Unit 2: Part 2 - Action Potential Propagation

Unit 2: Part 2 - Action Potential Propagation

• Unit covers action potential propagation (pages 179-193)

Describing Changes in Membrane Potential

• Normal resting potential is approximately -70mV
• Depolarization: Voltage increases relative to resting potential
• Hyperpolarization: Voltage decreases relative to resting potential
• Repolarization: Restoration of resting membrane potential

What Causes Changes in Membrane Potential?

• Stimulus: Any change in the cell's environment
• Ion channels open/close, causing ions to move in/out of the cell
• Resultant change is either a graded potential or an action potential

2 Types of Potentials

• Action potential
• Graded potential

Graded Potentials

• Stimuli cause voltage-gated ion channels to open.
• If the change in charge isn't enough to reach +30mV (action potential threshold), it's a graded potential
• Graded potentials are small changes in membrane potential (voltage across the membrane).
• Stronger stimuli produce larger changes in voltage
• Graded potentials dissipate as they move away from the stimulus area.

Graded Potentials aka Postsynaptic Potentials (PSP)

• Changes in membrane potential of postsynaptic membrane
• Can be either inhibitory (IPSP) or excitatory (EPSP)

Excitatory and Inhibitory Neurotransmitters

• Excitatory neurotransmitters increase the likelihood of an action potential (AP) occurring on the postsynaptic neuron, leading to an excitatory postsynaptic potential (EPSP)
  • Acetylcholine is a primary example
  • Works by opening Na+ ion channels
• Inhibitory neurotransmitters decrease the likelihood of an action potential occurring, leading to an inhibitory postsynaptic potential (IPSP)
  • GABA is a primary example in the brain
  • Works by opening Cl- ion channels

Threshold & the Development of an Action Potential

• For an AP to occur, the membrane potential must depolarize to -55mV (threshold)
• If a stimulus is strong enough to bring membrane potential from -70mV to -55mV, an action potential will occur
• Once the threshold (-55mV) is reached, more Na+ gates open, and more Na+ enters the cell until +30mV is reached.

Action Potential

• Action Potential is depicted as a graph. The graph shows the membrane potential fluctuating between -70mV (resting) and +30mV (depolarized ) over time
• Phases of the action potential:
• Phase 1 to 2
• Phase 2 to 3
• Resting potential

Action Potential

• Specialized channels (voltage-gated Na+ and K+ channels) exist on the cell membrane, opening/closing based on membrane potential changes.

Action Potential Sequence of Events

1. Stimulus (e.g., temperature, pressure, light, sound) alters resting membrane potential, causing chemically gated Na+ channels to open.
2. Strong graded potential reaches threshold (-55mV), triggering voltage-gated Na+ channels to open, allowing Na+ to rush into the cell
3. Membrane potential quickly climbs from -70mV to +30mV (action potential generated)
4. Na+ channels close, and K+ channels open
5. Repolarization occurs as K+ exits the cell.
6. K+ channels close slowly, leading to hyperpolarization for a short duration.

Action Potential-All or None Principle

• An action potential either occurs completely or doesn't occur at all. A certain threshold must be reached for an action potential.

Refractory Periods

• Absolute refractory period: No stimulus, regardless of strength, can generate a second action potential during this period
• Relative refractory period: A stronger-than-normal stimulus is necessary to generate a second action potential early in this period. A weaker-than-normal stimulus can trigger one later.

Linking the AP to a Nerve Cell

• Neurons detect stimuli and communicate with other cells.
• Functional cell in the nervous system: neuron
• Stimulus → receptor → integrating center→ effector → response

Details of the Neuron

• Part | Job
• Dendrites | Detect stimuli
• Axon | Develops and sends action potentials
• Cell Body | Nucleus (biosynthetic center)

Action Potential Propagation

• Continuous propagation: Action potential spreads along every section of the membrane and repeats
• Saltatory conduction: Action potentials “jump” from Node of Ranvier to Node of Ranvier

Myelinated Fibers vs. Non-Myelinated Fibers

• Myelinated fibers conduct impulses faster than non-myelinated fibers because the myelin sheath prevents signal decay. Action potentials “jump” between Nodes of Ranvier.
• Non-myelinated axons have voltage-gated channels along the entire length. As a result, the action potential has to travel down the entire membrane segment by segment.

Signal Decay

• In a bare plasma membrane (without voltage-gated channels), voltage decays as current leaks across the membrane

Speed of Nerve Conduction

• Table 7.4 provides conduction velocities based on different nerve fiber types (e.g., A alpha, A beta, A gamma, C), including the fiber diameter and myelination status.