Electrophysiology of Brain Signals Flashcards

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Resting Membrane Potential

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The steady-state voltage across a neuron's membrane when it is not firing. Typically around $-70~\mathrm{mV}$, due to the balance of ionic concentrations and selective permeability. It sets the stage for excitability.

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Action Potential Threshold

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The membrane voltage at which an action potential is triggered, roughly $-50$ to $-55~\mathrm{mV}$. Crossing this threshold opens voltage-gated channels, initiating a rapid depolarization. If threshold isn't reached, no action potential occurs.

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Depolarization

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Rising phase of the action potential caused by $\mathrm{Na}^+$ influx (inward current). The membrane voltage becomes less negative as more positive charge enters. This is followed by repolarization as $\mathrm{K}^+$ exits.

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Repolarization

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Return toward negative membrane potential due to $\mathrm{K}^+$ efflux (outward current). This helps terminate the depolarizing spike. It is followed by a brief hyperpolarization.

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Hyperpolarization

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A brief undershoot below the resting potential ($< -70~\mathrm{mV}$). The membrane becomes more negative than during rest before returning to baseline. Caused by continued $\mathrm{K}^+$ efflux and channel dynamics.

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Failed Initiations

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Subthreshold depolarizations (e.g., to about $-60~\mathrm{mV}$) that do not reach threshold. No action potential is produced. This helps illustrate the all-or-none nature of neuronal firing.

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All-or-None Law

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Neurons fire in an all-or-none fashion; the amplitude of the action potential is independent of stimulus strength. Subthreshold stimuli produce no response, while above-threshold stimuli trigger a full spike.

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Spike Size

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The amplitude of an action potential remains essentially constant, regardless of stimulus intensity. Stronger stimuli change firing rate, not spike height.

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Stronger Stimulus Effect

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Stronger stimuli increase the firing rate (frequency) rather than spike amplitude. This rate coding allows neurons to encode stimulus intensity.

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Baseline Firing Rate

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The spontaneous firing rate at rest is typically very low, often below $1$ Hz. Indicates baseline activity of neurons in a given preparation.

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Active Firing Rate

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During activity, firing rates can range from roughly $5$ to $100+$ Hz. This reflects real-time neuronal output under stimulation or task demands.

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Main Variable Measured

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In single-cell recordings, the firing rate is the main metric used to quantify neuronal activity. It summarizes how often a neuron fires over time.

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Single-Cell Recording Types

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Two main approaches: intracellular recording (inside the neuron) and extracellular recording (outside the neuron). Both reveal different aspects of neural signaling.

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Intracellular Recording

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An electrode is inserted inside the neuron to measure membrane potential. It provides detailed information about PSPs and the full action potential waveform. It is technically difficult and invasive and not usually feasible in human axons.

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Extracellular Recording

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An electrode placed just outside the neuron. It primarily measures action potentials (spikes) from nearby neurons. Less invasive than intracellular methods and widely used in awake animals and multi-neuron studies.

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Voltage-Clamp Technique

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A technique developed by Hodgkin & Huxley to hold the membrane potential at a set value and measure ionic currents. Enabled precise study of ion channels' roles in the action potential.

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Na+ Influx

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Depolarization results from Na+ influx, producing a positive current that drives the membrane potential toward the threshold and beyond.

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K+ Efflux

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Repolarization is driven by K+ efflux, a positive charge leaving the cell that helps restore the resting potential.

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Hodgkin & Huxley

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Pioneering researchers who established the ionic basis of the action potential and developed the voltage-clamp technique, identifying voltage-gated Na+ and K+ channels.

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Giant Squid Axon

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Giant squid axons have a huge diameter (~$1~\mathrm{mm}$), which allowed easy insertion of electrodes to study ion flow and laid groundwork for modern electrophysiology.

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Hubel & Wiesel Nobel Year

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Hubel & Wiesel won the Nobel Prize in $1981$, for discoveries on visual processing, including orientation-selective neurons in V1 and columnar organization.