Resting Potential, Action Potential Mechanisms, Synaptic Transmission, and Lab Review

Resting Membrane Potential and Ions

• Electrical Properties and Current-Voltage Relationship:

  • Electrical properties of cells are dictated by the fundamental relationship between voltage ($V$) and current ($I$) expressed in Ohm's Law.
  • A change in voltage necessitates a corresponding change in current, and vice versa.

• Resting Potential Characteristics:

  • The resting membrane potential of a cell is not zero; it maintains a stable negative value during resting conditions.
  • Historical Investigation (Hodgkin's cats in 1984 and '9, recognized with Nobel Prize honors for separate contributions):
  • Equilibrium potentials ($E_{ion}$) were calculated for potassium ($K^+$) and sodium ($Na^+$) ions and compared against the measured cellular resting membrane potential.
  • The calculated equilibrium potential for potassium ($E_K$) closely matched the measured resting membrane potential.
  • Extracellular Potassium Concentration Experiments:
  • Systematically varying extracellular potassium concentration ($[K^+]_{out}$) alters the electrochemical gradient across the membrane.
  • Increasing extracellular potassium concentration causes the resting potential to shift positively (depolarize) in direct proportion to the concentration change.
  • Conclusion: The resting potential is primarily generated by potassium ($K^+$) leaking out of the cell at rest down its concentration gradient through open leak channels.
  • Efflux of positively charged potassium ions ($K^+$) leaves behind uncompensated negative charges on the inner leaflet of the plasma membrane, generating intracellular negativity.

• Contributors to Resting Negativity:

  • Potassium Leak Channels: Act as the primary contributor, accounting for approximately $90\text{--}95\text{\%}$ of resting membrane conductance.
  • Sodium-Potassium Pump ($Na^+/K^+$ ATPase):
  • Electrogenic active transporter pumping unequal amounts of ions across the cell membrane.
  • Moves $3\text{ positive charges}$ ($Na^+$) out of the cell for every $2\text{ positive charges}$ ($K^+$) brought in per ATP cycle.
  • Generates a net loss of $1\text{ positive charge}$ per cycle, directly contributing to internal negativity.
  • Cytoplasmic Anions: Impermeant negatively charged proteins trapped within the cytoplasm directly maintain intracellular negativity.

Properties and Waveform of the Action Potential

• Fundamental Properties of Action Potentials:

  • All-or-None Phenomenon: Once triggered, an action potential fires completely at $100\%$ optimal amplitude. Partial action potentials (e.g., $25\%$ or $50\%$ amplitude) do not occur.
  • Threshold Voltage: An action potential is triggered only if the membrane potential reaches a critical threshold voltage, typically ranging between $-30\,\text{mV}$ and $-50\,\text{mV}$.
  • Propagation Without Decrement: The waveform propagates down the axon without loss of amplitude; the initial action potential appears identical to the final action potential.
  • Regenerative Nature: A single initial stimulus can generate a self-propagating series of action potentials along the entire length of the axon.
  • Refractory Period: Always followed by a mandatory refractory period that limits firing frequency and directionality.
  • Long-Distance Signaling: Serves as the principal mechanism for high-fidelity, long-distance communication in the nervous system.

• Action Potential Waveform Phases (Voltage vs. Time):

  • Resting Potential: Baseline negative potential prior to stimulation.
  • Depolarization to Threshold: Stimulus drives membrane voltage to threshold ($-30\,\text{mV}\text{ to }-50\,\text{mV}$).
  • Rising Phase: Rapid, steep depolarization driven by positive charge entry, moving membrane voltage rapidly into positive ranges.
  • Overshoot: Peak of the action potential where membrane potential surpasses $0\,\text{mV}$.
  • Falling Phase: Rapid repolarization phase where membrane potential drops steeply back toward negative values.
  • Undershoot (Hyperpolarization): Repolarization transiently dips below the baseline resting membrane potential toward $E_K$ before recovering.
  • Restoration: Membrane potential returns smoothly to baseline resting potential.

Ion Conductance and Ionic Basis of the Action Potential

• Transmembrane Currents (Voltage Clamp Experiments by Hodgkin and Huxley):

  • Strong depolarizing voltage steps elicit two distinct current components following brief capacitive charge alignment:
  • Inward Transient Current: An early, fast, inverted "U"-shaped inward current activated right at threshold, mediated by sodium ($Na^+$) influx.
  • Outward Delayed Current: A late, sustained outward current mediated by potassium ($K^+$) efflux.
  • Sodium Current ($Na^+$) Specifics:
  • Mediated by voltage-gated sodium channels that open rapidly at threshold and rapidly inactivate/close.
  • Inactivated channels enter a non-conducting state and remain transiently unavailable.
  • Specific blocker: Tetrodotoxin ($TTX$), found in pufferfish (requiring expert culinary dissection to avoid fatal poisoning of restaurant patrons).
  • At the sodium equilibrium potential ($E_{Na}$), net sodium current drops precisely to zero.
  • Potassium Current ($K^+$) Specifics:
  • Mediated by voltage-gated potassium channels that open with a delayed onset (slow activation) and do not rapidly inactivate.

• Conductance Changes and Sequential Timing:

  • Sequential conductance spikes: Sodium conductance ($g_{Na}$) increases rapidly first, followed sequentially by delayed potassium conductance ($g_K$).
  • Alternating timing prevents simultaneous equal entry and exit of positive ions, permitting rapid voltage transitions.

• Detailed Step-by-Step Mechanism:

  1. Rest: Voltage-gated $Na^+$ and $K^+$ channels closed; potassium leak channels open; $g_K$ high via leak; resting potential is negative.
  2. Threshold: Depolarization reaches threshold ($-30\,\text{mV}\text{ to }-50\,\text{mV}$).
  3. Rising Phase: Voltage-gated $Na^+$ channels open rapidly; $g_{Na}$ spikes; $Na^+$ rushes into cell down electrochemical gradient.
  4. Mid-Rising Phase: Voltage-gated $K^+$ channels begin opening around $1/2$ to $2/3$ up the rising phase, but massive $Na^+$ influx overwhelms early $K^+$ efflux.
  5. Falling Phase: Voltage-gated $Na^+$ channels rapidly inactivate/close; voltage-gated $K^+$ channels fully open; high $g_K$ drives massive $K^+$ efflux, repolarizing the cell.
  6. Undershoot: High $g_K$ drives membrane potential past baseline toward $E_K$, causing transient hyperpolarization.
  7. Return to Rest: Voltage-gated $K^+$ channels close, restoring resting conductances and baseline resting potential.

Unidirectional Propagation and Refractory Periods

• Mechanism of Axonal Propagation:

  • Action potentials travel strictly unidirectionally from soma (cell body) to axon terminal.
  • Regenerative spread: $Na^+$ entering open voltage-gated channels diffuses laterally inside the axon interior. This positive charge depolarizes the adjacent downstream membrane segment to threshold, triggering local $Na^+$ channel opening and regenerating the full action potential.

• Refractory Periods:

  • Absolute Refractory Period:
  • Begins during falling phase when voltage-gated $Na^+$ channels close into their inactivated state.
  • No stimulus, regardless of magnitude, can trigger a second action potential.
  • Analogy: Like a flushing toilet—once flushed, it cannot flush again until the flush mechanism resets.
  • Relative Refractory Period:
  • Follows absolute refractory period.
  • Some $Na^+$ channels have recovered to resting closed state, while others remain inactivated, and $K^+$ conductance remains elevated.
  • An action potential CAN be triggered, but requires a significantly larger stimulus than normal.
  • Evoked action potentials exhibit smaller peak amplitudes due to reduced $Na^+$ channel availability.
  • Functional Consequences:
  1. Guarantees strictly unidirectional signal propagation by rendering recently fired upstream membrane unresponsive.
  2. Sets maximum action potential firing frequency by establishing a minimum mandatory time spacing between consecutive action potentials (e.g., a $1\,\text{ms}$ action potential followed by a $3\,\text{ms}$ refractory period enforces a minimum interval of $4\,\text{ms}$ between spikes).

Axonal Conduction Types and Determinants of Velocity

• Modes of Axonal Conduction:

  • Continuous Conduction:
  • Occurs in unmyelinated axons.
  • Action potentials propagate continuously across every sequential segment of the axonal membrane.
  • Yields slower overall conduction velocity.
  • Saltatory Conduction:
  • Occurs in myelinated axons.
  • Action potentials "jump" between unmyelinated gaps called Nodes of Ranvier.
  • Yields dramatically faster conduction velocity.

• Myelin Structure and Function:

  • Myelin sheaths are formed by individual glial cells wrapping repeatedly around sections of the axon.
  • Glial cells leave small unwrapped spaces between adjacent sheaths termed Nodes of Ranvier.
  • Voltage-gated $Na^+$ channels are heavily concentrated at Nodes of Ranvier.
  • Myelin prevents transmembrane ion leakage, enabling passive internal current ($Na^+$) to travel rapidly down the axonal interior without loss.

• Determinants of Conduction Velocity:

  • Axon Diameter:
  • Increasing axon diameter decreases internal longitudinal resistance to ion flow.
  • Ions travel faster inside larger axons, depolarizing adjacent membrane segments sooner.
  • Largest axon diameter exhibits the highest conduction velocity; smallest axon diameter exhibits the lowest conduction velocity.
  • Myelination: Prevents ion leakage across axonal membrane, accelerating propagation speed relative to unmyelinated axons.

Synaptic Transmission: Electrical and Chemical Synapses

• Directional Synaptic Organization:

  • Neurons connect directionally: presynaptic neuron (signal provider, axon terminal) $\rightarrow$ synaptic cleft $\rightarrow$ postsynaptic neuron (signal receiver, dendrite/soma).

• Electrical Synapses:

  • Less abundant in the nervous system than chemical synapses.
  • Presynaptic and postsynaptic cells are physically joined by Gap Junctions.
  • Transmembrane gap junction proteins align between adjacent cells to form continuous physical aqueous pores.
  • Directly couples the cytoplasm of presynaptic and postsynaptic cells.
  • Extremely rapid transmission: presynaptic depolarization instantaneously depolarizes the postsynaptic membrane.
  • Primary function: Synchronizes electrical activity across groups of cells (e.g., cardiac muscle cells in the heart, coordinating unified organ contraction).

• Chemical Synapses:

  • Predominant synapse type in the nervous system.
  • Presynaptic and postsynaptic membranes do not touch; separated by a fluid-filled gap called the synaptic cleft.
  • Signal transmission occurs via chemical neurotransmitter diffusion across the cleft.

• Step-by-Step Chemical Synaptic Transmission:

  1. Action potential arrives at presynaptic axon terminal.
  2. Depolarization opens presynaptic voltage-gated calcium ($Ca^{2+}$) channels.
  3. Calcium ($Ca^{2+}$) rushes into the presynaptic terminal down its steep concentration gradient.
  4. $Ca^{2+}$ entry triggers a multi-step signaling cascade (~10 steps) directing neurotransmitter-filled synaptic vesicles to fuse with the presynaptic plasma membrane.
  5. Neurotransmitters are released into the synaptic cleft via exocytosis and diffuse across the cleft.
  6. Neurotransmitters bind to ligand-gated ion channel receptors on the postsynaptic membrane.
  7. Ion channels open, producing ion flux and graded postsynaptic potential responses.
  8. Vesicle membrane is recycled, refilled with neurotransmitter, and reset for future release.

Neurotransmitters, Receptors, and Postsynaptic Responses

• Classification of Neurotransmitters:

  • Small-Molecule Neurotransmitters:
  • Synthesized locally directly inside axon terminals and loaded into vesicles.
  • Examples: amino acids such as Glutamate and Glycine, as well as Acetylcholine ($ACh$).
  • Neuropeptides / Protein Neurotransmitters:
  • Synthesized via protein translation inside the cell body (soma) and transported down the axon to terminals.
  • Universal Dependency: Release of both small molecules and neuropeptides strictly depends on presynaptic $Ca^{2+}$ entry. Blocking $Ca^{2+}$ channels eliminates release.

• Postsynaptic Responses:

  • Excitatory Postsynaptic Potential (EPSP):
  • Graded depolarization bringing membrane potential closer to threshold.
  • Neurotransmitter binds ligand-gated channels permeable to $Na^+$ or $Ca^{2+}$, driving positive charge entry.
  • Increases probability of triggering an action potential.
  • Inhibitory Postsynaptic Potential (IPSP):
  • Graded hyperpolarization pushing membrane potential further away from threshold.
  • Neurotransmitter binds channels permeable to $K^+$ (efflux) or $Cl^-$ (influx).
  • Decreases probability of triggering an action potential.

• Postsynaptic Receptor Subtypes:

  • Ionotropic Receptors:
  • Receptor site and ion channel are part of the same protein complex (directly linked).
  • Fast, immediate postsynaptic response upon binding.
  • Metabotropic Receptors (G-Protein Coupled Receptors):
  • Receptor is separate from the ion channel; acts via intracellular G-protein cascades.
  • Slower, longer-lasting postsynaptic response.
  • Multiple Receptor Signaling: The same neurotransmitter (e.g., Glutamate, Acetylcholine) can bind to different receptor subtypes, producing distinct postsynaptic responses (fast vs. slow, excitatory vs. inhibitory).

Synaptic Integration and Axon Hillock

• Process of Integration:

  • Individual EPSPs are small and graded.
  • Neurons receive simultaneous inputs from hundreds to thousands of presynaptic terminals on dendrites and soma.
  • Integration (Summation): Postsynaptic neurons algebraically integrate (add) all incoming excitatory ($E_1, E_2$) and inhibitory ($I_1$) inputs.

• Axon Hillock Function:

  • The axon hillock is the specialized junction between the cell body and initial segment of the axon.
  • Contains the highest concentration of voltage-gated $Na^+$ channels, defining the lowest threshold region of the neuron.
  • Integration determines whether net summed depolarization at the axon hillock reaches threshold voltage to initiate an action potential.

Student Discussion, Lab Work, and Conceptual Review

• Cockroach Nerve Lab and Data Processing:

  • Lab work involved recording nerve activity spikes from cockroach leg preparations.
  • Data calculations included determining Root Mean Square ($RMS$) values for recordings.
  • Results section requirements involved completing data tables, documenting observations, and drawing action potential spikes.

• ChatGPT and Academic Integrity Policies:

  • Discussion regarding utilizing ChatGPT to generate, structure, or rephrase lab results sections.
  • Anecdotes on university AI enforcement policies:
  • Student example 1: Flagged for AI usage during freshman year; professor issued a warning and allowed a re-submission within a couple of days.
  • Student example 2: Denied AI usage after being flagged at over $70\%$ AI generation; case was escalated to university administration, resulting in an automatic failing grade ($F$) for the course that severely impacted GPA.

• Course Structures and Exam Strategies:

  • Organic Chemistry exam policy comparison:
  • Purely test-based grading structure with a forgiving final exam policy.
  • Example scenario 1: Mid-term exam average of $70\%$ combined with an $80\%$ score on the final exam (worth $50\%$ of total grade) raises overall course grade to $75\%$.
  • Example scenario 2: Mid-term exam average of $80\%$ combined with a $70\%$ score on the optional final exam drops the final exam score, preserving the overall $80\%$ average.
  • Neurobiology and Organic Chemistry exams scheduled back-to-back created major time constraints.
  • Preferred study locations: Spring Valley campus building selected over main campus ASEC building due to reduced crowding.
  • Planned study session scheduled for $5:20\,\text{PM}$ after final classes, focusing on labeling neuron anatomy diagrams.

• Conceptual Check - Sodium-Potassium Pump Inhibition:

  • Question: If the sodium-potassium ($Na^+/K^+$) pump is inhibited, what happens to membrane potential?
  • Explanation: The $Na^+/K^+$ pump continuously pumps $3\text{ positive charges}$ ($Na^+$) out for every $2\text{ positive charges}$ ($K^+$) in. Inhibiting the pump causes $Na^+$ to accumulate inside the cytoplasm. The resulting buildup of positive charges inside the cell decreases internal negativity, causing the membrane potential to depolarize (increase toward less negative values).

• Social Discussion:

  • Fraternity bid night event invitation sent via Telegram, featuring a house dog.
  • Shopping trip planned near Tysons (Tysons Corner), located approximately $20\text{--}27\,\text{minutes}$ away, with plans to dine at Olive Garden.