• Animals are characterized by having 2 broad types of muscle tissues: striated muscles
and smooth muscles
• Striated muscles are characterized by having banding patterns. There are alternating
light and dark bands on the cell.
• Smooth muscles lack that banding pattern.
• Smooth muscles and striated muscles are found in both vertebrates and invertebrates,
but the way they’re distributed may be different.
• The striations that occur in striated muscles reflect the fact that the myofillaments (actin
and myosin) are organized in a very regular way.
• In smooth muscles, the same proteins are present but they’re not organized in a regular
fashion so you don’t get the bands.
• Here we have a muscle. The basic building block of a muscle is a muscle cell which you
• Muscle cells go by different names but the most important ones are myofibre or muscle
• Myofibre, a single muscle cell, is bound by a plasma membrane. In the muscle the
plasma membrane is given the name sarcolemma.
• Each individual muscle cell is ensheathed by a layer of connective tissue called
• A whole bunch of individual muscle cells are bound up by a second layer of connective
tissue called perimysium.
• The structure the bundles of muscle fibres create is called a muscle fascicle.
• All of these fascicles are bound by another layer of connective tissue called epimysium,
and then we have a single muscle.
• So if you think it the other way, you have a muscle which is comprised of fascicles, which
are comprised of myofibres, and all of these things are bound by a connective tissue.
• The epimysium is continuous with the tendon. The tendons are connective tissues that
attach the muscle tissue to the bone. The connective tissue that makes up the tendon is
continuous with the layers of connective tissue that surround the entire muscle. • Within the myofibre are cylindrical structures called myofibril. Within each muscle cell,
there are many myofibrils. The myofibrils themselves are made up of the myofilaments
actin and myosin.
• The myofibrils are bounded by the sarcoplasmic reticulum
• Each myofibril is surrounded by the sarcoplasmic reticulum, which is the site that
calcium is stored inside the muscle cells.
• Calcium plays a role in regulating muscle contraction, and so the distance that calcium
has to travel from the storage site to the region of the myofibril needs to be kept as short
as possible so contraction can occur quick.
• Myofibrils are very small (around 1 micron in diameter) so you minimize the distance
calcium has to travel back and forth between the deepest parts of the myofibril.
• Actin and myosin are the major proteins involved in muscle contraction.
• Actin is referred to as thin filaments because it’s thinner in diameter than myosin.
• Each actin filament found in the muscle is made up of a protein called G-actin (globular
actin) which is an individual monomer actin.
• If you want to be specific, the type of actin found in muscles is known as alpha actin.
Actin plays a major role within cell cytoskeleton, in the muscle it’s a different kind of
actin, but in muscles it’s just alpha actin.
• A single monomer, an individual actin protein, is referred to as G-actin.
• In the muscle cells, the individual actin molecules are built into polymers and make up F-
actin (filamentous actin). In F-actin, individual G-actin molecules are linked together to
form this long filament, and 2 of these filaments are wound around each other (shown in
blue and purple) to form the F-actin.
• Actin is a polymer, so you can grow it in length or shorten it in length by adding or
stripping off G-actins. This is good in circumstances, like in cell’s cytoskeleton. But in the
context of a muscle, we want this to be precisely regulated and unchanging.
• To make sure G-actins aren’t popping on or off the filament, the ends of the actin
filament in the muscle are capped by proteins called capping proteins: CapZ and
• There’s a bit of polarity in the actin polymer. There’s a barbed end to which it’s easy for
new G-actins to grab on and add themselves to the chain. The barbed end is called the fast growing end because G-proteins can grab on and increase the length of the
filament. CapZ is specifically the capping protein for barbed end.
• The pointed end (slow-growing end). It’s unlikely that G-actins will attach themselves to
this end, and they’re more likely to pop off this end. Tropomodulin is the capping protein
for the pointed end.
• The distinct capping proteins on each end prevent any new actins from getting on and
getting off so the length of the filament doesn’t change.
• The major protein making up the thick filament is the myosin. In the muscle, myosin has
a dimer structure. This is where 2 individual molecules are wrapped around each other.
• The structure has 2 intertwined tails, and 2 heads. Each individual myosin is made of 3
basic proteins that interact with each other.
• The blue portion is known as the myosin heavy chain. The green and red bits are also
part of the myosin heavy chain. If you break an individual myosin molecule and break it,
the blue part comes off as one piece and it’s heavier than all the pieces which is why it’s
called the heavy chain.
• The heavy chain as a tail region, a neck region which has flexibility and allows the
structure of myosin to bend. The heavy chain also has a motor (also called head or
• The head of myosin has 2 distinct region:
o One region is able to bind to actin. The actin binding site of myosin has a
particular affinity of a certain part of actin which is important for muscle
o It also has a pocket where ATP can bind and hydrolyze. So ATP also plays an
important role in regulating muscle contraction.
• The 2 other parts of the myosin, one is called the essential light chain which is the yellow
protein you see here.
o The ability of myosin to generate force during muscle contraction depends in
large upon this protein, though no one knows what this protein actually does.
o This protein is found in the neck region of the myosin where bending occurs. So
the best guess is that this yellow protein probably gives some stability to the neck
region of myosin while it’s making these changes.
o If a myosin lacks the essential light chain, it can’t generate as much force. • The other supplemental part of the structure is called the regulatory light chain.
o This is the pink structure you see here.
o In vertebrate skeletal muscle the regulatory light chain doesn’t play that much of
a role. Its role is supplemental so it plays a background role in regulating how
well myosin and actin interact.
o Its role is more pronounced in smooth muscles where it can be phosphorylate
and calcium can bind to it, which can affect how well myosin and actin interact.
• So individual myosin are dimers. Myosins in actual muscle cells organize themselves
• They do this in such a way that all the shafts of the myosin are towards the centre and
the heads sticking out on one end. At the same time you have a mirror image at the
• This arrangement is called the end polar arrangement of myosin.
• A different arrangement characterizes smooth muscles.
• Here’s a striated muscle. There are light bands and dark bands. The light bands are
called I-bands (isotropic bands) and the dark bands are called A-bands (anisotropic
• In a light microscope you can only visualize the bands, but with an electron microscope
you can see much more details.
• What makes up the I-band?
o The I-band is characterized by the absence of myosin. In the region of the I-band
there’s no myosin. All that’s present here is largely actin.
o In the centre of the I-band is a very dark structure. This is the Z-disc. The Z-disc
is made of a number of proteins whose job is to anchor the actin filaments (thin
• In the A-band there are dark regions which is where myosin are present.
• You can see within the A-band there are dark regions next to a lighter region, what’s
responsible for that? o In the very dark regions, that’s where the myosin heads are present. In the lighter
regions the heads are absent, and that’s where the tails are.
• In the middle of the A-band is a region called the H-zone where it’s very light. This is the
region where no actin is present. In the centre of the H-zone is a very dark line called
the M-line which is made up of proteins whose job is to anchor the thick filaments.
• The sarcomere is considered the fundamental unit of muscle contraction.
• The sarcomere extends from one Z-disc to the next. During muscle contractions,
sarcomeres shorten in length.
• There are 2 other proteins. The first is titin (blue protein on the diagram) which extends
from the Z-disc all the way to the M-line.
• Actin is made up of individual proteins bound together, but titin is just a single protein. It
is recognized as the world’s largest known protein.
• It functions as 2 roles:
o In one of its functions as it runs along the length of the myosin, it provides that
region some stability to the myosin, so it helps stabilize the filament.
o Its other role, which is most important, is here in I-bands, titin is a very elastic
o This is important because if you take a muscle and stretch it out, and let go, it will
go back to its original length. This is because when you stretch the muscle out, it
extends the elastic component of titin and when you let go it snaps back to its
o So muscles can have a passive contractibility, an ability to contract without any
energy being invested simply because of the elastic component of titin.
• At the same time, we said the sarcomere shortens when muscles contract and pushes
towards each other. In that case the elastic region of titin becomes compressed, and
when muscle contraction ends, the titin pushes the muscle back to its resting length.
• So titin, at least in the region of the I-band, the elasticity allows muscle to return to its
original length when it’s stretched and when it’s contracted it helps the muscle relax.
• Nebulin is a protein that runs along the length of actin and helps stabilize the thin
• This is a cross section of a muscle • For every myosin thick filament, there’s a ring of 6 actin filament around them. This way
every myosin head is able to reach out and touch an actin.
• At the Z-disc, the 2 major proteins you’ll find are:
o alpha-actinin (the yellow protein), which anchors the thin filaments. It’s tethering
the thin filaments to each other at the Z-disc.
o Telethionin (the blue protein) tethers the elastic protein titin
• At the M-line, there’s a protein called myomesin. Its function is to tether the thick
filaments (myosin) to the M-line.
• So the thick and thin filaments aren’t free-floating, they’re anchored by these different
• What’s happening when muscles contract?
• In 1954, 2 groups of researchers proposed the mechanism to account for what’s
happening to the thick and thin filaments when muscles contract.
• This mechanism is called the sliding filament theory.
• The basic idea is that during a muscle contract, the length of the thin and thick filaments
don’t change. All that changes is the degree to which those filaments overlap.
• At the top you see a relaxed muscle, you have the A-band and I-band. The actin runs
from the Z-disc all the way to the H-zone. When the muscle contracts, the actin filaments
don’t shorten, they just overlap to a greater extent with the thick filaments.
• When the muscle contracts, you see a shortening of the I-band, the H-zone is gone in
the contracted muscle because the actin have been brought together.
• There’s no change in the length of the filaments, all that’s changing is the degree to
which they overlap.
• During muscle contraction, the thick filaments don’t move so the A-band doesn’t change
in length. It’s only the thin filaments that move, they’re pulled in towards the centre of the
• Starting with the state of rigor, where myosin head is bound to the actin. • The first thing that happens is that ATP binds to the ATP pocket on the myosin head.
When ATP binds, it causes a conformational change of the actin binding site such that
actin and myosin detaches because it loses its affinity for actin.
• Next, the ATP is hydrolyzed at that site, and the energy that’s released is used to change
the shape of the neck of myosin into what’s called a “high energy conformation state”.
The change of the shape of the myosin head is called cocking.
• So the energy that’s released from the ATP is used to transform the myosin from a low
energy conformation to a high energy confirmation.
• Because there’s no ATP bound in that ATP pocket and there’s only inorganic phosphate
and ADP, it changes the shape of the actin binding site on the myosin goes back to its
original conformation, and myosin can rebind to the actin.
• The binding of myosin to actin triggers a change in the conformation of the ATP pocket
and induces the release of phosphate. When the phosphate is lost, the head of the
myosin uncocks and goes back to its low energy configuration, and in doing so it
physically moves the actin relative to the myosin, and then ATP is released.
• As the head goes from a high energy conformation back its to low energy conformation,
it takes the filament and pushes it towards the centre of the sarcomere. This is called the
power stroke. ADP is eventually released.
• The binding of myosin to actin favors the release of phosphate through a change in the
conformation of the ATP pocket.
• This is important because if the molecule releases phosphate before it has a chance to
bind to actin and goes to low energy confirmation, it won’t get any work done. The fact
that the release of phosphate only occurs after myosin binds to actin ensures that the
energy from the hydrolyzed ATP is used to do some work.
NOVEMBER 19 RECAP:
• Vertebrate skeletal muscles are striated muscles. The striated arrangement results from
the actin and myosin being arranged in a regular way, which is not the case in smooth
• A-band is the region where there’s myosin. I-band is the absence of the myosin filament
and only actin is present.
• In the centre of the I-band is the Z-disc which is the boundary of the sarcomere. Z-disc is
made of protein like alpha actinin which anchors thin filaments and titin .
• In the H-zone there’s an absence of actin and only myosin is present. • Sliding Filament Theory was done independently by 2 groups of researchers. The idea
was that thin and thick filaments don’t change in length during contraction; they only
change in the extent to which they overlap.
• In the relaxed sarcomere you can see a wide I-band and the H-zone, when it’s
contracted, you lose the H-zone because the actin comes together in the centre. The I-
band become narrower because the thin filaments have moved towards the centre so
there’s less of a region where there’s only actin. The length of the A-band doesn’t
• This shows a better image of how there’s a change from the low energy conformation to
a high energy conformation.
• Let’s assume you have a row boat. All the people are rowing the boat synchronously.
Assume the boat is fixed to a dock by a giant elastic band. It wouldn’t make sense to row
the boat synchronously because when you take the oar out of the water at the same
time the elastic pulls us back to the dock and effort is wasted. Same principle applies to
• All the myosin heads within a thick filament work independently and in a highly
• If you have myosin bound to actin and they’re working synchronously and they cause a
power stroke, they’ll generate lots of force, but the elastic force from titin pushes that
muscle back to its original length.
• So you need to have myosin heads working independently and asynchronously so that
as one myosin detaches, other myosins are still bound to the thin filaments and they
help to maintain the tension whenever tension is generated.
• If all the my