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.