Nervous System Structure, Function, and Neural Signaling Study Guide

Nervous System Overview

Divisions of the Nervous System

The nervous system is divided into two main components, each with distinct roles in the body:

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for processing and integrating information, and coordinating activity throughout the body.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. It connects the CNS to limbs and organs, and is subdivided into sensory (afferent) and motor (efferent) divisions.

Example: Sensory nerves in the PNS carry information from the skin to the CNS, while motor nerves transmit commands from the CNS to muscles.

Cranial vs. Spinal Nerves

Cranial nerves emerge from the brain, while spinal nerves originate from the spinal cord. Both are part of the PNS, but differ in their anatomical location and functions.

• Cranial nerves: Control functions in the head and neck (e.g., vision, taste, facial movement).

• Spinal nerves: Serve the rest of the body, including limbs and trunk.

Neural Cells and Glial Cell Functions

Types and Functions of Glial Cells

Glial cells support and protect neurons in the nervous system. Major types include:

• Astrocytes: Maintain the blood-brain barrier, provide nutrients, and regulate ion balance.

• Oligodendrocytes (CNS) / Schwann cells (PNS): Form myelin sheaths around axons to increase conduction velocity.

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

• Ependymal cells: Line ventricles and produce cerebrospinal fluid.

Example: Schwann cells myelinate peripheral nerves, enabling rapid signal transmission.

Neurons: Structure and Function

Neuron vs. Nerve

A neuron is a single nerve cell specialized for communication, while a nerve is a bundle of axons (from many neurons) in the PNS.

Action Potentials

Neurons communicate via action potentials, which are rapid changes in membrane potential that travel along the axon.

• Importance: Action potentials allow neurons to transmit signals over long distances.

• Conduction velocity: Influenced by axon diameter and myelination.

Example: Myelinated axons conduct signals faster than unmyelinated ones.

Ion Channels and Membrane Potential

Neuronal signaling depends on the movement of ions across the membrane through specialized channels.

• Types of ion channels: Voltage-gated, chemically-gated, and mechanically-gated.

• Voltage-gated channels: Open in response to changes in membrane potential.

• Chemically-gated channels: Open in response to neurotransmitters.

• Mechanically-gated channels: Open in response to physical deformation.

Resting Membrane Potential

At rest, neurons maintain a negative membrane potential, typically around -70 mV, due to the distribution of Na+ and K+ ions.

• Key equation:

$V_{rest} \approx -70\ \text{mV}$

Threshold and Action Potential Initiation

The threshold is the membrane potential at which an action potential is triggered, usually around -55 mV.

• Reaching threshold: Sufficient depolarization opens voltage-gated Na+ channels.

• Depolarization: Na+ ions enter the cell, making the inside more positive.

• Repolarization: K+ ions exit the cell, restoring the negative potential.

Example: During an action potential, the membrane potential rapidly rises and falls.

Ion Movement During Action Potential

Phase	Ion Movement	Direction
Depolarization	Na+	Into cell
Repolarization	K+	Out of cell

Electrical Status During Action Potential

During depolarization, the inside of the neuron becomes more positive. During repolarization, the inside returns to a negative state.

• Depolarization: $Na^+$ influx

• Repolarization: $K^+$ efflux

Clinical Application: Lidocaine and Pain Signals

Mechanism of Lidocaine

Lidocaine is a local anesthetic that blocks pain signals by inhibiting voltage-gated Na+ channels in neurons. This prevents the initiation and propagation of action potentials, thereby blocking nerve transmission of pain.

• Example: Lidocaine is used in dental procedures to numb tissue.

Summary Table: Key Concepts

Concept	Definition	Example/Application
Glial Cells	Support cells in the nervous system	Astrocytes maintain ion balance
Action Potential	Rapid change in membrane potential	Neural signaling
Lidocaine	Blocks Na+ channels	Local anesthesia

Additional info: The notes above expand on brief question prompts to provide a comprehensive overview suitable for Anatomy & Physiology students, including definitions, examples, and tables for clarity.