Anatomy & Physiology 101
NERVOUS SYSTEM CELLS: SUPPLEMENT 1
I. INTRODUCTION TO THE NERVOUS SYSTEM
A. STRUCTURE
Central Nervous System (CNS): Brain, Spinal Cord
Peripheral Nervous System (PNS): Nerves
B. PRIMARY FUNCTIONS
1. COMMUNICATION - Sending Messages, Cells in the NS Send Messages to Cells in Other Systems
2. CONTROL - Regulation of Body Functions, The NS Regulates a # of Body Functions
(Important in Maintaining Homeostasis)
3. INTEGRATION - Unification of Body Functions, Allows the Body to Function as a Unit
*NOTE: Both the Nervous and the Endocrine Systems Have as Their Primary Functions, Communication,
Control, and Integration. The 2 Systems Differ in How They Communicate, Control, and Integrate (Nerve
Impulses – Rapid, Short-Lasting Vs. Hormones – Slow, Long-Lasting)
II. CELLS OF THE NERVOUS SYSTEM
There Are 2 Major Types of Nervous System Cells
1. GLIA (NEUROGLIA)
A. DEFINITION/NUMBER
Supporting Cells in the Nervous System / 900 Billion
B. TYPES
a. ASTROCYTES - “Star Cells” (Star-Shaped) Largest/Most Numerous Glia
Help Form the Blood-Brain Barrier (Protective Covering for Brain, Composed of Brain Capillaries
And Astrocytes)
b. MICROGLIA - “Small Glia” (Smallest) Phagocytes in Brain Inflammation
c. EPENDYMAL CELLS - Line Fluid Filled Spaces in the CNS
Help Produce/Keep Fluid Circulating (Cilia) Within the Spaces
d. OLIGODENDROCYTES - “Cells With Few Branches”
- Help Hold Together Nerve Fibers in the CNS (Nerve Fibers = Processes of Neurons)
- Produce the Covering for Nerve Fibers (Axons) in the CNS
(Many NF’s in the CNS Have 1 Covering: Myelin Sheath, Formed by Oligodendrocytes)
e. SCHWANN CELLS - Located Only in the PNS
- Hold Together Nerve Fibers in the PNS
- Produces the Coverings for Many Nerve Fibers (Axons) in the PNS
(Many NF’s in PNS Have 2 Coverings: Myelin Sheath and Neurilemma, Formed by Schwann
Cells; The Myelin Sheath is the Schwann Cell’s Plasma Membrane and the Neurilemma is the
Schwann Cell’s Cytoplasm and Nucleus)
- Cover and Support Neuron Cell Bodies in PNS
2. NEURONS
A. DEFINITION/NUMBER
Nerve Cells: Conduct NI / 100 Billion
B. STRUCTURE
a. GENERAL INFORMATION
1. PLASMA MEMBRANE
2. CYTOPLASM
3. CYTOSKELETON (NEUROFIBRILS)
Neurofibrils: Microscopic Threadlike Fibers that Extend Lengthwise Through the Neuron
Rapid Transport of Molecules From One End of the Neuron to the Other (i.e., Proteins)
b. MAJOR PARTS OF A NEURON
1. CELL BODY
Largest Part of Neuron
Contains Nucleus
Contains Typical Organelles
Contains Nissl Bodies - Rough ER of Neurons, Protein Synthesis
2. PROCESSES (NERVE FIBERS)
Thread-like Extensions from Cell Body
2 Types:
a. DENDRITE(S) - One or More/Neuron (Shorter)
Conduct NI Toward Cell Body
b. AXON - One/Neuron (Longer)
Conduct NI Away from Cell Body
1. AXON COLLATERAL(S) - Side Branches: 1 or More
2. TELODENDRIA & SYNAPTIC KNOBS - Terminal Branches and Bulges
3. COVERINGS (Axon)
a. ONE COVERING: MYELIN SHEATH (MYELINATED NERVE FIBERS)
Axons of Neurons in CNS Have 1 Covering, Myelin Sheath, Formed by
Oligodendrocytes
Known as Myelinated Nerve Fibers (White)
b. TWO COVERINGS: MYELIN SHEATH & NEURILEMMA (MYELINATED NERVE FIBERS)
Many Axons of Neurons in PNS Have 2 Coverings, Myelin Sheath and Neurilemma,
Formed by Schwann Cells
Also Known as Myelinated Nerve Fibers(White)
c. NO COVERINGS (UNMYELINATED NERVE FIBERS)
Some Axons of Neurons in PNS Have No Coverings (Axons are Embedded in
Schwann Cells, Rather than Schwann Cells Wrapping Around Axons)
Known as Unmyelinated Nerve Fibers (Gray)
Neurilemma Functions in Repair of Neurons
1) Mature Neurons Are Not Capable of Mitosis
2) Repair of Neurons Requires Intact Cell Body and the Presence of a Neurilemma
(Neurilemma Serves as the Guiding Tunnel)
3) Damage to Neurons in the CNS is Permanent
3. CLASSIFICATION OF NEURONS
a. STRUCTURAL CLASSIFICATION
Neuron Classified According to Number of Processes that Extend Off the Cell Body
1. MULTIPOLAR NEURONS - Several Dendrites, 1 Axon
2. BIPOLAR NEURONS - 1 Dendrite (Branched), 1 Axon
3. UNIPOLAR NEURONS - Several Dendrites, 1 Axon (Peripheral and Central Portions)
b. FUNCTIONAL CLASSIFICATION
Neuron Classified According to Direction It Conducts Nerve Impulses
1. AFFERENT (SENSORY) NEURONS
Conduct Nerve Impulses Toward CNS, Specifically From Receptors To CNS Receptors:
Distal Ends of Dendrites of Afferent (Sensory)Neurons Receives a Stimulus
Converts Stimulus into a Nerve Impulse
Located in Sense Organs
2. EFFERENT (MOTOR) NEURONS
Conduct Nerve Impulses Away From CNS, Specifically from CNS to Effectors
Effector:
Structure that Shows Action - Muscle or Gland
3. INTERNEURONS
“Between Neurons”, Conduct Nerve Impulses From Afferent Neurons to Efferent Neurons
Located Entirely in CNS
*MOST Afferent Neurons are Unipolar (A Few are Bipolar)
*Efferent Neurons are Multipolar
*Interneurons are Multipolar
4. GRAY MATTER
a. DEFINITION: Gray, Neuron Cell Bodies (CNS, PNS) and/or Unmyelinated Nerve Fibers (PNS)
b. NUCLEI(us) - Gray Matter in the CNS
c. GANGLIA (ion) - Gray Matter in the PNS
5. WHITE MATTER
a. DEFINITION: White, Myelinated Nerve Fibers
b. TRACTS - White Matter in the CNS
c. NERVES - White Matter in the PNS
1. CONNECTIVE TISSUE COMPONENTS
a. ENDONEURIUM - Connective Tissue that Wraps Around Each Individual Myelinated Nerve
Fiber
b. PERINEURIUM - Connective Tissue that Wraps Around Each Group of Myelinated Nerve
Fibers (Fascicle)
c. EPINEURIUM - Connective Tissue that Wraps Around the Entire Nerve
2. TYPES
a. MIXED NERVES - Contain Both Afferent and Efferent Nerve Fibers, Most Common
b. SENSORY NERVES - Contain Mainly Afferent Nerve Fibers
c. MOTOR NERVES - Contain Mainly Efferent Nerve Fibers
III. NEURON PATHWAYS
A. REFLEX ARC PATHWAYS (REFLEX ARCS)
1. DEFINITION
- Neuron Pathway To/Away From the CNS
- Nerve Impulse Always Begins in Receptors, Ends in Effector
2. TYPES
a. THREE NEURON ARC
Involves 3 Neurons: Afferent Neuron, Interneuron, and Efferent Neuron
Most Common Type of Reflex Arc Pathway
b. TWO NEURON ARC
Involves 2 Neurons: Afferent Neuron and Efferent Neuron (No Interneurons)
Simplest Type of Reflex Arc Pathway
3. REFLEX
- Response Produced When a Nerve Impulse Travels Over A Reflex Arc Pathway
- Involuntary (A Response to a Stimulus)
- Types (Based on Effector)
- Muscle Contraction
- Gland Secretion
*Reflex: Mechanism of Communication, Control in Nervous System
B. OTHER PATHWAYS
Many Other Neuron Pathways Exist in the Nervous System
Examples: 1) Receptors → Brain
2) Brain → Skeletal Muscles
3) Within the Brain
IV. PHYSIOLOGY OF NEURONS
How Neuron’s Conduct NI’s:
A.
GENERALIZATIONS ABOUT THE NERVE IMPULSE
1. STIMULUS INITIATES NI
- Stimulus Causes NI to Begin→ Indicates a Need for Communication
2. NI “FLOWS” IN WAVELIKE FASHION (P. MEMBRANE)
- Changes That Occur Involve Small Sections of the P.M. At a Time That Progress
3. ELECTROCHEMICAL
a. ELECTRICAL AS TRAVELS ALONG P. MEMBRANE (Like Electricity)
- Electrical Events Involve Changes in Ions
b. CHEMICAL AS TRAVELS ACROSS SYNAPSE (RELEASE OF NEUROTRANSMITTERS)
- Synapse = Space B/T Neuron and Next Structure
- Chemical Events Involves the Release of Chemicals (Neurotransmitters)
- Allows NI to Cross the Synapse
4. DOESN’T LOSE STRENGTH FROM START TO FINISH (DUE TO ELECTROCHEMICAL NATURE)
- If NI’s Were purely Electrical, They would be Stopped at Synapses; However at Synapses
NI’s Become Chemical
5. SPEED VARIES; DEPENDS ON
a. DIAMETER OF NEURON’S NERVE FIBERS (AXON)
- The Larger the Diameter of the Axon, the Faster the Speed of the NI (More Surface Area)
b. PRESENCE OF MYELIN SHEATH: MYELINATED NERVE FIBERS CONDUCT USING SALTATORY
CONDUCTION (FASTER)
- Saltatory Conduction: NI Leaps Along Nodes of Ranvier (Microscopic Gaps in Myelin
Sheath, Myelin Sheath acts as an Insulator)
B. BASIC EVENTS IN NERVE IMPULSE TRANSMISSION
1. CONDUCTION ALONG PLASMA MEMBRANE (ELECTRICAL)
a. RESTING MEMBRANE POTENTIAL (POLARIZATION)
-Neuron Resting (Not Conducting a Nerve Impulse)
- RMP for Neurons: The OUTER Surface of the P. Membrane Contains a Slight Excess of
POSITIVE Ions (Means INNER SURFACE is LESS POSITIVE/NEGATIVE Compared to OUTER
SURFACE)
- Major Mechanism That Maintains the RMP: Na+/K+ Pumps
- Membrane Potential
- Difference in Ion Concentration on Each Side of PM → Means a Difference in Electrical
Charges (Polarization) --------- Measured Like Electricity (Millivolts)
b. STIMULUS: Often* Leads to Action Potential
- Stimulus Often Causes NI to Begin (*Depends on Strength of Stimulus)
c. ACTION POTENTIAL (NERVE IMPULSE/REVERSE POLARIZATION)
- Neuron Active (Conducting a Nerve Impulse)
-A Reverse in Polarization Occurs: The INNER SURFACE Now Contains a Slight Excess of
POSITIVE Ions (Means the OUTER SURFACE is LESS POSITIVE/NEGATIVE in Relation to the
INNER SURFACE)
Mechanism:
1) Stimulus Causes Stimulus Gated Na+ Channels (At the Point of Stimulation) to Open –
------> Na+ Diffuses Into the Cell
2) If ENOUGH Stimulus Gated Na+ Channels Open and ENOUGH Na+ Ions Diffuse Into
the Cell*, then Voltage Gated Na+ Channels Open, and EVEN MORE Na+ Ions
Diffuse Into the Cell (Reverse Polarization)
* Depends Upon the Strength of the Stimulus
3) Na+ Channels Close Automatically Once a Maximum Number of Na+ Ions Move
Into the Cell (Voltage Peaks, All-or-None)
- How the Action Potential is Conducted: Along Small Sections of the Neuron at a Time
(Wavelike) (Reverse in Polarization Sets Up an Electrical Current Flow)
- A Neuron Can Conduct More Than 1 NI at a Time
d. REPOLARIZATION
- A Return to Polarization (RMP)
- Mechanism:
1) K+ channels Open* ------> K+ Diffuses Out of the Cell
(*Reaching the Peak in Voltage Causes Them to Open)
2) RMP Restored by Na+/K+ Pumps
- Repolarization Occurs Along Sections of the Neuron Just Behind the Action Potential
(Wavelike)
e. REFRACTORY PERIODS
1. ABSOLUTE REFRACTORY PERIOD
- Occurs Where the Action Potential is Occurring - This Area of P. Membrane
TOTALLY RESISTS STIMULATION
2. RELATIVE REFRACTORY PERIOD
- Occurs Where Repolarization is Ocurring - This Area of P. Membrane
PARTIALLY RESISTS STIMULATION
- The Refractory Periods are Important Because They
1) Prevent The Action Potential From Moving Backwards
2) Keep NI’s From Moving on Top of One Another
2. CONDUCTION ACROSS SYNAPSES (CHEMICAL)
a. LOCATION OF SYNAPSES
1. BETWEEN NEURONS (PRE & POSTSYNAPTIC)
2. BETWEEN MOTOR NEURON AND CELLS OF EFFECTOR
b. STRUCTURE OF CHEMICAL SYNAPSES
1. SYNAPTIC KNOBS - Bulges Located at the Terminal Ends of Telodendria
- Store Neurotransmitters
2. SYNAPTIC CLEFT- Space Between Neuron and Next Structure
3. PLASMA MEMBRANE OF NEXT STRUCTURE; EITHER:
a. POSTSYNAPTIC NEURON
b. CELLS OF EFFECTOR
- Plasma Membrane Contains Receptors to Which Neurotransmitters Bind
c. MECHANISM OF CHEMICAL CONDUCTION
1) The Nerve Impulse Reaches the Synaptic Knobs
2) Ca++ Channels (in the P. Membrane) Open and Ca++ Diffuses Into the Synaptic Knobs
3) Increased Intracellular Ca++ Causes the Release of Neurotransmitters (by Exocytosis)
4) Neurotransmitters DIFFUSE Across the Synaptic Cleft and BIND to Protein Receptors in the
P. Membrane of the Postsynaptic Neuron OR Effector Cells
5) Binding May Lead to Action Potential (Depends on Type(s) of Neurotransmitters and
Receptors Involved)
Lock and Key Fit
Excitatory/Inhibitory Neurotransmitters