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