Psychology 2: Resting Potential and Neuronal Communication

Semi-permeable: The cell membrane, which encloses the neuron, is semi-permeable, meaning it selectively allows certain substances to pass through while restricting others.
It separates the 'outside cell' environment from the 'inside cell' environment.

Ions: Electrically charged particles crucial for neuronal communication.
- Cation: An ion with a net positive charge (e.g., Na^+ , K^+ ).
- Anion: An ion with a net negative charge (e.g., Cl^- , A^- intracellular proteins).

Function: Specialized protein structures embedded in the cell membrane that allow ions to cross.
Ion selective: Each channel is typically permeable to specific types of ions (e.g., a Na^+ ion channel will primarily allow Na^+ to pass).
Gated: These channels can be either open or closed, regulating the flow of ions.
- Example: A gate on a Na^+ ion channel controls whether Na^+ can enter or exit the cell.

Two primary forces drive ions across an open ion channel:

Diffusion (Concentration Gradient)
- Ions tend to move from an area of higher concentration to an area of lower concentration to achieve equilibrium.
- This is a passive process driven by random molecular motion.
Electrostatic (Electrical Gradient)
- Ions are attracted to areas of opposite electrical charge and repelled by areas of like electrical charge.
- Positive ions (K^+ , Na^+ ) are drawn towards negatively charged regions.
- Negative ions (Cl^- , A^- ) are drawn towards positively charged regions.

Analogy: Similar to the difference in charge between the poles of a battery, which is referred to as the 'potential'. A voltmeter measures this difference.
In a neuron, it measures the charge difference between the inside and outside of the cell.

The resting membrane potential is typically between -65 and -70 millivolts (mV).
This means the inside of the neuron is 65 to 70 mV more negative than the outside of the neuron at rest.

The resting membrane potential is established and maintained by the unequal distribution of ions across the neuronal membrane.

Selective Permeability of the Cell Membrane
- The neuron's membrane exhibits differential permeability to various ions at rest:
  - It is moderately permeable to Potassium ions (K^+ ) and Chloride ions (Cl^- ).
  - It is relatively impermeable to Sodium ions (Na^+ ).
- This selective permeability allows K^+ to flow out more easily than Na^+ can flow in, contributing to the internal negativity.
The Sodium-Potassium (Na^+ /K^+ ) Pump
- This is an active transporter protein (requiring energy) embedded in the cell membrane.
- Function: It actively pumps $3$ Sodium ions (3 ext{ }Na^+ ) out of the cell and pumps $2$ Potassium ions (2 ext{ }K^+ ) into the cell for every cycle.
- Significance: The Na^+ /K^+ pump is crucial for maintaining the steep concentration gradients of Na^+ (higher outside) and K^+ (higher inside), which are essential for establishing and sustaining the resting membrane potential.
- Since it pumps out more positive charge (3 ext{ }Na^+ ) than it pumps in (2 ext{ }K^+ ), it contributes directly to the net negative charge inside the neuron.

Dendrites: Receive incoming signals from other neurons.
Cell body (Soma): Contains the nucleus and integrates incoming signals.
Axon: Transmits electrical signals (action potentials) away from the cell body to other neurons.
Axon terminals: Form synapses with other neurons, releasing neurotransmitters.

Presynaptic Neuron: The neuron sending the signal.
- Contains synaptic vesicles: Sacs that store and release neurotransmitters.
Synaptic Cleft: The small gap between the presynaptic and postsynaptic neurons.
Postsynaptic Neuron: The neuron receiving the signal.
- Features a postsynaptic receptor area: Contains receptors that bind to neurotransmitters. This binding can lead to changes in the postsynaptic neuron's membrane potential, potentially initiating a new electrical signal.