= Solution
At the initial negative <membrane potential>, most <voltage-gated potassium channels> have a closed activation gate. <Depolarization> moves charged residues in each <voltage-sensing domain>, particularly the S4 helices, through the <electric field>. Coupling between the <voltage-sensing domains> and the pore opens the intracellular gate. A classical <voltage-gated potassium channel> has four subunits, each with six transmembrane helices; S5, the pore loop and S6 contribute to the central pathway and <selectivity filter>. The rising open <probability> produces increasing outward <potassium> <electric current>, since the imposed positive <membrane potential> is above the <potassium> <Nernst equilibrium potential>. Here outward <electric current> is positive: $I_K=N\gamma p_O(V-E_K)$, where $N$ is channel number and $\gamma$ is single-channel <electrical conductance>.
The <electric current> subsequently falls because the rapidly inactivating Shaker form exhibits <ball-and-chain inactivation>. An intracellular N-terminal domain, tethered to the channel, blocks the open pore. The <ion channel> can therefore stop conducting while the <membrane potential> remains depolarized and the activation machinery remains activated. This is <ion-channel inactivation>, rather than reversal of the voltage step, loss of the ionic driving force, or ordinary gate closure. <Repolarization> favors removal of the block and recovery of channel availability. Slower <C-type inactivation> involving the outer pore can coexist, but is not the principal explanation of the rapid decay described here.
Several independent observations connect this functional explanation to structure. Small transient gating <electric currents> report movement of sensor <electric charge> before ionic conduction; charge-neutralization studies of S4/S2 change the measured gating charge and activation. Structural studies resolve separate sensor and pore regions.<Voltage-sensing residues in the S2 and S4 segments of the Shaker K+ channel by S. A. Seoh and others>[Gating-charge measurements identify voltage-sensing residues]{ref}<Crystal structure of a mammalian voltage-dependent Shaker family K+ channel by Stephen B. Long and others>[Voltage-sensor and pore architecture of Kv1.2]{ref} Historical N-terminal removal abolished rapid <ion-channel inactivation> while preserving activation, and the corresponding free peptide restored block from the cytoplasmic side. These results separate the activation gate from the tethered blocker and identify which side of the <cell membrane> contains the blocking domain.<Biophysical and molecular mechanisms of Shaker potassium channel inactivation by Toshinori Hoshi and others>[N-terminal perturbations distinguish rapid inactivation from activation]{ref}<Restoration of inactivation in mutants of Shaker potassium channels by a peptide derived from ShB by William N. Zagotta and others>[Cytoplasmic peptide restores inactivation to channels lacking the native N-terminal domain]{ref}
\b[Source-gap TODO:] the supplied PDF's figure invokes an undefined image object, so its actual pulse duration, pulse sequence, current scale, peak amplitude, decay time and any tail-current waveform cannot be read. The account above uses only the stated change from $-60$ to $+20\,\mathrm{mV}$ and the stated rise and rapid decay; no numerical kinetic fit or reconstruction of the missing trace is claimed.
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