Membranes and Receptors: Action Potentials

Membranes and Receptors: Electrical Excitability

• The lecture covers action potentials and their properties.
• Students will be able to identify the properties of action potentials and their ionic basis.
• They will outline changes in membrane ionic permeability.
• Students will describe the all-or-none law and refractoriness.
• They will demonstrate some properties of ion channels.
• Application of these facts to local anesthetics will be covered.

Action Potential (AP)

• Action potentials are changes in membrane voltage.
• They are nerve impulses and are not graded.
• Once initiated, they affect the entire excitable membrane.
• They propagate along axon surfaces.
• Membrane potential is the voltage difference across a cell membrane, where the cytoplasm is more negative than the exterior.

Voltage-Dependent Sodium and Potassium Channels

• These channels have specific stages.
• The first stage of the sodium channel includes activation.
• The second stage of the sodium channel is inactivation.
• The potassium channels open and the sodium channels close at the peak of the action potential.
• The channels then begin to reset.

Action Potential (AP) Steps

• A stimulus from a sensory cell or neuron depolarizes the target cell to threshold potential.
• If threshold is reached, sodium channels open and the membrane depolarizes.
• At peak action potential, potassium channels open, allowing potassium to leave the cell.
• Simultaneously, sodium channels close.
• The membrane hyperpolarizes as more K+ leaves.
• The membrane enters a refractory period, unable to fire.
• Potassium channels close, and the Na+/K+ transporter restores resting potential.

Action Potential Generation

• Action potentials are driven by changes in membrane permeability to sodium, bringing the membrane closer to the sodium equilibrium potential.
• The sodium hypothesis of action potentials describes this.
• The concentration of sodium ions in the external fluid greatly influences action potential.

Effects of Membrane Potential Changes

• Changes in membrane potential impact sodium and potassium currents.
• This effect is demonstrated graphically.

Resting Potential

• The resting potential is 65 mV.
• It's maintained by K+ leak channels and limited Na+ leak channels.
• The inside of the cell is more negatively charged than the outside.

Recovery After Action Potential

• After an action potential, most sodium channels are inactivated.
• They need time to recover before opening again—this process occurs when membrane potential returns to its resting level.
• The absolute refractory period is when all Na+ channels are inactivated, and the relative refractory period occurs as channels recover.

Action Potential Steps Summary

• Depolarization to threshold activates sodium channels.
• Sodium influx causes further depolarization.
• Potassium channels open, repolarizing the membrane.
• The membrane hyperpolarizes slightly before returning to resting potential.

Molecular Nature of Voltage-Gated Channels

• Voltage-gated ion channels are trans-membrane proteins.
• Channel function is regulated by changes in the membrane potential.
• Opening/closing is triggered by changes in ion concentration and charge gradient.

Types of Ion Channels

• Voltage-gated ion channels respond to voltage changes.
• Ligand-gated ion channels (activated by ligands).
• Mechanically-gated ion channels, responding to mechanical forces.

Voltage-Gated Ion Channel Structure

• The structure includes homologous repeats of transmembrane spanning domains.
• Some domains are responsive to the voltage field across the membrane.
• This is important for the function of Na+ and Ca++ channels.

Additional Notes

• The voltage-gated ion channels are regulated by the changes in the membrane potential, near the channel.
• Voltage-gated ion channels are important in maintaining neuronal homeostasis, secretion, endocytosis, muscle contraction, synaptic transmission, ciliary control, fertilization, etc.