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During an absolute neuronal refractory period, the excitable region cannot initiate a second distinct regenerative action potential under the physiological stimulation conditions being considered. Extensive sodium ion-channel inactivation is central for a fast sodium-dependent action potential. “Cannot fire” concerns regeneration; an imposed pulse can still move membrane potential or leave a recording artifact. Failure of just one fixed-amplitude test pulse does not establish an absolute neuronal refractory period: the same failure can occur in the relative neuronal refractory period.

The boundary is an operational question about distinguishable action potentials and the range of stimulation tested. A drawn vertical line is useful in the phenomenological paired-pulse recovery model, where it is imposed explicitly; it should not be mistaken for a universal sharp biochemical switch.

An action potential is a brief regenerative excursion of membrane potential, produced by changing electrical resistance and conductance through voltage-gated ion channels. In a typical fast neuronal action potential, sodium entry raises membrane potential, activating further sodium channels. This feedback produces the rapid rising phase. Inactivation of sodium channels and increased conductance to potassium then lower membrane potential again; persisting potassium conductance can produce an undershoot below the resting level. See Purves and colleagues: the ionic basis of action potentials.

For an illustrative electrical calculation, write inward sodium electric current as $I_{\mathrm{Na}}=g_{\mathrm{Na}}(E_{\mathrm{Na}}-V)$. At $V=-50\mathrm{mV}$, taking $E_{\mathrm{Na}}=60\mathrm{mV}$ and $g_{\mathrm{Na}}=10\mathrm{nS}$ gives $I_{\mathrm{Na}}=1.1\mathrm{nA}$ inward. A potassium current $I_{\mathrm{K}}=g_{\mathrm{K}}(E_{\mathrm{K}}-V)$ with $E_{\mathrm{K}}=-80\mathrm{mV}$ and $g_{\mathrm{K}}=20\mathrm{nS}$ gives $-0.6\mathrm{nA}$, outward. These illustrative values expose opposing driving forces; a waveform requires the conductances to evolve with membrane potential and time, as in the Hodgkin-Huxley model.

An action potential propagates along an axon because local electric currents bring neighboring regions of cell membrane into the regime where their voltage-gated ion channels regenerate the signal. It is a traveling pattern of excitation, not a packet of sodium ions traversing the entire axon. Regeneration allows an action potential to travel without the passive attenuation of a small local voltage change. See Ion Channels and the Electrical Properties of Membranes.

The neuronal refractory period constrains how soon another action potential can follow. Under comparable conditions, successive action potentials have roughly stereotyped shapes, making event count and timing useful for neural coding; waveform details can nevertheless change with channel state and conditions. The leaky integrate-and-fire model abstracts each action potential to a threshold event and reset. Its reset jump is a mathematical rule, not the actual regenerative waveform produced by sodium and potassium currents.

A physiologist who studied membrane potentials and action potentials.

Institutional biography: www.physoc.org/blog/prize-lecture-memoria-alan-hodgkin/

A physiologist who studied membrane potentials and action potentials.

Institutional biography: www.physoc.org/blog/prize-lecture-memoria-andrew-huxley/

A neuronal process that carries signals toward output sites, often by propagation of action potentials to synapses.

The axon initial segment is the specialized proximal region of an axon near the soma. In many neurons, its voltage-gated ion channels make it the site where an action potential begins. ^[ref]

A chemical synapse transmits a signal by releasing neurotransmitter from a presynaptic neuron to act on postsynaptic receptors across a synaptic cleft. The two cell membranes remain separate. A typical fast transmission sequence is:

See Chemical Synapses and 3.1 Synapses. The fast pathway uses synaptic vesicles prepared near release sites; it does not require a distant synaptic vesicle to travel across the terminal after every action potential. See Basic Neurochemistry: calcium entry and vesicles at release sites. Some chemical synapses release neurotransmitter in response to graded presynaptic membrane potential changes rather than action potentials.

Neurotransmitter clearance limits how long released neurotransmitter remains available. Depending on the neurotransmitter and location, diffusion away, uptake through membrane transport proteins into the sending or neighboring biological cells, and breakdown by enzymes contribute. Receptor binding and unbinding also affect response duration. Recovering synaptic vesicle membrane is a separate recycling task; uptake of released neurotransmitter is not the return of an intact synaptic vesicle. See General Principles of Cell Communication.

This electrical-to-chemical-to-electrical sequence offers several places to adjust a signal: synaptic release probability, receptor availability and neurotransmitter clearance can change its strength or duration. It helps explain why synaptic plasticity can modify communication between neurons, and why an arriving action potential need not cause a postsynaptic action potential.

Otto Loewi's 1936 Nobel lecture describes his 1921 experiment: fluid from a frog heart whose nerves had been stimulated produced the corresponding response when transferred to another heart. The transferable substance established chemical mediation in that preparation. This was evidence for released chemical signaling, not yet an identification of synaptic vesicle fusion and not a claim that every synapse is chemical. See Loewi, The Chemical Transmission of Nerve Action, Nobel lecture, 12 December 1936.

Current clamp prescribes injected electric current while recording membrane potential. A constant command does not force membrane potential to remain constant: a neuron can produce action potentials or synaptic potentials while that command is unchanged. At zero commanded electric current, the amplifier still measures membrane potential; intrinsic ionic electric currents continue. The name concerns current supplied by the instrument, not the cancellation of all currents in the biological cell.

In whole-cell recording, the measured pipette voltage during a current pulse includes the patch-clamp series resistance drop. A modern authored methods article explicitly describes current-pulse protocols and this distortion.^[ref] Check the access contribution with small pulses and distinguish the immediate pipette-voltage jump from the slower membrane capacitance response. A zero-input interval measures a baseline, not proof that the instrument has no voltage offset. The passive example below explains these distinctions without asserting any new physiological measurements.

A dendritic spike is a regenerative local voltage event in a dendrite. Its effect at the soma depends on propagation and attenuation; local regeneration does not by itself establish a somatic action potential. It is an active mechanism beyond the passive dendritic cable.

For a negative neuronal membrane potential, depolarization raises it toward zero or beyond, making the interior less negative relative to the exterior. This describes a voltage change, not necessarily increased action potential generation; see the depolarizing inhibition example under synaptic potential.

Sustained membrane potential depolarization can prevent repeated action potentials because voltage-dependent ion-channel inactivation leaves too few available sodium channels. This differs from a reduced response to a passive current step: active Hodgkin-Huxley model dynamics and recovery from inactivation are essential to the mechanism.

An excitatory postsynaptic potential is a synaptic potential associated with increased likelihood of a postsynaptic action potential. Its size and effect depend on the receiving neuron's state; the definition concerns firing, not a fixed neurotransmitter identity. See synaptic potential for the mechanism and source.

Extracellular single-unit recording uses an electrode outside a neuron to detect electrical events attributed to that unit's action potentials. It records a spike train, not the intracellular membrane potential or every input synapse. ^[ref]

The number of action potentials per unit time, interpreted as an average over a stated window or as an intensity estimated from a spike train. Window size changes which temporal information remains visible.

A conductance-based Hodgkin-Huxley model couples membrane potential to voltage-dependent channel-gating variables. It explains generation of action potentials through regenerative and opposing ionic electric currents, while its parameters and channel assumptions determine its biological domain of validity.

In the classical model, $m$ describes sodium activation, $h$ describes availability against sodium inactivation, and $n$ describes potassium activation. Each follows two-state ion-channel gating, with its own voltage-dependent rates. The available electrical conductances areThe barred quantities are maximum electrical conductances. The factors have the interpretation of independent permissive gates in an ion channel: three activation factors and a distinct inactivation-availability factor for sodium, and four identical activation factors for potassium. Large $h$ means availability; small $h$ means inactivation. These are model assumptions, not a literal structural description of every ion channel. See Gerstner, Kistler, Naud and Paninski, Neuronal Dynamics (2014), §2.2: model definition and stochastic opening.

With inward-positive electric current, a single-compartment form isHere $C$ is membrane capacitance, $I_{\mathrm{ext}}$ is imposed inward electric current, $g_L$ is leak electrical conductance, and each $E$ is a reversal potential. Use consistent units, and either whole-compartment quantities throughout or quantities per membrane area throughout. Opening changes electrical conductance; the reversal potential and present membrane potential determine the associated electric current. Coupling this equation to the voltage-dependent gates makes a generally nonlinear system of ordinary differential equations; a gate's fixed-voltage limit alone does not specify its evolving state during an action potential.

The original quantitative account is ^[ref]. The contrast with the leaky integrate-and-fire model is useful: here an action potential emerges from evolving ionic electric currents, whereas the simpler model records a threshold event and imposes a reset.

An inhibitory postsynaptic potential is a synaptic potential associated with decreased likelihood of a postsynaptic action potential. It can involve hyperpolarization or depolarization; an inhibitory conductance can even produce no measurable voltage change at rest. See synaptic potential and shunting inhibition for worked examples and sources.

A cochlear hair-cell class providing most afferent input to the auditory nerve. Mechanical stimulation changes receptor activity and transmitter release; downstream auditory nerve fibers carry action potentials.

An interspike interval is the elapsed time between consecutive action potentials in a spike train. Its probability distribution and dependence on previous activity can reveal a neuronal refractory period even when the average firing rate is unchanged.

In 1939, Alan Hodgkin and Andrew Huxley reported intracellular action potential recordings from squid giant axons at Plymouth.^[ref]

Alan Hodgkin's later account describes advancing a capillary electrode from a cut end into intact axon, avoiding scraping the cell membrane. Its Figure 3 uses a seawater-filled capillary internally, with external seawater as zero: the readout is internal relative to external electric potential. This supports a voltage across the surface cell membrane. As a separate ionic control, the account reports that raising external potassium toward its internal concentration reversibly abolished resting voltage; this was later work, not a control asserted for the 1939 recording. Injury and electrode-solution junction potentials limit interpretation. Voltage recording alone does not identify every conducting ion. A voltage clamp additionally uses feedback to impose voltage and measures the required electric current.^[ref]

In the schematic, connect the intracellular electrode to the positive input and the external reference electrode to the negative input of a voltage recorder. After electrode-offset correction, a negative reading means ${\varphi }_{\mathrm{inside}}<{\varphi }_{\mathrm{outside}}$. A useful baseline check is to compare both electrodes in the same external chemical solution before insertion; this is a measurement check, not an additional historical observation claimed here. Swapping inputs reverses the readout sign without changing the biological cell's membrane potential.

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. ^[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. ^[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 isFor 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.

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.^[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.^[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. ^[ref]