You can see smooth muscle is named
because it’s non-striated appearance.
Striations will be
lines that occur.
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.
Why this occurs is because there are
a number of different processes.
Smooth muscle has a larger
degree in which it can shorten.
So a smooth muscle can shorten to a
greater degree than skeletal muscle.
They also have a large number
of intermediate filaments
that help maintain its
They have thick filaments and thin
filaments just like skeletal muscle.
They also have these dense bodies
and these dense bodies help
maintain that structural relationship
of thick and thin filaments.
The other thing that’s different is
they are mechanically coupled at
least in the single unit or phasic
smooth muscle via gap junctions.
It can be seen here.
Now, let’s talk through the
various ways in which smooth
muscle develop force as
which there are a number.
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.
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.
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.
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.
A good example of that is
urinary bladder tissue
in which it’s relaxed most of the
time until one wants to urinate.
Other types of smooth muscle have a partially
contracted state almost all the time,
and therefore, they
have a tonic tone.
They can constrict to a greater degree,
but then will relax only a small amount.
Good examples of these are
blood vessels, where there
always will be a tonic
amount of vasoconstriction
and then you can still
but there will be a tonic
kind of contracted state.
And then finally, we have
very active type of smooth
muscle and where there are
lots of oscillations.
These oscillations are
oftentimes driven by
such things as slow
waves in the GI system.
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.
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
partially contracted, phasically
active, or normally relaxed.
How do you get a muscle to contract
in terms of smooth muscle?
There are two primary ways.
The first way is via causing an
electrical potential across the membrane,
which opens up calcium channels to
cause calcium-induced calcium release.
So calcium enters in through the cell from
the extracellular fluid into the cell,
which engages a calcium channel
to release more calcium.
So that’s done via
an electrical mean.
The other way to do it
is by using a ligand or
a particular substance
to bind to a receptor.
So good examples here are
things like hormones
that can bind to specific
receptors to cause an
enzymatic second messenger
signal pathway to release
calcium into the cell.
So let’s go through those two
in a little bit more detail.
So let’s go through
calcium-induced calcium release.
Oftentimes, these L-type
calcium channels are
located in small invaginations
in smooth muscle.
These are located
in close proximity
to calcium-induced calcium
release channels of the SR.
SR is the sarcoplasmic
Therefore, once there’s a
membrane potential change,
these calcium channels open to allow
little calcium influx into the cell.
Once the small amount of
calcium enters the cell,
it activates this
release channel to spill
out lots of calcium.
So the calcium that comes
into the cell isn’t really
involved with the contraction
to a great degree.
The most of the contraction is induced
from what is released from the SR.
That can be seen here
right below the caveolae.
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.
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.
There are a number of
calcium-binding proteins that are
located in smooth muscle that
help regulate its function.
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.
Graded contractions are a telltale
sign of what smooth muscle does.
You can have a little calcium release
and get a little contraction
or a lot of calcium release
and get a lot of contraction.
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.