Electrophysiology 1. Spikes, graded potentials and synaptic integration

Question 1

Define an evoked potential:

Answer 1

Electrical potential in a specific pattern recorded from a specific part of the nervous system

Average EEG waveform with respect to stimulus onset

Question 2

Describe the formation of a dipole that is detectable outside of the head:

Answer 2

When AP propagated along neurons, local currents produced outside of cell facilitate propagation

Currents too small to be detected by EEG + axons arranged randomly so many of the currents will cancel each other out

EC space becomes negative due to binding of NT and release of +ve ions into postsynaptic membrane, outward flow of +ve ions

Combination of above processes creates a dipole

Detectable outside of head due to summation of dipoles from simultaneous stimulation of cells

Question 3

Compare the action of recording voltages from a EC/IC environment of single cells:

Answer 3

EC – can’t record Vm - voltage difference across membrane

Spikes in nearby neurons cause local EC current flow = detected as transient voltage change

IC – record voltage difference across cell membrane = Vm

Question 4

Describe three examples of graded potentials:

Answer 4

EPSPs and IPSPs (post synaptic potentials)

Receptor potentials from sensory transduction

Subthreshold changes in Vm due to IC current injection in an electrophysiological experiment

Question 5

Compare neuronal inputs with outputs:

Answer 5

Neuronal outputs are APs (spikes), propagated down the axon = APs actively propagated

Neuronal inputs are EPSPSs and IPSPs, generated in dendrites and soma, in response to spikes in presynaptic neurons

Question 6

Define synaptic potentials and how they are propagated:

Answer 6

Synaptic potentials are graded potentials

They are passively propagated from dendrites to soma

Question 7

Describe the change in Vm due to an AP:

Answer 7

An AP in a presynaptic neuron triggers a 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

Question 8

Compare temporal summation with spatial summation:

Answer 8

Spatial summation: 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

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

Question 9

Compare APs with synaptic potentials:

Answer 9

Action potentials (spikes):

Large (~100mV)

Faster (c. 1ms)

All-or-nothing

Cannot summate

Active

Synaptic potentials (graded potentials):

Small (≤1 mV)

Slower (c. 10ms)

Graded

Can summate

Passive

Question 10

Define synaptic integration:

Answer 10

Synaptic integration is the computational process by which an individual neuron processes its synaptic inputs and converts them into an output signal

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

Question 11

Describe the electrical properties of the neuronal membrane that determine summation:

Answer 11

Space constant (or length constant), λ:

• Increases with diameter
• Increases with membrane resistance

Time constant, τ = RC (resistance * capacitance)

Question 12

Describe temporal summation in terms of the time constant:

Answer 12

Time constant of a postsynaptic cell affects amplitude of depolarization caused by consecutive EPSPs produced by single presynaptic neuron

If synaptic current generated by the presynaptic neuron is ~ same for both EPSPs

In a cell w/long time constant, first EPSP does not decay totally by the time 2nd EPSP triggered

Depolarizing effects of both potentials are additive = Vm above threshold + triggering AP

Short time constant = first EPSP decays to the resting potential before second EPSP triggered

Second EPSP alone does not cause enough depolarization to trigger AP

Question 13

Describe spatial summation in terms of the length constant:

Answer 13

Length constant of a postsynaptic cell affects amplitudes of two EPSPs produced by two presynaptic neurons

Both synapses are same distance from postsynaptic cell’s trigger zone in initial axon segment + current produced by each synaptic contact is same

And if distance between synaptic input site + trigger zone in post-SN cell is only one length constant

The postsynaptic cell has a long length constant of 1 mm

The synaptic potentials produced by each of two presynaptic neurons will decrease to 37% of their original amplitude

By the time they reach trigger zone

Summation of two potentials = in enough depolarization to exceed threshold, triggering AP

If the distance between the synapse and the trigger zone is equal to three length constants

The postsynaptic cell has a short length constant of 0.33 mm

Each synaptic potential will be barely detectable when it arrives at the trigger zone

Even the summation of two potentials is not sufficient to trigger AP