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Topic 2: Cells and Signalling - Coggle Diagram
Topic 2: Cells and Signalling
hyperpolarisation
negatively charged
polarised
negatively charged on inside, positive on outside
resting state
depolarised
positively charged on inside, negative on outside
active state
What occurs in the polarised neuron?
intracellular space negatively charged overall
extracellular space positively charged overall
the difference in charge across the membrane sets up electrostatic gradient that creates forces acting on different chemicals
opposites attract so anything with positive charge wants to be in negatively charged space + vice versa
K+ and Na+ are both positively charged ions so electrostatic gradient creates force that would have them move into or sat in the cell
given that most of K+ is already inside, electrostatic force would only act to keep it there
strong force on Na+ for them to move into cell
difference in concentration of individual ions across the membrane sets up a concentration gradient that creates forces acting on different chemicals
individual chemicals will be forced to move down their concentration gradient towards area where they are in lowest concentration
for K+ this creates force that would have them move out of cell
for Na+ this creates force that would have them move into cell
LO 2.2
The resting cell - leaky membranes
although Na+ and K+ ions don't freely pass through the membrane, at rest, some cane move through 'leaky channels'
the membrane is naturally more permeable to K+ so more can leak through the membrane
even though the two forces acting on K+ are opposing, there's still a small amount of movement of K+ out of the cell at rest which helps keep the cell negative because K+ takes it positive charge out of the cell when it leaks out
Na+ leaks through the cell less easily than K+ but it moves according to the diffusion + electrostatic forces so movement is into the cell, making the cell more positive
the cell must be kept negative + we must have more Na+ outside than in so this leaking must be countered
LO 2.2
The 'active' resting cell - sodium-potassium transporter
to prevent the concentrations of ions becoming incorrect + to help keep the inside of neuron negative, there's an active pump
active process so requires energy
pump transports 3 Na+ ions out of the cell, countering the leakage of Na+ in, for every 2 K+ ions it transports into the cell, countering leakage out
ensures Na+ and K+ stay at correct concentrations but also results in net loss of positive charge, keeping cell negative
LO 2.2
What are voltage-gated channels?
ion specific
open + close when potential difference or charge across membrane reaches specific levels
at rest they're all closed, leaving only leaky channels + pump to move ions across the membrane
with a small activation, they can burst into coordinated opening + closing
occurs when cell fires, spikes or produces action potential
LO 2.2
Phases of an action potential
depolarisation
if there's sufficient stimulation, there's depolarisation
polarisation is reversed so inside become positive
at threshold, voltage gated Na+ channels open + Na+ rushes into cell forced by electrostatic + diffusion forces
voltage gated K+ channels open a little later but open slower with little movement of K+ initially
at the peak, the sodium channels snap shut + so flow of sodium into cell immediately halted
the channels not only close but enter state where they can't be reopened
repolarisation
cell returns to its original polarised state with inside being more negative
voltage gated K+ channels remain open when Na+ channels have shut + K+ now flows freely out of cell down its concentration gradient but also now pushed out by electrostatic force created when influx of Na+ turned inside positive, repelling positively charged K+ ion
as membrane approaches its resting potential again, voltage-gated K+ channels close
Na+ channels reset + come out of their refractory period meaning they could open again if threshold reached
hyperpolarisation
period of hyperpolarisation when cell is even more negative than it was at rest
due to slow closure of voltage gated K+ channels, some extra K+ manages to escape, resulting in even more negative internal environment
to return to resting state, sodium-potassium transporter must work to switch ions back round to their original concentrations
relative refractory period
although Na+ channels now able to open again, they're further from threshold than they normally are at rest meaning much bigger stimulus required for them to fire again
LO 2.2
Propagation of the action potential
process of depolarisation, repolarisation happens at each segment of the axon all the way from the axon hillock to the axon terminals
moves down the axon because:
axon hillock has very high density of voltage gated Na+ channels so can start the process
depolarisation of one area depolarises adjacent areas because Na+ ions spread out once they enter axon
depolarisation can only move in one direction because previously depolarised area is in refractory state - creates wave down axon with each patch of axon in turn being depolarised, repolarised + hyperpolarised
LO 2.2
What are two additional features of the action potential?
amplitude
difference between resting potential + peak of action potential
always the same for a neuron
means size of activating stimulus can't be signalled by amplitude
signalled by frequency so bigger stimulus equals a bigger frequency
speed
speed at which action potential travels down the axon (conduction velocity) varies according to:
temperature
higher temperature allows greater kinetic energy in channels + ions so speeds everything up
axon diameter
greater diameter means less internal resistance to movement of ions so they can move quicker, increasing conduction velocity
myelination
presence of insulating substance prevents ions leaking from axon + so depolarisation spreads further without decaying
means action potential doesn't need to be regenerated so frequently, only regenerated at Nodes of Ranvier, speeding up transmission
LO 2.2
Loss of myelin - multiple sclerosis
importance of myelination is clear when you consider the impact if multiple sclerosis
MS is an autoimmune condition in which the body's immune system attacks the myelin that surrounds the axons
the myeline disappears + there are no or too few channels underneath where it should have been to allow conduction of the action potential
LO 2.2/3
The end of the electrical signal
once the action potential reaches the end of the axon terminal, it can't travel any further as there's a physical gap (synaptic gap) between neurons the signal must travel across
to do this, it must be converted into a chemical signal (a neurotransmitter)
neurotransmitters
chemical that must be synthesised in the neuron or otherwise be present in it
chemical that must be released when neuron is active + produce response in some targets
mechanism must exist for removing chemical from its site of activation after its work is done
amino acids
glutamate + GABA
monoamines
dopamine, noradrenaline + seretonin
other
acetylcholine
key chemical signals of the nervous system carried via vesicles which go through constant process of recycling so their contents can be packaged up + stored until they're required
the presynaptic neuron contains voltage gated calcium channels as well as re-uptake channels whereas the postsynaptic neuron has specific receptors that work on a lock + key system in which the right neurotransmitter is required to bind properly
when the action potential arrives at the end of the presynaptic neuron, this causes depolarisation resulting in voltage gated Ca+ channels opening
Ca+ floods into the cell + interacts with SNARE proteins, causing vesicles to dock + fuse to the membrane, releasing neurotransmitters into the synapse
neurotransmitters diffuse across the gap + some will bind to receptors
binding causes ion channels to open in the postsynaptic membrane where ions will flow in, changing polarisation of the cell
LO 2.2
What are the two types of postsynaptic receptors?
ionotropic/direct gating receptors
one neurotransmitter binding will open one channel
metabotropic/indirect gating receptors
neurotransmitter binding will set off cascade that can build into the process of amplification resulting in many ion channels opening
LO 2.2
Types of postsynaptic potential
excitatory postsynaptic potential (EPSP)
there are ion channels selective for Na+ which will move into the post-synaptic cell because it's at rest
brings positive charge into the cell + causes depolarisation
inhibitory postsynaptic potential (IPSP)
there are ion channels selective for K+ which would leave the cell + those selective for Cl- which would enter
results in more negative inside, hyperpolarisation
LO 2.2
Combining postsynaptic potentials
temporal summation
repeated inputs from the same place in quick succession will add up
spatial summation
inputs arriving in different places simultaneously are added together
if after adding up all EPSPs + IPSPs the result is a membrane potential over the threshold at the axon hillock then this will result in an action potential in the postsynaptic neuron
LO 2.2
What are the four different ways that an action potential signal can be turned off?
enzymes
break up neurotransmitter, ingredients can be recycled
reuptake
neurotransmitter taken back into presynaptic neurons, ingredients can be recycled
diffusion
neurotransmitter simply drifts away so can't bind
glia mop-up
astrocytes can mop up excess neurotransmitters