Electrophysiology 1. Spikes, graded potentials and synaptic integration

Question 1

Describe the various techniques for electrophysiological recording

Answer 1

-local EEG
-extracellular LFP
-intracellular recording

Question 2

Describe the local EEG

Answer 2

• Crudest/lowest resolution
• cortical thickness packed with neurones - mostly pyramidal cells
• regular orientation and layers and apical dendrites that go vertically in parallel

Question 3

Extracellular LFP

Answer 3

current flows → local field potential

Question 4

Intracellular recording

Answer 4

• finer/highest resolution
• as you increase stimulation = increased frequency of AP = rate coding
• AP frequency increases with stimulus intensity
• Membrane potential change → changes direction and amplitude of stimulus

Question 5

Distinguish between intracellular vs extracellular

Answer 5

• EC: occurrence of AP
  
  • 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
• IC: membrane potential and individual synaptic
  
  • records voltage difference across cell membrane (Vm)

Question 6

State the electrical properties of the neuron

Answer 6

1. Voltage: Battery -70mV Na/K equilibrium = ion pump
2. Capacitance: capacity to store in separate change ⇒ delayed response
   -determines rate of responses of membrane = membrane time constant
   -lipid bilayer membrane impermeable to ions - resistant BUT pores allow ion flow
3. Increase in ion channels opening = decreases resistance
4. Faster time constant- the more quickly it will approach final value = shorter time constant = longer it would take

Question 7

Explain the difference between action potentials and graded potentials

Answer 7

Graded potentials: subthreshold changes in Vm due to intracellular current injection in an electrophysiological experiment.

AP/SPIKES SYNAPTIC/GRADED
Large (~100mV) small (<1 mV)
Faster (c. 1ms) slower (c. 10ms)
All-or-nothing graded
cannot summate can summate
active passive

Question 8

What are neuronal inputs and outputs?

Answer 8

• Neuronal outputs: APs (spikes) propagated down the axon
  
  • APs potentials are actively propagated
• Neuronal inputs: EPSPs and IPSPs generated in dendrites and soma in response to spikes in presynaptic neurons
  
  • synaptic potentials are graded potentials
  • they are passively propagated from dendrites to soma

Question 9

An AP in a presynaptic neuron triggers a PSP in a postsynaptic neuron. Explain further.

Answer 9

• 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 de- or hyperpolarising
• changes in Vm due to single PSPs are usually small: in the mV range
• PSPs are graded potentials they can summate
• SUMMATION = longer time constant

Question 10

Describe Synaptic integration

Answer 10

• over any given (brief) time window spatial and temporal summation determine the value of Vm
• If positive to threshold, AP generated, otherwise not
  
  • EPSPs: +ve
  • IPSPs: -ve

Question 11

Describe Spatial summation

Answer 11

different presynaptic neurons with synapses on different spatial locations on the postsynaptic neuron can be active simultaneously - individual PSPs can summate

Question 12

Describe Temporal summation

Answer 12

• time course of AP is slower than PSP. If twi presynaptic APs are fired in rapid succession, PSP due to 2nd AP can sum with that due to 1st.
• Synaptic potentials slower to rise and decay due to passive electrical properties of membrane (time constant (C))
• if intraspike interval short enough ⇒ frequency high enough then 2nd AP before PSP

Question 13

Explain the effect of membrane space on synaptic integration

Answer 13

• increases with square root of diameter
• increases with membrane resistance
• decay of voltage along membrane

Question 14

Explain the effect of time constants on synaptic integration

Answer 14

T = RC

Question 15

Explain membrane leakage

Answer 15

• Vm +10 from presynaptic neuron ⇒ leaking occurs ⇒ less than -10 to soma and postsynaptic neuron = space constant
• The input signal to a neuron consists in in graded changes in Vm due to summation of excitatory and inhibitory postsynaptic potentials over a given time window.
• Time constant, tau, is given by membrane resistance multiplied by capacitance (t=RC). By opening more ion channels we can create faster responding neuron