Neuronal Electrical Signaling & Membrane Physiology – Core Vocabulary

Neuron Anatomy & Basic Signaling Framework

• Typical neuron layout
  • Cell body (soma) → dendrites on one side, single axon on the other
  • Axon can be much longer than shown in simple diagrams (some span from spinal cord to finger/toe tips)
• Two fundamental electrical events
  • Graded potentials (GPs)
  • Generated on dendrites
  • Variable in sign ("yes"/excitatory or "no"/inhibitory) and strength → graded
  • Action potentials (APs)
  • Initiated at the axon hillock only when threshold is met
  • All-or-none, stereotyped waveform, travel one direction to synaptic bulbs

Graded Potentials (Detailed)

• Represent local ion-flux changes across dendritic membrane
• Conveyed to axon hillock without regeneration → effective only over short distances
• Numerical traits
  • Amplitude scaled to stimulus strength; can be less than 1 mV or exceed 15 mV
  • Decay with distance and time unless reinforced
• Functional classification
  • Depolarizing GP (EPSP, "yes")
  • Caused mainly by \text{Na}^+ entry
  • Hyperpolarizing GP (IPSP, "no")
  • Caused mainly by \text{K}^+ exit (or \text{Cl}^- entry)

Axon Hillock: Summation & Threshold

• Hillock integrates all incoming GPs
  • Spatial summation: simultaneous inputs from different dendrites
  • Temporal summation: rapid, repeated input at one synapse before previous GP dissipates
• If net depolarization reaches threshold (≈ -55\ \text{mV}), voltage-gated \text{Na}^+ channels open → AP fired

Action Potentials vs. Graded Potentials

• Action potential
  • Fixed amplitude (≈ +30\ \text{mV} peak regardless of stimulus strength)
  • Regenerates along axon → long-distance signaling
  • Obeys refractory periods → unidirectional travel
• Graded potential
  • Variable amplitude
  • No refractory period, can summate
  • Confined to dendrite–soma region

Resting Membrane Potential (RMP)

• Recorded with intracellular microelectrode/voltmeter: -70\ \text{mV} (inside negative)
• Conceptualized as a biological battery
  • Membrane = separator wall
  • Outside = relatively more +; inside = relatively more -
  • Potential energy stored as electrical gradient

Battery Analogy (Chemical Example)

• Two chemical compartments (positive vs. negative ions) inside a 9\text{ V} battery mirror inside/outside of cell
• Connecting wire completes circuit, allowing electron/ion flow → work (light bulb illumination)
• Similarly, opening ion channels completes “circuit” across membrane → neuronal work (electrical signaling)

Ion Channels & Gates

• Leak channels (always randomly open)
  • Potassium leak: baseline \text{K}^+ efflux
  • Sodium leak: baseline \text{Na}^+ influx
• Gated channels
  • Ligand-gated (chemical) – open when neurotransmitter/ligand binds
  • Mechanical-gated – open under membrane distortion (touch, vibration, stretch)
  • Voltage-gated – open at specific membrane potentials (e.g., -55\ \text{mV} for \text{Na}^+, +30\ \text{mV} for \text{K}^+)

Sodium–Potassium Pump (Na⁺/K⁺-ATPase)

• Type: primary active transport; consumes 1 ATP/cycle
• Cycle summary
  1. Pump open to cytosol binds 3 \text{Na}^+
  2. ATP hydrolysis → conformational flip, releasing 3 \text{Na}^+ to extracellular space
  3. Pump now binds 2 \text{K}^+ outside
  4. Dephosphorylation → flips back, releases 2 \text{K}^+ inside
• Net effect per cycle: +1 positive charge moved out, maintaining inside negative

Gradients Established at Rest

• Chemical gradients
  • [\text{Na}^+]{\text{out}} \gg [\text{Na}^+]{\text{in}}
  • [\text{K}^+]{\text{in}} \gg [\text{K}^+]{\text{out}}
• Electrical gradient
  • Outside ≈ + relative to inside
• Combined: electrochemical gradients drive diffusion when channels open

Hyperpolarization vs. Depolarization Mechanics

• Hyperpolarization (IPSP)
  • \text{K}^+ exits or \text{Cl}^- enters
  • Membrane potential shifts to more negative (e.g., -80\ \text{mV})
  • Cell becomes less excitable → inhibitory signal
• Depolarization (EPSP)
  • \text{Na}^+ (or \text{Ca}^{2+}) enters
  • Membrane potential shifts to less negative (e.g., -60\ \text{mV})
  • Cell becomes more excitable → excitatory signal

Summation & Integration Examples

• Sequential EPSPs: \Delta V1 = 5\ \text{mV}, \Delta V2 = 8\ \text{mV} within same time window → total 13\ \text{mV} depolarization
• If RMP = -70\ \text{mV}, effective potential = -57\ \text{mV} (still sub-threshold)
• Add simultaneous IPSP of -4\ \text{mV} → net -61\ \text{mV}, preventing AP

Practical & Cross-Unit Connections

• Long peripheral axons (e.g., motor neurons to toes) rely on APs for rapid, faithful transmission over >1\ \text{m} lengths
• Previous muscle-physiology unit: motor neuron AP triggers \text{Ca}^{2+} release at neuromuscular junction → contraction
• Ion channelopathies or pump defects (e.g., hyperkalemic periodic paralysis) disturb RMP, altering excitability → clinical relevance

Ethical/Philosophical Considerations

• Neuro-electrical principles underlie treatments (deep brain stimulation, pacemakers, neuro-prosthetics)
  • Raises questions on enhancement vs. therapy
• Reliance on animal models (squid axon) prompted ongoing discussions about humane research standards

Key Numerical / Formula Reference List (Quick Recall)

• Resting membrane potential: -70\ \text{mV}
• Threshold potential: \approx -55\ \text{mV}
• AP peak: \approx +30\ \text{mV}
• Na⁺/K⁺-ATPase stoichiometry: 3\ \text{Na}^+{\text{out}} : 2\ \text{K}^+{\text{in}} : 1\ \text{ATP}
• Typical squid‐axon diameter used in experiments: \approx 1\ \text{mm} (giant compared to human \mu\text{m}-scale)

Study Tips

• Draw the neuron repeatedly, labeling ion channels and pumps
• Practice calculating net membrane potential changes given hypothetical ion fluxes
• Relate graded vs. action potentials to everyday analogies (dimmer switch vs. on/off light) for retention