Neurons Structure and Conduction of a Nerve Impulse

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NeuronsStructure and Conduction of a Nerve
Impulse
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Two coordinating systems which respond to
environmental stimuli

• Nervous System Endocrine (hormone) System
•  
• Begin with Nervous System (data processing
  system)
• 3 interconnected functions ? input / integration
  / output

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Basic Organization
sensory receptor (sensory input) ? integration ?
(motor output) ? effector

• Sensory Input triggered by stimuli
• conduction of signals to processing center
• Integration
• interpretation of sensory signals within
  processing centers
• Motor output
• conduction of signals to effector cells (i.e.
  muscles, gland cells)

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Neuron

• Dendrite - conducts signal toward the cell body
  -- input zone
• often short, numerous highly branched
• signal comes from sensory cell or neighboring
  neuron
• Axon - usually a single fiber -- conducting
  zone
• conducts signal away from cell body to another
  neuron or effector cell
• Axon Ending
• a cluster of branches (100s to 1000s)
• each with a bulblike synaptic knob
• relays signal to next neuron / effector cell

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Generation - Conduction of Neural Impulses

• Dependent on concentration gradients of Na K
• Na 14x greater outside
• K 28x greater inside
• Membrane permeability
• lipid bilayer bars passage of K Na ions
• protein channels and pumps regulate passage of
  K Na
• at rest more K move out than Na move in
• K ions diffuse out leave behind excess negative
  charge
• Sodium-potassium pump
• Na out - K in (more Na out than K in
• contributes to loss of ()

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Overview of Neural Impulse
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• Maintenance of negative charge within neuron
• resting membrane potential about -70 millivolts
• 5 voltage of AA battery
• Dissolved organic molecules negative charge
  kept inside
• Na - K balance

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• Stimulus causes opening of Na gates closing
  of K gates -
• Threshold 30 mV
• all - or - nothing response
• Action potential localized electrical event
• Changes permeability of region immediately ahead
• changes in K Na gates
• domino effect
• propagation of signal
• Intensity of stimuli (i.e. pinch vs. punch)
  number of neurons firing
• Speed on impulse based on diameter of axon
  amount of myelination wire for internet

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Myelin Sheath

• Resembles chain of beads
• Prevents ions from flowing through membranes
• Na channels highly concentrated at nodes
• Allows signal to travel faster because impulse
  jumps from node of Ranvier to node of Ranvier
  (with myelin sheath (225 mph / without 11 mph)
• MS ? destruction of mylin sheath by own immune
  system (progressive loss of signal conduction,
  muscle control brain function)

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Neurons Communicate at Synapses

• Electrical no synapse
• common in heart digestive tract - maintains
  steady, rhythmic contraction
• All cells in effector contain receptor proteins
  for neurotransmitters
• Chemical - skeletal muscles CNS
• presence of gap (SYNAPTIC CLEFT) which prevents
  action potential from moving directly to
  receiving neuron
• ACTION POTENTIAL (electrical) converted to
  CHEMICAL SIGNAL at synapse (molecules of
  neurotransmitter) then generate ACTION POTENTIAL
  (electrical) in receiving neuron

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Overview of Transmission of Nerve Impulse

• Action potential
• ? synaptic knob
• ? opening of Ca channels
• neurotransmitter vesicles fuse with membrane
• release of neurotransmitter into synaptic cleft
• binding of neurotransmitter to protein receptor
  molecules on receiving neuron membrane
• opening of ion channels
• triggering of new action potential
• Neurotransmitter is broken down by enzymes ion
  channels close -- effect brief and precise

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Nerve Impulse

• Presynaptic neuron
• Vesicles
• Calcium channels
• Synaptic cleft
• Postsynaptic neuron
• Neurotransmitter receptor

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Nerve Impulse

• Action potential
• ? synaptic knob
• ? opening of Ca channels
• neurotransmitter vesicles fuse with membrane
• release of neurotransmitter into synaptic cleft

Ca2
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Nerve Impulse

• Action potential
• neurotransmitter vesicles fuse with membrane
• release of neurotransmitter into synaptic cleft

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• Action potential
• binding of neurotransmitter to protein receptor
  molecules on receiving neuron membrane
• opening of sodium channels
• triggering of new action potential

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Neurotransmitters

• Catecholamine Neurotransmitters
• Derived from amino acid tyrosine
• Dopamine Parkinsons, norepinephrine,
  epinephrine
• Amine Neurotransmitters
• acetylcholine, histamine, serotonin
• Amino Acids
• aspartic acid, GABA, glutamic acid, glycine
• Polypeptides
• Include many which also function as hormones
• endorphins

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• Transmission of signals based on MULTIPLE STIMULI
• combined excitatory inhibitory neurons
• Inhibition in Pre-synaptic neuron
• Ca channels blocked
• stops release of neurotransmitter
• Inhibition in Post-synaptic neuron
• opens Cl- channels
• makes interior more -
• increase permeability of K ions
• makes interior more -