Video Transcript
In this video, we will learn how to
describe the structure of muscles by fast exploring the different types of muscle
before looking at skeletal muscles in more detail on both a macroscopic and
microscopic scale. We will learn how skeletal muscle
fibers are specialized for their function of providing movement to various parts of
the human body.
Each adult human body contains
around 650 muscles, making up about half of your total body weight. Muscles are structures in the body
that aid movement by contracting and relaxing. They help food move through our
digestive system after a meal, allow our legs and arms to move when running a race,
and make the pupils in our eyes become smaller when we look into a bright light. Without the ceaseless action of our
muscles, even our heart would stop beating. There are three main different
types of muscle which carry out these different functions: skeletal muscle, which is
sometimes called striated muscle, smooth muscle, and cardiac muscle.
While skeletal muscles are under
our conscious control and so are called voluntary muscles, smooth muscle and cardiac
muscle are involuntary muscles, so they’re controlled subconsciously and we cannot
decide to simply stop them from working. Some examples of voluntary skeletal
muscles can be found in our limbs, such as to allow our arms and legs to move. They are called skeletal muscles as
they are attached to the bones of our skeleton by tendons. This image displays what some
magnified skeletal muscle cells might look like. They are sometimes called striated
muscles due to their stripy appearance.
Let’s keep a checklist of the key
features that we’ve covered so far. Some skeletal muscles are described
as antagonistic as they work in a pair where when one muscle contracts, the other
relaxes, allowing a coordinated movement, such as in the arm. For example, the bicep and tricep
are antagonistic muscles in the upper arm. Here you can see a contracted bicep
and a relaxed tricep when the arm is bent. However, when the arm is stretched
out, the bicep relaxes and the tricep contracts. This allows the arm to move. In addition to helping our body
with movement, skeletal muscles also help us to maintain our posture.
If we were to look at the
individual cells in smooth muscle, we would see that they are nonstriated or not
stripy. Smooth muscle is sometimes more
generally known as involuntary muscle as it is not under conscious control as
skeletal muscle is. Smooth muscle is found in many
different organs. For example, there are smooth
muscle in the walls of hollow organs, like the esophagus pictured here, also in the
stomach and in the intestines to help food move through the digestive system. There’s also smooth muscle in
artery walls. In both the arteries and the
digestive system, the function of smooth muscle is to apply pressure to these organs
to help push substances through them, like blood or food, continuously without us
having to think about it.
Cardiac muscle, some of the cells
of which you can see in this image, are also involuntary but are only found in the
heart. And like skeletal muscle cells, you
can see that cardiac muscle cells also appear stripy or striated. The cells of cardiac muscle are
described as myogenic, which means that the impulse originates from the heart and
not externally like with voluntary muscles. This allows our heart to beat
continuously and tirelessly in a regular rhythm to pump blood around our body.
Let’s take a closer look at the
macroscopic structure of skeletal muscle and how all of its different components
function. The world macroscopic refers to
structures which are visible to the naked eye without the need to use a
microscope. Each muscle is considered an
individual organ, and each of these muscles contains different tissues, such as the
skeletal muscle tissue itself, nervous tissue, mostly consisting of motor neurons,
blood tissues, connective tissues like tendons which attach the muscle to bones. But let’s get rid of some of these
other tissues for now and focus on the muscle tissue itself.
Skeletal muscle consists of many
bundles of muscle fibers, which is sometimes called fascicles. These muscle fibers contain a
collection of tissues, cells, and organelles. Depending on its size, one muscle
may be made up of up to thousands of individual muscle fibers. Each bundle of fibers is surrounded
by a protective layer of connective tissue called the perimysium. The prefix peri- like perimeter
means that it’s surrounding something, while the “my” in the middle of the word
refers to muscle. The perimysium helps the cells to
withstand the pressure of muscle contraction. This connective tissue layer also
provides a place for the blood and nervous tissue to connect to the individual
muscle fibers.
Blood brings oxygen, glucose, and
other nutrients to the muscle cells to allow them to respire, release energy, grow,
and repair themselves. Although they might not actually be
visible without a microscope, motor neurons are nerve cells that carry out the
important function of stimulating muscle contraction. Each skeletal muscle fiber is one
very long cylindrical muscle cell, which is enclosed within a plasma membrane called
the sarcolemma. The prefix sarco- comes from the
Greek word for flesh, which is often used to describe the components of muscles. The suffix -lemma comes from the
Greek word for sheath as it forms a protective membrane around each fiber. The Sarcolemma is sometimes called
the myolemma, which contains the prefix myo-. And you may recall this refers to
the muscles.
Let’s look at the structure of the
muscle fiber on a microscopic scale. Muscle fibers have several
adaptations that make them effective for their function. We can already see the sarcolemma,
which we explored earlier, that forms a surrounding membrane all around the muscle
fiber. But don’t be concerned about the
complex-looking structures elsewhere as we’ll go through them all one by one.
Muscle fibers are part of one of
which you can see here are much longer than other cells. This is because they formed by many
individual muscle cells fusing together when you were only an embryo. This makes the muscles strong as
any junctions between cells add a point of weakness. So having long cells reduces the
number of weak points. This is also why a suit of armor is
strongest and most effective when it’s formed from one continuous sheet of metal as
every junction adds a weak point. As they’re formed from many cells,
one muscle fiber typically has many nuclei.
The cytoplasm within a muscle fiber
is called the sarcoplasm. In most animal cells, the main role
of the endoplasmic reticulum is a site of protein synthesis, modification, and
transport. Muscle fibers contain a specialized
endoplasmic reticulum called the sarcoplasmic reticulum which extends throughout the
muscle fiber. The sarcoplasmic reticulum of a
skeletal muscle fiber contains calcium ions which are needed to initiate muscle
contraction. Muscle cells require a large amount
of energy when they contract. So, they also contain many
mitochondria, which you may recall are the site of cellular respiration to release
the energy that’s needed for muscular contraction.
Parts of the sarcolemma surrounding
the muscle fiber fold inwards, which form structures called transverse or “T-”
tubules. This means that an impulse arriving
from the motor neuron can spread along the whole muscle fiber sarcoplasm so that all
the cells in the muscles can contract simultaneously. Each muscle fiber contains long
cylindrical organelles called myofibrils, which have been labeled here in pink. Myofibrils are made up of protein
fibers. There can be between 1000 and 2000
myofibrils in just one muscle fiber, which are arranged in parallel to each other
and to the muscle fiber along its interior. Myofibrils are specialized for
contraction.
You can think of the structure of
muscles like a rope. Ropes are made of individual
strings, much like muscles are made of muscle fiber bundles. And each of those strings is made
up of multiple threads, much like muscle fiber bundles are each made up of multiple
muscle fibers. The individual strands that make up
each thread can be thought of as myofibrils, which together provide the muscle with
their combined strength. Myofibrils are made up of many
repeating functional units called sarcomeres, which we can see as we’ve removed the
sarcoplasmic reticulum from this one region of the myofibril.
Let’s take a closer look at one
sarcomere so we can see how its different parts help the muscle to contract. The length of a single sarcomere is
marked as the distance between two Z lines. And this distance shortens, as does
the sarcomere as a whole, when the muscle contracts. Myofibrils have repeating patterns
of these sarcomeres, which are made up of two protein myofilaments; one of which is
called actin, shown here in red, and the other is myosin, which is shown here in
blue.
Actin is the thinner filament which
is made up of two strands of protein twisted together. Myosin is thicker than actin and
therefore appears darker in color. It’s a long rod-shaped fiber with
globular heads that project outwards. Myofibrils have alternating bands
which appear lighter and darker due to the composition of actin and myosin within
them in each sarcomere. This makes them look stripy or
striated.
Let’s simplify this diagram a bit
so you can see the different regions of the sarcomere more clearly. The I band is also known as the
isotropic band. This word means optically clear and
regular because it’s made of straight thin filaments of actin only, which are
represented here in red. If you forget which filament is
which, just check back in this key. As they only contain thin actin
filaments, the I bands appear considerably lighter than the rest of the
sarcomere. And for this reason, they’re
sometimes called the light bands.
Within the I band is a line that
marks the end of the sarcomere, which is called the Z line, so named for the German
word “zwischen,” which means between. The Z line always appears slightly
darker in micrograph images. As though it has been represented
here as a straight line, it actually consists of a high concentration of zigzagging
actin filaments and other proteins. Remember that each two adjacent Z
lines, which you might remember as Z for zigzagging, mark the end of the
sarcomere.
The A band is also known as the
anisotropic band, which means that it’s optically opaque because it contains both
types of filaments and the bulky globular heads of myosin. As the A bands contain these
thicker myosin filaments, they appear considerably darker in micrograph images than
the other bands, which is why they’re sometimes called the dark bands. The outer edges of the A band are
darkest as these are the regions where actin and myosin overlap.
The inner edges of the A band,
called the H zone, are not quite so dark as they only contain myosin filaments. The middle of the H zone is called
the M band. And you can remember this as M for
middle. There’s also a way to remember the
H band versus the I band. The letter H is wider than the
letter I. So, the H band is the one made of
the thick myosin filaments, while the I band is made of the thin actin filaments
only.
Now that we know a bit more about
the structure of the sarcomere, let’s see what happens when it contracts. In a contracted sarcomere, you can
see that the distance between the two Z lines has decreased. So, the length of the sarcomere has
also decreased. This is because the actin filaments
have been pulled by the myosin filaments closer towards the M line in the middle of
the sarcomere. And this also means that the length
of the H zone which only contains myosin has decreased. The decreasing length of the H
zone, which only contains myosin filaments, is what causes the whole sarcomere to
shorten. Let’s see how much we’ve learned
about the structure of muscles by having a go at a couple of practice questions.
There are three major types of
muscle tissue in the human body: skeletal, cardiac, and smooth. Which type of muscle is primarily
involved in conscious movements of the body?
The question asks us to identify
which type of muscle is involved in conscious movements. While some muscles are under
conscious control and so are called voluntary muscles, other muscles are controlled
subconsciously, so they’re involuntary. We cannot simply decide to stop
involuntary muscles from working. Smooth muscle is a type of
involuntary muscle involved in lots of different organ systems. For example, smooth muscles are
found in the walls of hollow organs like our stomach and intestines to help move
food through the digestive system. Smooth muscle is also found in the
walls of arteries and contracts to help push blood through these blood vessels
continuously.
Cardiac muscle cells are only found
in the heart. And like smooth muscles, these
muscles are also involuntary. They need to be controlled
subconsciously to ensure that the heart continuously beats in a consistent rhythm to
pump blood around the body. Voluntary muscles are skeletal
muscles such as those in our limbs which are our arms and legs, and they are those
which are attached to bones and can be controlled voluntarily to allow coordinated
movement of certain parts of the body. In addition to helping us to move,
skeletal muscles also help us to maintain our bodies’ posture. As they are the only type of
voluntary muscles, the type of muscle primarily involved in conscious movements of
the body is skeletal muscle.
Let’s have a go at another practice
question together.
The diagram provided shows the
basic structure of the sarcomere. Which letter indicates the I
band?
As we can see in this image of a
sarcomere, it is made up of two main filaments: actin myofilaments, which are shown
in red, and myosin myofilaments, which are shown in blue. Actin is a thinner filament made up
of two strands of protein fibers twisted together. Myosin is a thicker filament and is
a long rod-shaped fiber with globular heads that project outwards. The different letters in this
diagram show us the different regions of the sarcomere. So, let’s go through each one by
one to find out which is labeling the I band.
The region labeled V right in the
middle of the sarcomere is called the M line. The M line is found within a region
called the H zone, which has been labeled here with the letter X. The H zone contains myosin
filaments only. When the sarcomere contracts, the
actin filaments will be pulled closer towards the M line and the H zone will
shorten. The region labeled with a Z is
called the A band. The A band encompasses the H zone,
but it also contains regions around its outer edges where actin and myosin overlap,
which makes these regions appear slightly darker.
The length of a single Sarcomere is
measured as the distance between two Z lines, which on this diagram were indicated
by the letter W. As the only letter remaining is Y,
we can tell that this is labeling the I band. The I band is a region on the
sarcomere that only contains actin filaments, which makes it appear lighter in color
and is the reason why it’s sometimes called the light band comparatively to the
other regions of the sarcomere which will all contain some myosin filaments and will
therefore appear darker. Therefore, the letter indicating
the I band is Y.
Let’s review the key points that
we’ve covered in this video. Muscles are structures in the body
that aid movement by contracting and relaxing. Muscles can be voluntary skeletal
muscles or involuntary smooth or cardiac muscles. A single muscle consists of many
muscle fiber bundles which are surrounded by a protective layer called the
perimysium. Muscle fibers within these bundles
are specialized and elongated cells with many mitochondria, a sarcoplasmic reticulum
to store calcium ions, and myofibril organelles, which are specialized for muscular
contraction. Myofibrils contain repeating units
called sarcomeres, which in turn contain actin and myosin protein filaments.
