Conduction of Action Potentials

Local Current Flow

When a section of an axon depolarizes, positive charges move by local current flow into adjacent sections of the cytoplasm. This process is essential for the propagation of electrical signals along neurons.

• Depolarization: A temporary reversal of membrane potential, making the inside of the cell less negative.

• Local current flow: Movement of positive ions (mainly Na+) into adjacent regions, causing further depolarization.

• Extracellular current: Flows toward the depolarized region, completing the circuit.

• Example: The spread of depolarization along an unmyelinated axon.

Conduction of Action Potentials

Action potentials are conducted along the axon by the sequential opening of voltage-gated Na+ and K+ channels, creating a self-propagating electrical signal.

• Trigger zone: The initial segment where the action potential is generated.

• Propagation: Positive charges move to adjacent regions, depolarizing them to threshold and opening more Na+ channels.

• Unidirectional flow: Due to the refractory period, action potentials move in one direction.

• Example: Nerve impulse transmission from the spinal cord to a muscle.

Speed of Action Potential Conduction

The speed at which action potentials travel depends on axon diameter and myelination.

• Larger neurons: Conduct action potentials faster due to lower internal resistance.

• Myelinated neurons: Conduct faster because myelin increases membrane resistance and prevents ion leakage.

• Saltatory conduction: Action potentials "jump" between nodes of Ranvier, increasing speed.

• Equation: (velocity is proportional to the square root of axon diameter)

• Example: Motor neurons controlling skeletal muscles are large and myelinated for rapid response.

Saltatory Conduction

Saltatory conduction is the process by which action potentials jump from one node of Ranvier to the next in myelinated axons.

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

• Myelin sheath: Insulates the axon, preventing ion leakage and increasing conduction speed.

• Demyelinating diseases: Such as multiple sclerosis, reduce conduction speed and can block signal transmission.

• Example: Rapid reflexes rely on saltatory conduction for speed.

Chemical Factors Affecting Electrical Activity

Certain chemicals and changes in extracellular ion concentrations can alter neuronal excitability and action potential conduction.

• Channel blockers: Toxins or drugs that bind to ion channels and prevent their function.

• Ion concentration changes: Alterations in extracellular Ca2+ and K+ affect membrane potential.

• Hyperkalemia: Increased K+ brings the neuron closer to threshold, increasing excitability.

• Hypokalemia: Decreased K+ moves the neuron further from threshold, reducing excitability.

• Example: Cardiac arrhythmias can result from abnormal K+ levels.

Cell-to-Cell Communication in the Nervous System

Synapses

Neurons communicate with each other at specialized junctions called synapses, which can be electrical or chemical.

• Electrical synapses: Pass electrical signals directly through gap junctions; can be bi-directional and synchronize cell networks.

• Chemical synapses: Use neurotransmitters to cross the synaptic cleft and transmit signals to the next cell.

• Example: Electrical synapses are found in cardiac muscle; chemical synapses are predominant in the CNS.

Integration of Neural Information Transfer

Overview

Neural information transfer involves the integration of signals from multiple sources, allowing complex processing and response.

• Divergence: One neuron branches to communicate with multiple target neurons.

• Convergence: Multiple neurons synapse onto a single neuron, integrating information.

• Synaptic plasticity: The ability of synapses to strengthen or weaken over time, essential for learning and memory.

• Example: Reflex arcs involve both divergence (to activate multiple muscles) and convergence (to integrate sensory input).

Key Terms and Definitions

• Action potential: A rapid, transient change in membrane potential that propagates along the axon.

• Refractory period: The time during which a neuron is unable (absolute) or less able (relative) to fire another action potential.

• Saltatory conduction: The jumping of action potentials between nodes of Ranvier in myelinated axons.

• Chemical synapse: A synapse where neurotransmitters mediate signal transmission.

• Excitatory postsynaptic potential (EPSP): Depolarizing graded potential that brings the neuron closer to threshold.

• Inhibitory postsynaptic potential (IPSP): Hyperpolarizing graded potential that moves the neuron further from threshold.

Additional info: These notes are based on textbook figures and lecture slides from a standard Anatomy & Physiology curriculum, focusing on neural communication and action potential conduction.