BMS11004 - WEEK 2 TUESDAY, WEDNESDAY, THURSDAY Flashcards
plasma membrane properties, resting membrane potentials, electrochemical gradients, Nernst equation, Goldman equation, E ion, APs, Na+ channels, useful poisions
name the 2 types of electrical signal
action potential, graded potential
outline action potential - size, direction, coding style
fixed size, all-or-nothing signals that travel along (propagate) axons. can pass either way but tends to go one way
coded by frequency as are unit of size = lots of stimuli so larger AP per s
outline graded potentials - size, direction, coding style
variable size, local signals not propagated over long distances, passes both way along neuronal membrane
coded by size, varying according to strength of stimulus
why do neurons have resting potentials
membrane is selectively permeable to specific ions
unequal ion distribution
physical forces
what charge is an absolute requirement for nervous system function
negative charge
why is neuron membrane resting potentials selective and unequal
selective channels- passive, ions go along conc gradients
pumps assisting unequal distributions of charge (active, against conc gradient so require ATP)
what 2 forces control ion movements in aqueous solutions
diffusion, electrical field
what is equilibrium in regards to diffusion, membrane potentials
ion channels open so ions can flow down conc gradients, reaching equilibrium (equal charge per side)
how can we study movement of ions across membranes using currents
movement of charged ions causes electric current (I, measure in ampere). connect battery, introduce membrane channels and see movement of ions toward opposite electrode
how much current flow across a membrane is dependent on what 2 things
- electrical potential (voltage), reflecting difference in charge between anode-cathode
- electrical conductance, depending on number of ions abailable to carry charge and ease of which they can travel through space
define voltage
force exerted on charged particle
define electrical conductance
relative ability of charge to move from one point to another (g, measure in siemens)
what is symbol of electrical current and measured in what
I, amperes
define electrical resistance
relative ability of electrical charge to migrate (R, measure in Ohms. R = 1/g)
what is Ohms law
relationship between potential (V), conductance (g) and amount of current (I) that will flow
current is produce of conductance and PD so if conductance 0 then no current will flow even if a large PD
I=gV
what sets up an ionic concentration gradient
ion pump in membranes eg: Na+/K+ ATPase, Ca2+
what do Na/K pumps do
exchange internal Na+ for extracellular K, moves against conc gradient so require energy (breaks down ATP)
define membrane potentials
voltage across neuronal membrane at any given point, represented by Vm (measured by inserting microelectrode into cytosol)
what does Ca2+ pump do
transports Ca2+ out, maintains low intracellular Ca2+. is important:
1. Ca2+ are signalling ions. changes in Ca conc detected, is used in controlling cell functions
2. high intracellular Ca2+ is toxic and kills neurons
what is Eion
equilibrium potential. membrane potential that would be achieved in neuron if membrane were selectively permeable to that ion
how do ionic gradient influence membrane potentials
determines equilibrium potentials Eion
explain what Vm = Ek mean
(Ek = potassium equilibrium potentials)
equilibrium occur when electrostatic forces equal diffusional forces
if more K+ on left of membranes than right, why is there no net movement
phospholipid bilyaer has no channels so impermeable, no PD between inside/outside
Vm equal to 0mV as ratio of K+ to anions on either side of membrane equal 1 = neutral
what 4 key things does steady electrical PD across membrane require alongside ionic conc gradient and selective ionic permeability
- large membrane potential change caused by small ionic conc changes
- net difference in electrical charge across membrane due to thin membranes allowing ion interaction electrostatically meaning negative charge inside and positive charge outside attracted to cell membrane
- ions driven across membrane at a proportional rate to diff between MP and equil potential
- if conc diff across membrane known for ion, then Eion able to be calculated for specific ion
if more K+ on left than right of a membrane, what does introducing K+ channels do
allow movement from left to right down conc gradients via diffusion out of cell. corresponding anion too large to go through, so stay on left side. mean inside more negative, occur until reach electrical PD across membrane, so electrical force start to pull positive K+ into cell
if specific PD reached then electrical force pulling K+ ions in equal force of diffusion pushing K+ out = equilibrium
how can we measure resting membrane potential
connect neuron to voltmeter (measuring PD between 2 electrode), insert glass microelectrode filled with KCl to carry cahrge into neuron, and other electrode (silver chloride) into extracellular solut.
inside of membrane negative relative to outside
when electrode enter resting cell, value change from -65 to -90 mV
unequal charge distrib across membrane
what is Nernst equation used for
used to calculate equilibrium potential (Eion) for an ion
take into account temp, ion charge and ion conc outside of cell
in Nernst equation, what does R stand for
Eion = 2.303 RT/zF log (ion)o/(ion)i
gas constant
in Nernst equation, what does T stand for
Eion = 2.303 RT/zF log (ion)o/(ion)i
absolute temperature
in Nernst equation, what does z stand for
Eion = 2.303 RT/zF log (ion)o/(ion)i
ion charge
in Nernst equation, what does F stand for
Eion = 2.303 RT/zF log (ion)o/(ion)i
faradays constant
in Nernst equation, what does RT stand for
Eion = 2.303 RT/zF log (ion)o/(ion)i
ion charge, eg Ca2+ would be +2
in Nernst equation, what does increasing thermal energy of each particle do, relating to Eion
increasing diffusion = increase PD at equilibrium so Eion proportional temp
in Nernst equation, what does increasing electrical energy of each particle do, relating to Eion
decrease PD needed for balancing diffusion = Eion inversely proportional to ions charge
describe a potassium ion channel
4 sub-unit, form pore on membrane, transmembrane domains (located inside cells)
pore loop, contribute to selectivity filter which makes channels most permeable to K
key determinant for resting MP and neuron functions
what other ion does K channel allow entry
Na
increased Na permeability cause membrane potential of neuron to become less negative = disrupts neuron functions
what does increasing extracellular potassium cause?
depolarised membrane
what is importance of regulating external K+ conc
resting membrane mostly permeable for potassium, so MP close to Equilib potential
high K+ permeability mean MP particularly sensitive to extracellular K+ conc change, so increased extracellular K+ cause depolarisation
what are Goldman equations used for
calculate resting membrane potential (Vm), taking into consideration relative permeability of membrane to different ions
Goldman equation - what does Pk[K+]o, stand for
permeability for K+ outside of cell
Goldman equation - what does Pk[K+]i, stand for
permeability of K+ inside cell
what do ∝ mean
direct proportion
what is ionic driving force directly proportional to?
IDF ∝ Vm - Eion
define depolarisation
membrane potential is made less negative
define hyperpolarisation
membrane potential is made more negative
summarise how APs are generated
stimulus causes membrane channels permeable to cations to open (usually Na+), causes depolarising. if sufficient depolarisation occurs, neuron hits threshold
name key properties of AP
transient, rapid and reversible change in MP from neg to pos
all of same size and duration, don’t decrease as conducted along the axon
describe membrane permeability when its at rest
Na+ channels closed, K+ channel open as membrane mostly permeable to K+>Na+
membrane more neg = -80mV
describe membrane permeability when depolarising
EPSP depolarises membrane to above 0 (hitting excitation threshold), v/gated Na+ channels open. more depolarised = more channel open
when Na+ enters, polarity reverse to +30mV inside neurons
describe membrane permeability during repolarisation
Na+ channels begin to close, K+ start to open, relative permeability of membrane favours K+ meaning more K+ leave = more neg, decrease MP
after repolarisation, slowly close K+ channel = briefly flow out = hyperpolarisation
describe structure of v/gated Na+ channels
4 subunits, each with 6 transmembrane domains forming pore (pore is more permeable for Na+>K+ or Cl-)
transmembrane domains have many pos charged amino acids, start to move changing confirmation of entire molecule, opens pore
how do Na+ channel inactivate
depending on time, voltage. occurs quickly (1ms) and channel deactivation must occur before channels able to be activated again
-intracellular domain (ball) blocks pore from inside so ion cant enter
-to unblock have to bring back channel to neg (membrane repolarisation, by opening v/gated K+ channels)
outline the process of NT release
depolarising wave of AP reach PST, open Ca2+ channel
cause vesicle binding presynaptic membrane, release NT (exocytosis)
vesicle membranes recovered via endocytosis “pinching off”
how can poison be useful?
blocking ion channel, to regulate excitability and prevent neurons being overactive (can lead to neuronal death)
name 5 useful poisions
tetraethylammonium
lidocaine
tetrodotoxin
saxitoxins
dinoflagellates
what does tetraethylammonium (TEA), a useful poison do
block K channel
what does lidocaine, a useful poison, do
block Na channel, local anaesthetic
what does tetrodotoxin (TTX), a useful poison, do
block Na channel, puffer fish, neuroscience research - in order to study other channels need blocking of Na+ channel
how can diameter influence conduction velocity (AP propagation speed)
resistance to current flow is inversely proportional to cross-sectional area of axon. impact membrane permeability
how can myelination influence conduction velocity (AP propagation speed)
prevent current loss along axons via increasing Rm and Space constant
define space constant
distance from site of depolarisation where it has fallen to 37%
why are there many unmyelinated small axons (include space constant, membrane resistance and internal resistance)
space constant ∝ Rm/Ri so benefits of a high membrane resistance reduced by high internal resistance
metabolic and volume costs of myelination
explain axon propagation
- electrical sig at axon hillock, depolarises membrane beyond threshold, generates AP
- open v/gated Na channel, Na enter (pos charge) = depolarisation
- move along axon, depolarises threshold, keeps generating APs
- past areas return to resting potentials
what direction do ions and AP go in explain why
ions can travel both direction, AP only one = properties of v/gated channels having inactivation mechanisms (purple ball)
what can axon hillock also be known as
spike-initiation zone, if shifted further away from soma reduce neuronal excitability as polarisation has further distance to travel, used to regulate excitation levels
what type of potential do dendrites tend to use
mostly encodes info with graded potentials
what is frequency of APs dependent on
size of depolarising stimulus = stronger stimulus (more pos) = higher frequency
explain what rate of AP generation depends on
magnitude of continuous depolarising currents, which is reflected by depolarisation currents magnitude
but limit to frequency = “absolute refractory period”
what is absolute refractory period, and what follows it
1000Hz, once APs initiated neuron cannot initiate another for 1ms
often followed by relative refractory period= another AP can be fired but need stronger stimuli as threshold raised
give example for excitatory neuron
neurons receive glutamate and cause depolarisation
give example of inhibitory neuron
neurons receive GABA and cause hyperpolarisation
describe a GABA receptor
ionotropic channel and selective to Cl. when Cl channel opens, lead to hyperpolarisation (increase MP)
explain spatial summation
many APs received at same time on different axons, increased glutamate, and if reached threshold, results in EPSP
describe what spatial summation looks like on a graph
sharp increase, then tail off
explain temporal summation
AP travel one after another, each cause postsynaptic neuron to release glutamate. if they arrive in close succession, then amplitude of graded potentials are summed, and overall amplitude will be lots higher
describe what temporal summation looks like on graph
like some steps
explain how electrical synapse work
rapid, 2-way, electrical charge in cytosol of axon is carried by ion (not free electrons), meaning less conductive and not super insulated, but bathed in salty extracellular water (which conducts electricty)
where are electrical synapses used
reflexes
retinal neurons, few other adult CNS neuron, and cardiac and smooth muscle
