Nervous System Anatomy

Okay guys, in this lesson, we’re going to talk about the anatomy of the nervous system.

So, first I want to make sure you understand the purpose of the nervous system. This is what helps us coordinate and integrate all of our body functions. That’s everything from moving to breathing to digesting our food. It’s all coordinated through electrochemical signals that send signals from the nervous system out to other parts of the body. Now there are two main properties you need to know about the nervous system. One is irritability. That means that it can respond to some sort of stimulus. Another word is excitable. So it gets a stimulus and responds by starting a signal. Those signals are called Action Potentials. You’ll see me write AP. The second property is conductivity. This means that it can then take that signal or Action Potential and transmit it down nerve pathways to get it where it needs to go. So it ‘conducts’ the signal.

So let’s see what it looks like when it sends these electrochemical signals – again, remember they’re called Action Potentials or AP. So first the nerve will get a stimulus and respond to that stimulus – this could be almost anything. Like a pain sensation or stretching of the stomach. Then it converts that stimulus into an Action Potential – again that’s that irritability or excitability property. Then it’s going to send that action potential down a certain pathway, depending on what it’s trying to accomplish, and when it gets there, it’s going to produce an action. So an example might be – you touch a hot stove – so the pain is the stimulus. That tells your brain to initiate and send an action potential down your arm to pull your hand away. These types of electrochemical signals are happening constantly throughout your body and that’s how everything stays coordinated and working right.

So before we get into the cellular anatomy, I want to talk about the overall divisions of the nervous system. You’re going to see these terms a LOT moving forward, so make sure that you really understand this part. First, we have the Central and Peripheral Nervous Systems. The Central Nervous System or CNS is the Brain and Spinal Cord. So, if you’re looking at the nervous system as a whole – it is literally in the center. This is where the majority of our control centers are. Then we have Peripheral Nervous System or PNS, which is everything else. It is then divided into the somatic and autonomic nervous systems. When you see somatic I want you to think skeletal muscle and think voluntary actions. Autonomic – think “Automatic” – these are the unconscious involuntary things that happen in our bodies. So, somatic voluntary, autonomic involuntary. ONE more division – and this is the part you’re going to see a LOT. The Autonomic Nervous System is divided again into Sympathetic and Parasympathetic. For now – here’s what you need to know: Sympathetic Nervous System is “fight or flight” – this is what helps us fight off an attack or flee away from it – so all our senses are heightened, our heart rate and blood pressure go up, and non-essential functions like digestion are slowed down. The Parasympathetic Nervous System is “rest and digest” – so this slows us down a little and allows us to do the things we couldn’t do when we were fighting – like digestion and reproduction. So – fight or flight and rest and digest.

Now let’s talk generally about the cells of the nervous system. We have neurons and we have glial cells. The Neuron is the basic unit of the nervous system. If signals are being sent somewhere, it’s a neuron that’s involved. We classify them by their function – we have afferent and efferent neurons. Afferent is sensory, so it receives signals from the body. Efferent is motor – so it sends signals from the brain out to the organs to produce an action. You can remember this with the mnemonic SAME – S – sensory, A- afferent, M- motor, E – efferent. As far as glial cells there are quite a few types. We talk about these in detail in the neuro section of the med-surg course, and we’ve provided some detail in your outline. Big difference you’ll see here is that neurons don’t undergo mitosis – or cell division – and glial cells do.

Now, since the neuron is the basic unit of the nervous system – let’s talk about the different parts of the neuron and the role they play. This larger part here is called the cell body – this is where you’ll find the nucleus as well as other cellular structures like the mitochondria, etc. These finger like projections are called dendrites, this is what will receive the signal from another neuron. The long thin structure is called the axon – depending on where this particular neuron is in the body, the axon could be up to 2 feet long! Along the length of the axon, you may see what’s called a myelin sheath – it’s formed by these cells called Schwann cells. Now – you only have a myelin sheath if the cell wraps all the way around one axon. If you try to wrap the cell around multiple axons, it’s not actually going to create the myelin sheath because it can’t wrap all the way around them. The purpose of the myelin sheath is to make the transmission of the action potential much smoother down the length of the axon. So if you HAVE the myelin sheath, it’s called myelinated – otherwise you’d call it unmyelinated. We also have these little gaps between each of the Schwann cells in our myelinated axons – those are called Nodes of Ranvier – and their purpose is to speed up the signal by actually allowing it to JUMP from node to node. So we not only get a smoother signal, but a faster one. And finally, when the signal gets to the other end of the axon, it’s going to come to these axon terminals – also known as an “end foot”. So picture another neuron right on the other side of this terminal. The terminal backs right up to the dendrites on the other side – sends the signal across – and continues the action potential down the next neuron.

Now, in the lesson on nerve transmission, we’re going to talk in detail about how this signal goes from neuron to neuron – but the main thing I want you to see here is that the signal comes down the axon, to the terminal, and passes to another neuron. In that axon terminal we have little vesicles that hold our neurotransmitters – and that is what helps us send the signal. Again, check out the lesson on nerve transmission to see more details about that.

Last thing I want to review quickly is the membranes. If you’ve already watched the muscle and bone lessons, this is going to feel very familiar to you. You’ll have an individual neuron, covered in endoneurium – then a group of those bunched together in a fascicle by perineurium, then a group of fascicles held together by the epineurium. This is white fibrous connective tissue – and sometimes you might even see tiny blood vessels grouped in here as well.

So let’s recap what we’ve learned about the nervous system. Its main function is to receive and transmit electrochemical signals throughout the body to coordinate and integrate body functions. It is divided into the Central and Peripheral Nervous System, which is further divided into the Somatic and Autonomic Nervous Systems – which is divided ONE more time into the Sympathetic and Parasympathetic Nervous Systems. We have the neuron, which is the basic unit of the nervous system, as well as some glial cells in the central nervous system. And remember that at the end of the neuron is the axon terminal, which is where all the magic happens to send signals to other neurons. Make sure you check out the nerve transmission lesson to see more details about how that happens.

Check out the resources attached to this lesson as well. Now, go out and be your best selves today. And, as always, happy nursing!