Fundamentals of the Nervous System & Nervous Tissue (Chapter 11B) – Study Notes

Fundamentals of the Nervous System & Nervous Tissue

Overview

This section covers the essential structure and function of neurons, the role of myelin sheaths, neuron classification, neurophysiology, and the basis of electrical signaling in the nervous system. Understanding these concepts is foundational for studying the nervous system in human anatomy and physiology.

Neurons: Structure and Function

Definition and Components

• Neuron: The basic structural and functional unit of the nervous system, specialized for transmitting electrical signals.

• Key structural components:

  • Cell body (soma): Contains the nucleus and organelles.

  • Dendrites: Branching extensions that receive signals from other neurons.

  • Axon: Long projection that transmits impulses away from the cell body.

• Nucleus vs. Ganglion:

  • Nucleus: Cluster of neuron cell bodies in the CNS.

  • Ganglion: Cluster of neuron cell bodies in the PNS.

Myelin Sheath

• Myelin sheath: Whitish, lipoprotein covering around most long or large axons.

• Myelinated fibers: Axons with a myelin sheath; conduct impulses rapidly.

• Unmyelinated fibers: Axons without a sheath; conduct impulses slowly.

• Functions:

  • Protects axon

  • Electrically insulates fibers from one another

  • Increases transmission speed of nerve impulses

• Cells forming myelin sheaths:

  • CNS: Oligodendrocytes

  • PNS: Schwann cells

Myelin Sheath Formation in the PNS

• Schwann cells wrap around axons, forming the myelin sheath in successive layers.

• Neurilemma: Outer layer of Schwann cell cytoplasm.

• Nodes of Ranvier: Gaps between adjacent Schwann cells; occur at regular intervals (~1 mm) and allow axon collaterals to emerge.

Myelination in the CNS

• CNS contains both myelinated and unmyelinated axons.

• Oligodendrocytes can myelinate multiple axons simultaneously (up to 60).

• CNS myelin sheaths lack a neurilemma.

• White matter: Regions with dense collections of myelinated fibers (fiber tracts).

• Gray matter: Contains mostly neuron cell bodies and unmyelinated fibers.

Neuron Classification

Structural Classification

• Multipolar neurons: Three or more processes (one axon, multiple dendrites); most common type (99%).

• Bipolar neurons: Two processes (one axon, one dendrite); rare, found in special sense organs.

• Unipolar neurons: Single, short process that divides into proximal and distal branches.

  • Peripheral process: Associated with sensory receptors.

  • Central process: Enters CNS.

  • Found mainly in ganglia in PNS; sensory neurons.

Functional Classification

• Sensory (afferent) neurons: Transmit impulses from sensory receptors toward or into CNS; mostly unipolar, some bipolar (special senses).

• Motor (efferent) neurons: Carry impulses away from CNS to effector organs (muscles & glands); multipolar, cell bodies in CNS.

• Interneurons (association neurons): Lie between motor and sensory neurons in neural pathways; shuttle signals through CNS for integration; 99% of neurons, almost all multipolar.

Neurophysiology

Action Potentials and Nerve Impulses

• Neurons are highly irritable (excitable) and respond to stimuli by generating an action potential (nerve impulse).

• Action potentials are always the same regardless of stimulus source or type.

• Action potentials underlie all functional activities of the nervous system and are electrically measurable events.

Electrical Properties of the Body

Body Electrical Current

• The human body is electrically neutral (equal numbers of positive and negative charges).

• Membranes can separate charges, creating potential energy.

• Electrical currents in the body reflect the flow of ions rather than free electrons.

• A potential exists across membranes when ion numbers differ or the membrane provides resistance to ion flow.

Key Electrical Terms

• Voltage (V): Measure of potential energy generated by separated charge; action potentials measured in millivolts ($1~\text{mV} = 0.001~\text{V}$).

• Potential difference: Voltage measured between two points.

• Current (I): Flow of electrical charge between two points.

• Resistance (R): Hindrance to charge flow; insulators have high resistance, conductors have low resistance.

• Ohm's Law: Relationship between voltage, current, and resistance: $I = \frac{V}{R}$

Ion Channels and Membrane Permeability

Types of Ion Channels

• Leak (nongated) channels: Always open, allow passive ion movement.

• Gated channels: Open and close in response to specific signals.

  • Chemically gated (ligand-gated) channels: Open when a specific chemical (e.g., neurotransmitter) binds.

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

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

Operation of Gated Channels

• Chemically gated channels (e.g., ACh receptors):

  • Closed: No neurotransmitter bound; Na+ cannot enter, K+ cannot exit.

  • Open: Neurotransmitter bound; Na+ enters, K+ exits.

• Voltage-gated channels (e.g., Na+ channel):

  • Closed: Membrane potential is negative; Na+ cannot enter.

  • Open: Membrane potential becomes less negative (depolarization); Na+ enters, interior changes from negative to positive.

• Mechanically gated channels:

  • Closed: No physical stimulus.

  • Open: Pressure or touch deforms the receptor, opening the channel.

Ion Movement and Electrochemical Gradients

Principles of Ion Movement

• Ions move along concentration gradients (high to low concentration).

• Ions move along electrical gradients (toward opposite charge).

• When gated ion channels are open, ions move quickly across the membrane, following the electrochemical gradient (combined electrical and concentration gradients).

• This movement creates electrical currents and changes membrane voltage.

• Ion flows along electrochemical gradients underlie all electrical events in neurons.

Summary Table: Neuron Types and Functions

Classification	Type	Structure	Function	Location
Structural	Multipolar	3+ processes (1 axon, many dendrites)	Integration, motor	CNS, most neurons
Structural	Bipolar	2 processes (1 axon, 1 dendrite)	Sensory (special senses)	Retina, olfactory epithelium
Structural	Unipolar	Single process, divides into two branches	Sensory	PNS ganglia
Functional	Sensory (Afferent)	Unipolar/Bipolar	Transmit impulses to CNS	PNS to CNS
Functional	Motor (Efferent)	Multipolar	Transmit impulses from CNS to effectors	CNS to muscles/glands
Functional	Interneuron	Multipolar	Integration, connect sensory & motor	CNS

Key Equations

• Ohm's Law: $I = \frac{V}{R}$

• Action potential voltage: $1~\text{mV} = 0.001~\text{V}$

Example Application

• Example: In multiple sclerosis, myelin sheaths in the CNS are damaged, leading to slower nerve impulse conduction and neurological symptoms.

Additional info: These notes expand on the original slides by providing definitions, context, and a summary table for neuron types and functions, ensuring a self-contained study guide for exam preparation.