Cell Systems- Neuroscience PDF

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Summary

Nerve ImpulsesTransmitting information within single nerve fibres  Ionic basis of the resting potential  Generating an action potential (nerve impulse, spike)  Movement of action potentials along the axon  Frequency coding of signal strength  Recording methods: intracellular vs extracellularRec...

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Nerve Impulses Transmitting information within single nerve fibres  Ionic basis of the resting potential  Generating an action potential (nerve impulse, spike)  Movement of action potentials along the axon  Frequency coding of signal strength  Recording methods: intracellular vs extracellular Recap     

The nervous system is divided into CNS and PNS 2 key types of cells; neurons and glia cells Animal cells (including neurons) have phospholipid bilayers surrounding their contents This forms a barrier to water soluble ions as it has a hydrophobic regions Water soluble ions can pass the hydrophobic region via channel proteins (aka ion channels)

Important ions in neuron cytosol and the surrounding extracellular fluid include:  Potassium (K+)  Sodium (Na+)  Calcium (Ca2+)  Chloride (Cl-) They all carry an associated charge  The concentrations of these ions either side of the phospholipid bilayer and their ability to move across it are what generate the membrane potential and the equilibrium potential of a neuron  Membrane potential: the voltage (electrical potential), or the difference in charge between an anode (+) and a cathode (-), across a membrane  Equilibrium potential Membrane potential  Nerves, like many other cells, have a resting membrane potential of –40 to –80 mV (1 mV= 1/1000 volt)  Why is the inside of a nerve cell negative relative to the outside One KEY is the ionic concentration gradients  Particularly the K+ gradient

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The inside world- needs metabolic energy to make it different from outside The outside world- as produced by nature

Learn the direction of gradients, their approximate size and approximate actual concentrations

Inside- there is a high concentration relative to outside the cell, meaning in terms of the direction of the gradient- if there is a channel protein, direction of movement follows concentration gradientdiffusion out of the cell into extracellular fluid The resting membrane is relatively permeable to K+ ions, and impermeable to other ions (like Na+) So two key factors: 1. There is a higher concentration of K+ inside than outside the cell 2. K+ can move across the membrane, but not much else can Consequences of this...

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Membrane is selectively permeable to K+ =leakage channels Relatively impermeable to everything else K+ diffuses out of cell, down concentration gradient Takes positive charge with it, leaves inside negative Electrical gradient tends to pull K+ back into cell against the concentration gradient The more K+ leaves, the bigger the electrical gradient Not many ions have to move to set up the electrical gradient The chemical gradient is effectively unchanged Eventually (very quickly) electrical gradient= chemical gradient and system is in equilibrium

Nernst equation- calculating the equilibrium potential across membranes At equilibrium...

Can simplify this by dealing with the constants:

So at room temperate (20°C) Potassium equilibrium potential  e.g. if [K+]outside = 5 mM, [K+]inside= 50mM  Valency (z)= 1 for K+  Outside/inside = 0.1  Log10(0.1)= -1  K+ equilibrium potential = -58 mV Changing the concentrations (inside: outside ratios) will change the equilibrium potential  

Valency for Cl-= -1 Valency for Ca2+= 2

Typical equilibrium potential values in nerves  Every ion type has its own equilibrium potential, depending on its concentration ratio across the membrane  This equilibrium potential is given by the Nernst equation    

K+ Na+ Ca2+ Cl-

-80 mV +40 mV +120 mV -75 to –40 mV

Equilibrium potential can be calculated for any ion, whether or not the membrane is permeable to that ion So...

To a first approximation, the resting membrane potential is generated by  K+ concentration gradient  AND  The selective permeability to K+ If this was the whole story, the membrane potential value would BE the Nernst potential for K+ HOWEVER  

Real K+ equilibrium potential is typically –80 mV Real membrane potential is typically –65 mV

So something else going on besides (what is making the membrane potential less negative) Multiple ion types  The resting membrane is mainly permeable to K+, but is also a bit permeable to Na+  The actual membrane potential is a "compromise" between the K+ and Na+ equilibrium potentials, but weighted towards K+ because it has higher permeability  Therefore is the resting membrane potential. Ions are NOT in equilibrium, they are in a steady state Equilibrium and membrane potentials The resting membrane potential: steady state not equilibrium The resting potential can be calculated with the Goldman equation (this allows multiple ion types to be taken into account)  The membrane potential is mostly set by K+ and Na+

The Goldman equation (at body temp, 37°C)  PK and PNa is the permeability of the membrane to each ion  The terms in square brackets are the concentration inside and outside the membrane Say that the membrane is 40 times more permeable to K+ than Na+ and...

So... 350= 40(5) + 1(150) 4015= 40(100) + 1(15) May be written like this...

So... 8.75= (5) + 0.025(150) 100.375= (100)+ 0.025(15)

The resting membrane potential: steady state not equilibrium  There is steady leakage, K out of cell, Na into cell  Would cause gradients to run down  Gradients replenished by Na/K ATPase pump  

Gradients run down in >10 mins Membrane potential gradually lost without these pumps

Why have nerves? The job of the nervous system is rapid sensing, decision and action Step on a pin:  signal has to go from foot to head- or at least spine  Decision to withdraw has to be made- you can decide to step on it, if you're slightly mad  Signal has to go down to leg muscles lifting foot  Want to lift foot before impaled Signals are carried by nerve cells, rapidly, over quite long distances The nerve impulse How is information passed from one end of a nerve cell to the other  a wave of change in the membrane potential Two obvious questions:  What causes the inside to become positive?  Why does it spread along axon? Voltage-dependent ion channels  The resting membrane has a steady, relatively high-K low-Na permeability (leakage channels) which does not change. Overall leakage permeability is low  The membrane ALSO has voltage-dependent Na and K channels in cell membranes  These are SHUT at the resting potential, but OPEN if a stimulus makes the inside of the cell more +ve  The voltage dependent Na channels open quickly in response, but then automatically shut shortly after, even if the inside stays +ve (inactivation)  The K channels open more slowly and do not automatically shut, but shut if the inside goes back –ve  This sequence generates a nerve impulse (=action potential, =spike) Generating an action potential (summary)

g= conductance (the ability of the membrane to conduct or allow either Na or K through First graph: change in charge of intracellular fluid right beside the membrane Second graph: measure of stimulus input Third graph: conductance – ability of sodium (pink), potassium (blue) to pass through the membrane

There is a threshold 

Potassium channels now start to open

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Increase of potassium conductance Membrane potential more negative than rest- after-hyperpolarizartion (AHP)- ensuring a refractory period from one spike to the next and making sure that action potential propagates in one direction down the axon

Biotoxins and ion channels  Ion channels are crucial to neuron function and blocking them disrupts neuron ability to generate action potentials  Often target specific voltage gated ion channels  e.g.  Scorpions- numerous types + Na+, K+ and Cl Pufferfish- tetrodotoxin selectively bonds to Na+ - getting initial response to stimulus but Na channels can not open Refractory period 1  Early in the AHP (afterhyperpolarisation) after a spike (1-2 ms) it is impossible to get another spike, no matter how big the stimulus  Absolute refractory period  Due to voltage-dependent sodium channels still being shut from previous spike (inactivated)

Refractory period 2  Later in the AHP (about 3-10 ms) after a spike it is difficult to get another spike, needs a bigger than normal stimulus  Called the Relative refractory period  Due to some voltage-dependent potassium channels still being open from previous spike

Consequences of refractory period 1. Limits maximum frequency of action potentials  Once a neuron spikes, its cant spike again for a while  Maximum frequency usually ~300 Hz 2. Keeps propagation going in only one direction Movement of action potentials along the axon 

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Positive charge spreads out from the site of the spike- once there is a stimulus, voltage gated Na channels are opening- positive charge comes in to depolarise membrane and spreads from initial site of spike Stimulates the next region of axon Which generates a spike in turn

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Refractory period prevents spike going backwards (like a burning fuse)

Some vertebrates axons are myelinated Saltatory conduction speeds conduction up to 120 ms-1  Rather than creating action potentials all the way down entire length of axon, generation of action potentials jumps between myelinated portions made by Schwann cells between gaps called nodes of Ranvier Non-myelinated and myelinated conduction Non-myelinated: action potential regenerated at every point Conduction velocity: 0.1 - 10ms-1 (depends on diameter- fat axons faster) Myelinated: action potential jumps from node to node Conduction velocity: ~120 ms-1 (depends on diameter- fat axons faster) Frequency code for stimulus strength Advantages    

Unambiguous – all or none Can transmit long distances without loss of information Fast (but finite) conduction velocity: 0.1 to 120 m s-1 (fastest in a shrimp, ~330 m s-1)

Disadvantages  Can have problems interpeting weak signals with low frequencies  Needs time to integrate- weak signals, low frequency  Low frequency ceiling ~300 Hz  So needs range fractionation to cover wide range of signal strengths  Different neurons cover different ranges of signal strength  need different neurons to cover different ranges of signal strength to enable interpretations and detect different stimuli of different strengths Quantifying frequency: average  Measured in impulses per second (Hz)  Need horizontal scale bar to get time  Average frequency: count number of events in set time and scale to 1 sec Quantifying frequency: instantaneous  Gives you an idea of the intervals between firing events  Measure time interval between each spike in sec  Take reciprocal to get per sec (=Hz) (I.e. 1/x)  Measure time between spikes (100 in example below)  1/100= 0.01  Usually plot against time  X-value? Usually time mid-way between the 2 spikes generating the y-value

Two main electrical recording methods Intracellular  Place a microelectrode inside a neuron Extracellular  Place an electrode outside but near the neuron Intracellular  Place a microelectrode inside a neuron  Measure the voltage relative to the outside  I.e. the trans-membrane voltage

Can record      

Resting potential Synaptic potentials Spike All spikes same size and shape (all or none) But only from this neuron Difficult to do

Extracellular  Place an electrode outside but near to neurons  Measure the field potential in extracellular space relative to a distant site  Field generated by flow of current across membranes of nearby neurons as spike goes past

Cannot record  Resting potential (not inside)  Synaptic potentials (too small) Can record     

Spikes from any active nearby neuron Spike size depends on: Size of axon: big axons=big spikes Distance to electrode: near= big spikes Very different shape to intracellular

Relatively easy to do Comparison

Extracellular spike waveforms can add together  Main portion of spike is in the opposite direction because charges are being affected in different directions between intra and exta cellular

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If 2 neurons spike at same time Extracellular spikes can add together Can produce a distorted spike NO physiological significance Cannot happen with intracellular recording

How many neurons are active

1. 2. 3. 4. 5.

Biggest spike- non others this size and shape 3 similar spikes, probably all from same neuron Another single spike, different size from others The smallest clearly-defined spike, different from others Various small signals- may be distant or small neurons but probably just noise

So at least 4 neurons active near to recording electrode, but could be more Synapses    

Synaptic transmission- chemical vs electrical Transmitter release and presynaptic events Transmitter binding and postsynaptic events (EPSPs and IPSPs) Ionotropic ad metabotropic mechanisms

Basic information processing  Transmission over long distances happens within neurons by action potentials down the axons of neurons  At any point where a neuron must connect to another cell, information is passed between cells via synapses

Definitions Considering a synapse between two neurons  Presynaptic: the transmitting neuron that sends information to the other neuron  Postsynaptic: this is the receiving neuron that gets information from the other neuron

Synaptic transmission 1. Electrical synapses  Allow direct transfer of ions from one cell to another  Gaps between cells are bridged at sites called gap junctions  Distance between cells is extremely small and bridged by special proteins (connexins) that combine to form connexons  Two connexons (one from each cell) make up a gap junction channel  These channels allow movement of ions between cells Gap junction

Connexons are like tunnels linking inside of one cell to inside of the other  One connexon from one cell and another from the other meet in the gap between the cells, crossing extracellular space so ions can transfer from one cell to the other Transmission at electrical synapses  Any change in pre-synaptic membrane potential, +ve, -ve, can get transmitted to postsynaptic cell as post-synaptic potential (PSP)- influencing each other  Can be bidirectional  If there are lots of gap junctions  --> little loss of signal across synapse  --> may transmit 1:1  If only a few gap junctions  --> strong attenuation (gradual loss of flux intensity) (PSP used when responses need to be fast Electrical transmission occurs:  Where high speed is required – escape circuits (e.g. getting away from predators)  When group of cells need to produce more-or-less synchronous activity  -->Motorneurons innervating the same muscle  -->Not exactly synchronised, but "share" membrane potential changes between group 2. Chemical synaptic transmission is radically different 3 key areas of activity:

1. 2. 3. 4. 5.

Presynaptic terminals form swellings known as "boutons" Synaptic cleft widens to approx 20nm (but greater at nerve-muscle junctions) High density of mitochondria (to support high metabolic activity) in presynaptic terminal Many vesicles present in presynaptic terminal (containing many neurotransmitters) Protein accumulations called membrane differentiations in pre (Action zones) and post (postsynaptic density) synaptic membranes

Typical chemical synapse:

There are a few different ways to classify synapses in the CNS: 1. By the type of postsynaptic membrane and axon connects to

Examples below:  A) axodendritic synapse: axon is forming a synapse with a dendrite  B) axosomatic synapse: axon-soma  C) axoaxonic synapse: axon-axon 2. by the thickness of the membrane differentiations in the pre and post synaptic membranes  A) asymmetric protein thickness in membrane differentiations, where postsynaptic membrane has a lot more protein than pre-synaptic terminal  B) symmetrical membrane differentiations A chemical synapse in the brain Neuromuscular junction (NMJ)  Synapses can also be between neurons and muscle cells – motoneuron muscle junctions which allows neurons to send information to muscle cells to get them to move  Inward valleys shown on diagram below of muscle cell membrane that have receptors embedded in them  These increase surface area available for receptors to be present => when neurotransmitters are released, lots of receptors will be activated making it more likely for action potential to be converted into a chemical signal  

Synapses also pass membranes from nerves to muscle at motor end-plate regions These are the largest synapses in the body and always causes an action potential in the muscle

Chemical synapses use the movement of chemicals (neurotransmitters) to transfer information rather than ions What causes neurotransmitter release from a vesicle in the pre-synaptic cell? 1. Presynaptic cell activity     

Spike comes down axon- depolarising it Invades pre-synaptic terminal and depolarises it => lots of +ve charge in cell Depolarization opens voltage-gated Ca2+ channels present in terminal Ca2+ flows into presynaptic terminal This triggers exocytosis of vesicles – fusion of membrane of vesicle with membrane of presynaptic terminal

Depolarization of membranes -> Ca2+ inflow -> pre-synaptic cell releases neurotransmitter from vesicles into the cleft    

Complex set of proteins occur on vesicle and pre-synaptic plasma membrane Called snare proteins which grabs hold of vesicles full of neurotransmitters and holds them ready in position at the membrane of pre-synaptic terminal Ca2+ is essential link to lock snare proteins together and open fusion pore linking interior of vesicle to outside of cell => calcium is required for snare proteins to force the vesicles to fuse with membrane of presynaptic cell and release neurotransmitters

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Membrane potential in terminal is depolarized by arrival of the spike In most synapses, the calcium inflow has little effect on the membrane potential since concentrations are low => no electrical effect Although Ca2+ channels are voltage dependent, Ca2+ does NOT generate a spike Even though it does not generate electrical information, it does convert it into chemical information Too little calcium comes in, and its very local- in some animals, e.g jellyfish, calcium replaces sodium in generating action potentials- but not synapses

The job of Ca2+ is to act as an intracellular message converting the electrical depolarization caused by the spike into a chemical signal that triggers exocytosis Non-spiking synapses  We know that pre-synaptic depolarization opens voltage gated Ca2+ channels and Ca2+ inflow causes neurotransmitter release  However, some cells do not necessarily need a pre-synaptic spike  At some synapses, sub-threshold depolarization can open Ca2+ channels and release neurotransmitters without an action potential arriving at the terminal  At some synapses, neurotransmitter is released at normal resting potential  -->Can be increased by depolarization  -->Decreased by hyperpolarization  But can happen at normal resting potential- Like a leaky tap 2. Neurotransmitters and synaptic clef A single vesicle containing several thousand molecules of neurotransmitter The amplitude of the postsynaptic response to the neurotransmitter release in the synaptic cleft is therefore a multiple of the response to 1 vesicle of neurotransmitter e.g. 2 vesicles released will produce double the amount of molecules than 1

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How many vesicles?  In a typical synapse in our brain  Frequent fails (0 vesicles)  Often just 1 vesicle causing a successful transfer of information between cells  Sometimes 2-3 Various ways of increasing or decreasing number of...