Nervous System Structure and Neuronal Physiology Study Guide

Nervous System Overview

Divisions of the Nervous System

The nervous system is divided into two main components, each with distinct functions and I'm.

• 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. Subdivided into sensory (afferent) and motor (efferent) divisions. The PNS connects the CNS to limbs and organs.

Example: Sensory nerves in the skin detect touch and send signals to the CNS, which processes the information and may trigger a motor response via the PNS.

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 serve different regions and functions.

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

• Spinal nerves: Serve the trunk and limbs, transmitting sensory and motor information.

Neurons and Glial Cells

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 that insulate axons and speed up signal transmission.

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

• Ependymal cells: Line ventricles and produce cerebrospinal fluid.

Neurons vs. Glial Cells

Neurons are specialized for communication via electrical and chemical signals, while glial cells provide structural and metabolic support.

• Neurons: Transmit action potentials and process information.

• Glial cells: Support, protect, and nourish neurons.

Neuronal Physiology

Action Potentials

An action potential is a rapid change in membrane potential that allows neurons to transmit signals over long distances.

• Importance: Enables communication between neurons and with other cells (e.g., muscles).

• Conduction velocity: Influenced by axon diameter and myelination.

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 (e.g., Na+, K+ channels).

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

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

Resting Membrane Potential

At rest, neurons maintain a voltage difference across their membrane, typically around -70 mV.

• Maintained by: Sodium-potassium pump and selective permeability to ions.

• Equation:

$V_{m} = V_{inside} - V_{outside}$

Threshold and Depolarization

The threshold is the critical level of membrane depolarization required to trigger an action potential.

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

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

Example: During an action potential, voltage-gated Na+ channels open, followed by K+ channels.

Phases of Action Potential

Phase	Main Ion Movement	Membrane Potential Change
Depolarization	Na+ in	More positive
Repolarization	K+ out	More negative
Hyperpolarization	Excess K+ out	Below resting

Axon Hillock and Signal Initiation

The axon hillock is the site where action potentials are typically initiated due to a high density of voltage-gated channels.

Lidocaine and Pain Signal Blockage

Lidocaine blocks pain signals by inhibiting voltage-gated Na+ channels, preventing action potential propagation in sensory neurons.

• Application: Used as a local anesthetic in medical procedures.

Summary Table: Ion Channel Types and Functions

Channel Type	Location	Stimulus	Function
Voltage-gated Na+	Axon, axon hillock	Membrane depolarization	Initiate action potential
Voltage-gated K+	Axon	Membrane depolarization	Repolarize membrane
Chemically-gated	Dendrites, soma	Neurotransmitter binding	Synaptic transmission
Mechanically-gated	Sensory receptors	Physical deformation	Detect touch, pressure

Additional info: The study notes expand on brief question prompts by providing definitions, examples, and tables for clarity and completeness.