so this video we're gonna focus more on the structure and function of neurons member neurons are the nerve cells which you find a nervous tissue but the structural units of your nervous system neurons are these large highly specialized cells that conduct electrical impulses now they're special characteristics is that first of all they have extreme longevity I mean you're pretty much born with the neurons you're gonna have in your brain your whole life and we also called neurons am i taught ik because they typically don't divide a whole lot so you know you got to be kind to the neurons you have now because that's what you got and they're not going to divide if your brain gets damaged however there are some few exceptions where there is new neurogenesis in the brain so there's a little bit of hope now neurons also have a high metabolic rate you know we think about nervous tissues occupying maybe several percent of your entire body weight but your brain you know it's several pounds but it uses like over ten percent of your body's energy so it's disproportionally very active and requires a lot of energy now it consumes a tremendous amount of oxygen and glucose and if either of these get too low your brain starts shutting down so all neurons have a cell body which is just the main piece of it and it's going to have one or more processes that stick off now what you find in the neuron cell body is the peri carrion or soma that's just the main part and we think of this is the biosynthetic center of your neurons because they synthesize proteins membranes chemicals this is we're going to find things like rough ER and under stain it looks like Nissel bodies and it contains a spherical nucleus with nucleoli some contain pigments and and most the plasma membrane is part of the receptive region that receives information from other neurons so the cell body can also receive information from other neurons now most neuron cell bodies are located in the central nervous system when the new when the neuron cell body clusters in the central nervous system it forms what we call a nuclei so nuclei are these clusters of neurons cell bodies in the CNS like the brain and spinal cord has lots of nuclei and then ganglia are clusters of neuron cell bodies in the peripheral nervous system what they'll look like these kind of rounded collections of cells that are associated with nerves but what you find those ganglia are the cell bodies of peripheral system neurons now in terms of what they look like you know they have these arm like processes that extend from the cell body the CNS contains both neuron cell bodies and their processes the peripheral nervous system contains mostly just the neuron cell processes because that's what nerves are made of you know for the most part nerves are just made of the axons and processes of neurons now tracks are bundles of neurons in the central nervous system or axons rather and then nerves are bundles of axon processes in the peripheral nervous system now there's two types of processes you have axons and dendrites and this is showing just a typical neuron so you find here's our soma or cell body that's going to contain your nucleus and most of your organelles then we have the extensions of this cell that stick off into space you know in the tissue and these extensions are involved with communication so these ones out here called dendrites because the route dendro means tree so they kind of look like trees without leaves on them and these are typically going to be like the input regions of information so information is transmitted to the dendrites and the cell body and then coming off the cell body we have a long slender process called an axon and the axon is what generates electrical impulses and then transmits those impulses really rapidly down its length now this axon shows myelin sheath wrapped around little segments of the axon and each of these is a myelin sheath and basically what these myelin sheaths do is they serve as an insulation that way these impulses can travel even more quickly now the end of the axon is what we call axon terminals this is a secretory region because at these little blebs and as we saw in chapter 9 this is this is the axon terminal that contains vesicles that contain neurotransmitter and that's used for communication now the dendrites of motor neurons contain hundreds of these short kind of tapering diffusely branch processes you know contain same organelles as the cell body and you think of these is like the receptive or input region of the neuron so these convey incoming messages towards the cell body is what we call graded potentials which are short distance signals that get smaller over time and distance but in many brain areas finer dendrites are highly specialized to collect information and they contain these dendritic spines which are appendages with bulbous and spiky ends each of those spines being a communication point within another neuron in fact most neurons in the brain have up to about a thousand connections or so on average so what these dendrites look like then of these long processes that how these all these spines stick out and the number of spines here represents the number of communication points so each of these little spines would be a place where communication may be occurring so if they look more spiny that suggests then that this neuron is you know highly communicative and that means that it could be involved with like learning and memory or information processing you can see that there are probably thousands of connections on these dendrites now the axon is a long slender process that actually starts at this cone-shaped region called the axon hillock since some neurons axons are very short or even absent in others the axon can be like feet in length you know axons that go from your spinal cord down to the muscles your foot have to be as long as your leg in so you know that's up to several feet long now these long axons are called nerve fibers and the axons have occasional branches called collaterals now axons branch profusely at their end or terminus and they can number as many as 10,000 terminal branches which means that each axon can form a lot of different connections now the distal ends are called axon terminals or terminal Bouton's and basically the purpose of this axon is to conduct information rapidly from the cell body along the length of that axon all the way to the axon terminal now the neurotransmitters are stored and released by your axon terminals which are released into an extra sylars space called the synaptic cleft and these and these neurotransmitters are widely varied but there's lots of different types some can excite and some can inhibit now the purpose of these is to basically carry on many conversations with lots of different neurons at the same time and these axons rely on cell bodies to renew proteins and membranes so you know axons are not a self-sufficient part of the CEL they need to have new proteins and other ions transported down the length of that axon from the cell body so think of the cell bodies like the main processing center and the axon just next extension but that extension is dependent on the cell body so if the axons are cut they can't exist on their own that has to be connected to a cell body now these can quickly decay if they're cut or damaged but because of the structure of axons and how they're packaged by the myelin sheath it turns out that axons can regenerate as long as the main cell like a cell body doesn't die so in terms of functional characteristics of axons they can transmit information in both directions you know anterograde means away from the cell body retrograde means back towards the cell body and you need examples of like anterograde communication is basically where you want my transport things like mitochondria cytoskeletal elements membrane components and enzymes down the length of that axon and what's pretty cool is we actually have motor proteins that transmit this in this basically material and so they carry this material down the length of the axon essentially down the cytoskeleton retrograde is the transportation of material from the end of the axon all the way back towards the cell body and this includes things like organelles to be degraded signaling molecules but even viruses and bacterial toxins so when you think of like a viral infection that maybe begins in the skin you know these viruses can kind of piggyback on your own Seiler machinery and then basically get transported back to the cell body where they can replicate now the myelin sheath is basically a whitish protein lipid substance that functionally protects and electrically insulates the axon so the purpose of this is to actually increase the speed of nerve impulse transmission and so myelinated fibers which have a sheath that surrounds most or you know large segments of the axon basically transmit their action potentials significantly faster than non myelinated fibers in fact non melanin fibers can conduct their action potentials at like maybe a foot per second whereas the myelinated varieties can be up to 300 feet per second now myelination that pns is actually caused by Schwann cells remember Schwann cells wrap around the axon kind of a jelly-roll fashion in one cell forms one little segment of the myelin sheath so the outer collar of perinuclear cytoplasm is basically called in the neural lemma and it's this peripheral bolds that contains the nucleus and most of your cytoplasm here but what's interesting is that myelin is a living cell it's a living cell it surrounds your axons and basically helps to you know speed up action potentials now the plasma membranes of these of these myelin sheaths have far less protein there's no channels or carriers so that makes some really good electrical insulators because these interlocking proteins bind adjacent myelin membranes and it prevents the decay of electrical signals down the length of that hexam so wrapping of the myelin is kind of interesting process where the cell kind of folds around this axon and it starts to fold around itself and these wrappings can be on the order of hundreds so the cell might wrap itself around the axon hundreds of times which means you get lots and lots of layers of membrane that basically insulate the extra souther fluid from the axon internal environment so what this looks like in cross-section is you know this large sort of well looks aren't here but actually be fairly microscopic like this is one micrometer which is about one one thousandth of a millimeter and what we find here then is that you know there's a fluid-filled Center and there's gonna be satisfy elements that support the axon and if this is the membrane of the axon neuron piece itself all of these outer wrappings all these layers here that almost looked like you know kind of ridges on a record you know all of these are basically just an extra layer of cell wrapping in fact out here is the outer collar of perinuclear cytoplasm that contains the living suan cell components like the nucleus and or other organelles that basically allow you to maintain and regenerate this this mountain chief now myelination the peripheral nervous system also leaves gaps and we'll talk about why this is important later we used to call these nodes of ranvier but these gaps are found between myelin sheaths and they're important because you need to regenerate the action potential will make sense this in a future video now non-validated fibers are very thin they're not wrapped in myelin and there's the can be surrounded by Schwann cells but there's no coiling which means that they don't have like extra insulation now what just to summarize this member we've an axon here and along the length of the axon we have each of these is a little myelin sheath and so each of these would be its own separate Schwann cell and they actually leave little gaps in between called the node of ranvier err myelin sheath gap so the myelin sheath is formed by processes of oligodendrocytes in the central nervous system so there's a different cell type here in the brain spinal cord but each of these cells can wrap many different axons at once whereas a Schwann cell can only wrap one individual axon oligodendrocytes can wrap up to sixty axons at once and myelin sheath gap is also present there's no outer collar because the these wrappings are made of extensions of the cell rather than the entire cell wrapping around one piece of axon and the thinnest fibers are unmyelinated but can be covered by adjacent neuroglia so if we look in the brain we can differentiate two types of brain tissue we have white matter and gray matter white matter are basically areas of brain and spinal cord tissue that appear more light or kind of more whitish and so what you find that is a basically dense collections of myelinated fibers and they're usually fiber tracts so if there's white matter in the brain it's usually just a lot of myelinated axons now gray matters mostly neuron cell bodies and non myelinated fibers and because it lacks that extra lipid sheath what we find is that this tissue you know at a visual level appears more gray so remember it's the oligodendrocytes that wraps axons here you can see that one look and inter-site can wrap many axons at once and this is the major myelin cell type of your central nervous system so in terms of structural classifications of neurons we have multipolar bipolar and unipolar and structurally these are names for the number of processes that basically extend off of the cell so if it's a multipolar neuron you have three or more across season extend off the cell usually one axon and mini dendrites if it's bipolar we find two processes usually one accident one dendrite and unipolar is one T like process where essentially you have you know a short dendrite that links up with a long axon and the cell bodies kind of off to the side so it looks got a long telling process so just to summarize this what we see here is a multipolar neuron you can tell because you have at least one axon and lots of processes coming off we have our bipolar neurons which are dendrite and axon and then unipolar which is basically a long slender axon with a cell body off the side now functionally what these are involved with is you know a variety of things for one multipolar neurons because they have lots of processes they can get a lot of input right because they have lots of receiving regions so because they can receive lots of information multipolar neurons are really good at processing information you're gonna find lots of multipolar neurons like in their central nervous system so your brain is spinal cord bipolar neurons have one dendrite one axon so these are good at just moving information along not necessarily processing but just transmitting information so you're gonna find that bipolar neurons are very common in the special senses like in vision and olfaction or smell you defined bipolar neurons that basically just transmit action potentials from one cell to the next and then unipolar neurons are also just really good at transmitting information and you find these a lot in the peripheral nervous system where you might have receptors in the skin in a long axon that goes back towards the spinal cord or brain for communication of information now we can see the just different varieties of multipolar bipolar and unipolar remember multipolar can have you know they come as a lot two different shapes and sizes but ultimately what defines multipolar is the fact that there's lots of processes coming off bipolar can also be differently structured but ultimately has of one dendrite and one axon it's really good at transmitting sensory information like from your visual system and smell systems and then unipolar neurons are found mostly in the peripheral nervous system and these are really good at transmitting sensory information from like the skin or joints or muscle of your body all the way back to your brain spinal cord now and functionally we can classified neurons into two major categories we have sensory and motor neurons sensory neurons transmit impulses from you know basically receptors towards the central nervous system they're almost always unipolar and the cell bodies are located typically in ganglia of your peripheral nervous system motor neurons carry impulses from the central nervous system out to effectors and most cell bodies are located in the central nervous system you know like the spinal cord or brain however some autonomic motor neurons have their cell bodies in the peripheral nervous system or in ganglia so inter neurons are also called Association neurons now inter neurons are typically going to be highly branched located in the central nervous system and most of your body's neurons are these inter neurons now because they're multipolar they're really good at processing of information because they can receive lots of different inputs simultaneously and then send an output to a different part of your body but again most your body's neurons are these inter neurons because most of your body's neurons are found in the brain spinal cord which are mostly interneurons so just to kind of summarize what these look like you know multipolar neurons could be like a motor neuron which transmits information out towards muscle you know bipolar neurons are common in these special senses like for vision and smell but they basically transmit impulses back towards the brain spinal cord and then unipolar neurons are almost exclusively found in the peripheral nervous system and their video a really good job at transmitting information back towards the brain and spinal cord rapidly for basically you know transmission of sensory or afferent information