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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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3
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
new AP is generated there
• Continues along axon in one direction at
constant velocity
• Factors affecting conduction velocity:
– Axon diameter – larger diameter = faster conduction
– Degree of myelination
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5
The Action Potential
• Myelinated Neurons
– Nodes of Ranvier – only place where current can
pass through the membrane
– Saltatory conduction – 30X faster than continuous
conduction
The Action Potential
• Refractory period
– Neuron cannot respond to a second stimulus
• During repolarization
– Limits number of impulses per second
Figure 11.14
Stimulus
Absolute refractory
period
Relative refractory
period
Time (ms)
Depolarization(Na+ enters)
Repolarization(K+ leaves)
After-hyperpolarization
The Action Potential
• Coding for stimulus intensity
– All APs are independent of stimulus strength
– CNS must discern strong from weak signals to
initiate appropriate response
– Stimulus intensity is coded for by frequency of
action potentials
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 Synapse
• Nervous system operates through chains of neurons connected by synapses
• Syn = “to clasp or join”
• Junction between…
– Adjacent neurons
– Neuron and an effector cell
• Mediates information transfer
– Electrical
– Chemical
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The Synapse
• Presynaptic neuron
– Conducts impulses toward the synapse
• Postsynaptic neuron– Transmits impulses away from the synapse
• Most neurons are both
Dendrites
Cell body
Axon
Axodendriticsynapses
Axoaxonic synapses
Axosomaticsynapses
(a)
Axosomaticsynapses
Cell body (soma)of postsynaptic neuron
Axon of presynaptic
neuron
(b)
The Synapse
• Electrical Synapses
– Direct transmission of electrical signals from one cell to
another
– Less common than chemical synapses
• Neurons are electrically coupled (joined by gap junctions)
• Communication is very rapid
– May be unidirectional or bidirectional
• Important in
– Embryonic tissue
– Some brain regions
– Synchronizing groups of neurons (example: jerky eye movements)
Electrical Synapses The Synapse
• Chemical Synapses
– Indirect communication between cells
– Electrical signal of AP is changed to a chemical signal (neurotransmitter) in the presynaptic neuron
– Neurotransmitter is released into the synaptic space and diffuses toward the postsynaptic neuron
– Postsynaptic neuron changes chemical signal back to electrical signal for conduction along its own axon
– Unidirectional
– Here’s how it works…
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Chemical Synapses
1. Action potential
arrives at the
axon terminal in
the presynaptic
neuron
Chemical Synapses
2. Voltage-gated
Ca2+ channels
open, and Ca2+
enters the axon
terminal
Chemical Synapses
3. Ca2+ entry causes
synaptic vesicles
to release
neurotransmitter
by exocytosis
Chemical Synapses
4. Neurotransmitter
diffuses across the
synaptic cleft and
binds to specific
receptors on the
postsynaptic
neuron’s
membrane
Chemical Synapses
5-6. Binding of the
neurotransmitter
opens ion
channels and
creates graded
potentials in the
postsynaptic
neuron
Chemical Synapses
7. Graded
potentials
become nerve
impulses and are
conducted to the
next cell in the
chain
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The Synapse
• Postsynaptic membranes generally do not
generate action potentials
• When the neurotransmitter binds, one of two
types of graded potential occur
– Excitatory Postsynaptic Potentials (EPSP)
– Inhibitory Postsynaptic Potentials (IPSP)
EPSPs
• Excitatory Postsynaptic Potentials (EPSP)
– Depolarizes postsynaptic cell membrane
– Excitatory neurotransmitters
– Helps trigger AP at the axon hillock
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)
Figure 11.18a
An EPSP is a local
depolarization of the
postsynaptic membrane
that brings the neuron
closer to AP threshold.
Neurotransmitter binding
opens chemically gated
ion channels, allowing
the simultaneous pas-
sage of Na+ and K+.
Time (ms)
(a) Excitatory postsynaptic potential (EPSP)
Threshold
Stimulus
Me
mb
ra
ne
po
ten
tia
l (m
V)
IPSPs
• Inhibitory Postsynaptic Potentials (IPSP)
– Hyperpolarizes postsynaptic cell membrane
– Increases membrane permeability to K+ or Cl-
– Inhibitory neurotransmitters
– Decreases chance of AP
Figure 11.18b
An IPSP is a local
hyperpolarization of the
postsynaptic membrane
and drives the neuron
away from AP threshold.
Neurotransmitter binding
opens K+ or Cl– channels.
Time (ms)
(b) Inhibitory postsynaptic potential (IPSP)
Threshold
Stimulus
Me
mb
ra
ne
po
ten
tia
l (m
V)
1/1/2016
9
Neurotransmitters
• Excitatory– Acetylcholine
• Receptors also activated by
nicotine, muscarine
– Norepinephrine• Most common NT used by
the sympathetic nervous
system
– Glutamate • Most abundant NT in
vertebrates
• Component of concern in MSG
Neurotransmitters
• Inhibitory
– Serotonin (brain, GI tract)
– GABA (brain and retina)
– Glycine (spinal cord, brain,
and retina)
Thought Question
Strychnine is a pesticide that is used against small
vertebrates (birds, rodents). This chemical is an
antagonist to glycine. What symptoms might an
animal or human experience if they ingest this
substance?
The Synapse
• Inactivation of NT’s
1. Diffusion
2. Reuptake
3. Degradation (enzymatic inactivation)
• Examples
– Cholinesterase & Ach
• Found in synapses
– Monoamine Oxidase & Norepinephrine (many others)
• Bound to mitochondrial membrane in most cells
Figure 11.17
Action potential
arrives at axon terminal.
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Ca2+ entry causesneurotransmitter-containing synapticvesicles to release theircontents by exocytosis.
Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.
Ca2+
Synapticvesicles
Axonterminal
Mitochondrion
Postsynapticneuron
Presynapticneuron
Presynapticneuron
Synapticcleft
Ca2+
Ca2+
Ca2+
Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane.
Binding of neurotransmitteropens ion channels, resulting ingraded potentials.
Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.
Ion movement
Graded potential
Reuptake
Enzymaticdegradation
Diffusion awayfrom synapse
Postsynapticneuron
1
2
3
4
5
6
The Synapse
• Pharmacology
– Anticholinesterase neurotoxins
• Causes excitotoxicity (overstimulation)
• Example: Organophosphates, Nerve Gas
– Local Anesthetics
• Block Na+ channels
• Inhibitory
• Example: Lidocaine
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The Synapse
• Cocaine
– Prevents dopamine reuptake
– In response, the brain stops making dopamine
– User unable to experience pleasure without the
drug
Neural Integration
• Summation
– A single EPSP cannot induce an action potential
• EPSP’s can summate to reach threshold
– IPSP’s can also summate with EPSP’s
• Cancel each other out
Summation
• Types
– Temporal summation
• One or more presynaptic neurons transmit impulses in
rapid-fire order
– Spatial summation
• Postsynaptic neuron is stimulated by more than one
terminal at the same time
Figure 11.19a, b
Threshold of axon ofpostsynaptic neuron
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Resting potential
E1 E1 E1 E1
(a) No summation:
2 stimuli separated in time
cause EPSPs that do not
add together.
(b) Temporal summation:
2 excitatory stimuli close
in time cause EPSPs
that add together.
Time Time
E1 E1
Figure 11.19c, d
E1 + E2 I1 E1 + I1
(d) Spatial summation of
EPSPs and IPSPs:
Changes in membane
potential can cancel each
other out.
(c) Spatial summation:
2 simultaneous stimuli at
different locations cause
EPSPs that add together.
Time Time
E1
E2 I1
E1
Neuronal Integration
• Neurons function in groups, and each group
contributes to wider neuronal function
• There must be integration – the parts must
work together to form a more complex whole
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Neuronal Integration
• First level of neuronal integration: neuronal
pools
• Patterns of connections within a neuronal pool:
neuronal circuits
• Features
– Allow for a wide variety of neuronal interaction
– May consist of thousands of neurons
– Often include excitatory and inhibitory neurons
Figure 11.21
Presynaptic
(input) fiber
Facilitated zone Discharge zone Facilitated zone
Simple Neuronal Pool
Neuronal Circuits
• Diverging circuit
– One incoming fiber stimulates an ever-increasing
number of fibers
– May affect a single pathway or several
– Common in both sensory and motor systems
– Example: A single neuron in the brain can activate
100 or more motor neurons in the spinal cord and
thousands of skeletal muscle fibers
Figure 11.22a
Neuronal Circuits
• Converging circuit
– Opposite of diverging circuits
– Results in either strong stimulation or inhibition
– Also common in sensory and motor systems
– Example: Different sensory stimuli can elicit the
same memory
Figure 11.22c, d
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