NESC 2570 Lecture Notes - Lecture 2: Osmosis, Ion, Resting Potential
27 Jun 2018
School
Department
Course
Professor
Wednesday September 16th 2015
How can the nervous system carry out its job
Translate and modify information so that the organism can respond to its
environment
○
We need a nervous system that can take in the information from the exterior
environment
Make information on the basis of that information
○
Execute appropriate actions
○
Evolved to carry out processes as quickly as possible
○
Need to do these jobs as rapidly as possible
○
○
How can the nervous system carry out electrical signalling?
We have many of the same components of our nervous system as in a
computer
Wiring
○
Signaling
○
○
○
Nervous system has key building blocks
Action potential: large electrical potential and voltage changes that can
propagate with minimal decay over long distances
As the signal runs down the axon, the electrical charge is relatively
unchanged
All or nothing
○
○
Can also go up the dendrites
○
○
Short distance signaling is carried out by receptor potentials also known as
generator potentials and/or synaptic potentials
The computing power of the nervous system relies on the integration
of these
○
Each cell becomes a device
○
Microelectrodes are used to measure these electrical potentials
Active current carrying end is inside the cell and the outside is
attached to a voltmeter
○
When the microelectrode is taken out of the cell, the voltage
will drastically increase again
○
○
The resting membrane potential (between -40mV and -80mV)
How can a cell support this voltage change?
○
○
As soon as an electrical impulse crosses the threshold, the all or none
response is triggered and an action potential is fired
○
Polarized = resting potential
○
Depolarization = increase in electrical voltage
○
Hyperpolarization = decrease in electrical voltage
○
○
○
Where does the voltage come from?
Voltage is due to the movement of ions across the membrane
○
Two requirements in order for a cell to generate a voltage
Ions that are maintained at a concentration gradient across the membrane
Either more concentrated inside or outside
○
○
Permeability
○
○
Through osmosis across the semi permeable membrane, the voltage will shift
across the membrane
If there is an equal amount of ions on either side of the membrane, voltage
will be zero
○
It only goes as far as it goes because positive charges are attracted to
negative charges
○
○
Build up concentration levels
○
The number of ions that are needed to alter a voltage, in the grand scheme of
things, are quite few
○
○
Monday, September 21st 2015
Amplitude of the membrane potentials vary
○
The larger voltage change (action potential) is not variable, it is a constant
feature of that cell
Whether you have it or not is determined by the stimulus
○
In order to record these, we manufacture electrode with small tips to be
injected into the cell to measure the membrane potential
○
You can use a stimulated electrode to impose certain amounts of current
into the cell and determine its membrane potential
○
If you break into your cell, the electrode will cause a hyperpolarization
(from 0mV to -60mV)
○
If you inject negative current - anions are flowing out of the stimulating
electrode
The inside will become more negative (hyperpolarizing injection)
○
○
If you reverse this, and you inject positive current and you push out cations
then the inside will become more positive
Depolarization
○
If you exceed some intensity of positive current injections, the
response will be explosively qualitatively different that will lead to
an action potential
All-or-none response with an set threshold
○
○
○
In order for these electrical responses to occur, there are two critical
requirement
Cells must be able to create and sustain an ion gradient across the
membrane
Ion transporters
Actively move ions against concentration gradient
○
Create ion concentration gradients
○
You need something to make sure that the concentration
gradients are different between the outside and the inside
of the cell
○
○
The cell also has to have ion channels that establish the
permeability to that pumped ion
You need both of these in order for a membrane
potential to occur
Allow ions to diffuse down concentration gradient
○
Cause selective permeability to certain ions
○
○
They pass through this permeability through osmosis
If there is more ion on the one side than on the
other, then such passage through the channel will
occur more frequently going from high
concentration to low concentration
○
○
○
○
○
○
What happens when we have a membrane that is permeable to multiple ions?
Goldman-Hodgkin-Katz equation:
If there is an anion present (ex. Cl) you reverse the division ratio
○
The membrane sustains a voltage that may vary in time if any of the
concentrations or permeability's alter
○
It is just these changes across the membrane that are responsible to
the electrochemical functions of the cell
○
○
○
Extracellular and Intracellular ion concentrations
Potassium is higher on the inside - therefore it will be more inclined to
move to the outside, leaving the inside more negative
○
In contrast, sodium is abundant in extracellular space and depleted in the
intracellular space
These gradients are detected in the same way
○
Three sodium's are pumped out every time two potassium's are
pumped out
This would not happen if the sodium potassium pump was not
there to regulate the concentrations
○
○
○
Chloride varies from cell to cell
Typically it is higher on the outside than on the inside (but not
always)
○
In many nerve cells, there is very weak chloride pumping so it
distributes itself passively
○
It reaches the same equilibrium potential that it should be forced
into - so it doesn't satisfy the above two requirements
○
○
Calcium, the levels are much smaller
Intracellular concentrations are extremely minute
○
Calcium is kept at very low concentrations because it is an
intercellular messenger so they are very tightly controlled
○
It is used for signalling
○
Although it isn't abundant in the outside, the small concentration on
the inside proves that it is a very strong concentration gradient
○
○
○
Can pass currents across the membranes of a squid's axon
This is because the axon's are so large (1000x larger than ours)
○
The squid axon behaves like a pure potassium system
At rest, the potassium concentration was severely dominant at resting
potential
○
They were also able to force the axons to display an action potential
by stimulating a depolarization
○
○
With sodium, they found that lowering the outside sodium concentration
had no effect on the resting potential but reduced the peak amplitude of the
action potential
○
These observations were very dramatic and unexpected
Nerve impulses were associated with a brief introduction of
negativity
○
All investigators had imagined that the key event in an action
potential was an alteration in membrane potential
○
The membrane potential crossed 0mV and went positive
At rest, the membrane potential must be overwhelmed with
potassium so it is heavily permeable to potassium
○
In contrast during the action potential, the permeability to
sodium must become much larger and dominate all the
permeability's
○
This shift in permeability must be very brief (1-2 miliseconds)
which increases the action potential drastically and then shuts
off so the membrane potential can return to rest
○
○
Hodgkins and Hucksly wanted to know how the permeability's to
ions altered over time
Membrane permeability's must be variable that are determined
by the membrane potential
○
They are voltage sensitive permeability's
○
○
○
○
○
Chapter 2
September 16, 2015
2:31 PM
Wednesday September 16th 2015
How can the nervous system carry out its job
Translate and modify information so that the organism can respond to its
environment
○
We need a nervous system that can take in the information from the exterior
environment
Make information on the basis of that information
○
Execute appropriate actions
○
Evolved to carry out processes as quickly as possible
○
Need to do these jobs as rapidly as possible
○
○
How can the nervous system carry out electrical signalling?
We have many of the same components of our nervous system as in a
computer
Wiring
○
Signaling
○
○
○
Nervous system has key building blocks
Action potential: large electrical potential and voltage changes that can
propagate with minimal decay over long distances
As the signal runs down the axon, the electrical charge is relatively
unchanged
All or nothing
○
○
Can also go up the dendrites
○
○
Short distance signaling is carried out by receptor potentials also known as
generator potentials and/or synaptic potentials
The computing power of the nervous system relies on the integration
of these
○
Each cell becomes a device
○
Microelectrodes are used to measure these electrical potentials
Active current carrying end is inside the cell and the outside is
attached to a voltmeter
○
When the microelectrode is taken out of the cell, the voltage
will drastically increase again
○
○
The resting membrane potential (between -40mV and -80mV)
How can a cell support this voltage change?
○
○
As soon as an electrical impulse crosses the threshold, the all or none
response is triggered and an action potential is fired
○
Polarized = resting potential
○
Depolarization = increase in electrical voltage
○
Hyperpolarization = decrease in electrical voltage
○
○
○
Where does the voltage come from?
Voltage is due to the movement of ions across the membrane
○
Two requirements in order for a cell to generate a voltage
Ions that are maintained at a concentration gradient across the membrane
Either more concentrated inside or outside
○
○
Permeability
○
○
Through osmosis across the semi permeable membrane, the voltage will shift
across the membrane
If there is an equal amount of ions on either side of the membrane, voltage
will be zero
○
It only goes as far as it goes because positive charges are attracted to
negative charges
○
○
Build up concentration levels
○
The number of ions that are needed to alter a voltage, in the grand scheme of
things, are quite few
○
○
Monday, September 21st 2015
Amplitude of the membrane potentials vary
○
The larger voltage change (action potential) is not variable, it is a constant
feature of that cell
Whether you have it or not is determined by the stimulus
○
In order to record these, we manufacture electrode with small tips to be
injected into the cell to measure the membrane potential
○
You can use a stimulated electrode to impose certain amounts of current
into the cell and determine its membrane potential
○
If you break into your cell, the electrode will cause a hyperpolarization
(from 0mV to -60mV)
○
If you inject negative current - anions are flowing out of the stimulating
electrode
The inside will become more negative (hyperpolarizing injection)
○
○
If you reverse this, and you inject positive current and you push out cations
then the inside will become more positive
Depolarization
○
If you exceed some intensity of positive current injections, the
response will be explosively qualitatively different that will lead to
an action potential
All-or-none response with an set threshold
○
○
○
In order for these electrical responses to occur, there are two critical
requirement
Cells must be able to create and sustain an ion gradient across the
membrane
Ion transporters
Actively move ions against concentration gradient
○
Create ion concentration gradients
○
You need something to make sure that the concentration
gradients are different between the outside and the inside
of the cell
○
○
The cell also has to have ion channels that establish the
permeability to that pumped ion
You need both of these in order for a membrane
potential to occur
Allow ions to diffuse down concentration gradient
○
Cause selective permeability to certain ions
○
○
They pass through this permeability through osmosis
If there is more ion on the one side than on the
other, then such passage through the channel will
occur more frequently going from high
concentration to low concentration
○
○
○
○
○
○
What happens when we have a membrane that is permeable to multiple ions?
Goldman-Hodgkin-Katz equation:
If there is an anion present (ex. Cl) you reverse the division ratio
○
The membrane sustains a voltage that may vary in time if any of the
concentrations or permeability's alter
○
It is just these changes across the membrane that are responsible to
the electrochemical functions of the cell
○
○
○
Extracellular and Intracellular ion concentrations
Potassium is higher on the inside - therefore it will be more inclined to
move to the outside, leaving the inside more negative
○
In contrast, sodium is abundant in extracellular space and depleted in the
intracellular space
These gradients are detected in the same way
○
Three sodium's are pumped out every time two potassium's are
pumped out
This would not happen if the sodium potassium pump was not
there to regulate the concentrations
○
○
○
Chloride varies from cell to cell
Typically it is higher on the outside than on the inside (but not
always)
○
In many nerve cells, there is very weak chloride pumping so it
distributes itself passively
○
It reaches the same equilibrium potential that it should be forced
into - so it doesn't satisfy the above two requirements
○
○
Calcium, the levels are much smaller
Intracellular concentrations are extremely minute
○
Calcium is kept at very low concentrations because it is an
intercellular messenger so they are very tightly controlled
○
It is used for signalling
○
Although it isn't abundant in the outside, the small concentration on
the inside proves that it is a very strong concentration gradient
○
○
○
Can pass currents across the membranes of a squid's axon
This is because the axon's are so large (1000x larger than ours)
○
The squid axon behaves like a pure potassium system
At rest, the potassium concentration was severely dominant at resting
potential
○
They were also able to force the axons to display an action potential
by stimulating a depolarization
○
○
With sodium, they found that lowering the outside sodium concentration
had no effect on the resting potential but reduced the peak amplitude of the
action potential
○
These observations were very dramatic and unexpected
Nerve impulses were associated with a brief introduction of
negativity
○
All investigators had imagined that the key event in an action
potential was an alteration in membrane potential
○
The membrane potential crossed 0mV and went positive
At rest, the membrane potential must be overwhelmed with
potassium so it is heavily permeable to potassium
○
In contrast during the action potential, the permeability to
sodium must become much larger and dominate all the
permeability's
○
This shift in permeability must be very brief (1-2 miliseconds)
which increases the action potential drastically and then shuts
off so the membrane potential can return to rest
○
○
Hodgkins and Hucksly wanted to know how the permeability's to
ions altered over time
Membrane permeability's must be variable that are determined
by the membrane potential
○
They are voltage sensitive permeability's
○
○
○
○
○
Chapter 2
September 16, 2015 2:31 PM
Document Summary
How can the nervous system carry out its job. Translate and modify information so that the organism can respond to its environment. We need a nervous system that can take in the information from the exterior environment. Make information on the basis of that information. Evolved to carry out processes as quickly as possible. Need to do these jobs as rapidly as possible. We have many of the same components of our nervous system as in a computer. Action potential: large electrical potential and voltage changes that can propagate with minimal decay over long distances. As the signal runs down the axon, the electrical charge is relatively unchanged. Short distance signaling is carried out by receptor potentials also known as generator potentials and/or synaptic potentials. The computing power of the nervous system relies on the integration of these. Microelectrodes are used to measure these electrical potentials.
