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Notes The Nervous
Section 2 Notes
Notes The Nervous
Section 2 Lecture Outline – The Nervous System
textbook questions Keys
Keys Nervous System = Communication
Causes a response
stimuli (inside or
outside the body) What is the basic functional unit of
the nervous system?
What does the neuron do?
Carries messages throughout
How does it carry the messages?
By conducting electrical signals
What are these signals called?
Nerve impulses! What does a neuron look like?
What Neuron Anatomy
Cell Body Three parts to a
1. Cell body:
portion of the
neuron where all
located What is its job?
signals Neuron Anatomy
branched what is its job?
toward the cell body
Referred to as the
afferent Dendrites Neuron Anatomy
Long, slightly branched fiber
Long, What is its job?
Carry impulses away from the cell body
Referred to as the efferent process Neuron Anatomy
Three parts of the Axon:
A. Axon hillock:
= site of initiation of an action potential
(point where axon and cell body meet)
B. Axon fiber:
= the main portion of the axon
C. Axon terminal:
= branched end of the axon
= point of communication with other cells
point The Axon
Axon Hillock fiber Axon Terminals Neuron Anatomy
Additional Parts of a Neuron Structure of a Typical Neuron
Dendrite Axon terminal Cell body Nodes of
Nucleus Myelin sheath Neuron anatomy
What do you remember? 1
terminals 3 Types of Neurons
I. Sensory Neurons:
Receive incoming stimuli Five types of sensory neurons:
Thermo-receptors Location: Skin
Body Core Function: Sensation of hot and
Detects change in
body core temp.
Inner ear Function: Touch
Tongue (taste buds)
Blood vessels Function: Smell
Detects levels of CO2
Photo-receptors Location: Eyes Function: Allow vision thru detection of light Pain-receptors
Pain-receptors Location: Everywhere, except
the Function: Sensation of pain
released by damaged
cells Three Types of Neurons
II. Motor Neurons:
Carry impulses to muscles and glands
Cause a response to some stimuli
Connect sensory and motor neurons
Allow for quick response (reflex action) Three types of neurons
What is grey matter? Collective cell bodies and
dendrites of all neurons
dendrites What is white matter? Myelinated nerve fibers
Axons of all neurons
Can be approximately
one meter in length White matter Gray matter What is a Nerve?
Bundle of axons
held together by
What color is
are white matter
and they compose
nerves How are nerves held together?
What is this connective
Neuroglial cells (nerve glue)
Approximately half of the
volume of the brain is composed
of neuroglial cells
Most brain tumors develop in mesoglial
cells – NOT neurons
cells What do neuroglial cells do?
Support the axons
Insulate the electrical impulses
Like electrical tape insulates
electric wires this prevents “leaking”
of electric signals An example of a neuroglial cell
This wraps around the axon in multiple layers
It is composed of a fatty material called…
Regular breaks in the myelin sheath are called…
Node of Ranvier Impulse speed
Some neurons are fast, good conductors
Other neurons are slow, poor conductors
What distinguishes these
Diameter of the axon
Fastest axons have a large diameter and are
myelinated. How much faster?
Up to 100 times faster! (Reflexes are this type)
1. The Nerve Impulse
How are messages (impulses)
carried by the nervous system?
As electrical and chemical
How does the impulse develop?
Charged particles (ions) move
across the cell membrane
A neuron is ready to transmit an impulse when it
is in the resting state.
Resting membrane potential is -70 mV Resting Membrane Potential
The inside of the cell has
a negative charge as
compared to the outside
of the cell membrane.
How does this charge
There are more Na+
(sodium) ions outside
and fewer K+ (potassium)
ions The Nerve Impulse
How does the impulse begin?
The neuron is stimulated by another
neuron or by stimuli from the environment.
neuron The Nerve Impulse
If the stimulus is
strong enough, it
This stimulates an
If the stimulus is not
strong enough, no
impulse occurs (all-orimpulse
none principal) The Nerve Impulse
What happens if threshold is reached?
An action potential (nerve impulse) begins
What is an action potential?
Rapid reversal of membrane potential in
response to a stimulus
How does this happen?
Sodium channels open allowing
Na+ to flood into the cell.
The membrane potential rises to +30 mV (rising
phase) as inside of cell becomes more positive
phase) The Nerve Impulse
Please, please, tell us what happens next…
When the membrane potential reaches +30 mV,
the sodium channels close.
Potassium channels open and K+ flows out of
This causes the membrane
potential to become more
(falling The Nerve Impulse
leave cell K+ leaves
cell 0 Threshold of
excitation Membrane potential (mV) +50 -70 1
enters cell K+ channels
Excess K+ outside
diffuses away The Nerve Impulse
The The Nerve Impulse
potential -55 zatio
D ep olari 0 ation
Repolariz Voltage (mV) +40 Failed
initiations Threshold Resting state -70
Stimulus 0 1 Refractory
Time (ms) 4 5 The Nerve Impulse
What happens when the action potential
passes down the axon?
The resting potential is restored via the
Now the membrane is ready to transmit
K+ extracellular fluid Na+ ATP cytoplasm ADP The Nerve Impulse
An action potential only moves in one direction down the
From axon hillock, thru axon fiber to the axon terminal.
The action potential is regenerated at each Node of
Ranvier down the length of the axon.
Ranvier The Nerve Impulse
What happens when the impulse reaches the
It must pass thru
What is a synapse?
It is the gap between two
neurons, or between a neuron
and organ (effector).
Why is there a gap?
So the neurons don’t short each other out!! The Synapse
How do neurons pass the impulse across the
Using chemicals called neurotransmitters
What do neurotransmitters do?
Stimulate the dendrites of other neurons or
membranes of other cells.
If a dendrite is stimulated, it sends its message
to the cell body and the message is passed on
If a muscle or gland is stimulated, a reaction
occurs in that organ.
occurs The Synapse
The The Synapse
Model of the
Synapse synaptic vesicles Axon of
presynaptic cell Glycoprotein
neurotransmitters synaptic cleft receptors for
glycoprotein The Synapse
The Neuron Smear:
Identification: Note distinctive shape of neuron, with long
processes (dendrites and/or axons, 5) extending out from
main cell body.
Features to Know:
Features The large, irregularly shaped cell body (3) contains a
darker nucleus (2), which contains an even darker-staining
nucleolus (1). There are also numerous supporting
glial cells, though only
small dark nuclei
(4) are easily seen. Neuron
Nerve Myelin sheath ...
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- Winter '11
- axon hillock