Download begin

Classification of
Neurons
Nature of the Nervous System
Camillo Golgi
 The Synctium
 continuous
network
 no gaps ~

Nature of the Nervous System
Santiago Ramon y Cajal
 The Neuron Doctrine
 discrete cells
 communication across
gaps
 synapses ~

Nature of the Nervous System

Golgi won
 Proved himself wrong
 Golgi Stain silver chromate
Supported Neuron
Doctrine
Golgi & Cajal shared
Nobel Prize (1906) ~
Classification of Neurons

Structural – based on the number of
cytoplasmic processes
Multipolar neurons
Bipolar neurons
Unipolar neurons

Functional – based on the direction of
impulse transmission
Sensory neurons
Motor neurons
Interneurons (association)
Histology of Nervous
Tissue

2 types of cells
Neurons
– Structural & functional part of nervous system
– Specialized functions
Neuroglia (glial cells)
– Support & protection of nervous system
NEURONS
Basic functional unit of N.S.
 Specialized cell
 All cells have same basic
properties
 information processing
Transmits
Integrates
Stores
 Regulation of behavior ~

Neurons

Function
Conduct electrical impulses

Structure
Cell body
– Nucleus with nucleolus
– Cytoplasm (perikaryon)
Cytoplasmic processes
– Dendrites
– Axon
Review of Neuron
Structure
Nervous Tissue: Neurons
 Neurons = nerve cells
 Cells specialized to transmit messages
 Major regions of neurons
 Cell body – nucleus and metabolic center
of the cell
 Processes – fibers that extend from the
cell body (dendrites and axons)
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.8
Neuron Cell Body Location
 Most are found in the central nervous
system
 Gray matter – cell bodies and unmylenated
fibers
 Nuclei – clusters of cell bodies within the
white matter of the central nervous system
 Ganglia – collections of cell bodies
outside the central nervous system
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.13
Neuron Anatomy
 Cell body
 Nucleus
 Large
nucleolus
Figure 7.4a
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.9b
Neuron Anatomy
 Extensions
outside the cell
body
 Dendrites –
conduct
impulses toward
the cell body
 Axons – conduct
impulses away
from the cell
body (only 1!)
Figure 7.4a
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.10
Axon Structure

Long, specialized
Collaterals = branches
Telodendria = termination of axons &
collaterals


Cytoplasm = axoplasm
Plasma membrane = axolemma
Anatomy of a Neuron
Anatomy of a Neuron
Axons and Nerve Impulses
 Axons end in axonal terminals
 Axonal terminals contain vesicles with
neurotransmitters
 Axonal terminals are separated from the
next neuron by a gap
 Synaptic cleft – gap between adjacent
neurons
 Synapse – junction between nerves
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.11
Nerve Fiber Coverings
 Schwann cells –
produce myelin
sheaths in jelly-roll
like fashion
 Nodes of Ranvier –
gaps in myelin
sheath along the
axon
Figure 7.5
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.12
Application
In Multiple Scleroses the myelin
sheath is destroyed.
 The myelin sheath hardens to a
tissue called the scleroses.
 This is considered an autoimmune
disease.
 Why does MS appear to affect the
muscles?

Neuron Classification
Figure 7.6
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.15
Types of Neurons


Functional classification
Sensory or afferent: Action potentials toward CNS
Motor or efferent: Action potentials away from CNS
Interneurons or association neurons (CNS)
Principle neurons / projection neurons (CNS)
Structural classification
Multipolar, bipolar, unipolar
Shorthand
Neuron Classification
Categories overlap ~
Functional Classification of
Neurons
 Interneurons (association neurons)
 Found in neural pathways in the central
nervous system
 Connect sensory and motor neurons
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Functional Classification of
Neurons
 Sensory (afferent) neurons
 Carry impulses from the sensory receptors
 Cutaneous sense organs
 Proprioceptors – detect stretch or tension
 Motor (efferent) neurons
 Carry impulses from the central nervous
system
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
By function (connections)
Sensory
Motor
Interneuron
By morphology (# of neurites)
Multipolar
Bipolar
Unipolar
Structural Classification of Neurons
 Multipolar neurons – many extensions
from the cell body
Figure 7.8a
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Structural Classification of Neurons
 Bipolar neurons – one axon and one
dendrite
Figure 7.8b
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Structural Classification of Neurons
 Unipolar neurons – have a short single
process leaving the cell body
Figure 7.8c
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Neuroglia

Neuroglia of CNS
Astrocytes
–
–
–
–
Form the blood-brain barrier
Form a structural framework for the CNS
Repair damaged neural tissue
Control the interstitial environment of the CNS
Oligodendrocytes
– Form myelin sheaths CNS
Microglia
– Phagocytose foreign microbes, etc.
Ependymal
– Line ventricles of the brain, secrete cerebrospinal fluid

Neuroglia of PNS
Schwann cells
– Form myelin sheaths of PNS
Classification of Glial
Cells
Neuroglia of CNS
Nerve Fibers of the PNS

An axon and its sheaths
Myelinated axon
– Axon is surrounded by a myelin sheath
Unmyelinated axon
– Axon has no myelin sheath
Myelin



White matter of nerves, brain, spinal
cord
Composed primarily of phospholipids
Production
Developing Schwann cells wind around axon

Function
Increases speed of impulse conduction
Insulation and maintenance of axon
Schwann Cells and
Peripheral Axons
Myelin

Neurilemma
Peripheral cytoplasmic layer of the
Schwann cell enclosing the myelin
sheath

Nodes of Ranvier
Unmyelinated gaps between segments
of myelin
Impulses “jump” from node to node
A Myelinated Axon
Nerve Fibers of the CNS


Umyelinated
Myelinated
Production of myelin is from
oligodendrocytes
Nodes of Ranvier are less numerous
Nerve Fibers of the CNS
Structural Classification
of Neurons
Functional Classification
of Neurons
By Dendrite Structure
Pyramidal
vs.
Stellate
Dendritic Spines

Spiny or Aspinous ~
Other Classification Schemes
Axon length
 Golgi Type I - long
 Golgi Type II - short (local
signalling)
 Chemistry
 neurotransmitter released ~

How Neurons Function
(Physiology)
 Irritability – ability to respond to stimuli
 Conductivity – ability to transmit an
impulse
 The plasma membrane at rest is
polarized
 Fewer positive ions are inside the cell than
outside the cell
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.17
Starting a Nerve Impulse
 Depolarization – a
stimulus depolarizes the
neuron’s membrane
 A deploarized
membrane allows
sodium (Na+) to flow
inside the membrane
 The exchange of ions
initiates an action
potential in the neuron
Figure 7.9a–c
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.18
The Action Potential
 If the action potential (nerve impulse)
starts, it is propagated over the entire
axon
 Potassium ions rush out of the neuron
after sodium ions rush in, which
repolarizes the membrane
 The sodium-potassium pump restores
the original configuration
 This action requires ATP
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.19
Nerve Impulse Propagation
 The impulse
continues to move
toward the cell body
 Impulses travel
faster when fibers
have a myelin
sheath
Figure 7.9c–e
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.20


Stimuli
Receives information
Integrates information~



Axon carries information away from soma
Electrical signal
Axon terminal releases chemical message~
Continuation of the Nerve Impulse
between Neurons
 Impulses are able to cross the synapse
to another nerve
 Neurotransmitter is released from a nerve’s
axon terminal
 The dendrite of the next neuron has
receptors that are stimulated by the
neurotransmitter
 An action potential is started in the dendrite
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.21
Neuronal Communication:

Within a neuron, electrical signals called
action potentials travel along the axon.

Communication between neurons is
mediated via synaptic transmission (e.g.
by means of neurotransmitters such as
glutamate, GABA, acetylcholine).

Transfer of information from one part of
body to another.
How Neurons Communicate at
Synapses
Figure 7.10
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.22
Glia
Neural Support Cells
Nervous Tissue: Support Cells
(Neuroglia or Glia)
 Astrocytes
 Abundant, star-shaped cells
 Brace neurons
 Form barrier
between capillaries
and neurons
 Control the chemical
environment of
the brain (CNS)
Figure 7.3a
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.5
Nervous Tissue: Support Cells
 Microglia (CNS)
 Spider-like phagocytes
 Dispose of debris
 Ependymal cells
(CNS)
 Line cavities of the
brain and spinal cord
 Circulate
cerebrospinal
fluid
Figure 7.3b, c
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.6
Nervous Tissue: Support Cells
 Oligodendrocytes
(CNS)
 Produce myelin
sheath around
nerve fibers in the
central nervous
system
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 7.3d
Slide 7.7a
Neuroglia vs. Neurons
Neuroglia divide.
 Neurons do not.
 Most brain tumors are “gliomas.”
 Most brain tumors involve the
neuroglia cells, not the neurons.
 Consider the role of cell division in
cancer!

Support Cells of the PNS
 Satellite cells
 Protect neuron cell bodies
 Schwann cells
 Form myelin sheath in the peripheral
nervous system
Figure 7.3e
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 7.7b
Support Cells / Glia
CNS
PNS
Astrocytes
Provide physical
support
 Regulating chemical
content of
extracellular fluid
 localizes
neurotransmitters
 K+ concentration
 Blood-brain Barrier ~

Blood-Brain Barrier
Typical
Capillary
Brain
Capillary
BBB: Function
Maintains stable brain environment
large fluctuations in periphery
 Barrier to poisons
 Retains neurotransmtters & other
chemicals
 Regulates nutrient supplies
glucose levels
active transport ~

Myelin
Wrap around axon
 Saltatory Conduction
 faster transmission
 CNS: oligodendroglia
or oligodendrocytes
 PNS: Schwann cells ~

Myelinated and Unmyelinated
Axons

Myelinated axons
Myelin protects
and insulates
axons from one
another
Not continuous
Nodes of Ranvier

Unmyelinated
axons
Saltatory Conduction
Other non-neuronal cells
Microglia
 phagocytosis
 immune-like function
 Ependymal cells
 line walls of ventricles
 role in cell migration during
development ~

Chemical Synapses
Neuronal Pathways and
Circuits

Organization of neurons in CNS varies
Convergent pathways
Divergent pathways
Oscillating circuits