in this video we're going to talk about the synapse so the nervous system works because information flows from neuron to neuron and the way the information flows between cells are these functional units called synapses now synapses are the junctions that mediate the transfer of information from either neuron to neuron or from neuron to in an effector cell like a like a muscle cell or a glandular cell of the body now whatever we have a neuron that releases chemicals or sends information we call this the presynaptic neuron so it's the neuron that conducts impulses towards the synapse and the one that receives the information is called the postsynaptic neuron this is the neuron that transmits electro signals away from the synapse and receives that information so in the peripheral nervous system this could be like another neuron or muscle cell even a glandular cell but we'll still call this the postsynaptic cell it doesn't have to be a neuron like the postsynaptic cell could be muscle or glandular cells as well and a lot of these neurons function as both because if they're gonna send information along you know the postsynaptic neuron of one synapse could be the presynaptic neuron of another because it's all linked up in a long chain so if you look at this on an electron microscope we see this is the axon of a neuron and you can see that the axon collaterals can branch out and form these expanded tips which are the axon terminals full of you know synapses or I'm sorry a synaptic vesicles we're full of neurotransmitter that can be released directly onto the you know effector cell now in terms of where synapses can occur they can either be accident Recor now Akzo refers the axon dendritic refers the dendrite so Akzo dendritic synapses are between axon terminals of one neuron and the dendrites of another however a lot of other axon connections can also be somatic where the axon terminals of one neuron actually synapse on the soma or cell body of another now there are other types of connections as well like Akzo exonic done Joe dendritic and somatic dendritic but these are more rare now there's two major classes of synapses which are chemical and electrical now this is showing examples of chemical synapses where we have axons of three separate neurons that are actually branching out and forming connections with this postsynaptic neuron or effector cell so we see here that because this this axon is connecting to the cell body of this postsynaptic cell we call this type of communication point in acts of somatic synapse and because this axon is connecting the dendrites of our postsynaptic cell you know we call call this an accident' Hritik synapse but they can also be act so axon ik where one axon can direct directly connect to the axon another now this is important in terms of their arrangement because this helps determine how information is transferred so the farther away these axons are these synapses are from that axon of the neuron basically makes it less likely for these types of synapses to potentially cause an action potential we find that is that if an axon is directly on another axon it's probably going to cause an action potential with each time it communicates however with these ones that are more distant from the axon of this postsynaptic cell it's it's possible that the currents that are generated by these synapses may not generate an action potential in this postsynaptic cell so the what occurs in a synapse is the release of chemicals and we call these chemicals neurotransmitters so what happens is that the presynaptic neuron can release neurotransmitters through exits cytosis and those neurotransmitters are actually stored in synaptic vesicles now the receptor region on the postsynaptic neurons membrane has receptors and it receives the neurotransmitters from our presynaptic neuron and usually find these receptors like on the dendrite or cell body but we just saw in the last slide that if they can also be on the axon now that two parts of these cells interestingly enough are actually separated by a very small space called the synaptic cleft and electrical impulses change or change to chemical across the synapse and then back into electrical again so it's an action potential that causes the release of neurotransmitters or chemicals and those neurotransmitters can generate electrical currents in the synaptic cell now transmission across the synaptic cleft occurs in several key steps we find that the synaptic cleft prevents nerve impulses from directly passing from one cell to the next there's chemical events that are that are occurring and this depends on release diffusion and receptor binding and the reason why this is actually more regulatable is because there's lots of steps and it involves chemicals there are things that the cells can do to either enhance or diminish their sensitivity to these chemicals now this also ensures uni-directional communication between our neurons if they're separated by space and the only thing that can that can communicate between that space of chemicals it's far less likely that one neuron can excite itself so information transfer occurs in six key steps here we learn about this to some extent going back to the muscular chapter and so this would be a nice summary of that and then member the action potential travels down your axon eventually reaching the axon terminal of your presynaptic neuron now the arrival of this action potential triggers voltage-gated calcium channels to open and that due to its electrochemical gradient calcium actually flows into the axon terminal now once calcium entry occurs this actually triggers the release of synaptic vesicles because calcium causes synaptotagmin protein to react with snare proteins that cause the fusion of your synaptic vesicles with the axon membrane and the fusion of the vesicle actually triggers the exocytosis of this neurotransmitter into the synaptic cleft now at higher impulse frequency you release more vesicles and this actually is a greater effect on the postsynaptic cell now once those neurotransmitters are released into the synaptic cleft we see that they can diffuse and bind to specific receptors on the postsynaptic membrane and these are often chemically gated ion channels now the binding of neurotransmitter opens these ion channels which can create graded potentials and these graded potentials can be their excitatory or inhibitory depending on whether they depolarize or hyperpolarized the voltage now binding causes receptor protein to change shape causes these ion channels to open in that these ion channels can allow certain types of ions to flow like some ion channels may only allow sodium to flow others might only allow chloride to flow and this can help us determine whether it's gonna be excitatory or inhibitory in terms of its effect on the voltage of that cell now neurotransmitter effects are terminated by three key phases here now our presynaptic cell can actually react take that that a neurotransmitter and reuptake is a way to recycle those neurotransmitters otherwise we can degrade them with enzymes and this is what we saw at the neuromuscular junction with acetylcholinesterase it actually breaks down acetylcholine by degradation or the neurotransmitters just diffuse away from the synaptic cleft and this occurs within a few milliseconds so we're saying that the effective neurotransmitters is rapid and brief now just to summarize this process we see that the action potential can travel down the axon once it reaches the axon terminal it can trigger the opening of these voltage-gated calcium channels calcium flows into the axon terminal through its electrochemical gradient and then once calcium enters it actually interacts with synaptotagmin which then causes these synaptic vesicles to fuse with the membrane of the axon here now fusion with the membrane is actually what exocytosis is and so we see that these neurotransmitters are now in our static cleft where they can bind to receptors on the postsynaptic cells membrane and once these were their transmitters bind to the receptor this might open an ion channel now if this were like the muscle cell we learned how neurotransmitters allow for the acetylcholine receptors to open and that allows for you know the influx of sodium and a little bit of a flux of potassium but that's what created the endplate potential which was essentially this depolarizing current that might trigger an action potential the muscle cell now what's interesting though is it for the rest of the nervous system and like with other neurons it's not always excitatory what can happen is that sometimes these neurotransmitters can bind to receptors that allow for ions to flow that actually make it the voltage of that cell even more negative than rest which is actually going to inhibit that cell from pretend you lead generating action potentials so then we call that an inhibitory type of graded potential now remember the neurotransmitters are released or sorry removed in three key ways we can either reuptake those neurotransmitters back into the presynaptic neuron those neurotransmitters might just diffuse away into the exercise fluid or they can be broken down by enzymes now there's actually a synaptic delay because it takes time for neurotransmitter to be released and it takes time for that neurotransmitter to bind the receptors and it takes time for those receptors to allow for ions to flow in order for the postsynaptic cell to receive a response and this can be anywhere from 0.3 to 5 milliseconds I mean this is a really really short period of delay but there is a delay that occurs and this can be a rate limiting step of neural transmission so that transmission of the action potential down the axon can be very quick but because the synapse is more slow this ultimately causes slowing of action potential information spread so we find is that with neural networks that have many synapses they're gonna be a little bit longer in terms of information spread than neural networks that have very few synapses you know when we talk about reflexes later we'll learn how when it's a multi synaptic reflex those reflexes are going to be longer because it takes longer for the information to get to you know either the brain or the effector organ otherwise if it's like a monosynaptic reflex where there's only one synapse in the whole network we find that it's actually the fastest type of response so although these types of synaptic delays aren't noticeable to us they're still very fast and they can be experimentally determined now this other type of synapse is less common but we call this electrical synapse and so neurons some neurons can be electrically coupled and what we mean by this is that instead of having the release of neurotransmitter the presynaptic neuron is electrically coupled with the postsynaptic cell through gap junctions and so we're saying that the cytoplasm is then of these two cells would be you know communicated and so communication in electrical synapses is very rapid and it can either be unidirectional or bi-directional because ion's flow through the gap junctions based on their electrochemical gradients and so you find these electrical synapses in some brain regions that are responsible for really rapid types of information transfer like eye movements or the hippocampus which is involved with emotions and memory and the most abundant type of synapse you find in embryonic tissue is the electrical synapse and this is because of just rapid information transfer however in adults you find that electrical synapses are more rare however they are faster than chemical synapses and you might wonder well why do we even have chemical synapses if electrical synapses are quick well the problem with electrical synapses there's a trade-off here is that yes they're fast but they're so simple because all they are is just a tube that connects one cell to the next these electrical synapses aren't as regulatable I will learn about later how chemical synapses are more modifiable because you can change the number of neurotransmitters being released you can change the number of receptors on the postsynaptic cell you can change the amount in activity of the degradation enzymes or the reuptake of those neurotransmitters and so those chemical synapses can be modified and molded more dynamically whereas electrical synapses are really basic so you might not get the same type of modify ability in the nervous system if all of our neurons and all of our synapses relied on chemicals I'm sorry electrical synapses