>>Dr. Ketchum: This video will compare and contrast the two types of smooth muscle: single-unit,
also known as visceral smooth muscle, and multi-unit. Multi-unit is in the first column.
Single-unit is in the second column. Single-unit, again, is also called visceral smooth muscle.
So be sure you know single-unit as visceral smooth and use those terms interchangeably.
So let’s take a look at the difference between these two types of smooth muscle. We will
do this by looking at several characteristics. First, multi-unit smooth muscle functions
as discrete functional units. So that means each muscle fiber contracts independently
of all the other fibers. Is that similar to skeletal muscle? The answer is yes. Remember
skeletal muscles have motor neurons that tell this fiber over here to contract, and a motor
neuron over here telling this muscle fiber to contract. So these muscle fibers are contracting
independently of each other. Multi-unit is the same, and is therefore neurogenic. What’s
that mean? Remember to break down the words. The suffix “genic” means producing. So
what is causing or initiating the contraction of the muscle? The neuron. Remember—neurogenic.
The next characteristic to discuss is where in the body would one find multi-unit smooth
muscle? These are the places that you would find multi-unit muscles, some of which we’ve
already talked about. But we weren’t to muscle yet, so it wouldn’t have done any
good for me to tell you what kind of muscle it was. The following smooth muscles are multi-unit:
ciliary muscles. What are ciliary muscles important for? Hopefully you answered accommodation.
The circular and longitudinal muscles of the iris are also multi-unit. What are the circular
and radial muscles in the iris important for? The pupillary reflex. The smooth muscle at
the base of the hair follicle is also multi-unit smooth. What is it when your muscles contract
and the hair stands up—what do you call that? Those are goose bumps, right? So goose
bumps are caused by multi-unit smooth muscle. Okay, some of the smaller airways of the lungs
have multi-unit smooth muscle, and then the walls of your larger blood vessels—which
we haven’t talked about those quite yet—are also types of multi-unit smooth muscle. The
next characteristic for multi-unit smooth muscle is that these smooth muscle cells are
not self-excitable. This should make sense because they are neurogenic; they rely on
a neuron to tell them to contract. So these cells are not self-excitable, and since these
cells are not self-excitable, they have no gap junctions. This is the last characteristic
for multi-unit smooth muscle.
All right, so now let’s compare those characteristics to single-unit smooth muscle, which remember,
is also called visceral smooth muscle. So single-unit smooth muscle becomes excited
and contracts as a single-unit. This is called a functional syncytium. This is where the
whole muscle works as one unit. Rather than independent fibers contracting, the whole
muscle contracts. Let me give you an example. All right, let’s talk about the single-unit
smooth muscle of the uterus and childbirth. During birth, is it a good idea to have the
smooth muscle of the uterus function as one huge unit or for each fiber to independently
be stimulated while poor mom sits there waiting for those independent contractions? That will
not make mom happy. The single-unit smooth muscle of the uterus is an example of a functional
syncytium because all of those muscle cells together will function as one, and therefore
one huge contraction can occur all at once. So it makes it easier on mom to give birth.
This means that single-unit smooth muscle is myogenic. Which means what? Remember to
break down the word. “Genic” means producing and “myo” is muscle, therefore, it’s
the muscle cells that initiate contraction. And let’s say it’s this muscle cell here
that initiates contraction. How does this next cell receive that message? Through gap
junctions, that’s correct. See how the two go hand in hand? If the muscle is myogenic,
it will have gap junctions. If it is neurogenic, there is no need for gap junctions. So where
in the body would one find single-unit smooth muscle? Walls of your hollow organs. So I
gave you an example of the uterus, therefore the reproductive tract has single-unit smooth
muscle, but also the digestive tract and the urinary bladder are good examples.
In single-unit smooth muscle there are also these pacemaker potentials and slow-wave potentials.
I’m just going to briefly introduce you to them here, as we will discuss them in detail
later. Let’s look at the pacemaker potential first. A pacemaker potential is a type of
graded potential. So when you look at this pacemaker potential, does this cell stay at
rest for very long? No, it is only at rest momentarily. As soon as the membrane potential
comes back to rest, another action potential fires, but it has to come back down to rest
in order to fire another action potential. This entire graph we are going to call pacemaker
activity. We’ll talk about why those are important and the mechanisms involved at a
later time. Now let’s look at a slow-wave potential. These are also types of graded
potentials. These are important in the gut; they are important in digestion. Let’s look
at this graph of a slow-wave potential. So in a slow-wave potential, does the cell always
reach threshold? No. When the cell does reach threshold, however, does the cell fire only
one action potential or several action potentials? The cell fires several action potentials.
Furthermore, the cell is only at rest momentarily, and if the cell does not reach threshold (which
a lot of times it doesn’t) an action potential will not occur. If the cell does reach threshold,
multiple action potentials will occur. This is very different than a pacemaker potential.
We will come back to pacemaker potentials when we study the heart and slow-wave potentials
when we study the regulation of digestion.