2B) Neurophysiology: Structure and Function of Neurons

Neurophysiology of the Neuron

Overview of Neuron Physiology

The study of neuron physiology focuses on understanding how nerve cells (neurons) generate, propagate, and transmit electrical signals. Neurons are the fundamental units of the nervous system, responsible for communication within the body.

• Characteristics of Neurons: Neurons are highly specialized, long-lived cells that require a continuous supply of oxygen and glucose. They are generally amitotic (do not divide after maturity) and have a high metabolic rate.

• Structure of Neurons: Each neuron consists of a cell body (soma or perikaryon) and one or more processes (dendrites and axons).

Structure of Neurons

• Cell Body (Soma): Contains the nucleus and is the metabolic center of the neuron. It synthesizes proteins and other essential molecules. The cell body contains abundant rough endoplasmic reticulum (Nissl bodies), Golgi apparatus, and mitochondria.

• Dendrites: Short, branched extensions that serve as the receptive (input) region. They receive signals from other neurons and conduct impulses toward the cell body as graded potentials.

• Axon: The conducting region of the neuron, arising from the axon hillock. Each neuron has a single axon, which may branch at its end into terminal branches (telodendria) ending in axon terminals. Axons transmit electrical impulses (action potentials) away from the cell body to other neurons or effector cells.

• Myelin Sheath: A fatty, insulating layer composed of myelin that surrounds some axons. It increases the speed of nerve impulse transmission and protects the axon.

Example: Sensory neurons in the skin have long axons that transmit touch information to the spinal cord.

Principles of Electricity in Neurons

Basic Electrical Concepts

Neurons use electrical signals to communicate. These signals are based on the movement of ions across the plasma membrane, creating changes in membrane potential.

• Voltage (Potential Difference): The measure of potential energy generated by the separation of opposite charges. In neurons, this is measured across the plasma membrane in millivolts (mV).

• Current: The flow of electrical charge (ions) between two points. In neurons, current is carried by ions such as Na+ and K+.

• Resistance: The hindrance to charge flow. The plasma membrane acts as a resistor, controlling ion movement.

• Insulator: A substance with high electrical resistance (e.g., myelin).

• Conductor: A substance with low electrical resistance (e.g., cytoplasm).

Membrane Ion Channels

• Leakage (Nongated) Channels: Always open, allowing ions to move down their concentration gradients.

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

  • 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: Open in response to physical deformation (e.g., touch receptors).

Resting Membrane Potential (RMP)

Establishing the RMP

The resting membrane potential is the voltage difference across the plasma membrane of a resting neuron, typically around -70 mV (inside negative relative to outside).

• Selective Permeability: The membrane is more permeable to K+ than Na+ due to more K+ leakage channels.

• Ion Distribution: The cytoplasm contains more K+ and less Na+ than the extracellular fluid (ECF).

• Sodium-Potassium Pump (Na+/K+ ATPase): Maintains concentration gradients by pumping 3 Na+ out and 2 K+ into the cell.

Equation:

$V_m = 61.5 \, \log \left( \frac{[K^+]_{out} + \alpha [Na^+]_{out}}{[K^+]_{in} + \alpha [Na^+]_{in}} \right)$

Where $V_m$ is the membrane potential, and $\alpha$ is the relative permeability factor.

Key Properties of Resting Neurons

• The inside of the neuron is negative relative to the outside.

• A charge separation exists at the plasma membrane.

• The electrochemical gradient for Na+ is greater than for K+.

• The membrane is more permeable to K+ than to Na+.

Changes in Membrane Potential

Types of Changes

• Depolarization: Decrease in membrane potential (inside becomes less negative). Increases the probability of producing an impulse.

• Hyperpolarization: Increase in membrane potential (inside becomes more negative). Decreases the probability of producing an impulse.

Types of Signals

• Graded Potentials: Short-distance, localized changes in membrane potential. Their magnitude varies with stimulus strength and they decay with distance.

• Action Potentials: Long-distance signals of axons. They do not decay with distance and are all-or-none events.

Graded Potentials

Properties and Types

• Occur in dendrites and cell bodies.

• Triggered by a stimulus that opens gated ion channels.

• Can be depolarizing or hyperpolarizing.

• Types include:

  • Receptor (Generator) Potential: In sensory neurons.

  • Postsynaptic Potential: In response to neurotransmitter binding at synapses.

Action Potentials (AP)

Generation and Propagation

Action potentials are rapid, large changes in membrane potential that travel along axons. They are the principal means of long-distance neural communication.

• Only axons (and muscle cells) can generate action potentials.

• Also called nerve impulses.

• Do not decay with distance.

Phases of the Action Potential

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

2. Depolarization: Voltage-gated Na+ channels open, Na+ influx causes the inside to become less negative. If threshold is reached (about -55 mV), more Na+ channels open in a positive feedback loop, leading to a spike up to +30 mV.

3. Repolarization: Na+ channels inactivate, K+ channels open, K+ efflux restores negativity.

4. Hyperpolarization: Some K+ channels remain open, causing the membrane potential to become more negative than resting.

Propagation of the Action Potential

• APs are self-propagating and travel in one direction along the axon.

• Local currents depolarize adjacent membrane areas, opening voltage-gated Na+ channels in those regions.

• Channels behind the AP are inactivated, ensuring unidirectional propagation.

Stimulus Intensity and Refractory Periods

Encoding Stimulus Intensity

• All action potentials are alike; intensity is encoded by frequency (number per second).

• Stronger stimuli produce higher frequency of APs.

Refractory Periods

• Absolute Refractory Period: Time during which no new AP can be generated (Na+ channels are open or inactivated).

• Relative Refractory Period: Follows absolute period; a stronger-than-usual stimulus can trigger another AP (some K+ channels still open).

Conduction Velocity

Factors Affecting Conduction Speed

• Axon Diameter: Larger diameter fibers conduct faster due to lower resistance.

• Myelination: Myelinated axons conduct impulses much faster via saltatory conduction (APs jump between nodes of Ranvier) compared to continuous conduction in unmyelinated axons.

Example: Motor neurons controlling skeletal muscles are heavily myelinated for rapid response.

Summary Table: Key Properties of Neurons

Property	Description
Resting Membrane Potential	~ -70 mV (inside negative)
Main Ions Involved	Na+, K+, Cl-
Signal Types	Graded potentials (short distance), Action potentials (long distance)
Conduction Speed	Faster in large, myelinated axons
Refractory Periods	Absolute and relative, ensuring unidirectional AP propagation

Additional info: These notes are based on introductory slides for a college-level Anatomy & Physiology course, focusing on the cellular physiology of neurons and the principles underlying nerve impulse transmission.