Dual Innervation

00:01 Now a lot of our organs have dual innervation. Let's talk about dual innervation. And we will use two examples.

00:09 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.

01:53 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.

02:13 In the daytime you're relaxed, you're sitting by the campfire, you're looking at insects, so you need fine vision.

02:19 In terms of the heart, you have parasympathetic activity in the heart as well. That will slow down the heart rate.

02:26 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.