BIOL 2052 - Ionic basis of the membrane potential and synaptic integration

neurons

• functional unit of the nervus system

• generate and transmit electrical impulses

• allows motor control, sensory processing and proprioception

recap of physiology of an action potential

• the sodium potassium ATPase is electrogenic and generates the membrane potential

• 3 sodium ions pumped out for every 2K+ ions in

• the membrane potential is a measure of charge separation across the membrane

• membrane acts as a capacitor —> charge capacitance will be bigger in bigger cells and smaller in smaller cells

• each ions charge affects the membranes voltage by 16 nanovolts

• the concentrations do not flip, just the charge separation becomes sufficiently larger to create an action potential

movement of ions

• overall electrochemical gradient moves Na in an K out

• RMP is -70mV due to permanently expressed leak channels which allow K+ in

• when the inside becomes ore positive Na is repelled so it doesnt keep moving in but reaches an equilibirum

• at some point K ions will equilibrate (at a little lower than -70mV)

• sodium ions outside the cell are attracted to the potassium ions inside the cell

typical ion concentrations

ION	INSIDE	OUTSIDE
Na+	15 mM	145 mM
K+	150 mM	4 mM
Cl-	10	110 mM

the nernst equation to calculate equilibrium potential

• units in V

• F is faradays constant - 96485

• z is ionic charge

• T is temp in K

• R is the gas constant

this simplifies to

goldman hodgkin katz equation

• for neurons at rest pK:pNa:pCl is 1:0.05:0.45

• useful for when the concentrations change with time and determining membrane concs at a particular moment

summary of control of action potentials

firing of neurons

• neurons fire 10 times/s but some ultra rapid ones can fire up to 200x/s

• sodium is responsible for the AP which was proved by hodgkin and hatch in squid axons as decreasing the sodium concentration decreases the depolarisation of the membrane

• as you decrease the sodium conc the AP also moves to later as the concentration gradient is less steep

how can we measure neuronal activity

EXTRACELLULARLY

• local field potential

• electroencephalogram

INTRACELLULARLY

• action potentials

passive/active electrical properties

• inserting a voltage into an axon will give a response

• ohms law states that V=IR where current is in amps and R is in ohms

• once the threshold for activation is reached an action potential is formed which is an active property of an action potential

passive membrane properties

• the voltage effect of the membrane current takes time due to membrane capacitance - this is called the time constant

• the voltage effect of the membrane current decreases with distance from the injection site (the length constant )

• the voltage change as a response to current injection depends on the cells resistance (input resistance determined by V=IR)

• exponential equations can be used to model behaviour of many membrane properties

• we can refer to the constant extracted from these equations to qualify the properties of neurons

example of synaptic integration - dentate gyrus

• the dentate gyrus is involved in the generation of new neurons

• new neurons have different properties to the old neurons:

  NEW

  • small

  • high resistance

  • low capacitance

  • require less input to fire

    OLD

  • low input resistance

  • high capacitance

  • requires more input to fire

• if you multiply the capacitance and the resistance, this is a direct measure of how slow the cell will be at integrating inputs

capacitance

• the ability of membranes to store charge

• capacitors and resistors in parallel (which act as a model axon) has only passive properties, whilst actual axons have passive and active properties

how can we dissect the contribution of different currents to neuronal activity

• use of a voltage clamp to keep a set voltage

• you can then figure out which ions are flowing through the axon at set voltages so can tell at which voltage sodium channels are activated

• however, this experiment was run in squid axxons which differ from our brains as squid axons have slower sensory processing

propogation of signal

• delay in stimulation and change in current due to the membrane acting as a capacitor

• as the membrane becomes charged the current flows through the capacitance path

• gradually, more current flows through the resistance branch (the membrane potential)

• when the current is terminated the current through the capacitance branch flows through the resistance branch until the membrane potential returns to baseline

• this is represented by the “wave shaped” response

• as the input moves through the axon the change in the membrane potential will become smaller and smaller

summary of passive vs active responses of neurons

PASSIVE

• delayed response caused by capacitance of membrane

• attenuation across distance

ACTIVE

• excitatory input

• na channels and K channels opening

• does not attenuate over distance

active properties - the action potential

• usually, when current is injected the membrane depolarises with the magnitude of the injected current unless the injected current surpasses the threshold for activation, then an AP is generated which doesnt attenuate over distance

• this is an active property of neurons

• cells with a high resistance are more likely to surpass threshold

• the active properties will depend on the type of ion channel expressed and the specific localisation of these within cells

active properties of neurons

• bought about by ion channels —> either voltage gated or ligand gated

• ion channels have multiple subunits which must be activated which is why sometimes in equations why factors are raised to the power of 4

patch clamping

• allows us to study behaviour of ion channels

• a pipette is connected to an amplifier is used to isolate a patch of membrane from a cell and measure the current

• when the pipette is brought to the membrane it forms a bond which prevents the flow of ions - called a giga-ohm seal

• once the seal is formed the membrane patch can either be cut off or left attached

• to cute the cell - apply more suction

• when electrical potential is passed across the membrane the ion channels open and close and the amplifiers record the current

• pipette amplifiers are able to measure the current and clamp the voltage to keep it the same or they are able to keep it at the same current and record the voltage

• this info can be used to determine the resistance of the membrane

PATCH CLAMPING REQUIRES:

• clean glass

• air pressure

• membrane

• giga-ohm seal

ion current plotting: +ve in

• entry of positive current into the cell causes depolarisation

• this causes an inwards deflection

• the negative current exits the cell

• for example: glutamate receptors

ion current plotting: -ve in

• entry of negative ions

• exit of positive current

• hyperpolarisation of the cell causes an outward deflection

• for example: the GABA receptor

how do ligand gated ion channels encode electrical activity

• the electrical activity is dictated by the opening of ion channels

• current flowing through receptors = conductance x driving force

• 

• glutamate receptors have huge driving force and so the number of ion channels is equal to the current through the plasma membrane

• GABA has a smaller driving force because were closer to the reversal potential to chloride (RMP)

• however, the membrane potential always changes so we need to consider synaptic integration

synaptic integration

• neurons receive synaptic inputs and the resulting currents are summed by cells

• the net effect of the inputs modifies the output of the neuron

• 

• EI and I will give a smaller input as the result of EI is cancelled by I

• EI and E2 combined with I is not enough to generate an AP due to the inhibitory effects of I

• is synapses are more active and the connections stimulated by experience, studying etc, then the synapse expresses more AMPA receptors on the post synaptic membrane

many receptors are ion channels

• all or none response

• the larger the voltage/ further the voltage is from 0 the larger the current as the larger the driving force

• experiments often conducted in absence of magnesium but this is not physiological

• NDMA receptors are slow acting, AMPA receptors are fast acting

influence of magnesium - NDMA and AMPA receptor relationship

• physiological conc of Mg2+ is 1000um

• at low concentrations of magnesium it causes flickering of voltage as magnesium blocks NDMA receptors —> total block at high concentrations

• activation of NDMA receptors is caused by the activation of AMPA receptors and at resting membrane potential the NDMA receptors will be blocked by magnesium

• AMPA receptors activated by glutamate —> when the cell is depolarised the positive charge of the cell will repel the magnesium ion and will allow flux of cations and Ca2+ which will cause downstream effects

• therefore, NDMA receptors are voltage dependent and act as coincident detectors

• for them to be activated depolarisation and glutamate bound

• Mg2+ is expelled at around -50mV and

plasticity

• cells that fire together wire together

• as you continue to stimulate an axon, the excitatory post synaptic potential amplitude increases from a stable baseline

• receptor activation of AMPA and NDMA receptors results in synaptic plasticity

• continued activation will result in more AMPA receptors

• strengthening of synaptic function associated with growth of synaptic spine and increased cytoskeleton

back propogation of action potentials

• is you generate action potentials in the soma it can be detected in the dendrites dur to the back propagation of the action potential

• this does not mean that the AP can travel in both directions it can just travel to parts of the cell that haven’t experienced the action potential

GABA receptors

• when GABA receptors are activated it produces an inhibitory input

• causes these leaky channels which allows Na+ into the cell but not enough to cause an AP due to integration of inhibitory input