Download Communication within the Nervous System

Communication within the
Nervous System
The Cells that make us who we are
How neurons communicate with one another
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The Cells That Make Us Who We
Are
• How many are there?
• Neurons: 100 billion
• Make up 10% of brain volume
• Glia: Many more!
• Make up 90% of brain volume
• Neurons: Jobs include
• convey sensory information to the brain;
• carry out operations involved in thought and feeling;
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The Cells That Make Us Who We
• Send commands out to the body.
• Dendrites
• Cell body or soma
• Axons insulated with myelin (secreted by glia), with end terminals
that release neurotransmitters from vesicles into the synapse
Are
Figure 2.3: Components of a Neuron
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The Cells That Make Us Who We
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The Cells That Make Us Who We
Are
Figure 2.4 a,b: The Three Shapes of Neurons
•Unipolar neurons (a)
•Bipolar neurons (b)
•Multipolar neurons
• Figure 2.3, previous slide
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The Cells That Make Us Who We
Are
Table 2.1: The Three Types of Neurons
Figure 2.4c: The Three Shapes of Neurons
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The Cells That Make Us Who We
Type
Shape
Description
Motor neuron
Multipolar
Output to muscles/organs
Sensory neuron
Unipolar or Bipolar
Input from receptors
Interneuron
Multipolar
Within, axon
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The Cells That Make Us Who We
Are
Figure 2.5: Composition of the Cell Membrane
• Lipids
• Heads attracted to water in and outside the cell, tails repelled by water
• Creates a double-layer membrane
• Proteins
• Hold the cells together
• Controls the environment in and around the cell
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The Cells That Make Us Who We
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The Neural Membrane
• The neuron has a selectively-permeable
membrane.
• Water and gases pass freely through • Other substances are
barred from entry.
• Others pass through protein channels in the membrane
under certain circumstances.
• Polarization results from selective permeability of
the membrane.
• Polarization: difference in electrical charge between the
inside and outside of the cell.
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The Neural Membrane
• This difference in electrical charge is referred to as a voltage.
• A potential is any change in a membrane’s voltage.
•The resting potential is the difference in charge
between the inside and outside of the membrane
of a neuron at rest.
• Between -40 and -80 millivolts (mV) in different
neurons.
• A typical neuron’s resting potential is around -70 mV.
• Caused by unequal distribution of ions on either side of
the membrane.
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The Neural Membrane
• Outside contains mostly sodium (Na+) and chloride (Cl-) ions.
• Inside contains mostly potassium (K+) ions and organic anions (A-).
Figure 2.7: The Sodium-Potassium Pump
•Sodium-potassium pump
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The Neural Membrane
• Moves 3 Na+ outside for
every 2 K+ inside
•Force of diffusion:
• Ions flow from high to low
concentration
•Electrostatic pressure
• Ions attracted to the opposite
charge (+ to -), and repelled
by the same charge (+ from +)
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The Neural Membrane
•Excitatory signals cause a partial hypopolarization
(or depolarization), in a small area of the
membrane.
• The hypopolarization is caused by a change in ion
balance, which also affects the adjacent membrane.
• This spreading hypopolarization diminishes over
distance, so it is often referred to as a local potential.
•At the axon hillock, if hypopolarization reaches
threshold (around -60 mV), an action potential
will be triggered.
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The Neural Membrane
The Action Potential
Depolarization is the change in the resting neuron’s
polarity toward zero
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The Neural Membrane
+30 mV
0 mV
Hypopolarized
threshol
d
Resting
Potential
-70 mV
-80 mV
Hyperpolarize
d
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The Neural Membrane
Figure 2.8a: The Action Potential
1.
2.
4.
5.
Membrane depolarized past threshold through a
series of graded potentials.
Voltage-gated Na ions open, Na enters 3.
Voltage-gated K channels open, K exits.
K channels slowly close and membrane returns to resting
potential.
The Action Potential lasts about 1 millisecond
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The Neural Membrane
• Movement of action potentials down the axon is
not a flow of ions but a chain of events...
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The Neural Membrane
depolarizing adjacent membrane areas which
triggers another action potential.
• When the action potential reaches the terminals it
passes the message on to the next cell “in line”.
• The local potential is a graded potential, but the
action potential follows the all-or-none law.
• Always occurs at full strength and doesn’t vary with stimulus
intensity.
• Nondecremental
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The Neural Membrane
• Message travels over long distances at the same amplitude
• Rate Law: Firing rate of neuron proportional to stimulus
intensity
Absolute vs. Relative Refractory Period
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The Neural Membrane
+60 mV
+20 mV
-20 mV
-60 mV
Relative Refractory Period
0
1
2
3
4
5
Time between Action Potentials (ms)
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7
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The Cells That Make Us Who We
Are
Effects of Neurotoxins and Anesthetics
•Neurotoxins affect ion channels involved in the
action potential.
• Tetrodotoxin blocks sodium channels.
• Scorpion venom opens sodium channels, prolonging the
action potential.
•Beneficial drugs affect these ion channels as well.
• Local anesthetics block sodium channels.
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• General anesthetics work by opening potassium
channels.
•Optogenetics.
• Modified ion channels that are triggered by light.
Glial Cells
Myelination, Axon diameter, and conduction speed
• Myelin, secreted by glial cells, is a fatty tissue that
surrounds axons, providing electrical insulation and
support.
• CNS: oligodendrocytes
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• PNS: Schwann cells
• Increases the conduction speed from 1 m/s to over 120
m/s.
• Myelin gaps called nodes of Ranvier are where action
potentials occur... i.e. where sodium ions enter the axon.
• Transmission between nodes (under the myelin) is by local potential.
• Saltatory conduction: the action potential “jumps” from node to node.
• Multiple sclerosis is a disease in which myelin is destroyed,
reducing conduction speed.
• Axons with a larger diameter will conduct signals faster
than axons with a smaller diameter.
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Glial Cells
Figure 2.9: Glial Cells Produce Myelin for Axons
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Glial cells
Figure 2.11: Astrocyte Density Correlates With Behavioral Complexity
• Scaffolds for migrating neurons... guides new neurons
in fetal development
• Respond to injury and disease by removing debris.
• Provide energy to neurons.
• 7X more neural connections when glia are present
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Glial Cells
Figure 2.10: Glial Cells Increase the Number of Connections Between
Neurons.
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SOURCE: From “Synaptic Efficacy Enhanced by Glial Cells in vitro,” by F. W. Pfrieger and B. A. Barres, Science, 277, p. 1684. © 1997.
Used by permission of the author.
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How Neurons Communicate With One
Another
Figure 2.12: The Synapse Between a Presynaptic Neuron and a
Postsynaptic Neuron
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How Neurons Communicate With One
Another
•Synapse: connection
between a neuron and
another cell
• Presynaptic neuron
transmits the signal
• Postsynaptic cell receives the
signal
• Synaptic cleft (gap) between
the two
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How Neurons Communicate With One
Another
Figure 2.13: Loewi’s Experiment Demonstrating Chemical
How Neurons Communicate With One
Another
Transmission in Neurons •
One of two methods:
• Stimulated vagus nerve, slowed heart A
• Stimulated accelerator nerve, sped up heart A.
• Injected solution from heart A into heart B
• Heart B rate changed to match in similar ways
• Loewi’s conclusion
• Heart uses chemical messengers, not action potentials, to
change heart rate.
Steps of the Synaptic Event
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How Neurons Communicate With One
Another
2+
1. Action potential depolarizes
pre-synaptic membrane, Ca2+ channels
open and Ca enters cell
2. Neurotransmitter released into cleft
3. NT binds to post-synaptic receptors
4. Ionotropic receptor opens
post-synaptic ion channels, changing
Ca2+
EPSP
Receptor
Na+
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“Ionotropic effect”
How Neurons Communicate With One
Another
the potential
http://www.youtube.com/watch?v=http://www.yo
Postsynaptic Receptor Types
•Ionotropic receptors
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How Neurons Communicate With One
Another
• cause ion channels to open, which
• has a direct and rapid effect on the neuron.
•Metabotropic receptors
• open channels indirectly,
• producing slower but longer-acting effects.
•Synaptic transmission is much slower than axonal
(electrical) transmission.
How Neurons Communicate With One
Another
Excitatory & Inhibitory Postsynaptic Potentials
•Activation of receptors on the postsynaptic cell has
two possible effects on the membrane potential.
• Hypopolarization creates an excitatory postsynaptic
potential (EPSP).
• An EPSP opens sodium channels.
• This makes the postsynaptic neuron more likely to fire.
• Hyperpolarization creates an inhibitory postsynaptic
potential (IPSP).
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How Neurons Communicate With One
Another
• An IPSP opens potassium or chloride channels or both.
• This makes it less likely an action potential will occur.
Postsynaptic Potentials are Graded
•EPSPs and IPSPs are graded potentials.
• Accumulate over a short time (temporal summation)
• Combine inputs from different locations on dendrites
and cell body (spatial summation)
•The neuron acts as a(n)
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How Neurons Communicate With One
Another
• Information integrator (summation)
• Decision maker (excitatory and inhibitory inputs
combine algebraically, fires when above threshold)
• The Decision Point is the Axon Hillock, where the Axon
joins the cell body / Soma.
Figures 2.18 (left) & 2.17 (right): Temporal & Spatial Summation.
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How Neurons Communicate With One
Another
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How Neurons Communicate With One
Another
Removal of Neurotransmitters and Drug Effects
• Neurotransmitters must be removed to allow
frequent responding and to prevent it from affecting
nearby synapses.
• Reuptake: the transmitter brought back into the terminals
• Inactivation: enzymes break down the transmitter in the cleft
• Drugs
• Some mimic natural transmitters and stimulate receptors
themselves (agonists)
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How Neurons Communicate With One
Another
• Some block neurotransmitter receptors (antagonists) • Some
enhance or reduce transmitter effects. For example:
• Antidepressants block reuptake of serotonin (SSRIs)
• Some prevent neurotransmitter inactivation (MAOIs)
Regulation of Synaptic Activity
• One regulatory process occurs in axoaxonic synapses
• Presynaptic inhibition decreases the release of transmitter.
• Presynaptic excitation increases the release of transmitter.
• This regulation occurs by affecting calcium entry into the
terminal.
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How Neurons Communicate With One
Another
• Autoreceptors sense the amount of transmitter in
the cleft and cause the presynaptic neuron to reduce
excessive output.
• Glial cells
• prevent transmitter from spreading to other synapses;
• absorb and recycle transmitter for the neuron’s reuse;
• release glutamate to regulate presynaptic transmitter release.
A Variety of Neurotransmitters Multiplies the Possible Synaptic
Effects
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How Neurons Communicate With One
Another
•Receptor subtypes add more complexity
• Acetylcholine receptor has nicotine and muscarinic
subtypes.
•Neurons can release more than one chemical
• One fast-acting plus one or more slower-acting
neuropeptides
• Two or more fast-acting transmitters
• Excitatory & inhibitory transmitters at different synapses
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How Neurons Communicate With One
Another
• Contradicts Dale’s principle that neurons can only
release one neurotransmitter
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How Neurons Communicate With Each
Other
Table 2.2b: Some Representative Neurotransmitters
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Neurotransmitter
Function
Acetylcholine
Transmitter at muscles; in brain, involved in learning, etc.
Monoamines
Serotonin
Involved in mood, sleep and arousal, aggression, depression, obsessivecompulsive disorder, and alcoholism
Dopamine
Contributes to movement control and promotes reinforcing effects of food,
sex, and abused drugs; involved in schizophrenia and Parkinson’s disease.
Norepinephrine
Released during stress. Neurotransmitter in the brain to increase arousal and
attentiveness to events in the environment; involved in depression.
Epinephrine
A stress hormone related to norepinephrine; plays a minor role as a
neurotransmitter in the brain.
Amino Acids
Glutamate
The principal excitatory neurotransmitter in the brain and spinal cord. Vitally
involved in learning and implicated in schizophrenia.
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Gamma-aminobutyric
acid (GABA)
The predominant inhibitory neurotransmitter. Its receptors respond to alcohol
and the class of tranquilizers called benzodiazepines. Deficiency in GABA or
receptors is one cause of epilepsy.
Glycine
Inhibitory transmitter in the spinal cord and lower brain. The poison
strychnine causes convulsions and death by affecting glycine activity.
How Neurons Communicate With Each
Other
Table 2.2b: Some Representative Neurotransmitters
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Neurotransmitter
Function
Neuropeptides
Endorphins
Neuromodulators that reduce pain and enhance reinforcement.
Substance P
Transmitter in neurons sensitive to pain.
Neuropeptide Y
Initiates eating and produces metabolic shifts.
Gas
Nitric Oxide
One of two known gaseous transmitters, along with carbon monoxide. Can
serve as a retrograde transmitter, influencing the presynaptic neuron’s release
of neurotransmitter. Viagra enhances male erections by increasing nitric
oxide’s ability to relax blood vessels and produce penile engorgement.
How Neurons Communicate With One
Another
Neuronal Coding and Neural Networks
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•Neuronal coding and neural networks add further
complexity to neural processing.
• Trains of neural impulses encode additional information
in the intervals between spikes and length of bursts.
• Some information is encoded by segregating it to
specialized pathways known as “labeled lines.”
• In the brain, information is integrated and processed in
complex neural networks.
•One way of studying these networks is to simulate
their activity with computer-based artificial
neural networks.
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• Detecting cancer in a biopsy sample
Figure 2.23: Image of White Matter Fiber Tracts
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SOURCE: Courtesy of Jason Wolff.
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