Chapter Three
Cells of the Nervous System
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Glia and Neurons
• Glia
– Primary supporting cells of the CNS
• Macroglia (astrocytes, oligodendrocytes, Schwann
cells)
• Microglia
• Neurons
– Primary functioning cells of the CNS
– Information processing and communication
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Glia Are Classified by Size
• Macroglia
– Astrocytes, oligodendrocytes, Schwann cells
• Microglia
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Astrocytes
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Oligodendrocytes and Schwann Cells
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The Neural Membrane
• Phospholipid bilayer; ion channels/pumps
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The Cytoskeleton of Neurons –
Three Fiber Types
• Microtubules, neurofilaments, and
microfilaments
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Tau Phosphorylation
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The Neural Cell Body (Soma)
• Site of synapses and organelles
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Axons and Dendrites
• Dendrites receive signals from adjacent
neurons
– Dendritic spines
• Axons transmit signals
– Axon hillock
– Myelination
– Nodes of Ranvier
– Axon terminal
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Axons and Dendrites (cont’d.)
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Structural Variations in Neurons
• Unipolar
– Single branch extending from the cell body
• Bipolar
– Two branches extending from the neural cell
body: one axon and one dendrite
– von Economo neurons
• Multipolar
– Many branches extending from the cell body;
usually one axon and many dendrites
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Functional Variations in Neurons
• Sensory neurons
– Specialized to receive information from the
outside world
• Motor neurons
– Transmit commands from the CNS directly to
muscles and glands
• Interneurons
– Act as bridges between the sensory and
motor systems
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Structural and Functional Classification of
Neurons
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Structural and Functional Classification of
Neurons (cont’d.)
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Generating Action Potentials
• An action potential is an electrical signal
that begins the process of neural
communication
• Ionic composition of the intracellular and
extracellular fluids
– Differs in the relative concentrations of ions
inside vs. outside the cell
– The difference in ion composition between
these fluids provides the neuron with a source
of energy for electrical signaling
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Resting Potential
• Voltage difference across the resting
membrane = 70mV
• Extracellular environment is assigned a
value of 0
• Therefore, the resting potential = -70mV
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The Composition of Intracellular and
Extracellular Fluids
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Measuring the Resting Potential
of Neurons
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The Generation of the Action Potential:
The Movement of Ions
• Diffusion
– Molecules move from areas of high
concentration to areas of low concentration
(along a concentration gradient)
• Electrostatic pressure
– Like-signed ions repel each other
– Opposite-signed ions move toward each other
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Diffusion and Electrostatic Pressure
• Resting potential averages -70mV
– Resting membrane is permeable to potassium
– Some sodium leaks into the cell
– Resting potential is maintained by controlling
the movement of potassium ions
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Diffusion and Electrostatic Pressure
(cont’d.)
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The Action Potential – All-or-None
• Depolarization
– Ion movement decreases the membrane
potential toward 0 mV
• The membrane potential must reach the threshold
of about -65mV to produce an action potential
– When the threshold is reached, voltage-gated
sodium ion channels open to allow sodium to
flow into the neuron
– Voltage-gated potassium ion channels open
near the peak of the action potential to allow
potassium to flow out of cell
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The Action Potential – All-or-None (cont’d.)
• Once the cell returns to the resting level, it
actually hyperpolarizes
– Overshoots its target and becomes even
more negative than when at rest
• Refractory period
– Membrane potential returns to resting
potential
– Absolute versus relative refractory periods
• The rate of neural firing varies to reflect
stimulus intensity
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The Action Potential – The Sequence of
Events
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Propagating Action Potentials
• The signal reproduces itself down the
length of the axon
• Influenced by myelination
– Propagation in unmyelinated axon requires
reproduction of the action potential at each
successive axonal segment
– Propagation in myelinated axons requires
reproduction of the action potential in the
nodes of Ranvier: saltatory conduction
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Action Potentials Propagate Down the
Length of the Axon
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Propagation in Unmyelinated and
Myelinated Axons
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Neurons Communicate at the Synapse
• The action potential is transmitted to the
adjacent postsynaptic neuron at the
synapse
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A Comparison of Electrical and Chemical
Synapses
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The Electrical Synapse
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The Chemical Synapse
• Neurotransmitters are released from the
presynaptic cell
• Neurotransmitters bind to postsynaptic
receptor sites
• The chemical signal is then terminated
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Exocytosis Results in the Release of
Neurotransmitters
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Ionotropic and Metabotropic Receptors
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Methods for Deactivating Neurochemicals
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Postsynaptic Potentials
• Small, local, graded potentials
• Excitatory (EPSPs) or inhibitory (IPSPs)
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Neural Integration Combines Excitatory and
Inhibitory Input
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Neuromodulation
• Axo-axonic synapses between an axon
terminal and another axon fiber have a
modulating effect on the release of
neurotransmitter by the target axon
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