1/1/2016 1 Neural Signaling Chapter 11 The Nervous System The Nervous Impulse • Dependent upon a resting potential across the cell membrane – Magnitude of potential is determined by • Leakage channels for sodium and potassium • Active transport carriers (Sodium/Potassium pump) – The neuron is polarized • Impulse results from depolarization Factors that contribute to resting membrane potential Check out the A&P Flix on Mastering “Resting Membrane Potential” Figure 11.8 Finally, let’s add a pump to compensate for leaking ions. Na+-K+ ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential. Suppose a cell has only K+ channels... K+ loss through abundant leakage channels establishes a negative membrane potential. Now, let’s add some Na+ channels to our cell... Na+ entry through leakage channels reduces the negative membrane potential slightly. The permeabilities of Na+ and K+ across the membrane are different. The concentrations of Na+ and K+ on each side of the membrane are different. Na+ (140 mM) K+ (5 mM) K+leakage channels Cell interior –90 mV Cell interior –70 mV Cell interior –70 mV K+ Na+ Na+-K+ pump K+ K+ K+ K+ Na+ K+ K+ K Na+ K+ K+ Na+ K+ K+ Outside cell Inside cell Na+-K+ ATPases (pumps) maintain the concentration gradients of Na+ and K+ across the membrane. The Na+ concentration is higher outside the cell. The K+ concentration is higher inside the cell. K+ (140 mM) Na+ (15 mM) Figure 11.7 Voltmeter Microelectrode inside cell Plasma membrane Ground electrode outside cell Neuron Axon The Nervous Impulse • Polarization – Voltage across the plasma membrane – Inside of the cell is more negative than the outside • Resting potential – Polarization leads to attraction between opposite charges across the membrane – When a neuron is at rest, average potential is -70mV • Neurons use changes in membrane potential as signals to receive, integrate, and send information
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1/1/2016
1
Neural Signaling
Chapter 11
The Nervous System
The Nervous Impulse
• Dependent upon a resting potential across the
cell membrane
– Magnitude of potential is determined by
• Leakage channels for sodium and potassium
• Active transport carriers (Sodium/Potassium pump)
– The neuron is polarized
• Impulse results from depolarization
Factors that contribute to resting membrane potential
Check out the A&P Flix on Mastering “Resting Membrane Potential”
Figure 11.8
Finally, let’s add a pump to compensate
for leaking ions.
Na+-K+ ATPases (pumps) maintain the
concentration gradients, resulting in the
resting membrane potential.
Suppose a cell has only K+ channels...
K+ loss through abundant leakage
channels establishes a negative
membrane potential.
Now, let’s add some Na+ channels to our cell...
Na+ entry through leakage channels reduces
the negative membrane potential slightly.
The permeabilities of Na+ and K+ across the
membrane are different.
The concentrations of Na+ and K+ on each side of the membrane are different.
Na+
(140 mM )K+
(5 mM )
K+ leakage channels
Cell interior–90 mV
Cell interior–70 mV
Cell interior–70 mV
K+
Na+
Na+-K+ pump
K+
K+K+
K+
Na+
K+
K+K
Na+
K+K+Na+
K+K+
Outside cell
Inside cellNa+-K+ ATPases (pumps)
maintain the concentration
gradients of Na+ and K+
across the membrane.
The Na+ concentration
is higher outside the
cell.
The K+ concentration
is higher inside the
cell.
K+
(140 mM )Na+
(15 mM )
Figure 11.7
Voltmeter
Microelectrodeinside cell
Plasmamembrane
Ground electrodeoutside cell
Neuron
Axon
The Nervous Impulse
• Polarization
– Voltage across the plasma membrane
– Inside of the cell is more negative than the outside
• Resting potential
– Polarization leads to attraction between opposite
charges across the membrane
– When a neuron is at rest, average potential is -70mV
• Neurons use changes in membrane potential as
signals to receive, integrate, and send information
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The Nervous Impulse
• Types of changes
– Depolarization
• Decrease in membrane potential (interior becomes less
negative)
– Hyperpolarization
• Increase in membrane potential (inside becomes more
negative)
Figure 11.9a
Depolarizing stimulus
Time (ms)
Inside
positive
Inside
negative
Resting
potential
Depolarization
(a) Depolarization: The membrane potential
moves toward 0 mV, the inside becoming
less negative (more positive). This increases the
probability of nerve impulse production.
Figure 11.9b
Hyperpolarizing stimulus
Time (ms)
Resting
potential
Hyper-
polarization
(b) Hyperpolarization: The membrane
potential increases, the inside becoming
more negative. This decreases the probability
of nerve impulse production.
The Nervous Impulse
• Changes in polarization are produced by…
– Anything that changes ion concentration across the membrane
– Anything that changes membrane permeability to an ion (most important)
• Largely due to changes in the number of open ion channels
• Membrane channels
– Chemically gated (ligand gated)
– Voltage gated
Figure 11.6
(b) Voltage-gated ion channels open and close in response
to changes in membrane voltage.
Na+
Na+
Closed Open
Receptor
(a) Chemically (ligand) gated ion channels open when the
appropriate neurotransmitter binds to the receptor,
allowing (in this case) simultaneous movement of
Na+ and K+.
Na+
K+
K+
Na+
Neurotransmitter chemical
attached to receptor
Chemical
binds
Closed Open
Membrane
voltage
changes
The Nervous Impulse
• There also are mechanically gated membrane
channels
– Open in response to physical deformation (touch,
pressure, sound waves)
– Found in sensory receptors
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The Graded Potential
• Short lived, localized changes in membrane potential
– Due to incoming signals, usually energy or
neurotransmitters
• Channels open → ions flow
• Short distance signals
• May be depolarization or hyperpolarization events
Figure 11.10a
Depolarized region
Stimulus
Plasma
membrane
(a) Depolarization: A small patch of the
membrane (red area) has become depolarized.
Figure 11.10b
(b) Spread of depolarization: The local currents
(black arrows) that are created depolarize
adjacent membrane areas and allow the wave of depolarization to spread.
Figure 11.10c
Distance (a few mm)
–70
Resting potential
Active area
(site of initial
depolarization)
(c) Decay of membrane potential with distance: Because current
is lost through the “leaky” plasma membrane, the voltage declines
with distance from the stimulus (the voltage is decremental ). Consequently, graded potentials are short-distance signals.
Me
mb
ran
e p
ote
nti
al
(mV
)
The Action Potential
�Long distance signal
� Initiated by sufficient depolarization at site of graded potential
� Must reach threshold – usually a change of ~100mV
�Opening of specific voltage gated channels
�Does not decrease in strength with distance
� “All or none”
�A.K.A. nerve impulse
Stimuli that Initiate Action Potentials
• Light
• Heat
• Chemicals
• Mechanical energy
• Chemical stimuli
from other
neurons
Sensory Neurons Motor & Association Neurons
Threshold stimulus always required
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Dendrites
(receptiveregions)
Cell body
(biosynthetic centerand receptive region)
Nucleolus
Nucleus
Nissl bodies
Axon
(impulsegeneratingregion)
Axon hillock
NeurilemmaTerminalbranches
Node of Ranvier
Impulsedirection
Schwann cell(one inter-node)
Axon terminals(secretoryregion)
Dendriticspine
Neuron cell body
(a)
(b)
(impulseconductingregion)
The Action Potential
�The players:
Sodium Channel
• Opens instantly
• Can’t sustain (self-inhibits)
Na+
Potassium
channel
Sodium
channel
Activation
gates
Inactivation gateK+
Potassium Channel
� Slow to open
� Slow to close
Na+
Na+
Potassium
channel
Sodium
channel
1 Resting state
2 Depolarization
3 Repolarization
4 Hyperpolarization
The events
Activation
gates
Inactivation gateK+
K+
Na+
K+
Na+
K+
Action
potential
1 2 3
4
Resting state Depolarization Repolarization
Hyperpolarization
The big picture
1 1
2
3
4
Time (ms)
ThresholdMem
brane p
ote
nti
al
(mV
)
Figure 11.11 (1 of 5)
The Action Potential
• Resting potential is quickly restored
– Thousands of Na+/K+ pumps redistribute ions
– May seem like a huge task
– Only a small number of ions actually cross the
membrane
• Change in 0.012% of intracellular Na+ concentration
The Action Potential
• Once initiated, AP is self-propagating
– Once Na+ channels in one region are inactivated, no