Lecture 13 – Animal Nervous Systems

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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???

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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???

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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

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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

• Cell membranes are more permeable to K than to
  Na
• There is more K inside the cell than outside
• 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???

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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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Neuron Function Resting Potential
REVIEW

• 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 ChannelsWhy 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 ChannelsWhy 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 ChannelsWhy 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 cant 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???

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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 cant 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

• Resting Potential Na and K activation gates
  closed Na inactivation gate open on most
  channels
• Depolarization Na activation gates begin to
  open Na begins to enter cell
• Rising Phase threshold is crossed, Na floods
  into the cell, raising the membrane potential to
  35mV

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Resting Potential Na and K activation gates
closed Na inactivation gate open on most
channels
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Voltage Gate Activity

• Resting Potential Na and K activation gates
  closed Na inactivation gate open on most
  channels
• Depolarization Na activation gates begin to
  open Na begins to enter cell
• 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

• Resting Potential Na and K activation gates
  closed Na inactivation gate open on most
  channels
• Depolarization Na activation gates begin to
  open Na begins to enter cell
• 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

• Falling Phase Na inactivation gates close, K
  activation gates open Na influx stops, K
  efflux is rapid
• Undershoot K activation gates close, but not
  until membrane potential has gone a little bit
  below resting potential
• Refractory Period the Na inactivation gates
  remain closed during stages 4 and 5, limiting the
  maximum frequency of action potentials

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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

• Falling Phase Na inactivation gates close, K
  activation gates open Na influx stops, K
  efflux is rapid
• Undershoot K activation gates close, but not
  until membrane potential has gone a little bit
  below resting potential
• Refractory Period the Na inactivation gates
  remain closed during stages 4 and 5, limiting the
  maximum frequency of 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

• Falling Phase Na inactivation gates close, K
  activation gates open Na influx stops, K
  efflux is rapid
• Undershoot K activation gates close, but not
  until membrane potential has gone a little bit
  below resting potential
• Refractory Period the Na inactivation gates
  remain closed during stages 4 and 5, limiting the
  maximum frequency of action potentials

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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.
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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???

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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 the
  signal to the receiving cell

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Chemical Synapses

• A multi-stage process
• Neurons synthesize neurotransmitters, isolated
  into synaptic vesicles located at the synaptic
  terminal
• The action potential triggers the release of
  neurotransmitters into the synapse
• Neurotransmitters diffuse across the synapse
• Neurotransmitter binds to a receptor, stimulating
  a response (more later)

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Chemical Synapses

• Action potential depolarizes membrane at synaptic
  terminal
• Depolarization in this region opens Ca channels
• Influx of Ca stimulates synaptic vesicles to
  fuse with neuron cell membrane
• Neurotransmitters are released by exocytosis
• Neurotransmitters bind to the receiving cell
  membrane

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Chemical Synapses
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Chemical Synapses
REVIEW

• Action potential depolarizes membrane at synaptic
  terminal
• Depolarization in this region opens Ca channels
• Influx of stimulates synaptic vesicles to fuse
  with neuron cell membrane
• Neurotransmitters are released by exocytosis
• Neurotransmitters bind to the receiving cell
  membrane

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Chemical Synapses

• Direct synaptic transmission
• Neurotransmitter binds directly to ligand-gated
  channels
• Channel opens for Na, K or both
• Indirect synaptic transmission
• Neurotransmitter binds to a receptor on the
  membrane (not to a channel protein)
• Signal transduction pathway is initiated
• Second messengers eventually open channels
• Slower but amplified response

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Chemical synapses allow more complicated signals

• Electrical signals pass unmodified at electrical
  synapses
• Chemical signals are modified during transmission
• Type of neurotransmitter varies
• Amount of neurotransmitter released varies
• Some receptors promote depolarization some
  promote hyperpolarization
• Signals are summed over both time and space
• Remember that many, many neurons are responding
  to any given stimulus

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Chemical synapses allow more complicated signals

• Responses are summed at the axon hillock
• Action potential is generated and sent down axon
  or not

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Chemical synapses allow more complicated signals

• Summation is over both time and space
• Excitory and inhibitory signals can cancel each
  other

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Neurotransmitters review text and table, but
dont memorizeTable 48.1, 7th ed.
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CNS Organization in Vertebrates

• Brain integrates
• Spinal cord 1o transmits
• Both derived from hollow, dorsal embryonic nerve
  cord
• Hollow remnants remain in ventricles of brain and
  central canal of spinal cord
• Spaces are filled with cerebrospinal fluid that
  helps circulate nutrients, hormones, wastes, etc
• Fluid also cushions CNS
• Axons are aggregated white matter

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PNS Organization in Vertebrates

• Major role transmitting information from
  sensory structures to the CNS and from the CNS
  to effector structures
• Nerves always in left/right pairs that serve both
  sides of the body

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PNS Organization in Vertebrates

• Cranial nerves originate in brain and connect to
  the head and upper body
• Some have only sensory neurons (eyes, nose)
• Spinal nerves originate in spinal cord and
  connect to the rest of the body
• Contain both sensory and motor neurons

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Critical Thinking

• Can the eyes do anything besides see???
• Can the nose do anything besides smell???
• Can the ears do anything besides hear???

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Critical Thinking

• Can the eyes do anything besides see???
• Can the nose do anything besides smell???
• Can the ears do anything besides hear???

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PNS Organization in Vertebrates

• Cranial nerves originate in brain and connect to
  the head and upper body
• Some have only sensory neurons (eyes, nose)
• Spinal nerves originate in spinal cord and
  connect to the rest of the body
• Contain both sensory and motor neurons

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PNS Sub-divisions
All work together to maintain homeostasis and
respond to external stimuli
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PNS - Somatic

• Nerves that transmit signals to and from skeletal
  muscles
• Respond primarily to external stimuli
• Largely under voluntary control

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PNS - Autonomic

• Nerves that control the internal environment
• Respond to both internal and external signals
• Largely under involuntary control
• Three sub-divisions
• Sympathetic stress responses
• Parasympathetic opposes sympathetic
• Enteric controls digestive system

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PNS Autonomic
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Autonomic - Sympathetic

• Activates flight or fight responses
• Promotes functions that increase sensory
  perception and ATP levels
• Inhibits non-essential functions such as
  digestion and urination

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Autonomic Parasympathetic

• Returns body systems to base-line function
• Promotes digestion and other normal functions
• Usually antagonistic to sympathetic division

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Autonomic Enteric

• Specifically controls the digestive system
• Regulated by both the sympathetic and
  parasympathetic divisions

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Brain Development