Summary of "FISIOLOGÍA Clase 4: POTENCIAL DE ACCION"

Summary of “FISIOLOGÍA Clase 4: POTENCIAL DE ACCION”

This video lecture explains the concept of action potential in cells, focusing on its physiological basis, phases, and underlying ionic mechanisms. The lesson is delivered in an informal, conversational style and covers foundational concepts in membrane physiology, ion channel behavior, and the electrical changes during cell activation.

Main Ideas and Concepts

Resting Membrane Potential (-90 mV)

Cells maintain a resting electrical charge inside the membrane, typically around -90 mV. This polarization is due to the distribution and movement of ions, mainly:

Potassium (K⁺): High intracellular concentration (~140 mEq/L). Potassium tends to leave the cell, carrying positive charge out, making the inside more negative (~-94 mV).
Sodium (Na⁺): High extracellular concentration (~142 mEq/L). Sodium tends to enter the cell but the membrane is only 5% permeable to sodium, contributing a small positive charge (~+8 mV).
Sodium-Potassium ATPase Pump: Actively transports 3 Na⁺ out and 2 K⁺ in, against their gradients using ATP, contributing about -4 mV to the resting potential.

The combined effect results in the resting potential of approximately -90 mV.

Phases of the Action Potential

Resting (Polarized) State: Membrane potential is stable at -90 mV.
Depolarization (Activation):
- Triggered when ligand-gated sodium channels open in response to a chemical stimulus (ligand).
- Sodium ions enter, making the membrane potential less negative (towards zero).
- If the threshold of approximately -65 mV is reached, voltage-gated sodium channels open massively, causing a rapid influx of sodium and a sharp rise in membrane potential up to about +35 mV.
- This phase represents the “all-or-nothing” response.
Repolarization (Return to Rest):
- Voltage-gated sodium channels inactivate (close but do not allow sodium entry).
- Voltage-gated potassium channels open, allowing K⁺ to exit, carrying positive charge out and restoring negativity.
- Sodium channels (both ligand- and voltage-gated) close fully.
Hyperpolarization (Afterpotential):
- Potassium channels remain open longer than needed, causing the membrane potential to temporarily become more negative than the resting potential (up to -110 mV).
- The sodium-potassium pump restores the original ion distribution and resting potential (-90 mV).

Graphical Representation

Time (in tenths of a second) on the x-axis.
Membrane potential (mV) on the y-axis, with zero as a reference.
Depolarization is shown as a rapid upward spike from -90 mV to +35 mV.
Repolarization is a rapid downward slope back to resting potential.
Hyperpolarization dips below resting potential before returning to baseline.

Additional Concepts

Ligand-Gated Sodium Channels: Closed at rest; open when a specific chemical (ligand) binds, allowing sodium influx.
Voltage-Gated Sodium Channels: Open only after threshold potential is reached, allowing a large sodium influx.
Voltage-Gated Potassium Channels: Open at peak depolarization to help restore resting potential.
All-or-Nothing Principle: The action potential either occurs fully if threshold is reached or not at all.
Refractory Periods:
- Absolute Refractory Period: Sodium channels are inactivated; no new action potential can be initiated.
- Relative Refractory Period: Some sodium channels reset; a stronger-than-normal stimulus can trigger a new action potential.
Rheobase: Minimum stimulus strength to trigger an action potential.
Chronaxie: Time required for a stimulus of twice the rheobase strength to trigger an action potential.

Special Cases and Curiosities

Cardiac Muscle Action Potential:
- Features a plateau phase due to calcium influx balancing potassium efflux.
- Calcium channels open after sodium channels close, prolonging depolarization.
- This plateau supports the contraction phase of cardiac muscle.
Rhythmicity in Certain Cells:
- Some neurons (e.g., those controlling breathing or intestinal peristalsis) have a resting potential closer to threshold (-70 mV).
- They maintain some open sodium and calcium channels, allowing spontaneous rhythmic action potentials without external stimuli.
- Myelination increases conduction speed and efficiency in neurons.

Detailed Methodology / Process of Action Potential

At Rest:
- Potassium channels allow K⁺ to exit → negative interior.
- Sodium channels mostly closed; small Na⁺ leak inward.
- Sodium-potassium pump maintains gradients.
Activation (Depolarization):
- Ligand binds → ligand-gated sodium channels open.
- Sodium enters, membrane potential rises toward threshold (-65 mV).
- If threshold reached → voltage-gated sodium channels open.
- Massive sodium influx → membrane potential spikes to +35 mV.
Repolarization:
- Voltage-gated sodium channels inactivate (close).
- Voltage-gated potassium channels open → K⁺ exits.
- Membrane potential falls back toward negative.
Hyperpolarization:
- Potassium channels remain open briefly → potential dips below resting.
- Sodium channels fully closed.
- Sodium-potassium pump restores ionic balance → returns to resting potential.
Refractory Periods:
- Absolute: No new action potential possible.
- Relative: New action potential possible with strong stimulus.

Speakers / Sources Featured

Primary Speaker: The instructor/lecturer presenting the physiology class (unnamed).
References: The lecturer references physiology textbooks and prior classes (e.g., Goldman equation from previous lessons).
No other distinct speakers are identified in the subtitles.

Summary

The video provides a comprehensive explanation of the action potential mechanism, emphasizing ion movements, channel dynamics, and phases of electrical changes in the cell membrane. It introduces important physiological concepts such as the resting membrane potential, depolarization, repolarization, hyperpolarization, refractory periods, and special adaptations in cardiac and rhythmic neurons. The explanation is supported by graphical descriptions and practical examples, making it accessible for students learning cellular physiology.