Nerve Impulse Transmission

1. Overview of Neural Communication

• Fundamental mechanism for information transfer in the nervous system.
• Involves electrical and chemical signals.

2. Resting Membrane Potential

• Definition: The electrical potential difference across the neuron membrane at rest (~ -70 mV).
• Maintenance: Primarily by the sodium-potassium pump (Na⁺/K⁺-ATPase), pumping 3 Na⁺ out and 2 K⁺ in.
• Ion Distribution: High K⁺ inside, high Na⁺ outside.

3. Action Potential Generation

• Threshold Potential: Critical membrane potential (~ -55 mV) that must be reached to trigger an action potential.
• All-or-Nothing Principle: Action potentials either occur completely or not at all; no gradation in amplitude.

4. Phases of Action Potential

• Depolarization: Rapid influx of Na⁺ ions through voltage-gated Na⁺ channels, making the inside positive (~ +30 mV).
• Repolarization: K⁺ channels open, K⁺ ions flow out, returning the membrane potential towards resting level.
• Hyperpolarization: Brief period where membrane potential becomes more negative than resting potential due to delayed K⁺ channel closure.

5. Refractory Periods

• Absolute Refractory Period: No stimulus can trigger another action potential (Na⁺ channels inactivated).
• Relative Refractory Period: A stronger-than-normal stimulus is required to trigger an action potential.

6. Roles of Na⁺ and K⁺

• Sodium (Na⁺): Primarily responsible for depolarization (influx).
• Potassium (K⁺): Primarily responsible for repolarization and maintaining resting potential (efflux).

7. Saltatory Conduction

• Occurs in myelinated axons.
• Action potential "jumps" from one Node of Ranvier to the next.
• Speed Advantage: Much faster (1-120 m/s) and more energy-efficient than conduction in unmyelinated axons (0.5-2 m/s).

8. Synaptic Transmission

• Synapse: Junction between two neurons.
• Chemical Synapse Process:
  1. Action potential arrives at presynaptic terminal.
  2. Ca²⁺ influx triggers neurotransmitter release from vesicles into the synaptic cleft.
  3. Neurotransmitters bind to receptors on the postsynaptic membrane.
  4. Ion channels open, generating a new potential (EPSP or IPSP) in the postsynaptic neuron.

9. Neurotransmitter Receptors

• Ionotropic Receptors: Ligand-gated ion channels; fast response.
• Metabotropic Receptors: G-protein coupled; slower, longer-lasting response via intracellular signaling.

10. Postsynaptic Potentials

• Excitatory Postsynaptic Potentials (EPSPs): Depolarization, increases likelihood of action potential.
• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarization or stabilization, decreases likelihood of action potential.
• Summation: Temporal (from single synapse) and Spatial (from multiple synapses) integration of EPSPs/IPSPs.

11. Neurotransmitter Removal

• Mechanisms:
  1. Enzymatic degradation: (e.g., Acetylcholinesterase).
  2. Reuptake: By transporter proteins.
  3. Diffusion: Away from the synaptic cleft.

12. Factors Affecting Synaptic Transmission

• Presynaptic Factors: Ca²⁺ concentration, vesicle availability, previous activity.
• Postsynaptic Factors: Receptor density, receptor sensitivity, membrane potential.
• Environmental Factors: Temperature, pH, ion concentrations.

13. Clinical Significance

• Neurological Disorders: Multiple Sclerosis (demyelination), Myasthenia Gravis (AChR attack), Epilepsy.
• Pharmacological Targets: Local anesthetics (Na⁺ channel blockers), Antidepressants (reuptake inhibitors).

14. Key Numerical Values