突触结构

in this video I want to talk about the structure of the synapse synapses are where neurons contact and communicate with their target cells and the word synapse comes from Greek words meaning to clasp together so let me start by showing where synapses happened so first I'll draw the soma of the neuron and read and a few dendrites these branching processes of neurons in blue one dendrite and here's another dendrite and then I'll draw the axon of the neuron in green is this long unbranched process until it reaches the end and then it branches into multiple terminals I'll just draw a few here but it can have many many terminals and then let me just draw some target shapes to represent the types of target cells that a neuron may contact with the synapse and these cells may be another neuron they might be a muscle cell or they may be a gland cell and some neurons even have axon terminals that end on blood vessels to secrete substances into the bloodstream called hormones so synapses are these spots where the axon terminals of a neuron are contacting their target cell and there are a couple of types of synapses one of which has a gap like I've drawn here although it's actually a much smaller gap that I've drawn and the other one doesn't have a gap they're physically connected the type of synapse that has a gap is called a chemical synapse because it releases molecules or chemicals at the synapse that cross from the axon terminal to the membrane of the target cell the other type of synapse is called an electrical synapse and with this type of synapse the cells are actually physically connected so that the axon terminal physically connects with the membrane of the target cell and there are special channels called gap junctions that actually let the inside of the neuron communicate with the inside of the target cell the cytoplasm of the two are really connected and ions can flow directly from the neuron into the target cell in this set of videos I'm just going to talk about the chemical synapses where there is a gap because they're far and away more common than electrical synapses which are fairly rare at least in humans now a typical human neuron can have a massive number of connections through synapses it can connect to thousands of different target cells through its axon terminals and the typical neuron receives information through thousands of synapses most of those synapses come in to the dendrites so that the axon of another neuron is contacting and communicating with the dendrite of this neuron and there can be many many synapses on a dendrite and part of the reason its branched is just so they can have more surface area to form more synapses so that for most neurons most of the synapses are actually coming in through their dendrites however most neurons also do get a smaller number of synapses coming in to the soma and there are even synapses on to the axon but usually not just anywhere on there the axon it's usually on to the axon terminal so that this axon comes in in its axon terminal is synapsing on the axon terminal of this neuron and could be the same with this axon terminal it could have synapses and this axon terminal right here and so try to imagine thousands of synapses just literally covering this whole neuron and that it in turn is connecting with thousands of target cells through synapses and you can just imagine how complex the information is that's flowing into and out of the neuron from other parts of the nervous system but now let's zoom in and let's look at the structure of an individual synapse like for instance this synapse right here let me start by drawing a big axon terminal so I'll just blow up that axon terminal and make it really big here in green and then I'll just draw a target type of shape to represent the target cell which again could be another neuron could be a muscle cell or it could be a gland cell and then in the central nervous system covering most of the synapses are the end feet of astrocytes let me just draw that in purple and I actually just label that let me just write astrocyte and this would be one of the end feet to the Astro site that in the central nervous system are just plastered all over synapses so for a chemical synapse like this there's a gap here between the membrane of the axon terminal of the neuron and the membrane of the target cell and the gap is actually much smaller than this but I just needed a little room to draw it's a very very small gap but the cells are not actually physically touching each other there is a gap and we call this gap the synaptic cleft let me just write that out synaptic cleft and that's the space between the neuron and the target cell and then we have names for this piece of membrane over here and this piece of membrane this membrane facing the synaptic cleft on the axon terminal of the neuron we call the presynaptic membrane presynaptic synaptic presynaptic membrane is right here because it's before the synaptic cleft and then this piece of membrane on the target cell facing the synaptic cleft is the postsynaptic membrane postsynaptic membrane that's right over here and so it's presynaptic because it's before the synaptic cleft and it's postsynaptic because it's after the synaptic cleft just on the inside of the presynaptic membrane our vesicles which are little membrane enclosed bubbles inside the cytoplasm of the neuron and these are called synaptic vesicles synaptic vesicles that's each of these these bubble like structures just inside the presynaptic membrane of the neuron and these synaptic vesicles are full of molecules called neurotransmitter just rung couple of dots in here to represent these molecules and they're called collectively neurotransmitter because they transmit information from the neuron to the target cell so all of these molecules are collectively called neurotransmitters and there are different types of neurotransmitters that we'll get into in other videos on the postsynaptic membrane are receptors that are specific for the neurotransmitter in the synaptic vesicles the neurotransmitter will fit like a key and a lock to these neurotransmitter receptors on the synaptic membrane and in the next video we'll talk about how the neurotransmitter and the synaptic vesicles is released from the presynaptic membrane to cross the synaptic cleft and bind to its receptors on the postsynaptic membrane