Unformatted text preview: TRANSMISSION OF INFORMATION
IN THE NEURVOUS SYSTEM Vitaly Klyachko
Introduction to BME Sept 13nd, 2010 Transmission of information
Information must be transmitted
within each neuron
– action potential conduction
and between neurons
– synaptic transmission V.Gallese Neuron/Synapse Doctrine :
1836: Purkinje stained and identified first neurons,
suggested that nerve fibers originate from cell bodies
1839: Schwann proposed cell theory – all tissues
consist of cells
1842: Helmholz experimentally showed that nerves
originate from cell bodies
1873: Golgi invented silver nitrate staining method
to visualize neurons
1888: Cajal used Golgi staining to identify individual
1891: Walderyer proposed Neuron Doctrine Modern neuron staining
Modern golgi staining, using fluorescent marker genetically encoded in the mice Tim Murphy lectures, UBC Information
Transmission The Action
channels open. Na+ flow in.
The membrane potential changes
from -60mV to +40mV.
Sodium channels rapidly close.
Depolarization triggers opening of
K+ ions flow out, repolarizing the
M. Bear. Neuroscience: Exploring the brain. 3d ed. Action Potential Conduction
Orthodromic: AP travels
down axon to the synapse
Conduction velocity: 10 m/s
AP duration: 1-2 msec
Why natural AP propagates
only in one direction?
M. Bear. Neuroscience: Exploring the brain. 3d ed. Conduction Velocity:
The linear cable theory
1. Membrane parameters are linear: rm, ri, cm = constant, same for all parts of neuron
and passive (voltage-independent)
2. Current flows along a single spatial dimension x. x 3. Extracellular resistance r0 = 0; ∂ 2Vm
∂t λ= aR m
2 Ri τ m = Rm C m ii ri
equation Length constant
Time constant im
cm rm Parameters of the cable model
λ is the length constant:
λ determines the distance through which voltage spread along the cable. λ= aR m
2 Ri Potential spreads further in larger cables
For a 1 μm diameter dendrite
with Rm ≅ 104 Ω.cm2, λ ≅ 800 μm τm is the membrane time constant - determines how long it takes for
membrane potential to change in response to an injected current. τ m = RmCm Conduction velocity of a passively
propagating membrane potential
Conduction velocity: How does the fiber size influence conduction velocity? a – axonal diameter
For a 1 μm diameter axon with
Rm ≅ 20kΩ.cm2, Θ ≅ 700 μm/msec Action Potential Conduction
Passive conduction will ensure that adjacent
membrane depolarizes, so the AP “travels” down
But transmission by continuous AP is relatively
slow and energy-consuming (Na+/K+ pump).
A faster, more efficient mechanism has evolved:
Myelination provides saltatory conduction.
Myelination V.Gallese Myelination
Most mammalian axons are myelinated.
The myelin sheath is provided by oligodendrocytes
(CNS) and Schwann cells (PNS).
Myelin consists of multiple layers of membrane that
prevent passage of ions across the axon’s membrane. V.Gallese Saltatory Conduction
Myelinated regions of axon are electrically insulated. Electrical
charge moves along the axon rather than across the membrane.
Action potentials occur only at unmyelinated regions: nodes of
High Na+ channel
concentration High Rm,
only Ii Synaptic Transmission
Synaptic Transmission of information
Information must be transmitted:
within each neuron – saltatory AP conduction
and between neurons – synaptic transmission
and V.Gallese Why studying synaptic
transmission is important?
• Synapses are the computational units in the brain ~109 synapses / mm3
more synapses in the brain than stars in
• Synapses underlie all sensory experiences: vision, hearing, smell, taste and touch.
• Synaptic deregulation causes many neurological disorders Alzheimer's, Parkinson’s, mental retardation
• Synaptic transmission is essential for information
processing, learning and memory Synapse hypothesis
Charles Scott Sherrington
Discovered that neurotransmission
has (synaptic) delay – nerve is not a
Invented term synapse
Described excitatory and inhibitory
actions of neurotransmitters
Only excitatory propagate, inhibitory
1932 Nobel prize in Medicine
1932 Discovery of chemical
Showed that nerve stimulation
liberates a diffusible transmitter.
Perfusate from one stimulated
frog heart could be transferred
to another and changes beat
1936 Nobel prize in Medicine
Tim Murphy lectures, UBC Debate on synaptic transmission Debate
chemical or electrical.
Soup vs. Spark Debate:
Is synaptic transmission has a chemical nature (Soup) or is a
direct transfer of electrical charge (Spark)?
Pro-Chemical argument: Otto Loewi showed that acetylcholine
application mimics the effect of nerve stimulation
Pro-Electrical argument: synaptic transmission in some cases
is too fast (submillisecond), can only be electrical.
Eccles developed a model of electrical synapse for the
neuromuscular junction explaining current flow at “the point
Subsequently both chemical and electrical transmission was
shown to exist. Chemical Electrical Nobel Prize 1970 Nobel Prize 1963
From Kristin Harris Lectures.
http://synapses.mcg.edu/lab/harris/lectures.htm Electrical Synapses A hallmark of
electrical transmission is a very
between presynaptic stimulus
response. M. Bear. Neuroscience: Exploring the brain. 3d ed. Electrical Synapses
Gap junction: channel formed
by six connexins, 1-2nm pore
Cells are “electrically coupled”
Flow of ions from cytoplasm
to cytoplasm, bidirectionally
Very fast transmission
Synaptic delay: only ~0.2ms
Good for rapid behaviours
such as escape response.
Allows synchronizations of neural
populations (networks). Used in
inhibitory control, developing
M. Bear. Neuroscience: Exploring the brain. 3d ed. , Fundamental Neuroscience Electrical Synapses
No amplification mechanisms
Difficult to modulate properties
Can't change sign
No real mechanisms for long-term changes
Hard to implement memory
Cell size is important
Requires the presynaptic cell to be larger than the
postsynaptic cell to inject considerable charge Chemical Synaptic
Transmission A hallmark of
chemical transmission is a delay
between presynaptic stimulus
response. Tim Murphy lectures, UBC Chemical Synaptic
Electron Micrograph of a synapse Active zone Postsynaptic
neuron vesicles Venkatesh N. Murthy lectures, Harvard Chemical Synaptic
Synaptic vesicles, containing neurotransmitter, congregate at the
presynaptic active zone.
The action potential causes voltage-gated calcium (Ca2+) channels to open;
Ca2+ ions flood in.
Increase in Ca2+ concentration
causes vesicles to fuse with
Neurotransmitter is released
into synaptic cleft and diffuses
across to bind postsynaptic
Activation of postsynaptic receptors cause influx of ions into postsynaptic
cell, altering membrane potential. Chemical Synaptic
Multiple steps are required to release neurotransmitters
and for them to act on postsynaptic receptors
Synaptic delay: 0.2-2 msec.
Unidirectional, localized action
release machinery and receptors are localized
at active zone/postsynaptic specializations
Can change sign by release of inhibitory transmitter
Easy to modulate: has many steps at presynaptic and at
the postsynaptic sites. Types of Synapses
Types Types of Synapses
Axon to dendrite
Axon to cell Body
Axon to Axon
Dendrite to dendrite M. Bear. Neuroscience: Exploring the brain. 3d ed. Types of Synapses
By architecture/ultrastructure: Type I, asymmetrical, (excitatory) Type II, symmetrical, (inhibitory)
http://synapses.mcg.edu/atlas Types of Synapses
By the number of release sites: Single release site
(excitatory, CNS) Multiple release sites
Excitatory and inhibit., PNS)
Fundamental. Neurosci 2nd ed. Neurotransmitters
Three classes of neurotransmitters
– Amino acids, amines, and peptides
Many different neurotransmitters
- Several neurotransmitters can be released from the same
synapse. Example: GABA and Gly.
Defining particular transmitter systems
– By the molecule, synthetic machinery, packaging, reuptake
and degradation, etc.
– First identified neurotransmitter M. Bear. Neuroscience: Exploring the brain. 3d ed. Postsynaptic
Mechanisms Postsynaptic Receptors
Transmitter-gated ion channels Metabotropic:
G-protein-coupled receptors M. Bear. Neuroscience: Exploring the brain. 3d ed. Postsynaptic Receptors
Fast synaptic transmission
Sensitive detectors of chemicals
Regulate flow of large currents
Differentiate between similar
Basic structure: pentamer,
M. Bear. Neuroscience: Exploring the brain. 3d ed. Postsynaptic Receptors
Amino Acid-Gated Channels
Glutamate-Gated: AMPA, NMDA, kainite.
Mediate Excitatory transm.
GABA- and Glycine-Gated:
Mediate inhibitory transm.
Bind ethanol, benzodiazepines, barbiturates
Prime targets for drug
M. Bear. Neuroscience: Exploring the brain. 3d ed. Postsynaptic Receptors
G-Protein-Coupled Receptors and Effectors
3 basic operational steps:
Binding of the neuroBinding
transmitter to the receptor
Activation of G-proteins
Activation of effector systems
Activation The basic structure:
Single polypeptide with 7
membrane-spanning α-helices M. Bear. Neuroscience: Exploring the brain. 3d ed. Dendrites:
Integration of Information Function of Dendrites
Neurons have complex multiple compartment structure.
Note the differences in voltage spread across the dendrites and how voltage spreads through
the dendrtic tree with time.
Membrane potential spread in a model dendritic
tree of a Purkinje cell during dendritic AP Integration of Information
Depending on the type of ion channel which opens, the
postsynaptic cell membrane becomes either:
Depolarized, more prone to firing an AP, excitatory, EPSP
Hyperpolarized, less prone to firing an AP, inhibitory, IPSP M. Bear. Neuroscience: Exploring the brain. 3d ed. Integration of information
PSPs are small. An individual EPSP will not produce
enough depolarization to trigger an action potential.
IPSPs will counteract the effect of EPSPs at the same
Summation means the effect of many coincident
IPSPs and EPSPs at one neuron.
If there is sufficient depolarization at the axon
hillock, an action potential will be triggered.
axon hillock V.Gallese Integration of information
Synaptic Integration: Process by which multiple synaptic
potentials combine within one postsynaptic neuron M. Bear. Neuroscience: Exploring the brain. 3d ed. Integration of information
– Allows for neurons to perform sophisticated
– Integration: EPSPs added together to produce
significant postsynaptic depolarization
– Spatial: EPSP generated simultaneously in different
– Temporal: EPSP generated at same synapse in rapid
succession M. Bear. Neuroscience: Exploring the brain. 3d ed. ...
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This note was uploaded on 02/14/2012 for the course NUBITRY 3304 taught by Professor Various during the Spring '01 term at Albertus Magnus.
- Spring '01