis
PROFESSOR: DETLOFF
MUSCLE PROTEINS
Page 2 of 6
VI.
IMAGE: F-ACTIN HELIX [S6]
a. This structure actually has a helix to it with a pitch of 72 nanometers.
i. So the repeat distance is 36 nanometers.
b. This becomes important because a single catalytic step can push a myosin relative to that, by 36 nanometers
(And that’s actually been measured).
VII.
3 PART FIGURE: THIN FILAMENT [S7]
a. Now the thin filament isn’t JUST actin, there are regulatory proteins that kind of surround the actin.
i. One of them, called tropomyosin actually goes into the groove of the helix of the actin.
1. The actin itself is a double helical structure, and this tropomyosin goes into the groove.
ii. There’s also troponin, which is placed at regular intervals along this, and it’s involved in allowing myosin
heads to interact with actin.
b. So the actin itself is in a protected state (normally), protected by this tropomyosin and troponin complex.
VIII.
THE COMPOSITION AND STRCUTURE OF THICK FILAMENTS [S8]
a. The thick filaments are made entirely of myosin.
b. Myosin has 2 heavy chains and 4 light chains.
i. The heavy chains are fairly long (230 kilo-Daltons each).
ii. The light chains are fairly short and they are at the head of the myosin.
IX.
3 PART FIGURE: MYOSIN MOLECULE [S9]
a. This is what the myosin actually looks like.
b. The head is the circular portion on the left of the top picture, and the light chains are around it (the ovals
unattached to the helix, but surrounding the globular heads).
c. This is one of the subunits here (where the globular head and coiled rod join), and this is where ATP is
cleaved, giving a conformational change of that head.
i. What’s going to happen is you are going to see a conformational change based on ATP hydrolysis,
where you’re in the state of potential energy, it’s going to stick to an actin, and then snap back. It’s going
to push the myosin along the actin, and then the sarcomere will get shorter.
ii. They have to be attached to something, so there are these long coiled rods here (long alpha helices
intertwined with one another), giving a coiled coil domain.
iii. On the right of the top structure is the carboxy-terminus (COO-), the part of the myosin protein that’s last
synthesized at the ribosome (we will learn more about that later).
d. The N terminal (NH3+) domain (left of top structure) is what’s coded for first, and translated first when a
protein is made.
e. The head is shown by the red, green and purple (left side bottom image), and the light chains are shown in
yellow and magenta (structures wrapping around the dark coil).
f. This is the one molecule that can go like this, and keep on going, to give you your anchor area, and then
these other regulatory molecules here, at the light chains.
g. This is an Electron Micrograph (EM) of what a single myosin coiled coil chain would look like (middle image).
X.
REPEATING STRUCTURAL ELEMENTS ARE THE SECRET OF MYSOIN’S COILED COILS [S10]
a. Myosins are based on repeating structural elements. So there’s a 7-residue, 28-residue repeat, and when
those are repeated, they can become a 196-residue repeat.
b. The rule about the repeats is that the residues 1 and 4, of the 7-residue repeat, are hydrophobic. 2,3,and 6
are ionic.
i. Remember, this is part of the coiled coil domain.
c. Structures will collapse on one another when placed in solution (we talked about this in the quaternary
structure lecture).
i. Hydrophobic goes inside and on the outside are your more charged and polar residues.
ii. This is done to keep the water from becoming too structured around the hydrophobic regions.
iii. The repeating pattern favors that.
XI.
FIGURE 16.17 – AXIAL VIEW OF TWO-STRANDED ALPHA HELIX OF MYOSIN TAIL [S11]
a. These are alpha helices; this is one way to draw the alpha helix (you’re looking down the helix).
b. The alpha helices are right handed, as far as the way they twist around.
c. What is the difference between right-handed and left-handed helix?
i. Any of you who have ever worked with machines have had to put a wrench on a bolt, and you’re twisting
in a clockwise direction to try and get the bolt off (what many do naturally).
ii. But there are some bolts designed to be the opposite, so you have to crank them the other way.
iii. So one’s a right-handed helix (clockwise twist), the other’s a left-handed helix (counterclockwise twist).
(They’re formed by the groove that forms where the bolt goes into, the tread.)
1. This is a part of topology.
CLASS: Fundamentals I
Scribe: Nicola Gough
DATE: August 16, 2011 10:00 – 11:00
Proof: Frannie Davis
PROFESSOR: DETLOFF
MUSCLE PROTEINS
Page 3 of 6
iv. If you’re looking at a structure, and you take your finger and you go along the helix, and it’s going away
from you, and it goes in a clockwise direction as your finger is on the backbone of that helix, then it’s a
right-handed helix (clockwise).
v. If you’re looking at a structure, and you take your finger and you go along the helix, and it’s going
counterclockwise as it’s going away from you, that’s a left- handed helix (counterclockwise).
vi. It doesn’t matter which end you start from, you will ultimately have the same handedness to it.
vii. If you try to drive a right-handed helix with a screwdriver into a wall, you’d be turning it clockwise to get it
into the wall, and you’d turn it counterclockwise to get it out.
d. This is the topology of the way structural biologists would show something.
i. That topology is very important particularly for DNA, because DNA is a double helix.
1. So DNA has a certain amount of twist to it, that twist becomes very important in whether the two
strands of DNA come apart, and there are antibiotics that alter that twist. They can kill organisms
based on that. Therefore, topology does have some clinical significance as well.
e. These circular structures in the figure are the myosin tails.
i. If you look down the axis of this alpha helix, you will see that these “a” and “d” (1st and 4th) are
hydrophobic, and you’ll have a 1st and 4th (“a” and “d”) on other coil, and those will stick together.
ii. These two coils will curve around each other in a right-handed helix; it’s a coiled coil domain.
f. These other residues or amino acids on the outside (b, c, f) are polar (or charged) so they tend to go out into
the water.
g. This repeated pattern here allows staggering.
i. When you actually look at a thick filament, it has a staggered array of these types of things.
ii. The repeated nature of it means that one repeat unit can interact with another repeat unit of a different
myosin molecule, out of register.
XII.
MORE REPEATS! [S12]
a. Ultimately what happens is you see these heads that are on the end of myosin in a thick filament, and they’re
about 98 repeat units apart.
XIII.
FIGURE: MYOSIN MOLECULE IN THICK FILAMENT [S13]
a. This myosin head (on the left) would not ultimately end up with its C-terminus (located at the M-disk region),
the C-terminus would be very near…(He didn’t finish his sentence).
b. There’s a slight thickening here (at the bare zone?) because there is more protein material in the area here
(M-disk region), compared to the end.
XIV.
IMAGE: SKELETAL MUSCLE MYOFIBRIL [S14]
a. Now we have all the players that are going to contract the sarcomere.
b. As you can see at the top, this is one sarcomere (from the Z line to the Z line).
c. You can see how they are lined up with other sarcomeres, in a very ordered structure.
XV.
DISPOSITION OF MUSCLE MOLECULES IN ONE SARCOMERE [S15]
a. This shows a 3Dimensinal representation of what one thick filament (the myosin thick filament) would be
when surrounded by these thin filaments (the actin and tropomyosin complexes).
XVI.
DIAGRAM: SLIDING FILAMENT MODEL OF SKELETAL MUSCLE CONTRACTION [S16]
a. What I showed previously was an EM of a relaxed configuration, but you can take an EM of a sarcomere
when it’s in a contracted configuration as well.
b. What you see is these actin filaments at the I-Band getting pushed/pulled in towards the M zone structure.
i. Top of diagram is in the relaxed state, and the bottom of the diagram is in the contracted state.
ii. There’s a decrease in the width of these structures that are just single fibers.
iii. You are interdigitating the fibers.
iv. The actin is moving (being pulled) toward the center of sarcomere.
v. If this happens over and over and over, you can have quite a contraction of a muscle.
XVII. THE CONTRACTION CYCLE: ATP HYDROLYSIS DRIVES CONFORMATIONAL CHANGES IN MYOSIN [S17]
a. So how does the chemical power of ATP drive this?
i. If you start out in the pre-power stroke (bottom right of figure), it has thick filament (shown by how tightly
attached to bottom pink structure), and one myosin head on the thick filament. There are many, many
more myosin heads, but we are just going to show the one myosin head and what happens to that.
ii. The pre-power stroke is in a state of high potential energy, the head is fully extended, and it can snap
back. The head has been fully extended because you hydrolyzed ATP.
iii. So when ATP is hydrolyzed, it becomes in a state of lower potential energy. It’s a reverse spring (it
wants to pull back).
iv. Once this interacts, in resting muscles, you have this state right here (Top-of-the-power state). When
calcium gets released from the sarcomere and baths in regulatory elements that are stuck on this actin
molecule. You will then have an attachment, and a cross bridge will form. The myosin head will interact
CLASS: Fundamentals I
Scribe: Nicola Gough
DATE: August 16, 2011 10:00 – 11:00
Proof: Frannie Davis
PROFESSOR: DETLOFF
MUSCLE PROTEINS
Page 4 of 6
with the actin, and when it does that, it will then snap back, and that’s where the power is generated.
Now the actin is moving relative to the myosin (being pulled in) and then you will have the Rigor-like state
(no ADP or ATP bound).
v. You’ve all heard of Rigor mortis right? It’s the tensing up of all the muscles in a dead person; in fact
there’s been cases of bodies sitting up inside coffins, and pushing tops off coffins.
1. With this lack of energy, this person can’t create any more energy, and the muscles will tend to
tighten up, so they are left in this rigor-like state.
vi. In order to detach this (stop this), and get back to a resting state, you need ATP.
1. ATP will detach once it interacts with this light chain (which is located in the head of the myosin).
2. So the ATP will detach, and then the cleavage of ATP snaps the head forward a little bit.
3. And this can happen over and over and over again.
b. It requires an enormous amount of energy to contract muscles.
c. Do these ATP’s interact with a certain light chain? Yes, the certain light chain that ATP interacts with is LC-2.
d. The details of this conformational change are something that we’re studying now, it’s coming to light.
e. There’s the idea that the beta-sheet being twisted/folded on itself provides the potential energy.
i. The beta sheet twisted is shown in the diagram.
ii. In the Rigor-like state and the Top-of-power stroke (top left and top right in diagram), the actin cleft
closed and the beta sheet twisted is the conformation of the head (the protein structure inside the head is
changing and it’s stable in 2 states, but thermodynamically favored to be pulled back).
XVIII. THE CONTRACTION CYCLE: ATP HYDROLYSIS DRIVES CONFORMATIONAL CHANGES IN MYOSIN [S18]
a. The cross-bridge is between the actin and the myosin.
XIX.
DIAGRAM: CALCIUM PUMP [S19]
a. Calcium is the signal
i. Normally in this system (system of actin and myosin inside sarcomere), there’s hardly any calcium in
there because it’s all been sequestered and pushed into this sarcoplasmic reticulum (the membranous
structure that surrounds the myofibril)
ii. In order the get calcium… If you want to pump something out where there’s a low concentration, you
need energy.
iii. So there is a calcium pump on the lumen of the sarcoplasmic reticulum (it sucks calcium away from the
actin and myosin fibers), and it’s sequestered - it can’t get across a membrane because it’s too highly
charged.
iv. Membranes are made of a bunch of polar lipids, so it can’t just fly across there, it needs a hole to go
through; this hole happens to be a pump, its uses ATP, and that’s a large use of energy.
v. There’s a very high concentration in the resting state of the lumen calcium.
vi. But inside where the actual fibers are (the pink area at the bottom of the diagram), there is a very low
amount of calcium.
b. There are signals to contract that are electrical signals.
i. You have a nerve that goes to a muscle, and then at the end of the nerve, it typically releases
acetylcholine (a neurotransmitter).
ii. The neurotransmitter acetylcholine will send an electrical signal across the sarcoplasmic reticular
membrane, and that would open up a calcium channel; this tells the entire fiber to contract at this
particular time.
XX.
DIAGRAM: WHEN CALCIUM BINDS TO TROPONIN C [S20]
a. So how does calcium start this whole process of the myosin heads ratcheting along the actin filament to
contract?
i. In skeletal muscle, it is carried out by the troponin regulatory system.
ii. Remember that tropomyosin is curved around the actin filament.
iii. There are two different states:
1. When calcium is added to the system, it binds to troponin C (TnC) and that creates a shift in the
conformation of the troponin complex. It allows this surface of the actin to bind to the myosin head.
2. The fibers are coming straight out (in the diagram, you are looking at a cross-section of a fiber).
3. Tropomyosin is a very long molecule and binds longer
4. Troponin C (TnC) binds calcium, creates this conformational change, and then you can have this
cross-bridging happen, which is necessary. As soon as this happens, you can have the power
stroke, pulling it along. Once the calcium is pulled away by this pump, then everything can go back to
its resting state, and the myosin is a little bit further (its 35nm down the stretch); you can have this
flipping continue.
b. A lot of the energy of the muscle cell is not due to the ATP that’s created in (or changed in) the conformation
of myosin head; it’s due to the calcium pumps.
CLASS: Fundamentals I
Scribe: Nicola Gough
DATE: August 16, 2011 10:00 – 11:00
Proof: Frannie Davis
PROFESSOR: DETLOFF
MUSCLE PROTEINS
Page 5 of 6
c. So even after the contraction, it will take a while for the muscle to actually pump that calcium out – you are
still using a lot of ATP.
XXI.
SMOOTH MUSCLE CONTRACTION [S21]
a. Smooth muscle (peristaltic contractions you need for your digestive system to work – that’s smooth muscle)
i. Smooth muscle has a little different structure – it doesn’t have the striated sarcomeres one after another
inside a fibril.
b. Smooth muscle can contract as well, but it might have an actin-myosin complex looking like the picture below
(with anchors at each end) (the picture he drew on the chalkboard) (bubble formed around this stair-step
structure)
.___.
.___.
.___.
.___.
1. So if the intracellular calcium of smooth muscle increased, these would move relative to one another
and form a cell that is crunched more together (lines above will become more on top of each other)
2. These occur much more slowly.
3. Digestion doesn’t need to follow the fight or flight response.
4. There are different kinds of innervation of these, from the Central Nervous System.
5. There are a lot of smooth muscle cells that don’t have any innervation from the Central Nervous
System – they are totally worked based on hormones.
c. There’s no troponin complex inside the smooth muscle.
d. You still have tropomyosin, but it doesn’t act in the same manner.
e. The way a smooth muscle contracts is that calcium interacts with calmodulin.
i. Calmodulin looks like troponin C (troponin C is what binds calcium in the skeletal muscle).
ii. Calmodulin then activates/stimulates the Myosin Light Chained Kinase (MLCK).
1. The MLCK phosphorylates the light chain too, and that actually then sends the signal to start
contracting – it allows cross bridging and all the other steps of the skeletal muscle contraction I just
talked about.
XXII. SMOOTH MUSCLE EFFECTORS [S22]
a. Hormones regulate the contraction.
i. Epinephrine is one example of a smooth muscle relaxer.
1. It activates adenylyl cyclase (which makes lots of cyclic AMP in the cell)
2. Adenylyl cyclase then activates protein kinase, which inactivates the myosin-like chain (kinase).
b. If you look at the Myosin Light Chained Kinase (MLCK), when it phosphorylates LC-2, it activates LC-2. When
it’s phosphorylated itself, it becomes inactive.
c. There are several drugs out there that alter the smooth muscle contraction – they can alter it by altering this
phosphorylation (this inactivation) of Myosin Light Chained Kinase (MLCK) with cyclic-AMP (cAMP) being
mediated.
d. Epinephrine allows the relaxation of the muscles around the alveoli, opening up air passages.
e. Albuterol is the same, but it’s more selective for smooth muscles; it acts more on the lungs than the heart, so
it’s a little bit safer. Albuterol is inhaled.
f. Oxytocin is a natural stimulator of uterine smooth muscle; that is how you would induce labor; smooth muscle
of the uterus would be contracted by oxytocin.
XXIII. KINASES AND PHOSPHATASES [S23]
a. All a kinase does is it adds a phosphate group to another molecule, from a donor, such as ATP.
i. We talking about is kinases that work on proteins; on proteins, there are several residues, several amino
acids, that have hydroxyl groups on them (serine, threonine, and tyrosine)
1. Serine, threonine, and tyrosine can be phosphorylated (phosphate group can go on the end of them)
and that’s done by proteins that specifically recognize that serine, threonine, or tyrosine to be
phosphorylated; they alter the confirmation and the activity of several enzymes – its a way to
allosterically regulate an enzyme (you can turn an enzyme on with phosphorylation, you can turn an
enzyme off with phosphorylation)
b. The phosphate comes from a phosphate donor like a nucleotide triphosphate (like ATP).
c. There are other enzymes called phosphatases that remove that phosphate group and put a protein back into
it’s on or off state (depending on which is which).
XXIV. CREATINE KINASE AND PHOSPHOCREATINE PROVIDE AN ENERGY RESERVE IN MUSCLE [S24]
a. When the muscle is working incredibly hard (contracting for a long period of time), it’s going to run out of ATP.
i. A couple of systems have been developed to help provide ATP or an ATP equivalent for that muscle cell.
CLASS: Fundamentals I
Scribe: Nicola Gough
DATE: August 16, 2011 10:00 – 11:00
Proof: Frannie Davis
PROFESSOR: DETLOFF
MUSCLE PROTEINS
Page 6 of 6
ii. There are certain muscle cells that can respire at a higher rate (ex: skeletal muscle cells can respire very
quickly; they have storages of oxygen, they have myoglobin in them, they can store oxygen, their
mitochondria have lots of oxygen; but they go into an anaerobic state where there isn’t any oxygen at
many times, but all they need is that ATP though. So one of the ways this is done is with creatine.
iii. You all have heard about athletes taking a lot of creatine. This idea that creatine can help enhance
performance and weightlifting and so forth; that it just helps your muscles work better – well this is
probably true.
1. Your muscles have a lot of creatine. There is a creatine kinase that takes phosphorylated creatine
and then changes ADP to ATP, increasing the energy state there, allowing that ATP to then be used.
b. Creatine structure on the right of the slide, so if you build up a lot of that inside your muscles, it can become
phosphorylated slowly over time and then when you need that energy, creatine kinase can produce it for you.
c. Creatine is also being used as a potential treatment for Huntington’s
i. The idea behind these psychological disorders is that there’s an energy deficit in neurons, and so people
believe adding creatine can bulk the neuron up and protect it from these depletions of energy that occur
during the pathogenic state.
ii. It’s a very hopeful hypothesis, I hope it works, but it seems like it’s not necessarily going to help out.
XXV.
LACTATE FORMED IN MUSCLES IS RECYCLED TO GLUCOSE IN THE LIVER [S25]
a. There’s another system that’s used. Later we will talk about the Krebs cycle and glycolysis
i. Glycolysis is a way to take glucose and get it into the Krebs cycle – that’s the first thing that happens; it
can happen anaerobically (without oxygen).
b. We’ve all heard about lactate build up in a muscle that occurs after a while, and part of getting rid of that can
be helpful.
c. Vigorous exercise can lead to a buildup of lactate and NADH, and that’s just because of glycolysis (you will
learn more about the process of glycolysis later in the course).
d. NADH can be reoxidized to reduce pyruvate to lactate – that’s why lactate builds up inside the muscles.
e. Lactate can then be dumped into your nearest blood vessel and go to the liver; and then the liver can produce
glucose from it. This referred to as the Cori cycle.
XXVI. LACTATE FORMED IN MUSCLES IS RECYCLED TO GLUCOSE IN THE LIVER [S26]
a. The Cori cycle allows muscles to function to produce ATP (need the NADH+ to do this). It can go into the
mitochondria, and produce ATP.
b. Glycolysis produces some of it (not a lot, but some) so in an anaerobic state, muscle can take up glucose,
produce a couple ATP, lactate dehydrogenase produces lactate, it goes to the blood stream, so goes to the
liver, and that’s turned back into glucose.
c. It gives the muscle a little bit of time to run and produce a little ATP during that anaerobic state
XXVII. MUSCLE CONTROL BY RECEPTOR AGONISM AND ANTAGONISM [S27]
a. Smooth Muscle Relaxers – Future optometrists will be using something that alters the pupil size.
i. This individual here had some eye drops put in; that eye drop contained Tropicamide, which is affecting
the ability of the iris to contract or expand.
ii. There are 2 sets of muscles (one that’s circular- contraction that makes it smaller) (other is radial – pulls
it out) and what these work on is the acetylcholine receptors that send the signal into those muscles to
contract or not.
b. It’s an Antimuscarinic drug, which is a type of acetylcholine receptor.
c. An Antimuscarinic drug inhibits the circulatory muscles and allows those to relax, and redial muscles can pull
out a little bit. You can also add a little bit of phenylephrine HCl, which stimulates the radial muscles of the
eye to contract.
d. In different countries they use different methods. In the U.S., you can use both these sympathomimetics,
such as phenylephrine, which stimulate the redial muscles and relax the others.
e. It doesn’t affect how the muscles contract, it affects the control, it affects the calcium that comes in.
f. There may be other ways of relaxing muscles that more directly involve this mechanism here, or the other
mechanism I described in the skeletal muscles
g. THINGS TO REMEMBER:
i. There is a conformational change in the myosin head, myosin head is in a state of high potential energy
when it’s out, and it snaps back to low potential energy, pulling it along the actin fiber. ATP is then
hydrolyzed to snap it back into its higher state of potential energy. This can occur over and over and
over again; There are all of these systems involved that regulate that; the systems are very fast and it
involves calcium as the signal – in both smooth muscle and skeletal muscle (skeletal muscle has those
nice defined sarcomeres)
[End 43:56 mins]