Axonal Conduction: How an action potential moves along the axon

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Axonal Conduction How an action potential moves
along the axon

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What determines the conduction velocity, i.e.,
how fast action potentials move along the axon
membrane?

• 1. Large diameter (like the squids giant axon)
  favors rapid conduction. Why? The resistance of
  current flow in the center of the axon becomes
  smaller as the diameter becomes larger.
• 2. Insulation speeds conduction by allowing the
  signal to jump from uninsulated node to node down
  the axon (saltatory conduction). Myelination,
  the wrapping of axons by glial cells (Schwann
  cells or oligodendrocytes) eliminates the passive
  leakage so that the depolarization is relayed
  instantly as an electrical field effect to the
  next node, where exposed membrane allows the
  opening of Na channels that can recharge the
  action potential, to send it to the next node.
• (Myelination increases the rate of action
  potential spread most effectively, but the
  fastest conducting axons in our nervous system
  combine relatively large diameter and
  myelination. The smallest axons are not
  myelinated and their messages of dull pain and
  temperature arrive at the central nervous system
  very slowly.)

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Axonal conduction in the absence of myelination
every part of the membrane must go through the
changes in potential
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How axons get myelinated
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Myelination in central nervous system and
peripheral nerves allows saltatory conduction
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Saltatory conduction means jumping, and only the
regions of membrane exposed at the nodes of
Ranvier are actively involved in generating an
action potential.
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Multiple Sclerosis, a demyelinating disease
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Altered Conduction Patterns with MS
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What causes multiple sclerosis?

• It is an autoimmune disease in which the immune
  system attacks oligodendrocytes or the myelin
  itself within the central nervous system (the
  brain and spinal cord). Conduction rate drops
  and sometimes fails. Treatment usually involves
  drugs that suppress the immune system.

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What determines action potential frequency?

• The size of the input to the cell translates into
  the frequency of action potentials if the
  depolarization just rises above threshold, there
  will be one action potential. If it rises much
  higher, the action potentials will start as soon
  as it reaches threshold, and will continue to be
  generated as long as the membrane potential is
  above threshold this is called frequency coding.
  The example shown below is the response of a
  sensory cell to different levels of pressure,
  which result in different levels of
  depolarization.

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Firing frequency limitations?

• The limit to how high the frequency of action
  potentials rises is how fast the membrane of the
  axon can recover from the previous action
  potential there is a refractory period during
  which it is impossible to start off a new action
  potential because the membrane is occupied with
  the events of the previous action potential
• 1. the Na gates are inactivated,
• 2. the voltage-gated K channels are open.
• The stimulus depolarization cannot have an effect
  until the membrane potential returns to near its
  resting state. The fastest firing rate is seen
  in our spinal motor neurons, which can fire at
  frequencies approaching 1000/sec.

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Summary

• Action potentials can be generated only if the
  membrane possesses special voltage-gated
  channels.
• The action potentials role is to carry a signal
  from one part of the cell to another part (from
  input to output) in a reliable way. The signal is
  renewed all along the axon so the action
  potential that arrives at the axon terminals is
  the same size as the one that started out,
  i.e.,each action potential is all or nothing.
• Myelination and large diameter support more rapid
  conduction of the action potential.
• A neuron codes the intensity of the inputs it
  receives in terms of the frequency of its action
  potentials.
• Failures of conduction, as in multiple sclerosis,
  interfere with motor and sensory function.

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Action potentials are the simplest thing that
neurons do

• The information relayed by the action potential
  has value only if it can be passed along to
  target cells. At the axon terminals, the action
  potential invades the synaptic region. There,
  the details of what happens depends on what kind
  of contacts the neuron makes with its target
  cells.

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Definitions

• The word synapse is from the Greek word for
  clasp, connect or join. Synapses are the
  functional connections of neurons
• 1.with other neurons or
• 2. muscle cells or
• 3. gland cells.
• These functional connections can involve
• chemical transmission or
• electrical transmission.
• They can be
• discrete (exclusively between the two synaptic
  partners) or
• 2. diffuse, allowing the impact to spread out to
  more than one target cell.

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Electrical vs. Chemical Synaptic Transmission
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Characteristics of Electrical Communication

• Functions
• 1. Speed Utilized at synapses between decision
  output command neurons and motor neurons that
  activate muscle. First described in the escape
  reflex of crayfish.
• 2. Synchrony Electrical fish extend their
  wing-like fins synchronously when jumping out of
  the water.
• 3. Developmental cell linkage allows spread of
  regulatory signals (peptides up to 1000 mol. wt.
  can move across).
• Mechanism
• Gap junctions in a planar array
• allow current to flow between
• cells Sometimes the current flow
• is effectively unidirectional and
• other times it can flow either
• direction equally well.

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Electrical Synapses advantages and disadvantages

• Advantages
• 1. fast 0.1msec (vs. 200msec for chemical
  synapses)
• 2. economical
• 3. can be bidirectional
• Disadvantages
• 1. limited ability to be integrated with other
  inputs.
• 2. cannot have inhibitory effect.
• 3. Cannot have longer-term actions mediated by
  second messengers.