Psychology 2: Resting Potential and Neuronal Communication

To develop a comprehensive understanding of the resting potential of neurons.
To recognize that our behavior is a direct result of intricate communication between neurons.

Cell Membrane: A fundamental component of neuronal communication, it is described as semi-permeable, meaning it selectively allows certain substances to pass through while restricting others. This permeability difference is crucial for maintaining the distinct intracellular and extracellular environments.
The membrane separates the inside of the cell from the outside environment.

Neuronal communication relies on the movement of charged particles called ions.
- Cation: An ion carrying a positive charge.
- Anion: An ion carrying a negative charge.

Ions, due to their charge and usually hydrophilic nature, cannot freely pass through the lipid bilayer of the cell membrane.
Ion Channels: Specialized protein structures embedded within the cell membrane that enable ions to cross.
- Ion Selective: Each channel is typically selective for a particular type of ion (e.g., a K^{+} channel allows potassium ions to pass, but not sodium).
- Gated: Many ion channels are gated, meaning they can be either open or closed. This gating mechanism controls the flow of ions, regulating neuronal activity.
- Example: An Na^{+} ion channel is depicted with a gate, illustrating how its opening or closing can control sodium movement.

When an ion channel is open, two primary forces dictate the direction of ion movement:

Diffusion (Concentration Gradient):
- Ions tend to move from an area of high concentration to an area of low concentration.
- This force seeks to equalize the distribution of ions across the membrane.
Electrostatic (Electrical Gradient):
- Ions are attracted to areas of opposite charge and repelled by areas of like charge.
- This force is driven by the difference in electrical potential across the membrane.

What is a Potential?: In the context of electricity, a potential refers to a difference in charge between two points, similar to the voltage across the poles of a battery. A voltmeter is used to measure this difference.
Resting Membrane Potential (RMP):
- The electrical potential difference across the neural cell membrane when the neuron is at rest (not actively firing an action potential).
- The typical value is approximately -65 to -70 mV (millivolts).
- This means the inside of the neuron is about 65 mV more negative than the outside of the neuron.

The existence of the RMP is fundamentally due to the unequal distribution of ions between the intracellular and extracellular fluids.
Key Ions and Their Distribution/Forces:
- Potassium (K^{+}):
  - High concentration inside the cell.
  - Diffusion: Tends to push K^{+} out of the cell.
  - Electrostatic: The negative interior of the cell tends to pull positive K^{+} into the cell.
  - At rest, the membrane is moderately permeable to K^{+} through specific leak channels.
- Sodium (Na^{+}):
  - High concentration outside the cell.
  - Diffusion: Tends to push Na^{+} into the cell.
  - Electrostatic: The negative interior tends to pull positive Na^{+} into the cell.
  - At rest, the membrane is relatively impermeable to Na^{+}.
- Chloride (Cl^{-}$)$:
  - High concentration outside the cell.
  - Diffusion: Tends to push Cl^{-} into the cell.
  - Electrostatic: The negative interior of the cell tends to push negative Cl^{-} out of the cell.
  - At rest, the membrane is moderately permeable to Cl^{-}.
- Large Anions (A^{-}): (e.g., negatively charged proteins)
  - High concentration inside the cell.
  - These are generally impermeable to the membrane and contribute significantly to the negative charge inside the neuron.

Two primary mechanisms establish and maintain the unequal ion distribution crucial for the RMP:

Selective Permeability of the Cell Membrane:
- The membrane is moderately permeable to K^{+} and Cl^{-} ions (via leak channels).
- The membrane is relatively impermeable to Na^{+} ions at rest.
- This differential permeability allows K^{+} to slowly leak out, making the inside more negative.
Sodium-Potassium (Na^{+}/K^{+}) Pump (or Transporter):
- This is an active transport mechanism that utilizes energy (ATP) to move ions against their concentration gradients.
- For every cycle, the pump:
  - Expels 3 sodium ions (Na^{+}) from the inside of the cell to the outside.
  - Brings 2 potassium ions (K^{+}) from the outside of the cell to the inside.
- This action results in a net removal of one positive charge from the cell with each cycle (3 positive out, 2$$ positive in), directly contributing to the cell's negative resting potential.

Dendrites: Receive signals from other neurons.
Cell body (Soma): Integrates incoming signals.
Axon: Transmits electrical signals (action potentials) away from the cell body.
Axon Terminals: Form synapses with other neurons, releasing neurotransmitters.

Presynaptic Neuron: The neuron sending the signal.
- Contains Mitochondria (for energy) and Synaptic Vesicles (containing neurotransmitters).
Synaptic Cleft: The small gap between the presynaptic and postsynaptic neurons.
Postsynaptic Neuron: The neuron receiving the signal.
- Features a Postsynaptic Receptor Area where neurotransmitters bind, leading to a response in the postsynaptic neuron.