6.5 Neurons

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6.5 Neurons Synapses
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The Nervous System

• Divided into 2 parts
• CNS Central Nervous System
• Consists of brain and spinal cord
• Composed of neurons
• PNS Peripheral Nervous System
• Consists of all the nerves that branch from the
  CNS and go to the rest of the body
• Made up of sensory receptors and nerves
• Afferent and efferent

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Neurons
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Neurons transmit electrical impulses

• Send out rapid electrical impulses called action
  potentials
• must be changed into a chemical message in order
  to cross the synapse (space between 2 neurons)
• Carry message to a relay neuron inside the CNS
• Relay neuron turns it into an electrical impulse
• Then it travels to an effector cell (elicits a
  response)

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Resting potential Action Potential

• Difference in electrical charge inside
  (cytoplasm) and outside (extracellular fluid)
• Membrane potential
• Controlled by sodium NA and potassium k
• Neurons pump sodium and potassium ions across
  their membrane to generate a resting potential

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Resting potential

• The charge of a cell at rest. The neuron is not
  sending an action potential. Resting potential is
  maintained by the sodium-potassium pump.
• At rest charge inside cells is more negative by
  -70mv
• Uses ATP to pump 3Na out of cell and 2K into
  cell
• Some leaky channels (always working to
  maintain resting potential)

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Action Potential

• The changing charge in a cell due to the opening
  of sodium-gated ion channels (depolarization)
  followed by the opening of potassium-gated ion
  channels (repolarization)
• Occur when a stimulus is detected
• If cells depolarized to a charge around -50 mv
  and action potential is propagated (threshold
  potential)

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Propagation of an action potential
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Nerve Impulses

• When action potential is propagated it moves down
  the axon to the terminal buttons
• Axon is depolarized and then repolarized
• Wave of traveling membrane charge changes
• Myelinated axons speed up the transmission
• Axon potential generated between nodes (between
  myelination, across Schwann cells)
• Jumping of the impulse from one node to another
  is called saltatory conduction

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Graph of an Action Potential
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Synaptic Transmission

• Synapse space between two neurons or a neuron
  and a muscle cell
• In order to carry and impulse across a synapse it
  needs to be changed from and electrical signal to
  a chemical signal
• Chemical form is called a neurotransmitter
• Stored in the terminal buttons of neurons
• Arrival of action potential at terminal buttons
  triggers the opening of the calcium gated
  channels in the presynaptic membrane
• Causes the release of neurotransmitters

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Neurotransmitters

• Excitatory when they open sodium channels
• Inhibitory when they open potassium channels
• Examples
• Acetylcholine (cholinergic synapses)
• Cholinergic synapses control muscle contractions
• Dopamine
• Serotonin
• Epinephrine
• norepinephrine

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Useful applications for blocking cholinergic
synapses

• Pesticides neuroactive insecticides bind to
  acetylcholine receptors causing the paralysis of
  insects.
• Botox toxin derived from the bacteria that
  causes botulism, stops muscle contractions by
  blocking the release of acetylcholine. The
  relaxed muscles reduce the appearance of wrinkles.

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Removal of neurotransmitters

• Must be removed from the synapse to stop the
  message being sent
• Removed by enzymes
• Removed by terminal buttons by reuptake
  (endocytosis)

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• Receptor ? Interneuron ? Effector Reflex
• Does not involve the brain
• Involuntary (ex. pain)
• Adaptive behavior for survival