00:01
You can see smooth muscle is named
because it’s non-striated appearance.
00:07
Striations will be
lines that occur.
00:10
And when we went through the skeletal
muscle, you noticed that there were
varying components that you can see
individual lines in the muscle itself.
00:19
Why this occurs is because there are
a number of different processes.
00:24
Smooth muscle has a larger
degree in which it can shorten.
00:28
So a smooth muscle can shorten to a
greater degree than skeletal muscle.
00:34
They also have a large number
of intermediate filaments
that help maintain its
structural integrity.
00:40
They have thick filaments and thin
filaments just like skeletal muscle.
00:46
They also have these dense bodies
and these dense bodies help
maintain that structural relationship
of thick and thin filaments.
00:56
The other thing that’s different is
they are mechanically coupled at
least in the single unit or phasic
smooth muscle via gap junctions.
01:05
It can be seen here.
01:08
Now, let’s talk through the
various ways in which smooth
muscle develop force as
which there are a number.
01:15
The first we’ll talk through
is in this type of graph where we have force
on the Y-axis and time on the X-axis.
01:26
This is one type of force production in which
force production is high almost all the
time until you see a relaxation and then
it returns back to a high amount of force.
01:39
The good example for this are sphincters
in which they are normally contracted
and then relaxed for a very short
period of time and contract again.
01:51
Other types of smooth muscle work almost in
the opposite way in which they are normally
relaxed and then they have bursts of activity
and then they go back to a relaxed state again.
02:03
A good example of that is
urinary bladder tissue
in which it’s relaxed most of the
time until one wants to urinate.
02:12
Other types of smooth muscle have a partially
contracted state almost all the time,
and therefore, they
have a tonic tone.
02:22
They can constrict to a greater degree,
but then will relax only a small amount.
02:28
Good examples of these are
blood vessels, where there
always will be a tonic
amount of vasoconstriction
and then you can still
vasoconstrict more,
but there will be a tonic
kind of contracted state.
02:41
And then finally, we have
very active type of smooth
muscle and where there are
lots of oscillations.
02:48
These oscillations are
oftentimes driven by
such things as slow
waves in the GI system.
02:55
These can be phasically active and
intestinal muscle is a great example
in which you get waves of contraction and
relaxation, contraction and relaxation,
but the relative
force is pretty low.
03:09
So combining all of these
together, you can see
that smooth muscle has a
wide range of functions
and that’s probably why it’s present
in so many different tissues
because it could be
contracted normally,
partially contracted, phasically
active, or normally relaxed.
03:29
How do you get a muscle to contract
in terms of smooth muscle?
There are two primary ways.
03:37
The first way is via causing an
electrical potential across the membrane,
which opens up calcium channels to
cause calcium-induced calcium release.
03:48
So calcium enters in through the cell from
the extracellular fluid into the cell,
which engages a calcium channel
to release more calcium.
03:58
So that’s done via
an electrical mean.
04:02
The other way to do it
is by using a ligand or
a particular substance
to bind to a receptor.
04:08
So good examples here are
things like hormones
and neurotransmitters
that can bind to specific
receptors to cause an
enzymatic second messenger
signal pathway to release
calcium into the cell.
04:23
So let’s go through those two
in a little bit more detail.
04:26
So let’s go through
calcium-induced calcium release.
04:30
Oftentimes, these L-type
calcium channels are
located in small invaginations
in smooth muscle.
04:36
These are located
in close proximity
to calcium-induced calcium
release channels of the SR.
04:43
SR is the sarcoplasmic
reticulum.
04:45
Therefore, once there’s a
membrane potential change,
these calcium channels open to allow
little calcium influx into the cell.
04:55
Once the small amount of
calcium enters the cell,
it activates this
calcium-induced calcium
release channel to spill
out lots of calcium.
05:05
So the calcium that comes
into the cell isn’t really
involved with the contraction
to a great degree.
05:11
The most of the contraction is induced
from what is released from the SR.
05:16
That can be seen here
right below the caveolae.
05:24
Now, a ligand-gated contraction,
seen up here, can be done via again
a hormone or a neurotransmitter
activating let’s say a
G-coupled protein receptor,
and the example that we have here activates
phospholipase C, which is an enzyme,
and that converts a number of
substances, but it creates IP3 and DAG,
and it’s IP3 that then goes to
the SR to bind to an IP3-gated
channel to open up to allow
calcium afflux into the cytosol.
05:58
So both of these two mechanisms
both involve calcium in the
cytosol but how they get it out
is via a different mechanism:
one through calcium-induced
calcium release and the other one
through an enzymatic process
involving phospholipase C and IP3.
06:19
There are a number of
calcium-binding proteins that are
located in smooth muscle that
help regulate its function.
06:26
These smooth muscle-binding proteins will
help activate to cause contraction to occur,
as well as there are calcium-binding
proteins that are there to help
sequester calcium or to bind it
up so it’s not in its free form.
06:42
Graded contractions are a telltale
sign of what smooth muscle does.
06:48
You can have a little calcium release
and get a little contraction
or a lot of calcium release
and get a lot of contraction.
06:55
So it’s all graded by the amount of
calcium release, whether it’s done via
calcium-induced calcium release or
whether it’s done via an IP3 mechanism.