Neurons and Neural Transmission: Resting Potential, Action Potential, and Myelination

Neurons & Neural Transmission

Introduction to Neurons

Neurons are the fundamental units of the nervous system responsible for receiving, processing, and transmitting information through electrical and chemical signals. Understanding their structure and function is essential for comprehending neural communication.

• Dendrites: Branch-like extensions that receive signals from other neurons.

• Cell Body (Soma): Contains the nucleus and integrates incoming signals.

• Axon: Long projection that transmits electrical impulses away from the cell body.

• Axon Hillock: The region where action potentials are initiated.

• Myelin Sheath: Insulating layer that speeds up signal transmission.

• Synaptic Terminals: Release neurotransmitters to communicate with other cells.

Neural Communication

Neurons communicate via two main processes: electrical signaling within the cell and chemical signaling between cells.

1. Action Potential: An electrical signal that travels along the axon of a neuron.

2. Synapse: The junction between two neurons where neurotransmitters are released to transmit signals to the next cell.

Resting & Action Potentials

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the neuronal membrane when the cell is not actively transmitting a signal. This potential is typically around -70mV (some sources may state -60mV).

• Definition: The difference in charge between the inside and outside of the cell.

• Typical Values:

  • Outside the cell (extracellular): 110mV

  • Inside the cell (intracellular): 40mV

• Formula: $V_{m} = V_{inside} - V_{outside}$ Example: If $V_{inside} = 60mV$ and $V_{outside} = 80mV$, then $V_{m} = 60mV - 80mV = -20mV$

What Causes the Resting Potential?

• Concentration Gradient: Ions move from areas of high concentration to low concentration.

• Semi-Permeable Membrane: The cell membrane allows selective movement of ions via channels.

• Key Ions: Sodium (Na+), Potassium (K+), Chloride (Cl-).

• Impermeable Membrane: Some ions cannot cross the membrane freely, maintaining the potential.

Action Potential

Generation and Phases of Action Potential

An action potential is a rapid, temporary change in the membrane potential that travels along the axon. It is an all-or-none response, meaning it either occurs fully or not at all.

• Threshold: The minimum level of depolarization required to trigger an action potential.

• Depolarization: Sodium channels open, Na+ rushes in, making the inside of the cell more positive.

• Repolarization: Potassium channels open, K+ exits the cell, restoring the negative potential.

• Hyperpolarization (Undershoot): The membrane potential temporarily becomes more negative than the resting potential.

• Refractory Period: A brief period during which another action potential cannot be initiated.

Action Potential Graph

• Resting potential: -70mV

• Threshold: typically around -55mV

• Peak: +50mV

• Return to resting potential after the refractory period

Key Terms and Concepts

• All-or-None Response: Once the threshold is reached, the action potential fires completely.

• Refractory Period: Ensures unidirectional propagation of the action potential.

• Hyperpolarization: Occurs during the undershoot phase.

Axonal Transmission and Myelination

Role of Myelination

Myelination is the process by which axons are coated with a fatty substance called myelin, produced by glial cells (Schwann cells in the peripheral nervous system, oligodendrocytes in the central nervous system). Myelin increases the speed and efficiency of electrical signal transmission.

• Saltatory Conduction: In myelinated axons, action potentials 'jump' from one node of Ranvier to the next, greatly increasing conduction speed.

• Unmyelinated Axons: Action potentials travel continuously along the axon, resulting in slower transmission.

• Nodes of Ranvier: Gaps in the myelin sheath where ion exchange occurs, enabling saltatory conduction.

Comparison Table: Myelinated vs Unmyelinated Axonal Transmission

Feature	Myelinated Axon	Unmyelinated Axon
Conduction Speed	Fast (saltatory conduction)	Slow (continuous conduction)
Energy Efficiency	High	Low
Action Potential Propagation	Jumps between nodes	Moves along entire axon
Susceptibility to Signal Loss	Low	Higher

Clinical Relevance

• Multiple Sclerosis: A disease where myelin is damaged, leading to impaired neural transmission.

Summary of Key Points

• Neurons transmit signals via action potentials and synapses.

• Resting membrane potential is maintained by ion gradients and selective permeability.

• Action potentials are all-or-none responses that propagate along axons.

• Myelination increases the speed and efficiency of neural transmission.

Additional info: Some values and terms were clarified and expanded for academic completeness. The table was inferred from the comparison of myelinated and unmyelinated axons in the notes and images.