Basic Properties Of The Action Potential

The basic properties of the action potential can be studied using a microelectrode constructed from a glass capillary tube with a fine tip and containing artificial intracellular solution. This microelectrode,

Encyclopedia of the Human Brain Volume 1

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inserted into the cell body or axon of a neuron (Fig. 1a, inset), measures the value of membrane potential relative to the extracellular space. At rest, typical values of membrane potential range from —40 to —90 mV. Passing positive electrical current into the cell depolarizes it (i.e., makes membrane potential less negative). In response to small depolarizing stimuli, the neuron's response is small as well (Fig. 1a, bottom). In response to larger stimuli, above a threshold value, the response is fundamentally different; the membrane potential quickly rises to a value well above 0 mV and then falls over the course of 1-5 msec to its resting value (Fig. 1a, middle). Often, the falling phase of the action potential undershoots resting potential temporarily. The action potential is said to be all-or-nothing because it occurs only for sufficiently large depolarizing stimuli, and because its form is largely independent of the stimulus for suprathreshold stimuli. In some neurons, a single action potential can be induced by the offset of a hyperpolarizing stimulus

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