Nervous System: Resting Membrane Potential and Action Potential

Structure of a Neuron

• Consists of three main parts:
  • Dendrites: Receive signals (can be stimulatory or inhibitory).
  • Cell Body: Integrates incoming signals from dendrites and processes them. Sends signals to the axon if the threshold is reached.
  • Axon: Transmits electrical impulses away from the cell body.
  • Axon Terminals: Release neurotransmitters to communicate with target cells.

Electrical Signalling in Neurons

• Neurons function via electrical signalling, primarily by moving cations across plasma membranes.
• Cations involved in signalling:
  • Sodium (Na+), Potassium (K+), Calcium (Ca2+)
  • Sodium: High concentration outside the cell; low inside.
  • Potassium: High concentration inside the cell; low outside.
  • Calcium: High concentration outside the cell; low inside.

Resting Membrane Potential

• Neurons have a resting membrane potential that is more negative inside compared to outside:
  • This difference is created by the sodium-potassium pump and ion leakage.
  • Potassium ion (K+) leakage is predominant, causing the inside of the membrane to be negatively charged.

Generation of Action Potential

• When a neuron is stimulated and reaches a threshold:
  1. Depolarization Phase: Sodium channels open, allowing Na+ to flow into the cell, making the inside more positive.
  2. Repolarization Phase: Sodium channels close, and potassium channels open, allowing K+ to exit the cell, restoring the negative charge inside.
  3. Hyperpolarization Phase: Too much K+ exits, temporarily making the inside more negative than the resting potential.

Action Potential Phases

• Three main phases include:
  • Depolarization: Due to Na+ ions entering the cell.
  • Repolarization: Following the exit of K+ ions.
  • Hyperpolarization: Excessive K+ exit results in a temporary increase in negative charge inside.

Refractory Periods

• Absolute Refractory Period: The neuron cannot generate a new action potential regardless of stimulus strength. This occurs immediately following an action potential.
• Relative Refractory Period: A new action potential can occur, but it requires a stronger than usual stimulus, as the membrane is still hyperpolarized.

Factors Affecting Action Potential Transmission

• Myelination: Myelin sheaths, made by oligodendrocytes and Schwann cells, insulate axons and facilitate faster propagation of action potentials via saltatory conduction.
• Action potentials only occur at nodes of Ranvier.

Synaptic Transmission

• When an action potential reaches the axon terminal, it triggers the opening of voltage-gated Ca2+ channels, allowing Ca2+ to enter the cell and stimulate neurotransmitter release.
• Example: Acetylcholine is released from synaptic vesicles, binds to receptors on the postsynaptic cell:
  • Enzyme acetylcholinesterase breaks down acetylcholine, terminating its action. If absent, acetylcholine accumulates and continuously stimulates the target, leading to possible dysfunction.

Summary of Key Concepts

• Resting Membrane Potential: Inside of the neuron is more negative than the outside due to ion distribution.
• Action Potential Generation: Triggered by a stimulus that causes depolarization, followed by repolarization and possible hyperpolarization.
• Synapses: Region where neurotransmitter release occurs to pass signals to other cells.
• Refractory periods ensure that action potentials only travel unidirectionally along the axon, maintaining the integrity of the signal transmission.