Information Flow and the Neuron

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Information Flow and the Neuron

• Starr/Taggarts
• Biology
• The Unity and Diversity of Life, 9e
• Chapter 34

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Key Concepts

• Neurons are basic units of communication in
  nervous system
• Neurons are excitable cells
• With stimulation, the polarity of charge across
  the membrane reverses from negatively charged
  inside the cell to being positively charged

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Key Concepts

• Action potentials are changes in the polarity of
  the cell membrane
• Information flow starts with action potentials,
  which self propagate along the plasma membrane
• Chemical signals released from a neuron stimulate
  or inhibit the adjoining neuron, muscle cell, or
  gland cell
• Information flow depends on integration of signals

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INPUT stimulus
RECEPTORS sensory neurons
INTEGRATORS interneurons (e.g., the brain)
motor neurons
EFFECTORS muscle cells, gland cells
OUPUT response
NERVOUS SYSTEM
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dendrites
INPUT ZONE
cell body
TRIGGER ZONE
axon
axon endings
CONDUCTING ZONE
OUPUT ZONE
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Types of Neurons
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Resting Membrane Potential
Negative inside, positive outside Can be
detected by electrodes placed inside and outside
axon Usually about -70 millivolts
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Restoring and Maintaining Readiness
Potential is established and maintained by
pumping and leaking of Na and K
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Restoring and Maintaining Readiness
Ion Concentrations At Resting Potential
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A Closer Look at Action Potentials

• Voltage changes
• Na, K
• Graded response
• Local response
• Threshold level
• Action Potential

• All or nothing
• Action potential
• Depolarization
• Repolarization

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Action Potential All-or-Nothing Spike
Triggered when threshold potential is
reached Always the same size
During action potential, inside of patch of
membrane becomes more positive then outside
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Propagation of an Action Potential
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Propagation of an Action Potential
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Chemical Synapses

• Neurotransmitters
• Acetylcholine (ACh)
• Junctions
• Neuron - cell
• Neuron - neuron
• Neuron - muscle
• Neuron - gland
• Excitatory effect
• Inhibitory effect

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A Closer Look at a Chemical Synapse
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Channels Open
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Examples of Signals

• Neurotransmitters
• Acetylcholine (Ach)
• Norepinephrine (NE)
• Dopamine
• Gamma amino butyric acid (GABA)
• Neuromodulators

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Synaptic Integration

• EPSP
• Depolarizing effect
• IPSP
• Hyperpolarizing effect
• Synaptic integration
• Excitatory or Inhibitory
• Acetylcholinesterase
• Breaks apart ACh
• ACh is removed from the synaptic cleft

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axon
myelin sheath
nerves outer wrapping
blood vessels
nerve fascicle
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Structure of a Nerve
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The Myelin Sheath
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Reflex Arcs

• Reflex - Automatic movement made in response to
  stimulus
• Simplest reflexes - Sensory neurons synapse
  directly on motor neurons
• Stretch reflex
• Most reflex pathways include interneurons

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axon
axon
axon ending
receptor endings
peripheral axon
cell body
cell body
cell body
axon
axon endings
dendrites
dendrites
INTERNEURON
MOTOR NEURON
SENSORY NEURON
Spinal cord
STIMULUS Biceps stretches.
RESPONSE Biceps contracts
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In Conclusion

• The nervous system consists of sensory neurons,
  interneurons, and motor neurons, which activate
  muscle and glands
• A neurons dendrites and cell body are input
  zones and the axon ending is the output zone
• Channels exist in the membrane for Na and K

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In Conclusion

• There is a voltage difference across the plasma
  membrane in a resting neuron
• An action potential is a reversal of the voltage
  difference across the membrane
• Stimuli can cause local graded potentials or to
  the threshold of an action potential
• When Na channels open, allowing Na to enter a
  cell, the voltage difference across the membrane
  reverses abruptly

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In Conclusion

• Sodium/Potassium Pumps restore the gradients
  after an action potential
• Neurotransmitters released into a synaptic cleft
  may excite or inhibit the cells plasma membrane
• developed by M. Roig