神经元结构概要

in this video I want to provide an overview of neuron structure neurons and adults have a soma it's also called a cell body soma and they have processes called neurites which are divided into dendrites and axons dendrites are usually short branched processes that are often covered in small spines that increase their surface area and perform some other functions so these are dendrites dendrites and then the other Nurik they have is called an axon which is usually long and unbranched until it reaches its end so this is the axon the area where the axon leaves the soma is called the axon hillock the axon may be short or it may be very long up to one meter or more and it usually is unbranched from most or all of that length until it gets to the end in these structures which are called axon terminals and at this point it will often branch and create multiple axon terminals the first part of the axon is called the axon initial segment where it's also called the trigger zone and we'll get into the reason for that in the next video axons can be so long that they are dependent on systems that transport substances from the soma which contains most of the organelles to the axon terminals and vice versa things have to be transported both directions and the axon is dependent on those systems large axons are usually wrapped in a sheath of a material called myelin and axons that have a myelin sheath have little gaps between these segments of myelin called nodes of ranvier so the sheath I've drawn and yellow is the myelin each of these little segments of sheath here and these gaps that it regularly interrupt the sheath are called nodes of ranvier nodes of ranvier these little gaps in the myelin sheath the axon terminals will come very close to the target cells of the neuron which I'll just draw here so these are the target cells and these target cells maybe another neuron they may be a muscle cell or they may be a gland cell a few neurons even have axons that terminate on capillaries to secrete substances called hormones into the bloodstream the place where an axon terminal comes close to touching the target cell is called a synapse this is a pretty typical structure for a neuron but there are multiple structural types of neurons each of which can be further divided into subtypes so let's go over some of the big categories of structural types of neurons in the central nervous system neurons start as neural stem cells which turn into most of the cell types of the central nervous system and these neural stem cells then differentiate into cells called neuroblasts and don't worry about the details here because we'll go into a lot more detail in other videos on development of the nervous system but neural stem cells and neuroblasts look pretty similar they're basically just shapeless cells without processes neural stem cells can become almost any neural cell of the central nervous system while neuroblasts can only become neurons neuroblasts will then migrate away from the neural stem cells to the location that their somas will have after development neuroblast then extend a process which is an axon toward their target cell and that axon is tipped with this structure called a growth cone growth cone the axon growth cone follows guidance cues in the environment until it reaches the target cell of the neuron a similar process occurs for neurons in the peripheral nervous system but the original and the migrating cells for those neurons are neural crest cells instead of neural stem cells and neural blasts neurons at this stage have only one process which is an axon so they are now called unipolar neurons unipolar that's the structural type of this neuron because there's one pole to the cell one process giving a sense of direction on this otherwise shapeless cell unipolar neurons are present in humans mainly during development the next structural type of neuron has a soma and it has one axon but it also has one dendrite so since this structural type of neuron has two processes or two poles it's called a bipolar neuron by polar the next structural type of neuron has a soma just like the others and one axon but it has multiple dendrites and so since it's gonna have multiple poles it's called a multipolar neuron multipolar and this is the most common structural type of neuron in adult humans the last big category of structural types of neurons is a little different it has a soma like all the rest and then it has one short process coming out of the soma that then divides into two long processes going in different directions and these are both axons the axon bringing information in from the periphery is called the peripheral axon and the axon bringing information into the central nervous system is called the central axon the very end of the peripheral axon acts a lot like dendrites do on the other structural types of neurons and will start to go over the function of dendrites and axons in the next video and then this part of the peripheral axon near the end is the axon initial segment where the trigger zone just like this part is on a multipolar neuron close to the soma and just like in these neurons where this is the trigger zone and then the end of the axon has the axon terminals in this type of neuron this is the trigger zone of the axon and then the axon terminals are all the way at this end of the central axon so this type of neuron is a big long funny name it's called a pseudo pseudo unipolar neuron pseudo unipolar and the reason is that it's kind of sort of like a unipolar neuron with only one process coming out of the soma but that little short process immediately splits into these two long axons so it's really a different shape than the unipolar neurons