anatomy and physiology i electrical signals in neurons action potentials the synapse instructor:...
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Anatomy and Physiology I
Electrical Signals in Neurons
Action Potentials
The Synapse
Instructor: Mary Holman
Electrical Signals in Neurons
• Two basic features of cell membranes of neurons and other excitable cells
Membrane potential
Ion channels
• Allow neurons to communicate using electrical signals
Action potentials
Fig. 10.14c
+
+
––
+
+
––
+
+
–+–
–+–+
–
+–
+–
+ –
+–
+–
+–
+–
–70 mV
Low Na+
Low K+ High K+
High Na+
Na+
K+Pump
The Membrane Potential of the Resting Neuron
-70 millivolts
1 mV= 0.001Volts
Impermeantanions
TriggerZone
Membrane Potential = unequal distribution of + and - ionson either side of a cell membrane
Resting Membrane Potential of the Neuron
From: Principles of Anatomy & Physiology by Tortora & Grabowski
Ion Channels
Gated channels that open and close in response to a stimulus
1. Voltage-gated ion channels as in action potentials
2. Ligand-gated ion channels as in neurotransmitters
3. Mechanically-gated ion channel
Fig. 10.13
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Gate-like Mechanism of Ion Channels
Protein
(b) Channel open(a) Channel closed
Cellmembrane
Fatty acidtailPhosphate head
Stimulus-action potential-chemical-mechanical
ions
Membrane potentialchanges due to move-ment of ions when channels are open
• A cell that has a membrane potential is said to be polarized
• When the ion flow due to the opening of an ion channel produces a more negative membrane potential it is called
hyperpolarized
• If the membrane potential becomes more positive, it is called depolarized
Membrane Polarization
Action Potential• If the membrane is depolarized to
~ -55mV it is said to have reached threshold potential
• When a threshold potential is reached, a nerve impulse or action
potential is generated
Fig. 10.15a
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–62 mV
Na+
Na+
Neurotransmitter
Ligand-gatedNa+ channel
Presynapticneuron
Sub-threshold DepolarizationDoes not Result in an Action Potential
Trigger Zone
Fig. 10.15b
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–55 mV
Na+Na+
Na+Na+
Na+
Trigger zone
Voltage-gatedNa+ channel
An Action Potential is Generated WhenDepolarization Reaches ~55mV
Ligand-gatedNa+ channel
Fig. 10.16
(a)
Region of depolarization(b)
Region of repolarization
–70
–0
–70
–0
–70
–0K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+
K+
K+
K+ K+
K+ K+
Na+ Na+ Na+
Na+ Na+ Na+
Thresholdstimulus
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+
K+ K+ K+ K+ K+
Na+ Na+ Na+
Na+ Na+ Na+
K+
K+
K+ K+ K+
K+ K+ K+
Na+ channels openK+ channels closed
K+ channels openNa+ channels closed
ActionPotential& NerveMembrane
An Action Potential = Nerve Impulse
• All-or-none response
all impulses are the same strength
more stimuli leads to more impulses
• Refractory Period Absolute vs Relative
period of time between impulses when membrane is unresponsive to
stimuli ~1--30 milliseconds
Fig. 10.17
Milliseconds
10
0
+20
+40
2 3 4 5 6 7 8
Mem
bra
ne
po
ten
tial
(m
illi
volt
s)Action potential
Hyperpolarization
–40
–20
–60
–80
Restingpotential
Resting potentialreestablished
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Oscilloscope Recording of an Action Potential
Fig. 10.18
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Direction of nerve impulse
+ +
+ +
+
– – – – – – – – –
– – – – –– – – –
– – – – –– – – –
– – – – – – – – –
– – – – – – – – –
– – – – – – – – –
+ + + + + + + +
+ + + + + + + + +
+ +
+ +
++ + + + + + + +
++ + + + + + + +
+ +
+ +
++ + + ++ + + +
++ + + ++ + + +
Region ofaction potential
A Nerve Impulse = moving Zone of Depolarization
• Myelinated nerves conduct action potentials from node to node
• This mode of transmission is called saltatory conduction
Impulse Conduction
Fig. 10.19
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+ +
+ ++++
+ +
+ +
+ +
+ +
+ +
+ +
Electric current Nodes Axon
Schwann cells(a)
+++++
++
+ +
+ +
+ +
+ +
+ +
+ +
(b)
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ ++++
– –– –
– –– –
– –– –
– –– –
– –– –
– –– –
– –– –
– –– –
– –– –
– –– –
– –– –
– –– –
Action potential
Action potential
Action potential
A Nerve Impulse Moving along a Myelinated Axon
The Synapse
• Nerve impulses pass from neuron to neuron at synapses, moving from a pre-synaptic neuron to a post-synaptic neuron.
• Neurotransmitters are released when the impulse reaches a synaptic knob
Fig. 10.11
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Dendrites
Impulse
Impulse
Impulse
Synapticcleft
Axon ofpresynapticneuron
Cell body of postsynaptic neuron
Axon of postsynaptic neuron
Axon ofpresynapticneuron
Synapses ofThree Neurons
Events at the Synaptic Cleft
• Action potential races along axon to the terminal knobs or varicosities.• The change in polarization of the membrane of the knob opens voltage sensitive Ca++ gates & Ca++ floods in.• The increased [Ca++] stimulates vesicles to move to the cell membrane and release neurotransmitters via exocytosis.• Neurotransmitter molecules bind to receptors on post-
synaptic neuron membrane which opens ligand-gated ion channels. Depending on which ion channels are opened, membrane is hyperpolarized or depolarized.
Fig. 10.12a
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Mitochondrion
Synaptic knob
Synaptic cleftNeurotransmitter
Axon
Ca+2
Presynaptic neuron
Direction ofnerve impulse
Synapticvesicles
Cell body or dendriteof postsynaptic neuron
Synapticvesicle
Vesicle releasingneurotransmitter
Axonmembrane
Polarizedmembrane
Depolarizedmembrane
Ca+2Ca+2
Activity at a Synaptic Cleft
Fig. 10.12b
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Mitochondrion
Synapticvesicle
Synapticcleft
Postsynapticmembrane
© Don W. Fawcett/Photo Researchers, Inc.
A Synaptic Cleft
TEM 37,500x
Neurotransmitters
• Released from vesicles in terminal knobs of presynaptic axons
• React with receptors on membrane of postsynaptic neuron
• Many different molecules act as neuro-transmitters
• Many serious diseases result from
imbalances of neurotransmitters
• Can cause excitatory (EPSP) or inhibitory (IPSP) post-synaptic membrane potentials by opening or closing various ion channels
• Usually the net effect the EPSPs and IPSPs is determined in the region of the trigger zone of the neuron
Actions of Neurotransmitters
Fig. 10.20
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Nucleus
Neuroncell body
Presynapticknob
Presynapticaxon
Neurons Receive Multiple Signals Simultaneously
Impulse Processing• Neuron pools in CNS interpret
impulses from many neurons
• Convergence
Multiple axons delivering impulses to the same neuron
• Divergence
An axon that branches and delivers impulses to several neurons
Fig. 10.21a
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1 2
3
Impulse Convergence
Fig. 10.21b
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4
5
6
Impulse Divergence
Fig. 10A
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Ion channel
Nicotine
Outsidenerve cell
Membranelipid bilayer
Insidenerve cell
Addictions
• Some addicting compounds bindto sites that bindintrinsic pain re-lievers (endorphins)
• Others alter bindingof neurotransmittersacting either as anagonist or antagonist