Nervous System: Structure, Function, and Action Potentials

Nervous System

Overview

The nervous system is a complex network of cells responsible for transmitting signals throughout the body. It is essential for coordinating bodily functions, responding to stimuli, and maintaining homeostasis.

• Key cell types: Neurons and glial cells

• Associated tissues: Nervous tissue, including gray and white matter

• Main function: Rapid communication via electrical and chemical signals

Neuron Structure and Function

Neuron Anatomy

Neurons are specialized cells that transmit nerve impulses. Their structure enables efficient signal transmission.

• Soma: The cell body, containing the nucleus and organelles

• Axon: Long projection that carries impulses away from the soma

• Dendrites: Branch-like structures that receive signals from other neurons

• Axon terminal: End of the axon where neurotransmitters are released

Neuronal Signaling

Types of Signals

Neurons communicate using both electrical and chemical signals.

• Electrical messages: Changes in cell membrane charge due to movement of ions

• Chemical messages: Release of neurotransmitters into the synaptic cleft

Example: The transmission of a signal from one neuron to another at a synapse involves both electrical changes and chemical neurotransmitter release.

Membrane Potential and Ion Gradients

Resting Membrane Potential

The resting membrane potential is the voltage difference across the neuronal membrane when the cell is not transmitting a signal.

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

• Key ions: Sodium (Na+), Potassium (K+), Chloride (Cl-)

• Gradient establishment: Na+ and K+ ions are distributed unequally across the membrane, maintained by the sodium-potassium pump

Formula:

$E_{K^+} = \frac{RT}{zF} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Where $E_{K^+}$ is the equilibrium potential for potassium, R is the gas constant, T is temperature, z is the charge, F is Faraday's constant, and [K+] are concentrations.

Action Potentials

Phases of the Action Potential

An action potential is a rapid change in membrane potential that propagates along the axon.

• Depolarization: Na+ channels open, Na+ enters the cell, making the inside less negative

• Repolarization: Na+ channels inactivate, K+ channels open, K+ leaves the cell, restoring negativity

• Hyperpolarization: K+ channels remain open briefly, causing the membrane potential to become more negative than resting

• Return to resting potential: Ion channels reset, and the sodium-potassium pump restores original ion concentrations

Key formula:

$I = g(V - E)$

Where I is the ionic current, g is conductance, V is membrane potential, and E is equilibrium potential.

Propagation of Action Potentials

Action potentials travel along axons, sometimes jumping between nodes of Ranvier in myelinated neurons (saltatory conduction).

• Continuous conduction: Occurs in unmyelinated axons

• Saltatory conduction: Occurs in myelinated axons, increasing speed and efficiency

Ion Channels

Types of Ion Channels

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

• Leak channels: Always open, allowing passive ion movement

• Gated channels: Open only in response to specific stimuli (e.g., voltage, ligand)

Voltage-Gated Ion Channels

Voltage-gated ion channels are crucial for action potential generation and propagation.

• Location: Found in axons and skeletal muscle cells

• Activation: Open in response to changes in membrane potential

• States: Closed (resting), open (activated), inactivated (temporarily non-conducting)

Action Potential Sequence

Stepwise Channel Activity

The firing of an action potential involves a precise sequence of channel openings and closings:

1. Voltage-gated Na+ channels open, Na+ enters the cell

2. Na+ channels inactivate, stopping Na+ influx

3. Voltage-gated K+ channels open, K+ leaves the cell

4. Membrane potential returns to resting value

Refractory periods: After an action potential, the neuron cannot immediately fire again (absolute and relative refractory periods).

Synaptic Transmission

Chemical and Electrical Synapses

Neurons communicate at synapses, which can be chemical or electrical.

• Chemical synapse: Neurotransmitters released into the synaptic cleft

• Electrical synapse: Direct ion flow between cells via gap junctions

Effect on dendritic membrane: Synaptic transmission can depolarize or hyperpolarize the postsynaptic cell, affecting its likelihood of firing an action potential.

Lab Practical Preparation

Key Concepts for Lab

Students should be familiar with vertebrate anatomy, physiology, and classification, as well as the practical aspects of nervous system function.

• Action potential phases and ion channel states

• Ion concentrations and their effect on membrane potential

• Relationship between stimulus amplitude and action potential frequency

Summary Table: Ion Channel States During Action Potential

Phase	Na+ Channel	K+ Channel	Membrane Potential
Resting	Closed	Closed	Negative (approx. -70 mV)
Depolarization	Open	Closed	Becomes less negative
Repolarization	Inactivated	Open	Returns toward negative
Hyperpolarization	Closed	Open	More negative than resting
Return to Resting	Closed	Closed	Restored to -70 mV

Additional info: Some details, such as the exact values for equilibrium potentials and the sodium-potassium pump mechanism, were inferred for completeness.