ACTION POTENTIALS
Chapter 11 Part 2
HONORS ANATOMY & PHYSIOLOGY
The Brain and Behavior
Key terms
• action
potential: A short-term change in the electrical potential that
travels along a cell (such as a nerve or muscle fiber); the basis of
neural communication.
• neural
impulse: The signal transmitted along a nerve fiber, either in
response to a stimulus (such as touch, pain or heat), or as an
instruction from the brain (such as causing a muscle to contract).
• Plasticity:
The ability to change and adapt over time.
• Polarity:
The spatial differences in the shape, structure, and
function of cells.Almost all cell types exhibit some sort of polarity,
which enables them to carry out specialized functions.
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The Brain and Behavior
• Reuptake:
The reabsorption of a neurotransmitter by a
neuron after the transmission of a neural impulse across
a synapse.
• Depolarization:
the act of depriving of polarity, or the
result of such action; reduction to an unpolarized
condition
• resting
potential: The membrane potential of inactive
cells. The voltage that exists across plasma membranes
during the resting state of excitable cells; ranging from:
• -90 to -20 millivolts
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Neurons

use pulses of electrical current to
receive
transmit
regulate
the flow of information over long distances w/in
the body
Neuron Organization
Types of Neurons

Sensory Neurons



transmit information (senses) from body  brain
are afferent
specialized dendrites that initiate action
potential when stimulated
Types of Neurons
2. Motor Neurons

transmit signals to muscle fibers & glands
 are efferent
ACTION POTENTIALS
nerve impulses
occur in neurons only
neurons are electrically excitable
 communicate with one another using 2 types of
electrical signals:
1.
2.
graded potentials (short distances only)
action potentials (short or long distances)
Synapse

junction between axon terminal & next cell
(another neuron, muscle fiber, gland cell)

neurotransmitters are chemical messengers
released @ most synapses that pass action
potential to receiving cell
Synapse

presynaptic cell: cell releasing neurotransmitter
& passing on action potential

postsynaptic cell: receiving neurotransmitter

synaptic cleft: physical space between the 2;
neurotransmitter released into this space &
diffuses across it attaching to receptors on
postsynaptic cell
Synapse
Ion Pumps

ions unequally distributed across plasma
membrane


inside of cell slightly (-) compared to outside cell
source of potential nrg

called the membrane potential

resting potential: the membrane potential of
neuron @ rest =
 -60 to –80 mV
Resting Potential
Formation of Resting Potential


Na+/K+ pump generates & maintains the
ionic gradients of membrane potential
1 turn of pump



1 ATP
3 Na+ out
2 K+ in
Membrane Potential
Ion Channels

pores that span the membrane allowing ions to
diffuse across (in or out)

membranes are selectively permeable and
variations in how easily any particular ion can
cross a membrane depends on the # of
channels & how often they are open
Types of Ion Channels
Action Potentials

neurons have gated ion channels that open or
close in response to stimuli


open/close changes permeability for that ion
neurons have K+ channels


when open K+ diffuses out of cell
changes resting potential from:
-60 mV to -90 mV
Hyperpolarization

when K+ channels open & resting potential
decreases to -90 mV inside of cell becoming
more (-) than normal resting potential called:
hyperpolarization
K+ Ion Channels in Neurons
Depolarization



when Na+ ion channels open Na+ diffuse into
cell making inside less (-) compared to
outside cell
membrane potential shifts toward (+) mv
this reduction in magnitude of membrane
potential called
depolarization
Graded Potentials

any shift in membrane potential

magnitude of shift varies with strength of
stimulus

induce a small electrical current that flows
along the membrane leaking out of the cell

so only lasts short distance from source
Action Potential

electrical signal that propagates along the
membrane of a neuron as a nongraded (all or
nothing) depolarization

have a constant magnitude & can regenerate in
adjacent regions of the membrane

travel long distances
Voltage-Gated Ion Channels

ion channels that open/close based on
membrane potential passing a particular level

Na+ channels in neurons are voltage gated:
open when depolarization occurs  Na+
diffuses into cell  becomes more depolarized
 more Na+ channels open (+ feedback)
Threshold

Action potentials occur when a depolarization
increases the membrane voltage to a particular
value (the threshold)

for mammals the threshold is a membrane
potential ~ -55mV

once started the action potential has a
magnitude independent of the strength of
triggering stimulus



+ feedback loop of depolarization & channel
opening triggers an action potential whenever
the membrane potential reached the threshold
membrane depolarization opens both Na+ & K+
channels but Na+ opens faster initiating the
action potential
Na+ channels become inactivated as action
potential proceeds (gates close) & remain so
until after membrane returns to resting
potential
Refractory Period

(-) membrane potential restored by inactivation
of Na+ channels, which increases K+ outflow

This is followed by a refractory period:

no matter how strong the stimulus to initiate next
action potential is cannot initiate one during
refractory period
Conduction of Action Potentials
Action Potentials

An electrical impulse travels along the axon
via depolarized voltage-gated ion channels
in the membrane, and can either "jump"
along a myelinated area or travel
continuously along an unmyelinated area.

http://highered.mheducation.com/sites/0072495855/stude
nt_view0/chapter14/animation__the_nerve_impulse.html

http://www.sumanasinc.com/webcontent/animations/cont
ent/action_potential.html

https://www.youtube.com/watch?v=U0NpTdge3aw

https://www.youtube.com/watch?v=ifD1YG07fB8