Excitatory Postsynaptic Potentials vs. Inhibitory Postsynaptic Potentials

The nervous system coordinates behavior, perception, and homeostasis through an intricate communication network governed by electrical and chemical signaling. At the core of this processing capability is the synapse, a specialized junction where an upstream presynaptic neuron communicates with a downstream postsynaptic cell. When an action potential reaches the presynaptic terminal, voltage-gated calcium channels open, initiating neurotransmitter exocytosis into the synaptic cleft. These chemical messengers diffuse across the interstitial space and bind to specific postsynaptic receptors, inducing conformational shifts that alter the electrical potential of the postsynaptic membrane. These resulting voltage fluctuations are known as postsynaptic potentials (PSPs).

Postsynaptic potentials are divided into two fundamental, opposing classes: excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). Rather than operating as binary on-off triggers, these electrical fluctuations act as the analog currency of the central and peripheral nervous systems. An EPSP depolarizes the local neuronal membrane, moving the intracellular potential closer to the threshold required to discharge an action potential. Conversely, an IPSP hyperpolarizes or stabilizes the membrane potential, counteracting depolarizing inputs and preventing unwarranted firing. Every motor movement, sensory experience, and cognitive calculation emerges from the continuous arithmetic summation of these microscopic electrical events across billions of interconnected neurons.

Understanding the interplay between EPSPs and IPSPs requires analyzing ion selectivity, thermodynamic driving forces, receptor kinetics, and the structural morphology of dendrites. Synaptic inputs undergo physical decay as they travel along dendrites, governed by the passive biophysical properties of the lipid bilayer and cytoplasm. The final computation determining whether a neuron discharges an action potential takes place at the axon initial segment, where thousands of arriving EPSPs and IPSPs are integrated over space and time. This article provides a comprehensive biophysical, physiological, and computational analysis of excitatory and inhibitory postsynaptic potentials, dissecting their molecular foundations, spatial arrangements, integration dynamics, and clinical significance.

1. Introduction: Foundations of Postsynaptic Membrane Dynamics

1.1 Defining Graded Potentials in the Central and Peripheral Nervous Systems

Graded potentials represent localized, non-propagating fluctuations in membrane electrical charge that vary continuously in amplitude and duration. Unlike the regenerative, all-or-none action potentials that travel down axonal pathways, graded potentials arise primarily within dendritic spines, dendritic shafts, and the somatodendritic arborizations of neurons. In both the central nervous system (CNS) and the peripheral nervous system (PNS), these subthreshold electrical responses serve as the primary analog currency of neuronal computation, capturing subtle gradations in synaptic input before digital conversion at the spike trigger zone.

The biophysical signature of a graded potential depends on ligand-receptor interactions rather than the opening of voltage-gated channels. When a neurotransmitter binds to an ionotropic receptor, the channel pore opens, allowing a specific flux of ions across the postsynaptic density. The resulting current alters the electrical charge across the phospholipid bilayer, producing a voltage deflection whose amplitude directly reflects the quantity of bound ligand, channel open probability, and single-channel conductance. If a low concentration of neurotransmitter is cleared from the synaptic cleft, only a small cohort of channels will open, generating a modest deflection of a fraction of a millivolt. If a high-frequency burst triggers substantial vesicular release, thousands of channels open concurrently, producing a much larger electrical event.

Graded potentials exhibit no intrinsic threshold and do not possess absolute or relative refractory periods. Successive graded potentials can stack upon one another, producing composite waveforms whose temporal profiles depend on the opening and closing kinetics of the underlying receptor channels and the passive electrical parameters of the host dendritic branch. Because these potentials do not regenerate through positive feedback loops of voltage-dependent conductances, they decay exponentially as they spread passively away from their site of origin, attenuating across distance and dissipating over time according to core cable theory principles.

1.2 Conceptual Distinctions Between Synaptic Excitation and Inhibition

Synaptic excitation and inhibition are commonly differentiated by whether they drive membrane depolarization or hyperpolarization. However, a strictly functional definition relies on how each input alters the probability that the postsynaptic neuron will fire an action potential. Synaptic excitation describes any process that shifts the neuronal membrane potential closer to the threshold of voltage-gated sodium channel activation, or otherwise increases the likelihood of action potential generation. In contrast, synaptic inhibition acts to suppress firing, either by moving the membrane potential away from the threshold or by stabilizing it below that threshold.

A frequent point of academic confusion arises when classification relies solely on the sign of the membrane voltage deflection. While classical excitation involves an inward positive current that depolarizes the membrane, and classical inhibition involves an outward positive current (or inward negative current) that hyperpolarizes it, there are conditions where an inhibitory input causes no measurable hyperpolarization or may even cause a slight depolarization. This phenomenon, known as shunting or silent inhibition, occurs when the reversal potential of the activated channel matches or sits slightly positive to the cell’s resting membrane potential. Even without driving the membrane potential in a negative direction, the massive increase in membrane conductance clamps the membrane voltage, dissipating concurrent excitatory currents into the extracellular space. Thus, inhibition is defined by functional suppression of spike generation rather than hyperpolarization alone.

Neurocomputational output represents a dynamic equilibrium between these competing depolarizing and hyperpolarizing forces. Individual pyramidal neurons receive thousands of excitatory and inhibitory synaptic contacts across their arborizations. Neural circuits orchestrate these forces to regulate sensory gating, prevent runaway recurrent excitation, synchronize regional oscillations, and filter incoming information. The downstream response of the neuron is never determined by a single input in isolation, but by the ongoing integration of excitatory drives and inhibitory constraints.

1.3 The Axon Hillock as the Critical Arbiter of Synaptic Computation

The axon hillock and its continuous downstream segment, the axon initial segment (AIS), represent the physiological decision-making center of the neuron. While synaptic inputs arrive across distal dendrites, the soma, and proximal dendritic bifurcations, the downstream action potential initiates at the AIS. This region possesses the lowest electrical threshold for excitation in the entire neuron, primarily due to an exceptionally high local density of voltage-gated sodium channels (principally the NaV1.2 and NaV1.6 isoforms), tethered to the actin cytoskeleton by scaffolding proteins such as ankyrin-G and beta-IV spectrin.

Synaptic currents generated at distant dendritic postsynaptic sites cannot trigger local action potentials under typical conditions, as dendrites generally lack the density of low-threshold voltage-gated sodium channels required for regenerative, all-or-none propagation. Instead, these currents must spread passively along the interior of the dendritic cable, through the soma, and ultimately into the axon initial segment. As these currents travel, they experience significant attenuation in both amplitude and temporal sharpness, losing high-frequency components to the capacitance of the lipid bilayer and losing current across leaky resting membrane channels.

Consequently, the axon hillock acts as a spatiotemporal comparator. Excitatory currents arriving from multiple dendrites depolarize the initial segment toward its threshold (often around -55 mV to -45 mV, depending on channel subtype and distribution). Concurrently, inhibitory currents arriving at the soma or proximal axon hyperpolarize the initial segment or shunt incoming excitatory current to the extracellular bath. If the net somatic membrane potential at the axon initial segment reaches threshold, the high concentration of NaV channels activates synchronously, initiating a self-sustaining wave of depolarization down the axonal shaft. If inhibitory inputs prevent the membrane from reaching this threshold, the integrated signal dissipates harmlessly without producing an output spike.

2. Excitatory Postsynaptic Potentials (EPSPs): Mechanisms and Molecular Characteristics

2.1 Neurochemical Mediators and Receptor Subclasses

The neurochemical basis of synaptic excitation in the mammalian nervous system is dominated by the amino acid L-glutamate, which serves as the primary excitatory neurotransmitter throughout the central nervous system. Upon release from presynaptic vesicles, glutamate diffuses into the synaptic cleft, where it interacts with a diverse family of ionotropic and metabotropic postsynaptic receptors. The principal mediators of classical, fast-decaying EPSPs are the ionotropic glutamate receptors, organized into three primary functional families: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, N-methyl-D-aspartate (NMDA) receptors, and kainate receptors. Each subclass exhibits distinct structural properties, opening kinetics, and ion selectivity profiles.

AMPA receptors, composed of heterotetrameric assemblies of GluA1 through GluA4 subunits, drive the initial, rapid phase of the EPSP. These receptors possess a relatively low affinity for glutamate, open within microseconds of ligand binding, and desensitize rapidly. The presence or absence of the edited GluA2 subunit dictates whether the AMPA channel is impermeable or permeable to calcium ions; the vast majority of principal neurons in the mammalian forebrain express RNA-edited GluA2(R) subunits, making their AMPA-mediated currents primarily driven by monovalent sodium and potassium flux. Kainate receptors, formed from GluK1 through GluK5 subunits, play a specialized role, exhibiting slightly slower kinetics and contributing to both postsynaptic depolarization and presynaptic modulation of transmitter release.

NMDA receptors, assembled from GluN1, GluN2 (A through D), and GluN3 subunits, introduce a critical layer of voltage dependency and high divalent cation conductance to the excitatory response. Under resting membrane potentials (-70 mV to -65 mV), extracellular magnesium ions (Mg²⁺) enter the channel pore and become lodged within its selectivity filter, physically obstructing the passage of other ions. When the postsynaptic membrane is sufficiently depolarized by concurrent AMPA receptor activation, the positive charge inside the cell repels the divalent magnesium ion out of the pore, relieving the block. Once open, NMDA receptors permit significant calcium (Ca²⁺) influx alongside sodium and potassium, driving prolonged postsynaptic currents and initiating second-messenger signaling cascades essential for synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Structurally complex postsynaptic densities, heavily populated by scaffolding proteins such as PSD-95 and SAPAP families, tightly tether these glutamate receptors in precise spatial registers opposite presynaptic active zones (Welch et al., 2007).

Outside of central glutamatergic transmission, acetylcholine acts as a vital excitatory transmitter. In the peripheral nervous system, acetylcholine is released at the neuromuscular junction, activating ionotropic nicotinic acetylcholine receptors (nAChRs) on muscle fibers to generate an end-plate potential, an analog of the neuronal EPSP. Within the central nervous system, acetylcholine acts through both ionotropic nicotinic receptors and G-protein coupled muscarinic receptors to drive regional network excitability. Serotonin (5-hydroxytryptamine) also contributes directly to fast excitation via the 5-HT₃ receptor, the sole ionotropic member of the serotonin receptor family, while its metabotropic counterparts modulate ongoing glutamatergic EPSP amplitudes across diverse cortical regions (Nichols, 2016).

2.2 Ionic Permeability, Driving Forces, and Inward Currents

The generation of an EPSP relies on the opening of non-selective cation channels that allow the simultaneous movement of multiple positive ion species across the lipid bilayer. Unlike voltage-gated sodium channels that feature an exceptionally narrow, highly specific selectivity filter, ionotropic AMPA, NMDA, and nicotinic receptors possess a wider, water-filled central pore that allows both sodium (Na⁺) and potassium (K⁺) ions—and in select cases, calcium (Ca²⁺)—to pass along their respective electrochemical gradients.

The magnitude and direction of the net ionic current traversing an open channel are governed by Ohm’s law applied to biological membranes, formulated as:

I_ion = g_ion × (V_m – E_ion)

where I_ion is the ionic current, g_ion is the membrane conductance for that ion, V_m is the instantaneous membrane potential, and E_ion is the equilibrium potential for that ion species calculated via the Nernst equation. For a channel permeable to multiple monovalent cations, the reversal potential (E_rev) represents the membrane potential at which the inward flux of one ion precisely balances the outward flux of the other, resulting in zero net current:

I_net = g_channel × (V_m – E_rev)

For classical ionotropic glutamate receptors such as AMPA receptors, the channel pore exhibits roughly equal permeability to sodium and potassium ions. The equilibrium potential for sodium (E_Na) typically hovers between +55 mV and +65 mV, reflecting the steep concentration gradient driving extracellular sodium into the cell. Conversely, the equilibrium potential for potassium (E_K) is situated between -90 mV and -100 mV, driving intracellular potassium out of the cell down its chemical gradient. When both ions move through the open pore simultaneously, the reversal potential E_rev falls close to 0 mV.

At standard neuronal resting membrane potentials (-70 mV), the driving force acting on sodium ions, calculated as (V_m – E_Na) = (-70 mV – (+60 mV)) = -130 mV, is exceptionally large, driving a substantial inward movement of positive charge. In contrast, the driving force acting on potassium ions, calculated as (V_m – E_K) = (-70 mV – (-90 mV)) = +20 mV, is relatively weak, generating only a modest outward movement of positive charge. Because the inward sodium current vastly exceeds the outward potassium current at resting potentials, the net current through the channel is inward and positive, which accumulates positive charge along the inner leaflet of the lipid bilayer and depolarizes the postsynaptic membrane toward 0 mV.

2.3 Biophysical Dynamics: Kinetics, Amplitude, and Spatial Attenuation

The temporal morphology of a typical EPSP is characterized by asymmetric kinetics: a rapid, near-instantaneous rising phase followed by a protracted, multi-millisecond decay phase. When glutamate binds to AMPA receptors, channel opening occurs within sub-millisecond timescales, driving a sharp inward current that peaks within 0.2 to 1.0 milliseconds. The subsequent decay phase is shaped by multiple factors, including channel deactivation, receptor desensitization, the rate of neurotransmitter clearance from the cleft by astrocytic transporters, and the membrane time constant of the postsynaptic dendrite. In fast-spiking interneurons, AMPA-mediated EPSPs may exhibit decay time constants as brief as 1 to 2 milliseconds, whereas in cortical pyramidal neurons, decay constants often range from 10 to 30 milliseconds.

Individual unitary EPSPs—those generated by the release of neurotransmitter from a single presynaptic vesicle (a quantum) or a single presynaptic axonal varicosity—produce remarkably small somatic voltage changes. At the soma of a mature mammalian pyramidal neuron, a unitary EPSP typically registers an amplitude of only 0.5 to 2.0 millivolts. Because a resting neuron must be depolarized by 15 to 25 millivolts to reach the action potential threshold, a solitary unitary EPSP is rarely sufficient to discharge a post-synaptic spike. Instead, dozens or hundreds of synchronous EPSPs must summate to drive output signaling.

As an EPSP propagates from its origin in a dendritic spine toward the soma, it undergoes passive electrotonic attenuation governed by core cable equations. The passive spread of voltage along a uniform cylindrical process is described by the linear cable equation:

λ² × (∂²V / ∂x²) – τ × (∂V / ∂t) – V = 0

where λ represents the space or length constant, and τ represents the membrane time constant. The space constant, defined as:

λ = √(r_m / r_i)

measures the distance over which a steady-state voltage deflection decays to 1/e (approximately 37%) of its initial value, where r_m is the membrane resistance and r_i is the internal axial resistance per unit length. Because dendrites possess finite membrane resistance and substantial internal resistance, much of the inward excitatory current leaks through resting background channels as it travels longitudinally down the core cytoplasm. Consequently, EPSPs generated in distal apical tufts suffer extensive amplitude attenuation and temporal broadening by the time they arrive at the soma, whereas proximal dendritic inputs reach the axon initial segment with minimal loss of fidelity (DeFelipe, 2011).

Dendritic spines play a critical role in shaping local EPSP biophysics. The thin, constricted spine neck provides a high-resistance barrier (often exceeding 100 to 500 megaohms) that connects the expanded spine head to the parent dendritic shaft. This structural configuration creates an isolated microdomain. When glutamate activates AMPA and NMDA receptors on the spine head, the high neck resistance limits the rapid escape of current, generating a massive local EPSP within the spine head (often 10 to 40 millivolts) that facilitates the relief of the voltage-dependent magnesium block from NMDA receptors. However, as this current crosses the high-resistance spine neck into the low-resistance dendritic trunk, the voltage drops sharply, delivering only a small, attenuated EPSP to the primary dendritic shaft.

3. Inhibitory Postsynaptic Potentials (IPSPs): Mechanisms and Molecular Characteristics

3.1 Neurochemical Mediators and Inhibitory Receptor Diversity

Synaptic inhibition in the mammalian central nervous system is mediated predominantly by two neurotransmitters: gamma-aminobutyric acid (GABA) and glycine. GABA acts as the primary inhibitory neurotransmitter throughout the brain, cerebral cortex, hippocampus, and basal ganglia, while glycine dominates inhibitory transmission in the spinal cord, brainstem, and retina. The coordinated release of these neurotransmitters regulates neural gain, sharpens sensory representations, establishes network oscillatory patterns, and prevents pathological runaway excitation.

GABA exerts its inhibitory actions through two structurally and pharmacologically distinct receptor classes: GABA-A and GABA-B receptors. GABA-A receptors are ionotropic, ligand-gated ion channels belonging to the Cys-loop superfamily of pentameric receptors, which also includes glycine receptors, nicotinic acetylcholine receptors, and 5-HT₃ receptors. Typically assembled from combinations of alpha, beta, and gamma subunits (most commonly a stoichiometry of two alpha-1, two beta-2, and one gamma-2), GABA-A receptors possess an integral central channel pore selectively permeable to chloride anions, and to a lesser extent, bicarbonate anions (Merali et al., 2004). Binding of two GABA molecules induces rapid channel opening, generating fast inhibitory postsynaptic potentials (fast IPSPs) that peak within 1 to 5 milliseconds and decay over 10 to 100 milliseconds.

Glycine receptors share significant architectural homology with GABA-A receptors, assembling as pentamers composed of alpha and beta subunits. They similarly open a chloride-selective pore upon agonist binding, mediating fast inhibitory currents that are blocked by the alkaloid antagonist strychnine. Fast glycinergic IPSPs are vital for coordinating reciprocal inhibition during motor reflex arcs, sensory filtering in the cochlear nucleus, and respiratory pacemaking in the brainstem.

In contrast to the rapid ionotropic actions of GABA-A and glycine receptors, GABA-B receptors mediate slow, prolonged inhibitory responses through metabotropic, G-protein-coupled mechanisms. GABA-B receptors exist as obligate heterodimers consisting of GABA-B1 and GABA-B2 subunits. Upon agonist binding, the associated G-protein complex dissociates into G-alpha and G-beta-gamma subunits. The G-beta-gamma dimer diffuses intracellularly along the inner leaflet of the plasma membrane to directly bind and activate G-protein-coupled inwardly rectifying potassium (GIRK/Kir3) channels. Activation of these channels allows potassium to flow out of the cytoplasm down its chemical gradient. Because of the multi-step biochemical cascades involved, GABA-B-mediated IPSPs exhibit a delayed onset (30 to 50 milliseconds) and an extended duration lasting hundreds of milliseconds to several seconds. Furthermore, presynaptic GABA-B receptors inhibit voltage-gated calcium channels, attenuating transmitter release from excitatory and inhibitory terminals alike.

3.2 Anion Influx, Potassium Efflux, and Hyperpolarizing Dynamics

The polarity and magnitude of an ionotropic IPSP depend on the transmembrane concentration gradient of chloride (Cl^–) and the resulting chloride equilibrium potential (E_Cl), calculated using the Nernst equation:

E_Cl = (R × T / z × F) × ln([Cl^–]_out / [Cl^–]_in)

Because chloride carries a negative valence (z = -1), this formulation simplifies at physiological temperature (37 °C) to:

E_Cl = -61.5 mV × log₁₀([Cl^–]_out / [Cl^–]_in)

In mature central neurons, intracellular chloride concentration is maintained at low levels (typically 4 to 8 mM) relative to the extracellular environment (approximately 110 to 120 mM). This concentration difference is established and sustained by cation-chloride cotransporters embedded within the plasma membrane, most notably the potassium-chloride cotransporter 2 (KCC2). KCC2 uses the favorable outward potassium gradient to extrude chloride ions from the cytoplasm, maintaining E_Cl at hyperpolarized values, generally between -70 mV and -85 mV. When GABA-A or glycine receptors open in a neuron whose resting membrane potential is -65 mV, the membrane potential sits positive relative to E_Cl. Consequently, a strong thermodynamic driving force acts on chloride ions, driving them inward through the open receptor pore. As negatively charged chloride anions enter the cell, the intracellular potential becomes more negative, producing a hyperpolarizing IPSP that pulls the membrane potential toward E_Cl, away from the action potential threshold.

In metabotropic inhibition mediated by GABA-B receptors, the underlying ionic mechanism shifts from anion influx to cation efflux. The opening of GIRK channels increases the membrane conductance to potassium (K⁺). Because the intracellular potassium concentration is exceptionally high (~140 mM) compared to the extracellular fluid (~3 to 5 mM), the equilibrium potential for potassium (E_K) resides deep at negative potentials, around -90 mV to -105 mV. At any physiological membrane potential, the net driving force pushes potassium ions outward. The resulting outward positive current strips positive charge from the inside of the cell, generating a hyperpolarizing waveform that drives the postsynaptic membrane toward -90 mV. This metabotropic hyperpolarization establishes a long-lasting physiological floor that counteracts sustained excitatory volleys.

3.3 Shunting Inhibition and Biophysical Membrane Clamping

While classical inhibition involves overt membrane hyperpolarization, synaptic inhibition can operate effectively without hyperpolarizing the neuron. This occurs through shunting inhibition (also termed silent or divisive inhibition), a biophysical phenomenon governed by localized changes in membrane conductance rather than voltage deflections alone.

Shunting inhibition arises when the chloride equilibrium potential (E_Cl) sits at or near the resting membrane potential (V_rest), a common baseline state in many central neurons where both values hover near -70 mV. When GABA-A receptor channels open in this state, the electrical driving force on chloride is negligible:

(V_m – E_Cl) ≈ 0 mV

Because there is no net driving force, no net ionic current flows across the membrane, and no visible hyperpolarization or depolarization appears on an intracellular voltage trace. The resting membrane potential remains unchanged.

Despite the absence of a voltage deflection, the opening of thousands of GABA-A channels produces a substantial increase in total membrane conductance (g_membrane), which corresponds to a sharp decrease in input resistance (R_in = 1 / g_membrane). According to Ohm’s law (ΔV = I × R_in), the voltage change (ΔV) produced by any subsequent or concurrent excitatory current (I_EPSP) is directly proportional to the membrane input resistance. By opening a low-resistance pathway across the phospholipid bilayer, shunting inhibition causes concurrent depolarizing currents entering via nearby excitatory synapses to leak rapidly out into the extracellular space before they can spread to the soma or axon initial segment.

This localized decrease in input resistance acts as an electrical “short circuit” or shunt. If an excitatory synapse is located on a distal dendrite and an active shunting inhibitory synapse is positioned along the path between that dendrite and the soma, the inhibitory conductances divert the incoming excitatory current. The EPSP is attenuated, preventing the membrane potential at the axon initial segment from reaching threshold. Furthermore, shunting inhibition reduces both the membrane space constant (λ) and time constant (τ), narrowing the spatial range over which excitatory potentials can travel and accelerating their decay. Through this mechanism, shunting inhibition performs divisive mathematical operations on incoming excitatory signals, scaling neural gain without requiring energetic expenditure to recover hyperpolarized ionic deficits.

4. Key Differences: Comparative Analysis of EPSPs and IPSPs

4.1 Polarity Shift and Reversal Potential Relationships

The primary functional difference between an EPSP and an IPSP lies in the relationship between their respective reversal potentials (E_rev) and the action potential threshold (V_th). An electrical event is defined as excitatory if its reversal potential is more positive than the action potential threshold, and inhibitory if its reversal potential is more negative than that threshold. This rule holds true regardless of whether the membrane potential depolarizes or hyperpolarizes during the event.

Because ionotropic glutamate receptors admit both sodium and potassium ions, their reversal potential sits near 0 mV. Because resting membrane potentials range from -75 mV to -60 mV and action potential thresholds sit between -55 mV and -40 mV, an open AMPA or NMDA channel pulls the postsynaptic membrane toward 0 mV. This value is far positive to the action potential threshold, driving the membrane toward spike initiation.

In contrast, the reversal potential for fast ionotropic IPSPs is governed by chloride, which typically sits between -70 mV and -85 mV, while slow metabotropic IPSPs are governed by potassium, which sits near -95 mV. Both reversal potentials lie negative to the action potential firing threshold. Consequently, opening these channels pulls the membrane potential toward values below threshold, or clamps it at a subthreshold potential. While an EPSP consistently shifts the membrane in a depolarizing (positive) direction, an IPSP typically shifts it in a hyperpolarizing (negative) direction or stabilizes it near the resting baseline, preventing other inputs from reaching the firing threshold.

4.2 Selectivity Filters and Ionic Channel Architecture

The distinct reversal potentials of excitatory and inhibitory responses reflect fundamental architectural differences within their channel selectivity filters. Ionotropic glutamate receptors assemble as tetramers (complexes of four subunits), whereas ionotropic GABA and glycine receptors assemble as pentamers (complexes of five subunits). This structural divergence produces pore dimensions and electrostatic environments optimized for fundamentally different classes of charge carriers.

In excitatory AMPA and NMDA channels, the inner pore of the tetramer is lined with negatively charged amino acid residues (such as aspartate and glutamate side chains) that form an electrostatic barrier against anions. These negative charges attract positively charged cations, stripping a portion of their hydration shells as they pass through the channel pore. The diameter of the central cavity is sufficiently wide to permit the passage of hydrated sodium and potassium ions, and in unedited AMPA or wild-type NMDA receptors, hydrated divalent calcium ions as well. Anions such as chloride are repelled by this negative electrostatic ring, preventing negative charge from moving through the excitatory channel.

Conversely, ionotropic GABA-A and glycine receptors feature a pore lined with neutral and positively charged basic amino acid residues, including conserved arginine and lysine motifs. These positively charged residues create a welcoming electrostatic environment for halides, facilitating the passage of chloride ions while repelling cations such as sodium, potassium, and calcium. The diameter of the open GABA-A channel pore measures approximately 0.5 to 0.6 nanometers, matching the hydration radius of chloride. Furthermore, the G-protein coupled cascades triggered by metabotropic GABA-B receptors modulate inwardly rectifying potassium channels that feature an exceptionally conserved amino acid sequence (the TVGYG signature motif) within their selectivity filter. This motif mimics the hydration shell of potassium, allowing dehydration and selective conduction of potassium ions while excluding smaller sodium ions and all anions.

4.3 Subcellular Spatial Distribution on Neuronal Morphology

Neurons display precise subcellular segregation in the spatial distribution of their excitatory and inhibitory synaptic contacts. Rather than being scattered randomly across the plasma membrane, EPSPs and IPSPs are targeted to distinct anatomical microdomains, shaping how these electrical signals interact and integrate across the somatodendritic arbor.

In neocortical and hippocampal pyramidal neurons, excitatory glutamatergic synapses are concentrated almost exclusively on dendritic spines—small, bulbous protrusions extending from apical, oblique, and basal dendrites. A single pyramidal neuron can possess tens of thousands of spines, each serving as an isolated electrical and biochemical reaction chamber. Distal dendritic arborizations collect sensory and contextual signals from wide cortical areas, generating localized EPSPs that undergo spatial summation as they travel toward the primary shaft. This structural distribution prevents excessive metabolic stress at the soma and allows localized dendritic branches to perform non-linear computations.

In contrast, inhibitory GABAergic synapses are concentrated along proximal dendritic shafts, cell bodies (somata), and the axon initial segment. This architecture is established by distinct subtypes of GABAergic interneurons. For example, basket cells project axons that wrap around the somata of pyramidal neurons, forming dense rings of inhibitory synapses. Chandelier cells (axo-axonic cells) target the axon initial segment, forming specialized synaptic rows known as cartridges (DeFelipe, 2011). This proximal placement gives inhibitory inputs substantial control over neuronal output. Because all excitatory currents traveling from distal dendritic spines must traverse the soma to reach the axon initial segment, an IPSP positioned at the soma or proximal shaft acts as a physical gatekeeper. An inhibitory current can intercept and shunt depolarizing currents from thousands of distal excitatory synapses, vetoing action potential generation before the signal reaches the trigger zone.

4.4 Comparison Dimensions and Parameter Summary

The following head-to-head comparison table summarizes the biophysical, molecular, kinetic, and pharmacological properties distinguishing excitatory from inhibitory postsynaptic potentials.

Biophysical Dimension	Excitatory Postsynaptic Potential (EPSP)	Inhibitory Postsynaptic Potential (IPSP)
Primary Neurotransmitters	L-Glutamate (CNS), Acetylcholine (PNS / CNS), Serotonin (5-HT via 5-HT3 receptors).	GABA (brain / forebrain), Glycine (spinal cord / brainstem).
Major Receptor Subtypes	Ionotropic: AMPA, NMDA, Kainate, Nicotinic AChR. Metabotropic: Group I mGluRs.	Ionotropic: GABA-A, Glycine receptors. Metabotropic: GABA-B receptors (linked to GIRK channels).
Permeant Ionic Species	Inward Na+, outward K+, and variable inward Ca2+ (NMDA, GluA2-lacking AMPA).	Inward Cl– (and HCO3–) for ionotropic; outward K+ for metabotropic.
Net Current Direction	Net inward positive current (depolarizing).	Net inward negative current (hyperpolarizing/shunting) or outward positive current.
Reversal Potential (Erev)	Near 0 mV (above action potential threshold).	Near -70 mV to -85 mV (Cl–) or -95 mV (K+) (below action potential threshold).
Membrane Voltage Shift	Depolarization (shifts Vm toward threshold, e.g., from -70 mV to -65 mV).	Hyperpolarization (e.g., from -70 mV to -78 mV) or conductance clamping (shunting).
Rise Time Kinetics	Extremely fast: 0.2 to 2.0 milliseconds (AMPA); intermediate: 5 to 20 ms (NMDA).	Fast: 1 to 5 milliseconds (GABA-A); slow: 30 to 100 milliseconds (GABA-B).
Decay Time Kinetics	Fast: 5 to 30 ms (AMPA); slow: 50 to 500 ms (NMDA).	Intermediate: 10 to 100 ms (GABA-A); protracted: 200 to 2000 ms (GABA-B).
Primary Subcellular Location	Dendritic spine heads, distal dendritic arbors, peripheral motor endplates.	Soma (perisomatic), proximal dendritic shafts, axon initial segment (axo-axonic).
Pharmacological Antagonists	CNQX, NBQX (AMPA/Kainate); APV / D-AP5, Ketamine, MK-801 (NMDA).	Bicuculline, Picrotoxin (GABA-A); Strychnine (GlyR); CGP55845, Saclofen (GABA-B).
Functional Objective	Drive postsynaptic membrane to action potential firing threshold; information relay.	Prevent unwarranted firing, modulate gain, enforce temporal precision, shape rhythms.

5. Similarities and Shared Neurobiological Principles

5.1 Graded Biophysical Nature and Absence of Refractory Periods

Despite their opposing effects on membrane polarity and action potential generation, EPSPs and IPSPs operate under the same fundamental biophysical principles. Both waveforms represent graded potentials, which vary continuously in amplitude based on the intensity of the presynaptic stimulus. Neither event relies on the regenerative, threshold-dependent positive feedback cycles characteristic of voltage-gated sodium channels. Instead, the amplitude of both EPSPs and IPSPs scales with the quantity of neurotransmitter released into the synaptic cleft, the number of postsynaptic receptors bound, and the single-channel conductance of those receptors.

Because neither potential requires the activation of voltage-dependent sodium channels that enter an inactivated, non-conductive state following depolarization, neither EPSPs nor IPSPs exhibit absolute or relative refractory periods. In an axonal membrane, an action potential must subside and the membrane must repolarize before sodium channels reset, imposing a mandatory refractory window during which no subsequent spike can be initiated. In contrast, postsynaptic membranes can receive and process successive inputs instantaneously. If a second presynaptic action potential triggers neurotransmitter release while a postsynaptic potential is still decaying, the channels recruited by the second release event add their current directly to the ongoing voltage change. This process of summation occurs without a delay or mandatory refractory gap.

Furthermore, both potential classes decay passively as they spread electrotonically away from the synaptic junction. Neither potential contains intrinsic biochemical machinery to regenerate its amplitude along passive dendrites. As a result, both EPSPs and IPSPs lose amplitude, broaden in duration, and attenuate according to the passive core cable properties of the dendritic and somatic membrane.

5.2 Dependency on Passive Membrane Properties

Both EPSPs and IPSPs are subject to the same passive cable properties of the host neuron, defined by membrane resistance (R_m), axial intracellular resistance (R_i), and membrane capacitance (C_m). These biophysical parameters govern how any electrical current, whether depolarizing or hyperpolarizing, travels through the cytoplasm and charges the phospholipid bilayer.

The membrane resistance (R_m) represents the opposition offered by the lipid bilayer and open background leak channels to the passage of ionic current. When membrane resistance is high, fewer leak channels are open, keeping current confined within the intracellular core and enabling both EPSPs and IPSPs to travel greater distances with minimal loss of amplitude. The axial resistance (R_i) represents the cytoplasmic resistance to longitudinal current flow down the length of the dendrite. Axial resistance is inversely proportional to the cross-sectional area of the process; thicker dendrites have lower axial resistance, facilitating the longitudinal spread of both excitatory and inhibitory currents.

The membrane capacitance (C_m) measures the ability of the lipid bilayer to store electrical charge. Because the phospholipid bilayer acts as a dielectric separating the conductive intracellular and extracellular salt solutions, any incoming ionic current must first charge or discharge this biological capacitor before producing a shift in transmembrane voltage. This capacitive charging introduces an identical temporal delay for both EPSPs and IPSPs, slowing the rising phase and lengthening the decay tail of the postsynaptic potential. The membrane time constant:

τ = R_m × C_m

dictates the rate of voltage change across time for both potential types, while the space constant:

λ = √(r_m / r_i)

dictates the rate of electrical decay across distance. Because both signal classes share this shared biophysical medium, modifications to background leak conductances alter the propagation and temporal decay of EPSPs and IPSPs in an identical mathematical fashion.

5.3 Regulation by Transporter Clearance and Enzymatic Breakdown

Both excitatory and inhibitory postsynaptic potentials require rapid, coordinated mechanisms to terminate their signaling and clear the synaptic cleft. If neurotransmitters remained within the cleft indefinitely, postsynaptic receptors would enter prolonged desensitized states, causing synaptic transmission to fail. Furthermore, extended receptor activation would prolong postsynaptic potentials, degrading the temporal precision of neural signaling and potentially triggering cellular damage.

Termination of excitatory glutamatergic EPSPs depends on high-affinity sodium-dependent transporters known as excitatory amino acid transporters (EAATs), specifically EAAT1 (GLAST) and EAAT2 (GLT-1), which are densely expressed on neighboring astrocytic processes. These transporters leverage the sodium electrochemical gradient to clear glutamate from the cleft within milliseconds of its release, keeping ambient extracellular concentrations low and preventing spillover to adjacent synapses. Once inside astrocytes, glutamate is converted to glutamine by the enzyme glutamine synthetase, transferred back to the presynaptic terminal, and hydrolyzed back into glutamate for vesicular repackaging. At the neuromuscular junction, acetylcholine clearance relies on enzymatic hydrolysis by acetylcholinesterase, which cleaves the transmitter into acetate and choline within microseconds.

Inhibitory postsynaptic transmission relies on an analogous clearance system. Ionotropic GABA-A and metabotropic GABA-B signaling are terminated by high-affinity GABA transporters (GAT-1, GAT-2, and GAT-3) distributed across presynaptic terminals and surrounding astrocytes. Transporters couple the movement of GABA to the inward cotransport of sodium and chloride, rapidly stripping the neurotransmitter from the synaptic cleft. In astrocytes, GABA is metabolized by GABA transaminase into succinic semialdehyde, entering the tricarboxylic acid cycle. Similarly, glycinergic IPSPs are terminated by specialized glycine transporters (GlyT1 and GlyT2). Pathological disruptions or pharmacological interventions that impair these clearance cascades lead to abnormally prolonged postsynaptic decay tails, impairing temporal summation and driving network dysregulation across both excitatory and inhibitory systems (Atkin et al., 2018).

6. Synaptic Integration: How Neurons Synthesize EPSPs and IPSPs

6.1 Mechanisms of Temporal and Spatial Summation

Synaptic integration is the computational process by which a postsynaptic neuron combines hundreds to thousands of individual EPSPs and IPSPs arriving across its dendritic tree to determine its downstream firing output. Because a unitary EPSP rarely possesses sufficient amplitude to depolarize the axon initial segment to threshold, neurons rely on two primary forms of algebraic summation: temporal summation and spatial summation.

Temporal summation occurs when a single presynaptic bouton fires repeatedly at high frequencies, releasing successive bursts of neurotransmitter before the prior postsynaptic potentials have decayed. If the inter-spike interval is shorter than the postsynaptic membrane time constant (τ), the second EPSP rides atop the decaying tail of the first, producing an additive, staircase-like depolarizing waveform. If this sequence continues, the cumulative depolarization can cross the action potential threshold at the initial segment. Conversely, high-frequency inhibitory inputs can undergo temporal summation, driving a sustained hyperpolarization or long-lasting shunting conductance that suppresses firing for extended periods.

Spatial summation describes the simultaneous integration of postsynaptic potentials generated at distinct, spatially segregated synapses across the dendritic arbor. When multiple excitatory synapses fire synchronously, their individual inward currents converge within the parent dendritic branches and soma. If the spatial distribution of these inputs falls within the length constant (λ) of the dendrite, their currents combine to produce a larger net depolarization at the axon hillock than any single input could generate in isolation.

However, the interaction between concurrent EPSPs and IPSPs rarely follows simple linear arithmetic. When an EPSP and an IPSP are activated within the same dendritic branch, the large conductance increase from the inhibitory channel reduces local input resistance and draws the local membrane potential toward the inhibitory reversal potential. This shunting effect dissipates the excitatory current, resulting in sublinear summation, where the net somatic voltage deflection is smaller than the mathematical sum of the two independent potentials. Spatial placement relative to the soma dictates the final calculation: an IPSP placed proximal to an excitatory input acts as a spatial veto, dissipating the distal EPSP as it travels toward the axon initial segment.

6.2 Excitation-Inhibition (E/I) Balance in Neural Circuits

Cortical and subcortical microcircuits maintain a regulated ratio between excitatory and inhibitory synaptic conductances, a state known as excitation-inhibition (E/I) balance. Rather than operating in isolation, pyramidal projection neurons and local GABAergic interneurons are linked through structured microcircuit motifs that enforce this equilibrium over millisecond timescales.

A primary structural motif is feedforward inhibition. In this arrangement, an upstream excitatory axon projects to both a principal pyramidal neuron and a local inhibitory interneuron. The interneuron rapidly fires, targeting the same pyramidal cell with a brief delay (typically 1 to 2 milliseconds). This configuration establishes a narrow temporal integration window. The pyramidal cell can only integrate incoming EPSPs during the brief interval before the onset of the disynaptic IPSP. Once the inhibitory input arrives, the membrane is clamped or hyperpolarized, preventing further summation. This mechanism enforces temporal precision, ensuring that the target neuron fires only in response to tightly synchronized inputs while filtering asynchronous noise.

A complementary architecture is feedback (recurrent) inhibition. Here, an active pyramidal neuron sends axon collaterals to recruit neighboring inhibitory interneurons, which loop back to synapse onto the original pyramidal cell and its surrounding network. As global network excitation rises, feedback inhibition increases proportionally, preventing runaway recurrent excitation. This reciprocal feedback loop between excitatory and inhibitory populations is essential for generating synchronized network oscillations, including gamma rhythms (30 to 80 Hz). These rhythmic oscillations align action potential timing across distributed neural ensembles, supporting sensory perception, spatial navigation, and cognitive processing (Muthukumaraswamy et al., 2013; Carhart-Harris & Friston, 2010).

6.3 Pathological Consequences of E/I Disequilibrium

Disruptions in the balance between excitatory and inhibitory postsynaptic transmission underlie a wide spectrum of neuropsychiatric and neurological disorders. When the delicate equilibrium between EPSPs and IPSPs is altered, neural circuits lose their functional stability, resulting in cognitive impairment, sensory distortions, or cellular damage.

The most direct manifestation of E/I disequilibrium is epilepsy. When GABAergic inhibition fails—whether due to mutations in GABA-A receptor subunits, loss of parvalbumin-positive interneurons, or impaired chloride extrusion—recurrent glutamatergic EPSPs spread unchecked through cortical networks. This loss of inhibition triggers paroxysmal depolarizing shifts, characterized by massive, synchronous depolarization across entire neuronal populations, producing clinical seizure activity. Conversely, excessive inhibitory tone or impaired NMDA receptor signaling can suppress cortical circuits, contributing to the cognitive deficits, impaired working memory, and negative symptoms observed in schizophrenia (Greven et al., 2019).

Another severe consequence of uncontrolled excitation is glutamate excitotoxicity. During ischemic stroke or traumatic brain injury, energetic collapse prevents astrocytic EAAT transporters from clearing glutamate from the synaptic cleft. The resulting continuous activation of NMDA and calcium-permeable AMPA receptors drives massive calcium influx into postsynaptic dendrites. This intracellular calcium overload activates calpains, calcineurin, and other degradative enzymes, damages mitochondria, generates reactive oxygen species, and initiates apoptotic or necrotic cell death pathways.

Furthermore, alterations in chloride cotransporter expression can invert the polarity of postsynaptic potentials. Following peripheral nerve injury or chronic inflammation, down-regulation of the potassium-chloride cotransporter KCC2 in dorsal horn spinal neurons impairs chloride extrusion, causing intracellular chloride concentrations to rise. When GABA-A or glycine receptors open under these conditions, the chloride reversal potential (E_Cl) shifts to a value more positive than the resting membrane potential. Consequently, GABA and glycine release triggers outward chloride efflux, producing depolarizing and excitatory responses rather than hyperpolarizing IPSPs. This pathological inversion transforms inhibitory circuits into drivers of excitation, contributing to the intractable allodynia and hyperalgesia characteristic of chronic neuropathic pain states (Garakani et al., 2020).

7. Frequently Asked Questions

7.1 Can an EPSP and an IPSP Cancel Each Other Out Completely?

From an algebraic perspective, if an inward depolarizing current from an EPSP matches the magnitude of an outward hyperpolarizing current from an IPSP at the exact same spatial location and time, the two currents cancel out, resulting in zero net change in membrane voltage. In living neurons, however, complete cancellation rarely occurs, because excitatory and inhibitory synapses are typically segregated into distinct anatomical compartments.

Excitatory synapses are primarily located on distal dendritic spines, while inhibitory synapses are concentrated on proximal shafts and the cell soma. Because of this spatial separation, an IPSP positioned at the soma does not cancel the distal EPSP at its origin. Instead, the distal dendrite experiences local depolarization, while the soma experiences a decrease in input resistance (shunting) or hyperpolarization. The somatic IPSP intercepts and dissipates the arriving EPSP current before it can depolarize the axon initial segment. Furthermore, because EPSPs and IPSPs exhibit different rise and decay kinetics, their interaction is dynamic: the rapid peak of an EPSP may propagate past a synapse before the slower peak of an arriving IPSP reaches its maximum, allowing a brief fraction of the excitatory signal to escape.

7.2 Why Can Chloride Influx Cause Inhibition Without Causing Hyperpolarization?

Inhibition is defined functionally by a reduction in the probability of action potential discharge, not by hyperpolarization of the membrane. When the chloride equilibrium potential (E_Cl) is identical to the resting membrane potential (V_rest), opening GABA-A or glycine channels produces zero net driving force (V_m – E_Cl = 0 mV). Consequently, no net current flows across the membrane, and no visible hyperpolarization occurs on a voltage trace.

However, opening these receptor channels introduces thousands of open pores into the phospholipid bilayer, increasing the membrane conductance (g) and reducing the membrane input resistance (R_in = 1 / g). According to Ohm’s law (ΔV = I × R_in), the voltage change produced by any concurrent or subsequent excitatory current is directly proportional to this input resistance. With input resistance reduced, depolarizing currents entering via nearby excitatory synapses leak out across the membrane rather than traveling to the axon initial segment. This shunting inhibition clamps the local membrane potential near E_Cl, suppressing excitation without requiring any downward deflection of the resting potential.

7.3 How Do Neuromodulators Dynamically Alter EPSP and IPSP Amplitudes?

Neuromodulators such as dopamine, serotonin, norepinephrine, and acetylcholine alter the amplitudes, kinetics, and integration of postsynaptic potentials by activating G-protein-coupled receptors (GPCRs). Unlike fast ionotropic receptors that conduct ions directly, GPCRs initiate intracellular enzymatic cascades that alter the phosphorylation states of ion channels, regulate vesicle release, and modify resting membrane properties.

For example, activation of G-alpha-s-coupled receptors stimulates adenylyl cyclase, raising intracellular cyclic AMP (cAMP) and activating protein kinase A (PKA). PKA phosphorylates specific serine residues on the GluA1 subunits of AMPA receptors, increasing their open probability and channel conductance, which amplifies subsequent EPSPs. Conversely, G-alpha-i-coupled receptors inhibit adenylyl cyclase, reducing kinase activity and diminishing postsynaptic potential amplitudes. Neuromodulators also regulate background leak channels (such as KCNQ or TASK potassium channels). Closing these leak channels increases total membrane resistance, which broadens the length and time constants of the dendrite and allows both EPSPs and IPSPs to travel farther with less passive attenuation.

At the presynaptic level, neuromodulators act on receptors localized to axon varicosities to inhibit voltage-gated calcium channels. By reducing calcium influx during the arrival of an action potential, neuromodulators decrease vesicle release probability, reducing the amplitude of downstream EPSPs or IPSPs. Through these coordinated presynaptic and postsynaptic mechanisms, neuromodulatory pathways dynamically alter the gain and balance of synaptic circuits across behavioral states.

7.4 What Distinguishes Graded Postsynaptic Potentials from Action Potentials?

Graded postsynaptic potentials and action potentials represent the two primary electrical signaling modes of the nervous system, differing in their generation mechanisms, propagation characteristics, and computational roles:

• All-or-None vs. Graded Amplitude: Action potentials are all-or-none events. Once the membrane at the axon initial segment reaches threshold, an action potential of fixed amplitude (~100 mV) discharges. Postsynaptic potentials are graded; their amplitudes range continuously from 0.1 mV to tens of millivolts, scaling directly with neurotransmitter quantity and channel availability.
• Propagation Mechanisms: Action potentials propagate actively along axons via the regenerative, sequential opening of voltage-gated sodium and potassium channels, traveling long distances without loss of amplitude. Postsynaptic potentials spread passively (electrotonically) through dendritic cytoplasm, decaying exponentially across distance and time according to cable properties.
• Refractory Periods: Action potentials possess absolute and relative refractory periods caused by the inactivation of voltage-gated sodium channels and prolonged potassium channel activation, preventing overlapping spikes. Postsynaptic potentials lack refractory periods, allowing successive inputs to summate continuously.
• Underlying Channel Types: Action potentials rely on voltage-gated ion channels (NaV, KV) distributed along unmyelinated axons or nodes of Ranvier. Postsynaptic potentials are generated by ligand-gated ion channels (ionotropic receptors) or G-protein coupled cascades concentrated at the postsynaptic density.

8. Conclusion: The Duality of Neural Computation

8.1 Summary of Core Neurocomputational Trade-offs

The nervous system operates through a continuous dialogue between excitation and inhibition. Excitatory postsynaptic potentials provide the driving current for communication, carrying sensory inputs, executing motor commands, and enabling associative plasticity across neural networks. However, unconstrained excitation leads to run-away amplification, cellular metabolic exhaustion, and excitotoxic damage. Inhibitory postsynaptic potentials provide the counterbalancing control required for functional computation, establishing temporal firing windows, sharpening receptive fields, routing signal traffic, and synchronizing distributed ensembles into coherent rhythms.

This biological duality reflects a trade-off between sensitivity and stability. If a circuit is biased too heavily toward excitation, it becomes prone to instability, saturation, and seizure activity. If it is biased too heavily toward inhibition, its responsiveness diminishes, impairing information transfer and network throughput. The nervous system manages this trade-off by placing distinct receptor classes across specific subcellular compartments—housing EPSPs within compartmentalized dendritic spines to perform local analog computations, while stationing IPSPs along proximal shafts and somata to regulate global output. This balance allows complex computations to unfold within the somatodendritic arbor before conversion into all-or-none action potentials at the axon initial segment.

8.2 Emerging Paradigms in Electrophysiological Research

Technological developments continue to refine our understanding of postsynaptic mechanics. Modern methodologies, such as two-photon glutamate uncaging paired with high-resolution patch-clamp electrophysiology, permit the stimulation of individual dendritic spines, revealing the nonlinear biophysical rules that govern local EPSP generation and compartmentalized calcium signaling. Genetically encoded voltage indicators (GEVIs) and advanced optical voltage imaging allow researchers to visualize subthreshold electrical events simultaneously across thousands of individual dendritic spines and interneuron terminals in behaving animals, bypassing the spatial limitations of traditional somatic electrode recordings.

Concurrently, optogenetics and chemogenetics enable cell-type-specific interrogation of distinct interneuron subtypes, isolating the individual contributions of parvalbumin-positive, somatostatin-positive, and VIP-positive GABAergic inputs to circuit computation. Computational neuroscience leverages these biophysical datasets to build detailed compartmental models of whole neurons, illuminating how morphology, spine neck resistance, and channel distributions shape synaptic integration. These insights are advancing clinical pharmacology, guiding the development of subunit-selective GABA-A receptor positive allosteric modulators, AMPA receptor potentiators, and KCC2 gene therapies designed to restore excitation-inhibition equilibrium across conditions including epilepsy, autism spectrum disorders, schizophrenia, and neuropathic pain.

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