Human Morphology and Function: Action Potential & Excitation

What type of ions maintain internal negativity in the cell?

Impermeant anions

What are examples of impermeant anions inside the cell?

Proteins, organic phosphate, and sulfate compounds

What causes a deficit of positive ions like K+ inside the cell?

Impermeant anions

What is the result of a deficit of positive ions inside the cell?

Negative charge inside the cell

What ion causes prolonged depolarization similar to sodium?

Ca²⁺ (Calcium)

What role do pumps play during the resting phase of Ca²⁺?

They move ions to the cell exterior

What initiates depolarization related to calcium?

Voltage-gated Ca²⁺ channels

Why are voltage-gated Ca²⁺ channels called "slow channels"?

They gate 10-20x more slowly than Na⁺ channels

Where are slow Ca²⁺ channels particularly important for prolonged stimuli?

Cardiac and smooth muscle

What happens when there is a decrease in Ca²⁺ levels regarding Na⁺?

Easier Na⁺ activation

How does a decrease in Ca²⁺ affect the action potential trigger level?

Triggered at lower levels of depolarization

Where does Ca²⁺ bind to affect channel activation?

Ca²⁺ binds to Na⁺ channels

How does Ca²⁺ binding normally affect the threshold for Na⁺ channels?

Increases threshold to trigger opening of Na⁺ channels at normal levels

What condition does a significant drop in calcium lead to?

Spontaneous firing or tetany

At what percentage reduction in normal calcium levels does tetany typically start?

50% reduction

What severe complication may arise from calcium deficit and tetany?

Lethal respiratory muscle spasms

How does a depolarized area propagate the action potential?

It excites the adjacent membrane

What must the adjacent membrane reach to trigger depolarization during propagation?

Threshold membrane potential

What spreads during the propagation of an action potential?

Local current flow

In how many directions does the local current flow travel?

Both directions unless blocked

Can an action potential travel backwards during propagation?

No, not backwards

What principle states that reaching the threshold yields a full response?

All-or-Nothing Principle

What happens if the threshold is not reached in the All-or-Nothing principle?

No signal and propagation ends

Is "partial" depolarization possible under the All-or-Nothing principle?

No

Where are K⁺ and Na⁺ located immediately after firing?

K⁺ outside, Na⁺ inside

Approximately how many action potentials can occur before concentration differences prevent firing?

100,000 to 50 million

What restores ion gradients after firing?

Na⁺/K⁺ ATP pumps

What cellular resource is required by Na⁺/K⁺ ATP pumps?

ATP

What is the energy cost result of increased heat production during recovery?

Increased heat production equals energy cost

In which type of muscle are plateau potentials typically seen?

Cardiac muscle

What causes plateau potentials?

Ca²⁺ slow channels (slightly permeable to Na⁺) and delayed K⁺ opening

How long does a plateau potential prolong muscle contraction?

200 to 300 ms

How does the length of a plateau potential compare to the action potential?

It is the same length as the action potential

What is rhythmicity in cellular physiology?

Self-excitation for repeated discharge of a signal

What are examples of locations or cells where rhythmicity occurs?

Pacemaker cells, smooth muscles of intestines, and CNS for breathing

What causes repetitive discharge in rhythmicity?

Enough Na⁺/Ca²⁺ leak leading to automatic depolarization

What determines the downtime in rhythmic self-excitation?

Hyperpolarization

What ion movement triggers sufficient inward current for excitation?

Inward movement of Na⁺ ions

What is a mechanical stimulus for excitation?

Disturbance of membrane (e.g., excite nerve endings in the skin)

What is a chemical stimulus for excitation?

Neurotransmitters moving between neurons and muscles

What is an example of an electrical stimulus for excitation?

Stimuli between muscle cells in the heart

What are acute local potentials that fail to trigger depolarization called?

Acute subthreshold potentials

What can accumulated acute local potentials cause?

Action potentials

How does the strength of a local potential affect the latent period?

The stronger the local potential, the shorter the latent period before depolarization

How does Ca²⁺ help inhibit excitability?

Stabilizes membrane and decreases membrane permeability to Na⁺

How do local anesthetics affect excitability?

They block the activation gate of Na⁺ channels

What is the overall effect of local anesthetics on Na⁺ channels?

Reduces the likelihood of an action potential