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Nervous System
AP Biology Chap 48
Neuron
The basic structural unit of the nervous
system
The job of the neurons
Neurons transfer long-distance
information via electrical signals and
usually communicate between cells
using short-distance chemical signals.
• The higher order processing of nervous
signals may involve clusters of neurons
called ganglia or most structured groups of
neurons organized into a brain.
Types of neurons
• Sensory (afferent) – receive stimulus
• Motor (efferent) stimulate effectors
which are target cells, muscles, sweat
glands, stomach, etc.
• Association (interneurons) located in
spinal cord or grain integrate or
evaluate impulses for appropriate
responses.
• The transmitting cell is called the
presynaptic cells
• The receiving cell is the
postsynaptic cell
Neuron Structure
• Cell body which contains the nucleus and
organelles and numerous extensions
• Dendrites receive signals
• Axon longer, transmits signals (axon-away)
• Ends of axons end in synaptic terminals
which release neurotransmitters across a
synapse
• Glial cells nourish and support the neurons
Fig. 48-4
Dendrites
Stimulus
Nucleus
Cell
body
Axon
hillock
Presynaptic
cell
Axon
Direction of impulse
Synapse
Synaptic terminals
Postsynaptic cell
Neurotransmitter
Glial Cells
• Nourish neurons
• Insulate axons
• Regulate the extracellular fluid around
the neuron
Nerve conduction
• In order to conduct an electrical nerve
impulse, a voltage or membrane
potential, exists across the plasma
membrane of all cells.
• For a typical non-transmitting neuron,
this is called the resting potential and
is between -60 and -80 mV. So
essentially -70 mV.
My neurons are resting!
-70 mV
Membrane Potential
• Principal cation (+) inside of cell K+
• Principle anion (-) inside of cell:
negatively-charged proteins, amino
acids, PO4- and SO4-. Symbol is A-.
iNside is NEGATIVE!
Outside of the cell
• Principal ion is Na+
• Some Cl- outside too.
• Outside is overall positive!
The salty banana!
Get it?
These large neg
ions and molecules
do not move out.
Measuring membrane potential
How is the membrane potential
established?
• Ion channels
• Concentration of ions
• Size of particles (proteins too large
– semipermeable nature of
membrane)
• Na-K pump maintains Na+ outside
and K+ inside
Fig. 48-6b
Key
Na+
K+
OUTSIDE
CELL
INSIDE
CELL
(b)
Sodiumpotassium
pump
Potassium
channel
Sodium
channel
What causes the generation of
a nerve signal?
• Neurons and muscle cells are excitable
cells – they can change their membrane
potentials due to
• gated ion channels* – can be
chemically gated which respond to
neurotransmitters or
• voltage-gated which respond to a
change in membrane potential.
* Found only in nerve cells
• Upon receiving a stimulus, Na+ channels
open and Na+ flows into the cells and thus
they become more positive inside and
more negative outside and the charge on
the membrane becomes depolarized.
• The stronger the stimulus, the more Na
gated Ion channels open.
Production of an Action Potential
• Once depolarization reaches a certain
membrane voltage called the threshold level
(-50 mv), more Na gates open and an action
potential is triggered that results in
complete depolarization.
• This stimulates neighboring Na gates,
further down the neuron, to open. The
action potential is an all or none event,
always creating the same voltage spike
once the threshold is reached.
Fig. 48-10-1
Key
Na+
K+
Membrane potential
(mV)
+50
Action
potential
–50
Plasma
membrane
2
4
Threshold
1
5
1
Resting potential
Depolarization
Extracellular fluid
3
0
–100
Sodium
channel
Time
Potassium
channel
Notice, gates are closed!
Cytosol
Inactivation loop
Undershoot
1
Resting state
Fig. 48-10-2
Key
Na+
K+
Membrane potential
(mV)
Some Na+ gates open!+50
Action
potential
–50
2
Plasma
membrane
2
4
Threshold
1
5
1
Resting potential
Depolarization
Extracellular fluid
3
0
–100
Sodium
channel
Time
Potassium
channel
Notice, gates are closed!
Cytosol
Inactivation loop
Undershoot
1
Resting state
Fig. 48-10-3
Key
Na+
Na+ gates open!
K+
A lot of Na+ gates open!
3
Rising phase of the action potential
Membrane potential
(mV)
+50
Action
potential
–50
2
2
4
Threshold
1
5
1
Resting potential
Depolarization
Extracellular fluid
3
0
–100
Sodium
channel
Time
Potassium
channel
Plasma
membrane
Cytosol
Inactivation loop
Undershoot
1
Resting state
• In response to the inflow of Na, the
gated K channels begin to open,
allowing K to rush to the outside of the
cell. Na gates close. This creates a
reverse charge polarization, (neg
outside, positive inside) called
repolarization.
Fig. 48-10-4
Key
Na+
K+
3
4
Rising phase of the action potential
Falling phase of the action potential
Membrane potential
(mV)
+50
2
4
Threshold
1
5
1
Resting potential
Depolarization
Extracellular fluid
3
0
–50
2
Na closes, K opens
Action
potential
–100
Sodium
channel
Time
Potassium
channel
Plasma
membrane
Cytosol
Inactivation loop
Undershoot
1
Resting state
In fact more K ions go out than is
actually needed to return to
threshold,
resulting in an increased negative
charge inside called a
hyperpolarization or undershoot.
This keeps the direction of the
nerve impulse going one way and
not backing up.
Fig. 48-10-5
Key
Na+
K+
3
4
Rising phase of the action potential
Falling phase of the action potential
Membrane potential
(mV)
+50
Action
potential
–50
2
2
4
Threshold
1
1
5
Resting potential
Depolarization
Extracellular fluid
3
0
–100
Sodium
channel
Time
K just keeps
flowing out.
Potassium
channel
Plasma
membrane
Cytosol
Inactivation loop
5
1
Undershoot
Resting state
Hyperpolarization
Refractory Period
• After the impulse, the Na channels
remain inactivated
• Since the neuron cannot respond to
another stimulus with the reversal of
charges, the Na-K pump has to restore
the original charge location. This is
called the refractory period.
https://www.youtube.com/watch?v=iBDXOt_uHTQ
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impul
se.html
Action Potentials Video | DnaTube.com - Scientific Video Site
Requires the Na-K pump
Fig. 48-11-3
Axon
Plasma
membrane
Action
potential
Cytosol
Na+
K+
Action
potential
Na+
K+
https://www.youtube.
com/watch?v=9euDb
4TN3b0
K+
Action
potential
Na+
K+
Boseman video on action potential
https://www.youtube.com/watch?v=HYLyhXRp298
Watch this for review!
Properties of an Action
Potential
• Are all or none depolarization – once
threshold is reached (-50 mV) – always
creates the same voltage spike regardless of
intensity of the stimulus.
• The frequency of the action potentials
increases with intensity of stimulus.
• Action potentials travel in only ONE
direction!
• The greater the axon diameter, the faster
action potentials are propagated.
Importance of myelin
• Acts as insulators.
• Gaps in the myelin are called nodes of
Ranvier and serve as points along
which the action potential is
propagated, increasing the speed.
• This is called saltatory conduction.
The myelin sheath is composed of
Schwann cells (PNS) or
oligodendrocytes (CNS) that
encircle the axon in vertebrates.
Saltatory Conduction
• Voltage channels concentrated at the
nodes of Ranvier - jumping action
potentials
Multiple Sclerosis
http://www.youtube.com/watch?v=o4YkqRUErPY
The Synapse
• Area between two neurons, between
sensory receptors and neurons or
between neurons and muscle cells or
gland cells
Types of synapses
• Electrical – via gap junctions such as in
giant axons of crustaceans
• **Chemical – electrical impulses
changed into chemical signals
Fig. 48-15
What happens at the synapse?
5
Synaptic vesicles
containing
neurotransmitter
Voltage-gated
Ca2+ channel
Na+
Presynaptic
membrane
Postsynaptic
membrane
1 Ca2+
4
2
Synaptic
cleft
K+
6
3
Ligand-gated
ion channels
http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter44/transmission_acros
s_a_synapse.html
What happens at the synapse?
Arrival of an action potential (1) opens Ca+
channels (2) (membrane signaling cAMP),
causes synaptic vesicles full of NT’s to fuse
with pre-synaptic cell membrane (3) and pop
open (4) releasing the NT’s which travel across
the synapse to the receptor on the postsynaptic cell (4) opening the Na gates (5) to
continue the nerve impulse. After the threshold
is reached, the Na gates close (6).
What happens to the NT’s?
• They can be transported (transporters)
back to the presynaptic cell
• They can be broken down by enzymes.
• They can diffuse out of the cell.
Types of neurotransmitters
Each neuron secretes only ONE type of
NT.
• Excitatory (EPSP) NT’s depolarize the
post-synaptic cell
• Inhibitory (INSP) NT’s hyperpolarize
and thus the potential is not carried on.
EPSP Excitatory Post Synaptic Potential
EPSP and IPSP
Integration of impulses
Axon hillock
summation
• Through summation, an IPSP can
counter the effect of an EPSP
• The summed effect of EPSPs and
IPSPs determines whether an axon
hillock will reach threshold and
generate an action potential
Summation of impulses
• Temporal summation occurs with
repeated release of nt’s from one
or more synaptic terminals before
RP
• Spatial summation occurs when
several different presynaptic
terminals release NT’s
simultaneously
Assume a single IPSP has a negative
magnitude of -0.5 mV at the axon
hillock and that a single EPSP has a
positive magnitude of +0.5 mV, for a
neuron with initial membrane
potential of -70 mV, the net effect of 5
IPSP’s and 2 EPSPs spatially would
be to move the membrane potential
to? Would the impulse continue?
-71.5 mV
Neurotransmitters
(a) Affect ion channels
(b) Affect signal transduction pathways
How? Involve cAMP, cAMP protein
kinases, GTP, GTP binding proteins
Neurotransmitters
• The same neurotransmitter can
produce different effects in different
types of cells
• There are five major classes of
neurotransmitters: acetylcholine,
biogenic amines, amino acids,
neuropeptides, and gases
a. ACETYLCHOLINE
• Found in vertebrate neuromuscular
junctions
- excitatory at skeletal muscles
- inhibitory at heart
b) Biogenic Amines (derived from amino
acids)
• epinephrine, norepinephrine (fight or
flight),
• dopamine, serotonin (involved in
sleep, mood, attention, and learning).
Blocking epinephrine
c) Amino Acids
• Types:
GABA – most common inhibitor
Glutamate - excitatory
d) Neuropeptides (short chains of
amino acids)
Types
• Endorphins – inhibitory, relieves pain
• Opiates – mimic endorphins
e) Gaseous signals
• Gases such as nitric oxide and
carbon monoxide are local regulators
in the PNS – can dilate blood vessels
How do drugs work?
• Agonists – mimic neurotransmitters such as
in nicotine mimicking acetycholine
• Antagonists – block action of NT’s such as
atropine and curare (poisons) – block
acetylcholine and thus prevent nerve firing in
muscles – leads to paralysis and death
• Cocaine and amphetamines block the
reuptake of NT’s at synapses
Many antidepressants block reuptake of
serotonin so serotonin lingers longer in
synaptic cleft.