Sequence of Events That Occur During Continuous Propagation

Learning Targets

Students can describe the events involved with the propagation of an action potential.

Terms

Action potentials - the change in electrical potential associated with the passage of an impulse along the membrane of a nerve cell.
Nerve impulses - signals transmitted along a nerve fiber. It consists of a wave of electrical depolarization that reverses the potential difference across the nerve cell membrane.
Threshold -
All or none principle - the strength by which a nerve responds to a stimulus is the independent of the strength of the stimulus. If the stimulus is independent of the strength of the stimulus. If the stimulus exceeds the threshold potential, the nerve or muscle fiber will give a complete response.
Refractory period  - the brief period immediately following the response especially of a nerve before it recovers the capacity to make a second response.
Propagation - Moves action potentials generated in axon hillock along the entire length of the axon.
Continuous propagation - of action potentials along an unmyelinate axon and affects one segment of the axon at a time.
Saltatory propagation - Action potential along myelinated axon and is faster and uses less energy than continuous propagation.

Checkpoints

Briefly describe the events that occur in the propagation of an action potential.

• Step 1: Action potential in segment to threshold
• As an action potential develops at the initial segment (1), the transmembrane potential at this site depolarizes to +30 mV
• Step 2: Depolarizes second segment to threshold
• As the sodium ions entering at (1) spread aeway from the open voltage-gated channels, a graded depolarization quickly brings the membrane in segment (2) to threshold.
• Step 3: First segment enters refractory period
• An action potential now occurs in segment (2) whiles segment (1) beings repolarization.
• Step 4: Local current depolarizes nest segment
• As the sodium ions entering at segment (2) spread laterally, a graded depolarization quickly brings the membrane in segment (3) to threshold, and the cycle is repeated.

What is the role of the sodium potassium pump in the propagation of action potentials.

• Power the sodium - Potassium Exchange Pump
• To maintain concentration gradients of Na+ and K+ over time.
• Requires energy (1 ATP for each 2 K+/3 Na+ exchange)
• Without ATP
• Neurons stop functioning

Compare and contrast continuous and saltatory propagation.

Notes

• Action Potentials
• Propagated changes in transmembrane potential (electrical signals)
• Affect an entire excitable membrane (the entire axon and cell membrane)
• Link cell body with motor end plate actions (cell body -> axon->synaptic terminal)

• Initiating Action Potential
• Initial stimulus
• A depolarization(return to 0 or +) of axon hillock large enough to change resting potential to threshold level of voltage-gated sodium channels.
• Repolarization (return rest -)
• Hyperpolarization (exceed rest more -)
• All-or-none principle
• If a stimulus exceeds threshold amount
• The action potential is the same
• No matter how large the stimulus
• Action potential is either triggered, or not

• Four steps in the generation of action potentials
• Step 1: Depolarization to threshold
• Stimulus initiates action potential large enough to open sodium channels (threshold).

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• Step 2: Activation of Na+ channels
• Rapid depolarization Na+ ions rush into cytoplasm. Inner membrane changes from negative to positive.

• 

  
  
• Step 3: Inactivation of Na+ channels and activation of K+ channels
• Inactivation gates close (Na+ channel inactivation). K+ channels open repolarization begins.

• 

  
  
• Step 4: Return to normal permeability
• K+ channels begin to close. When membrane reaches normal resting potential K+ channels finish closing. Membrane is hyperpolarized to transmembrane potential returns to resting level. Action potential is over.

• 

  
  
• The Refractory Period
• The time period
• From beginning of action potential
• To return to resting state
• During which membrane will not respond normally to additional stimuli.
• Power the sodium - Potassium Exchange Pump
• To maintain concentration gradients of Na+ and K+ over time.
• Requires energy (1 ATP for each 2 K+/3 Na+ exchange)
• Without ATP
• Neurons stop functioning

• Propagation of Action Potentials
• Propagation
• Moves action potentials generated in axon hillock
• Along entire length of axon
• Two methods of propagating action potentials
1. Continuous propagation (unmyelinated axons)
2. Saltatory propagation (myelinated axons)

• Continuous Propagation
• Of action potentials along an unmyelinated axon
• Affects one segment of axon at a time
• Steps in propagation
• Step 1: Action potential in segment to threshold
• As an action potential develops at the initial segment (1), the transmembrane potential at this site depolarizes to +30 mV.

• 

  
  
• Step 2: Depolarizes second segment to threshold
• As the sodium ions entering at (1) spread aeway from the open voltage-gated channels, a graded depolarization quickly brings the membrane in segment (2) to threshold.

• 

  
  
• Step 3: First segment enters refractory period
• An action potential now occurs in segment (2) whiles segment (1) beings repolarization.

• 

  
  
• Step 4: Local current depolarizes nest segment
• As the sodium ions entering at segment (2) spread laterally, a graded depolarization quickly brings the membrane in segment (3) to threshold, and the cycle is repeated.

• 

  
  
• Cycle repeats
• Action potential travels in one direction (1m/sec)

• Saltatory Propagation
• Action potential along myelinated axon
• Faster and uses less energy than continuous propagation
• Myelin insulates axon, prevents continuous propagation
• Local current "jumps" from node to node
• Depolarization occurs only at nodes

Picture

Video

josephssweves.blogspot.com

Source: https://sites.google.com/site/brookehamiltonneuraltissue/propagation-of-an-action-potential

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