Study Guide: Nervous System Structure and Function

Nervous System: Structure and Function

Anatomical and Functional Divisions of the Nervous System

The nervous system is divided into anatomical and functional components, each with distinct roles in maintaining homeostasis and coordinating bodily functions.

• Anatomical Divisions:

  • Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for integrating, processing, and coordinating sensory data and motor commands.

  • Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Divided into the somatic and autonomic nervous systems.

• Functional Divisions:

  • Somatic Nervous System (SNS): Controls voluntary movements of skeletal muscles.

  • Autonomic Nervous System (ANS): Regulates involuntary functions such as heart rate, digestion, and respiratory rate. Subdivided into sympathetic and parasympathetic divisions.

• Examples: The CNS processes sensory information, while the PNS transmits signals to and from the CNS.

Glossary of Key Terms

Understanding key terminology is essential for mastering neuroanatomy and neurophysiology.

• Nerve: A bundle of axons in the PNS that transmits electrical impulses.

• Node of Ranvier: Gaps in the myelin sheath along an axon where action potentials are regenerated.

• Tract: A bundle of axons within the CNS with a common origin and destination.

• Nucleus (CNS): A cluster of neuron cell bodies within the CNS.

• Ganglion: A cluster of neuron cell bodies in the PNS.

Structure and Classification of Neurons

Neurons are the primary functional units of the nervous system, specialized for communication.

• Typical Neuron Structure: Consists of a cell body (soma), dendrites (receive signals), and an axon (transmits signals).

• Functions of Each Component:

  • Dendrites: Receive incoming signals from other neurons.

  • Cell Body: Contains the nucleus and organelles; integrates signals.

  • Axon: Conducts electrical impulses away from the cell body.

• Neuron Classification:

  • Multipolar: Many dendrites, one axon (most common in CNS).

  • Bipolar: One dendrite, one axon (found in sensory organs).

  • Unipolar: Single process that splits into two branches (sensory neurons in PNS).

Neuroglia: Types and Functions

Neuroglia (glial cells) support and protect neurons, contributing to the maintenance of the nervous system.

• Astrocytes: Maintain the blood-brain barrier, provide structural support, and regulate ion concentrations.

• Oligodendrocytes: Form myelin sheaths in the CNS, aiding in rapid signal transmission.

• Schwann Cells: Form myelin sheaths in the PNS and assist in nerve regeneration.

• Microglia: Act as immune cells, removing debris and pathogens.

• Ependymal Cells: Line ventricles of the brain and produce cerebrospinal fluid.

• Functions: Neuroglia are involved in myelination, repair of nerve damage, and regulation of the interstitial environment.

Resting Membrane Potential

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

• Creation and Maintenance: Maintained by ion channels and active transport (e.g., sodium-potassium pump).

• Forces Involved: Chemical gradients (differences in ion concentration) and electrical gradients (differences in charge).

• Sodium-Potassium Pump: Actively transports 3 Na+ ions out and 2 K+ ions into the cell, maintaining a negative internal environment.

• Typical Resting Potential: Approximately -70 mV in neurons.

Transmembrane Potential Changes

Changes in the transmembrane potential are essential for nerve signal transmission.

• Leak Channels: Allow passive movement of ions, contributing to resting potential.

• Gated Channels: Open or close in response to stimuli, allowing rapid changes in membrane potential.

• Phases:

  • Depolarization: Membrane potential becomes less negative (Na+ influx).

  • Repolarization: Return to resting potential (K+ efflux).

  • Hyperpolarization: Membrane potential becomes more negative than resting.

Action Potential: Generation and Propagation

An action potential is a rapid, temporary change in membrane potential that travels along the axon.

• Threshold: The minimum membrane potential required to trigger an action potential.

• All-or-None Principle: Once threshold is reached, an action potential occurs fully; if not, it does not occur.

• Refractory Period: Time during which a neuron cannot fire another action potential (absolute and relative phases).

• Propagation:

  • Saltatory Conduction: In myelinated axons, action potentials jump from node to node (Nodes of Ranvier), increasing speed.

  • Continuous Conduction: In unmyelinated axons, action potentials propagate along every part of the membrane, which is slower.

Factors Affecting Action Potential Speed

The speed of action potential propagation is influenced by axon diameter and myelination.

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

• Myelination: Myelinated axons conduct impulses more rapidly via saltatory conduction.

• Comparison: Myelinated, large-diameter axons are the fastest; unmyelinated, small-diameter axons are the slowest.

Summary Table: Myelinated vs. Unmyelinated Axons

Feature	Myelinated Axon	Unmyelinated Axon
Conduction Type	Saltatory	Continuous
Speed	Fast	Slow
Energy Efficiency	High	Low
Location	CNS and PNS (e.g., motor neurons)	Some sensory fibers

Key Equation: Nernst Equation (for equilibrium potential)

The Nernst equation calculates the equilibrium potential for a particular ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Where: Eion = equilibrium potential for the ion R = universal gas constant T = temperature in Kelvin z = charge of the ion F = Faraday's constant [ion]outside and [ion]inside = ion concentrations outside and inside the cell