Neurophysiology of the Neuron

Overview of Neuron Physiology

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 how the nervous system operates.

• Characteristics of Neurons: Neurons are highly specialized, long-lived cells that require a continuous supply of oxygen and glucose. Most neurons have a cell body and one or more processes.

• Structure of Neurons: Neurons consist of a cell body (soma), dendrites, and an axon. Each region is associated with specific functions: reception (dendrites), propagation (axon), and transmission (axon terminals).

• Principles of Electricity: Neurons use electrical signals generated by the movement of ions across their membranes to communicate.

• Gated Channels: Specialized proteins in the membrane act as ion channels, which can be open or closed depending on various stimuli.

• Resting Membrane Potential (RMP): The difference in electrical charge across the neuron's membrane at rest.

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

• Refractory Periods and Conduction Velocity: These determine how quickly and efficiently neurons can transmit signals.

Structure of Neurons

Neurons have distinct regions that contribute to their function:

• Cell Body (Soma/Perikaryon): Contains the nucleus and is the metabolic center, synthesizing proteins and other essential molecules.

• Dendrites: Short, branched extensions that receive incoming signals and conduct them toward the cell body. They increase the surface area for synaptic input.

• Axon: A long, singular process that conducts impulses away from the cell body. Axons may be myelinated, which increases conduction speed.

• Axon Terminals: Branches at the end of the axon that transmit signals to other neurons or effector cells.

Functional Regions of the Neuron

• Receptive Region: Dendrites and cell body; receive and integrate incoming signals.

• Conducting Region: Axon; propagates action potentials.

• Secretory Region: Axon terminals; release neurotransmitters to communicate with other cells.

Principles of Electricity in Neurons

Neurons generate and conduct electrical signals based on the movement of ions:

• Voltage: The measure of potential energy generated by separated charges, measured in volts (V) or millivolts (mV).

• Current: The flow of electrical charge (ions) between two points.

• Resistance: Hindrance to charge flow; insulators have high resistance, conductors have low resistance.

Membrane Ion Channels

Ion channels are proteins that allow specific ions to pass through the membrane:

• Leakage (Nongated) Channels: Always open, allowing passive movement of ions.

• Gated Channels: Open or close in response to specific stimuli:

  • Chemically Gated: Open with binding of a specific chemical (e.g., neurotransmitter).

  • Voltage-Gated: Open and close in response to changes in membrane potential.

  • Mechanically Gated: Open in response to physical deformation (e.g., pressure).

Resting Membrane Potential (RMP)

The RMP is the electrical potential difference across the plasma membrane when the neuron is not transmitting a signal.

• Typical Value: Approximately -70 mV (inside negative relative to outside).

• Establishment: Due to differences in ion concentrations and permeability:

  • Membrane is more permeable to K+ than Na+.

  • Na+/K+ ATPase Pump: Maintains gradients by pumping 3 Na+ out and 2 K+ in.

Equation for Resting Membrane Potential

The Nernst equation can be used to calculate the equilibrium potential for a particular ion:

Where Eion is the equilibrium potential, R is the gas constant, T is temperature, z is the charge of the ion, and F is Faraday's constant.

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential that occur in response to stimuli opening gated ion channels.

• Depolarization: Membrane potential becomes less negative (closer to zero).

• Hyperpolarization: Membrane potential becomes more negative (further from zero).

• Characteristics: Graded potentials decrease in strength as they spread from the point of origin (decremental).

Action Potentials

Action potentials are rapid, long-distance electrical signals used for neural communication.

• All-or-None Principle: Once threshold is reached, an action potential is generated and propagates without decrement.

• Phases:

  1. Resting State: All gated Na+ and K+ channels are closed.

  2. Depolarization: Voltage-gated Na+ channels open, Na+ influx causes membrane potential to become positive.

  3. Repolarization: Na+ channels inactivate, K+ channels open, K+ efflux restores negative membrane potential.

  4. Hyperpolarization: Some K+ channels remain open, causing membrane potential to dip below resting value.

Action Potential Graph

Typical action potential changes from -70 mV (resting) to +30 mV (peak depolarization), then returns to -70 mV.

Propagation of Action Potentials

Action potentials propagate along the axon by opening voltage-gated Na+ channels in adjacent regions, ensuring unidirectional flow.

• Self-Propagating: Each segment of the axon depolarizes the next.

• Unidirectional: Due to refractory periods, action potentials only move forward.

Refractory Periods

Refractory periods ensure that each action potential is a separate, all-or-none event and enforce one-way transmission.

• Absolute Refractory Period: No new action potential can be generated while Na+ channels are open or inactivated.

• Relative Refractory Period: A stronger-than-usual stimulus can initiate another action potential as K+ channels remain open.

Conduction Velocity

The speed of action potential conduction depends on axon diameter and myelination.

• Continuous Conduction: Occurs in unmyelinated axons; slower.

• Saltatory Conduction: Occurs in myelinated axons; action potentials jump between nodes of Ranvier, increasing speed (up to 30x faster).

Summary Table: Types of Ion Channels

Example: Neuron Signal Transmission

When a sensory neuron detects a stimulus, mechanically gated channels open, generating a graded potential. If the graded potential reaches threshold at the axon hillock, a voltage-gated action potential is triggered and propagates along the axon to the axon terminals, where neurotransmitters are released to communicate with the next cell.

Additional info: These notes expand on the brief points in the original slides, providing definitions, explanations, and a summary table for clarity and completeness.