PHYSIOLOGY 1

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PHYSIOLOGY 1

• LECTURE 12
• Graded Potentials
• Action Potential Generation

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Graded PotentialsAction Potential

• Objectives Student should know
• 1. Graded potential
• 2. Types of graded potentials
• 3. Action potential
• 4. Three stages of action potential
• 5. Types of action potential
• 6. Generation of action potential
• 7. Properties of action potentials
• 8. Differences graded vs action potential

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Graded Potentials

• A. Subthreshold electrical stimuli that do not
  produce a true action potential but do generate
  electrical signals
• B. Stimuli may be electrical, chemical, or
  mechanical
• C. Stimuli produce two types of physiochemical
  disturbances

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Graded Potentials

• 1. Local, graded, non propagated potentials
  called receptor or generator potentials, synaptic
  potentials or electrotonic potentials
• 2. Action potentials (complete depolarization)
  or nerve impulses which are propagated down the
  axon to cause the release of neurotransmitters

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ACTION POTENTIAL
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Graded Potentials
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Graded Potentials
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Graded Potentials Local Response

• A. Subthreshold response
• B. Characteristics of graded potentials
• 1. It is Local - changes in membrane potential
  are confined to relatively small regions of the
  plasma membrane
• 2. It is graded - Refers to the magnitude of the
  potential change and that the signal can be
  reinforced.

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Graded Potentials Local Response

• A. Magnitude can vary (is graded) with the
  magnitude of the stimulus
• B. Graded events can be hypopolarizing
  (depolarizing - decrease in potential difference)
  or hyperpolarizing

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Graded Potentials Local Response

• 3. Graded potentials are conducted with
  decrement. (conduction magnitude falls off the
  further you get from the point of origin)

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Graded Potentials Local Response

• A. Charge is lost across the membrane because of
  leaky channels and the magnitude of the
  potential decreases with distance from the site
  of origin (charge density falls).
• B. Graded potentials and the local current they
  generate can function as signals over very short
  distances
• C. Graded potentials die out in 1 - 2 mm of the
  origin

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Types of Graded Potentials

• 1. Characteristics of Graded Potentials
• a. Only type of communication by some neurons
• b. Play an important role in the initiation and
  integration of long distance signals by neurons
  and other cells

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Types of Graded Potentials

• 2. Specific types of graded potentials
• a. Receptor (Generator) potentials
• 1) Sensory receptors respond to stimuli from
  mechanoreceptors, thermoreceptors, nociceptors
  (pain), chemoreceptors, and electromagnetic
  receptors (vision)
• a) Graded potential from stimuli is called
  receptor potential
• b) If graded potential reached threshold an
  action potential is generated and sensory
  information is sent to the spinal cord and brain

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Types of Graded Potentials

• b. Pacemaker potential - heart
• 1) Specialized coronary muscle cells in the
  cardiac pacemaker region (SA node) have leaky
  ion channels graded potentials can potentially
  induce a true cardiac action potential
• 2) Graded potential is responsible for cardiac
  automaticity

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Types of Graded Potentials

• c. Postsynaptic membrane potentials
• 1) Graded potentials that develop on the
  postsynaptic membrane during synaptic
  transmission (stimuli from other nerves - can be
  stimulatory or inhibitory)
• 2) If graded potentials reach threshold action
  potential develops

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Types of Graded Potentials

• D. EPP End Plate Potential
• Post synaptic graded potential that develops at
  the neuromuscular junction (always stimulatory
  and always reach threshold if generated by an
  action potential in the innervating alpha motor
  neuron). Postsynaptic membrane potentials are
  important in AP generation in nerve to nerve and
  nerve to muscle communication.

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Action Potential Generation

• Graded Potentials which reach threshold generate
  action potentials
• 1. Much larger response - Membrane polarity
  reverses (complete depolarization)
• 2. AP are propagated without decrement
• a. Size and shape of AP are constant along nerve
  fiber

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Action Potential Generation

• All or None Response - Size and shape of AP are
  not influenced by the size of the stimulus
• Action Potential - Rapid but transient change in
  a membrane potential - Change in local membrane
  polarity -
• Polarized___Depolarized___Polarized

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Action Potential GenerationThe Action Potential
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Action Potential GenerationThe Action Potential

• Characteristics of action potentials
• 1. Requires specific voltage- gated ion channels
• 2, AP are the result of rapid changes in ion
  conductance
• 3. AP occur only on regions of cell membranes
  that are electrically excitable
• 4. AP generally are a standard size and shape
  for a specific cell type
• 5. All or none - when membrane reaches threshold
  an AP is generated (Not-Graded)

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Action Potential GenerationThe Action Potential

• 6. Time - AP not only have a specific size and
  shape but also exists within a specific time
  frame , ave. 1 to 5 msec.- (ie time duration of
  the action potential is always the same for a
  specific tissue)
• Specific to transport protein cycle times

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Action Potential Generation

• Importance of Action Potentials
• Nerve traffic, muscle contraction, hormone
  release, G.I. secretions, Cognitive thought, etc.
• Action Potentials are required for the senses -
  Sight, hearing, and touch are all dependent on
  action potentials for transmission of information
  to the brain
• Threshold stimuli (Graded Potential) cause
  the.generation of an action potential

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Action Potential Generation

• Three Stages of the Action Potential
• 1. Resting stage - Polarized stage - This is the
  normal resting membrane potential and varies with
  the cell type nerve -90 mV, heart pacemaker
  -60 mV, and skeletal muscle -83 mV
• 2. Depolarization stage - Sodium ions (Na) flow
  into the cell as the threshold for voltage gated
  Na channels are exceeded.

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Action Potential Generation

• 3. Repolarization stage - Potassium (K) ions
  flow out of the cell as voltage gated K channels
  are opened and the cell membrane potential moves
  back toward the resting membrane potential.

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Action Potential GenerationThe Action Potential

• Three Stages
• 1. Resting Stage
• (Polarized State)
• 2. Depolarization
• 3. Repolarization

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Action Potential Generation

• Components of an Action Potential
• 1. Threshold - Membrane potential at which
  voltage gated channels will open
• 2. Rising phase - as Na channels open membrane
  potential begins to shift toward the equilibrium
  potential for Na (Nernst Potential for Na)
• 3. Overshoot - The point at which the membrane
  potential becomes positive. The greater the
  overshoot potential the further the membrane will
  stay above threshold

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Action Potential Generation

• 4. Peak - At the peak of the action potential
  the sodium conductance begins to fall (Closure of
  the slow gate)
• 5. Repolarization - Inactivation of sodium
  channels and opening of the K channels (Opening
  of the K voltage channel slow gate) causes
  repolarization
• 6. Threshold - As the membrane potential passes
  back through threshold the voltage gated channels
  reset (both the Na and K channels)

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Action Potential Generation

• 7. After - hyperpolarization - The Na voltage
  gated channels have a fast gate and a slow gate
  passage of the membrane potential back through
  threshold causes the fast gate to close too
  rapidly for any Na ions to pass while the slow
  gate opens. The K voltage gate with its single
  slow gate begins to close slowly so for a period
  of time K still flows out of the cell
  hyperpolarizing the cell. Return to resting
  membrane voltage is due to NaKATPase

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Action Potential Generation
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The Action Potential Types
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Action Potential GenerationProperties

• Properties of Action Potentials -
• Refractory periods are times when it is either
  impossible or more difficult than normal to
  generate a second action potential.

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Action Potential GenerationProperties

• Absolute Refractory
• During this period the voltage gated channels
  responsible for the action potential have not
  reset and therefore, do not respond to
  stimulation.

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Action Potential GenerationProperties

• Relative Refractory
• This period corresponds to the positive after
  potential period and due to the hyperpolarization
  of the cell it is more difficult to generate a
  second action potential.

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Action Potential GenerationProperties

• Voltage Inactivation - If a cell membrane is
  maintained at a voltage potential above threshold
  than the voltage gated channels are not reset
  and, hence, inactivated and no action potentials
  can be generated.
• Accommodation to Slow Depolarization - If a slow
  depolarization occurs the voltage gated channels
  do not respond and no action potential occurs.