Monday April 11, 2014. Nervous system and biological electricity III 1. No pre-lecture quiz 2. A...
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Transcript of Monday April 11, 2014. Nervous system and biological electricity III 1. No pre-lecture quiz 2. A...
Monday April 11, 2014.
Nervous system and biological electricity III
1. No pre-lecture quiz2. A review of Action potentials3. Myelin4. Synapses and neurotransmitters
The Action Potential Is a Rapid Change in Membrane Potential
1. Depolarization phase
2. Repolarization phase
3. Hyperpolarization phase
Resting potential
Threshold potential
Voltage-gated sodium channels allow the action potential to occur
• https://www.youtube.com/watch?v=ifD1YG07fB8
Voltage-gated channels
How voltage-gated channels work
At the resting potential, voltage-gated Na+ channels are closed.
Conformational changes openvoltage-gated channels whenthe membrane is depolarized.
Two important types:1.) Na+ voltage gated channels2.) K+ voltage gated channels
Resting Potential - Both voltage gated Na+ and K+ channels are closed.
Initial Depolarization - Some Na+ channels open. If enough Na+ channels open, then the threshold is surpassed and an action potential is initiated.
Na+ channels open quickly. K+ channels are still closed.
PNa+ > PK+
Na+ channels self-inactivate, K+ channels are open.
PK+ >> PNa+
Emembrane ≈ E K+
PK+ > PK+ at resting state
Resting Potential - Both Na+ and K+ channels are closed.
Action Potentials Propagate because Charge Spreads down the Membrane
PROPAGATION OF ACTION POTENTIAL
NeuronAxon
1. Na+ enters axon.
2. Charge spreads;membrane“downstream”depolarizes.
Depolarization atnext ion channel
3. Voltage-gatedchannel opens inresponse todepolarization.
Why does the membrane potential increase during stage 3 of the action potential?
A. Both the voltage-gated Na+ channels and voltage gated K+ channels are open.
B. All of the K+ channels (both leak and voltage gated) are open.
C. The voltage gated Na+ channels are open, but the voltage gated K+ channels have
not opened yet.
D. The voltage gated Na+ channels are open, but the K+ channels (both voltage gated and leak) have not opened yet.
Why does the membrane potential decrease during stage 4 of the action potential?
A. The voltage gated K+ channels open.B. The voltage gated Na+ channels open.C. The voltage gated K+ channels close.D. The voltage gated Na+ channels close.E. A and D
Action Potentials Propagate Quickly in Myelinated Axons
Action potentials jump down axon.
Nodes of Ranvier Schwann cells (glia)wrap around axon,forming myelin sheath
Axon
Schwann cell membranewrapped around axon
Action potential jumpsfrom node to node
The process of coating axons with myelin is incomplete when humans are born. This is part of the reason why
babies are uncoordinated and slow learners.
Babies need lots of fat – not only for energy storage but also to myelinate their neurons.
Multiple Sclerosis (MS)
• Disease results in damage to myelin and impairs electrical signaling.
• Muscles weaken and coordination decreases.
Presynaptic
Postynaptic
Axon Terminal(pre-synapse)
Dendrite(post-synapse)
Synapse
neurotransmitter
Synaptic vesicle
Voltage-gatedCa++ channel
Neurotransmittertransporter
NeurotransmitterReceptor
Don’t worry about this
ACTION POTENTIAL TRIGGERS RELEASE OF NEUROTRANSMITTER
Na+ and K+
channels
Presynapticmembrane(axon)
Postsynapticmembrane(dendrite orcell body)
Actionpotentials
1. Action potential arrives;triggers entry of Ca2+.
2. In response to Ca2+, synapticvesicles fuse with presynapticmembrane, then releaseneurotransmitter.
3. Ion channels open whenneurotransmitter binds; ionflows cause change inpostsynaptic cell potential.
4. Ion channels will close asneurotransmitter is brokendown or taken back up bypresynaptic cell (not shown).
Synapse animationhttps://www.youtube.com/watch?v=LT3VKAr4roo
Ion Channels on Post-synaptic Cell at Synapse
• Some only let Na+ pass through.
• Some let Na+/K+ pass through.
• Some only let K+ pass through.
• Some increase the permeability of Cl-.
Excitatory vs. Inhibitory Synapses
• Excitatory synapses cause the post-synaptic cell to become less negative triggering an excitatory post-synaptic potential (EPSP)– Increases the likelihood of firing an action potential
• Inhibitory synapses cause the post-synaptic cell potential to become negative triggering an inhibitory post-synaptic potential– Decreases the likelihood of firing an action potential
Postsynaptic Potentials Can Depolarize or Hyperpolarize the Postsynaptic Membrane
Postsynaptic potentials can depolarize or hyperpolarize thepostsynaptic membrane.
Depolarization,Na+ inflow
Hyperpolarization, K+
outflow or Cl– inflowDepolarization andhyperpolarizationstimuli applied
Excitatorypostsynapticpotential(EPSP)
Inhibitorypostsynapticpotential(IPSP)
EPSP IPSP
Resting potential
Neurons Integrate Information from Many Synapses
Most neurons receive information from many other neurons.
Axons ofpresynaptic neurons
Dendrites ofpostsynaptic neuron
Cell body ofpostsynaptic neuron
Axonhillock Axon of postsynaptic cell
Excitatory synapseInhibitory synapse
Neurons Integrate Information from Many Synapses
Postsynaptic potentials sum.
Action potential
ThresholdRestingpotential
Neurotransmitters
• More than 100 neurotransmitters are now recognized, and more will surely be discovered.
• Acetylcholine is important and one of the first ones discovered because its involvement in muscle movement.
• Dopamine and serotonin hugely important for many behaviors.
• The workhorses of the brain are glutamate, glycine, and γ-aminobutyric acid (GABA).
Acetylcholine
• Stimulates muscles
• Also found throughout nervous system
• Usually excitatory, but can be inhibitory depending on the receptor
Acetylcholine
Dopamine• Excitatory (but sometimes inhibitory) depending
on the location in the nervous system
• Associated with the reward system!!
• Requires a transport protein to inactivate
Dopamine
Serotonin
• Excitatory or inhibitory depending on area of CNS
• Ecstasy (MDMA) causes increased release
• Involved in sleep, appetite, mood
• Drugs like prozac (SSRIs – selective serotonin reuptake inhibitor) slows down transport protein
• Transporter also binds cocaine and amphetamines.
The Autonomic Nervous System Controls Internal ProcessesPARASYMPATHETIC NERVES
“Rest and digest”SYMPATHETIC NERVES
“Fight or flight”
Constrict pupils
Stimulate saliva
Slow heartbeat
Constrict airways
Stimulate activityof stomach
Inhibit release ofglucose; stimulategallbladder
Stimulate activityof intestines
Contract bladder
Promote erectionof genitals
Sacralnerves
Lumbarnerves
Thoracicnerves
Cervicalnerves
Cranialnerves
Dilate pupils
Inhibit salivation
Increase heartbeat
Relax airways
Inhibit activityof stomach
Stimulate releaseof glucose; inhibitgallbladder
Inhibit activityof intestines
Relax bladder
Promoteejaculation andvaginal contraction
Secreteepinephrine andnorepinephrine(hormones thatstimulate activity;see Chapter 47)
Sympathetic chain:bundles of nervesthat synapse withnerves from spinalcord, then sendprojections to organs
The Functions of the PNS Form a Hierarchy
Central nervous system (CNS)Information processing
Peripheral nervous system (PNS)
Sensoryinformation
travels inafferent division
Most informationtravels in
efferent division,which includes…
Somaticnervoussystem
Autonomicnervous system
Parasympatheticdivision
Sympatheticdivision