The Nerve Impulse

1
The Nerve Impulse

• The nerve impulse is like electricity moving
  through a wire
• The production of nerve impulses depends on the
  movement of positively charged ions across the
  cell membrane.
• Sodium (Na) Potassium (K)

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Resting Potential
3
Resting Potential

• There is more potassium (K ) ions in the neuron
  than in the fluid outside
• There are more sodium (Na ) ions in the fluid
  outside the neuron than inside the neuron.
• The resting neuron has a positive charge outside
  the cell, and a negative charge inside the cell

4
Resting Potential

• Because both sodium and potassium ions can move
  across the cell membrane, the unequal
  distribution of these ions must be maintained by
  active transport.
• Proteins in the cell membrane actively pump
  sodium ions out of the neuron and actively pump
  potassium ions into the neuron.
• Some K leaks back out of the cell through
  channelsthis maintains the negative charge
  inside the cell

5
Sodium Potassium Pump
6
Sodium Potassium Pump

• Pumps more sodium out of the cell than potassium
  in
• Compare the charge inside the neuron to outside
  the neuron?

7
Resting Potential

• As a result of active transport (K in, Na out)
  and diffusion (K out, Na in), a negative charge
  builds up on the inside of the membrane and a
  positive charge builds up on the outside of the
  membrane.
• The difference in electrical charge across the
  cell membrane of a resting neuron is called is
  resting potential.
• A neuron has a resting potential of about -70
  millivolts (mV)

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A Nerve Impulse
9
Nerve Impulses on the Move
Nerve impulses are like ripples caused by
throwing a pebble into water. The ripple is
caused by the up and down movement of water. The
nerve impulse is the movement of ions.
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Moving Impulses

• A nerve impulse begins when a neuron is
  stimulated by another neuron or its environment.
• The cell membrane in axon contains thousands of
  protein channels. Generally the sodium channels
  are closed.

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• At the leading edge of an impulse, sodium
  channels open allowing sodium ions to flow into
  the cell.
• This flow of positive ions causes a temporary
  change in the charges on the cell membrane.
• The inside of the membrane gains a positive
  charge and the outside of the membrane gains a
  negative charge.
• This reversal of charges across the membrane
  along the length of an axon is called an action
  potential.

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• As the impulse passes through the axon, potassium
  channels open allowing K ions to flow out of the
  cell.
• The resting potential is now reestablished with
  the negative charge inside the membrane and the
  positive charge outside the membrane.
• A neuron has an action potential of about 30 mV.

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Action Potential

• An action potential is caused by positive ions
  moving in and then out of the neuron at a certain
  spot on the neuron membrane.
• An action potential is initiated by a stimulus
  above a certain intensity or threshold.

14
Action Potential

• Not all stimuli initiate an action potential. The
  stimulus could be a pin prick, light, heat, sound
  or an electrical disturbance in another part of
  the neuron.
• Action potential is an all or nothing mechanism,
  just like a mousetrap or stack of dominoes.

15
Action Potential Depolarization

• A stimulus causes a gate in the Na Channel to
  open. Since there is a high concentration of Na
  outside, Na diffuses into the neuron. The
  electrical potential changes to 40 mV.

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Action Potential Repolarization

• Depolarization causes the K Channel gate to
  immediately open. K diffuses out of the neuron.
  This reestablishes the initial electrical
  potential of -60 mV.

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Action Potential Refractory Period

• During this time ( 1 msec), the Na and K
  Channels cannot be opened by a stimulus.
• The Na/K Pump actively pumps Na out of the
  neuron and K into the neuron. This reestablishes
  the initial ion distribution of the resting
  neuron.

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Summary of Action Potential
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Action Potential

• This single action potential acts as a stimulus
  to neighboring proteins and initiates an action
  potential in another part of the neuron.
• Ultimately a wave of action potentials travels
  from the dendrites all the way to the axon
  terminals.
• At the axon terminal, the electrical impulse is
  converted to a chemical signal that stimulates a
  neighboring neuron.
• These chemical signals are called
  neurotransmitters.

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The Synapse

• Synapse Small space where a neuron can transfer
  an impulse to another cell.
• It is a small gap that separates the axon
  terminal from the dendrites of the next neuron.
• The axon terminals contain tiny sacs filled with
  neurotransmitters.
• Neurotransmitters Chemicals used by a neuron to
  transmit an impulse to another cell.

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How does everything work together.
22
The Neuromuscular Junction

• Definition the junction between motor neurons
  and muscle fibers
• Electrical impulses in a motor neuron travel
  along the axon to the synaptic knobs
• At the synaptic knobs the chemical
  neurotransmitter acetylcholine is released and
  diffuses across the gap between the motor neuron
  and muscle fiber

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• Acetylcholine stimulates the sarcoplasmic
  reticulum in the muscle fibers (myofibrils) to
  release stored calcium
• The stored calcium is released and bonds to
  troponin on the actin fibers

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• When the calcium bonds to troponin, this causes a
  shift in the tropomyosin protein strand
• When the tropomyosin shifts, myosin bonding sites
  along the actin are exposed
• The myosin heads can freely bond to the
  actinthey begin to slide along the actin like
  hands pulling on a rope

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• http//outreach.mcb.harvard.edu/animations/actionp
  otential.swf
• Action Potentials
• http//www.blackwellpublishing.com/matthews/channe
  l.html
• (Action potentials)
• http//www.blackwellpublishing.com/matthews/nmj.ht
  ml
• (synapse and neurotransmitters)