Ch 3

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Action Potential
- Fundamental electrical signal generated by nerve cells
- Arises from changes in membrane permeability to specific ions
- Changes characterized through voltage clamp technique
Ion Permeability Changes in Neurons
- Initial rapid rise in sodium (Na+) permeability
- Followed by a slower rise in potassium (K+) permeability
- Both permeabilities increase with membrane potential depolarization
- Kinetics and voltage dependence explain action potential generation
Voltage Clamp Technique
- Permits systematic characterization of permeability changes
- Essential for studying mechanisms of action potentials
Threshold Level
- Action potentials initiated when membrane potential exceeds a threshold
- Mechanism behind Na+ permeability sensitive to membrane potential
Mathematical Models
- Help predict ion permeability behaviors accurately
- Enable understanding of action potential propagation in axons

Neuronal Membrane Permeability
- Differentially permeable to various ion species
- Transient increase in Na+ permeability initiates action potentials
History of Research
- Hodgkin and Huxley, late 1940s: first to use voltage clamp on squid axons
- Investigated voltage-dependent Na+ and K+ permeability changes
Voltage Clamp Method
- Developed by Kenneth Cole; allows control of membrane potential
- Measures current needed to maintain specific membrane potential
Experiments on Ion Currents
- Showed that Na+ and K+ permeability changes correlate with action potentials

Figure 3.1 A (Current Flow)
- Membrane hyperpolarization produces brief capacitive current
- Very little current flows during hyperpolarization
Figure 3.1 B (Depolarization)
- Depolarizing to 0 mV results in a rapid inward ion current, then delayed outward current
- Evidence for voltage-dependent permeability in axons

Types of Ion Currents
- Initial inward current (early current) followed by delayed outward current
- Changes in membrane potential affect current flow characteristics
Equilibrium Potentials
- Equilibrium potential for Na+ derived predicts no current flows at +52 mV
- Experimental evidence identifying Na+ carrying early current
K+ Outward Current
- K+ outflow confirms K+ role in observed late endogenous current

Pharmacological Evidence
- Tetrodotoxin (blocks Na+ current) vs. Tetraethylammonium (blocks K+ current)
- Indicates Na+ and K+ currents flow through independent pathways
Time-Dependent Changes in Conductance
- Hodgkin and Huxley calculated Na+ and K+ conductance changes mathematically
- Observed that Na+ conductance activates quickly but inactivates; K+ conductance activates more slowly and does not inactivate

Hodgkin-Huxley Model
- Describes activation and inactivation of Na+ and K+ conductance
- Generates action potentials with remarkable accuracy
Na+ and K+ Conductance Characteristics
- Voltage and time-dependent, with significantly different activation timelines

Conductance Changes and Action Potential
- Models accurately simulate action potentials, depicting Na+ influx and K+ efflux
- Shapes understanding of Na+ and K+ conductance contributions to action potential behavior

Positive Feedback Loop
- Activating voltage-sensitive Na+ channels increases Na+ entry, leading to membrane depolarization
- Results in continuous voltage-sensitive feedback for action potential generation

Long-Distance Signaling
- Discusses how action potentials propagate across axons despite poor conductivity
Propagation Dynamics
- Local depolarizations along the axon trigger action potentials in adjacent segments
Refractoriness
- Prevents backward propagation of action potentials and ensures directionality

Myelination and Conduction Velocity
- Myelin acts as an insulator, increasing conduction velocity by reducing current leakage
- Saltatory conduction occurs at nodes of Ranvier, where Na+ channels are localized
Conduction Velocity Comparison
- While unmyelinated axons conduct at low speeds, myelinated axons can reach significantly higher speeds

Action Potential Dynamics
- Comprehensive explanation of how action potentials arise from changes in Na+ and K+ permeabilities
Voltage Clamp Technique
- Empirical technique fundamental to understanding ionic currents and membrane dynamics

Multiple Sclerosis (MS)
- A central nervous system disease characterized by myelin damage and neurological deficits
- Symptoms range from motor paralysis to sensory dysfunction and abnormalities in cerebrospinal fluid
- Proposed autoimmune mechanisms of action relating to inflammatory demyelination

Overview of Findings
- Comprehensive understanding of ion channel dynamics and action potential generation can inform future research into neurophysiology and related disorders.