The Mechanics of Neuron Firing and Nerve Impulses

Introduction

• Scenario: Playing catch—how the brain helps you catch a ball
• Key Question: How does information quickly flow from the eyes to the brain and back to the hand?
• Mechanism: Neurons convert environmental information into electrical signals (action potentials or nerve impulses)

Neuron Structure and Function

• Neurons: Tens of billions in the human brain
• Components:
  • Dendrites: Receive signals
  • Cell body: Integrates signals
  • Axon: Transmits signals to the next cell
• Analogy: Axon is like a wire carrying electricity from a switch to a bulb

Ionic Environment

• External: Salty solution with positive sodium (Na⁺) and negative chloride (Cl⁻) ions
• Internal: Positive potassium (K⁺) ions and negatively charged proteins
• Electrical Gradient: Inside is slightly more negative than outside; resting potential ~ -70 millivolts

Ion Channels and Voltage Changes

• Ion Channels: Protein pores that selectively allow ions to pass
  • Voltage-gated ion channels: Open/close at specific voltages
• Graded Potential: Small, insignificant voltage fluctuations
• Threshold Potential: Neuron fires when voltage reaches -55 millivolts

Action Potentials

• Depolarization: Sodium channels open, sodium rushes in, inside becomes positive (+30 millivolts)
• Repolarization: Potassium channels open, potassium exits, voltage drops
• Hyperpolarization: Voltage dips below resting potential due to slow potassium channel closure
• Reset (Resting Potential): Sodium-potassium pumps restore resting potential using ATP

Characteristics of Nerve Impulses

1. All-or-Nothing: Binary nature; occurs once threshold is crossed
2. Unidirectional: Signal only moves forward due to refractory period

Enhancing Signal Speed

• Myelin Sheath: Coating that speeds up signaling
• Nodes of Ranvier: Gaps in myelin sheath where signals jump—saltatory conduction

Sensory Responses and Reflexes

• Sense Organs: Respond to stimuli (e.g., light in the retina, pain from stepping on a pin)
• Brain: Interprets nerve impulses and sends commands
  • Example: Reflex action in the spinal cord to quickly pull away from a pin

Interpretative Role of the Brain

• Pattern Recognition: Detects patterns, frequency, and origin of impulses
• Digital Language: Nerve impulses are the brain's 'digital language' for communication