Histology: Bone, Muscle, Nerve

Histology: Bone, Muscle, Nerve


[MUSIC PLAYING] Hello, and welcome to this video lecture on
bone, muscle, and nerves, where we will explore both the macro and micro structures of these
three main categories within histology. My name is Christopher Demas and I’ll be guiding
you through these topics. [Music] Throughout your histology course so far, you’ve
been learning about each of these structures individually. So, therefore, I wanted to take a moment to
step back and appreciate them together. Throughout this lecture, I’m going to show
the similar layered structures of bone, muscle, and nerves. These layers are often separated by connective
tissue that has similar naming schemes for these structures. In addition, this lecture should give you
a better understanding of how the micro structures in functional units come together to form
the macro structures you are observing in the anatomy lab. We are going to take a closer look at the
structure of the femur, a muscle in the leg known as the adductor longus, and the sciatic
nerve that runs along the posterior of your leg. These structures have been chosen purely for
composition, and can serve as a model for most muscle nerves and long bones within the
human body. I’ll start off by drawing the structure of
the long bone in the upper leg, the femur. There are two types of bone that I would like
to point out — compact bone, which is seen as a dense white rind around the marrow space,
and spongy bone, which is the more delicate inner structure in the marrow space. Compact bone makes up approximately 80% of
the weight to the bone, and provides the strength. The spongy reduces the weight the bone, and
provides a space for hematopoiesis, or blood cell maturation. Now, let’s focus on the structural unit of
compact bone, known as an osteon, or haversian system. As we zoom in and focus on a single osteon,
you’ll notice that there are a number of concentric cylinders that the osteon is organized into. Looking a little closer, notice that at the
center of each of these osteons is a canal, known as a haversian canal. Within these canals, there are blood vessels
and nerves. So I’m going to go ahead and label in the
vein, the nerve in the center, and the artery. Notice that embedded in the osteons are a
cell type known as osteocytes. The osteocytes are mature osteopaths that
have become embedded in the bone. Before we move on, let us not forget the connective
tissue layers associated with bone. First, there is a connective tissue layer
lining the medulary cavity, and within the haversian canal, known as endosteum. Also, there is a layer of connective tissue
on the outside of bone known as pariosteum which forms the outermost perimeter of the
bone. Endosteum is actually debated whether or not
it’s found within the haversian canal. So make sure you’re familiar that it’s located
within the medulary canal. Those are the two layers that you should be
familiar with for bone. One additional piece of information that I’d
like to add is that there are also perpendicular canals running through the compact bone. And this is called Volksman’s canals, which
is perpendicular, of course, to the haversian canals. Now, let us move on to muscle. And, in particular, we’re going to focus on
skeletal muscle. Just like bones, skeleton muscle is also composed
of a number of layers. I’ll start by drawing in the main part of
the muscle belly. Each whole muscle is held together by connective
tissue layer known as epimysium. Within each skeletal muscle are a number of
bundled muscle cells known as fascicles. I’ve drawn a fascicle here in an exploded
view. Notice that each deep red colored cylinder
represents one cell within the fascicle. Each fascicle is wrapped in perimysium. Looking within each fascicle are a number
of muscle fiber cells. Each cell has its own outer coating known
as the endomysium. Within each skeletal muscle cell are multiple
nuclei that you can see along the periphery. And I’m going to draw that in purple. This is actually specific to skeletal muscles
only, as cardiac muscle and smooth muscle are mononuclear. Each muscle fiber cell contains a number of
structural units called myofibrils, which are responsible for the muscle contractions,
which I have drawn in an exploded view here. The myofibril is composed of the microfilaments
myosin and actin that ultimately participate in organized contractions. The functional unit within the muscle fibril
is called a sarcomere. The details of the sarcomere can only be visualized
under an electron microscope. The repeating nature of the sarcomere creates
a pattern of alternating light and dark bands that we can actually see with a regular light
microscope. The darker bands are A bands. And those are where the thicker myosin filaments
are located. The lighter bands are the I bands, and are
the areas where no myosin is located. The Z bands, or dis, delineate the borders
of each individual sarcomere. But they are quite thin, and can only be seen
on electron microscopy. Lastly, let’s take a closer look at the nerve
and its structure. The larger bundles are often situated between
the bones and the muscles. However, it’s important to recognize that
muscles and nerves must work together. And these systems are ultimately integrated. For the purpose of this video, we will be
focusing specifically on the structure of the nerve bundle, like the one we see here. The outermost layer of connective tissue surrounding
the large nerve bundles is called the epineurium. Within this structure, in this case, is four
nerve fascicles, each of which are encompassed by perineurium. Within this perineurium is a vast number of
neuron fibers that are surrounded by another connective tissue layer called endoneurium. Within the endoneurium is the mylon-coated
axons. This should be sufficient level of detail
for now, as we will be discussing nerves more in a future lecture. Now that we have finished drawing this segment,
let’s take a moment to put everything together. There are three connective tissue and casements
within the muscle and nerve that follow the same prefix naming scheme of largest to smallest,
epi, peri, and endo. Bone has a peri and endo layer, but lacks
the epi layer. Make sure you are familiar with the name of
the muscle structures and the connective tissue wrappings. For example, that a muscle cell contains a
number of myofibrals, and nuclei wrapped in the endomysium. A fascicle consists of a bundle of muscle
cells surrounded by perimysium. And a muscle belly is made of a collection
of fascicles surrounded by epimysium. Also remember that muscle, bone, and nerves
all work together, and are integrated structures, even though they have been drawn separately
in this segment. Thanks so much for watching. And I hope you found this to be helpful.

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