Transcript How neurons communicate ACTION POTENTIALS

How neurons communicate
ACTION POTENTIALS
• Researchers have used
the axons of squids to
study action potentials
• The axons are large
(~1mm) and extend
the length of the
squid’s body
Electrochemical messages
• Neurons use
electrochemicals to
communicate
• The cells use ions most important ones;
Na+, Cl-, Ca2+, and K+.
Neuron at Rest
• At rest the neuron is
said to have a resting
potential.
• Intracellular (inside)
area is negative
relative to
extracellular (outside)
area.
Resting Potential
• The cell membrane
selectively allows
some ions to pass
through (ion channels)
• Passive transport
• K+, pass freely
• Na+ and Cl- less freely
• A- protein cannot pass
Resting Potential
• Active transport also
occurs
• It moves 3 Na+ out of cell
for every 2 K+ that move
in to the cell
• Resting potential is
-70mV (millivolts)
• At rest there are more Na
ions outside and more K
ions inside
ACTION POTENTIALS
• Passed down axon - away
from cell body
• They are created when a
stimulus causes a
depolarizing current
• The resting potential
moves toward 0mV
• When it reaches a level of
-55 mV, it will “fire” an
action potential
All or None
• Once an axon’s
threshold potential has
been reached it will
fire an action potential
of a fixed size.
• This size will not vary
• Thus it is called the
All or None Principle
Action Potentials:
•
Caused by the exchange of ions across the
neuron membrane
1. Na channels open, and Na+ rushes into the cell
(due to the negative charge and [] difference).
2. Inside the neuron becomes more positive and
depolarizes
3. K channels open, causing K to rush out of cell
(follows [] gradient)
Action Potentials:
4. The K leaving the cell reverses the
depolarization
5. At this time the Na channels close, and the
neurons begins a repolarization.
6. The potential actually depresses below -70mv
for a short period because the K channels are
still open.
7. Gradually the potential returns to resting levels
and the cell is ready to fire again. (refractory
period)
ttp://faculty.washington.edu/chudler/ap.html
http://www.blackwellpublishing.
com/matthews/channel.html
What are some real-world implications of this?•
Some nerve poisons (e.g., scorpion venom) open Na+
channels and shut K+ channels & disrupts any action
potentials.
Local anesthetic drugs (Novocain, Xylocaine) block
the Na+ channels and prevent action potentials along
sensory neurons.
General anesthetics used in hospitals,open some K+
channels in the brain a bit wider than usual. This
counter-acts the effects of Na+ channels being opened
and prevents action potentials from propagating.
Using your textbook, (page 418-420) find and define the
following terms:
•Action potential
•Polarized membrane
•Depolarization, repolarization
•Sodium-potassium pump
•Refractory period
Explain the contributions of the following researchers:
•Galvani, Einthoven, Dubois-Raymond, Berger,
Bernstein, and Cole and Curtis
Propagating an Action Potential:
Method and Speed
• Domino effect:
• Influx of positive ions
causes adjacent Na+ gate
to open and, in turn, this
causes the next Na+ gate
to open, and so on.
• Hence, an action potential
is actually selfpropagating.
However, myelin sheaths permit
speeds up to 100 m/s.
• How?
• Saltatory
conduction
• Saltus is Latin for
jump
Mylin Sheath
• Produced by Glial
Support Cells
• Myelin wraps
around the axon
and helps electrical
current flow
• does not cover the
entire axon – nodes
of Ranvier
• distance between
these nodes is
between 0.2 and 2
mm
• Allows AP to jump
•Myelin allows the Na+ ions to move quickly to the
next Na+ gate at a node of Ranvier
•The action potential jumps from one node to the
next
How fast does an action potential
move along an axon?
• The thinnest axons
propagate an action
potential at less than 1
meter per second (1
m/s).
• Thick axons propagate
action potentials at
about 10 m/s.
WHAT YOU SHOULD KNOW
•Identify the three major components of an neuron and their
function
•Identify the benefits of myelination to a neuron
•Distinguish between the basic types of neurons
•Explain the role of sodium and potassium in an action
potential
•Explain the components of an action potential
•Explain the concept of threshold
•Explain how salutatory conduction enhances neuron activity
Neuron to Neuron
Communication
•
Two types of events
1. Electrical
2. Chemical
• Electrical events
propagate a signal
within a neuron
• Chemical processes
transmit the signal
from one neuron to
another or to a
muscle cell.
Chemical process occurs at end of axon in a
structure called a synapse
• Touching very close against
the dendrite of another cell
• The axon releases chemical
substances called
neurotransmitters
• Neurotransmitters attach to
chemical receptors in the next
neuron and promote
excitatory or inhibitory
changes in its membrane.
Chemical Synapses
NEUROTRANSMITTER
1. The release of a
neurotransmitter is
triggered by the arrival
of an action potential
2. AP causes cellular
secretion, known as
exocytosis:.
Neurotransmitter Release
A. Synaptic vesicles store
neurotransmitters
B. When action potential
arrives at presynaptic
axon bulb, synaptic
vesicles merge with
presynaptic membrane.
C. Release chemicals into
cleft
D. Neurotransmitter molecules diffuse across
synaptic cleft to bind with receptors on
postsynaptic membrane
E. The type of neurotransmitter and/or receptor
determines if the response is excitation or
inhibition.
•
The action that follows activation of a
receptor site may be either depolarization
(an excitatory postsynaptic potential - EPSP)
or hyperpolarization (an inhibitory
postsynaptic potential - IPSP).