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neuron vs neuroglia???
- NEURON: functional cell of nervous system; no division
- communication to muscles or glands, etc.
- produce action potentials!!!
- no division... so once it's damage, it can't really be repaired, cannot be regenerated and can loose function
- NEUROGLIA: "support cells"; may divide
- repair, regulate, protect, and support
- outnumber neurons ~20:1!!! but don't produce action potentials!!!
- may divide... need to repair and regulate during infection or trauma
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what are the CNS neuroglia (4) vs PNS neuroglia (2)???
- CNS glial cells: astrocytes, microglia, ependymal cells and oligodendrocytes
- PNS glial cells: schwann cells and satellite cells
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1st CNS neuroglia: ASTROCYTES... general features? (6)
- 1. largest and most numerous CNS neuroglia
- - structural framework/support for CNS (very abundant and have an extensive cytoskeleton, they have their own internal cytoskeleton)
- 2. maintain extracellular environment
- - regulate osmolarity of K+, Na+ and CO2 (basically of ions and gases)
- - conduit for nutrients, ions and dissolved gas from vasculature to/from neurons
- - 1 process is wrapped around both the axon and the capillary! acts like a channel from nutrients from blood to neurons w/c is great cause neurons need lots of ions and gases
- - communicate with each other and neurons via gap junctions
- 3. enhance or supress synaptic communication by absorbing and recycling neurotransmitters (especially glutamate and GABA)
- - shows how it has hands on everything
- - since involved with synapse, can absorb NTs released
- - impt bec the key to neuronal transmission is to be fast and to react, can't reverse signal until NTs are absorbed, NTs shouldn't be hanging around because signal gone
- - figure shows an astrocyte surrounding a SYNAPSE... unmyelinated regions on NODES, astrocyte surrounds these nodes
4. contribute to neuronal development in utero
- 5. components of the blood brain barrier.... feet of astrocytes cover capillaries!!!
- - limits movement in & out... also limits blood flow and volume via chemicals secreted by astrocytes
- 6. stabilize damaged neuronal tissue
- - migrates to damaged area to wall of injured tissue and stabilize area
- - astrocytes allow more blood to enter damage site to bring in cells needed for repair
- - can move to site of damage so if you have a trauma, it tries to stop it from spreading
- - brain is like butter... astrocytes bring structure to something to prevent damage
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2nd CNS neuroglia: MICROGLIA... general features?
- 1. wandering police force and janitors
- - phagocytose debris & pathogens
- - monitor and patrol the brain for infection
- 2. monocyte/macrophage lineage: enter CNS during embryogenesis
- - originally an immune cell, makes their way to the brain during development
- - can upregulate and increase in concentration in times of need
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3rd CNS neuroglia: EPENDYMAL CELLS... general features?
- - line the brain ventricles and central canal of spinal cord
- - produce, circulate (through cilia in ependymal cells) & monitor composition of CSF
- - cushions the brain
- - transports dissolved nutrients, gases and waste
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4th CNS neuroglia: OLIGODENDROCYTES... general features?
 
- - only in CNS!!! for PNS = schwann cells
- - produce myelin for CNS neuron to insulate them
- - it insulates neuron to prevent ion loss is neuron, important to neuronal communication
- - myelin = layers of oligodendryte cell membrane (shown)
- - concentric layers of oligodendrocytes cell membrane wraps around axon (shown above)
- - many oligodendrocytes contribute to myelination of 1 axon
- - 1 oligodendrocyte can wrap around several axons
- - but 1 oligodendrocyte can't myelinate a whole axon so needs several oligodendrocytes to myelinate a whole axon
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unmyelinated region; "spaces"; term used in both CNS and PNS
node of ranvier
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single myelinated region
internode
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acts as insulation around axon; limits leakage of ions out of and into the axon; therefore, increasing conduction of electrical signals; usually found where speed is vital ex. cortex of spinal cord (reflexes) and peripheral nerves; made of lipids (gives brain its white color!!!)
myelin
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white matter vs grey matter
- white matter: are regions of CNS & PNS containing numerous myelinated axons; myelin = lipid = fat; so the fat gives it the white color
- grey matter: regions of CNS & PNS contains neuronal cell bodies, dendrite, and unmyelinated axons ; has clusters of rough ER and ribosomes called NISSL BODIES that give grey matter its grey color
- SUMMARY: white = myelinated = bec of fat in myelin
- SUMMARY: grey = unmyelinated = bec of nissl bodies
notice a FLIP: grey matter on periphery of CNS but white matter on periphery of PNS
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how is the PNS neuroglia different from the CNS neuroglia?
- PNS are all the neuronal tissue outside the brain and spinal cord
- delivers sensory info to CNS & carries out motor commands
- PNS is not protected by blood brain barrier, blood-CSF barrier or cranium SO...
- it's more readily exposed to toxins and mechanical trauma (more vulnerable than CNS)
- so the entire neuron is protected or covered by neuroglia
- in PNS, the neurons are engulfed by neuroglia bec not as protected as CNS
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1st PNS neuroglia: SATELLITE CELLS... general features?
- protects ganglia (neuronal cell bodies in PNS) from surrounding environment (from the ECF) which might not be always perfect
- regulates gases, nutrients and NT surrounding ganglia
- similar to astrocyte in CNS!!!
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2nd PNS neuroglia: SCHWANN CELLS... general features?
- produce myelin for PNS axons
- a schwann cell can only cover one axon (diff from oligodendrocytes!!!)
- axons are covered by multiple schwann cells
- unmyelinated axons are also covered by schwann cells... they enclose the axons and not myelinate them... don't make concentric layers of cell membrane of schwann cell... to protect from ECF
- similar to oligodendrocytes in CNS
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unmyelinated PNS axon vs myelinated PNS axon?
- unmyelinated: has one coat of myelin... schwann cell spreads a thin layer of myelin to surround axon in 1 loop not to insulate it but to protect it from ECF
- myelinated: has mutiple coats of myelin... schwann cell spreads layers of myelin for insulation
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demyelination disorder: primary damage vs secondary damage???
- primary damage: myelin and/or myelinating neuroglia... basically the 1st even to go wrong, damage usually done here
- secondary damage: axons --> poor neurotransmitters; causes cognitive, sensory and motor problems... axons are naked where they shouldn't be naked... and axons can't effectively communicate
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demyelination disorder: guillian-barre syndrome
- immune mediated loss of PNS myelin that causes impaired neurotransmission
- incidence: 1 in a million
- etiology: bacterial or viral infection (respi or GI) or vaccine
- symptoms: progressive paralysis, but 70% of people usually resolve this problem... starts hard but eventually goes away
- treatment: (1) plasmapheresis... basically wash plasma to get rid of antibodies since the immune cells are doing the damage; (2) supportive treatment - ex glucocorticoid to decrease infection
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demyelinated disorder: multiple sclerosis
- immune mediated loss of CNS myelin
- incidence: 1 in 1000... if you're in 3rd decade of life... females have a 2-4X more chance... if you have a 1st degree relative, 15X more chance... geographic location at first 15 yrs of life, farther from equatior are more likely to get this
- etiology: (a) immune disease, (b) viral mimicry
- contributing factors: sunlight, diet, genetics and hormones (so better to get more sun, eat more fish, get more vitD and male)
- pathology: chronic T cell-mediated demylenation of axons... cytokines recruit macrophage which stirs things up and does further damage... peri-neuronal scarring... impaired neurotransmission... periods of remission
- treatment: glucocorticoids to decrease inflammation, interferon-beta to decrease immune system, muscle relaxants to decrease twitching and tetani
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review: which ions are more on the inside and more on the outside of the cell???
- more Na+ outside
- more Cl- outside
- more Ca2+ outside
- more K+ inside
- more A- inside
A- = negatively charged proteins, polypeptides, organophosphates that can't migrate across the membrane (DNA, RNA, etc...)
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concentration gradient for individual ions across the selectively permeable cell membrane
- ionic gradient aka chemical gradient
- causes the membrane to be polarized... the differences across the membrane is energy waiting to be utilized
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the inside of the cell is more negatively charged, ex -70mV for neurons
membrane potential
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membrane potential of an undisturbed cell
- resting membrane potential
- is 'consistent' but ions are always moving across membrane
- so basically stays around 70mV even though ions are constantly moving in and out
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what creates the membrane potential??? why is the cell interior more negative???
- 1. permeability: K+ is more permable than Na+ so more K+ leak channels take K+ OUT of the cell... loss of positively charged K+ inside the cell (aka more positively charged K+ outside)
- 2. Na+/K+ pump: 2K+ inside, 3Na+ outside = more positively charged cell exterior
- 3. fixed negatively charged proteins: A- are stuck inside the cell
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differential distribution of ions across cell membrane
chemical gradient
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differential distribution of charge across cell membrane; opposites attract (think of magnets)
- electrical gradient
- electrical gradients can either reinforce or oppose the chemical gradient for each ion
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chemical gradient + electrical gradient!!! chemical gradients and electrical gradients have the potential to do work... they're combined forces that influence the movement of ions
- ELECTROCHEMICAL GRADIENT
- ECG for K and Na are the primary factors affecting membrane potential!!!
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describe what happens to K and Na during (1) chemical gradient, (2) electrical gradient, (2) overall electrochemical gradient
- K chemical: a lot of K+ exits (down chemical gradient)
- K electrical: little K+ enters (because inside of cell is more negative)
- K electrochemical: K+ exits
- Na chemical: a lot of Na+ enters (down chemical gradient)
- Na electrical: little Na+ enters (because of negative charge inside)
- Na electrochemical: A LOT MORE Na+ enters (if it could)
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equal but opposite chemical and electrical gradients; driving force of gradient AND ion charge are opposite but equal? this potential for every ion is different!
- EQUILIBRIUM POTENTIAL
- reminder: at rest the membrane permeability for K is high and Na is low
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what would happen if the membrane became a lot more permeable to Na?
Na+ would rush into cell moving toward equilibrium potential and makes the cell more positive
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charge across membrane is less polar; membrane potential shifts toward a more positive one; positive ions (Na) rush into the cell; membrane potential moves from -70mV --> -60mV --> 0mV --> +40mV
DEPOLARIZATION
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re-stores resting membrane potential; gets back to -70mV; positive ions (Na and K) leave the cell
REPOLARIZATION
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membrane potential moves away from 0; cell interior becomes more negative than normal resting membrane potential; positive ions (K) leave cell (efflux) ORRRRR negative ions (Cl) enter; decreases the chances of generating an action potential because further from the likelihood of reaching threshold
HYPERPOLARIZATION
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graphical representation of depolarization, repolarization and hyperpolarization:
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ligand gated channels VS voltage gated channels in relation to neurons???
- ligand gated channels: in dendrites and soma... results in graded potential (red area on graph)
- voltage gated channels: in axon... results in action potential (green area on graph)
remember: voltage gated Na channels have an activation and inactivation gate that function independently
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how does presynaptic cell communicate with postsynaptic cell?
- SYNAPTIC ACTIVITY
- basically, neurotransmitter released which then binds to postsynaptic receptors on B
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what causes presynaptic cell to release neurotransmitter at synapse?
- ACTION POTENTIAL
- happens all the way down length of axonal projection
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membrane potential of a cell in 'action;' rapid, large depolarization followed by repolarization at the membrane potential; (1) transmits electrical signal long distances, self regenerating at nodes, (2) AP will only happen if threshold is reached
ACTION POTENTIAL
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membrane potential (-60mV for neurons) at which many voltage gated Na channels open up to initiate an action potential; Na can rush in cell to generate action potential if this is reached
- THRESHOLD
- action potential can't take place w/o reaching threshold
- impt to remember: not every depolarization is big enough to reach threshold and result in an action potential
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what causes action potentials?
GRADED POTENTIALS
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how do graded potentials generate an action potential?
- key = threshold
- enough or strong enough graded potentials are generated such that threshold is reached
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graded potential... basic characteristics???
- a local change in membrane potential that occurs in dendrites, some and axon hillock
- stimulus opens a ligand or mechanical gated channel
- local current... change in membrane potential doesn't travel far, it only affects local regions of cell membrane
- can be DEpolarizing or HYPERpolarizing because it depends on what channel is actually opened (Na or K or Cl)
- many different types of cells have graded potentials... ex motor end plate and glands
- it DECAYS as it moves... it's only a short distance signal and doesn't regenerate
- neighboring cells become depolarized bec of Na going in and it dissipates slowly... affects stuff next to it depending how close it is to the localized area
- graded potentials are smaller (smaller change in mV) compared to an action potential
- magnitude of depolarization depends on amount of stimulus (larger, longer, stimulus leads to a larger, longer graded potential)
- can be summed to reach threshold... graded potentials are additive!!! can combine to all lead to an action potential
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self generating wave of electrochemical activity due to opening of many voltage gated Na channels on the axon; dramatic change in membrane potential (from -60mV to +30mV)
ACTION POTENTIAL
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action potential... basic characteristics???
- only occurs in excitable membranes... ex in neurons and muscle fibers... begins at initial segment and travels down axons
- regenerated at unmyelinated regions (nodes) of axons
- always DEpolarizing!!! (remember: difference... graded can be de or hyper!)
- all or non response... once threshold is reached, can't loo back!
- magnitude of stimulus is 'negligible': maximum membrane potential is +30mV... can't get any more depolarized than this (diff from graded!!!)
- doesn't decay!!! long distance signal that regenerates at nodes, duration is always the same in a given tissue (diff from graded!!!)
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depolarization positive feedback?
- graded potentials increase the amount of intracellular Na to cause further depolarization... neuron may now be depolarized enough to reach threshold
- at threshold, there is enough Na inside the cell via graded potentials, to open voltage gated ion channels to cause an action potential
- POSITIVE FEEDBACK = adding more Na inside causes more voltage gated Na channels to open!!!
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generation of an action potential: what happens during DEPOLARIZATION???
- ligand or mechanical gated Na channels open at SOMA: causes Na to enter soma and create a local current, graded potentials are generated at hillock, membrane potential approaches -60mV
- action potential BEGINS as voltage gated Na channels open up at initial segment open (activation gate opens)... Na floods into neuron and membrane potential moves from -60mV to +30mV... triggers voltage gated Na channels to open all along axon (propagation of AP)
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generation of an action potential: what happens during REPOLARIZATION??
- 1. neuron membrane potential starts at =30mV
- voltage gated Na channels close via inactivation gate and no more Na enters cell
- voltage gated K channels open and K rushes out of cell to bring down to resting membrane potential
- 2. neuron membrane potential at -70mV and goes to -90mV
- voltage gated K channels slowly shut and membrane potential reaches -90mV (hyperpolarization!!!)
3. Na/K pump normalizes levels to return to -70mV
4. neuron is now ready for another action potential... can generate 1,000 action potential per second
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when neuron absolutely can't generate another action potential; voltage gated Na channels need to fully recover
ABSOLUTE REFRACTORY PERIOD
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neuron can generate another action potential if stimulus is strong enough; when hyperpolarized!; BUT stronger stimulus is needed to reach threshold
RELATIVE REFRACTORY PERIOD
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graphical representation of absolute refractory period and relative refractory period:
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