<P> Intracellular recording involves measuring voltage and / or current across the membrane of a cell . To make an intracellular recording, the tip of a fine (sharp) microelectrode must be inserted inside the cell, so that the membrane potential can be measured . Typically, the resting membrane potential of a healthy cell will be - 60 to - 80 mV, and during an action potential the membrane potential might reach + 40 mV . In 1963, Alan Lloyd Hodgkin and Andrew Fielding Huxley won the Nobel Prize in Physiology or Medicine for their contribution to understanding the mechanisms underlying the generation of action potentials in neurons . Their experiments involved intracellular recordings from the giant axon of Atlantic squid (Loligo pealei), and were among the first applications of the "voltage clamp" technique . Today, most microelectrodes used for intracellular recording are glass micropipettes, with a tip diameter of <1 micrometre, and a resistance of several megohms . The micropipettes are filled with a solution that has a similar ionic composition to the intracellular fluid of the cell . A chlorided silver wire inserted into the pipet connects the electrolyte electrically to the amplifier and signal processing circuit . The voltage measured by the electrode is compared to the voltage of a reference electrode, usually a silver chloride - coated silver wire in contact with the extracellular fluid around the cell . In general, the smaller the electrode tip, the higher its electrical resistance, so an electrode is a compromise between size (small enough to penetrate a single cell with minimum damage to the cell) and resistance (low enough so that small neuronal signals can be discerned from thermal noise in the electrode tip). </P> <P> The voltage clamp technique allows an experimenter to "clamp" the cell potential at a chosen value . This makes it possible to measure how much ionic current crosses a cell's membrane at any given voltage . This is important because many of the ion channels in the membrane of a neuron are voltage - gated ion channels, which open only when the membrane voltage is within a certain range . Voltage clamp measurements of current are made possible by the near - simultaneous digital subtraction of transient capacitive currents that pass as the recording electrode and cell membrane are charged to alter the cell's potential . </P> <P> The current clamp technique records the membrane potential by injecting current into a cell through the recording electrode . Unlike in the voltage clamp mode, where the membrane potential is held at a level determined by the experimenter, in "current clamp" mode the membrane potential is free to vary, and the amplifier records whatever voltage the cell generates on its own or as a result of stimulation . This technique is used to study how a cell responds when electric current enters a cell; this is important for instance for understanding how neurons respond to neurotransmitters that act by opening membrane ion channels . </P> <P> Most current - clamp amplifiers provide little or no amplification of the voltage changes recorded from the cell . The "amplifier" is actually an electrometer, sometimes referred to as a "unity gain amplifier"; its main job is to change the nature of small signals (in the mV range) produced by cells so that they can be accurately recorded by low - impedance electronics . The amplifier increases the current behind the signal while decreasing the resistance over which that current passes . Consider this example based on Ohm's law: A voltage of 10 mV is generated by passing 10 nanoamperes of current across 1 MΩ of resistance . The electrometer changes this "high impedance signal" to a "low impedance signal" by using a voltage follower circuit . A voltage follower reads the voltage on the input (caused by a small current through a big resistor). It then instructs a parallel circuit that has a large current source behind it (the electrical mains) and adjusts the resistance of that parallel circuit to give the same output voltage, but across a lower resistance . </P>

Which of the following is not a characteristic of a 3 channel or multi-channel electrocardiogram