Neuronal Membrane Potentials and Synaptic Transmission

Resting Membrane Conditions

The resting membrane potential is a fundamental property of neurons, reflecting the difference in electrical charge across the plasma membrane when the neuron is not actively transmitting a signal.

• Ion Distribution:

  • Sodium (Na+): Higher concentration outside the neuron.

  • Potassium (K+): Higher concentration inside the neuron.

  • Chloride (Cl-): Higher concentration outside the neuron.

  • Proteins (A-): Negatively charged proteins are more abundant inside the neuron.

• Charge Distribution: The inside of the neuron is negatively charged relative to the outside.

• Typical Resting Membrane Potential: Approximately -70 mV (millivolts).

Polarization and the Role of Na+-K+ Pumps

The resting membrane is described as "polarized" because there is a separation of charge across the membrane.

• Polarization: The inside of the cell is negative compared to the outside.

• Na+-K+ Pump: This active transport protein moves 3 Na+ ions out and 2 K+ ions into the cell, using ATP. This maintains the concentration gradients and contributes to the negative resting potential.

Equation:

Types of Ion Channels

Ion channels are proteins that allow specific ions to move across the membrane. They are classified by their gating mechanisms:

• Leak Channels: Always open; allow passive movement of ions (mainly K+).

• Voltage-Gated Channels: Open or close in response to changes in membrane potential; essential for action potentials.

• Ligand-Gated Channels: Open or close when a specific chemical (ligand) binds to the channel protein; important in synaptic transmission.

Comparison Table:

Synaptic (Graded) Potentials

Synaptic or graded potentials are small, localized changes in membrane potential that occur in response to neurotransmitter binding.

• Definition: Local changes in membrane potential, varying in size and not propagated over long distances.

• Importance: They determine whether a neuron will reach threshold to fire an action potential.

• Difference from Action Potentials: Graded potentials are decremental (decrease with distance), can be excitatory or inhibitory, and are not all-or-none. Action potentials are all-or-none and propagate without decrement.

Summation: Spatial and Temporal

Summation refers to the process by which multiple graded potentials combine to influence the likelihood of an action potential.

• Spatial Summation: Multiple synaptic inputs from different locations on the neuron occur simultaneously.

• Temporal Summation: Multiple synaptic inputs from the same location occur in rapid succession.

Example: If two excitatory postsynaptic potentials (EPSPs) arrive at the same time from different synapses, their effects add up (spatial summation). If one synapse is activated repeatedly in quick succession, the effects also add up (temporal summation).

Action Potentials: Location and Channels

Action potentials are rapid, all-or-none electrical signals that travel along the axon.

• Location: Action potentials are initiated at the axon hillock and propagate along the axon.

• Voltage-Gated Channels: Axons contain voltage-gated Na+ and K+ channels.

• Channel Gating: A change in membrane potential (depolarization) opens voltage-gated Na+ channels, followed by opening of K+ channels as the membrane repolarizes.

Events of an Action Potential

The action potential consists of a series of rapid changes in membrane potential:

1. Resting State: All voltage-gated channels are closed; membrane at -70 mV.

2. Depolarization: Voltage-gated Na+ channels open; Na+ enters the cell, making the inside more positive.

3. Repolarization: Na+ channels inactivate; voltage-gated K+ channels open; K+ leaves the cell, restoring negativity inside.

4. Hyperpolarization: K+ channels remain open slightly longer, causing the membrane potential to become more negative than resting.

5. Return to Resting Potential: K+ channels close; Na+-K+ pump restores original ion distribution.

Action Potential Graph:

• Rising phase: Depolarization due to Na+ influx.

• Peak: Maximum positive voltage, Na+ channels inactivate.

• Falling phase: Repolarization due to K+ efflux.

• Undershoot: Hyperpolarization before returning to resting potential.

Equation (Nernst Equation for Equilibrium Potential):

Additional info: The Nernst equation is used to calculate the equilibrium potential for a particular ion, which helps explain the resting membrane potential.