Lecture #13 – Animal Nervous Systems
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Transcript Lecture #13 – Animal Nervous Systems
Lecture #13 – Animal Nervous Systems
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Key Concepts:
• Evolution of organization in nervous
systems
• Neuron structure and function
• Neuron communication at synapses
• Organization of the vertebrate nervous
systems
• Brain structure and function
• The cerebral cortex
• Nervous system injuries and diseases??? 2
All animals except sponges have some kind
of nervous system
• Increasing complexity accompanied increasingly
complex motion and activities
• Nets of neurons bundles of neurons cephalization
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First split
was tissues;
next was
body
symmetry;
echinoderms
“went back”
to radial
symmetry
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Derived radial symmetry and nerve network
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Cephalization
• The development of a brain
• Associated with the development of
bilateral symmetry
• Complex, cephalized nervous systems are
usually divided into 2 sections
Central nervous system (CNS) integrates
information, exerts most control
Peripheral nervous system (PNS) connects
CNS to the rest of the body
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Critical Thinking
• What is the functional advantage of
cephalization???
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Critical Thinking
• What is the functional advantage of
cephalization???
• All the sensory, processing, eating and
many feeding structures are located at the
advancing end of the animal
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Cephalization
• The development of a brain
• Associated with the development of
bilateral symmetry
• Complex, cephalized nervous systems are
usually divided into 2 sections
Central nervous system (CNS) integrates
information, exerts most control
Peripheral nervous system (PNS) connects
CNS to the rest of the body
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PNS CNS PNS
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Specialized neurons support
different sections
• Sensory
Transmit information from the sensory
structures that detect the both external and
internal conditions
• Interneurons
Analyze and interpret sensory information,
formulate response
• Motor
Transmit information to effector cells – the
muscle or endocrine cells that respond to input
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Critical Thinking
• Which type of neuron would have the most
branched structure???
Sensory neurons
Interneurons
Motor neurons
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Critical Thinking
• Which type of neuron would have the most
branched structure???
Sensory neurons
Interneurons
Motor neurons
• Interneurons have the most connections of
all neurons
• They make “all the connections”
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Neuron structure is complex
100 billion
nerve cells in
the human
brain!
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Basic Neuron Structure
• Cell body
• Dendrites
• Axons
• Axon hillock
• Myelin sheath
• Synaptic terminal
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Cell Body
• Contains most cytoplasm and organelles
• Extensions branch off cell body
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Dendrites
• Highly branched extensions
• Receive signals from other neurons
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Axons
• Usually longer extension, unbranched til end
• Transmits signals to other cells
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Axon Hillock
• Enlarged region at base of axon
• Site where axon signals are generated
Signal is sent after summation
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Myelin Sheath
• Insulating sheath around axon
• Also speeds up signal transmission
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Synaptic Terminal
• End of axon branches
• Each branch ends in a synaptic terminal
Actual site of between-cell signal generation
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Synapse
• Site of signal transmission between cells
• More later…
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Supporting Cells - Glia
• Maintain structural integrity and function of
neurons
• 10 – 50 x more glia than neurons in
mammals
• Major categories
Astrocytes
Radial glia
Oligodendrocytes and Schwann cells
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Glia – Astrocytes
• Structural support for neurons
• Regulate extracellular ion and
neurotransmitter concentrations
• Facilitate synaptic transfers
• Induce the formation of the blood-brain
barrier
Tight junctions in capillaries allow more control
over the extracellular chemical environment in
the brain and spinal cord
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Glia – Radial Glia
• Function mostly during embryonic
development
• Form tracks to guide new neurons
out from the neural tube (neural
tube develops into the CNS)
• Can also function as stem cells to
replace glia and neurons (so can
astrocytes)
This function is limited in nature;
major line of research
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Glia – Oligodendrocytes (CNS) and
Schwann Cells (PNS)
• Form the myelin sheath around axons
• Cells are rectangular and tile-shaped,
wrapped spirally around the axons
• High lipid content insulates the axon –
prevents electrical signals from escaping
• Gaps between the cells (Nodes of
Ranvier) speed up signal transmission
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The nerve signal is electrical!
• To understand signaling process, must
understand the difference between resting
potential and action potential
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Resting Potential
• All cells have a resting potential
Electrical potential energy – the separation of
opposite charges
Due to the unequal distribution of anions and
cations on opposite sides of the membrane
Maintained by selectively permeable
membranes and by active membrane pumps
Charge difference = one component of the
electrochemical gradient that drives the
diffusion of all ions across cell membranes
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Neuron Function – Resting Potential
• Neuron resting potential is ~ -70mV
At resting potential the neuron is NOT actively
transmitting signals
Maintained largely because cell membranes
are more permeable to K+ than to Na+; more
K+ leaves the cell than Na+ enters
An ATP powered K+/Na+ pump continually
restores the concentration gradients; this also
helps to maintain the charge gradient
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Resting Potential Ion Concentrations
1. Cell membranes are
more permeable to
K+ than to Na+
2. There is more K+
inside the cell than
outside
3. There is more Na+
outside the cell than
inside
• Both ions follow their
[diffusion] gradients
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Critical Thinking
• If both ions follow their
diffusion gradients, what
is the predictable
consequence???
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Critical Thinking
• If both ions follow their
diffusion gradients, what
is the predictable
consequence???
• A dynamic equilibrium
where both charge and
concentration were
balanced
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Resting Potential Ion Concentrations
• A dynamic equilibrium
is predictable, but is
prevented by an ATP
powered K+/Na+ pump
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Neuron Function – Resting Potential
• Neuron resting potential is ~ -70mV
At resting potential the neuron is NOT actively
transmitting signals
Maintained largely because cell membranes
are more permeable to K+ than to Na+; more
K+ leaves the cell than Na+ enters
An ATP powered K+/Na+ pump continually
restores the concentration gradients; this also
helps to maintain the charge gradient
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Resting Potential Ion
Concentrations
• ATP powered pump
continually transfers 3
Na+ ions out of the
cytoplasm for every 2 K+
ions it moves back in to
the cytoplasm
• This means that there is
a net transfer of + charge
OUT of the cell
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Resting Potential Ion Concentrations
• Thus, the membrane
potential is maintained
• Cl- and large anions
also contribute to the
net negative charge
inside the cell
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REVIEW
Neuron Function – Resting Potential
• Neuron resting potential is ~ -70mV
At resting potential the neuron is NOT actively
transmitting signals
Maintained largely because cell membranes
are more permeable to K+ than to Na+; more K+
leaves the cell than Na+ enters
An ATP powered K+/Na+ pump continually
restores the concentration gradients; this also
helps to maintain the charge gradient
Cl-, other anions, and Ca++ also affect resting
potential
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Gated Ion Channels
Why Neurons are Different
• All cells have a membrane potential
• Neurons can change their membrane
potential in response to a stimulus
• The ability of neurons to open and close
ion gates allows them to send electrical
signals along the extensions (dendrites
and axons)
Gates open and close in response to stimuli
Only neurons can do this!
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Gated Ion Channels
Why Neurons are Different
• Gated ion channels manage membrane
potential
Stretch gates – respond when membrane is
stretched
Ligand gates – respond when a molecule
binds (eg: a neurotransmitter)
Voltage gates – respond when membrane
potential changes
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Gated Ion Channels
Why Neurons are Different
• Hyperpolarization = inside of neuron
becomes more negative
• Depolarization = inside of neuron becomes
more positive
Either can occur, depending on stimulus
Either can be graded – more stimulus = more
change in membrane potential
• Depolarization eventually triggers an
action potential = NOT graded
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Depolarization eventually triggers an
action potential – action potentials
are NOT graded
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Action Potentials ARE the Nerve Signal
• Triggered whenever depolarization reaches
a set threshold potential
• Action potentials are all-or-none responses
of a fixed magnitude
Once triggered, they can’t be stopped
There is no gradation once an action potential
is triggered
• Action potentials are brief depolarizations
1 – 2 milliseconds
• Voltage gated ion channels control signal
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Critical Thinking
• If the action potential is of a fixed
magnitude, how do we sense different
levels of a stimulus???
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Critical Thinking
• If the action potential is of a fixed
magnitude, how do we sense different
levels of a stimulus???
• They can occur with varying frequency
Frequency is part of the information
• They can occur from a large number of
nearby neurons
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Action Potentials ARE the Nerve Signal
• Triggered whenever depolarization reaches
a set threshold potential
• Action potentials are all-or-none responses
of a fixed magnitude
Once triggered, they can’t be stopped
There is no gradation once an action potential
is triggered
• Action potentials are brief depolarizations
1 – 2 milliseconds
• Voltage gated ion channels control signal
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Fig. 48.13; p. 1019, 7th Ed.
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Voltage Gate Activity
1. Resting Potential – Na+ and K+ activation
gates closed; Na+ inactivation gate open
on most channels
2. Depolarization – Na+ activation gates
begin to open – Na+ begins to enter cell
3. Rising Phase – threshold is crossed, Na+
floods into the cell, raising the membrane
potential to ~ +35mV
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1. Resting Potential – Na+ and K+
activation gates closed; Na+
inactivation gate open on most
channels
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Voltage Gate Activity
1. Resting Potential – Na+ and K+ activation
gates closed; Na+ inactivation gate open
on most channels
2. Depolarization – Na+ activation gates
begin to open – Na+ begins to enter cell
3. Rising Phase – threshold is crossed, Na+
floods into the cell, raising the membrane
potential to ~ +35mV
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2. Depolarization – Na+ activation gates
begin to open – Na+ begins to enter cell
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Voltage Gate Activity
1. Resting Potential – Na+ and K+ activation
gates closed; Na+ inactivation gate open
on most channels
2. Depolarization – Na+ activation gates
begin to open – Na+ begins to enter cell
3. Rising Phase – threshold is crossed, Na+
floods into the cell, raising the membrane
potential to ~ +35mV
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3. Rising Phase – threshold is crossed,
Na+ floods into the cell, raising the
membrane potential to ~ +35mV
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Voltage Gate Activity
4. Falling Phase – Na+ inactivation gates
close, K+ activation gates open – Na+
influx stops, K+ efflux is rapid
5. Undershoot – K+ activation gates close,
but not until membrane potential has gone
a little bit below resting potential
6. Refractory Period – the Na+ inactivation
gates remain closed during stages 4 and
5, limiting the maximum frequency of
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action potentials
Membrane
repolarizes
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4. Falling Phase – Na+ inactivation gates
close, K+ activation gates open – Na+
influx stops, K+ efflux is rapid
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Voltage Gate Activity
4. Falling Phase – Na+ inactivation gates
close, K+ activation gates open – Na+
influx stops, K+ efflux is rapid
5. Undershoot – K+ activation gates close,
but not until membrane potential has gone
a little bit below resting potential
6. Refractory Period – the Na+ inactivation
gates remain closed during stages 4 and
5, limiting the maximum frequency of
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action potentials
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5. Undershoot – K+ activation gates close,
but not until membrane potential has
gone a little bit below resting potential
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Voltage Gate Activity
4. Falling Phase – Na+ inactivation gates
close, K+ activation gates open – Na+
influx stops, K+ efflux is rapid
5. Undershoot – K+ activation gates close,
but not until membrane potential has gone
a little bit below resting potential
6. Refractory Period – the Na+ inactivation
gates remain closed during stages 4 and
5, limiting the maximum frequency of
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action potentials
6. Refractory Period – the Na+ inactivation
gates remain closed during stages 4
and 5, limiting the maximum frequency
of action potentials
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Fig. 48.13, 7th Ed.
Conduction of
Action Potential
• Electrical signal moves
along the axon by
depolarizing adjacent
regions of the membrane
past the threshold
• The depolarization effect
is NOT directional – the
cytoplasm becomes more
+ in both directions
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Critical Thinking
• If the depolarizing effect is bilateral, why
does the signal travel in one direction
only???
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Critical Thinking
• If the depolarizing effect is bilateral, why
does the signal travel in one direction
only???
• The refractory period!!!
• Na+ gates are locked shut at the signal
source end and the depolarization can
only affect the leading end of the axon
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Conduction of
Action Potential
• Electrical signal moves
along the axon by
depolarizing adjacent
regions of the membrane
past the threshold
• Depolarization zone
travels in one direction
only due to the refractory
period (Na+ gates locked)
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Speed!
• Diameter of axon
Larger = less resistance faster signal
Found in invertebrates
Max speed ~ 100 m/second
• Nodes of Ranvier
Signal jumps from node to node
Found in vertebrates
Saves space – 2,000 myelinated axons can fit
in the same space as one giant axon
Max speed ~ 120 m/second
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Synapses – the gaps between cells
• Electrical synapses occur at gap junctions
Action potential is transmitted directly from cell
to cell
Especially important in rapid responses such as
escape movements
Also with controlling heart beat (but with specialized muscle tissue)
• Most synapses are chemical
The signal is converted from electrical
chemical electrical
Neurotransmitters cross the synapse and carry
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the signal to the receiving cell