PHCL2610 Lecture 3: Nervous System

Lecture 3 PHCL2610: Nervous System

• The lecture covers the nervous system, specifically focusing on action potentials.
• The primary reference material is Vander's Human Physiology 16th Edition, Copyright 2023, Chapter 6, Section C: Synapses.
• The lecturer is Ana Maria Oyarce Ph.D, Department of Pharmacology - HEB280C.

Action Potential Consists of Phases

• Phase 1: Resting membrane potential.
• Phases 2 and 3: Depolarization—membrane potential moves from resting membrane potential (RMP) towards less negative or positive values.
• Phases 4 and 5: Repolarization—membrane potential returns to the RMP (becomes negative)
• Phase 6: Hyperpolarization—membrane potential moves away from the RMP in a more negative direction.

Resting Membrane Potential (Phase 1)

• Na+ and K+ leak channels are open, allowing the flow of ions.
• Voltage-gated Na+ and K+ channels are closed.

Depolarization (Phases 2 and 3)

• Voltage-gated Na+ channels open following a stimulus.
• Depolarization begins when the membrane potential shifts from resting membrane potential towards a less negative potential; Na+ moves across the membrane.
• The depolarization needs to reach a threshold (-55 mV) to produce an action potential.
• When the membrane potential reaches threshold, the rest of the voltage-gated Na+ channels open.
• Na+ rushes into the neuron, causing the membrane potential to become positive.
• As the membrane potential approaches +30 mV, voltage-gated Na+ channels start to close.

Repolarization (Phases 4 and 5)

• Na+ channels become inactive and begin to close.
• K+ channels open.
• K+ ions leave the neuron, creating a positive charge buildup inside the neuron, causing the change in membrane potential from +30 mV to -70 mV.
• Inactivated Na+ channels return to their resting state.

Hyperpolarization (Phase 6)

• Voltage-gated K+ channels close slowly, remaining open longer than needed to return to RMP.
• More K+ than needed for RMP leaves the cell.
• Na+ channels are in the resting state or closed.
• K+ channels close, returning the potential to resting levels.
• This phase results in a more negative membrane potential than at rest.

Characteristics of Action Potentials

• Action potentials rely on the distribution of ions and changes in membrane permeability to ions (mainly Na+ and K+, with Cl- playing a lesser role).
• These changes are driven by the action of voltage-gated channels.
• Action potentials are short-lived events.

Action Potential Propagation

• Action potentials are initiated at the axon hillock and propagate towards the axon terminal.

• They can only proceed down the axon as long as the space behind it is in its refractory period.

• There are two types of propagation:

  • Continuous propagation - occurs in unmyelinated axons and muscle fibers. Depolarization and repolarization happens step-by-step in each adjacent segment of the membrane. It is a slower process.

  • Saltatory propagation - occurs in myelinated axons. Propagation is faster because the signal jumps between the nodes of Ranvier (gaps between myelin sheaths).

Refractory Periods

• A refractory period is a period where a cell does not respond to further stimulation; no action potential is produced.
• Two types of refractory periods exist for action potentials, absolute and relative.
• The conformation of the voltage-gated Na+ channels is important during these periods.
• Absolute refractory period: No amount of stimulation can produce a second action potential. During this period, the Na+ channels are open or inactivated.
• Relative refractory period: A stronger-than-threshold stimulus can produce a second action potential in this period. The K+ channels are still open.

Behavior of Voltage-Gated Channels

• Na+ channels have different conformations based on membrane potential.
  • At rest, Na+ channels are closed.
  • During activation, Na+ channels open and then quickly inactivate.
  • After inactivation, Na+ channels return to their closed state, becoming available for activation again.
• K+ channels open and close relatively slowly at different membrane potentials