3. Different Types of Neural cells
What are the Schwann cells?
-They are the glia cells of the peripheral nervous system -they are derived from neural crest cells -The main purpose of the Schwann cells is to isolate axon by myeline sheets. Remember that axons in the peripheral NS are super long. The myeline in the peripheral NS has the same structure and function as in the CNS but its produced by different cells.
What are some structures that neurons and glia cells share?
-most have the Soma/cell body that contain nucleus and organelles -coming out of the body you have processes which are long extensions that come out of soma
Neural cells (cells of the nervous system) are divided into 2 big categories:
1. Neurons (or nerve cells) 2. Glia cells
Sum up:
Astrocytes nourish neurones and produce Brain Blood Barrier. Ependymal cells produce Cerebrospinal fluid. Microglia ingest and brake down waste products and pathogens. Oligodendrocytes produce myelin in CNS Schwann cells produce myelin in PNS
Structure of myeline sheet:
AT the end you can see SOMA of the Schwann cell. And membrane is wrapped around the axon. That membrane contains myeline.
What is the purpose of AXON HILLOCK?
Axon hillock integrates excitatory and inhibitory signals from the dendrites and it fires Action potential if the excitatory signals are strong enough to reach threshold.
What is the purpose of AXON?
Axon transmits electrical signal/action potential from the soma to synaptic knob.
What is the collection of bodies called in CNS vs. PNS?
CNS: NUCLEUS PNS:GANGLION
What is the purpose of DENDRITE?
Dendrites receive incoming signals and carry them to soma.
So what makes up nerve?
Multiple neurons might be bundle together to make up nerves in the peripheral nervous system. These nerves might be: Sensory, Motor and mixed. The cell bodies of neurons that are the same type are cluster into ganglia.
What are Neuronal processes?
Neuronal processes may refer to: dendrites or axon. Things that come off soma.
Thing to notice:
Neurons are not physically connected to one another. Between the neurons there is a small space in which neurotransmitters are secreted that bind to dendrites of the post-synaptic neuron. This small space is called: SYNAPTIC CLEFT
What are neurons?
Neurons are specialized cells that are capable of transmitting electrical signals and then translating these electrical impulses into chemical signals.
Let's start with Resting membrane potential:
Normally neurons have charge difference across the membrane. Where there is no action potential and neuron is resting (Resting membrane potential). Inside the neuron's soma the charge is more negative (-70mV) and outside of the soma the charge is more positive.
What is the purpose of SYNAPTIC BUTTON?
Synaptic button lies at the end of the axon and releases neurotransmitters.
There are different ways in which we can categorized neurons in here:
The direction of information flow between CNS and PNS. -If the neuron brings the information from periphery into CNS we call that afferent neuron or it can also be called sensory neuron. Since it brings information about stimuli into CNS. -Neurones that carry information from CNS into periphery we call them efferent neurons. There are 2 main kind of efferent neurons: 1. Motor neurones control skeletal muscle 2. Autonomic neurones control smooth muscles, cardiac muscles and glands cells. -Neurones that connect 2 other neurons together are called interneurones and they are mostly found in CNS.
Lets look at the ependymal cells structures:
These cells form: simple cuboidal epithelium Simple-one layer of cells Cuboidal-cells are shaped like cubes Epithelium-they line cavity
Lets look at the specific glia cells:
-Astrocytes -Microglia -Ependymal cells -Oligodendrocytes and Schwann cells
Lets now move on to ASTROCYTES a type of glia cell:
-Astrocytes are glia cells of the CNS -star shaped glia cells -at the end of their processes they have special structures called: END FEET
Another Glia cell is: MICROGLIA
-Glia cells of CNS -Microglia derived from circulating monocytes from the bone marrow that enter the CNS. Therefore they are derived from mesoderm part of the embryo and not from ectoderm that all the other neural cells are derived from.
Another functions of the astrocytes are:
-Homeostasis is another main job of astrocytes cells. They keep ion concentration especially potassium in a specific rage. So they constantly monitor intrastitual fluid (fluid between the cells) and keep the fluid in specific range. Sometimes they will secrete lactate into that fluid. Neurons constantly need glucose and oxygen so when neurons are low on glucose they will use a bit of lactate from astrocytes. -Astrocytes form blood brain barrier with controls the transmission of solutes from the bloodstream into nervous tissue. Astrocytes will wrap their processes/END feet all over the blood vessels and prevent anything diffusing from the vessels into CNS.
Microglia cells come in few different shapes:
-Resting microglia have small soma and lots of large and branched processes. At this stage they are just resting and monitoring the environment. When they sense inflammation they convert into active form. They remove all the processes and they expand their somas. The inflammation might be from injury or infection (bacteria or other pathogens. -Active microglia have large somas and they look and act as microphages. So active microglia will move/migrate through CNS and look for bacteria or pathogens and if they find pathogens they will secrete a special chemicals that are toxic (Reactive Oxygen Species) that can kill the bacteria. After bacteria or other pathogens is killed they will take up that death cell. It actually does not even have to be pathogen, they will also take up dead cells of CNS just to remove them. This is called PHAGOCYTOSIS.
Lets look at the neuron structure?
-Soma/cell body that contain nuclei (DNA) -Dendrites come off soma and they receive incoming signals from other cells. -Axons are long or might be short. Usually axons are unbranched
Lets move on to OLIGODENDROCYTES:
-They are glia of the CNS -They are derived from neural steam cells -Oligodendrocytes extend few processes toward the axons in CNS and wrap the myeline sheet around the axon. So this is how segments of myeline sheets are created on the axons. -Notice that myeline sheet is connected to the soma of the oligodendrocyte cell by processes and to maintain the myeline sheet the soma/cell must be alive.
Types of Neurons:
-Unipolar neuron has one axon (one processes). These neurons are mostly present during development. -Bipolar neuron has one dendrite and one axon (2 processes) -Multipolar neurone has multiple dendrites. These neurons are the most common in adult human.
What are some of the functions of the astrocytes?
-they occupy huge space of the CNS and they are the foundation that forms the structure of the brain. Neurons and other glia cells then fill up that structure -If there is injury somewhere in the CNS astrocytes will divide and migrate to the area of the injury and surrender the injured area. Then they grow very large and thick processes that will wall off that area of injury. This is called: Glial scar and the formation of glia scar is called: ASTROGLIOSIS
What are END FEET on Astrocytes:
Astrocytes form the BLOOD BRAIN BARRIER by wrapping their feet around capillarities and preventing diffusion of large molecules to CNS.
How Axon Hillock works?
Axon Hillock plays important role in generating Action Potential in neuron. Signal that is arriving from the dendrites can be ether excitatory or inhibitory depending on which neurotransmitter is secreted. The Axon Hillock sums up these signals and if the sum of these signals is high enough (ether excitatory or inhibitory) and it reaches the threshold axon hillock will initiate action potential in neuron. Action potential then will travel down axon to axon terminal and neurotransmitter is released into synaptic cleft.
Notice that things must be moved up and down axon. From soma to axon terminal and back.
Axoplasmic transport, also called axonal transport, is a cellular process responsible for movement of mitochondria, lipids, synaptic vesicles, proteins, and other cell parts (i.e. organelles) to and from a neuron's cell body, through the cytoplasm of its axon (the axoplasm).
Ependymal cells separate:
Cerebral Spinal Fluid from Interstitial Fluid of the CNS. So they form barrier but that barrier is much more leaky compared to Blood Brain Barrier. This is actually useful for medical purposes. There are areas that we can sample the cerebral spinal fluid and test it and this will tell us what is happening in the brain tissue. Since the barrier is very leaky the fluid will move into the brain and things from the brain will move to the fluid. So we can see if any pathogens are present, blood or other things. This is called: Spinal Tap. Cells On the side that face Cerebral Spinal Fluid have small processes called microvilli that increase the surface area of the ependymal cells. They also have cilia that help move the CSF around.
Thing to know about neurones:
Each neuron has specific shape that matches its function.
So to sum up:
Each oligodendrocyte processes forms one myeline sheet on the axon but each oligodendrocyte might myeline many axons.
What are neurons derived from?
From population of cells called: Neuro-steam cells or Neurocrest cells (embryonic development).They are derived from ectoderm. Most neurons and Glia found in CNS come from neuro-steam cells while neurons and glia found in peripheral NS come from neurocreast cells.
Generating of the action potential:
If the membrane potential crosses a value called THRESHOLD POTENTIAL information then will fire down the axon in a form of ACTION POTENTIAL. Action potential will then travel to the end of the axon (axon terminal) and the way that information will cross to another cell is by crossing the small gap between the axon terminal and target organ called synaptic cleft. The way that happens is by usage of neurotransmitters molecules that will bind to the receptor on the target cell. Binding of the neurotransmitter will change the behavior of the target cell.
How do neural stem cells differentiate?
In central nervous system neural steam cells differentiate into neuroblast. Neuroblast will then migrate away to the location where their somas will be permanently. Then neuroblast will extend axon to its target cell, and that axon has a special structure on top of it called GROWTH CONE. Growth cone follows guidance cues in the environment. Neurons that have only one axon are called UNIPOLAR neurons.
Moving on to Ependymal cells:
Inside the brain and spine there are spaces that are full of cerebrospinal fluid. These spaces are connected to the small canal that goes down the spinal cord. The lining of these spaces that host the fluid is called: EPENDYMA and the cells that line that space are EPENDYMAL CELLS.
What happens after microglia phagocytosis pathogen?
It will take the pieces of the pathogen that just eat it and place them on its surface for other cells to see. This is mainly done for other cells-cells of the immune system. So lymphocytes will come in and recognize the pieces of the bacteria that microglia just displaced on its surface and this will start the immune system respond. So this will increase inflammation even more since more microglia will be activated in that process.
What is the main function of neurons?
Its to process and transmit information.
So ependymal cells:
Line the ventricles of the brain and produced the Cerebrospinal fluid which physically supports brain and serves as shock absorbent.
Lets look how neurons function together:
Look at the picture. You can see different neurons and their axons and dendrites. You can also see a line that separates CNS from Periphery NS.
Usually long axons are myelinated:
Most mammalian nerves are myelinated. Sheets of Myelin are wrapped around the axon and this prevents lost of signal while action potential is traveling long distances (some axons might be 1 meter long) or crossing of signal from one axon to another. In addition myelin also increases speed of conduction of axon. So action potential will travel faster in myelinated axon.
What is myeline composed of?
Mostly of lipids. So its a fatty shed that is wrapped around axons many times. This isolate axon. Which makes the transmission of Action potential fast.
What is the purpose of Myelin Sheets?
Myelin sheets acts as insulation around axon and speeds up conduction.
What are Axon terminals?
Neurotransmitters are packaged into synaptic vesicles that cluster beneath the axon terminal membrane on the presynaptic side of a synapse. The axonal terminals are specialized to release the electrical impulse of the presynaptic cell.
What are the small brakes between the myelin sheets on axon?
Nodes of Ranvier. Notice: we are talking in here about GAPS in myeline sheets. That expose axon and are essential for rapid signal conduction.
What is the difference between the Schwann cells vs oligodendrocytes?
Oligodendrocytes produce myeline for many axons. But one Schwann cell produces one myeline sheet. So the Schwann cells are glued to axons in the peripheral nervous system, wrapping the axon around and isolating it. SO the cell membrane is the myeline sheet on the axon.
What is the name of cells that produce myeline?
Oligodendrocytes produce myeline in central nervous system. Schwann cells produce myelin in peripheral nervous system.
But there is a problem when we call neuron a nerve cell. Why?
Peripheral nervous system is made up of nerves. But nerves in periphery contain both neurons and glia cells. SO referring to neurons nerve cells is not 100% accurate. Neurons are found both in the central nervous system and in the peripheral nervous system but CNS have specific types of Glia cells that are different then peripheral Nervous system.
What is the purpose of SOMA?
Soma is cell body that contains nucleus, ER and ribosomes.
What happens when there is demyelination of neurones?
Sometimes body will attack (immune system will attack) its myeline on neurons. The absence of the myelin will result in slower transfer of information between neurons. Multiple Sclerosis MS is common diseases of demyelination. These patients might experience: lost of balance, weakness, vision problems.
How neurons process information?
Summation of all the excitatory and inhibitory stimuli that come in to the neuron through dendrites occurs at the trigger zone. The summation of the graded potential is the way neurons PROCESS INFORMATION.
Draw a synapse:
TOGETHER: NERVE TERMINAL, SYNAPTIC CLEFT AND POST-SYNAPTIC MEMBRANE IS CALLED SYNAPSE.
Axons in the central nervous system make up:
TRACKS. Unlike nerves tracks only carry one type of information. The cell bodies of these neurons are grouped together into NUCLEI.
Where is Axon Hillock?
The axon hillock is a specialized part of the cell body (or soma) of a neuron that connects to the axon.
So lets get the main point on here:
The input information is converted into the size and duration of the graded potentials, is then converted into temporal pattern of firing of action potentials in the axon, this information is then converted to the amount and temporal pattern of neurotransmitter released at synapse.
Graded potential size and duration depends on:
The size and duration of the size and duration of the inputs that are coming into dendrites.
Neurons can receive excitatory or inhibitory signals from other neurons:
These signals enter neuron through dendrites. Then the information in transmitted through Soma of the neuron to the axon. These signals that are coming in from dendrites cause small membrane potential changes called: GRADED POTENTIALS. These are changes of the membrane potential away from the resting potential. They are small is size and short in duration. They also travel very short distances.
The last function of astrocytes is:
They also help clean up synapses between neurons. They do this by extending their processes into the synapse space so that the end feet plaster the synapse space and they clear up synapses by removing neurotransmitter.
What are Axon Terminals?
They come close to target cells, which can be another neuron, gland cell or muscle cell. Some neurons also synapse onto capillarities and control the secretion of the hormone into the blood stream.
What is the main function of microglia?
They detect and kill pathogens and infectious agents in the CNS. They take up dead cells of CNS and pathogens using PHAGOCYTOSIS.
What is the main function of Glia cells?
They support neurons.
What is SYNAPSE?
This is the place where axon terminal comes close to touching target cell. Remember that target cell can be another neuron, gland or muscle.
There is a special type of the neuron called: PSEUDOUNIPOLAR NEURON.
This neuron has some and then axon that goes in both directions. One side of the axon brings information from periphery (peripheral axon) and the other part of the axon brings information into central nervous system is called central axon. The end of the peripheral axon acts a lot like dendrites would do, and the end of the central axon acts like axon terminals.
What are Synaptic Bouton?
This structure is enlarged and flatten to maximized neurotransmission and ensure proper neurotransmitter release.
Lets get back and now focus on the function of the neurons.
What is the main function of the neurons? To process and transfer the information.
What is the function of Axon Hillock?
When signals come in from other neurons into the dendrites these signals are transmitted through cell body before it gets to axon hillock. Axon hillock integrates the information that are coming in from the dendrites.
