Transcript file

Nervous System Lecture
(Day 2: How Nerve Impulses are Conducted)
Two Functions of Neurons:
1) Irritibility: ability to respond to a stimulus and convert
it into a nerve impulse.
2) Conductivity: ability to transmit the impulse to other neurons,
muscles and glands.
o Many different types of stimuli excite neurons (eg. light,
pain, sound, etc.)
o This causes neurons to become active and generate an impulse
(known as an action potential).
o An action potential involves the change in the permeability of
the neuron’s plasma membrane, allowing the movement of
two ions, which results in changes in the polarity (charges) of
the neuron.
The two ions that are involved in nerve impulse conduction are:
Sodium (Na+)
Potassium (K+)
If the stimulus is strong enough, depolarization activates the
neuron to initiate and transmit an action potential (nerve impulse).
Nerve Impulse Activation (step by step):
(see pg. 232)
1) At resting state (when neurons are not active), the inside of the
nerve cell (inner surface of plasma membrane) is more negative
than on the outside of the cell (due to the permeability of the
membrane allowing K+ ions to move outside the cell).
The cell is said to be POLARIZED.
Inside cell: - charged (mostly K+)
Outside cell: + charged (mostly Na+)
2) When a nerve cell is adequately stimulated,
a) Sodium gates open on plasma membrane.
b) Na+ ions rush in (known as the “sodium rush”).
The cell is now DEPOLARIZED.
Inside cell: + charged (Na+ and K+)
Outside cell: - charged
Na+ rushing in
3a) Right after the sodium rush, the sodium gates close.
Now the membrane is not permeable to Na+ but is permeable
to K+.
b) K+ moves out of the cell.
The cell has now REPOLARIZED. (Note: the charges
have been restored to how they were in the beginning but the ions
are not in the proper location).
K+ moving out
Inside cell: - charged (Na+)
Outside cell: + charged (K+)
sodium gate closed
4) A neuron is not ready to send another message until
resting state has been reestablished.
To do so, a “sodium-potassium pump” uses energy (ATP) to
bring K+ back into cell and Na+ back out of the cell
(note: energy is required to bring ions across membrane
when it is not permeable to these ions).
Sodium Potassium Pump
Nerve Impulse Conduction Animation
Conduction of Nerve Impulse from one neuron to next:
(Conductivity)
(see pg. 233)
•The electrical impulse doesn’t exactly travel from one neuron
to the next. Instead, the message is passed between neurons with the
help of neurotransmitters.
•Chemicals get released at the axon terminals into the synapse,
bind to the dendrites of the next neuron, and trigger an action
potential on that next neuron.
•Thus, transmission of nerve messages is an Electrochemical event!!
Dysfunctions of Nerve Impulse Conduction:
Problems with Sodium
•Without Na+, nerve impulses cannot be properly
transmitted.
•If the sodium gates couldn’t open properly, a message
would not be transmitted. Alcohol, sedatives, and
anesthetics make nerve cells less permeable to Na+, thus
less capable of sending messages effectively and quickly.
Problems with Neurotransmitters
•If proper amounts of neurotransmitters are not released, or are not
received by next neuron, nerve impulse conduction will be impaired.
•Drugs such as cocaine, interfere with neurotransmitter reception by
blocking uptake receptors. This can result in a “flood” of a
neurotransmitter in the synapse which can create a temporary “high”
but can also interfere with proper nervous system functioning.
Interesting fact:
Lithium ions (Li+) are chemically similar to sodium ions.
Lithium is frequently used to treat severe depression.
Giving a patient Lithium adds even more positive ions on
the outside of the nerve cell which raises the resting
potential, thus making it easier to initiate an action
potential (ions are even less in balance thus easier to get
action potential going). The effect often makes the person
more active and less depressed as nerve impulses
(stimulation) are increased.
The End of Part 2