>>Dr. Ketchum: The purpose of this video is to provide an overview of the items that you
will need to be able to compare and contrast for smooth and cardiac muscle. Let’s begin
with what is called striated muscle versus smooth muscle. These are micrographs taken
from a compound microscope. Notice the lines running through this skeletal muscle fiber?
Those are what we call striations. Hence, skeletal muscle is striated. In cardiac muscle
you can also see these striations, hence cardiac muscle is also striated. And finally, here
is smooth muscle. There are no lines running through it, meaning no striations. Hence smooth
muscle is not striated, and that’s why we call it smooth. The next item for comparison
is whether or not the muscle is myogenic or neurogenic. Remember, break the word down.
The suffix “genic” means producing, “myo” means muscle and “neuro” means neuron.
Now ask yourself—how is the contraction initiated? In other words, what causes the
muscle to contract? Is it a neuron, or is it the muscle itself? So let’s take a closer
look.
Let’s first discuss neurogenic muscles; that’s the picture on your left. So here’s
the motor neuron coming down to the muscle fiber, and it tells the skeletal muscle to
contract. So this muscle is neurogenic; there’s a neuron telling the muscle to contract. The
neuron is what initiates the muscle contraction, so we know that skeletal muscle is neurogenic.
There is also a type of smooth muscle called multi-unit smooth muscle that is neurogenic.
So once again, neurogenic means the neuron stimulates the muscle to contract. Okay, now
let’s look at the image on your right. This image illustrates a myogenic muscle. “Myo”
means what? Muscle, right? What you have here are muscle cells. If this muscle cell spontaneously
depolarizes, that spreads to additional muscle fibers, and the end result is contraction
of the muscle. So there is no neuron telling that muscle to contract; the cell spontaneously
does it. In one of your laboratory exercises, you will hear your TA say that your frog is
brain dead—but the heart still contracts. So is there a neuron telling the heart muscles
to contract? No! The frog is brain dead. So this means that cardiac muscle is myogenic.
There is also another type of myogenic muscle called single-unit smooth muscle. If you have
one muscle cell that spontaneously depolarizes—so let’s take this muscle cell here—we’ll
mark it with a red “x.” If this cell is going to depolarize, we want that depolarization
to spread into the adjacent muscle fiber. How is that depolarization going to spread
to that muscle fiber? What does it have to spread through? What does the cell require?
Gap junctions. These cells have to have gap junctions. In cardiac muscle, this gap junction
is part of this intercalated disc. The other part of the intercalated disc is the desmosome.
So is a gap junction similar to a drinking straw? Yes it is. So depolarization is spreading
to the next muscle cell through the gap junction. And why does cardiac muscle also have these
desmosomes? What’s their function? It allows the muscle to stretch without tearing. So
when you think about an intercalated disc, an intercalated disc contains both gap junctions
and desmosomes. Okay, but it is the gap junction that the depolarization is spreading through.
So when the depolarization spreads from this muscle cell to this muscle cell, and then
spreads to this muscle cell and to another, this allows the muscles to contract all at
once. This is called a functional syncytium.