Nerve Impulse Conduction Theories

Electrical Excitability of Nerve Impulse Conduction

• The lecture aims to explain how the resting nerve fiber membrane potential is raised to threshold, describe extracellular recordings, demonstrate local circuit theories of nerve impulse propagation, apply the effect of nerve fiber diameter and myelin on nerve conduction, and distinguish clinical applications like demyelination.

Electrical Stimulation

• Electrical stimulation occurs under a cathode (negative charge), reducing excitability.
• Conversely, it decreases under an anode (positive charge).
• This process can initiate an action potential in an axon or group of axons by reaching threshold.

Extracellular Recording

• Extracellular recordings of action potentials provide information about conduction velocity under various conditions.
• Diphasic recording shows a biphasic waveform.
• Monophasic recording displays a single waveform shape, revealing differences between intact and damaged axons.

Conduction Velocity

• Conduction velocity is calculated by dividing the distance between stimulating and recording electrodes by the time gap between stimulus and action potential recording.
• Velocity = Distance/Time

Action Potential Conduction

• Changes in membrane potential in one axon part spread to adjacent areas due to local currents.
• Conduction velocity depends on how far local currents spread along the axon.
• Depolarization of an axon section to threshold initiates an action potential at that location.

Length Constant

• The length constant (λ) measures the distance a potential change travels to reach 37% of its original value.

Factors Affecting Conduction Velocity

• High membrane resistance and low membrane capacitance enhance conduction velocity by enabling local currents to propagate further.
• High axonal diameter decreases cytoplasmic resistance, improving conduction velocity.

Capacitance

• Capacitance (C) is the ability of the lipid bilayer to store charge.
• High capacitance results in needing more current for charging or longer charging time, affecting local current spread, especially with short current pulses.

Membrane Resistance

• Membrane resistance is influenced by the number of open ion channels.
• Lower resistance correlates with more open ion channels and more local current loss across the membrane, reducing current spread.

Action Potential Propagation

• Local currents propagate the action potential down the axon.
• An axon area that has just fired an action potential is refractory, so it cannot fire another until recovery.

Refractory Period

• The refractory period prevents backward propagation of action potentials.

Myelin Sheath

• Myelin significantly increases conduction velocity in axons.
• Larger-diameter axons (e.g., motoneurons) are myelinated, while smaller axons (e.g., sensory fibers) are often not.
• Myelin reduces capacitance and increases axon resistance.

Schwann Cells and Oligodendrocytes

• Schwann cells myelinate peripheral axons, while oligodendrocytes myelinate axons in the central nervous system.

Fiber Diameter and Velocity Relationship

• Conduction velocity is proportional to fiber diameter in myelinated axons.
• The highest velocity in mammals is achieved by large-diameter myelinated axons.
• In unmyelinated axons, conduction velocity correlates with the square root of fiber diameter.

Demyelination

• Demyelination is a condition where sections of axons lose their myelin sheath.
• Multiple sclerosis is a prominent demyelinating disease.
• Damage to myelin can dramatically reduce the ability of axons to conduct action potentials, influencing and sometimes blocking conduction velocity.