Action Potentials and Neural Signaling: Study Notes for Anatomy & Physiology

Neural Signaling and Action Potentials

Overview of Action Potentials

An action potential is a rapid, temporary change in a cell's membrane potential, essential for neural communication. It is a fundamental process in the nervous system, allowing neurons to transmit signals over long distances.

• Resting Membrane Potential: The typical resting membrane potential of a neuron is about -70 mV, maintained by ion gradients and selective membrane permeability.

• Depolarization: A stimulus causes the membrane potential to become less negative, reaching a threshold that triggers the action potential.

• Repolarization: The membrane potential returns toward the resting value after the peak of the action potential.

• Hyperpolarization: The membrane potential temporarily becomes more negative than the resting potential before stabilizing.

Key Ions: Sodium (Na+) and potassium (K+) are the primary ions involved in generating action potentials.

Phases of the Action Potential

The action potential consists of several distinct phases, each characterized by specific ion channel activity and changes in membrane potential.

1. Resting State: Voltage-gated Na+ and K+ channels are closed. The membrane is at -70 mV.

2. Depolarization: A stimulus opens voltage-gated Na+ channels. Na+ rushes into the cell, making the inside more positive. The membrane potential rises toward +30 mV.

3. Repolarization: Na+ channels inactivate, and voltage-gated K+ channels open. K+ exits the cell, returning the membrane potential toward negative values.

4. Hyperpolarization: K+ channels remain open slightly longer, causing the membrane potential to become more negative than the resting potential before stabilizing.

Example: The classic action potential graph shows a rapid spike (depolarization), a sharp drop (repolarization), and a brief undershoot (hyperpolarization) before returning to baseline.

Ion Channel Dynamics

Ion channels are proteins that allow specific ions to cross the neuronal membrane, crucial for generating and propagating action potentials.

• Voltage-Gated Sodium Channels: Open rapidly in response to depolarization, allowing Na+ influx.

• Voltage-Gated Potassium Channels: Open more slowly, allowing K+ efflux to repolarize the membrane.

• Leak Channels: Always open, contributing to the resting membrane potential.

Formula: The Nernst equation can be used to calculate the equilibrium potential for each ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Where: Eion = equilibrium potential for the ion R = gas constant T = temperature (Kelvin) z = charge of the ion F = Faraday's constant

Sequence of Events in an Action Potential

The following steps summarize the sequence of events during an action potential:

1. Depolarization brings an area of membrane to threshold.

2. Action potential begins.

3. Sodium floods into the cell.

4. Membrane potential goes from -70 mV to +30 mV.

5. Potassium channels close.

6. Sodium channels close and voltage-gated potassium channels open.

7. Sodium channels open (for the next action potential).

Example: The provided graph shows the changes in membrane potential over time, with labeled phases corresponding to the above steps.

Distribution of Ions and Membrane Potential

The resting membrane potential is established by the differential distribution of ions across the membrane and the selective permeability of the membrane to these ions.

• Na+-K+ Pump: Actively transports 3 Na+ ions out and 2 K+ ions into the neuron, maintaining the concentration gradients.

• Charge Distribution: The inside of the neuron is negatively charged relative to the outside.

Formula: The Goldman-Hodgkin-Katz equation can be used to calculate the resting membrane potential considering multiple ions:

$V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

Where: Vm = membrane potential P = permeability of the membrane to the ion

Summary Table: Ion Channel States During Action Potential

Phase	Na+ Channels	K+ Channels	Membrane Potential
Resting	Closed	Closed	-70 mV
Depolarization	Open	Closed	Rising toward +30 mV
Repolarization	Inactivated	Open	Falling toward -70 mV
Hyperpolarization	Closed	Open (then closing)	Below -70 mV

Clinical Relevance

• Disruptions in ion channel function can lead to neurological disorders, such as epilepsy or channelopathies.

• Local anesthetics block voltage-gated Na+ channels, preventing action potential propagation and thus pain sensation.

Additional info: The notes reference support sessions and office hours, which are not directly relevant to the scientific content but indicate this is a class or lecture note set. The main focus is on the physiology of action potentials in neurons, a core topic in Anatomy & Physiology.