Notes On Action Potentials and Reaction Time

Reaction Time and Excitable Cells

• Example: catching a falling ruler → measures reaction time.

• Involves neurons + skeletal muscle cells (excitable cells).

• Excitable cells: generate and transmit electrical signals in response to a stimulus.

• Basis of quick reactions = action potential.

Resting Membrane Potential (RMP)

• Cell membrane controls ion movement (Na⁺, K⁺).

• Sodium-potassium pump (uses ATP):

  • Moves 3 Na⁺ out / 2 K⁺ in.

  • More positive ions outside → inside of cell relatively negative.

• Resting membrane potential ≈ -70 mV (inside negative vs. outside).

• Leaky channels:

  • Na⁺ leaks in (higher conc. outside → inside).

  • K⁺ leaks out (higher conc. inside → outside).

• Pump + leak channels maintain steady RMP.

Action Potential Phases

1. Stimulus depolarizes cell toward threshold (≈ -55 mV).

2. If threshold reached → all-or-nothing AP triggered.

   • No partial APs.

3. Depolarization (rising phase):

   • Voltage-gated Na⁺ channels open → Na⁺ rushes in → inside becomes positive.

   • Membrane potential can reach +30 mV.

4. Repolarization (falling phase):

   • Na⁺ channels inactivate.

   • Voltage-gated K⁺ channels open → K⁺ leaves cell → potential drops.

5. Hyperpolarization:

   • K⁺ efflux overshoots → potential dips below RMP.

6. Return to RMP:

   • Na⁺/K⁺ pump + channel closure restore -70 mV.

Types of Gated Channels

• Ligand-gated: open when neurotransmitter binds (e.g., neuromuscular junction).

• Mechanically gated: respond to physical stimuli (e.g., touch, pressure).

• Voltage-gated: open/close at specific membrane voltages (key in AP).

Propagation of Action Potentials

• AP at one segment → voltage change triggers next segment’s Na⁺ channels.

• Previous segment enters refractory period:

  • Na⁺ channels inactive briefly (~2 ms).

  • Prevents backward APs → ensures one-way flow.

• In myelinated axons: AP jumps between nodes of Ranvier (saltatory conduction) → faster transmission.

Clinical Tie-ins

• Voltage-gated channel dysfunction → disorders (e.g., epilepsy, seizures).

• Drugs may target these channels to manage conditions.

Big Picture

• Everyday actions (moving, thinking, catching rulers) depend on excitable cells.

• Action potentials = foundation of rapid neural and muscular responses.