Chapter 4: Neural Conduction and Synaptic Transmission

I. Resting Membrane Potential

• Definition: Electrical charge difference across a neuron's membrane when at rest (~ -70mV).

• Maintained by:

  • Sodium-Potassium Pump (Na⁺/K⁺ pump) → Moves 3 Na⁺ out, 2 K⁺ in.

  • Selective Ion Channels → Allow movement of ions like Na⁺, K⁺, and Cl⁻.

• Modern Example: Like a smartphone on standby mode, maintaining energy but ready to activate instantly.

II. Action Potential: The Neural Signal

• All-or-none Principle: If the threshold (-55mV) is reached, an action potential occurs.

• Phases:

  • Depolarization: Na⁺ channels open → Na⁺ rushes in → Membrane becomes positive.

  • Repolarization: K⁺ channels open → K⁺ exits → Restores negativity.

  • Hyperpolarization: Neuron temporarily more negative than resting potential.

• Propagation:

  • Unmyelinated Axons: Slower signal transmission.

  • Myelinated Axons: Uses saltatory conduction (jumps between Nodes of Ranvier) for faster transmission.

• Modern Example: Like sending a text message; once sent, it cannot be taken back, and it travels instantly if connected to high-speed Wi-Fi (myelinated) or slower if on low bandwidth (unmyelinated).

III. Synaptic Transmission: Neuron Communication

• Synapse Structure:

  1. Presynaptic Neuron: Sends signal.

  2. Synaptic Cleft: Space between neurons.

  3. Postsynaptic Neuron: Receives signal.

• Steps:

  1. Action potential reaches terminal → Opens Ca²⁺ channels.

  2. Ca²⁺ influx triggers vesicle fusion → Neurotransmitter release.

  3. Neurotransmitters bind to receptors on the postsynaptic membrane.

  4. Excitatory (EPSP) or Inhibitory (IPSP) response is generated.

• Modern Example: Like pressing ‘send’ on an email, neurotransmitters carry the message to the recipient, who then decides how to respond.

IV. Major Neurotransmitters & Receptors

• Neurotransmitters:

  • Glutamate → Main excitatory transmitter.

  • GABA → Main inhibitory transmitter.

  • Dopamine → Reward, motivation, movement.

  • Serotonin → Mood, sleep, appetite.

  • Acetylcholine → Muscle control, learning.

• Receptor Types:

  • Ionotropic Receptors: Fast, direct ion flow (e.g., NMDA for glutamate).

  • Metabotropic Receptors: Slower, uses G-proteins for longer effects.

• Modern Example: Dopamine is like social media likes—each one gives a small reward, reinforcing the desire to keep posting.

V. Neurotransmitter Termination

• Reuptake: Transporters take neurotransmitters back into the presynaptic neurons.

• Enzymatic Degradation: Enzymes break down neurotransmitters (e.g., acetylcholinesterase for acetylcholine).

• Diffusion: Neurotransmitters drift away from the synapse.

• Modern Example: Similar to unread emails getting archived or deleted after some time.

VI. Drugs and Synaptic Transmission

• Drugs modify neurotransmission by:

  • Agonists: Mimic neurotransmitters (e.g., heroin mimics endorphins).

  • Antagonists: Block receptors (e.g., naloxone blocks opioid receptors).

  • Reuptake Inhibitors: Increase neurotransmitter levels (e.g., SSRIs for serotonin).

• Modern Example: Caffeine acts as an adenosine antagonist, blocking sleep signals and keeping you awake like a ‘Do Not Disturb’ mode on your phone.

Conclusion

• Neural conduction and synaptic transmission are fundamental for brain function.

• Understanding these mechanisms helps explain behavior, drug effects, and neurological disorders.

• Modern Example: Our brain functions like a high-speed internet network, where efficient communication between neurons is essential for quick decision-making and response to stimuli.