Monday April 11, 2014.
Nervous system and biological electricity III
1. No pre-lecture quiz
2. A review of Action potentials
3. Myelin
4. Synapses and neurotransmitters
The Action Potential Is a Rapid Change
in Membrane Potential
1. Depolarization
phase
2. Repolarization
phase
Threshold potential
Resting potential
3. Hyperpolarization phase
Voltage-gated sodium channels allow
the action potential to occur
• https://www.youtube.com/watch?v=ifD1YG07
fB8
Voltage-gated channels
Two important types:
1.) Na+ voltage gated channels
2.) K+ voltage gated channels
How voltage-gated channels work
At the resting potential, voltagegated Na+ channels are closed.
Conformational changes open
voltage-gated channels when
the membrane is depolarized.
Resting Potential - Both voltage gated Na+ and K+ channels are
closed.
Initial Depolarization - Some Na+ channels open. If enough Na+
channels open, then the threshold is surpassed and an action
potential is initiated.
Na+ channels open quickly. K+ channels are still closed.
PNa+ > PK+
Na+ channels self-inactivate, K+ channels are open.
PK+ >> PNa+
Emembrane ≈ E K+
PK+ > PK+ at resting state
Resting Potential - Both Na+ and K+ channels are closed.
Action Potentials Propagate because Charge Spreads down the Membrane
PROPAGATION OF ACTION POTENTIAL
Axon
Neuron
1. Na+ enters axon.
2. Charge spreads;
membrane
“downstream”
depolarizes.
Depolarization at
next ion channel
3. Voltage-gated
channel opens in
response to
depolarization.
3
4
2
1
6
5
Why does the membrane potential increase during stage 3 of the action potential?
A. Both the voltage-gated Na+ channels and voltage gated K+ channels are open.
B. All of the K+ channels (both leak and voltage gated) are open.
C. The voltage gated Na+ channels are open, but the voltage gated K+ channels have
not opened yet.
D. The voltage gated Na+ channels are open, but the K+ channels (both voltage gated
and leak) have not opened yet.
3
4
2
1
6
5
Why does the membrane potential decrease during stage 4 of the
action potential?
A.
B.
C.
D.
E.
The voltage gated K+ channels open.
The voltage gated Na+ channels open.
The voltage gated K+ channels close.
The voltage gated Na+ channels close.
A and D
Action Potentials Propagate Quickly in Myelinated Axons
Action potentials jump down axon.
Action potential jumps
from node to node
Nodes of Ranvier
Schwann cells (glia)
wrap around axon,
forming myelin sheath
Axon
Schwann cell membrane
wrapped around axon
The process of coating axons with myelin is incomplete
when humans are born. This is part of the reason why
babies are uncoordinated and slow learners.
Babies need lots of fat – not only for energy storage but also to myelinate their neurons.
Multiple Sclerosis (MS)
• Disease results in damage to myelin and
impairs electrical signaling.
• Muscles weaken and coordination decreases.
Presynaptic
Postynaptic
neurotransmitter
Synaptic vesicle
Voltagegated
Ca++
channel
Don’t worry
about this
Neurotransmitter
transporter
Axon Terminal
(pre-synapse)
Neurotransmitter
Receptor
Synapse
Dendrite
(post-synapse)
ACTION POTENTIAL TRIGGERS RELEASE OF
NEUROTRANSMITTER
Na+ and K+
channels
Presynaptic
membrane
(axon)
Postsynaptic
membrane
(dendrite or
cell body)
Action
potentials
1. Action potential arrives;
triggers entry of Ca2+.
2. In response to Ca2+, synaptic
vesicles fuse with presynaptic
membrane, then release
neurotransmitter.
3. Ion channels open when
neurotransmitter binds; ion
flows cause change in
postsynaptic cell potential.
4. Ion channels will close as
neurotransmitter is broken
down or taken back up by
presynaptic cell (not shown).
Synapse animation
https://www.youtube.com/watch?v=LT3VKAr4roo
Ion Channels on Post-synaptic Cell at
Synapse
• Some only let Na+ pass through.
• Some let Na+/K+ pass through.
• Some only let K+ pass through.
• Some increase the permeability of Cl-.
Excitatory vs. Inhibitory Synapses
• Excitatory synapses cause the post-synaptic cell
to become less negative triggering an excitatory
post-synaptic potential (EPSP)
– Increases the likelihood of firing an action potential
• Inhibitory synapses cause the post-synaptic cell
potential to become negative triggering an
inhibitory post-synaptic potential
– Decreases the likelihood of firing an action potential
Postsynaptic Potentials Can Depolarize or Hyperpolarize
the Postsynaptic Membrane
Postsynaptic potentials can depolarize or hyperpolarize the
postsynaptic membrane.
Depolarization,
Na+ inflow
Hyperpolarization, K+
outflow or Cl– inflow
Excitatory
postsynaptic
potential
(EPSP)
Resting potential
Inhibitory
postsynaptic
potential
(IPSP)
Depolarization and
hyperpolarization
stimuli applied
EPSP  IPSP
Neurons Integrate Information from Many Synapses
Most neurons receive information from many other neurons.
Axons of
presynaptic neurons
Dendrites of
postsynaptic neuron
Cell body of
postsynaptic neuron
Axon
hillock
Axon of postsynaptic cell
Excitatory synapse
Inhibitory synapse
Neurons Integrate Information from Many Synapses
Postsynaptic potentials sum.
Action potential
Threshold
Resting
potential
Neurotransmitters
• More than 100 neurotransmitters are now
recognized, and more will surely be discovered.
• Acetylcholine is important and one of the first ones
discovered because its involvement in muscle
movement.
• Dopamine and serotonin hugely important for
many behaviors.
• The workhorses of the brain are glutamate, glycine,
and γ-aminobutyric acid (GABA).
Acetylcholine
• Stimulates muscles
• Also found throughout nervous system
• Usually excitatory, but can be inhibitory
depending on the receptor
Acetylcholine
Dopamine
• Excitatory (but sometimes inhibitory) depending
on the location in the nervous system
• Associated with the reward system!!
• Requires a transport protein to inactivate
Dopamine
Serotonin
• Excitatory or inhibitory depending on area of CNS
• Ecstasy (MDMA) causes increased release
• Involved in sleep, appetite, mood
• Drugs like prozac (SSRIs – selective serotonin reuptake
inhibitor) slows down transport protein
• Transporter also binds cocaine and amphetamines.
The Autonomic Nervous System Controls Internal Processes
PARASYMPATHETIC NERVES
SYMPATHETIC NERVES
“Rest and digest”
“Fight or flight”
Constrict pupils
Dilate pupils
Stimulate saliva
Inhibit salivation
Slow heartbeat
Constrict airways
Cranial
nerves
Cervical
nerves
Relax airways
Stimulate activity
of stomach
Inhibit release of
glucose; stimulate
gallbladder
Inhibit activity
of stomach
Thoracic
nerves
Stimulate release
of glucose; inhibit
gallbladder
Stimulate activity
of intestines
Inhibit activity
of intestines
Lumbar
nerves
Secrete
epinephrine and
norepinephrine
(hormones that
stimulate activity;
see Chapter 47)
Sacral
nerves
Contract bladder
Promote erection
of genitals
Increase heartbeat
Sympathetic chain:
bundles of nerves
that synapse with
nerves from spinal
cord, then send
projections to organs
Relax bladder
Promote
ejaculation and
vaginal contraction
The Functions of the PNS Form a Hierarchy
Central nervous system (CNS)
Information processing
Peripheral nervous system (PNS)
Sensory
information
travels in
afferent division
Somatic
nervous
system
Most information
travels in
efferent division,
which includes…
Autonomic
nervous system
Sympathetic
division
Parasympathetic
division