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Chapter 4 Neural Conduction and Synaptic Transmission How Neurons Send and Receive Signals This multimedia product and its contents are protected under copyright law. The following are prohibited by law: any public performance or display, including transmission of any image over a network;

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chapter 4 neural conduction and synaptic transmission

Chapter 4Neural Conduction and Synaptic Transmission

How Neurons Send and Receive Signals

  • This multimedia product and its contents are protected under copyright law. The following are prohibited by law:
  • any public performance or display, including transmission of any image over a network;
  • preparation of any derivative work, including the extraction, in whole or in part, of any images;
  • any rental, lease, or lending of the program.

Copyright © 2006 by Allyn and Bacon

the neuron s resting membrane potential
The Neuron’s Resting Membrane Potential
  • Inside of the neuron is negative with respect to the outside
  • Resting membrane potential is about -70mV
  • Membrane is polarized, it carries a charge
  • Why?

Copyright © 2006 by Allyn and Bacon

ionic basis of the resting potential
Ionic Basis of the Resting Potential
  • Ions, charged particles, are unevenly distributed
  • Factors influencing ion distribution
    • Homogenizing
    • Factors contributing to uneven distribution

Copyright © 2006 by Allyn and Bacon

ionic basis of the resting potential4
Ionic Basis of the Resting Potential
  • Homogenizing
    • Random motion – particles tend to move down their concentration gradient
    • Electrostatic pressure – like repels like, opposites attract
  • Factors contributing to uneven distribution
    • Membrane is selectively permeable
    • Sodium-potassium pumps

Copyright © 2006 by Allyn and Bacon

ions contributing to resting potential
Ions Contributing to Resting Potential
  • Sodium (Na+)
  • Chloride (Cl-)
  • Potassium (K+)
  • Negatively charged proteins (A-)
    • synthesized within the neuron
    • found primarily within the neuron

Copyright © 2006 by Allyn and Bacon

the neuron at rest
The Neuron at Rest
  • Ions move in and out through ion-specific channels
  • K+ and Cl- pass readily
  • Little movement of Na+
  • A- don’t move at all, trapped inside

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equilibrium potential
Equilibrium Potential
  • The potential at which there is no net movement of an ion – the potential it will move to achieve when allowed to move freely
  • Na+ = 120mV
  • K+ = -90mV
  • Cl- = -70mV (same as resting potential)

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the neuron at rest8
The Neuron at Rest
  • Na+ is driven in by both electrostatic forces and its concentration gradient
  • K+ is driven in by electrostatic forces and out by its concentration gradient
  • Cl- is at equilibrium
  • Sodium-potassium pump – active force that exchanges 3 Na+ inside for 2K+ outside

Copyright © 2006 by Allyn and Bacon

something to think about
Something to think about
  • What would happen if the membrane’s permeability to Na+ were increased?
  • What would happen if the membrane’s permeability to K+ were increased?

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generation and conduction of postsynaptic potentials psps
Generation and Conduction of Postsynaptic Potentials (PSPs)
  • Neurotransmitters bind at postsynaptic receptors
  • These chemical messengers bind and cause electrical changes
    • Depolarizations (making the membrane potential less negative)
    • Hyperpolarizations (making the membrane potential more negative)

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generation and conduction of postsynaptic potentials psps11
Generation and Conduction of Postsynaptic Potentials (PSPs)
  • Postsynaptic depolarizations = Excitatory PSPs (EPSPs)
  • Postsynaptic hyperpolarizations = Inhibitory PSPs (IPSPs)
  • EPSPs make it more likely a neuron will fire, IPSPs make it less likely
  • PSPs are graded potentials – their size varies

Copyright © 2006 by Allyn and Bacon

epsps and ipsps
EPSPs and IPSPs
  • Travel passively from their site of origination
  • Decremental – they get smaller as they travel
  • 1 EPSP typically will not suffice to cause a neuron to “fire” and release neurotransmitter – summation is needed

Copyright © 2006 by Allyn and Bacon

integration of psps and generation of action potentials aps
Integration of PSPs and Generation of Action Potentials (APs)
  • In order to generate an AP (or “fire”), the threshold of activation must be reached at the axon hillock
  • Integration of IPSPs and EPSPs must result in a potential of about -65mV in order to generate an AP

Copyright © 2006 by Allyn and Bacon

integration
Integration
  • Adding or combining a number of individual signals into one overall signal
  • Temporal summation – integration of events happening at different times
  • Spatial - integration of events happening at different places

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what type of summation occurs when
What type of summation occurs when:
  • One neuron fires rapidly?
  • Multiple neurons fire at the same time?
  • Several neurons fire repeatedly?
  • Both temporal and spatial summation occur simultaneously

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the action potential
The Action Potential
  • All-or-none, when threshold is reached the neuron “fires” and the action potential either occurs or it does not.
  • When threshold is reached, voltage-activated ion channels are opened.

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the ionic basis of action potentials
The Ionic Basis of Action Potentials
  • When summation at the axon hillock results in the threshold of excitation (-65mV) being reached, voltage-activated Na+ channels open and sodium rushes in.
  • Remember, all forces were acting to move Na+ into the cell.
  • Membrane potential moves from -70 to +50mV.

Copyright © 2006 by Allyn and Bacon

the ionic basis of action potentials21
The Ionic Basis of Action Potentials
  • Rising phase: Na+ moves membrane potential from -70 to +50mV.
  • End of rising phase: After about 1 millisec, Na+ channels close.
  • Change in membrane potential opens voltage-activated K+ channels.
  • Repolarization: Concentration gradient and change in charge leads to efflux of K+.
  • Hyperpolaization: Channels close slowly - K+ efflux leads to membrane potential <-70mV.

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refractory periods
Refractory Periods
  • Absolute – impossible to initiate another action potential
  • Relative – harder to initiate another action potential
  • Prevent the backwards movement of APs and limit the rate of firing

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the action potential in action
The action potential in action
  • http://intro.bio.umb.edu/111-112/112s99Lect/neuro_anims/a_p_anim1/WW1.htm
  • http://bio.winona.msus.edu/berg/ANIMTNS/actpot.htm

Copyright © 2006 by Allyn and Bacon

psps vs action potentials aps
EPSPs/IPSPs

Decremental

Fast

Passive (energy is not used)

Action Potentials

Nondecremental

Conducted more slowly than PSPs

Passive and active

PSPs Vs Action Potentials (APs)

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conduction in myelinated axons
Conduction in Myelinated Axons
  • Passive movement of AP within myelinated portions occurs instantly
  • Nodes of Ranvier (unmyelinated)
    • Where ion channels are found
    • Where full AP is seen
    • AP appears to jump from node to node
      • Saltatory conduction
      • http://www.brainviews.com/abFiles/AniSalt.htm

Copyright © 2006 by Allyn and Bacon

structure of synapses
Structure of Synapses
  • Most common
    • Axodendritic – axons on dendrites
    • Axosomatic – axons on cell bodies
  • Dendrodendritic – capable of transmission in either direction
  • Axoaxonal – may be involved in presynaptic inhibition

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synthesis packaging and transport of neurotransmitter nt
Synthesis, Packaging, and Transport of Neurotransmitter (NT)
  • NT molecules
    • Small
      • Synthesized in the terminal button and packaged in synaptic vesicles
    • Large
      • Assembled in the cell body, packaged in vesicles, and then transported to the axon terminal

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release of nt molecules
Release of NT Molecules
  • Exocytosis – the process of NT release
  • The arrival of an AP at the terminal opens voltage-activated Ca++ channels.
  • The entry of Ca++ causes vesicles to fuse with the terminal membrane and release their contents
  • http://www.tvdsb.on.ca/westmin/science/sbioac/homeo/synapse.htm

Copyright © 2006 by Allyn and Bacon

activation of receptors by nt
Activation of Receptors by NT
  • Released NT produces signals in postsynaptic neurons by binding to receptors.
  • Receptors are specific for a given NT.
  • Ligand – a molecule that binds to another.
  • A NT is a ligand of its receptor.

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receptors
Receptors
  • There are multiple receptor types for a given NT.
  • Ionotropic receptors – associated with ligand-activated ion channels.
  • Metabotropic receptors – associated with signal proteins and G proteins.

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ionotropic receptors
Ionotropic Receptors
  • NT binds and an associated ion channel opens or closes, causing a PSP.
  • If Na+ channels are opened, for example, an EPSP occurs.
  • If K+ channels are opened, for example, an IPSP occurs.

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metabotropic receptors
Metabotropic Receptors
  • Effects are slower, longer-lasting, more diffuse, and more varied.
  • NT (1st messenger) binds > G protein subunit breaks away > ion channel opened/closed OR a 2nd messenger is synthesized > 2nd messengers may have a wide variety of effects

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reuptake enzymatic degradation and recycling
Reuptake, Enzymatic Degradation, and Recycling
  • As long as NT is in the synapse, it is active – activity must somehow be turned off.
  • Reuptake – scoop up and recycle NT.
  • Enzymatic degradation – a NT is broken down by enzymes.

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small molecule neurotransmitters
Small-molecule Neurotransmitters
  • Amino acids – the building blocks of proteins
  • Monoamines – all synthesized from a single amino acid
  • Soluble gases
  • Acetylcholine (ACh) – activity terminated by enzymatic degradation

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amino acid neurotransmitters
Amino Acid Neurotransmitters
  • Usually found at fast-acting directed synapses in the CNS
  • Glutamate – Most prevalent excitatory neurotransmitter in the CNS
  • GABA –
    • synthesized from glutamate
    • Most prevalent inhibitory NT in the CNS
  • Aspartate and glycine

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monoamines
Monoamines
  • Effects tend to be diffuse
  • Catecholamines – synthesized from tyrosine
    • Dopamine
    • Norepinephrine
    • Epinephrine
  • Indolamines – synthesized from tryptophan
    • Serotonin

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soluble gases and ach
Soluble-Gases and ACh
  • Soluble gases – exist only briefly
    • Nitric oxide and carbon monoxide
    • Retrograde transmission – backwards communication
  • Acetylcholine (Ach)
    • Acetyl group + choline
    • Neuromuscular junction

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neuropeptides
Neuropeptides
  • Large molecules
  • Example – endorphins
    • “Endogenous opiates”
    • Produce analgesia (pain suppression)
    • Receptors were identified before the natural ligand was

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pharmacology of synaptic transmission
Pharmacology of Synaptic Transmission
  • Many drugs act to alter neurotransmitter activity
    • Agonists – increase or facilitate activity
    • Antagonists – decrease or inhibit activity
    • A drug may act to alter neurotransmitter activity at any point in its “life cycle”

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agonists 2 examples
Agonists – 2 examples
  • Cocaine - catecholamine agonist
    • Blocks reuptake – preventing the activity of the neurotransmitter from being “turned off”
  • Benzodiazepines - GABA agonists
    • Binds to the GABA molecule and increases the binding of GABA

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antagonists 2 examples
Antagonists – 2 examples
  • Atropine – ACh antagonist
    • Binds and blocks muscarinic receptors
    • Many of these metabotropic receptors are in the brain
    • High doses disrupt memory
  • Curare - ACh antagonist
    • Bind and blocks nicotinic receptors, the ionotropic receptors at the neuromuscular junction
    • Causes paralysis

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