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Lecture 8 Recording Neural Ac3vity Today: • Gap junc3on overview • Synap3c integra3on • Intracellular (single unit recordings) – Voltage Clamp – Current Clamp • Extracellular (mul3 unit recordings) – Mul3-‐unit recordings – Local Field Poten3als (EEG, or ECoG) Gap Junc3ons Gap Junc3ons Cj Cj Rj Gap Junc3ons Rj Cj Cj Gap Junc3ons Rj C j = cA j Cm m = cAm Cj Cj Am > 10 4 * A j Cm >> C j Gap Junc3ons Rj C j = cA j Cm m = cAm Am > 10 4 * A j Cm >> C j Gap junctions have low-pass filtering characterstics → Spikes are more attenuated than bursts spikelets burstlet 2 mV 50 mV 50 ms Burst firing Measuring Neuronal Ac3vity Intracellular (single unit) Measuring Neuronal Ac3vity Intracellular (single unit) “Local” field poten3als EEG ECoG Measuring Neuronal Ac3vity Intracellular (single unit) “Local” field poten3als EEG Extracellular (mul3 unit) ECoG Intracellular electrodes Diameter ~1 micron Intracellular electrodes Intracellular recording techinques • Voltage Clamp (voltage source) – Voltage is specified by user – Current required to maintain that voltage is measured (equal and opposite to 3me-‐dependent ionic currents) • Current Clamp (current source) – Current is specified by user – Voltage required to maintain that current is measured – Measures changes in the membrane poten3al – At I=0 records approximately the true Vm Func3onal Proper3es of Op-‐Amps Property 1: If an op amp has sufficiently high gain A, then with the op-‐amp is operated in its linear region, v p ≈ v n Property 2: If the op amp has a very high input resistance, then ideally the input stage of the op amp draws no current • These two proper3es generally simplify amplifier design using op amps as major components ip = in ≅ 0 VSat eo vn vp A in − e 0 = A(v p − v n ) ip + vp − vn − VSat Input-‐Output Rela3onship of OP-‐Amp Ideal Voltage Clamp Circuit Ideal Current Clamp Circuit Patch clamp configura3ons Good and Bad Seals Why is the seal important? Why is the seal important? Why is the seal important? Why is the seal important? Why is the seal important? Measuring Neuronal Ac3vity Intracellular (single unit) “Local” field poten3als EEG Extracellular (mul3 unit) ECoG Extracellular Recording Vm Extracellular Recording I ΔVm Extracellular Recording j ≡ I / A = current density I= ∫∫ j • r̂ r 2 sin θ dθ dφ I Spherical symmetry I j= r̂ 2 4π r ΔVm r Extracellular Recording j ≡ I / A = current density Ohms Law 2 j=σE I= ∫∫ j • r̂ r sin θ dθ dφ I Spherical symmetry I j= r̂ 2 4π r ΔVm r Defini3on of Voltage r ∞ V ≡ − ∫ E• dS = ∫ ∞ r ∞ I V=∫ dr ' 2 r 4πσ r V= I 4πσ r j(r ') dr ' σ Potassium Sodium dominated Potassium Sodium dominated Waveform depends on electrode position Gold C et al. J Neurophysiol 2006;95:3113-3128 ©2006 by American Physiological Society Waveform depends on electrode position Gold C et al. J Neurophysiol 2006;95:3113-3128 ©2006 by American Physiological Society Note: Unlike intracellular recordings, here individual EPSPs cannot be recorded Far from the cell, only Local Field Poten3als (LFPs) are measured EEG ECoG Far away: average ac3vity EEG correlates with PSPs • Simultaneous Intracellular and EEG Strong S3mulus Intracellular (AC) EEG Intracellular (DC) EEG correlates with PSPs • Simultaneous Intracellular and EEG Weak S3mulus Intracellular (AC) EEG Intracellular (DC) EEG correlates with PSPs • Simultaneous Intracellular and EEG Intracellular (AC) EEG Intracellular (DC) What what happened to the spikes? Inhomogeneous conduc3vity leads to high pass filtering in neural 3ssue In Vivo Measurement of Cor8cal Impedance Spectrum in Monkeys: Implica8ons for Signal Propaga8on Nikos K. Logothe3s , Christoph Kayser , Axel Oeltermann Neuron Volume 55, Issue 5 2007 809 -‐ 823 hip://dx.doi.org/10.1016/j.neuron.2007.07.027 Grey maier is homogeneous and no strong frequency response Non-‐point-‐neuron models show low-‐pass filtering at long distances Debate over LFP localiza3on December 2009 December 2011 LFP signal originates within 250 microns of electrode LFP signal integrates ac3vity over cen3meters LFP Consensus • LFP dominated by ensemble averaged PSPs • Integrated over some area 100s microns -‐ cms • Individual spikes (or PSPs) cannot be measured Today: • 3 measurement regimes: – Intracellular • Ac3on poten3als and PSPs in individual cells – Extracellular (nearby) • Ac3on poten3als from many cells (dis3nguishable) – Extracellular (far away) • Averaged ac3vity of many thousands of cells (mostly PSPs) So Far: • Ionic concentra3on differences inside and outside the cell lead to membrane poten3als • Voltage gated ion channels enable these cells to generate “rapid” voltage signals • Chemical and electrical synapses enable cells to communicate to one another • Micro electrodes can measure the spiking ac3vity of individual cells and cell assemblies in vivo • Next: How do we process these measured signals?
