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Neuronal Anatomy
and Communication
Cells of the Nervous System:
Neurons

Three types of neurons:
 Sensory
neurons
 Motor neurons
 Interneurons
Neuronal structure
Soma
 Dendrites
 Axon
 Terminal buttons
 Synaptic cleft

Neuronal classifications

Multipolar neuron

Bipolar neuron (a)

Unipolar neuron (b)
Internal structure
Cell membrane
 Cytoplasm
 Mitochondria
 Nucleus

 Chromosomes
 Proteins

Microtubules
Cells of the Nervous System:
Glia

Glial cells support neural function
Glial Cells

Astrocytes
 Arms
wrap around blood
vessels
and neuronal structures
 Isolate the synaptic cleft
 Maintain chemical
composition
of extracellular space
 Clean up following cell
death
Glial cells

Oligodendrocytes (CNS) &
Schwann cells (PNS)
 Provide
support and insulation
in the form of the myelin
sheath
Myelin
 Nodes of Ranvier

Glial cells

Microglia
 Smallest
glial cells
 Brain’s immune cells
Blood-Brain Barrier
Composed of tightly-packed cells of the
cerebral blood vessels.
 Regulates chemicals in the CNS
 Protects the brain from toxins
 Semipermeable

The withdrawal reflex: an example of
neuronal communication
1. Sensory neuron detects
2. Message is sent
3. Neurotransmitter is released
4. Interneuron
5. Motor neuron sends a
message
Communication within a neuron
Based on changes in the membrane
potential of the neuron.
 Neurons have two basic potentials

Resting membrane potential
The inside of a neuron is negatively
charged relative to the outside
 Due to concentrations of positively and
negatively charged ions in the brain

 Outside
the cell
 Inside the cell
Resting membrane potential


A concentration gradient pulls the sodium,
potassium and chloride ions toward the
membrane; electrostatic forces prevent them
from crossing it.
The balance between potassium and sodium
ions in and out of the neuron is maintained
Membrane potential

The membrane potential can change:
 Depolarization
 Hyperpolarization
-40
-50
-60
Depolarization
-70
-80
Hyperpolarization
Resting Membrane
Potential
Action potential
A hyperpolarization _________
communication within a cell.
 A depolarization _________ the cell, and
_________ the chances of communication
within the cell.
 Threshold of excitation

Action potential
A massive, momentary reversal of the
membrane potential.
 Carried down the axon from the cell body
to the terminal buttons.
 Results in the release of a chemical
message into the synapse.

Action potential

Chemical messages from other neurons
affect the neuron’s charge.

Excitatory Post-Synaptic Potentials (EPSPs)
 Inhibitory Post-Synaptic Potentials (IPSPs)

When the cell is depolarized to -65mV, an
action potential begins.
Steps of the action potential
1.
2.
3.
4.
5.
6.
Ion channels in the
membrane rapidly open and
Na+ enters the cell (-65mV 
+40mV)
As Na+ rushes in, K+ is
forced out of the cell.
As the action potential peaks,
Na+ channels close, and no
more Na+ enters the cell.
K+ is forced out of the cell,
which decreases the charge
inside the cell and K+
channels close.
K+ ions trapped outside of the
cell result in a temporary
hyperpolarized membrane
potential.
Ion channels reset and the
Na+/K+ pump returns the ions
to the normal gradients.
All-or-None law

An action potential either occurs or it
doesn’t.
 Magnitude
is the same.
 Does not diminish in strength.
Rate law
The strength of a response depends on the
firing rate of the cell.
 More action potentials/second = strong
response, fewer = weak response.

Action potential conduction

Action potentials depend on sodium influx
from the extracellular fluid.
 Nodes

of Ranvier.
Saltatory conduction
Communication between
neurons
Within-neuron communication: electrical
signal
 Between-neuron communication: chemical
signal
 Synaptic transmission

Synaptic structure

Presynaptic membrane
 Terminal
button
 Vesicles
 Transporter
molecules
Synaptic cleft
 Postsynaptic membrane

 On
the dendrite, soma or axon
 Receptors
Neurotransmitter binding
Binding sites
 Ligands

 Molecule
that fits into a specific binding site
Endogenous ligands
 Exogenous ligands

Synaptic firing
1.
2.
Initiated by an action potential in the cell
Neurotransmitter (NT) binds to the
receptor

Prompting specific ion channels to open
Types of receptors


Ionotropic receptor
Metabotropic receptor
Neurotransmitter
Ion Channel
Joins the
ion channel
Receptor
G-protein
Enzyme
Gate
Second
messenger
Synaptic firing
3.

Postsynaptic potentials are produced by the
flow of ions in and out of the cell.
Each NT produces a specific postsynaptic
potential

Excitatory NTs
 Inhibitory NTs
Synaptic firing
4.
Neural integration is the summation of all
postsynaptic potentials.

Determines the response to PSPs.
Synaptic firing

Remember – Each neuron has synaptic
connections with hundreds of other
neurons, and must summate all incoming
PSPs thousands of times each second!
Synaptic firing
5.
Removal of NT from the synapse
terminates PSPs

Reuptake
 Enzymatic deactivation
Autoreceptors

Found on the presynaptic
membrane
Types of synapses
Axodendritic
 Axosomatic
 Axoaxonic
