Neuron Structure and Function

Four Main Parts of a Neuron

• Dendrites: Receive incoming information.
• Cell Body: Processes and integrates the information.
• Axon: Carries information over long distances within the neuron.
• Axon Terminal: Transmits information to the next cell.

Nerve

• A bundle of axons.
• Transmits information over long distances.

Signal Transmission in Neurons

• Dendrites: Receive signals; decide whether to pass them based on the strength of stimulation.
• Action Potential: Occurs when strong enough stimulation causes the neuron to "fire".
• Ions Involved: Sodium (Na⁺), Potassium (K⁺), Chloride (Cl⁻).

Ionic Movement and Membrane Potential

• Resting State: Sodium is more concentrated outside the neuron, while potassium is more concentrated inside.
• Electrochemical Gradient: Combination of the chemical and electrical gradients across the membrane.
• Membrane Potential: Around -70mV at rest; inside is less positive than outside.

Ion Channels

• Ion Movement: Requires protein channels; passive diffusion along concentration gradient.
• Types of Channels:
  • Voltage-Gated: Open at specific membrane voltages.
  • Ligand-Gated: Open when a specific molecule binds.
  • Mechanically-Gated: Open in response to physical forces.
• Selectivity: Channels allow specific ions (e.g., sodium or potassium channels).

Graded vs. Action Potentials

• Graded Potentials: Small changes, transient, do not typically involve voltage-gated channels.
• Action Potentials: Triggered when membrane potential reaches threshold (e.g., -55mV).

Action Potential Process

1. Depolarization: Sodium channels open, Na⁺ enters, potential becomes positive.
2. Overshoot: Potential goes beyond zero to around +30mV.
3. Repolarization: Potassium channels open, K⁺ exits, potential returns to negative.
4. Hyperpolarization: Potential briefly becomes more negative than resting potential.
5. Restoration: Sodium-potassium pump restores gradients.

Refractory Periods

• Absolute Refractory Period: No new action potential can occur.
• Relative Refractory Period: New action potential possible but needs stronger stimulus.

Factors Affecting Signal Velocity

• Myelin Sheaths: Increase conduction speed.
  • Saltatory Conduction: Action potential "jumps" from node to node (nodes of Ranvier).
  • Schwann Cells: Form myelin in the peripheral nervous system.
  • Oligodendrocytes: Form myelin in the central nervous system.

Summary

• Without stimulus, membrane is at resting potential.
• Small stimulus causes graded potential.
• Above-threshold stimulus triggers action potential; neuron fires.
• Action potential frequency can vary based on stimulus intensity.