BIOB34H3 Chapter Notes -Voltage-Gated Potassium Channel, Threshold Voltage, Tandem Pore Domain Potassium Channel

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8 Apr 2012
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BIOB30 Chapter 8 (258-266)
Neurons: Cellular and Network Properties
1
Graded Potentials Reflect Stimulus Strength
Graded potentials depolaraizations or hyperpolarizations that occur in the
dendrites and cell body (or sometimes in the axon terminals)
o The changes in membrane potential have an amplitude directly
proportional to the strength of the triggering event
What causes graded potential
o In the CNS and the efferent division, chemical signals from neurons
open chemically gated ion channels
Allows ions to enter or leave the neuron
o In some sensory neurons, a mechanical stimuli (e.g. stretch) opens or
closes ion channels
Closing ion gates can cause depolarization too
o E.g. closing the K+ gate and retaining K+
Figure 8.7 show graded potential where:
o A stimulus opens a monovalent cation channel allowing sodium influx
o Positive charge sends a wave of depolarization through the cytoplasm
Local current the wave of depolarization that moves through
the cell
Convention: current is the net movement of positive
electric charge
o Initial amplitude (or depolarization strength) in a graded potential is
determined by how much charge enters the cell
Stronger initial means it will spread farther without dying
Why graded potentials lose strength as they move through the cytoplasm
1. Current Leak positive ions leak out as the depolarization wave
moves (membrane has open leak channels so it is a bad insulator)
2. Cytoplasmic resistance the cytoplasm provides resistance to the
flow of electricity
Trigger zone the integrating center of the neuron which contains a high
concentration of voltage-gated sodium channels in its membrane
o In efferent neurons and interneurons: is at the axon hillock
At the initial segment (the very first part of the axon)
o In sensory neurons, it is immediately adjacent to the receptor (where
the dendrite joins the axon)
If the trigger zone is depolarized to threshold voltage, the voltage-gated Na+
channels open and an action potential is initiated
Excitatory depolarizing graded potentials
Inhibitory hyperpolarizing graded potentials
Subthreshold graded potential one that is below threshold by the time it
reaches the trigger zone
Action Potentials Travel Long Distances
Action potentials or spikes do not diminish in strength through the neuron
o Amplitude is around 100mV
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BIOB30 Chapter 8 (258-266)
Neurons: Cellular and Network Properties
2
o All-or-none they occur as a maximal depolarization or don’t occur at
all
Excitability the ability of a neuron to respond rapidly to a stimulus and fire
an action potential
A suprathreshold (above threshold) stimulus causes an action potential that
requires only 2 types of gated ion channels
o Voltage gated sodium and potassium channels
o Also need a leak channel for resting membrane potential
Signaling in the CNS can be much more complex
o Some neurons fire without an external stimulus
Tonically active neurons that fire regular trains of action
potentials (beating pacemakers)
Bursting bursts of action potentials rhythmically alternating
with intervals of quiet (rhythmic pacemakers)
o The different firing patterns are created by ion channel variants that
differ in their activation and inactivation voltages, opening and closing
speeds, and sensitivity to neuromodulators
Na+ and K+ Move Across the Membrane During Action Potentials
Figure 8.9 shows voltage and ion permeability changes during an action
potential
o There are 3 phases: the rising phase, the falling phase, and the after-
hyperpolarization phase
Rising Phase of the Action Potential
Due to a sudden temporary increase in permeability to Na+
Steps:
o An action potential begins (graded potential reaches the trigger zone)
o Membrane is depolarized to threshold (-55mV)
o Voltage-gated sodium channels open
o Na+ moves down the electrochemical gradient (into the cell)
o Further depolarizes the membrane
o Overshoot the portion of the action potential above 0mV
o When membrane potential becomes positive, electrical gradient for
sodium disappears
Concentration gradient remains so influx continues
Membrane potential moves toward +60mV (ENa) as long as it is
permeable to Na+
o Before ENa , sodium channels in the axon close a permeability
decreases a lot
o The action potential peaks at +30mV
Falling Phase of the Action Potential
Due to an increase in K+ permeability
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