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Acetylcholine is a neurotransmitter that acts through nicotinic ionotropic receptors and muscarinic metabotropic receptors. The receptor and the target cell's machinery determine the response; acetylcholine itself does not specify whether coupling is direct or mediated by intracellular signal transduction. ^[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.

An electrical synapse passes ionic electric current directly between neurons through gap junctions, whose paired channels connect the two cytoplasms. It does not require synaptic vesicle release and postsynaptic neurotransmitter binding. Transmission across the junction is very fast; the receiving cell membrane's capacitance and other conductances still determine its voltage response. Electrical coupling can coordinate the timing of activity across neurons. See Purves and colleagues: electrical synapses and gap junctions.

For a passive junction with constant conductance $g_j$, the inward electric current into cell 2 iswhere each $V_k$ is the cell's membrane potential, measured against a common extracellular reference. Taking $g_j=2\mathrm{nS}$, $V_1=-55\mathrm{mV}$ and $V_2=-65\mathrm{mV}$ gives $20\mathrm{pA}$ into cell 2 and the equal opposite electric current into cell 1. Exchanging the voltages reverses the electric current. Both depolarization and hyperpolarization can therefore be transmitted; an electrical synapse is not inherently an excitatory connection.

Many electrical synapses conduct in both directions, but this does not guarantee equal voltage responses: the cells can have different effective electrical resistance and capacitance. Some junctions rectify, meaning they conduct preferentially in one direction, so the constant-$g_j$ formula is insufficient. A concrete molecular example is the original study ^[ref]. Bidirectionality is thus a common property with important qualifications, rather than part of the definition.

Endocytosis takes material into a biological cell by inward budding and separation of part of its cell membrane. At a chemical synapse, it helps recover membrane and proteins after synaptic vesicle exocytosis. This recovery is distinct from uptake of individual neurotransmitter molecules through membrane transport proteins.

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.

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]

A metabotropic receptor couples ligand recognition to intracellular signal transduction, which can regulate a separate ion channel, alter enzyme activity or change gene expression. In the usual neurotransmitter context, metabotropic receptors are G protein-coupled receptors: receptor activation is relayed by a G protein. There need not be a change in membrane potential at all. ^[ref]

Two routes should be distinguished. An activated G protein can regulate an ion channel locally at the cell membrane, without an obligatory diffusible second messenger. Alternatively, it can regulate an enzyme that changes the availability of a second messenger, such as cAMP or inositol trisphosphate. Downstream proteins then change channel opening, phosphorylation or other cellular activities. The receptor, G protein, second messenger and final target are distinct components; an activated G protein is a relay protein, rather than itself a small-molecule second messenger.^[ref]

Do not turn this useful chemical synapse classification into the claim that every receptor is a GPCR, or that every indirect response must pass through a G protein. For example, the insulin receptor is an enzyme-linked cell-surface receptor whose intracellular catalytic activity initiates protein phosphorylation. Intracellular receptors supply another category. “Metabotropic” commonly names the GPCR branch in neuroscience; the broader idea of biochemical coupling does not make all non-channel receptors structurally identical. ^[ref]

Separating recognition from execution allows signal amplification and branching. A persistently activated receptor can activate several relay proteins; a regulated enzyme can produce many second messenger molecules. As an explicitly invented count example, suppose one activated receptor activates four relay proteins during one interval, and each relay activates a distinct enzyme that produces fifteen second messenger molecules during the next interval. With sufficient substrates and no loss, this gives $4\times 15=60$ molecules. Those counts do not specify a physiological gain or response duration. A directly gated ion channel also passes many ions while open, so amplification is not an exclusive property of metabotropic receptors: always name the input and output being compared. ^[ref]

A particularly instructive electrophysiology experiment separated a chemical trigger from the ion channel carrying its eventual electric current. In 1987, Hirono, Ito and Sugiyama injected messenger RNA from rat brain into frog egg cells, then tested acquired responses to neurotensin and muscarinic acetylcholine stimulation. Under voltage clamp at $-60\mathrm{mV}$, neurotensin elicited transient, oscillatory inward electric currents after a delay of several tens of seconds. This delay refers to that response and preparation. Reversal potential measurements implicated increased permeability to chloride anions, rather than a pore specific to the applied ligand. Both neurotensin and acetylcholine responses were inhibited by pertussis toxin, which interferes with susceptible G proteins, and by intracellular EGTA, which binds free calcium. Injecting inositol trisphosphate mimicked the responses; a large prior dose of inositol trisphosphate reduced subsequent responses to either extracellular signal. These interventions implicate a shared intracellular relay and calcium-dependent channel opening.^[ref]

The informative comparison with the nicotinic acetylcholine receptor experiment is mechanistic: an extracellular ligand can produce an inward electric current either by directly gating the pore or by mobilizing an intracellular second messenger and stored calcium that activate another pore. Intracellular inositol trisphosphate bypasses reception, whereas EGTA tests the need for free calcium. However, neither inhibition nor mimicry alone identifies every molecular connection: a perturbation can affect several targets, and an injected mediator can activate a pathway without proving that it is the only natural route. The egg cell's endogenous machinery and the mixed messenger RNA preparation also limit extrapolation to an intact neuron. The experiment establishes functional coupling in its preparation, rather than the exact molecular composition or timing of every metabotropic receptor.

Even G protein-dependent channel regulation need not involve a lengthy second messenger cascade. Pfaffinger and colleagues reported in 1985 that acetylcholine-evoked potassium channel activation in embryonic chick atrial cells required intracellular GTP, a guanine-containing nucleotide, and was abolished by pertussis-toxin pretreatment. Changes in cyclic nucleotides, including cAMP, did not account for the channel response. The report discusses intrinsic activation delays of roughly $30$–$100\mathrm{ms}$, illustrating relatively rapid metabotropic receptor action. These observations support a G protein coupling step without cyclic-nucleotide second messengers; they do not by themselves exclude every possible intermediary or prove the specific subunit-channel interaction.^[ref]

A small timing model makes the extra steps visible without inventing an experimental trace. Let $u(t)$ jump from zero to one at $t=0$ and remain on. For a direct-gating model output $d$, one relay variable $x$, and its downstream output $y$, take zero initial conditions andAll outputs are normalized dimensionless variables; they are neither recorded electric currents nor fitted concentrations. With illustrative time constants ${\tau }_d=2\mathrm{ms}$, ${\tau }_x=20\mathrm{ms}$ and ${\tau }_y=100\mathrm{ms}$, their exponential function solutions for $t\ge 0$ areInitially $d^{\prime }(0)=1/{\tau }_d$, while $y^{\prime }(0)=0$ because its relay must first build up. Both outputs eventually approach one. Equal normalized final values do not imply equal biological amplitudes or absence of signal amplification; normalization deliberately removes that question.

The model's useful feature is the delayed buildup created by sequential relaxation, not its arbitrary numbers. Actual kinetics also depend on ligand delivery, relay deactivation, neurotransmitter clearance and the final target. A local G protein route can act promptly, while an ionotropic receptor can remain active during continued exposure or feed a longer-lasting intracellular response. The beauty of metabotropic receptor coupling is its flexibility: the same recognition event can adjust electrical responsiveness and other cellular tasks through different targets. To classify an observed response, combine timing with molecular identity and controlled perturbations.

A neuron is a specialized biological cell that receives, integrates and transmits signals. Its shape separates places where inputs arrive from places where outputs reach other biological cells: this functional asymmetry is neuronal polarity. Understanding a neuron therefore means connecting its anatomy to changes in membrane potential, rather than treating it as a wire that merely forwards every input.

In a common multipolar neuron, branching dendrites receive inputs at synapses; the soma contains the cell nucleus and most major organelles; the axon carries signals to distant or nearby targets; and axon terminals form output contacts. All these regions belong to one biological cell enclosed by a continuous cell membrane. An axon can branch, allowing one neuron to influence several receiving biological cells.^[ref]

At an input chemical synapse, a neurotransmitter acts on a receptor, changing an electric current directly or through signal transduction.^[ref] A resulting synaptic potential contributes to synaptic integration in the dendrites and soma. In many spiking neurons, sufficiently strong depolarization recruits voltage-gated ion channels in the axon initial segment, initiating an action potential that propagates along the axon. At an output chemical synapse, calcium entry through voltage-gated ion channels promotes synaptic vesicle exocytosis, releasing neurotransmitter. An action potential is thus regenerated electrically along a cell membrane; it is not a packet of neurotransmitter travelling from the dendrites to the axon terminals.^[ref]

The compartments are specializations, not exclusive jobs. Inputs can contact the soma or axon as well as dendrites; dendrites can support active voltage-gated ion channels. Paired recordings from a pyramidal neuron's soma and dendrite or axon established that an action potential can begin in the axon and propagate back into the dendrites. The usual output direction therefore does not make every electrical signal strictly one-way.^[ref] An electrical synapse, which directly couples cytoplasms through gap junctions, also differs from the sending-and-receiving arrangement of a chemical synapse.^[ref]

Nor does every neuron have the upper diagram's shape. A bipolar neuron has two main processes, one dendrite and one axon; a multipolar neuron has several dendrites; and a pseudounipolar neuron has a single process from the soma that divides into peripheral and central branches. In the last arrangement, sensory signals can travel past the soma rather than obligatorily through it. These shape categories do not specify every neuron's receptor, connectivity or firing behavior.^[ref] Some neurons in the retina communicate through graded changes in membrane potential without generating action potentials.^[ref]

Why timing changes the computation. Synaptic integration can combine inputs at different locations, called spatial summation, and inputs arriving at different times, called temporal summation. A minimal example uses the leaky integrate-and-fire model with the illustrative parameters already given there: resting and reset membrane potential $-65\mathrm{mV}$, firing threshold $-50\mathrm{mV}$, electrical resistance $R=100\mathrm{M\Omega }$, capacitance $C=200\mathrm{pF}$, time constant $\tau =RC=20\mathrm{ms}$ and neuronal refractory period $2\mathrm{ms}$. These are chosen model values, not measured properties of a particular neuron. Before firing, a constant inward electric current $I$ starting from rest givesThe exponential function expresses the competition between charging the cell membrane's capacitance and leakage through its electrical resistance.^[ref]

Two $100\mathrm{pA}$ inputs, each lasting $40\mathrm{ms}$, produce different outputs depending on their timing. Either input alone reaches only $-56.3534\mathrm{mV}$. Applied together, they give $I=200\mathrm{pA}$ and reach threshold after $20\ln 4\mathrm{ms}\approx 27.7259\mathrm{ms}$, producing one action potential event during the pulse. If instead the second input begins $20\mathrm{ms}$ after the first ends, the earlier depolarization has partly decayed: the second pulse peaks at approximately $-55.9229\mathrm{mV}$ and neither pulse produces an action potential. The two-input protocols deliver the same total electric charge, $8\mathrm{pC}$, yet only the coincident protocol fires. Counting total input alone misses this timing dependence.

This example makes a neuron a detector of sufficiently clustered input, not a perfect logical gate: different parameters change the result. Actual dendrites introduce location-dependent filtering and active responses, and an inhibitory postsynaptic potential or shunting inhibition can change whether another input succeeds. The single-compartment leaky integrate-and-fire model omits these mechanisms.^[ref] Across connected neurons, branching axons distribute outputs while converging synapses combine inputs; spike trains can support temporal coding and rate coding, and synaptic plasticity changes the influence of an input. Connectivity and cellular dynamics together determine the computation.^[ref][ref]

Testing the connection between structure and function. Whole-cell recording connects anatomy to a measured membrane potential. In a published mouse protocol, a two-photon microscope guides solution-filled glass pipettes to neurons; an amplifier and bath reference electrode complete the electrical recording apparatus. A high-resistance seal forms against the cell membrane, then brief suction opens the membrane patch beneath the tip. In current clamp, the amplifier supplies a chosen electric current and records the resulting membrane potential, unlike voltage clamp, which controls voltage and measures electric current. The investigators used $500\mathrm{ms}$ current steps of several amplitudes to characterize excitability and firing, and negative current pulses to estimate input electrical resistance.^[ref]

The observable is a voltage trace, including graded responses and action potentials. Comparing injected electric current with that trace tests how input becomes output; it does not identify every underlying ion channel. The protocol checks access electrical resistance, which can distort voltage and action potential measurements, and includes intervals without stimulation to record spontaneous activity. In a teaching replication, repeated zero-input trials and monitoring baseline drift would provide further controls; these are recommended comparisons, not additional results attributed to the paper. A recording at the soma also cannot establish the local voltage everywhere in a branching dendrite, and this anesthetized preparation does not establish behavior in every awake neuron.^[ref]

A complementary experiment used simultaneous whole-cell recordings from the soma and an apical dendrite or axon of the same pyramidal neuron in a brain slice. The relative onset times of action potentials, together with sodium currents in excised dendrite membrane patches, supported axonal initiation followed by active propagation into dendrites. Pairing sites within one neuron is a useful comparison because it avoids confusing differences between cells with differences between compartments. Instrument timing and comparable recording quality would be essential controls when reproducing the inference. Only the published abstract was accessible here, so no unverified timing values, drug controls or channel distributions are supplied.^[ref]

Neurotransmitter clearance removes released neurotransmitter from the region where it can activate receptors, through diffusion, cellular uptake or enzyme-mediated breakdown. Its routes and rates help delimit a chemical synapse's response in space and time; see the mechanisms and source under chemical synapse.