Resting Membrane Potential and Action Potentials

Resting Membrane Potential

• Presenters: Othman Al-Shboul, Faris Katbi, Mo-Rami
• Version: 3

Excitable Tissues

• Definition: Excitable tissues respond to various stimuli by rapidly changing their resting membrane potentials (RMP) and generating electrochemical impulses.
• Types of Excitable Tissues:
  • A. Nerve
  • Function: Conduct messages
  • B. Muscle
  • Function: Contract

Neuron Basics

• Resting Membrane Potential (RMP):
  • Typical value for neurons: $-70 ext{ mV}$
• Impact of Stimulation:
  • Stimuli such as touch, smell, or sight can change the RMP.
  • Types of changes:
  1. Depolarization: Membrane potential becomes more positive.
  2. Repolarization: Membrane potential returns back to RMP after depolarization.
  3. Hyperpolarization: Membrane potential becomes more negative than RMP.

Mechanism of Excitation

• Changes in ion permeability (Na+ and K+) lead to excitation.
• Involves effects on voltage-gated (VG) channels sensitive to changes in membrane potential (MP).

Action Potential

• Definition: A wave of MP changes that sweeps along the membrane.
• Characteristics:
  • Action potential spreads throughout the membrane in an undecreasing fashion.

Mechanism of Action Potential

1. Resting State:

   • Membrane potential = $-70 ext{ mV}$
   • All gated sodium and potassium channels are closed.

2. Threshold Activation:

   • When the stimulus reaches the threshold of $-55 ext{ mV}$, VG Na+ channels open significantly, leading to rapid Na+ influx.
   • Membrane becomes $600$ times more permeable to Na+ than to K+.

3. Depolarization:

   • Na+ influx changes membrane potential from $-70 ext{ mV}$ to a peak potential of $+30 ext{ mV}$.

4. Repolarization:

   • Closure of Na+ VG channels and opening of K+ VG channels occur, leading to K+ efflux.
   • Cell attempts to return to resting state.

5. Hyperpolarization:

   • Voltage-gated K+ channels remain open after repolarization, causing a potential of around $-90 ext{ mV}$.

6. Return to Resting State:

   • The cell regains its resting state eventually.

Additional Notes

• Diffusion: Both depolarization and repolarization occur via diffusion and do not require active transport.
• Graded Potential: Before reaching the threshold, changes in potential are classified as graded potentials.
• The Na+/K+ pump restores normal conditions after action potentials, pushing Na+ out and bringing K+ in.

Characteristics of Action Potential

1. All-or-None Response:
   • An action potential will only be produced if the membrane is depolarized to its threshold level.
   • If the membrane does not reach threshold, no action potential occurs.
2. Stimulus Intensity Coding:
   • Increasing stimulus strength does not change the magnitude of the action potential.
   • A stronger stimulus increases the frequency of action potentials but not their magnitude.
3. Refractory Period:
   • Defined as the time during which a cell cannot repeat an action potential.
   • Divided into Two Phases:
     • a) Absolute Refractory Period:
     • No second action potential can be produced regardless of the stimulus strength due to inactivated Na+ channels.
     • Begins at the threshold period and ends at the resting potential.
     • b) Relative Refractory Period:
     • A second action potential can occur but requires a stronger stimulus due to continued K+ efflux.
     • This phase prevents backward current flow.

Pharmacological Blockers of Na+ & K+ Channels

1. Tetrodotoxin (TTX):
   • Sourced from pufferfish, it blocks VG Na+ channels.
   • Action potentials cannot occur with TTX present.
2. Tetraethylammonium (TEA):
   • K+ channel blocker.
   • Action potentials can still occur but have an abnormally long duration due to dependence on Na+ channel inactivation alone.

Structure of Neuron

1. Cell Body:
   • Contains the nucleus and DNA.
2. Dendrites:
   • Receive signals from other neurons, sending inputs toward the cell body.
3. Axon Hillock:
   • The first part of the axon and the region where it exits the cell body.
4. Axon:
   • The conduction zone that transmits signals away from the cell body.
5. Myelin Sheath:
   • Serves as an insulator to prevent ion leakage, increasing conduction speed.
6. Axon Terminals:
   • Output zone that sends signals to other neurons.

• Direction of Signal Transmission: Dendrites → Cell body → Axon → Axon terminals

Myelinated vs. Unmyelinated Axons

• Myelin:
  • A lipid-rich substance that insulates and prevents ionic leakage, enabling faster conduction of action potentials.

Site of Action Potential Initiation

• Axon Hillock:
  • Contains an abundance of voltage-gated Na+ channels and has the lowest threshold for action potential initiation, hence termed the neuron's trigger zone.

Historical Perspectives on Nervous System Signaling

1. Galen (129-210 AD): Proposed hydraulic mechanism for muscle contraction due to fluid flowing through hollow nerves.
2. René Descartes (1596 – 1650): Suggested animal spirits flowed from the brain through nerves to muscles.
3. Luigi Galvani (1794): Showed that nerves and muscles could be activated by charged electrodes, indicating electrical signaling in the nervous system.
4. 1930s Advances: Development of electronic amplifiers and recording devices that enabled the recording of electrical signals.
5. H.K. Hartline (1956): Recorded electrical signals in the optic nerve of the horseshoe crab (Limulus).

Resting Membrane Potential in Different Cells

• Values:
  • Skeletal muscle cells: $-90 ext{ mV}$
  • Nervous cells: $-70 ext{ mV}$
  • Smooth muscle: $-60 ext{ mV}$
  • Adipocytes: $-58 ext{ mV}$
  • Epithelial cells: $-53 ext{ mV}$
  • Fibroblasts: $-20 ext{ to } 30 ext{ mV}$
  • Blood cells: $-10 ext{ mV}$