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