Chapter 2 module 2.2 Flashcards
Polarization
A difference of electrical charge between two locations
The neural inside the membrane has a slightly negative electrical potential with respect to the outside
Electrical gradient
A difference of electrical charge between the inside and the outside of the cell
Resting potential
The difference in voltage in a resting neuron
The resting potential is mainly the result of negatively charged proteins inside the cell
The membrane is selectively permeable
That means that some chemicals can pass through it more freely than others can
More large or electrically charged ions and molecules cannot cross the membrane at all
Chemicals that can cross are: water, Origen, carbon dioxide, and urea. They cross through Channels that are always open.
What are the major ions that cross through the selective channels?
Sodium, potassium, calcium, and chloride
When the membrane is at rest…
Sodium channels are closed.
Sodium-potassium pump
A protein complex that repeatedly ran sport three sodium ions out of the cell while drawing two potassium ions into it
This requires energy
Concentration gradient
The difference in distribution of ions across the membrane
Sodium is more concentrated outside than inside, so just by the laws of probability, sodium is more likely to enter the cell than to leave it
Potassium
Potassium is positively charged and the inside of the cell is negatively charged, so the electrical gradient tend to pull potassium in. However, potassium is more concentrated inside the cell the outside, so the CONCENTRATION GRADIENT tends to drive it out.
For potassium, the electrical gradient and concentration gradient are almost in balance
Chloride
Negatively charged
Not actively pump in and out
And it’s channels are not voltage dependent
Therefore chloride ions are not the key to action potential
Why resting potential?
The advantage is that the resting potential prepares the neuron to respond rapidly to a stimulus
-70mV
When an axon membrane is at rest…
The recordings show a steady negative potential inside the axon
Hyperpolarization
Means increase polarization
Depolarization
Reduction of the polarization in the neuron
Threshold of excitation
Any stimulation beyond the threshold of excitation produces a sudden, massive depolarization of the membrane
When the potential reaches the threshold, the membrane suddenly opens it’s sodium channels and permit a rapid, massive flow of ions across the membrane
Action potential or neural firing
A rapid depolarization and slight reversal of the usual polarization
+30 mV
Voltage-activated channels
Membrane channels whose permeability depends on the voltage difference across the membrane
At the resting potential, the channels are closed.
When the potential across the membrane reaches threshold…
the sodium channels open wide. Sodium ions rush into the neuron explosively until the electrical potential across the membrane passes beyond zero to a reversed polarity.
From -70mV To +30mV
What happens at the pick of the action potential?
The sodium gates quickly close and resist reopening for about the next millisecond
What brings the membrane to its original state of polarization?
Not the sodium-potassium pump which transports three sodium ions outside of the cell while drawing two potassium ions inside it, this would prove to slow. Instead, after the action potential is underway, the potassium channels open. Potassium ions flow out of the cell simply because they are much more concentrated inside than outside and they are no longer held inside by the negative charge.
As they flow out of the axon they carry with them a positive charge.
What is the all-or-non law?
The amplitude an intensity of an action potential are independent of the intensity of the stimulus that initiated it
What does an axon do in order to signal the difference between a weak stimulus and a strong stimulus?
The axon can’t send bigger or faster action potentials, all it can change is the timing
Researchers have long known that a greater frequency of action potentiall per second indicates ‘‘stronger’’ stimulus. In some cases, a different rhythm of response also carries information .. Page, 44
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Refractory period
There are two: absolute and relative
The membrane resists the production of further action potentials
Absolute refractory period
The membrane cannot produce another action potential regardless of the stimulation
Relative refractory period
A stronger that usual stimulus is necessary to initiate an action potential
Absolute, about 1ms
Relative, about 2-3
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Axon hillock
A swelling whrere the axon exits the soma
Propagation of the action potential
It describes the transmission of an action potential down the axon
Electrical conduction in a copper wire with free electrons travels at a rate approaching the speed of light, 300 million meters per second.
In an axon, transition relies on the flow of charged ions through a water medium. In thin axons, action potentials travel at a velocity of less than 1m/s. Thicker axons and those covered with an insulating shield of myelin conduct with greater velocities . Though greater diameters only give axons the capacity to transmit the action potential at a speed of 10 m/s
Myelin
It’s an insulating material composed of fats and proteins that help increase the speed up to about 100 m/s
Made of glial cells ( Greek, ‘‘glue’’ )
Saltatory conduction
The jumping of action potentials from node to node
A flow of ions
The diffuse within the axon pushing a chain of positive ions along the axon to then next node where they regenerate the action potential
Question: what is there in the myelin shield that facilitates the movement of ions across the axon
Local neurons
Neuron with short axons that share information only with adjacent neurons
They do not produce action potentials but graded potentials
