Neurophysiology: Structure and Function of Neurons

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 grasping 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 in nerve impulse transmission.

• 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 regulate ion flow, crucial for generating electrical signals.

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

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

• Propagation: The movement of the action potential along the neuron.

• Refractory Periods and Conduction Velocity: Mechanisms that ensure unidirectional and efficient transmission of nerve impulses.

Structure of Neurons

Functional Regions of the Neuron

Neurons are composed of distinct regions, each with specialized roles in the reception, propagation, and transmission of nerve impulses.

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

• Dendrites: Short, branched extensions that serve as the receptive (input) region, conducting graded potentials toward the cell body.

• Axon: The conducting region, typically a single long process that transmits action potentials away from the cell body to other neurons or effectors.

• Axon Terminals: The secretory region, where neurotransmitters are released to communicate with other cells.

Example: Sensory neurons have long dendrites to receive signals from sensory organs, while motor neurons have long axons to transmit signals to muscles.

Principles of Electricity in Neurons

Electrical Properties and Ion Movement

Neurons generate and transmit electrical signals through the movement of ions across their membranes, governed by basic principles of electricity.

• Voltage: The measure of potential energy generated by the separation of charged ions, expressed in volts (V) or millivolts (mV).

• Current: The flow of electrical charge (ions) between two points, dependent on voltage and resistance.

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

Equation:

$I = \frac{V}{R}$

Where $I$ is current, $V$ is voltage, and $R$ is resistance.

Membrane Ion Channels

Types and Functions of Ion Channels

Ion channels are membrane proteins that regulate the movement of specific ions, crucial for neuronal electrical activity.

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

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

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

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

  • Mechanically Gated Channels: Respond to physical deformation (e.g., pressure).

Example: Voltage-gated sodium channels are essential for the initiation and propagation of action potentials.

Resting Membrane Potential (RMP)

Establishment and Maintenance of RMP

The resting membrane potential is the electrical charge difference across the neuron's membrane when the cell is not actively transmitting a signal.

• Selective Permeability: The membrane is more permeable to potassium (K+) than sodium (Na+), leading to a net negative charge inside the cell.

• Sodium-Potassium Pump (Na+/K+ ATPase): Maintains concentration gradients by pumping three Na+ out and two K+ in.

Equation:

$\text{Na}^+/\text{K}^+ \text{ ATPase}: 3\ \text{Na}^+_{\text{out}} : 2\ \text{K}^+_{\text{in}}$

Key Points:

• Inside of the neuron is negative relative to the outside.

• Cytoplasm contains more K+ and less Na+ than extracellular fluid (ECF).

• Charge separation exists at the plasma membrane.

Changes in Membrane Potential

Depolarization, Hyperpolarization, and Signal Generation

Neurons change their membrane potential to generate signals, which can be either graded or action potentials.

• Depolarization: Decrease in membrane potential (less negative), increasing the probability of impulse generation.

• Hyperpolarization: Increase in membrane potential (more negative), decreasing the probability of impulse generation.

• Graded Potentials: Localized, short-distance changes in membrane potential, varying in strength.

• Action Potentials: Long-distance, all-or-none electrical signals that do not decrease in amplitude with distance.

Action Potential Generation and Propagation

Phases and Mechanisms of Action Potentials

Action potentials are rapid, transient changes in membrane potential that propagate along the axon, enabling neural communication.

1. Resting State: All gated Na+ and K+ channels are closed; only leakage channels are open.

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 level.

Threshold: The critical level of depolarization (typically -55 mV) required to trigger an action potential.

Equation:

$\text{Threshold} \approx -55\ \text{mV}$

Propagation of Action Potentials

Mechanisms and Directionality

Action potentials propagate along the axon by sequential opening of voltage-gated channels, ensuring unidirectional signal transmission.

• Local currents depolarize adjacent membrane areas, triggering new action potentials.

• Propagation is self-sustaining and moves only in the forward direction due to refractory periods.

Refractory Periods

Absolute and Relative Refractory Periods

Refractory periods ensure that each action potential is a separate, all-or-none event and prevent backward propagation.

• 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 trigger an action potential as K+ channels remain open.

Conduction Velocity

Factors Affecting Speed of Nerve Impulse Transmission

The speed at which action potentials travel along axons depends on axon diameter and the presence of myelin.

• Continuous Conduction: Occurs in unmyelinated axons; slower due to sequential opening of channels along the entire axon.

• Saltatory Conduction: Occurs in myelinated axons; action potentials jump between nodes of Ranvier, increasing speed up to 30 times faster than continuous conduction.

Summary Table: Types of Ion Channels

Channel Type	Stimulus	Function
Leakage (Nongated)	None (always open)	Maintains resting membrane potential
Chemically Gated	Binding of neurotransmitter/hormone	Initiates graded potentials
Voltage-Gated	Change in membrane potential	Generates and propagates action potentials
Mechanically Gated	Physical deformation (pressure, stretch)	Detects sensory stimuli

Additional info: These notes expand on the brief points in the original slides, providing definitions, examples, and equations for clarity and completeness.