Download Biology 212: January 30, 2002

Biology 212: January 14 (begin) and January 16, 2004
Nervous System 1
Disclaimer: This is meant to be a lecture outline and study guide, not a complete transcript
of the lecture. However, I have included more detail than usual for section III to help
reinforce the key concepts. Don’t just memorize, THINK AND UNDERSTAND!
I.
Overview of the nervous system [Fig. 48.1]
A. Sensory, integrative and motor
II. Structure of the neuron and associated cells
A. Basic parts: structure and function [Fig. 48.2]
1. cell body
2. dendrites
3. axons
B. Glial cells
C. Examples of neurons
III. Membrane potentials: the key to electrical signals
A. What is meant by the membrane potential?
B. What determines the value of the membrane potential? Focus on the neuron at
rest (i.e. the resting potential) which is approximately -70 mV.
1. Chemical gradients
a. Ions are distributed differentially across the cell membrane [Fig. 48.7a]
 The distribution of the two major ions responsible for resting potential and
action potential, potassium ions (K+) and sodium ions (Na+), are
maintained by a specialized “pump” that requires ATP. (Na+- K+ pump)
[Fig. 48.7b]
2. Selective permeability of the membrane
a. The contribution of a particular ion to the membrane potential of the neuron at
any given time is correlated with its permeability. Impermeable ions cannot
move and thus cannot contribute to the potential.
b. At rest, the neuron is relatively permeable to K+ and almost impermeable to
Na+, thus K+ is nearly solely responsible for the RP of neurons.
 These differences are due to differences in numbers of channels (proteins
allowing particular ions through) that are “open” at rest.
(i) At rest, the membrane is about 25 times more permeable to K+ than
Na+ because there are so many more “passive” channels for K+ than
Na+
(ii) Note that these are “resting” or “passive” channels. They are not
voltage-gated or chemically-gated, and are generally always open.
 OK, I admit that there are ways to close them, but that is beyond
the scope of this course ;-)
3. Putting it together: Actual value of the resting potential is the balance of
electrical and chemical gradients among all ions to which the membrane is
permeable.
a. Intuitive look at why resting potential is negative (approx. -70 mV).

Potassium (K+) ions leave the cell along their chemical gradient because
the membrane is relatively permeable to it. Since potassium is positively
charged, the result is that the inside is negative.
 It would be even more negative (about -85 mV is where K+ chemical
gradient matches the electrical gradient) but the membrane is slightly
permeable to sodium ions, and those enter and raise the potential a bit…
 I think your text does a nice job with RP, pp. 1026-1028.
 Note that the gradients don’t appreciably change because it takes very few
little movement of ions before the chemical gradient is balanced by the
electrical gradient.
(i) Also, the Na+- K+ pump consistently operates to maintain the
gradients.
4. NOTE: You have absolutely must understand how chemical gradients and
selective permeability, and resultant (though minimal) movement of ions at any
given time determine the particular electrical potential of the membrane at that
time!!!
C. The action potential
1. What is an action potential (AP)?
a. Defined as a rapid, transient change in the membrane potential from negative
to positive, and then negative again (back to resting potential)
b. This is the nerve impulse, the “signal” of the neuron that travels along the
axon.
c. It is basically, “all-or-nothing”, on or off. Think digitally!
2. What allows the changes in membrane potential to occur during an AP?
a. AP is due to rapid changes in selective permeability of the membrane
throughout its course [custom drawing!; Fig. 48.9]
 First, permeability to sodium ions rapidly increases because…
(i) Voltage-gated sodium channels open when a threshold potential
(approximately -55, clearly less negative than the resting potential)
is reached
 This threshold is reached due to depolarization somehow occurring
in the part of the membrane about to generate the AP (a.k.a.
“depolarization). See later sections on propagation of AP and
IPSPs/EPSPs to understand how the membrane can depolarize
enough to reach threshold.
 Once the voltage-gated sodium channels open, the membrane is
nearly 1000 times more permeable to sodium than to potassium!
(ii) Sodium ions enter the cell. It is not “lots” of sodium; just a teeny
amount will change the membrane potential from negative to positive
(around +40 mV)
 Note: the sodium gradient isn’t appreciably changed. After even
many APs, there are still far more sodium ions outside the cell than
inside.
 After that, permeability to sodium ions rapidly decreases again. This
is because each channel is programmed to be open for only about 0.5
msec. It then closes and inactivates.
 Don’t dwell on the whole “two gate” thing, just think “open or
closed?”
 And at about the same time as permeability to sodium decreases,
permeability to potassium ions increases because…
(i) Voltage-gated potassium channels open
 Just like voltage-gated sodium channels, they respond to increases
in membrane potential, but there is a time delay (i.e. they are
slower to open) so that gives the sodium the chance to do its thing.
(ii) Potassium exits the cell. It is not “lots” of potassium; just a small
amount will change the membrane potential from positive back to
negative
 Note that an “undershoot” to an even more negative value than the
RP occurs because now, the membrane is even more permeable to
potassium than it was at rest. All the “passive” channels are still
open, plus now, all these additional channels are open.
 Permeability returns to its level at rest…
(i) Once the voltage-gated potassium channels close.
 So, membrane is once again 25 times more permeable to
potassium, with just the “passive” channels open.
(ii) And the cell is once again back at its RP!
3. Propagation of the AP down the axon to nerve terminal [Fig. 48.11]
a. The basics
b. Saltatory conduction
IV. The synapse [Fig. 48.12]
A. Structure of the chemical synapse
B. Events that occur at the synapse
1. Review of events from time membrane is depolarize until the time neurotransmitter
binds to post-synaptic cells
a. Be sure you understand role of calcium!
2. Effects of neurotransmitter binding
a. EPSPs, IPSPs and what causes them
C. How is neurotransmitter activity stopped?
D. Neural integration [Figs. 48.13, 48.14]
Study questions (for these lectures only)
1. Explain what is meant by sensory, integrative and motor functions of the nervous system.
Think especially about the types of information carried by sensory vs. motor neurons.
2. Sketch an idealized neuron and label its key parts. Also, briefly describe how other neurons
may differ from this idealized neuron.
3. What are the major functions of glial cells?
4. What is the myelin sheath and it function?
5. What two key factors determine the value of the membrane potential at any given time?
6. Explain why the resting potential of neurons is negative (HINT: Your answer should be
based on the factors you listed in the previous question.)
7. You are studying the neurobiology of an alien species, and find that their neurons are quite
similar to ours, except that their cell membranes are far more permeable to sodium ions than
to potassium ions. If all else (i.e. concentration gradients, basic principles of the RP)
remained the same, would the neuron be negative or positive at rest? Briefly explain your
answer.
8. If at rest, the concentration gradients of sodium and potassium were reversed (i.e. relatively
high potassium on the outside compared to the inside, relatively high sodium on the inside
compared to the outside) but permeabilities remained the same, would the resting potential be
positive or negative. Briefly explain your answer.
9. What is meant by “passive” channels, “voltage-gated” channels and “chemically-gated”
channels? (Brief descriptions: just a few words should do it!)
10. Give a step-by-step description of the action potential once the threshold potential is reached.
You can include a diagram, but remember to use words to explain your diagram. Be sure to
describe:
 All relevant channels involved, and when they open and close (refer to your “custom
drawing” from class)
 Which ions move when, in which direction, and what they do to the membrane potential
11. Briefly explain how an action potential is propagated down an axon.
12. Describe, step-by-step, what happens at a chemical synapse from the time the signal reaches
the nerve terminal until the neurotransmitter binds to the receptor on the post-synaptic cell.
13. What generally happens in the post-synaptic cell as a result of neurotransmitter binding?
14. Describe three ways that neurotransmitter activity within the synapse can be terminated.
15. What are IPSPs and EPSPs and what is their function? You should include the concept of
“integration” in your answer. How might they lead to an action potential? How might they
reduce the chances of an action potential?