Which neurotransmitters are found in the ANS?

ANStmits.jpg (29465 bytes) Actually, Figure 11.41 in your textbook.

This figure shows the different neurotransmitters of the ANS.

All preganglionic neurons use the same neurotransmitter

You see, preganglionic neurons have their cell bodies in the spinal cord and project their axons out the ventral horn. Does that remind you of something? That's just like motor neurons. And, just like motor neurons, preganglionic neurons release ACh at their synapses.

2 different neurotransmitters exist for postganglionic neurons

All parasympathetic postganglionic neurons release ACh onto their target tissue.

Most sympathetic postganglionic neurons release NE (norepinephrine) onto their target tissue.

Why should we care about the different neurotransmitters?

Think about this scenario. You have a patient who is panicky and hypertensive. You need to give your patient a medication that will not increase levels of panic, but only relax that patient. What do you do?

Obviously, you DO have a medication for your patient. Why? Because you can give your patient a medication, a chemical, that blocks the action of the sympathetic nervous system or that simulates the action of the parasympathetic nervous system.

Remember, neurotransmitters are chemicals. Let's consider how these chemicals work on one particular organ, your heart. When postganglionic neurons release NE onto your heart, your heart rate increases. This is the effect that the sympathetic nervous system has on the heart. When postganglionic neurons release ACh onto your heart, by contrast, your heart rate decreases. This is the effect of the parasympathetic nervous system. Now, if you could give a patient with a racing heartbeat a chemical like ACh, or a chemical that blocks NE, that patient's heart rate should slow down.

How come these two chemicals (ACh and NE) can have a different effect on the same tissue? You have to think back to how these chemicals work. All neurotransmitters work by crossing a synapse and interacting with a postsynaptic receptor. Once the neurotransmitter binds to the receptor, that causes some sort of specific postsynaptic effect. The specific effect of the neurotransmitter is due to the exact receptor that it associates with. You learned about this last semester when we covered postsynaptic potentials (the postsynaptic response could be small or large, excitatory or inhibitory).

What neurotransmitter receptors are involved?

Your book has a figure that tmitrec.jpg (22913 bytes) can help us with this... Figure 11.42. Here is that figure (although it is numbered incorrectly):

The first thing that I'd like you to notice about this figure is that no matter whether the CNS neuron is a preganglionic neuron or a somatic motor neuron, it not only releases ACh, but that the ACh it releases interacts with a specific type of ACh receptor, called a nicotinic ACh receptor. It is called nicotinic because this receptor can be artificially activated by nicotine.

In parts A and B, you can also see the receptors for the neurotransmitters released by the postganglionic neurons. In part A, even though the postganglionic neurons use ACh for their neurotransmitter, you see that the receptors for this ACh is different than that found on somatic muscles. In the parasympathetic nervous system, the postganglionic neurons' ACh is received by muscarinic ACh receptors. Nicotine cannot activate these receptors. Other drugs that can activate the nicotinic ACh receptors typically will also NOT activate muscarinic receptors. So, if a patient needed a drug to decrease their heart rate and calm down, for example, a drug that activates muscarinic ACh receptors would only affect the parasympathetic nervous system. The difference between the nicotinic ACh receptors (of the somatic system) and the muscarinic ACh receptors (of the parasympathetic system) is is lucky; if there were no difference, giving someone a drug to increase the effects from a parasympathetic synapse would also cause that person to start jerking around uncontrollably as their somatic muscles contracted!

Nicotinic ACh receptors lead to very short but strong muscle contractions. Muscarinic ACh receptors lead to slower and weaker muscle contractions. Does it make sense to you why the nicotinic ones must be in our voluntary system while the muscarinic one is in our involuntary system?

In part B, you see that the postganglionic neurons of the sympathetic nervous system, which use NE instead of ACh, affect their targets through adrenergic receptors. I don't know why your textbook didn't correct the error in this picture (it was in the last edition, too). NE works through adrenergic receptors, not nicotinic receptors.

As you read through the section of your textbook called "Actions of Autonomic Neurotransmitters" you will read about some familiar-sounding drugs or chemicals. They mention alpha and beta types of adrenergic receptors. You have probably heard of beta-blockers... They mention that monoamine oxidase decreases quantities of NE, and you have probably heard of MAO inhibitors... Also, epinephrine can also affect the sympathetic nervous system, and its common name is adrenaline; I'm sure you've heard of that.

I hope you understand the importance of learning about ANS neurotransmitters and their receptors!