Neurotransmitter anatomy

Voiceover: In this video I want to just introduce some of the anatomy involved in neurotransmitters. Recall that neurotransmitters are molecules that communicate between neurons and their target cells and chemical synapses. And some neurotransmitters are released by neurons distributed widely throughout the nervous system, while others are specific to certain areas. For examples of some neurotransmitters that are released by neurons distributed throughout the nervous system, the first one is glutamate, which is the most common excitatory neurotransmitter of the entire nervous system. Then there's GABA, which is short for gamma-aminobutyric acid, and glycene, and these are the most common inhibitory neurotransmitters of the nervous system, GABA in the brain and glycene in the spinal cord in particular. Neurons releasing these neurotransmitters are widely distributed through the nervous system, and these neurotransmitters are really involved in most functions of the nervous system. Next I want to just introduce some of these areas of the brain that have collections of neurons that send axons diffusely to release specific neurotransmitters onto widespread areas of the cerebral cortex. And often other areas as well, but these widespread projections to the cerebral cortex, let me just represent these by a few axons coming up toward the cerebral cortex, and these widespread projections that are dumping a bunch of a specific kind of neurotransmitter all over your big areas of the cerebral cortex are really important for a bunch of the higher functions of the nervous system, including the cognition, emotion, and consciousness. So let me start introducing a few of these, and I'm going to start with glutamate, because there are specific areas in the reticular formation of the brain stem and parts of the thalamus that project axons diffusely up to the cerebral cortex and release glutamate all over neurons of the cerebral cortex. And this collection of neurons that has this diffuse projection of glutamate to the cerebral cortex, we call this the reticular activating system. And this is actually required for consciousness. Without this system, there usually is no consciousness. The next one I want to mention is acetylcholine. There are certain nuclei in the frontal lobe that actually send diffuse projections up to a number of areas in the cerebral cortex, releasing acetylcholine. These are called the basalis nucleus and the septal nuclei. The next one I want to mention is histamine, and there are a number of neurons in the hypothalamus that send projections to release histamine all over the cerebral cortex. So the hypothalamus is here, and these neurons send these diffuse projections up to the cerebral cortex to release histamine. Next is norepinephrine, and there's an area here in the pons where there are a bunch of neurons that send diffuse projections of norepinephrine up to the cerebral cortex. That area is called the locus ceruleus, and I've seen a few different ways of spelling this, but this is the one I'm familiar with. I'm sure any of them are fine. Then there's serotonin, and there are a number of nuclei at all levels of the brain stem, up here in the midbrain, down here in the pons, and in the medulla that are all called the raphe nuclei. These raphe nuclei release serotonin, and my understanding is that it's mostly the ones that are higher up that send diffuse projections up to the cerebral cortex to release serotonin, and that these raphe nuclei also send serotonin to other part of the nervous system. And last up, although I'm sure there's probably more, but last up that I want to talk about of these diffuse projection systems releasing neurotransmitters onto the cerebral cortex, involves the neurotransmitter dopamine. For dopamine, there's an area here-ish in the midbrain that diffusely projects dopamine onto the cerebral cortex, and that area is called the ventral tegmental area. I'll just right VTA for short for the ventral tegmental area. So all of these diffuse projection systems sending neurotransmitters through widespread areas of the cerebral cortex are, again, very important to the higher functions of the nervous system, like aspects of cognition and emotion and consciousness, and many psychiatric disorders appear to involve dysfunction of these neurotransmitter systems. Many of the psychoactive medications appear to influence neurotransmitter release from these diffuse projection systems to the cerebral cortex. Now, for dopamine in particular, there are a couple of other projection systems of dopamine that aren't to the cerebral cortex, but that are important for functions of the nervous system and can become problems for medications that affect dopamine neurotransmission. One of these is right next to the ventral tegmental area in the midbrain. I'll actually draw them overlapping because they're so close together. And it sends axons releasing dopamine to a couple of nuclei deep in the cerebral hemisphere, and these are all actually parts of the basal ganglia. So this collection of neurons in the midbrain that is projecting dopamine is called the substantia nigra. It's actually projecting dopamine up to another part of the basal ganglia called the striatum. If there's some problem with this system, some problem with the substantia nigra getting dopamine to the striatum, that appears to be what happens with Parkinson's disease. I won't go more into that now, but that's very important for Parkinson's disease and other disorders that have Parkinsonism, or the symptoms and signs of Parkinson's disease. There are also dopaminergic neurons in the hypothalamus. These dopaminergic neurons in the hypothalamus are actually sending dopamine down to the pituitary gland to control the release of one of the hormones in the pituitary gland. I won't go any more into that, but that's another important dopamine projection system. So all of that's in the central nervous system in the brain and the spinal cord, but there are neurotransmitters involved in the function of the peripheral nervous system too, all the nerves going out to structures in the rest of the body. Our main ones there are acetylcholine and norepinephrine. Acetylcholine has several jobs. It's the neurotransmitter released from lower motor neurons coming out of the spinal cord or the brain stem, and their axons will then synapse on skeletal muscle cells and release acetylcholine, that's the neurotransmitter they use. But acetylcholine is also involved in the function of the autonomic nervous system. Let me just write ANS for short for autonomic nervous system. Most of the neurons of the autonomic nervous system release acetylcholine as their neurotransmitter, and a smaller number of neurons of the autonomic nervous system release norepinephrine as their neurotransmitter. So I'll stop there in my overview of the anatomy of neurotransmitters to just give you a feel for how many different types of neurotransmitters are located in lots of different areas. And I just want to make the point that some neurotransmitters are used widely throughout the nervous system, whereas others have these more kind of discrete collections of neurons that are either projecting widely to areas of the nervous system like the cerebral cortex, or more specifically between one specific area and another specific area.