Neuronal Signal Transmission

Mechanisms of Signal Movement Along the Neuron

Neurons transmit signals along their plasma membrane by two primary mechanisms, each with distinct properties and physiological roles.

• Graded, Electrotonically Conducted Potentials: These are local changes in membrane potential that spread passively, similar to how electrical current moves through a wire. They decrease in amplitude with distance from the point of origin.

• Action Potentials: These are all-or-none impulses that propagate without decrement along the axon, enabling long-distance communication within the nervous system.

Key Terms: Graded potential, action potential, axon, plasma membrane

Action Potential Propagation

Steps in Action Potential Propagation

The action potential is a rapid, transient change in membrane potential that travels along the axon. Its propagation involves several key steps:

1. Na+ Influx: The opening of voltage-gated Na+ channels allows sodium ions to enter the neuron, causing local depolarization and the spread of positive charge.

2. Positive Feedback: Depolarization triggers more Na+ channels to open, amplifying the response.

3. Unidirectional Propagation: The action potential does not travel backward due to the refractory period of the channels.

4. Factors Enhancing Speed: Larger axon diameter, myelination, and increased temperature all increase the speed of action potential propagation.

Example: In myelinated axons, action potentials jump between nodes of Ranvier, a process called saltatory conduction.

Membrane Properties of the Axon and Cell Body

Trans-Membrane Resistance and Capacitance

The efficiency of signal transmission in neurons depends on two key membrane properties:

• Trans-membrane resistance: This describes how easily charge leaks across the membrane. High resistance means less leakage and allows charge to travel farther within the neuron.

• Membrane capacitance: The ability of the membrane to store charge. Lower capacitance (greater distance between charges) allows for faster changes in membrane potential and more efficient signal propagation.

Key Point: High resistance and low capacitance favor rapid and long-distance signal transmission.

Length Constant (λ)

The length constant () quantifies how far a graded potential can travel before decaying to 37% of its original value. It is determined by membrane resistance and internal resistance:

• Formula: where is membrane resistance and is internal (axoplasmic) resistance.

• Application: Larger means signals travel farther along the axon.

Example: In the diagram, current injection at one point of the axon shows how potential decays with distance, illustrating the concept of the length constant.

Factors Affecting Action Potential Velocity

Axon Diameter, Myelination, and Temperature

The speed at which action potentials travel along an axon is influenced by several factors:

• Axon Diameter: Larger diameter reduces internal resistance, allowing local potentials to travel farther and faster.

• Myelination: Myelin sheaths (formed by Schwann cells in the PNS and oligodendrocytes in the CNS) increase membrane resistance and decrease capacitance, enabling saltatory conduction and rapid signal transmission.

• Temperature: Higher temperature increases conduction velocity; a 10°C increase can double the speed.

Example: The velocity of nerve impulse conduction increases with axon diameter in both myelinated and unmyelinated axons, as shown in comparative graphs of different animal species.

Saltatory Conduction

In myelinated axons, action potentials "jump" from one node of Ranvier to the next, greatly increasing conduction speed compared to unmyelinated axons.

• Nodes of Ranvier: Gaps in the myelin sheath where voltage-gated channels are concentrated.

• Effect: Myelination increases trans-membrane resistance and decreases capacitance, reducing current leakage and allowing rapid signal transmission.

Comparative Table: Myelinated vs. Unmyelinated Axons

Summary

• Neurons transmit signals via graded potentials and action potentials.

• Action potential propagation depends on Na+ influx, positive feedback, and membrane properties.

• Membrane resistance and capacitance are critical for efficient signal transmission.

• Axon diameter, myelination, and temperature all affect conduction velocity.

• Saltatory conduction in myelinated axons enables rapid nerve impulse transmission.