Electrophysiology I: Spikes, Graded Potentials and Synaptic Integrations

Question 1

What are the differences between extracellular and intracellular recording?

Answer 1

Extracellular:
- Usually done on live subjects
- Cannot record voltage difference across a membrane
- Spikes in nearby neurons cause local extracellular current flow which can be detected as small transcient voltage changes
- Records spikes not membrane potential (Vm)

Intracellular:
- Can record both spikes and Vm

Question 2

What are graded potentials?

Answer 2

Temporary changes in a membrane voltage created by a stimuli opening a voltage-gated channel

Question 3

What are examples of graded potentials?

Answer 3

• Excitatory postsynaptic potentials
• Inhibitory postsynaptic potentials
• Receptor potentials from sensory transduction

Question 4

What is capacitance?

Answer 4

The ability to store and separate charge

Question 5

How is the cell membrane like an electrical circuit?

Answer 5

• It has a current and charge when there is a flow of ions
• It has resistance dependent on the open and closed ion channels
• Has a capacitance by being able to separate charge between outside and inside cell (Vm)

Question 6

What happens in synaptic transmission?

Answer 6

• An AP in the presynaptic neuron triggers a postsynaptic potential
• Neurotransmitters secreted from the presynapse binds to the R on the post synapse
• Causes the ion channels to open and allows for current to flow
• There is a local change in Vm and causes either de/hyperpolarisation

Question 7

Can AP and PSPs summate?

Answer 7

• AP no
  -PSPs Yes

Question 8

What is synaptic integration?

Answer 8

Where over any given time window, spatial and temporal summation determine the value of Vm

Question 9

What are the differences between AP and Graded spikes?

Answer 9

• AP are larger, faster, cannot summate, active and have an all-or-nothing theory
• Graded spikes are smaller, slower, graded (can have multiple sizes), can summate and are passive

Question 10

What are the two types of summation that affect synaptic integration?

Answer 10

Spatial and Temporal Summation

Question 11

What is Temporal Summation?

Answer 11

Where summation occurs over time

• If 2 presynaptic Ads are fired in rapid succession, PSP due to 2nd AP

Question 12

What is Spatial Summation?

Answer 12

Different presynaptic neurons with synapses are on different spatial locations on the post synaptic neuron can be active simultaneously

Question 13

How can time distance and time constants affect synaptic integration?

Answer 13

• Longer the length distance, it means that spatial summations are more effective as a shorter length causes more leakage
• Longer time constant means it allows the current to decrease and increase more slowly

Question 14

What are the electrical properties of the neuronal membrane that determines summation?

Answer 14

Space constant (lambda): increase with square root of diameter and membrane resistance

Question 15

Define an evoked potential:

Answer 15

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

Average EEG waveform with respect to stimulus onset

Question 16

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

Answer 16

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 17

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

Answer 17

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 18

Describe three examples of graded potentials:

Answer 18

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 19

Compare neuronal inputs with outputs:

Answer 19

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 20

Define synaptic potentials and how they are propagated:

Answer 20

Synaptic potentials are graded potentials

They are passively propagated from dendrites to soma

Question 21

Describe the change in Vm due to an AP:

Answer 21

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 22

Compare temporal summation with spatial summation:

Answer 22

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 23

Compare APs with synaptic potentials:

Answer 23

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 24

Define synaptic integration:

Answer 24

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 25

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

Answer 25

Space constant (or length constant), λ: - Increases with diameter - Increases with membrane resistance Time constant, τ = RC (resistance * capacitance)

Question 26

Describe temporal summation in terms of the time constant:

Answer 26

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 27

Describe spatial summation in terms of the length constant:

Answer 27

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