Another week, another blog post!
Today I’m going to finish going through all the material that I have learned on the nervous system. There are bits coming up that I am worried about having to explain it in an easy way to understand, but I can’t always pick and choose what I want to do. Am I right?
Let’s get going then!
I finished the previous post by going through all the divisions and the subdivisions of the nervous system, where they are found in the body, and what roles they have. Today I’m going to talk about the different cells that are found throughout the nervous system and how information in received.
I think most people have a general idea of what nerve cells look like. Long branch-shaped cells in the brain with light flashing here and there. That was my extent of knowledge on nerve cells before I properly learned about them in university. But it turns out those branchy looking cells have such an amazing physiology going on inside of them at all times!
In the nervous system, there are 2 types of cells. Neurons and glial cells. Neurons, as shown above, are the cells we most commonly think of when we think of nerve cells. They are the cells responsible for receiving and sending signals. Neurons are highly specialized to conduct cell nerve impulses through electrical cell signaling. Neurons are also very special in that they are amitotic, which means they cannot regenerate, however, they have the extreme longevity to make up for being amitotic. Another interesting fact about the neurons is that most of the cell bodies of these neurons are located in the CNS! This means that a single neuron can easily be the length of your toes to you back! Crazy! This explains why a horrible back injury can lead to the loss of use in the lower body.
As you can see above, there are 3 different types of neurons. Motor, inter, and sensory neurons. The different types of neurons depend on where the cell body is located. The first type is the motor neurons, also known as multipolar neurons, which are the most common and biggest nerve cell of them all. They have 1 axon with their cell body located very close to the abundance of dendrites; their main functions are to carry signals from the CNS to the rest of the body. Interneurons, also known as bipolar neurons, are quite rare compared to motor neurons. They also have 1 axon but 1 dendrite attached to the cell body. Impulses from the interneurons move between sensory and motor neurons in the CNS. And lastly, sensory neurons, also known as unipolar neurons are commonly found in the sensory receptors, which are in the PNS. They have more than one dendrite and axon in which the cell body is located between the 2 axons. Sensory neurons transmit impulses from sensory receptors to the CNS.
Although these neurons look different and have different functions, every neuron has 3 things in common. They receive information through the dendrites which are the shorter branch looking bits. The received information then passes down through the long axon, then to the longer branch-looking axon terminal where dendrites from other neurons are positioned very close to receive information.
Moving onto the glial cells!
Glial cells are basically cells that help and protect neurons. There are roughly 9 times more glial cells than neurons because glial cells do reproduce. So say for example someone has a brain tumor, those tumor cells would be formed by glial cells, not neurons. This is why brain tumors are called gliomas. In the first diagram with the big neuron, you can see the blue Schwann cells (a type of glial cell) wrapped around the axon and some other types of glial cells in the diagram below. These different glial cells all have different functions in the nervous system, hence the different shapes and sizes. For example, microglial cells help with immune defense, Schwann cells insulate and protect the neurons, while oligodendrocytes build up the myelin sheath which I will explain soon. These glial cells, however, are only found in certain divisions of the nervous system. Astrocytes and microglial cells are neuroglial cells meaning they are found in the CNS. The rest are not neuroglial cells meaning they can be found in the PNS.
Because Schwann cells, that build up to be the myelin sheath have lots of important functions, I am going to go more in depth about what they do. Schwann cells wrap around axons of neurons kind of like a swiss roll or a sushi roll I guess?Yes, I know my current hunger is reflecting on my writing style. But anyways, this insulates the axon and helps increase the speed of which nerve impulses pass through the axon. In the CNS, oligodendrocytes have this role for the axons in the neurons, and in the PNS is where Schwann cells have this role. As you can see in the diagrams above, there is always more than one Schwann cell wrapped around an axon. And between those Schwann cells are tiny gaps called nodes of Ranvier. These nodes help nerve impulses jump from one node to the next, through saltatory action (like a frog jumping on lily pads to cross a stream) which is very energy efficient for the neuron sending the nerve impulse.
Now onto synapse. What is synapse? It is the end of the action potential. What is action potential then? Well, because I didn’t properly learn about action potential yet, here is a link to a great video explaining action potential: https://www.youtube.com/watch?v=OZG8M_ldA1M&t=516s. If you don’t know what action potential is, I highly recommend you watch this video before reading through what I will be explaining next!
Do you understand what action potential is now? Let’s get going then!
Synapse occurs after the action potential and there are electrical and chemical synapses. Electrical synapses send signals much more quickly because the electrical signal does not get chemically translated and therefore is less efficient than chemical synapses. Chemical synapses are a bit slower than electrical synapses however, they are more efficient, precise, selective, and common. So I will be focusing on chemical synapse today. Synapse occurs when action potential stimulates the release of neurotransmitters(chemical signals) from vesicles at the axon terminal that then diffuses across the synaptic cleft(or gap) to deliver its message. The arrival of action potential opens the voltage-gated calcium ion channels which allow external calcium ions to flow into to the neuron. Calcium ions entering the neuron causes the neurotransmitter containing vesicles to fuse with the presynaptic membrane and release their contents into the synapse.
As you can see in the diagram above, neurotransmitters are protected by the synaptic vesicles which carry them down to the cell membrane. You will then notice that once the neurotransmitters reach the cell membrane, they are no longer inside the synaptic vesicle to diffuse across the synaptic cleft. The diffused neurotransmitters then attach to the neurotransmitter receptors in the postsynaptic neuron which starts the action potential for the postsynaptic neuron. If my explanation was not clear, here is a video link that explains synapse very well: https://www.youtube.com/watch?v=L41TYxYUqqs.
Now that I have finished explaining about the synapse, let’s take a few steps back to the PNS. As I mentioned in my previous post, the PNS controls sensory neurons. These sensory neurons have sensory receptors that are specialized to respond to stimuli. There are different types of sensory receptors to respond according to the 5 types of stimulus or sense. The first receptor is the mechanoreceptor which responds to the sense of touch. Any types of stimuli from pressure, itch, vibration, to stretch. The next is the thermoreceptor and yes, it responds to changes in temperature. Then comes the photoreceptor which responds to light. The chemoreceptors respond to chemicals and pH, and last but not least, nociceptors respond to potential danger that can be interpreted as pain. One thing to keep in mind about the sensory receptors is that the sensation of whatever you are experiencing does not happen in the sensory receptors. The receptors simply receive the sensory information and send it to the brain to perceive the sense.
And that is it people! This post ended being my longest one yet as I was trying to avoid writing another post on the nervous system so I do apologize for the great length of this post. I hope this is helpful and easy to understand and I’ll come back soon with a surprise subject!