Chapter 4: The Action Potential — Comprehensive Study Notes (Bullet-Points)

CHAPTER 4: THE ACTION POTENTIAL — COMPREHENSIVE STUDY NOTES

• Overview and big picture

  • The action potential (AP) is the signal that conveys information over distances in the nervous system.
  • Resting membrane potential: inside of the membrane is negatively charged relative to outside (Vm ≈ -65 mV).
  • An AP is a rapid reversal of this polarity: the inside becomes briefly positive relative to the outside.
  • APs generated by a patch of membrane are uniform in size/duration and do not diminish as they propagate down the axon.
  • The code for information transfer is the frequency and pattern of APs (temporal pattern encodes information).
  • Subtopics include: mechanisms generating APs, their propagation, and how experimental tools reveal underlying ion-channel function.

• Box 4.1: Methods of Recording Action Potentials

  • Intracellular recording vs extracellular recording
  • Intracellular: impales neuron with microelectrode; measures potential difference between intracellular electrode and ground; uses an amplifier and oscilloscope; APs are measured as membrane potential changes
  • Extracellular: records from near membrane with fine-capillary electrode or thin insulated wire; detects brief voltage differences as positive charges move across membrane; typically smaller signals than intracellular recordings
  • Oscilloscope (or modern digital recorders) capture voltage versus time; APs can be heard as a characteristic sound when connected to a speaker
  • For teaching: APs are a sequence of ionic movements across the membrane; electrical currents can be interpreted as ion fluxes through channels
  • Concepts introduced: ground reference, electrode impedance, and the idea that APs are driven by ion channel dynamics

• The Ups and Downs of an Action Potential

  • Typical AP waveform on an oscilloscope: rising phase (rapid depolarization) → overshoot (inside becomes positive) → falling phase (rapid repolarization) → undershoot or after-hyperpolarization → gradual return to resting Vm
  • Peak Vm during overshoot ≈ +40 mV; duration ≈ 2 ms
  • Resting Vm ≈ -65 mV
  • APs are brief and stereotyped in amplitude/duration across neurons

• The Generation of an Action Potential

  • Trigger mechanism: depolarization must reach threshold; threshold is a critical depolarization level that opens enough Na+ channels to favor inward Na+ current.
  • Real-life trigger: stretch-activated Na+ channels at nerve endings can be opened by mechanical stimulation (e.g., a thumbtack piercing the skin) leading to Na+ influx and depolarization above threshold
  • Two major routes to depolarization to threshold:
  • Mechanical (stretch-sensitive Na+ channels) at sensory endings
  • Neurotransmitter-gated Na+ channels at interneurons (postsynaptic depolarization)
  • Alternative method used in experiments: injecting current via a microelectrode to depolarize membrane to threshold
  • Concept: APs are an all-or-none event—once threshold is crossed, an AP is generated; increasing depolarization beyond threshold does not change the amplitude of the AP

• The Generation of Multiple Action Potentials

  • If continuous depolarizing current is applied, a neuron fires multiple APs in succession (repetitive firing)
  • Firing rate depends on the magnitude of the depolarizing current:
  • Near threshold: ~1 Hz
  • Higher current: higher firing rate (e.g., 50 Hz or more)
  • The maximum firing rate is limited by the absolute refractory period (~1 ms): after an AP, Na+ channels are inactivated and cannot fire another AP immediately
  • Absolute refractory period limits the fastest possible APs; relative refractory period follows during which a larger depolarization is needed to reach threshold
  • Relationship to encoding: firing frequency encodes the strength of depolarizing input
  • Figures (referenced): AP waveform (Fig. 4.1) and firing-rate relationships (Fig. 4.3) illustrate these concepts

• The Action Potential, in Theory

  • Membrane currents and conductances: the AP arises from changes in membrane permeability to key ions (Na+, K+) and their gradients maintained by pumps
  • Idealized neuron model (Fig. 4.5): three key membrane components are Na+ channels, K+ channels, and Na+/K+ pumps maintaining concentration gradients
  • Equilibrium potentials (Nernst): at 37°C, EK ≈ -80 mV and ENa ≈ +62 mV; driving forces are (Vm - EK) and (Vm - ENa)
  • Driving force concept: ionic current through a channel is Iion = gion (Vm − Eion)
  • Key relationships (Ohm’s law style): IK = gK (Vm − EK) and, more generally, Iion = gion (Vm − Eion)
  • When only K+ channels are open (Vm initially 0 mV, EK ≈ -80 mV): K+ leaves the cell until Vm ≈ E_K; this is the basis of the resting negative potential and the hyperpolarization mechanism
  • Conceptual flipping: if Na+ channels open (g_Na increases) with ENa ≈ +62 mV, Na+ influx depolarizes Vm toward ENa; if K+ channels dominate again, Vm returns toward EK
  • The rising phase can be explained by transient increase in g_Na and Na+ influx; falling phase by inactivation of Na+ channels and the delayed opening of K+ channels
  • The role of gating and voltage dependence is central to the AP waveform

• The Action Potential, in Reality

  • Two-gate concept (Hodgkin–Huxley): Na+ channels open briefly then inactivate; this brief opening is necessary to produce the short AP duration
  • Sodium channels exhibit fast activation (opening within microseconds to milliseconds), brief open time (~1 ms), and inactivation that prevents re-opening until the membrane returns to near threshold (deinactivation)
  • Patch-clamp revolution: enabled direct measurement of ionic currents through individual channels (see Box 4.3)
  • Summary: Na+ influx drives the rising phase; K+ efflux drives the falling phase; Na+ channel inactivation and delayed K+ channel activation shape the brief AP

• The Voltage-Gated Sodium Channel

  • Structure: four homologous domains I–IV; each domain contains six transmembrane segments S1–S6; the four domains form the pore
  • Voltage sensor: S4 contains regularly spaced positively charged residues; depolarization moves S4, opening the activation gate
  • Pore selectivity: selectivity filter makes Na+ permeable about 12 times more than K+; ions pass with partial dehydration and interaction with the filter and a hydration shell that acts as a molecular chaperone
  • Inactive vs resting states: Na+ channels open on depolarization and inactivate rapidly; deinactivate when Vm returns to negative values
  • Figures: Structural diagrams (Fig. 4.7) and models of Na+ channel gating (Fig. 4.8, 4.9) illustrate gating and ion selectivity

• Functional Properties of the Sodium Channel

  • Patch-clamp findings (Box 4.3): Na+ channels open with little delay when Vm depolarizes from around −65 to −40 mV; stay open ~1 ms and inactivate; cannot be opened again until deinactivated after repolarization
  • Channel density: axonal membranes contain thousands of Na+ channels per μm^2; the concerted opening of many channels generates the whole-axon AP
  • Consequences for AP properties:
  • Threshold arises from the requirement to surpass the point where sufficient Na+ conductance exceeds K+ conductance
  • Rapid opening explains the fast rising phase
  • Brief open time explains the brief duration of the AP
  • Inactivation explains the refractory periods
  • Genetic variation: multiple Na+ channel genes exist; differences in expression contribute to subtle AP property variations across neurons

• Box 4.3: The Patch-Clamp Method

  • Patch-clamp allows recording currents through single channels
  • Procedure: bring a glass recording pipette to a small patch of membrane, form a gigaohm seal (~10^9 Ω), then rupture the patch to measure currents across the patch
  • Applications: observe Na+ channel opening with depolarization; single-channel conductance and open duration
  • Key finding: most channels oscillate between open/closed states with a fixed unitary conductance; ions can pass through single channels at very high rates

• The Effects of Toxins on the Sodium Channel

  • Tetrodotoxin (TTX) from puffer fish blocks the Na+ pore by binding externally, preventing Na+ currents and APs; often fatal if ingested
  • Saxitoxin (dinoflagellates) similarly blocks Na+ channels and can accumulate in shellfish during red tides
  • Other toxins: batrachotoxin (frogs) forces channels to stay open longer and at more negative potentials; veratridine and aconitine have similar effects
  • Toxin insights: binding sites help infer channel structure; toxins are useful experimental tools to block or alter APs; some toxins have clinical or ecological relevance

• Voltage-Gated Potassium Channels

  • Hodgkin–Huxley observations showed a transiently increased K+ conductance during the AP rise, contributing to repolarization
  • Delayed rectifier concept: potassium gates open about 1 ms after depolarization, providing outward current to reset Vm toward EK
  • Diversity: many different voltage-gated K+ channels exist; common architecture includes four subunits forming a pore; gating modulated by membrane voltage
  • General role: K+ channels provide a path for K+ to leave the cell during depolarization, helping to terminate the AP and hyperpolarize (contribute to the refractory periods)

• Putting the Pieces Together: Key Properties of the Action Potential

  • Threshold: Vm at which enough voltage-gated Na+ channels open so Na+ conductance exceeds K+ conductance
  • Rising phase: large Na+ inward current depolarizes Vm rapidly
  • Overshoot: Vm approaches ENa, becoming positive
  • Falling phase: Na+ channel inactivation + delayed K+ channel activation cause K+ efflux; strong driving force on K+ during peak depolarization
  • Undershoot: continued K+ permeability drives Vm toward EK, hyperpolarizing relative to rest
  • Absolute refractory period: Na+ channels inactivate and cannot be reactivated until deinactivation occurs at negative Vm
  • Relative refractory period: most K+ channels close; more depolarizing current needed to reach threshold
  • The Na+/K+ pump contributes in the background to restore gradients after APs, maintaining long-term ionic gradients
  • Figure reference: cumulative waveform and currents illustrate the combined Na+ inward and K+ outward currents (Fig. 4.12)

• Action Potential Conduction

  • Propagation concept: APs must be conducted down the axon to transmit information to synapses
  • Analogy: a fuse burning down a line; once the AP is generated at a patch, the local depolarization spreads to adjacent patches, triggering subsequent APs
  • Orthodromic conduction: APs typically travel from soma toward the axon terminal; antidromic conduction can be elicited experimentally (AP travels in the reverse direction)
  • Directionality arises because the membrane behind the AP is refractory due to Na+ channel inactivation
  • Conduction velocity: typically around 10 m/s, though it varies with axon properties
  • Length of membrane involved during an AP: v × t; for v ≈ 10 m/s and t ≈ 2 ms, distance ≈ 0.02 m = 2 cm
  • Factors influencing conduction velocity
  • Axonal diameter: larger diameter reduces internal resistance and increases conduction speed; velocity roughly increases with diameter
  • Leakage: current loss across membrane reduces propagation efficiency; fewer open membrane pores improves speed
  • Myelination (see Box 4.5): insulation speeds up conduction by reducing membrane leaks and enabling saltatory conduction
  • Example: squid giant axon (1 mm diameter) used historically to study AP biophysics; demonstrates scaling of conductance properties
  • Important clinical note: smaller axons have higher sensitivity to local anesthetics (see Box 4.4)

• Box 4.4: Local Anesthesia

  • Local anesthetics temporarily block APs in axons; common examples include lidocaine
  • Smaller axons (often these encode pain signals) are more susceptible to conduction block by local anesthetics due to safety margins in AP generation
  • Lidocaine mechanisms:
  • Binds to voltage-gated Na+ channels, specifically the S6 segment of domain IV, inside the pore after crossing the lipid membrane and entering via the open channel
  • Blocking Na+ flow prevents depolarization; thus, APs cannot propagate
  • Clinical implications: topical, infiltration, nerve block, and spinal anesthesia strategies

• Box 4.5: Myelin and Saltatory Conduction

  • Myelin insulation increases conduction velocity by reducing current leakage and forcing current to travel inside the axon
  • Myelin is produced by glial cells: Schwann cells (PNS) and oligodendroglia (CNS)
  • Nodes of Ranvier are gaps in the myelin where voltage-gated Na+ channels cluster; inter-nodal distance depends on axon size (roughly 0.2–2.0 mm)
  • Saltatory conduction: APs “jump” from node to node, dramatically increasing conduction velocity compared to unmyelinated fibers
  • Visual aid: saltatory conduction diagram (Fig. 4.15) and the myelin sheath/node arrangement (Fig. 4.14)

• Box 4.6: The Eclectic Electric Behavior of Neurons

  • Neurons vary in electrical behavior, not just morphology
  • Cortical neuron types: aspinous stellate cells vs. spiny pyramidal cells exhibit different firing patterns
  • Stellate cells: relatively steady firing during sustained depolarization
  • Pyramidal cells: rapid burst at onset, then adaptation (slowing of firing rate over time) or rhythmic bursts in some subtypes
  • Reasons for diversity in firing patterns:
  • Different ion channel types and densities; many ion channel types (>12) contribute to the overall pattern
  • Interactions among multiple channel types yield the unique “electrical signature” of a neuron class
  • Implication: neuronal computation arises from the complex interplay of multiple channels across membranes

• Box 4.2: Path of Discovery — The Discovery of the Channelrhodopsins (G. Nagel et al.)

  • Channelrhodopsin-1 (ChR1): identified as a light-gated cation channel; initial discoveries showed small photocurrents in oocytes
  • Channelrhodopsin-2 (ChR2): later discovered to produce large photocurrents; enables robust blue-light–induced depolarization
  • Development path:
  • Expression in frog oocytes and measurement of light-activated currents
  • Realization that ChR2 is a light-activated cation channel permeable to Na+ and Ca2+ with rapid opening in response to blue light
  • Engineering to shorten the channel and improve expression and functionality; YFP tagging for visualization
  • Cross-species collaborations (e.g., with C. elegans muscle cells) to demonstrate functional consequences of light activation
  • Optogenetics: the fusion of genetics and optics to control neural activity with light, enabling precise temporal control of neuronal firing
  • Impact: catalyzed a new field for noninvasive neural control and opened broad biomedical research opportunities (Fig. 4.4 illustrates ChR2-based control)

• Box 4.1 (expanded): Recording and Interpretation Details

  • Intracellular recording specifics:
  • Microelectrode filled with KCl; high conductivity; measurement of intracellular Vm relative to ground
  • Extracellular recording specifics:
  • Recording near the membrane; signals are smaller but less invasive; can still reveal action-potential timing and amplitude relationships
  • Practical note: APs appear very differently on intracellular vs extracellular recording traces due to the nature of the measurement reference and the field/current distribution

• Box 4.3 (Patch-Clamp) — The Technique in Depth

  • Patch-clamp method enables recording from single ion channels or small patches of membrane
  • Procedure steps:
  • Bring a glass pipette tip to the membrane; apply suction to form a tight seal (gigaohm seal ~10^9 Ω)
  • Optionally rupture the patch to access the interior (whole-cell configuration) or study single-channel currents in the patch
  • Outcomes:
  • Observe channel opening and closing (unitary conductance)
  • Examine how changes in membrane potential affect single-channel currents
  • Significance: established the quantitative link between single-channel properties and macroscopic AP features

• Box 4.2 (Channelrhodopsins) — Practical and Conceptual Takeaways

  • Channelrhodopsins provide precise, fast, and reversible control of neuronal depolarization with light
  • Halorhodopsin (NpHR): light-activated chloride pump that hyperpolarizes neurons with yellow light, inhibiting AP firing
  • The optogenetic toolkit expands experimental capabilities for circuit-level manipulations

• Box 4.4: Local Anesthesia (Special Interest)

  • Mechanism: reversible blockade of APs by local anesthetics (e.g., lidocaine) via binding to voltage-gated Na+ channels
  • Binding site: S6 alpha helix of domain IV; access requires the channel to be in the open state
  • Clinical nuance: smaller-diameter axons (often pain fibers) are more susceptible, which is advantageous for pain relief strategies
  • Practical notes: various delivery methods (topical, infiltration, nerve block, spinal) depend on clinical needs
  • Chemical structure note: lidocaine binds within the pore and prevents Na+ flow during depolarization

• Box 4.5: Myelin and Saltatory Conduction (Special Interest)

  • Myelin increases conduction velocity by insulating the axon, reducing membrane capacitance, and minimizing current leakage
  • Nodes of Ranvier host dense Na+ channel clusters; saltatory conduction skips gaps, leaping from node to node
  • Typical inter-nodal distance: about 0.2–2.0 mm depending on axon size
  • Functional implication: myelination is a critical adaptation that allows rapid signaling without excessive axon diameter
  • Clinical relevance: demyelinating diseases (e.g., Multiple Sclerosis) slow conduction velocity and disrupt timing of neural signaling

• Box 4.6: The Eclectic Electric Behavior of Neurons (Special Interest)

  • Neurons display diverse firing patterns, including steady firing, adaptation, bursts, and rhythmic bursts
  • The diversity arises from a broad repertoire of ion channels and their dynamic interactions
  • Understanding neuronal behavior requires considering the full complement of ion channels and their regulatory networks

• Box 4.5 (MS context) — Additional clinical insights

  • Myelin loss slows conduction leading to impaired sensation and motor function
  • Guillain–Barré syndrome is a demyelinating disease affecting peripheral nerves; conduction deficits are diagnostic

• Spike-initiation zone and specialization of axonal compartments

  • Spike-initiation zone: high density of voltage-gated Na+ channels, typically at the axon hillock or sensory nerve endings
  • Dendrites and soma generally have fewer voltage-gated Na+ channels and do not typically generate regenerative Na+-dependent APs
  • The axonal membrane is molecularly specialized for excitability due to channel density differences
  • Figure concepts: spike initiation at axon hillock; spike-initiation zones in sensory neurons vs cortical pyramidal neurons

• Concluding synthesis: wiring the brain as a membrane-based signaling network

  • Neurons possess enormous membrane surface area (~250,000 μm^2 per neuron); human brain contains ~85 billion neurons with an aggregate surface area ~21,250 m^2, roughly the size of three soccer fields
  • APs and synaptic transmission together create the neural code that underpins cognition and behavior

• Key terms (summary capture from the chapter end)

  • rising phase, overshoot, falling phase, undershoot, after-hyperpolarization
  • threshold, absolute refractory period, relative refractory period
  • optogenetics, channelrhodopsin-2 (ChR2)
  • voltage clamp, voltage-gated sodium channel, patch clamp, channelopathy
  • tetrodotoxin (TTX), voltage-gated potassium channel
  • saltatory conduction, degenerate conduction, spike-initiation zone, axon hillock

• Review questions (to test understanding)

  • Define membrane potential (Vm) and sodium equilibrium potential (E_Na). Which changes during an AP?
  • Which ions carry the early inward and late outward currents during the AP?
  • Why is the AP described as all-or-none?
  • What would happen if delayed rectifier K+ channels opened much later than normal?
  • If tetrodotoxin (TTX) labels a neuron, which parts would be labeled and what would be the consequence of applying TTX?
  • How does AP conduction velocity vary with axonal diameter? Why?

• References and further reading (selected portions mentioned in the text)

  • Nagel et al. (Channelrhodopsin-2, 2003) and related optogenetics literature
  • Hille (Ionic Channels of Excitable Membranes, 1992) for channel structure and function
  • Neher, Sakmann, and the patch-clamp technique (1992; Nobel Prize recognition)
  • General neurophysiology texts for voltage-clamp and Hodgkin–Huxley framework

• Equations to memorize and use

  • General ionic current: I{ ext{ion}} = g{ ext{ion}} (Vm - E{ ext{ion}})
  • Specific currents during AP: IK = gK (Vm - EK), I{ ext{Na}} = g{ ext{Na}} (Vm - E{ ext{Na}})
  • Driving force for a given ion: Vm - E{ ext{ion}}
  • Distance covered by an AP traveling at velocity v for duration t: distance = v imes t; e.g., for v = 10 \, ext{m/s} and t = 2 \, ext{ms}, distance = 2 \, ext{cm}
  • Equilibrium potentials (typical): EK \,\approx -80 \, \text{mV},\quad E{Na} \,\approx +62 \, \text{mV}

• Quick mapping of concepts to figures (to aid study)

  • Fig. 4.1: AP waveform and its components (rising phase, overshoot, falling phase, undershoot)
  • Fig. 4.2 & 4.3: relationship between injected current, threshold, and firing rate
  • Fig. 4.5: membrane currents and conductances; driving forces; basic Ohm’s law relation
  • Fig. 4.6: dynamic gating model of Na+ and K+ channels during the AP
  • Fig. 4.7–4.9: sodium channel structure, voltage sensing (S4) and selectivity filter
  • Fig. 4.10: sodium channel opening/inactivation dynamics
  • Fig. 4.12: molecular basis of AP with separate ion currents and summed currents
  • Fig. 4.13: conduction of AP down the axon
  • Fig. 4.14–4.15: myelin, nodes of Ranvier, and saltatory conduction

• How to study effectively from these notes

  • Map each AP phase to the underlying ionic currents and channel states
  • Tie waveform features to gating kinetics (Na+ inactivation, K+ delayed rectifier)
  • Use Box sections to recall experimental approaches (intracellular/extracellular recording, patch clamp)
  • Link clinical relevance (local anesthetics, demyelinating diseases) to underlying membrane physiology
  • Practice with the equations above to solidify how ionic currents generate observed Vm changes