Download The Nervous System

Notes
The Nervous
System
Chapter 35
Section 2
Notes
The Nervous
System
Chapter 35
Section 2
Nervous System = Communication
Function:
Receives Stimuli
Interprets information
Causes a response
-Controls and
coordinates
bodily functions
-Responds to
stimuli (inside or
outside the body)
What is the basic functional unit of
the nervous system?
Neuron!
What does the neuron do?
Carries messages throughout
the body
How does it carry the messages?
By conducting electrical signals
What are these signals called?
Nerve impulses!
What does a neuron look like?
Neuron Anatomy
Cell Body
Three parts to a
NEURON:
1. Cell body:
Large, central
portion of the
neuron where all
organelle are
located


What is its job?
Interpret incoming
signals
Neuron Anatomy
2. Dendrites:
Short, highly
branched fibers



what is its job?
Carries impulses
toward the cell body
Referred to as the
afferent process
Dendrites
Neuron Anatomy
3. Axon:
Long, slightly branched fiber



What is its job?
Carry impulses away from the cell body
Referred to as the efferent process
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
Ranvier
Axon
Nucleus
Myelin sheath
Neuron anatomy
What do you remember?
1
dendrites
6
Myelin
sheath
2
nucleus
Direction
of impulse
3
Cell body
5axon
Nodes of
Ranvier4
Axon
7
terminals
Three Types of Neurons
II. Motor Neurons:
Carry impulses to muscles and glands
Cause a response to some stimuli
III. Interneurons:
Connect sensory and motor neurons
Allow for quick response (reflex action)
Three types of neurons
Neurons
What is grey matter?

Collective cell bodies and
dendrites of all neurons
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
connective tissue.
What color is
a nerve?
White!
Why?
Because axons
are white matter
and they compose
nerves
How are nerves held together?
Connective tissue
What is this connective
tissue called?
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
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
Schwann cell
This wraps around the axon in multiple layers
It is composed of a fatty material called…
Myelin
Regular breaks in the myelin sheath are called…
Node of Ranvier
Impulse speed
Some neurons are fast, good conductors
of impulses
Other neurons are slow, poor conductors
of impulses
What distinguishes these
two types?
Diameter of the axon
2.
Myelination
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
signals.
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
difference develop?
There are more Na+
(sodium) ions outside
and fewer K+ (potassium)
ions inside.
The Nerve Impulse
How does the impulse begin?
The neuron is stimulated by another
neuron or by stimuli from the environment.
The Nerve Impulse
If the stimulus is
strong enough, it
reaches threshold
level.
This stimulates an
impulse.
If the stimulus is not
strong enough, no
impulse occurs (all-ornone 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
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
the cell.
This causes the membrane
potential to become more
negative again
(falling phase).
The Nerve Impulse
Na+ channels
close
2
K+ channels
open, K+
begins to
leave cell
K+ leaves
cell
0
Threshold of
excitation
Membrane potential (mV)
+50
-70
1
Na+
channels
open, Na+
enters cell
K+ channels
close
3
Excess K+ outside
diffuses away
The Nerve Impulse
Action
potential
Voltage (mV)
+40
0
-55
Failed
initiations
Threshold
Resting state
-70
Stimulus
0
1
Refractory
period
2
3
Time (ms)
4
5
The Nerve Impulse
What happens when the action potential
passes down the axon?
The resting potential is restored via the
sodium/potassium pump.
Now the membrane is ready to transmit
another impulse.
K+
extracellular fluid
Na+
ATP
cytoplasm
ADP
The Nerve Impulse
An action potential only moves in one direction down the
axon.
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.
The Nerve Impulse
What happens when the impulse reaches the
axon terminal?
It must pass thru
the synapse.
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
synapse?
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.
The Synapse
The Synapse
Model of the
Synapse
synaptic vesicles
Axon of
presynaptic cell
Glycoprotein
(ligands)
neurotransmitters
synaptic cleft
receptors for
neurotransmitters
receptors for
glycoprotein
The Synapse
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:
 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 their
small dark nuclei
(4) are easily seen.