Biology 3201

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Biology 3201

• Unit 1 Maintaining Dynamic Equilibrium II
• Section 1 Nervous System
• The Neuron

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The Neuron

• The basic functional unit of the nervous system.
• Function Send impulses to and from the CNS and
  PNS and the effectors (muscles/glands)

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Neuron Structure
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• Dendrite ? Fine hair-like extensions on the end
  of a neuron.
• Function receive incoming stimuli.
• Cell Body or Soma ? The control center of the
  neuron.
• Function Directs impulses from the dendrites to
  the axon.
• Nucleus ? Control center of the Soma.
• Function Tells the soma what to do.
• Axon? Pathway for the nerve impulse (electrical
  message) from the soma to the opposite end of the
  neuron.
• Myelin Sheath ? An insulating layer around an
  axon. Made up of Schwann cells.
• Nodes of Ranvier ? Gaps between schwann cells.
• Function Saltatory Conduction (Situation where
  speed of an impulse is greatly increased by the
  message jumping the gaps in an axon).

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Types of Neurons

• There are 3 types of neurons.
• Sensory Neurons ? Neurons located near receptor
  organs (skin, eyes, ears).
• Function receive incoming stimuli from the
  environment.
• Motor Neurons ? Neurons located near effectors
  (muscles and glands)
• Function Carry impules to effectors to initiate
  a response.
• Interneurons ? Neurons that relay messages
  between other neurons such as sensory and motor
  neurons. (found most often in Brain and Spinal
  chord).

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Types of Neurons
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Nerves

• Nerves ? Collections of neurons that are joined
  together by connective tissue.
• Responsible for transferring impulses from
  receptors to CNS and back to effectors.

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How Do Neurons Operate?

• Neuron at Rest ? Resting Potential
• Occurs when the neuron is at rest.
• A condition where the outside of the membrane is
  positively() charged compared to the inside
  which is negatively(-) charged.
• Neuron is said to be polarized.
• Neuron has a voltage difference of -70 mV

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How is resting potential maintained?
Ion Distribution
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How is resting potential maintained?

• At rest, the sodium gates are closed.
• Membrane is 50 times more permeable to K ions
  causing them to leak out.
• This causes outside of membrane to have an
  abundance of charges compared to inside. The
  inside of the membrane is negative compared to
  the outside. This is helped by the (-) proteins
  etc.
• The sodium-potassium pump pulls 2 K ions in
  for 3 Na ions sent out. This further creates a
  charge difference!!

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Action Potential

• The mechanism by which neurons send impulses.
  They are comprised of electrical signals
  generated at the soma and moving along the axon
  toward the end opposite the soma (motor neurons)
• Action potentials occur in two stages
• Depolarization
• Repolarization

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Depolarization in an action potential

• When the neuron is excited past its Threshold
  the following events occur
• Sodium ions (Na) rush into the axon.
• This neutralizes the negative ions inside.
• The inside of the axon becomes temporarily ()
  while the outside becomes temporarily (-). The
  reversal of charge is known as depolarization
• Nearby Sodium (Na) channels open to continue the
  depolarization.

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Lets Have a look.

• http//www.mcgrawhill.ca/school/booksites/biology/
  studentresources/toc/unit4homeostasismaintaini
  ngdynamicequilibrium/chapter12thenervoussyst
  em/coolstufftoseeanddo/moviechannelbehaviou
  r.php

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Repolarization

• This is the restoring of the () charge on the
  outside of the axon and (-) on the inside.
• Potassium gates open and potassium floods out.
• This generates positive charge on the outside of
  membrane.
• Sodium Channels Close (no charges can get
  inside)
• The Sodium/Potassium pump rapidly moves Sodium
  out of the cell.
• Further creates the () charge outside with a (-)
  charge inside.

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Refractory Period

• Brief period of time between the triggering of an
  impulse and when it is available for another.
• NO NEW action potentials can be created during
  this time.

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Saltatory Conduction

• The jumping of an impulse between the Nodes of
  Ranvier thus dramatically increasing its speed.
• Only occurs in axons having Myelin.
• 2m/s ? 120 m/s

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All or None Response

• If an axon is stimulated above its threshold it
  will trigger an impulse down its length.
• The strength of the response is not dependent
  upon the stimulus.
• An axon cannot send a mild or strong response. It
  either responds or does not!!!

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Homework

• View ILO 06 Structure of a Neuron
• Complete Activity 1 and 2 (Go to Work)
• View ILO 07 Nerve Tansmission
• Answer Q. 2, 4, 5, 7
• Explain the critical role Myelin plays to an
  axon. Include at least two roles it plays.

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