Electrophysiology 1: Spikes, graded potentials and synaptic integration

Question 1

which type of recording has the highest resolution?

Answer 1

Intracellular recordings

Question 2

what is the difference between intracellular and extracellular recordings?

Answer 2

Extracellular recordings can’t record voltage difference across membranes , Vm so it doesn’t record the true membrane potential. However, spikes in nearby neurons cause local extracellular current flow, which can be detected as small transient voltage changes but doesn’t detect graded potentials.

Intracellular recordings can record voltage difference across the cell membrane. Intracellular recordings measures the voltage directly across the cell membrane so you can see both action potentials and sub-threshold events (graded potentials)

Question 3

single cell recordings can be taken by which two techniques

Answer 3

intracellular and extracellular recordings

Question 4

explain why extracellular recordings can’t measure membrane potentials

Answer 4

Because they only detect the voltage Changes in the surrounding environment and not inside the neuron. Since the extracellular recordings are made by placing the electrode in the extracellular space (outside the neuron) and so we don’t get a measure of the electrical difference between inside and outside the neuron, we don’t get the Vm.

Question 5

We can insert microelectrodes and stimulate the membrane potential by injecting current. This can result in an inward or outward current. Define the two

Answer 5

Inward current= Causes a + membrane potential (a bigger inward current results in a bigger change in + membrane potential)

Outward current= results in hyperpolarising the membrane potential and a larger pulse

Question 6

what happens when we increase the amplitude of the injected current?

Answer 6

We don’t generate further depolarisation rather we generate more action potentials.

Question 7

what process underlies the concept of rate coding (average spike frequency/ n.u of spikes over some integration of time)?

Answer 7

As you increase the stimulation you get an increased frequency of action potentials.

Question 8

rather than being a straight flat line what does the resting membrane potential look like?

Answer 8

Fluctuations

Question 9

why can current not flow across the lipid bilayer and what property of the membrane deals with this problem?

Answer 9

The lipid bilayer is impermeable to ions and so the current can not pass through the membrane without protein channels

Question 10

what are the 2 electrical properties of neurons?

Answer 10

1) Resistance of the membrane due to ion channels- the more ion channels open - decreased resistance /higher conductance/ permeability

2)Capacitance= the capacity of the cell membrane to store and separate charge

Question 11

what feature of the membrane explains the delayed response of a neuron firing?

Answer 11

The capacitance = the delayed reaching of the new voltage value because capacitance stores charge

Question 12

explain capacitance

Answer 12

As the inward current is switched on a portion of the current rather than flowing out of the circuit would go to charge in the capacitor and so there would be a build up of positive charge. So at the initial point there is no change in the voltage across the membrane but as the charging of the capacitor progresses less of the current is gone into charging the capacitor

Question 13

How does the capacitor get discharged?

Answer 13

As more current flows across the membrane resistance (through ion channels) the current flows and membrane potential increases and stays at that lvl till the current is removed, which discharges the capacitor

Question 14

what property of the neuron membrane determine the rate of response of the membrane?

Answer 14

The membrane capacity

Question 15

define membrane constant (T)

Answer 15

The time it takes for the membrane potential to change by approximately 63% of its final value after a current is applied

Question 16

what is the equation for membrane time constant?

Answer 16

resistance x capacitance

Question 17

what does a larger time constant mean?

Answer 17

The membrane takes longer to respond to changes in current, resulting in slower changes in membrane potential.

Question 18

what is a graded potential?

Answer 18

A small, local change in membrane potential that occurs in response to a stimulus. The magnitude of this change is proportional to the strength of the stimulus

Question 19

list some examples of graded potentials

Answer 19

1) Excitatory post synaptic potentials (EPSPs)
2) Inhibitory post synaptic potentials (IPSPs)
3)Receptor potentials from sensory transduction

Question 20

what are EPSP?

Answer 20

Type of graded potential that makes the postsynaptic membrane more likely to fire an action potential.

Occurs when excitatory neurotransmitters (e.g. glutamate) bind to receptors, causing depolarisation by allowing + ions to enter the cell. This moves the membrane potential closer to the threshold for triggering an action potential

Question 21

What are IPSPs?

Answer 21

A graded potential that makes the postsynaptic membrane less likely to fire an action potential

Occurs when inhibitory neurotransmitters (e.g GABA and glycine) bind to receptors, causing hyperpolarisation by allowing negative ions to enter or positive ions to exit the cell. This moved the membrane potential further away from the threshold, inhibiting the chance of an action potential.

Question 22

what are neuronal outputs?

Answer 22

Action potentials (spikes), propagated down the axon

Question 23

why are action potentials actively propagated?

Answer 23

due to the use of ion channels, which requires ATP for function

Question 24

what are neuronal inputs?

Answer 24

EPSPs and IPSPs

Question 25

where are neuronal inputs, EPSPs and IPSPs generated?

Answer 25

Dendrites and soma in response to spikes in presynaptic neurons

Question 26

what is the difference between the way action potentials and graded potentials (synaptic potentials) are propagated?

Answer 26

Action potentials are actively propagated down the axon whilst synaptic potentials/graded potentials are passively propagated from dendrites to soma

Question 27

why are graded potentials passively propagated?

Answer 27

because they are not amplified by voltage gated ion channels (although there r a few that amplify them but we just say they're passive)

Question 28

Describe the process of synaptic transmission

Answer 28

1) An AP in a presynaptic neuron triggers a PSP in a postsynaptic neuron 2)NT secreted from presynaptic neuron binds to receptors on postsynaptic membrane, opening ion channels and allowing a local current 3)Results in local change of Vm, which may be de or hyperpolarising 4) Changes In Vm due to single PSPs are usually small in mv range 5) PSPs are graded potentials - they can summate

Question 29

define spatial summation

Answer 29

Different presynaptic neurons with synapses on different spatial locations on the presynaptic neuron can be active simultaneously, in which case the individual PSP can summate, reaching threshold potential to trigger an AP.

Question 30

define temporal summation

Answer 30

when a single neuron fires multiple action potentials in rapid succession causing its postsynaptic potentials to add up over time. If these signals arrive closely enough their combined effect can reach the threshold to trigger an action potential.

Question 31

define synaptic integration

Answer 31

Over any given (brief) time window the sum of the inputs (graded potentials , IPSP/ESPS) depending on spatial and temporal summation determine the actual value of Vm. If positive to threshold, AP generated otherwise not.

Question 32

what is space constant?

Answer 32

the amount of decay of voltage along the length of the membrane

Question 33

what does a longer length constant do? and why?

Answer 33

It makes spatial summation more effective because a longer length constant means the electrical signal can travel further along the neuron without significant decay, allowing inputs on the neurons dendrites/soma to summate more effectively at the axon hillock, where AP is initiated.

Question 34

what effect does a shorter length constant have on spatial summation?

Answer 34

With a shorter length constant the loss in amplitude would be greater- signal would fade more quickly with the length so the spatial summation won't be sufficient to reach the threshold

Question 35

what is the difference when we have a short and a long time constant?

Answer 35

1)long time constant for a synaptic current = the voltage rises more slowly and falls more slowly 2)Short time constant = the voltage rises and falls more rapidly