Hey it’s Professor Dave, let’s talk about
muscles. When you hear the word muscle, what comes to mind? You might think about the muscles in your
legs that help you to run fast, or maybe body-builders working on their biceps in the gym. These muscles belong to a particular type
of muscle called skeletal muscle, but we actually have three different types of muscles in our
bodies, and the other two are not as obvious. In addition to skeletal muscle, humans also
have cardiac muscle, and smooth muscle, so let’s discuss all three of these types right now. First, what these different types of muscle
have in common is that they are all vital for movement. Muscle tissue is predominately comprised of
muscle cells, and these are highly specialized for a process called contraction, which is
what allows for mobility. The three types of muscles are quite different
in their functions, but there are some similarities in their structure, so let’s go through
them one at a time. Let’s start with skeletal muscle, as it’s
the most abundant. These are the muscles that cover our skeleton
and give our body its shape. The exact number of skeletal muscles in the
body is not precisely known, but it’s thought to be over six hundred and fifty. These attach to bones either directly or via
connective tissue called tendons, and they are responsible for a variety of functions. Skeletal muscles produce voluntary movements,
which are the ones that are under our control. This includes simple movements like extending
limbs, or more coordinated movements like running or swimming, and they achieve this
by pulling on the bones of the skeleton. Our skeletal muscles also maintain our posture. When you’re sitting down, it might feel
as though you’re totally relaxed, but in actuality, many of these muscles are constantly
working by making small adjustments to maintain your precise body position. Skeletal muscle even forms sphincters in the
digestive and urinary tracts to allow for the control of actions such as swallowing
and urination. Beyond these functions, they help us maintain
optimal body temperature by releasing heat as a byproduct of muscle contraction. So what makes up skeletal muscle? We will see that skeletal muscle is different
from the cardiac and smooth muscles that will be examined later, in that skeletal muscle
cells are multinucleated. This means they have multiple nuclei. Cells called myoblasts, which have one nucleus
each, fuse together to form a long cylindrical multinucleated cell called a muscle fiber,
with its nuclei located at the periphery, and this is surrounded by a sheath of connective
tissue called the endomysium. Inside each muscle fiber are many myofibrils,
and these contain myofilaments, which are themselves organized into units called sarcomeres. These are the contractile machinery which
actively shorten, and are therefore responsible for skeletal muscle contraction. It is also these sarcomeres that give skeletal
muscle a striated or striped appearance, and they are the smallest functional unit in the
muscle. The myofilaments within are of two types,
thinner actin filaments and thicker myosin filaments, and the sliding of these filaments
past one another is what produces contraction, which we will discuss in more detail later. So that’s what a muscle fiber is made up of. Now zooming out from there, each muscle fiber
is arranged into a bundle of muscle cells called a fascicle, which is surrounded by
a layer of fibrous connective tissue called the perimysium, and multiple muscle fascicles
will in turn group together to form an even larger structure, which is surrounded by a
layer of dense irregular connective tissue called the epimysium. This group of fascicles, along with blood
vessels, nerves, and all the tissue, comprise an entire skeletal muscle, and these will
span joints and attach to bones in at least two places. Attachments can be direct, where the epimyseum
of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage, or
it can be indirect, where the muscle’s connective tissue extends beyond the muscle as a ropelike
tendon, or a sheetlike aponeurosis, which anchors to a skeletal element, or other muscles. For a skeletal muscle to be able to contract,
it needs to be activated or innervated by neurons in the central nervous system, and
in particular the somatic nervous system, which essentially means the voluntary nervous system. We will learn about the nervous system a little
bit later, for now, we can just understand that skeletal muscle is the only muscle type
under voluntary control, which means these are the muscles that move when we decide to
move them. Now let’s move onto cardiac muscle. Cardiac means “relating to the heart”,
so as one might guess, cardiac muscle is only found in the heart, and cardiac muscle cells
are called cardiomyocytes. There is just one role for cardiac muscle,
and that is to contract in order to push blood out of the heart to the rest of the body,
which will supply tissues around the body with vital oxygen and nutrients. Cardiac muscle is similar to skeletal muscle
in the sense that they also have a striated appearance, which is due to being comprised
of myofibrils and sarcomeres for contractility. However, cardiac muscle is different from
skeletal muscle in other ways. Cardiac muscle cells are short, branched cells,
which is a structure that allows them to communicate with other cells, and they typically have
one or maybe two centrally located nuclei. In between each cardiomyocyte there is an
intercalated disc, which contains gap junctions. These gap junctions allow the electrical stimulus
that is required for contraction to rapidly spread across the cardiac tissue, by allowing
ions to move from one cell to the next. This means that the cardiomyocytes can contract
in a coordinated fashion, allowing the heart to work as an efficient pump. Another difference from skeletal muscle is
that we do not voluntarily contract the muscles in our heart, meaning that we don’t consciously
think about our heartbeat, it just happens automatically. This means that our cardiac muscle is innervated
by the autonomic nervous system, which regulates the speed at which electrical stimuli are
generated by specialized cardiac muscle cells called pacemaker cells. More on the autonomic nervous system later. Lastly, let’s take a look at the third type
of muscle tissue, known as smooth muscle. Smooth muscle is found in most of our organs,
and it contracts to regulate many of our bodily functions as we go about our daily business. Smooth muscle in blood vessel walls will contract
or relax to regulate blood pressure and the distribution of blood to our different organs. It is present in our airways to allow more
or less air to pass through. It contracts in the walls of our digestive
tract to push material along. The smooth muscle in our bladder contracts
to expel urine out of the body, and it has numerous functions in the reproductive system,
including contraction of the walls of the uterus to push a baby out of its mother during childbirth. Smooth muscle is typically organized into
two layers. First, a longitudinal layer, where muscle
fibers run parallel to the long axis of the organ, such that contraction will dilate and
shorten the organ. Then there is a circular layer, where muscle
fibers run around the circumference of the organ, such that contraction will constrict
and lengthen the organ. When these layers alternate contraction, it
can push material through the organ, a process called peristalsis. Similarly to cardiac muscle, smooth muscle
is innervated by the autonomic nervous system, so it contracts and relaxes automatically. Smooth muscle cells also have a single centrally
located nucleus, but they have a different shape than cardiomyocytes. The cells are short but spindle-shaped, which
is referred to as fusiform. Interestingly, smooth muscle differs from
skeletal muscle and cardiac muscle because it is not striated, due to the lack of myofibrils
and sarcomeres. Their myofilaments are scattered throughout
the cytoplasm of the muscle cells instead. So with that, we now know a bit about the
three different types of muscle tissue, those being skeletal muscle, cardiac muscle, and
smooth muscle. We can clearly see how these three types are
quite different in terms of functionality, but remarkably, they are all extremely similar
in structure. Now that we have a decent grasp on the structure
of a muscle, let’s talk about muscle contraction on the molecular level, so that we can better
understand this incredible process.
