The Nervous System

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The Nervous System
AP Biology Unit 6
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Branches of the Nervous System

• There are 2 main branches of the nervous system
• Central Nervous System
• Brain
• Spinal Cord
• Peripheral Nervous System
• All nerves leading to rest of body

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Anatomy of a Neuron

• Dendrites where a signal is received by the
  neuron
• Cell body contains the organelles, nucleus of
  the cell
• Axon signal travels down this to get to the
  other end of the neuron

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Anatomy of a Neuron

• Myelin surrounds the axon to speed up the
  signal
• Synaptic Terminal end of the neuron
• Synapse gap/space between neurons

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Question

• What is the general pathway of a signal through a
  neuron?
• Dendrites ? cell body ? Axon ? Synaptic Terminals
  (then into the synapse to get to the next neuron
  or other cell)

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Sending Signals

• The signal sent through a neuron is an
  electrical signal
• Based on the movement of ions into and out of the
  cell
• Causes changes in the and charges inside the
  cell

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A Neuron at Rest

• A neuron at rest (unstimulated) has a difference
  in charge (voltage) across the plasma membrane
  -70 mV resting potential
• This means that it is more negative inside than
  outside
• The resting potential is caused by the
  distribution of ions on either side of the
  membrane

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A Neuron at Rest

• Resting potential (-70 mV) is maintained by the
  Sodium-Potassium Pump
• Pumps Na out of cell
• Pumps K into the cell
• Active transport

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Ion Concentrations at Rest

• At rest, there are also open K channels in the
  membrane ? Allows some K to escape
• Leaves negatively charged molecules behind (Cl-
  ions, etc.) ? more negative on the inside than on
  the outside.

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Sending a Signal Action Potential

• Na channels are embedded in the membrane of the
  neuron
• Usually, these Na channels are closed, but can
  be triggered to open when the correct stimulus is
  received
• Voltage gated channels open in response to a
  particular change in voltage (charge)
• Chemical gated channels open in response to a
  chemical binding to them

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Action Potential

• STEP 1 To start an action potential, some kind
  of stimulus (light, pressure, chemical, etc.)
  causes Na channels in the dendrite to open.
• This causes Na to flood into the neuron from
  outside ? DEPOLARIZATION

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Questions

• Why does Na diffuse in from the outside?
• Higher concentration on the outside
• When depolarization occurs, how is the charge
  inside the neuron affected?
• Becomes more positively charged inside
• What would happen if Cl- channels are also
  opened?
• Cl- would also flow in makes the inside more
  negative (cancels out the charge from Na coming
  in) -- HYPERPOLARIZED

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Action Potential

• STEP 2 The change in voltage triggers the next
  Na channel (voltage gated channel) to open.
• STEP 3 As Na diffuses down the neuron, it
  continues to trigger voltage gated Na channels
  to open.
• This is what sends a signal down the neuron
  towards the axon terminal.

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Action Potential

• STEP 4 Na voltage gated channels only open
  temporarily. After a short period of time, they
  close and an inactivation gate opens to prevent
  them from opening again for a little while ?
  REFRACTORY PERIOD

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Action Potential

• STEP 5 The neuron is reset (REPOLARIZED) by
  the opening of voltage gated K channels.
• K flows out of the neuron, making the inside
  more negative again.
• Why does K flow out?
• Higher K concentrations inside neuron
• The Na/K pump also helps reestablish resting
  potential.

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Saltatory Conduction

• Depolarization Repolarization happens over and
  over down the axon, so the nerve impulse travels.
  
• Myelin sheaths insulate the axon, keeping ions
  from flowing out except at Nodes of Ranvier.

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Saltatory Conduction

• Wider axons yield faster conduction because there
  is less resistance.
• Action potentials jump from one Node of Ranvier
  (space between myelin sheaths) to the next,
  speeding up the signal.

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Communication between Neurons

• When the signal reaches the axon terminal, it
  triggers voltage gated Ca2 channels to open.
• This causes vesicles that contain
  neurotransmitter molecules to fuse with the
  plasma membrane and expel the neurotransmitters
  into the synaptic cleft (space between neurons)

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Communication between Neurons

• The neurotransmitters will diffuse across the
  cleft and bind to receptors on the next neuron
  (postsynaptic neuron).
• This triggers a Na chemical gated channel to
  open on the postsynaptic neuron, triggering an
  action potential in that neuron.

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Communication between Neurons

• After the signal has been sent, neurotransmitters
  are eliminated from the synaptic cleft by
• Diffusion diffuse away
• Reuptake
• Enzyme degradation

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Communication between Neurons

• Reuptake
• Neurotransmitters are actively transported back
  into the presynaptic neuron ? repackaged into
  vesicles to be released again
• Recycling neurotransmitters ?
• Enzyme degradation
• Enzymes in the synaptic cleft break down the
  neurotransmitter

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Question

• Why is it important that our bodies / medications
  control nerve communication?
• So that signals are only sent to neurons when
  needed.

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Control of Communication

• How can nerve communication be varied?
• Change conduction of impulse
• Change synaptic cleft size
• Change volume of neurotransmitters or vesicles
• Change number of ligand-gated ion channels on
  post synaptic neuron
• Add a chemical that binds to ligand-gated ion
  channels to block them or always keep them open
• Add a chemical that binds to neurotransmitters,
  so they cannot bind to the ligand-gated ion
  channels