1. 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

2. The Action Potential Is a Rapid Change in Membrane Potential
1. Depolarization
phase
2. Repolarization
phase
Threshold potential
Resting potential
3. Hyperpolarization phase

3. Voltage-gated sodium channels allow the action potential to occur

4. 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, voltage-
gated Na+ channels are closed.
Conformational changes open
voltage-gated channels when
the membrane is depolarized.

5. Resting Potential - Both voltage gated Na+ and K+ channels are closed.

6. Initial Depolarization - Some Na+ channels open
Initial Depolarization - Some Na+ channels open. If enough Na+ channels open, then the threshold is surpassed and an action potential is initiated.

7. Na+ channels open quickly. K+ channels are still closed.
PNa+ > PK+

8. Na+ channels self-inactivate, K+ channels are open.
PK+ >> PNa+

9. Emembrane ≈ E K+
PK+ > PK+ at resting state

10. Resting Potential - Both Na+ and K+ channels are closed.

11. Before the end of the semester you are going to learn how
A. how the nerve cell interprets the information incoming from other neurons at the dendrites & axon hillock
B. how the signal is propagated along the axon
C. how the signal is transferred via neurotransmitters to the next neuron (or muscle as shown in this graph)
In order to understand any of this, we need to understand the nature of biological electricity - which is the point of this lecture. Then, we’re going to discuss B (how the signal is propagated along the axon). Then A. and Then C.

12. 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.

13. 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.

14. Why does the membrane potential decrease during stage 4 of the action potential?
A. The voltage gated K+ channels open.
B. The voltage gated Na+ channels open.
C. The voltage gated K+ channels close.
D. The voltage gated Na+ channels close.
E. A and D

15. Action Potentials Propagate Quickly in Myelinated Axons
Action potentials jump down axon.
Action potential jumps
from node to node
Nodes of Ranvier
Axon
Schwann cells (glia)
wrap around axon,
forming myelin sheath
Schwann cell membrane
wrapped around axon

16. 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.

17. Multiple Sclerosis (MS)
Disease results in damage to myelin and impairs electrical signaling.
Muscles weaken and coordination decreases.

18. Presynaptic
Postynaptic

19. neurotransmitter Synaptic vesicle Neurotransmitter transporter
Axon Terminal
(pre-synapse)
Voltage-gated
Ca++ channel
Neurotransmitter
Receptor
Synapse
Don’t worry about this
Dendrite
(post-synapse)

20. ACTION POTENTIAL TRIGGERS RELEASE OF NEUROTRANSMITTER
Na+ and K+
channels
Presynaptic
membrane
(axon)
Postsynaptic
(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).

21. Synapse animation

22. 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-.

23. 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

24. Postsynaptic Potentials Can Depolarize or Hyperpolarize the Postsynaptic Membrane
Depolarization,
Na+ inflow
Hyperpolarization, K+
outflow or Cl– inflow
Depolarization and
hyperpolarization
stimuli applied
Excitatory
postsynaptic
potential
(EPSP)
Inhibitory
postsynaptic
potential
(IPSP)
EPSP  IPSP
Resting potential

25. 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

26. Neurons Integrate Information from Many Synapses
Postsynaptic potentials sum.
Action potential
Threshold
Resting
potential

28. 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).

29. Acetylcholine Stimulates muscles Also found throughout nervous system
Usually excitatory, but can be inhibitory depending on the receptor

30. Acetylcholine

31. Dopamine
Excitatory (but sometimes inhibitory) depending on the location in the nervous system
Associated with the reward system!!
Requires a transport protein to inactivate

32. Dopamine

33. 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.

34. The Autonomic Nervous System Controls Internal Processes
PARASYMPATHETIC NERVES
“Rest and digest”
SYMPATHETIC NERVES
“Fight or flight”
Constrict pupils
Dilate pupils
Stimulate saliva
Inhibit salivation
Slow heartbeat
Cranial
nerves
Increase heartbeat
Cervical
nerves
Constrict airways
Relax airways
Stimulate activity
of stomach
Inhibit activity
of stomach
Thoracic
nerves
Inhibit release of
glucose; stimulate
gallbladder
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
Sympathetic chain:
bundles of nerves
that synapse with
nerves from spinal
cord, then send
projections to organs
Relax bladder
Promote erection
of genitals
Promote
ejaculation and
vaginal contraction

35. The Functions of the PNS Form a Hierarchy
Central nervous system (CNS)
Information processing
Peripheral nervous system (PNS)
Sensory
information
travels in
afferent division
Most information
travels in
efferent division,
which includes…
Somatic
nervous
system
Autonomic
nervous system
Sympathetic
division
Parasympathetic
division