Saltatory Conduction

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Saltatory Conduction

• - speeding up of neuronal signalling

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Myelin

• an insulating layer
• made of Schwann cells
• protective
• speeds signal transmission in myelinated neurons
  action potentials only happen at the nodes of
  Ranvier

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• Exposed areas on neuron with many voltage-gated
  sodium channels, able to depolarize and pass on
  signal
• Action potential jumps from node to node,
  causing voltage-gated Na channels to open

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Figure 2.20  Saltatory conduction in a myelinated
axonAn action potential at the node triggers
flow of current to the next node, where the
membrane regenerates the action potential.
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• Results in nerve impulse
• Passing on of signal down the neuron
• Much faster in myelinated neurons (120 m/s) than
  in non-myelinated (0.5 m/s)

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The Synapse

• The synapse is the junction between neurons
• Synapses are found between sensory receptors and
  sensory neurons and between motor neurons and
  muscle cells (effectors)
• The transmitting cell is called the presynaptic
  cell and the receiving cell is the postsynaptic
  cell
• There are two main types of synapses
• 1) electrical synapses
• 2) chemical synapses

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a) Electrical Synapse

• Pre and post synaptic cells are connected by gap
  junctions that allow action potentials to pass
  directly from one cell to another

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b) Chemical Synapse
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• The electrical signal is converted into a
  chemical signal, and then back to an electrical
  signal in the post cell
• Within the cytoplasm of the presynaptic cell
  there are numerous sacs filled with
  neurotransmitters, the messenger molecules that
  will be released into the synaptic cleft, as a
  result of an action potential
• The neurotransmitters will bind to receptors on
  the postsynaptic neuron, causing its
  depolarization (action potential)
• Neurotrasmitters in cleft are degraded by enzymes
  or taken up by neurons for re-use

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Chemical Synapse
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Neurotransmitters

• 1) Dopamine and serotonin (amines)
• Technically the only two things you enjoy!
• Mood regulators, linked to sensations of pleasure
• Dopamine has been linked to schizophrenia and
  Parkinsons disease.
• Not enough serotonin is linked to depression
• Serotonin and LSD (hallucinogen) have similar
  structures

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• Endorphins (peptides, endogenous morphines)
• Natural painkillers, affect emotions
• Released during exercise runners high
• Similar to opiates, morphine and heroin bind to
  the same receptors

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• Norepinephrine (amine)
• Complements the actions of epinephrine, readies
  the body to respond to danger/stress

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• OK, but why do some things feel kind of good and
  others REALLY good?

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• The All-or-None Law
• The size, amplitude, and velocity of an action
  potential are independent of the intensity of the
  stimulus that initiated it.
• How then is stimulus intensity coded?

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Summation

• The figure below shows a typical neural pathway
• When neuron A and Neuron B fire together an
  action potential is triggered in neuron D
• Note that neuron C is inhibitory

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Temporal and Spatial Summation
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Peripheral Nervous System
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• divided into two parts
• Somatic System
• controls voluntary movement of skeletal muscles
• myelinated

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2. Autonomic System

• controls involuntary glandular secretions and
  functions of smooth and cardiac muscle
• divided into

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i. sympathetic nervous system

• activates fight or flight response
• norepinephrine (neurotransmitter) works with
  epinephrine (hormone) to activate stress response
• blood pressure increases (vasoconstriction),
  heart beats faster, digestion slows down, etc.
• released by modern stressors anxiety

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ii. parasympathetic nervous system

• involuntary control in opposition to the
  sympathetic nervous system
• activated when the body is calm and at rest (ex.
  meditation)
• the break to sympathetic NSs gas pedal

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HOMEOSTASIS!!!!