lecture 3 kds sept 21

8/8/2019 Lecture 3 KDS Sept 21

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Graded or Passive PotentialsGraded or Passive Potentials


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Graded/Passive PotentialsGraded/Passive PotentialsLength Constant, Time ConstantLength Constant, Time Constant

The length constant is the distance over which a spreading electrotonic

potential declines to 37% of its initial value. With too short a length constant, EPSPs would decline to negligiblevalues before they spread to the trigger zone; synapses located on the

dendrites would be ineffective. The time constant specifies the time it takes an electrotonic potential to riseto 63% (or fall to 37%) of its original value.

Too short a time constant lessens the chances for summation becausethe individual PSPs are very brief.


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Spatial and TemporalSpatial and Temporal

SummationSummation

What happens with IPSPs? See 12-1

Chapter 12 of your Text


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Trigger Zone: Cell Integration and Initiation of APTrigger Zone: Cell Integration and Initiation of AP


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Trigger Zone: Cell Integrationand Initiation of AP


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Trigger ZoneTrigger ZoneAxon of HillockAxon of Hillock

1. Site of initiation of active potentials (Action Potentials)2. Thought to have higher Na+ channel density, however

some reports are against this idea3. Require threshold which is higher Na+ current compared to

K+ current at the membrane.

4. Creates electrical cascade of all-or-nothing, where ActionPotential will note degrade until it reaches the terminal

5. Orthodromic conduction6. Antidromic conduction impossible due to inactivation of Na+

channels leading to absolute refractory period.


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ENa+

EK+

Vm

Threshold

The Action PotentialThe Action Potential


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Course of the Action Potential The action potential begins with a partial depolarization (e.g.

from firing of another neuron ) [A].

When the excitation threshold is reached there is a suddenlarge depolarization [B].

This is followed rapidly by repolarization [C] and a briefhyperpolarization [D].

There is a refractory period immediately after the actionpotential where no depolarization can occur [E]

Membranepotential(mV)

[A]

[B] [C]

[D] excitation threshold

Time (msec)-70

+40

0

0 1 2 3

[E]


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9-3 In your Text

Isolating Sodium and Potassium Channel CurrentsIsolating Sodium and Potassium Channel Currents


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The Na Action PotentialThe Na Action Potential-- Ion ChannelIon Channel

ContributionsContributions

Rising Phase Overshoot

Undershoot

Inactivation

9-10 from your Text


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Direction of PropagationDirection of Propagation

Orthodromic vs. Antidromic

Describe the direction of propagation of an electricalpotential.

Orthodromic

Propagation of signal is travelling in physiologicallyintended direction.

Motor Neuron vs Sensory Neuron

Antidromic Propagation of signal is travelling in the oppositephysiologically intended direction.


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Threshold of Na Action PotentialThreshold of Na Action Potential

The initiation of an action potential

requires a threshold to be met.

There is no specific value for all

thresholds. This is determined by anumber of factors:

(1)Membrane input resistance, (2)membrane

capacitance, (3)Nernst values, and (4)ionchannel types and (5)population inmembrane.


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9-11 B in your Text


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StrengthStrength--Duration CurveDuration Curve -- ChronaxieChronaxie andand RheobaseRheobase

There is a strong relationship between the

strength (voltage) and duration (time) of a

stimulus to charge a membrane.

Because of this, there are minimum andmaximum values for the use of them in

unison represented by this curve.

Anything to the right and above the curveshows that threshold was reached.


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9-8 From your Text

Refractory Period


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Refractory Period

1) For action potentials this is a Na+ channel phenomenon.

2) There are two types of refractory periods:a) Relative Refractory Period

Some Na+ channels are inactiveb) Absolute Refractory Period

ALL Na+ channels are inactive

3) Propagation direction and termination relies on Refractory Period


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lecture 3 kds sept 21

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