Electrophysiology

Question 1

What are the pros of fast vs slow time constants for a neuron?

Answer 1

• slow time constant gives you more time for a temporal summation so best to pick up slower change.
  -fast time constant would give you more temporal precision.

Question 2

Why can’t graded signals be directly transmitted as output to postsynaptic neurons? Why do we need action potentials?

Answer 2

• graded signals decay as you go down the axon/neuron so they are not strong enough to go the whole distance from neuron to synaptic terminals.
  -if an axon is short enough, a graded signal is enough to reach synaptic terminals so no action potential is needed as the signal goes directly to the voltage gated calcium channel

Question 3

example(typical value for Cl- in mammanlian CNS (mM)
-extracellular : 100-120
-intacellular : 5-10

In what direction is the concentration gradient of CL?

Answer 3

Conc gradient of Cl is into the cell because there is a high conc of CL outside the cell

Question 4

example(typical value for Cl- in mammanlian CNS (mM)
-extracellular : 100-120
-intacellular : 5-10

In which direction is the voltage gradient on Cl?

Answer 4

Voltage gradient of Cl is outwards because Cl is negative and inside the cell is negative.

Question 5

example(typical value for Cl- in mammanlian CNS (mM)
-extracellular : 100-120
-intacellular : 5-10

the transmembrane voltage (Vm) at which Cl- would be in euilibrium would be
a)negative
b)positive
c)zero

Answer 5

a) negative

Question 6

Assume Cl equilibrium is -65mV and Vm is -70mV.

What would the net driving force result in?

Answer 6

Cl outflow.
This is because the voltage gradient (-70mV) is greater than the equilibrium, therefore voltage gradient wins.

Question 7

How is current direction defined?

Answer 7

Current direction is defined from in to out, so negative current can be +ve charge entering or -ve charge leaving the cell.

Question 8

Assume internal Cl concentration is doubled.

What would happen to the driving force of Cl due to concentration gradient?

Answer 8

driving force would become decreased. therefore the value of equilibrium of Cl would be less negative.

Question 9

What is the relationship between the Na pump and the Action Potential (AP)?
a) no direct relationship
b) responsible for repolarising phase
c) generates AHP

Answer 9

a) no direct relationship

Question 10

What is the unit for driving force?

Answer 10

Volts

Question 11

INa= gNa (VM -ENa)
Which term represents driving force on the sodium current?

Answer 11

(Vm-Ena)

Question 12

difference between extracellular and intracellular recording from single cells

Answer 12

EXTRACELLULAR:
- cannot record voltage difference across membrane (membrane potential,Vm)
-spikes in nearby neurons cause local extracellular current flow, which can be detected as small transient voltage change.

INTRACELLULAR:
- records voltage different across cell membrane (Vm)

Question 13

difference effects of injecting a cell with inward vs outward current.

Answer 13

-Injecting a cell with outward current causes hyperpolarization in the cell. a larger current causes a larger hyperpolarization

-similarly an inward current causes a positive change in the membrane potential, causing depolarization. Once depolarization passes a specific threshold, an action potential is generated.
AS you increase the stimulation, you get an increased frequency of action potentials.

Question 14

examples of graded potentials

Answer 14

• subthreshold changes in Vm due to intracellular current injection in an electrophysiological experiment
  -Excitatory and inhibitory postsynaptic potentials (EPSPs & IPSPs)
  -receptor potentials from sensory transduction

Question 15

What are neuronal outputs

Answer 15

neuronal outputs are action potentials (spikes), propagated down the axon.
action potentials are actively propagated.

Question 16

what are neuronal inputs

Answer 16

neuronal inputs are EPSPs & IPSPs, generated in dendrites and soma, in response to spikes in presynaptic neurons.
-synaptic potentials are graded potentials which are passively propagated from dendrites to soma

Question 17

describe synaptic transmission

Answer 17

An AP in a presynaptic neuron triggers a post-synaptic potential (PSP) in a postsynaptic neuron.

• neurotransmitter secreted from presynaptic binds to receptor on postsynaptic membrane, opening ion channels and allowing local current flow.

-results in local change in Vm, which may be de- or hyperpolarizing

-changes in Vm due to single PSPs are usually small (in mV range)

-PSPs are graded potentials: they can summate

Question 18

what is spatial summation

Answer 18

different presynaptic neurons with synapses on different spatial locations on the postsynaptic neuron can be active simultaneously, in which case the individual PSPs can summate.

• synaptic potentials can be excitatory or inhibitory

Question 19

what is temporal summation

Answer 19

in temporal summation, the time course of AP is slower than PSP. If two presynaptic APs are fired in rapid succession, PSP due to 2nd AP can sum with that due to 1st.

Question 20

compare differences between AP and synaptic potentials (graded)

Answer 20

AP
-large (~100mV)
-Faster
-all-or-nothing
-Cannot summate
-Active

Graded
-Small (-<1mV)
-slower
-graded
-can summate
-passive

Question 21

what is synaptic integration

Answer 21

over any given (brief) time window spatial and temporal summation determine the value of Vm. If positive to threshold, AP generated, otherwise not

Question 22

define time constant and space constant

Answer 22

time constant is determined by the capacitance of the membrane is found by equation:
- timecontant = resistance * current

Space constant is the amount of decay of voltage along the length of the membrane. and it increases with:
-square root of diameter
-membrane resistance

Question 23

how does time constant affect temporal summation

Answer 23

a long time constant gives more time for summation to occur, whereas inputs would have to occur more quickly and more closely spaced together to generate summation when there is a short time constant.

Question 24

how does length constant affect spatial summation

Answer 24

a long length constant makes spatial summation more effective as there is less decrement in the amplitude of voltage change.

a shorter length constant means the loss in the amplitude would be greater, making the spatial summation unable to meet threshold.

Question 25

ion transporters (pumps) vs ion channels

Answer 25

ion transporters maintain conc gradients over the long term & are not involved in neuronal signalling.
-actively transport ions against conc gradient
-create ion conc gradients

ion channels allow selective movement of ions across the membrane (‘downhill’-so not energy consumption). the direction depends on both concentration & electrical gradient.
-ion flow constitutes an electrical current
-cause selective permeability to certain ions
-allows ions to diffuse down conc gradient