Now a lot of our organs have dual innervation. Let's talk
about dual innervation. And we will use two examples.
The first is the iris and the second is the sinoatrial node
in the heart. Now in both cases, there is sympathetic
activation through either alpha or beta activity. In the eye,
the sympathetic system causes a contraction of the radial
muscle and dilation of the iris. Now, it's hard to remember
that. So let's think of it this way. The sympathetic system
is the system that we use to run away from the sabertooth
tiger as a cavemen. So you need it to be able to see far
and you need it to be able to see in the dark. So what we
want it to do with our eyes is we want it to open up the iris
so that we can see really really well. So it makes sense that
the radial muscle which is the spoke like muscle on the outside
of the eye will contract, opening up the iris and it also
relaxes the ciliary muscle which is the constrictor muscle
of the eye. With respect to the heart, we want a fast heart
rate because we are going to be running. So it makes sense
that we're going to accelerate the sinoatrial node through
the beta 1 receptor. We also will increase the activity
of ectopic pacemakers. Now why would we want that from an
evolutionary point of view. That's because in case the SA
node fails we have other ectopic pacemakers that would pick
up the job. The opposite of sympathetic activity is
parasympathetic activity. Now in the eye we act through
the M receptors or muscarinic receptors causing circular
muscle contraction. It also results in iris constriction.
So we contract that ciliary muscle to get a smaller pupil.
Now why is that useful? It's because, imagine you are a
cavemen or a cavewomen sitting around the fire and you
want to watch ants build a house, you're going to be able
to focus on small small features and it's bright day out
and you are nice and relaxed. So in the night time, you're
scared, you're afraid, your eyes have to open up.
In the daytime you're relaxed, you're sitting by the campfire,
you're looking at insects, so you need fine vision.
In terms of the heart, you have parasympathetic activity in
the heart as well. That will slow down the heart rate.
It does'nt really affect the ectopic pacemakers at all but it
does cause a calming effect overall in the heart. Okay, in both
cases the dominant resting tone is parasympathetic. So if you
block all nervous input to the eye or to the heart,
you're going to result in pupil dilation and tachycardia
because the resting tone is always the vagal tone or the
parasympathetic tone. Okay, let's have a look at how this
different neurotransmitters are made. You have the cholinergic
terminal button and the noradrenergic terminal button. So
what's a terminal button? Button is French for button
and the end of the nerve looks like a button. That's why we
call it a terminal button. And each case, we manufacture
the neurotransmitter within the terminal button. I have listed
every single possible effector on there, so that you can
look at it at one glance but we're just
going to focus on one portion at each time.