Free Biology NEET Video Lectures for Neural Control And Coordination

00:00:42 and welcome to today's lecture on neural control and coordination part one. So today we're going to talk about the nervous system and how it regulates the various uh functions of the body. In part one, we are going to only discuss about the very basic concepts about what constitutes the nervous system, the basic unit of the nervous system which is a neuron and how it generates the electrical impulse and then conducts it. So let's see in order to maintain homeostasis in our body the functions of

00:01:17 different organs have to be coordinated. Now what do I mean by homeostasis? Homeostasis is a steady state that is maintained in the human body of various parameters. Let me give you some examples. For example, your heart rate is always at a steady level of 70 to 80 beats a minute and your breathing rate is also constant as well as your body temperature. Unless something happens to make these things change then the body tries to bring back normaly by changing the conditions and bringing back these

00:01:54 parameters to the normal level. So this is called maintenance of homeostasis. For maintaining homeostasis you need to have coordination between the various organs in the body. An example of this is during physical exercise. So when you're performing physical exercise, your um limbs need to move. And how do the limbs move? They move with the help of muscles. Now the muscles need to contract in order for the limbs to move. And for that they need energy and they need oxygen and they need glucose. So

00:02:28 they get their oxygen from the lungs. So the blood has to travel through the vessels to the lungs where it is exposed to the air from the outside and there is an exchange of gases and oxygen from the air gets into the blood and into the hemoglobin and there it is carried from there through the heart where it is pumped and goes through the circulatory system to the muscles and then the muscles are ready to perform their activity. So physical exercise therefore involves the coordination between all

00:03:06 these various systems in the body. The respiratory system, the cardiovascular system and the muscular system. So this was an example of how coordination between different organs is required for any activity performed by the human body. Now what regulates this? So this coordination is taking place because of two systems in our body which are called the control systems. The first one is the neural system which we are going to discuss today and the other one is the endocrine system which we are going to

00:03:37 discuss in a few lectures later. So to begin with the neural system let's see what is this neural system. So this neural system provides an organized network of pointto-point connections which is required for quick coordination. So in these organs that I just talked about, the heart, the lungs, and the blood vessels and the muscles, they're all located in various locations within your body, but they need to communicate with each other in order to act in a coordinated manner. So for this

00:04:10 purpose, the uh the communication between these organs is partially carried out through the nervous system. So this forms an organized network which connects these systems. Now the basic organization of the neural system has been conserved across evolution. So from the very simplest organisms to the most complex organisms, the human beings, there is a nervous system which is in place to carry out the functions that I just talked about. For example, if you look at a simple organiz uh organism such as the hydra

00:04:47 which or jellyfish, they have a very simple nervous system which just consists of a network of neurons that are just connected to each other. Now if we move higher uh organisms such as insects, they have a nervous system which is slightly more complex than the one that is present in the jellyfish. Here the organism has not only neurons they are organized into a structure which is called as the brain and then there are structures called ganglia which are collections of these neurons and there are other neural tissues

00:05:20 associated with it. Then if we move ahead into mammals of which uh human beings are a type of mammals you see that the nervous system has become even more complex. it has many more parts and it has it consists of many many more types of neurons which are connected to each other through very complicated networks. So the bottom line is this process is conserved across evolution. Today's lecture we are going to talk about three things. First we will talk about the neuron which is the basic structure and functional unit of

00:05:57 the neural system. So just like in an organism the basic structural and functional unit is a cell within the nervous system the cell that makes up the nervous system is called the neuron. Then we are going to talk about how this neuron functions. So in order for the neuron to do what it does that is communicate between different organs in the body it needs to generate what is known as a nerve impulse. So we're going to talk about how this nerve impulse is generated and then how it is transmitted. The third thing that we're

00:06:32 going to discuss today is about the conduction of this impulse across a synapse. So now I have introduced a new word which is the syninnapse. And I will explain later in the lecture what it actually means. But for now what you need to understand is the syninnapse is the connection between two neurons. So when one neuron is uh wanting to talk to the next neuron or pass on the information from one neuron to the next, it does it through what is known as the syninnapse which is the gap or uh space between the two synapses uh between the

00:07:06 two neurons. So let us begin with first understanding what is the neuron itself and as I said before it is the structural and functional unit of the neural system. So the nervous system is just made up of billions of these neurons all connected to each other. The three main parts of the neuron are as follows. A cell body, dendrites and axons. And I will explain to you what each of these are. So here is a typical diagram of a neuron. So what you see here is that this is the cell body. These are the

00:07:49 dendrites and this part is called the axon. Now let me explain to you in little bit more detail. So cells that you are familiar with looking at before usually have this kind of a shape. So you know all cells have what is known as a cell membrane and they have a nucleus and then they have cytoplasm and lots of things are present within the cell which are known as organels. So this is a typical ukareotic cell. Now each cell in a multisellular organis organism has to be specialized to perform certain functions. So the

00:08:31 functions of the nervous system are communication of information from one point to the other. So how is this achieved? For this the cell has to take on a particular shape. And what the neuron does is it has very long extensions of the cytoplasm bound by the membrane. These are called processes. Some of these processes are not so long and they are called as dendrites and some of these processes are very long and functionally also they are different. So they are called as the axon. So this is the cell body where the

00:09:17 nucleus resides and this part is called as the dendrite which is the short process and this is called as the axon which is the long process. Now we'll talk about each of these parts of the neuron one by one. The cell body as I told you this is the cell body. This is the place where the nucleus resides along with several other organals. So obviously it has the nucleus. It has the cytoplasm and it has various organels. And there is one particular characteristic that has been observed within the cell body of neurons

00:09:57 of neuronal cells which may not be present in other cell types. These are called as Nissles granules. So Nissles granules represent RNA. So you might have heard of this molecule called RNA. So RNA is produced in the uh cell because it has an intermediate between the DNA and the proteins. So the neuronal cell has to make lots of proteins all the time. So these proteins have to be generated by copying the sequence of DNA into an complimentary RNA sequence and which is then translated into a chain of amino

00:10:36 acids which makes up the proteins. Now because of this there is huge amounts of RNA granules that are present in the cytoplasm and these get stained as nissle granules and this was named after France Nissle who discovered the stain that actually stains these nissle granules within the cell body of the neuron. So as to just to summarize the cell body contains the nucleus it contains the cytoplasm. It contains various organels such as the endopplasmic reticulum, the GGI body, the lossome, the mitochondria

00:11:16 etc. and along with that it contains a specific kind of granules made up of RNA which are called as nissle granules. Now let us look at the other feature of neurons which are these cytoplasmic processes which are bound by membrane. They are called as dendrites. If you look at this uh diagram you can see that this short membrane bound processes are known as dendrites. They also contain nissle granules and they project out of the cell body. For example, if this was the cell body, they project outwards,

00:11:51 but they are not as long as the axons and they sometimes contain branches. In fact, they're very often highly branched structures. Right? So, these fibers are characterized by the fact that they conduct nerve impulses towards the cell body. So there is a directionality in the way nerve impulses travel within the neuron. So the nerve impulses usually travel from the tip of the dendrite or from one point in the dendrite towards the cell body and then from there they go on to the axon until they reach the end of the

00:12:35 axon and from there they actually go on to the dendrite of the next neuron. So this is the axon. This is the dendrite of the next neuron and this is what is the sinapse which I mentioned before. Now we will come back to this once again towards the end of this lecture where where I will explain to you how this conduction of the nerve impulse takes place. Here all you need to remember is the dendrites are the processes where the nerve impulse is transmitted towards the cell body not away from it. Now the axon so this axon is a long

00:13:21 fiber which extends from one end of the cell body. So the cell body had a lot of dendrites and then one of these cytoplasmic processes grows really long and it becomes specialized to transmit the nerve impulse and this is known as an axon. By rule the axon always transmits the nerve impulses away from the cell body towards a sinapse or what is known as a neuromuscular junction. Sometimes these neurons do not communicate to another neuron instead they communicate to a muscle. So in that case the sinapse is known as a

00:14:04 neuromuscular junction. So this axon if it communicates to a muscle it will make a neuromuscular junction. And the end of this axon is a knob-like structure which is known as the sinaptic knob. And here are the synaptic knobs. There may be more than one present at the end of the axon of any particular neuron. Another fact that you need to remember here is that the axon has fewer branches than the dendrites. The dendrites are extensively branch structures. Now let us talk about another feature of the axon. You might

00:14:46 have heard of this. Certain axons are known to be mileelinated and certain axons are known to be non-milinated. Now what do we mean by mileelination? So mileination is actually a kind of insulation. So you have seen wires in an electrical circuit and you've seen that these wires are usually made up of metals but they're covered by some kind of plastic material which acts as insulation that doesn't allow the current to leak out and it will not shock you if you touch the wire. Right? So this mileination is

00:15:22 equivalent to that insulation. Now in the nervous system these dendrites and axons particularly the axons have very long they are very long. So the nerve impulse or electrical impulse is being conducted across this just like in an electrical circuit. So just like the wires are insulated in an electrical circuit. These axons have to be insulated with something which is called as myelin. So this is a cross-section of an axon. So it it's a cylindrical long structure. So cross-section will look

00:15:57 like a circle roughly. So if you look at a mileinated axon, you will see a layer of insulation around it which is known as myelin. In non-minated neurons, the axon is also protected by another cell wrapping around it which is known as a schwan cell. But it does not form this multiple rounds of myelin membrane which is a specialized kind of cell membrane that is wrapped around the axon. So non-minelinated neuron will have a schwan cell just surrounding it. But a mileinated um axon will have a schwan

00:16:38 cell which became very long and then wrapped itself around multiple times around this axon to make this like ring-like structure like the layers in an onion. You have seen that right? So this is what myelin actually looks like. So this is a picture of an axon which is mileelinated by a schwan cell. So this is a single schwan cell which is wrapped around one segment of the axon and if it forms multiple rounds around the axon the membrane becomes very specialized and this forms a compact membrane myelin. Okay, which is very

00:17:17 specific. This is a sheath around the axon and there are gaps between adjacent shuan cells which have wrapped around two uh segments of a axon. So the gap in between them is called as nodes of ranvir. Okay. So here is a node of ranvir. Then the mileinated nerve fibers are found usually in spinal and cranial nerves. In the next part of this uh lecture, I'm going to actually explain what are spinal and cranial nerves. So for now, you don't need to understand that there are certain kind of nerves in

00:17:55 which these uh axons are found. Now non-melinated axons as I said they also have a schwan cell wrapped around them in segments but they do not form this multi-layered specialized myelin. Okay. and they are found commonly in the autonomous and sumatic neural systems. So there are three kinds of neurons with respect to their shape and their organization. So these neurons can be classified into uniolar neurons. So uniolar neurons usually look like this. They have one process which is coming out from the cell body which is of

00:18:38 course bound by the membrane but it is a single process and this kind of a neuron is usually found in the embryionic stage meaning during development when the neuron is actually forming is not yet become a mature neuron it is in a uniolar form. The second kind of neuron that can be observed in the nervous system is called as a bipolar neuron. Now the name itself tells you bipolar means it has two processes. So one side it has a process which is the dendrite and the other side it has a process which is known as the

00:19:17 axon. So it has one axon and one dendrite and usually this kind of bipolar cells are found within the retina of the eye. So again I'm going to discuss about the retina of the eye in the next part of this lecture. Third type of neurons are called multipolar neurons. Now multipolar neurons look like the typical example of the neuron that I showed you before and it is characterized by the presence of one axon and two or more dendrites. So these are the dendrites and obviously they are more than two in

00:19:53 number and these kind of neurons are usually found in the cerebral cortex which is present the outermost part of your brain is the cerebral cortex. So I'm going to describe more about the cerebral cortex and the kind of neurons that make up this in the next part of the lecture. Now that we have described the neuron to you in great details, let us talk about how the nerve impulse is generated. How do these neurons talk to each other? They have to communicate. Not only they have to communicate with each other,

00:20:28 they have to communicate with the different organs. So the information has to be taken from one system to the other. So for this the neurons use a electrical impulse or a nerve impulse for this communication. So the reason why neurons can generate an electrical impulse is because they are excitable cells and their membranes are in a polarized state. I will explain to you what this exactly means. So we have to first understand why are the neuronal membranes in a polarized state. So to understand this you have to

00:21:11 understand how neurons signal or how do they talk to each other. So neuronal signaling depends on the ability of neurons to generate rapid differences in electrical potential across their membranes. So there is a voltage or a membrane potential that is present or a distribution of charge that is different across the membrane of a neuronal cell. Okay. And the neurons and the muscles are the only cells in the body that have an ability to change this potential that is present across the membrane. Most

00:21:49 cells in the body have this membrane potential which is a asymmetric distribution of ions across their membrane which gives a membrane potential. But most cells in the body cannot change this membrane potential. Only neurons and muscle cells have the ability to do this and they do this in response to certain stimuli. Okay. So these rapid changes in membrane potential are produced by what is known as ion channels and this is what distinguishes excitable cells such as neurons and muscles from nonexitable

00:22:28 cells which are the most of the other cells in the body. So then what are these ion channels? As the name itself suggests, if this is the plasma membrane of a cell, embedded in the plasma membrane is a protein which has a pore through it. So this pore allows ions or charged particles to go inside. And if such ion channels are present in a cell membrane and the ability to open and close these channels is there is regulated then that cell can rapidly change the distribution of ions across the membrane and thus

00:23:12 change the membrane potential in response to a stimulus. And therefore ion channels are very very important for the conduction of nerve impulses. So we'll see more details about it in the following slides. So why do nerve cells use channels? So the that addresses the basic question that the cell membrane is made up of a phospholipid billayer which is embedded with proteins including the ion channels. Now what is this phospholipid billayer? The phospholipid billayer has a single unit which is a

00:23:51 phospholipid. So the phospholipid is a molecule which has a polar head and two hydrophobic tails. So these phosphoipids are arranged in this manner and this forms a phospholipid billayer. So you have the polar heads all on the outside or in the inside of the membrane and the hydrophobic portion is tucked inside. So this forms a hydrophobic barrier. So if this is the external surface um which is facing the extracellular fluid and this is the cytoplasmic surface then the polar heads are present towards each of these while

00:24:53 the interior of the membrane is all hydrophobic. So this acts as a barrier and normally does not allow any charged molecules to go through. Right? So how will you then allow charges to go through? The only way it can be done is if there are some proteins that are embedded inside this phosphoipid billayer which allow these ions to go through. So this is a picture of the same. So you see this is the phosphoipid billayer here the polar head groups and the two hydrophobic tails facing inwards and across this is embedded an ion

00:25:36 channel. So usually ions positively charged ions like sodium and potassium have a shell of water molecules around them. Right? So this is making them polar and they are charged so they are polar anyway and therefore they cannot pass through the hydrophobic core of the plasma membrane. So they can only pass through if there are open channels which are filled with water which allow these to go through. So therefore ion channels are very very important to change the distribution of these charged ions

00:26:13 across the plasma membrane from the inside and the outside of the cell. Now what is the key important fact about these ion channels? These ion channels are selectively permeable to different ions. For example, you can have a particular ion channel that only allows sodium ions to come through. There is another particular ion channel which only allows potassium ions to go through. And there might be a third one which allows chloride ions to go through. So these ion channels are all very selective. They don't you know take

00:26:50 anybody and everybody that comes. So let's see when a neuron is not conducting any impulse. It is called to be in a resting state. Now in the resting state what is the condition of these uh membranes. The cell membrane is not that much permeable to sodium. Instead it is quite highly permeable to potassium ions. That means at this stage the ion channels that are open are potassium ion channels. So potassium ion channels allow the flow of potassium ion from the outside to the inside or from the inside to the outside. What drives

00:27:33 whether the ions will go from the inside to the outside is the difference between the concentration of ions across the membrane. So if there is a higher number of ions inside they will tend to diffuse outside. If there are higher number of ions outside they will tend to diffuse inwards. Also what you should uh know that in the resting neuron the sodium ion channels are closed. So there are very few open sodium channels. So therefore sodium ions cannot easily pass across the membrane. And the membrane is also

00:28:06 completely impermeable to negatively charged proteins which are present in the exoplasm within the cytoplasm. The cytoplasm contains a lot of proteins and these are mostly negatively charged but they can never move out of the cell even though ion channels may be open because ion channels only allow ions and that to certain kinds of ions and certainly not any proteins. So this results in a certain distribution of ions across the membrane. Now what do you see? You see the axon inside has a high concentration

00:28:40 of potassium ions and negatively charged proteins and a low concentration of sodium ions. Whereas the fluid outside the axon the membrane contains a low concentration of potassium and a high concentration of sodium and therefore there is a opposite concentration gradient of sodium and potassium across the axon membrane. So this is what it exactly looks like. So you have high sodium outside, low potassium whereas you have high potassium inside and low sodium and of course lot of uh uh negatively charged proteins which are

00:29:18 only present inside. So this is a representation of the axonal membrane and this is the fluid outside the axon and this is the axoplasm or the cytoplasm of the axon. Right? So now with this we see that there are ionic gradients across the membrane and this ionic gradient across the resting state is maintained in this way because of a particular pump. So this is a sodium potassium pump. So this is another protein which actually is called a pump because it transports ions across the membrane using energy in the form of ATP

00:30:04 and it does so against the concentration gradient. For example, if this is the plasma membrane and this is the sodium potassium pump, it will be open towards the inside and it will bind to one ATP molecule and it will bind to three sodium ions because uh it is using the ATP because you know already there are more sodium ions outside than inside. So it has to use ATP or energy to transport it against the concentration gradient. So it is transporting three sodium molecules and as soon as the ATP is

00:30:45 hydraized to ADP and inorganic phosphate this energy changes the confirmation of this protein and it now opens towards the outside. So the three sodium ions can now diffuse towards the outside and now when it is open towards the outside it binds to potassium ions. So two potassium ions will bind and now again the confirmation will change and now it will again open towards the inside of the membrane and deliver these two potassium ions to the inside and then it will go back to the default state where

00:31:21 it will bind another ATP molecule and hydraulize it and then transport three sodium ions outside and two potassium ions inside. So this sodium potassium pump continues to maintain a high amount of sodium outside the cell and a high amount of potassium inside the cell. So as a result what happens is the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarized. Why? because you have a lot of negatively charged proteins which are present inside which

00:31:58 can never go out of the cell because of the phospholipid barrier. This electrical potential difference or charge difference across the resting membrane plasma membrane is known as the resting membrane potential. So this is very important and we will see that this has a negative value. So this is a cartoon showing you that right outside the membrane there is a high concentration of positive charges and right inside the membrane there is a high concentration of negative charges because of which the voltage or the

00:32:36 membrane potential difference across the plasma membrane which is denoted by this equation here V in voltage inside minus V out has a negative value because of this high amount of negative charge inside. So this in the resting state is maintained as a negative membrane potential which has a value of -60 to -70 m volts. Now let's see how does the neuron actually change the membrane potential. So the nerve impulse is generated and conducted along the axon by changing this membrane potential. So

00:33:17 when there is an external stimulus, there is a change in the resting membrane potential. How does this happen? This happens by the way of ion channels. So suddenly the stimulus changes the permeability of the membrane to certain ions. Usually it is to positively charged sodium ions. So sodium ion channels will open up. Remember there were not so many sodium ion channels that were open in the resting state. But now new sodium ion channels will open up and the flood of sodium ions can therefore come in. And

00:33:52 now what will happen? The membrane potential will change because the distribution of ions across the membrane is changing. Now let's see how that will result in the generation of an electrical impulse. Say this is a schematic which shows you the membrane and you see sorry this is the membrane. This line is the membrane and this is the exoplasm. This is the other side. The membrane on the other side. Now you see normally there was a negative charge which was present in the inner side of the membrane and a

00:34:31 positive charge which was present on the outer side of the membrane. Right? However, in a certain region of this membrane, there is a stimulus which changes the permeability of this membrane to sodium ions. So, sodium ions suddenly become able to go freely into the cell and because of the concentration being higher, they tend to move from outside of the cell to the inside of the cell. So here they're going across the membrane and what happens next is this leads to a rapid influx of sodium followed by the

00:35:05 reversal in the polarity. So instead of being highly negative inside it becomes positively charged inside and the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. Now the polarity of the membrane at site A is reversed and hence it is called depolarized. So a portion of the axonal membrane gets depolarized. The electrical potential difference across this plasma membrane at the site A is called an action potential because it is generated as the

00:35:41 result of a stimulus which is actually termed as the nerve impulse. So this change in the membrane potential will have to be transmitted across the axon. So how is this transmitted? We will see in the next slide. So what happens is at sites that are immediately after site A say site B the membrane has a uh positive charge on site A but then there are negatively charged ions present still in site B. So the what will happen these ions will tend to diffuse from here to here. These positively charged

00:36:22 sodium ions that just came in from this portion of the membrane will change tend to diffuse from here to here and as a result of this the inner surface there will be a flow of current in this direction. So current is the flow of positive charges. So since sodium ions are positively charged the current is moving in this direction. On the outer surface of the membrane, there will be an opposite movement because there is a deficit of positive charge here on the outside. The ne the positively charged ions from the

00:36:55 B side will move towards the A side and will now change the polarity of the membrane. So the polarity of the site gets reversed and now the action potential has moved from site A to site B. Right? Thus the impulse which was generated in side A now arrives at side B. This sequence keeps on repeating across the entire length of the axon. So further and further segments such as C, D and E will become depolarized and this will lead to the propagation of the depolarization which is a change in the membrane potential or the action

00:37:37 potential. However, this change that initial stimulus which change the permeability of these membranes to the sodium ion is very very short-lived. Meaning the sodium ion channels will open only for a very short duration and as soon as they close the potassium ion channels will open up and the potassium will follow the opposite direction of movement. Since there were more potassium ions inside the cell, they will move out and this will remove the excess positive charge that had come in with the sodium

00:38:13 ions to the outside and restore back the membrane potential to its resting negative value. Right? Then there is now an asymmetric distribution of sodium and potassium ions across the membrane. So the sodium potassium pump will come into action and it will move out the excess sodium and bring in the potassium again and restore the balance to what it was before the impulse was being conducted. So now I have told you about how this nerve impulse is actually generated by changing the distribution of ions across the plasma membrane of

00:38:55 the axon. Now this impulse has to be transmitted. What the nervous system is made up of is long chains of individual units which are these neurons. So there are many neurons that are connected to each other and they form a long chain and the communication begins with the first neuron right up to the last neuron in the chain and this is forming what is known as a nerve. Okay. So this entire network has to communicate the information in the form of electrical impulses from one neuron to the next to

00:39:34 the next. So how does this happen? This takes place through the synapses. What are these synapses? These synapses are these regions that I have highlighted here in these red boxes. These are the gaps between the axonal knobs. these knobs which I mentioned before and the dendrites of the next neuron. So therefore a syninnapse can be defined as a structure which is present at the junction of one neuron and another neuron and therefore it consists of a preynaptic neuron which is this guy and a post synaptic neuron which is this

00:40:18 guy and it may be separated by a gap which is known as the synaptic clft. So synapsis can be of two kinds electrical and chemical synapsis. But before I go into this, let me just explain to you one very basic thing about this. That is in order for the electrical impulse to go from one neuron to the next, there has to be a mechanism of bridging this gap because these are changes in membrane potential. How are you going to communicate this change in membrane potential from one neuron to the next?

00:40:56 it cannot just jump across the gap. So the way this is done is very different in an electrical synapse and in a chemical synapse and I'll explain to you one by one. So in an electrical synapse what happens is you have this is the pre saptic membrane and there is an ion channel which is present here and this is the post sinaptic membrane which also has an ion channel here. Now as you see the two ion channels are closely opposed to each other so that they form an almost continuous pore that is communicating

00:41:58 from the interior of one particular uh presinaptic neuron to the postsinaptic neuron. So this forms a continuous channel. So now you can understand how this electrical impulse which is nothing but positive charges moving towards the uh end of the axon can go on into the next one. So they they can move easily through these pores into the next post synaptic neuron and depolarize the membrane over there. Right? So when this moves along, this is how electrical synapses transfer the impulse from one

00:42:37 neuron to the next. So the characteristics of the electrical synapse are as follows. The membranes of the pre and post synaptic neurons are very very close to each other. They almost touch. So the synaptic clft is negligible. The electrical current can flow directly from one neuron to the other. The impulse transmission across the electrical synapse is always much faster. You will see why it is faster than the chemical synapse in a minute. Electrical synapses are however rare in our system. We have very very few

00:43:14 electrical synapses in the entire nervous system and I will kind of explain that towards the end of this class. Let's look at the typical synapse which is a chemical synapse. So chemical synapse looks very very different from an electrical synapse. So here this is the pre-sinaptic neuron. The knob that is present at the end of the axon right and you see this is the membrane of the post synaptic cell and here is the membrane of the presinaptic neuron. Now there is a certain gap between the two and the

00:43:53 basic difference here is that ions cannot pass directly from the presinaptic to the post synaptic cell. Instead there is a change of guard meaning there is a change in the communication system. So before this within a particular neuron from the dendrite to the axon there was electrical impulses which is being carried. Then as it reaches the end of the axon, this electrical impulse now gets converted into a chemical signal. So this chemical signal is in the form of what is known as neurotransmitters. So what are these

00:44:35 neurotransmitters? Neurotransmitters are very small uh amino acid derived molecules or very small peptides made up of few amino acids in a chain and they are packaged into these vesicles. So there are these vesicles which are membranebound packets of neurotransmitters that are stored within the end of the presinaptic neuron in the in the form of these neurotransmitter vesicles. Now when a depolarization wave has reached this part it will lead to the fusion of these synaptic vesicles to this membrane towards the postsinaptic

00:45:23 cell and this releases the neurotransmitters into the space or the synaptic clft between the two neurons. Now these neurons are chemical in nature. So the electrical signal has now been transferred to a chemical signal. Now these neurons the post synaptic ones will have certain proteins on their membrane which are known as neurotransmitter receptors meaning they can receive the signal. So how do they receive the signal? The neurotransmitter molecule comes and binds to them and they are very specific. A particular

00:45:58 neurotransmitter only binds to a particular neurotransmitter receptor and not to any protein. So only if the neurotransmitter receptor is present on the post-sinaptic cell can it bind there. And this binding of the neurotransmitter to the neurotransmitter receptor can do very wonderful things. It can change the membrane potential of the postsinaptic cell. How does this do this? This can do it by opening up ion channels in the postsinaptic membrane and allowing positively charged ions to get in. This will result in

00:46:36 depolarization of the post synaptic cell. And so the message has been communicated from the presinaptic neuron to the postsinaptic neuron through a chemical messenger which is the neurotransmitter. So if you compare a chemical synapse to an electrical one, you will see that the membranes of the pre and postsinaptic neurons are separated by a synaptic clft which was almost non-existent in the case of an electrical synapse. The chemicals such as neurotransmitters are involved which are not involved in the

00:47:11 case of an electrical signapse. Impulse transmission across uh chemical signapse is much slower. Now it is obvious why it is slower because it involves the transfer of a electrical impulse into a chemical signal and then again into an electrical impulse. So this takes time. Most synapses are chemical in nature in our system. Towards the end of this lecture, I will ask you certain questions and when I answer those questions, you will get to know why most synapses in uh our human body are chemical in nature and not

00:47:49 electrical. Okay. So this is a summary of the process of synaptic transmission which I just described across a chemical synapse. So this is a cartoon showing you a chemical synapse and the various steps that are involved in the transmission of the signal from the presinaptic to the postsinaptic cell. So this is a uh cross-section of a presinaptic neuron. Here is the synapse and here are the neurotransmitter vesicles which are filled with neurotransmitters. And what you see here is a specific ion channel

00:48:27 which is a calcium ion channel. Now ion channels as I told you are regulated. They're opening does not happen all the time. Certain ion channels are open all the time such as the resting potassium ion channels that are present in the neuronal membrane. However, certain ion channels are open only on a stimulus. So they are called as gated ion channels. So these gated ion channels can be gated by various things. They can be gated by binding of a chemical. They can be gated by the membrane potential. So such ion channels

00:49:02 are usually called voltage gated ion channels. So an example of that voltage gated ch ion channel is this calcium ion channel. So when there is a nerve impulse coming in the preynaptic cell from the dendrite through the cell body through the axon it reaches the end of the syninnapse and here it changes the membrane potential to a more positive value. It depolarizes the cell. Now this opens up these voltage gated calcium channels. Okay. So these voltage gated calcium channels then allow a lot of calcium ions to get

00:49:39 into the presinaptic end of the neuron. So this calcium ions are a trigger for these neurotransmitter vesicles to fuse to this membrane and they fuse to this membrane and release the neurotransmitters. Now these neurotransmitters will bind to the postsaptic cell membrane because there are certain receptors present on the post synaptic cell membrane which are recognized by these neurotransmitters. Now these neurotransmitter receptors can also be ion channels themselves. So these are again gated ion channels but they are

00:50:19 not voltage gated. They are gated by lians or things that can bind to them. So only when the neurotransmitter binds to them they open up and when they open up they allow positively charged ions such as sodium to get in to the post synaptic neuron the dendrite. This is the dendritic end of that neuron remember. So now what happens is this gets depolarized and the nerve impulse is generated now in the dendrite of the second neuron in the series. So what I just told you is summarized in here. When an impulse such as an action

00:50:56 potential arrives at an axon terminal, it stimulates the movement of synaptic vesicles towards the membrane where they fuse with the plasma membrane and this is because of voltage gated calcium channels opening up. Then the release neurotransmitter binds to specific receptors on the postsinaptic membrane and this opens ion channels allowing the entry of ions into the post synaptic neuron which generates a new membrane potential there. The new potential developed may be however excitatory or inhibitory. So

00:51:37 again when I discuss certain questions at the end of today's lecture I will explain to you how the postsaptic potential can be either excitatory or inhibitory. So with that I would like to end today's lecture and I will now move on to questions. Let us try to answer certain questions that might have risen in your mind as well. So the first question that I would like to answer is why are ion channels needed to transport ions across a cell membrane. So if you think about this as I told you the cell membrane is made up

00:52:17 of a lipid billayer. So a lipid billayer has a hydrophobic interior region which is made up of these hydrophobic tails and has a polar region which is hydrophilic. Now the environment outside and inside the cell are both water rich. So these are hydrophilic environments. But the barrier in between is hydrophobic. So what are charged molecules such as ions to do? They cannot get into the cell. Therefore, they need these specialized pores within the membrane which allow a charged particle like the

00:53:02 iron to go through. Otherwise, it would be repelled away from the hydrophobic core of the plasma membrane. Now, let me explain to you how the postsinaptic potential can be inhibitory in nature. So when I told you that there is a pre-sinaptic neuron say this is the axonal knob and this is the dendritic end of the postsinaptic cell. I told you that there are some neurotransmitters which will fuse to this membrane and release neurotransmitters into the space. When there is a depolarization arriving here, this opens

00:53:49 up calcium channels which are voltage gated. Calcium gets into the axon terminal and this triggers the release of this neurotransmitters. Now the neurotransmitter receptor which is present on the post synaptic cell membrane is very specific. And what kind of receptor is present here will determine if this is excitatory or inhibitory. What do I mean by excitatory or inhibitory? Excitatory is something that will lead to depolarization or more positively charged ions getting in. Inhibitory on the other hand would be something that

00:54:40 changes the membrane potential and makes it more negative. So that is called as hyperpolarization. So if these are these receptors lead to opening of ion channels which are allowing negatively charged ions to come in like chloride ions or they are potassium channels which allow the potassium to move out of the cell then the negative charge inside can increase and this state is called as a hyperpolarized state. So instead of this neuron getting excited as a result of an electrical impulse coming from the

00:55:23 presinaptic neuron it will get inhibited and therefore the post synaptic potential in this case would be inhibitory. So then we can ask another question why are chemical synapses more common as compared to electrical synapses. The basic fundamental difference between a chemical and an electrical signapse is two. One that it allows the uh neuron to communicate directly with the next one through direct passage of ion channels through these pores which are between the two spanning the two neurons. So this is

00:56:06 fast sure compared to the chemical signapse. However, it cannot be very easily regulated. So once you have activated the presinaptic neuron, the electrical impulse will go through to the postsinaptic neuron and you can't regulate it at all. However, in a chemical synapse, it is very easy to regulate the transfer of this impulse from one to the next neuron. How? Because it is getting converted into a chemical uh signal. This chemical signal can be amplified. For example, a certain amount of depolarization can lead to different

00:56:47 amounts of neurotransmitter being released which will amplify the signal. And depending on how many neurotransmitter receptors are present on the post synaptic cell, more and more ion channels can be opened up. So the signal can be really amplified. Besides this, you can control it at multiple steps. You can control the release of the synaptic uh vesicle fusion to the membrane. You can control how much neurotransmitter is being released. Whether this neurotransmitter is getting degraded here, which will control the

00:57:23 signal and then further how much neurotransmitter receptor is there will dictate how much of this signal goes to the next neuron. So this is why this is a much more advantageous system and therefore evolution has selected chemical synapses over electrical synapses. The last question is what is the function of the sodium potassium pump? Now if you remember after the production of an electrical impulse the membrane distribution of charges gets changed. So the potassium ion has moved out the sodium ion has come in. Now to

00:57:57 restore it back to the default state you need something and that function is performed by the sodium potassium pump. Of course it needs energy because it is going to do the ion transfer against the concentration gradient. So three sodiums will move out two potassiums will come in and therefore this pump restores the potential to the resting value so that the next stimulus is able to depolarize it again. So with this I'm going to end today's lecture. Thank you for listening.