= Ionotropic receptor
An <ionotropic receptor> is a <receptor> in which binding of a <ligand> directly gates an <ion channel> within the same <protein> complex. The binding site detects the chemical signal; the pore supplies a path across the <cell membrane>. At a <chemical synapse>, a <neurotransmitter> therefore changes <synaptic conductance> without an obligatory intervening <G protein> or <second messenger>. <Neuroscience, 2nd edition edited by D. Purves and others/Two Families of Postsynaptic Receptors>{ref}
A familiar example is the nicotinic <acetylcholine> <receptor> of skeletal muscle: <acetylcholine> binding opens a pore passing <cations>, including <sodium> and <potassium>. The muscarinic <acetylcholine> <receptor> instead uses intracellular <signal transduction>; the same <neurotransmitter> can engage different mechanisms. Direct gating does not mean that an <ionotropic receptor> pumps <ions>: <electrochemical gradients> drive passage through the open <ion channel>. Nor does “ionotropic” specify excitation. The <reversal potential>, initial <membrane potential> and other conductances determine whether the effect favors an <action potential>, causes <hyperpolarization>, or supplies <shunting inhibition>. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/Ion Channels and the Electrical Properties of Membranes>{ref}
A concrete electrical approximation separates opening from drive. Let $N$ identical <ion channels> each have open-channel <electrical conductance> $\gamma$ and opening <probability> $p(t)$. At fixed <membrane potential> $V$, with common <reversal potential> $E_{\mathrm{rev}}$, the <expected value> of inward-positive <electric current> is
$$
I_{\mathrm{in}}(t)=N\gamma p(t)(E_{\mathrm{rev}}-V).
$$
For stipulated values $N=100$, $\gamma=20\,\mathrm{pS}$, $p=0.2$, $V=-60\,\mathrm{mV}$ and $E_{\mathrm{rev}}=0$, the mean open count is $20$, the population <electrical conductance> is $0.4\,\mathrm{nS}$ and $I_{\mathrm{in}}=24\,\mathrm{pA}$. One open <ion channel> carries $1.2\,\mathrm{pA}$. These are illustrative values, not measurements. At $V=E_{\mathrm{rev}}$, the net <electric current> vanishes despite open <ion channels>; neither binding nor opening alone determines the readout. The approximation omits changes in <ion> <concentrations> and voltage-dependent open-channel <electrical conductance>.
\Image[/ionotropic-metabotropic-receptors-mechanisms.png]
{title=Direct <ionotropic receptor> gating and two routes of <metabotropic receptor> action}
{description=Original mechanism cartoons, not molecular structures or measured pathways. Solid blue arrows show one possible net movement of positive <ions> when the <electrochemical gradient> favors entry; dashed orange arrows relay a signal. In the first panel binding and pore functions belong to one <protein> complex. In the other panels a separate <ion channel> is regulated through a <G protein>, either locally or through an <enzyme>, <inositol trisphosphate> and stored <calcium>. Positions, shapes and arrow lengths encode no measured distances or times.}
A motivating experiment made the elementary electrical events visible. As Bert Sakmann recounts, chronically denervated frog muscle fibers had <acetylcholine>-sensitive <receptors> spread beyond the original nerve contact. Mild <enzyme> treatment exposed the <cell membrane>; a polished glass pipette sealed over a small patch to reduce background noise. Two intracellular electrodes held the local <membrane potential> through <voltage clamp>, while the patch pipette recorded tiny <electric currents>. Applying suberyldicholine, a <ligand> that activates the <acetylcholine> <receptor> and prolongs openings, revealed abrupt, approximately rectangular current events. Their amplitudes agreed reasonably with estimates from whole-membrane current fluctuations. This established discrete opening and closing events; it did not make every <ligand> response instantaneous. Background noise and finite recording bandwidth concealed sufficiently small or brief events.<Elementary Steps in Synaptic Transmission Revealed by Currents through Single Ion Channels by Bert Sakmann>[pp. 33–35]{ref}
Recording an intact membrane patch alone does not exclude intracellular relays. Tank, Huganir, Greengard and Webb supplied a complementary isolation test in 1983: they purified nicotinic <acetylcholine> <receptors> from an electric ray's electric organ, incorporated them into artificial <lipid> membranes, and fused small vesicles into larger vesicles by freezing and thawing. Sealed glass-pipette recordings from isolated membrane patches resolved functional <ion channels> with properties resembling those in muscle. Removing the intact <cytoplasm> and its signaling machinery supports the sufficiency of the <receptor> complex for channel function; an artificial <lipid> environment still cannot reproduce every native regulatory interaction.<Patch-recorded single-channel currents of the purified and reconstituted Torpedo acetylcholine receptor by D. W. Tank and others>[abstract]{ref}
Direct gating supports millisecond communication at many <chemical synapses>. Response duration also depends on <neurotransmitter clearance>, binding and channel-state transitions. Continued <ligand> exposure can prolong activity. Classify an <ionotropic receptor> by its direct connection between recognition and pore gating, rather than a universal duration cutoff. <Neuroscience, 2nd edition edited by D. Purves and others/Two Families of Postsynaptic Receptors>{ref}
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