Action Potentials and Synaptic Transmission (3A)

SECTION 1

Historical Context & Motivation

The quest to understand how the nervous system communicates information has spanned centuries, bridging anatomy, physics, and biochemistry in ways that continue to shape modern neuroscience and clinical medicine. Early natural philosophers debated whether nerves carried "animal spirits" or some form of fluid, but the true electrical nature of neural signaling did not emerge until careful experimentation revealed that living tissues could generate and propagate electrical impulses. The discovery that action potentials are discrete, all-or-none electrical events — and that communication between neurons depends on chemical synaptic transmission — fundamentally transformed our understanding of how organisms sense, integrate, and respond to environmental stimuli.

1791

Galvani's "Animal Electricity"

Luigi Galvani demonstrated that electrical stimulation could cause frog leg muscles to contract, establishing that biological tissues possess intrinsic electrical properties and launching the field of electrophysiology.

1902

Bernstein's Membrane Hypothesis

Julius Bernstein proposed that nerve cells maintain a resting membrane potential due to selective permeability to potassium ions, and that action potentials result from a transient breakdown of this selective permeability.

1921

Loewi's Vagus Nerve Experiment

Otto Loewi demonstrated chemical neurotransmission by transferring fluid from a stimulated frog heart to a second heart, showing that a chemical substance — later identified as acetylcholine — mediated the signal.

1952

Hodgkin & Huxley Model

Alan Hodgkin and Andrew Huxley used the giant axon of the squid to develop a quantitative model describing how voltage-gated Na⁺ and K⁺ conductances produce the action potential, earning them the 1963 Nobel Prize.

1970s

Molecular Characterization of Ion Channels

Advances in patch-clamp electrophysiology (Neher and Sakmann) and molecular biology enabled the identification and cloning of individual ion channel proteins, connecting biophysics to molecular structure.

These discoveries converged on a central question that remains at the heart of MCAT-level physiology: how do neurons convert graded electrochemical signals into rapidly propagated digital impulses, and how do those impulses translate into precisely regulated chemical messages at the synapse? Answering this question requires an integrated understanding of membrane biophysics, ion channel gating, and the molecular machinery of vesicle release — all of which are high-yield topics for Foundational Concept 3.

SECTION 2

Core Principles & Definitions

Understanding action potentials and synaptic transmission requires a firm grasp of several interrelated biophysical and biochemical principles. At the most fundamental level, neurons are excitable cells whose plasma membranes maintain an unequal distribution of ions — primarily Na⁺, K⁺, Cl⁻, and organic anions — that establishes the resting membrane potential of approximately −70 mV. This potential difference represents stored electrochemical energy that, when released through the coordinated opening and closing of voltage-gated ion channels, generates the action potential.

Electrochemical Gradient

Each ion experiences two forces: a chemical gradient (concentration difference) and an electrical gradient (voltage difference). The net driving force on an ion is determined by the difference between the membrane potential and that ion's equilibrium (Nernst) potential.

Na⁺/K⁺-ATPase

This primary active transporter extrudes 3 Na⁺ for every 2 K⁺ imported per ATP hydrolyzed, maintaining low intracellular [Na⁺] and high intracellular [K⁺]. It is electrogenic, contributing approximately −3 to −5 mV to the resting potential.

Voltage-Gated Ion Channels

These transmembrane proteins undergo conformational changes in response to membrane depolarization. Voltage-gated Na⁺ channels open rapidly (activation), then self-inactivate via a "ball-and-chain" mechanism, while voltage-gated K⁺ channels open more slowly (delayed rectifiers).

All-or-None Principle

Once threshold (approximately −55 mV) is reached, a full action potential fires with a stereotyped amplitude. Stimulus intensity is encoded not by the size of individual action potentials, but by their frequency and the number of neurons recruited.

Synaptic Transmission

At chemical synapses, arrival of an action potential at the presynaptic terminal triggers Ca²⁺ influx through voltage-gated Ca²⁺ channels, prompting SNARE-mediated vesicle fusion and neurotransmitter release into the synaptic cleft.

✦ KEY TAKEAWAY

Think of the neuron as a loaded spring: the Na⁺/K⁺-ATPase winds the spring by maintaining ion gradients (potential energy), voltage-gated channels release it in an explosive, self-propagating cascade (the action potential), and at the synapse the kinetic energy of the electrical signal is converted into a chemical parcel (neurotransmitter) that crosses the gap like a courier delivering a message to the next cell. The entire system is designed for speed, fidelity, and modulability — analogous to how fiber optic converters translate light signals into electrical ones at relay stations in a telecommunications network.

SECTION 3

The Action Potential: A Visual Explanation

The following diagram illustrates the characteristic waveform of a neuronal action potential, plotting membrane potential (mV) against time (ms). Each phase corresponds to specific ion channel events: subthreshold depolarization toward threshold, rapid Na⁺ influx during the rising phase, Na⁺ channel inactivation and delayed K⁺ efflux during repolarization, transient hyperpolarization (undershoot) below resting potential, and final restoration of resting conditions. Understanding this waveform is essential because MCAT questions frequently require you to identify which phase is occurring based on a description of channel states or pharmacological intervention.

The action potential waveform showing five phases: resting state at −70 mV, depolarization to threshold (−55 mV), the rising phase driven by Na⁺ influx to +35 mV, repolarization via K⁺ efflux, and the hyperpolarizing undershoot to approximately −90 mV before return to resting potential.

During the resting phase, leak K⁺ channels dominate membrane permeability, holding V_m near E_K. A stimulus that depolarizes the membrane to threshold activates a critical mass of voltage-gated Na⁺ channels, initiating a positive feedback loop: Na⁺ influx → further depolarization → more Na⁺ channels open. This explosive rising phase terminates when Na⁺ channels enter an inactivated state (distinct from closed) and delayed-rectifier K⁺ channels open, driving repolarization. Because K⁺ channels close slowly, V_m transiently overshoots the resting value, producing the hyperpolarizing undershoot that corresponds to the relative refractory period.

SECTION 4

Mathematical Framework: Nernst & Goldman Equations

The quantitative foundation for understanding membrane potentials rests on two key equations. The Nernst equation calculates the equilibrium potential for a single ion species — the voltage at which the electrical and chemical driving forces on that ion exactly balance. The Goldman-Hodgkin-Katz (GHK) equation extends this to account for the relative permeabilities of multiple ion species simultaneously, yielding the actual resting membrane potential. Mastery of these equations is non-negotiable for MCAT success: you must be able to predict how changes in ion concentration or membrane permeability shift V_m.

NERNST EQUATION

E_ion = (RT / zF) × ln([ion]_outside / [ion]_inside)

At 37°C this simplifies to: E_ion = (61.5 mV / z) × log₁₀([ion]_out / [ion]_in). R = gas constant (8.314 J·mol⁻¹·K⁻¹), T = temperature (K), z = ion valence (charge), F = Faraday constant (96,485 C·mol⁻¹).

GOLDMAN-HODGKIN-KATZ EQUATION

V_m = (RT/F) × ln((P_K[K⁺]_o + P_Na[Na⁺]_o + P_Cl[Cl⁻]_i) / (P_K[K⁺]_i + P_Na[Na⁺]_i + P_Cl[Cl⁻]_o))

P_K, P_Na, P_Cl = relative permeabilities of each ion. Note that Cl⁻ (an anion) has its concentration terms inverted relative to the cations. At rest, P_K ≫ P_Na, so V_m is close to E_K (−90 mV). During the AP peak, P_Na dominates and V_m approaches E_Na (+60 mV).

CONDUCTION VELOCITY

v ∝ √(d / Rₘ × Rₐ)

Conduction velocity (v) increases with axon diameter (d) and membrane resistance (R_m), and decreases with axoplasmic resistance (R_a). Myelination dramatically increases R_m and decreases membrane capacitance, enabling saltatory conduction with velocities up to 120 m/s.

⚡ MCAT High-Yield Tip

The Nernst equation predicts that doubling the extracellular K⁺ concentration will make E_K less negative by approximately 18.5 mV (= 61.5 × log₁₀ 2). Because V_m tracks E_K closely at rest, hyperkalemia depolarizes neurons and cardiomyocytes, potentially causing dangerous arrhythmias — a concept that bridges physiology and clinical medicine.

SECTION 5

Synaptic Transmission: Detailed Breakdown

When an action potential arrives at the presynaptic terminal, it must be converted from an electrical signal to a chemical one — and then back to an electrical signal in the postsynaptic cell. This process, known as chemical synaptic transmission, involves a precisely orchestrated sequence of molecular events: Ca²⁺ influx, vesicle docking and fusion mediated by the SNARE complex (synaptobrevin, syntaxin, SNAP-25), neurotransmitter release into the cleft, binding to postsynaptic receptors, and signal termination through enzymatic degradation, reuptake, or diffusion.

Overview of chemical synaptic transmission. Presynaptic vesicles containing neurotransmitter (yellow circles) fuse with the membrane upon Ca²⁺ influx through voltage-gated channels. Released neurotransmitter crosses the cleft and binds either ionotropic receptors (fast, direct ion flow) or metabotropic receptors (slower, second-messenger cascades). Signal termination occurs via enzymatic degradation, reuptake transporters, or diffusion.

Comparison of ionotropic and metabotropic postsynaptic receptors

Feature	Ionotropic Receptor	Metabotropic Receptor
Structure	Ligand-gated ion channel (receptor IS the channel)	7-transmembrane domain G-protein-coupled receptor (GPCR)
Speed	Fast (milliseconds)	Slow (seconds to minutes)
Mechanism	Direct ion flow through the channel pore	Activates G-protein → second messengers (cAMP, IP₃, DAG)
Duration	Brief: effect ceases when ligand dissociates	Prolonged: amplified and sustained via signaling cascades
Examples	Nicotinic AChR, GABA-A, NMDA/AMPA glutamate receptors	Muscarinic AChR, GABA-B, α/β adrenergic receptors

The postsynaptic response depends on the type of receptor and the ion it conducts. An excitatory postsynaptic potential (EPSP) results from Na⁺ (or mixed cation) influx, depolarizing the postsynaptic membrane and bringing it closer to threshold. An inhibitory postsynaptic potential (IPSP) results from Cl⁻ influx or K⁺ efflux, hyperpolarizing the membrane and reducing the probability of an action potential. These graded potentials undergo temporal summation (repeated inputs from the same synapse over time) and spatial summation (simultaneous inputs from multiple synapses) at the axon hillock, the site of highest voltage-gated Na⁺ channel density and therefore the decision point for action potential initiation.

SECTION 6

Worked Example: Predicting Membrane Potential Changes

Consider a neuron with the following ionic conditions at 37°C: [K⁺]_out = 5 mM, [K⁺]_in = 140 mM, [Na⁺]_out = 145 mM, [Na⁺]_in = 12 mM. At rest, the membrane permeability ratio is P_K : P_Na = 1 : 0.04. Calculate (a) E_K, (b) E_Na, and (c) the approximate resting V_m using a simplified Goldman equation (ignoring Cl⁻).

Calculating Equilibrium and Resting Potentials

Step 1 — Calculate E_K using the Nernst equationE_K = (61.5 mV / z) × log₁₀([K⁺]_out / [K⁺]_in). Since z = +1 for K⁺: E_K = 61.5 × log₁₀(5 / 140) = 61.5 × log₁₀(0.0357) = 61.5 × (−1.447).

E_K ≈ −89 mV

Step 2 — Calculate E_Na using the Nernst equationE_Na = (61.5 / +1) × log₁₀(145 / 12) = 61.5 × log₁₀(12.08) = 61.5 × 1.082.

E_Na ≈ +67 mV

Step 3 — Apply the simplified Goldman equationV_m = 61.5 × log₁₀[(P_K[K⁺]_out + P_Na[Na⁺]_out) / (P_K[K⁺]_in + P_Na[Na⁺]_in)]. Substituting: V_m = 61.5 × log₁₀[(1 × 5 + 0.04 × 145) / (1 × 140 + 0.04 × 12)] = 61.5 × log₁₀[(5 + 5.8) / (140 + 0.48)] = 61.5 × log₁₀(10.8 / 140.48) = 61.5 × log₁₀(0.0769) = 61.5 × (−1.114).

V_m ≈ −68.5 mV — close to the typical resting potential of −70 mV, confirming that V_m lies much closer to E_K than E_Na because P_K ≫ P_Na at rest.

Step 4 — Interpret physiological significanceThe driving force on Na⁺ at rest = V_m − E_Na = −68.5 − (+67) = −135.5 mV (strong inward driving force, but few channels open at rest). The driving force on K⁺ = V_m − E_K = −68.5 − (−89) = +20.5 mV (modest outward driving force, many leak channels open). This explains why even a small increase in Na⁺ permeability can trigger a large depolarization.

The large electrochemical driving force on Na⁺ is the "loaded spring" that powers the explosive rising phase of the action potential.

SECTION 7

Graded Potentials vs. Action Potentials

A common source of MCAT confusion is the distinction between graded potentials (including EPSPs, IPSPs, receptor potentials, and generator potentials) and action potentials. The following comparison highlights the critical differences along every dimension you might be tested on.

Graded potentials versus action potentials — a high-yield MCAT comparison

Property	Graded Potential	Action Potential
Amplitude	Variable — proportional to stimulus strength	Fixed — all-or-none (~100 mV swing)
Propagation	Decremental — decays with distance (passive spread)	Non-decremental — regenerated at each point along the axon
Direction	Bidirectional from point of origin	Unidirectional (refractory period prevents backpropagation)
Summation	Temporal and spatial summation possible	No summation — cannot be added together
Channels involved	Ligand-gated, mechanically gated, or leak channels	Voltage-gated Na⁺ and K⁺ channels
Refractory period	None	Absolute (Na⁺ inactivation) and relative (hyperpolarized)
Location	Dendrites, cell body, sensory receptors	Axon hillock → along the axon

✦ KEY TAKEAWAY

Graded potentials are the analog input signals — like the volume knob on a mixing console, they vary continuously in magnitude. Action potentials are the digital output — like a binary switch that is either fully on or fully off. The axon hillock functions as the analog-to-digital converter: it integrates graded inputs (EPSPs and IPSPs) and, if the net signal exceeds threshold, fires a standardized all-or-none pulse. This dual analog-digital architecture allows neurons to perform sophisticated computation at dendrites while ensuring high-fidelity, long-distance signal propagation along the axon.

SECTION 8

Clinical & Advanced Connections

The basic principles of action potentials and synaptic transmission have profound implications for understanding pathology and pharmacology — topics that increasingly appear in MCAT passages. Diseases and drugs that target ion channels, neurotransmitter synthesis, vesicle release, receptor binding, or signal termination represent some of the most therapeutically important classes in medicine. The table below connects the molecular mechanisms discussed in this lesson to their clinical counterparts.

Clinically relevant disruptions of neural signaling

Target / Mechanism	Clinical Example	Consequence / Mechanism of Action
Na⁺ channel blockade	Local anesthetics (lidocaine), anti-epileptics (carbamazepine)	Block voltage-gated Na⁺ channels → prevent AP generation → nerve block or reduced seizure activity
K⁺ homeostasis disruption	Hyperkalemia (renal failure)	↑ [K⁺]_out → depolarized resting V_m → inactivation of Na⁺ channels → cardiac arrhythmia
Demyelination	Multiple sclerosis, Guillain-Barré syndrome	Loss of myelin → ↓ R_m, ↑ capacitance → slowed/failed saltatory conduction
Neurotransmitter reuptake inhibition	SSRIs (fluoxetine) for depression	Block SERT → ↑ serotonin in synaptic cleft → enhanced serotonergic transmission
Vesicle release inhibition	Botulinum toxin (Botox), tetanus toxin	Cleave SNARE proteins → block ACh release (botulinum = flaccid paralysis; tetanus = spastic paralysis via inhibitory interneuron blockade)
AChE inhibition	Myasthenia gravis treatment (pyridostigmine), nerve agents (sarin)	Block acetylcholinesterase → ↑ ACh at NMJ → improved (or excessive) neuromuscular transmission

Looking forward, the principles covered here extend naturally into topics such as synaptic plasticity (long-term potentiation and depression, which underlie learning and memory), neural circuits (reflex arcs, central pattern generators), and neuropharmacology (agonists, antagonists, allosteric modulators). For the MCAT, the most important next step is understanding how these principles apply to specific organ systems — the autonomic nervous system, the neuromuscular junction, and sensory transduction — all of which are covered under Foundational Concept 3.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

A researcher applies tetrodotoxin (TTX), which selectively blocks voltage-gated Na⁺ channels, to a neuron. The neuron can still be depolarized to −55 mV by current injection, but no action potential is generated. Explain why TTX prevents action potential firing even though the membrane reaches threshold, and describe what would happen to the resting membrane potential.

PROBLEM 2 — BASIC CALCULATION

Using the Nernst equation at 37°C (61.5 mV / z), calculate the equilibrium potential for Ca²⁺ given [Ca²⁺]_out = 2.0 mM and [Ca²⁺]_in = 0.0001 mM (100 nM). Note that z = +2 for Ca²⁺.

PROBLEM 3 — INTERMEDIATE

A neuron receives three simultaneous EPSPs of +8 mV, +6 mV, and +5 mV at different dendritic locations, along with one IPSP of −10 mV. If the resting potential is −70 mV and threshold is −55 mV, will this neuron fire an action potential? What type of summation is occurring? Assume negligible decay of the graded potentials before reaching the axon hillock.

PROBLEM 4 — APPLIED

A patient with myasthenia gravis has autoantibodies against nicotinic acetylcholine receptors at the neuromuscular junction (NMJ). The treating physician prescribes pyridostigmine, an acetylcholinesterase inhibitor. Explain the pathophysiology of the weakness in myasthenia gravis and the mechanism by which pyridostigmine improves muscle strength. Why might overly aggressive dosing cause paradoxical weakness (cholinergic crisis)?

PROBLEM 5 — CRITICAL THINKING

Consider an experimental manipulation in which the extracellular K⁺ concentration is gradually increased from 5 mM to 40 mM. Predict, using the Nernst and Goldman equations, what happens to (a) E_K, (b) resting V_m, (c) action potential amplitude, and (d) the neuron's ability to fire repetitive action potentials. Integrate your knowledge of Na⁺ channel gating states (closed, open, inactivated) in your analysis.

SUMMARY

Lesson Summary

Neurons maintain a resting membrane potential of approximately −70 mV, established by the Na⁺/K⁺-ATPase and dominant K⁺ leak channel conductance. The Nernst equation predicts the equilibrium potential for individual ions, while the Goldman-Hodgkin-Katz equation integrates multiple ion permeabilities to yield V_m. When graded depolarization reaches threshold (~−55 mV), voltage-gated Na⁺ channels open in a positive feedback loop, producing the rapid rising phase of the action potential. Na⁺ channel inactivation and delayed K⁺ channel opening drive repolarization and a transient hyperpolarizing undershoot, creating absolute and relative refractory periods that enforce unidirectional propagation and limit firing frequency.

At the chemical synapse, depolarization of the presynaptic terminal opens voltage-gated Ca²⁺ channels, triggering SNARE-mediated vesicle fusion and neurotransmitter release. Postsynaptic responses depend on receptor type: ionotropic receptors produce fast EPSPs or IPSPs via direct ion flow, while metabotropic receptors activate slower G-protein cascades. These graded postsynaptic potentials undergo spatial and temporal summation at the axon hillock, where the decision to fire the next action potential is made. Signal termination occurs via enzymatic degradation, reuptake, or diffusion — each mechanism a target for pharmacological intervention in clinical neuroscience.