Nerve Impulses

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Nerve Impulses
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Membrane Potentials

• All living cells maintain a difference in the
  concentration of ions across their membranes.
• There is a slight excess of positives on the
  outside and a slight excess of negatives on the
  inside.
• This results in a difference in electrical charge
  across the plasma membrane called membrane
  potential.

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The Voltmeter
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Resting Membrane Potentials

• When a neuron is not conducting electrical
  signals, it is said to be resting.
• At rest, a neurons membrane potential is
  typically maintained at about -70 mV.

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Sodium-Potassium Pump

• Active transport mechanisms in the plasma
  membrane that transports sodium ions (Na) and
  potassium ions (K) in opposite directions at
  different rates.
• Three Na out for every two K in
• Maintains an imbalance in the distribution of
  positive ions, thus maintaining a difference in
  electrical chargethe inside becomes slightly
  less positive (slightly negative).

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The Sodium-Potassium Pump at Work
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Role of Channels in Membrane

• Some K channels are open when at rest
• K diffuses down its concentration gradient
• Adds to the positive on the outside of the cell
• Na channels are closed

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Local Potentials

• In neurons, membrane potentials can fluctuate
  above or below the resting membrane potential in
  response to certain stimuli.
• A slight shift away from the RMP in a specific
  region of the plasma membrane is often called a
  local potential.

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• Excitation of a neuron occurs when a stimulus
  triggers the opening of stimulus-gated channels
  allows Na to enter the cell
• Depolarization movement of the membrane
  potential towards zero
• Inhibition occurs when a stimulus triggers the
  opening of stimulus-gated K channels. As K
  diffuses out of the cell the positive ions
  outside the cell increases
• Hyperpolarization movement of the membrane
  potential away form zero (thus below the usual
  RMP)

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Action Potential

• An action potential is a nerve impulse in which
  an electrical fluctuation travels along the
  surface of a neurons plasma membrane.
• Voltage Gated Channels (-59 mV threshold
  potential)
• All or nothing response

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Steps of the Mechanism that Produces an Action
Potential
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Step 1

• A stimulus triggers stimulus gated Na channels
  to open and allow inward Na diffusion. This
  causes the membrane to depolarize.

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Step 2

• As the threshold potential is reached, voltage
  gated Na channels open.

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Step 3

• As more Na enters the cell through voltage
  gated Na channels, the membrane depolarizes even
  further.

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Step 4

• The magnitude of the action potential peaks (at
  30 mV) when voltage gated Na channels close.

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Step 5

• Repolarization begins when voltage gated K
  channels open, allowing outward diffusion of K.

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Step 6

• After a brief period of hyperpolarization, the
  resting potential is restored by the
  sodium-potassium pump and the return of ion
  channels to their resting state.

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Action Potential
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Refractory Period

• Absolute can not respond to any stimulus no
  matter how strong
• Relative -- the membrane is repolarizing and can
  only respond to very strong stimuli

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Conduction of the Action Potential

• The action potential causes voltage gated
  channels to open in adjacent areas of the axon
  membrane causing the action potential to move
  down the length of the axon.
• In myelinated fibers, electrical changes in the
  membrane can only occur at gaps in the sheath
  (nodes of Ranvier). This is called saltatory
  conduction.

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Action Potential Travels Along the Axon
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Saltatory Conduction
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Conduction Speed

• The speed of conduction of a nerve fiber is
  proportional to its diameter. The larger the
  diameter, the faster it conducts impulses.
• Myelinated fibers conduct impulses more rapidly
  than unmyelinated fibers.

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Synaptic Transmission
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Synapses

• A synapse is where signals are transmitted
  between neurons.
• Presynaptic and postsynaptic neurons.
• Electrical synapse occur when two cells are
  joined end to end by gap junctions. The action
  potential just continues on between cells
  (cardiac cells).

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Chemical Synapse

• Three structures make up a chemical synapse
• A synaptic knob contains vesicles with
  neurotransmitters
• A synaptic cleft space in between (20-30 nm)
• The plasma membrane of a postsynaptic neruon.

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Electrical and Chemical Synapses
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• Action potential can not cross synaptic cleft
• Neurotransmitters are released from the synaptic
  knob where they travel across the cleft to the
  receptors on the plasma membrane of the
  postsynaptic neuron.
• Neurotransmitters cause either depolarization
  (excitatory) or hyperpolarization (inhibitory) of
  the postsynaptic membrane.

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The Chemical Synapse

1. Calcium influx
2. Neruotransmitter vesicles move to membrane and
   open
3. Neurotransmitter diffuses across synaptic cleft
   and bind to receptor molecules.

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