Neurophysiology of Neuronal Signaling: Local and Action Potentials

Local potentials are graded changes in membrane potential that spread only short distances via passive propagation. They vary in both amplitude and duration and can be either excitatory or inhibitory.

Types of Local Potentials

• Receptor Potentials: These are generated at the peripheral receptors of a sensory neuron when they are exposed to stimuli like stretching, compression, or thermal changes. For example, stretching a muscle can open $\text{Na}^{+}$ or $\text{Ca}^{2+}$ channels, generating a receptor potential.
• Synaptic Potentials: These occur at the postsynaptic membrane when neurotransmitters bind to ligand-gated channels.
  • Excitatory Postsynaptic Potentials (EPSPs): These cause depolarization, making the membrane potential more positive than the resting state.
  • Inhibitory Postsynaptic Potentials (IPSPs): These cause hyperpolarization, making the membrane potential more negative.

Summation

Because local potentials are small, they must often be integrated to reach the threshold required for an action potential. This occurs through:

• Temporal Summation: A single presynaptic neuron fires a series of small potentials in rapid succession (within milliseconds).
• Spatial Summation: Multiple different presynaptic neurons fire simultaneously at different locations on the same postsynaptic neuron.

Action Potentials: The "All-or-None" Signal

While local potentials are passive and short-lived, the action potential is an active, large-scale depolarization that is repeatedly regenerated to transmit information over long distances. An action potential is "all-or-none," meaning that once a stimulus reaches the threshold potential (approximately $-55\text{ mV}$), an action potential will fire with a constant amplitude regardless of the stimulus strength.

The phases of an action potential include:

1. Depolarization: Upon reaching threshold, voltage-gated $\text{Na}^{+}$ channels open rapidly, allowing an influx of $\text{Na}^{+}$ that makes the interior of the cell positive.
2. Repolarization: $\text{Na}^{+}$ channels inactivate, and voltage-gated $\text{K}^{+}$ channels open, allowing $\text{K}^{+}$ to exit the cell and restore a negative charge.
3. Hyperpolarization: $\text{K}^{+}$ channels often remain open longer than necessary, causing the membrane potential to drop below the normal resting level.

Refractory Periods and Propagation

Following an action potential, the neuron enters a refractory period. During the absolute refractory period, it is impossible to trigger another action potential because the $\text{Na}^{+}$ channels are inactivated. During the relative refractory period, a second action potential can be triggered, but it requires a much stronger stimulus because the cell is hyperpolarized.

To ensure rapid communication, neurons use structural adaptations:

• Myelination: Glial cells (Oligodendrocytes in the CNS and Schwann cells in the PNS) wrap axons in insulating myelin.
• Saltatory Conduction: Instead of continuous depolarization, the signal "jumps" between the nodes of Ranvier (gaps in the myelin), significantly increasing conduction velocity.

Neural Communication: Neurotransmitters and Neuromodulators

Information is passed between neurons at the synapse, which consists of the presynaptic terminal, the synaptic cleft, and the postsynaptic terminal.

Neurotransmitters (Fast-Acting)

Neurotransmitters carry chemical signals across the synapse to induce rapid changes in membrane potential.

• Ionotropic (Fast-acting): These bind directly to ligand-gated ion channels. The receptor and the channel are a single functional unit, allowing for millisecond-scale responses.
  • Glutamate: The primary fast excitatory neurotransmitter in the CNS.
  • GABA: The major inhibitory neurotransmitter in the CNS.
  • Acetylcholine (ACh): Essential for muscle contraction at the neuromuscular junction.

Neuromodulators (Slow-Acting)

Neuromodulators are a subset of chemicals released into extra-synaptic sites (outside the synapse). They do not cause rapid signal transmission but instead alter the properties of ion channels to adjust the activity of many neurons simultaneously.

• Metabotropic (Slow-acting): These bind to G-protein coupled receptors (GPCRs), which activate a second messenger system. This can lead to long-term changes, such as altering gene expression or increasing the number of receptors on a dendrite.
• Examples: Dopamine (linked to Parkinson's), Serotonin (mood regulation), and Endorphins (pain modulation).

💡 Exam Tip: Agonists vs. Antagonists

• Agonists (e.g., Levodopa for Parkinson's) mimic the effects of naturally occurring neurotransmitters by binding to and activating receptors.
• Antagonists (e.g., Botulinum Toxin) impede the effects of neurotransmitters by either blocking the release of the chemical or preventing it from binding to the receptor.

Clinical Correlations

Demyelinating Diseases

When the insulating myelin is destroyed, signal conduction is disrupted, leading to weakness, loss of coordination, and sensory deficits.

• Guillain-Barré Syndrome: An autoimmune attack on the myelin of the Peripheral Nervous System (PNS), often following an infection.
• Multiple Sclerosis (MS): An autoimmune attack on the oligodendrocytes in the Central Nervous System (CNS), causing plaques in the white matter.

Neuromuscular Junction Disorders

• Lambert-Eaton Myasthenic Syndrome (LEMS): Antibodies attack the voltage-gated $\text{Ca}^{2+}$ channels in the presynaptic terminal, reducing the release of Acetylcholine.
• Myasthenia Gravis (MG): Antibodies attack and destroy the nicotinic ACh receptors on the postsynaptic muscle membrane, leading to muscle weakness that worsens with repetitive use.

🔍 Clinical Case Study: Myasthenia Gravis

A patient presents with ptosis (drooping eyelids) that worsens in the evening and improves with rest. This is a hallmark of Myasthenia Gravis because repetitive use of the muscle depletes the available receptors, leading to increased weakness as the day progresses.