Membrane and Action Potential Quiz

Membrane Potential and Action Potential

• Membrane Potential (Polarization): The resting membrane is polarized, meaning the inside is negative relative to the outside, which is positive. The reason for this polarization is an unequal distribution of ions on both sides of the membrane.
• Resting Neuron Ions: In a resting neuron, sodium (Na+) concentration is high outside the cell (extracellular fluid, ECF), while potassium (K+) concentration is high inside the cell (intracellular fluid, ICF). There are also large, negatively charged proteins and ions like phosphates (PO43-) within the cytosol that cannot easily cross the plasma membranes.
• Resting Membrane Potential (RMP): The difference in charge across the membrane (polarization) is called resting membrane potential (RMP). In neurons, the RMP averages around -70mV.
• Action Potential: An "All-or-None" Response. The action potential shows an "all-or-none" response, where the weakest stimulus needed to activate a neuron to generate a nerve impulse is called the threshold stimulus. Nerve impulses are identical in their electrical change, meaning all impulses are similar.

Depolarization

• Strong Stimulus: If a stimulus is strong enough, it changes the membrane's permeability, allowing sodium (Na+) ions to rapidly enter the cell.
• Polarity Reversal: This rapid influx of Na+ causes the polarity to reverse, making the inside of the membrane positively charged relative to the outside (+30mV). This reversal is called depolarization. The plasma membrane now becomes depolarized.
• Initiation of Action Potential: This sudden depolarization initiates the nerve impulse or action potential.

Propagation of the Action Potential

• Adjacent Area: Depolarization of the first membrane area triggers permeability changes in the adjacent area, leading to further depolarization. The events of depolarization are repeated.
• Rapid Propagation: This initiates the rapid propagation of the action potential along the entire length of the membrane.

Repolarization

• K+ Channels Open: Immediately after depolarization, potassium (K+) channels open, and sodium (Na+) channels close.
• K+ Diffusion: K+ ions diffuse out of the cell into the extracellular fluid. This restores the negative charge on the inside of the membrane and the positive charge on the outside.
• Voltage Change: As a result, the membrane voltage changes from +30mV back to -70mV. The membrane repolarizes and the resting membrane potential (RMP) is re-established.

Hyperpolarization

• More Negative: Hyperpolarization is a change in the membrane potential that makes it even more negative than the resting potential. This is the opposite of depolarization.
• Inhibition of Action Potentials: Hyperpolarization inhibits further action potentials by increasing the stimulus needed to reach the action potential threshold.

Restoring Ionic Conditions

• Sodium-Potassium Pump: The ionic conditions of the resting state are restored later through the activity of the sodium-potassium pump.
• Ion Exchange: The pump ejects three sodium (Na+) ions for every two potassium (K+) ions it brings back into the cell.

Transmission at Synapses

• Synapse Definition: A synapse is a junction between a neuron and another neuron or an effector cell (muscle or gland).
• Calcium Channels: When the action potential reaches the axon terminal of the presynaptic neuron, calcium (Ca2+) channels open.
• Neurotransmitter Release: Calcium ions cause synaptic vesicles containing neurotransmitters (like acetylcholine) to fuse with the membrane and release their contents into the synaptic cleft.
• Postsynaptic Neuron Response: Neurotransmitters bind to receptors on the postsynaptic membrane, triggering a response in the postsynaptic neuron. The nerve impulse is then carried along the axon of the postsynaptic neuron to the next synapse.

Clinical Notes

• Multiple Sclerosis (MS): An autoimmune disease where the immune system attacks the myelin sheath, the protective covering around nerve fibers in the CNS. MS leads to hardened sheaths and disrupted nerve signals. Consequences can include visual/speech problems and loss of muscle control.
• Local Anesthetics: Drugs that block pain signals by preventing voltage-gated sodium (Na+) channels from opening, which stops propagation of action potentials to the central nervous system (CNS). Examples include procaine and lidocaine.
• Puffer Fish Neurotoxin (Tetrodotoxin, TTX): A potent neurotoxin found in pufferfish viscera. TTX blocks the opening of voltage-gated sodium (Na+) channels, inhibiting action potentials and causing paralysis, often leading to suffocation.
• Localized Cooling: Localized cooling of a nerve can produce an anesthetic effect, as lower temperatures reduce the speed of action potential propagation, which can help alleviate pain.