Membrane Potentials
Introduction
1. Describe the neuron function
Neurons act to facilitate the transfer of data amongst the body’s cells
sustaining homeostasis.
2. Contrast graded potentials and action potentials.
Graded potentials are electrical signals that have variable amplitude, are localized, and travel short
distances along the membrane. In contrast, an action potential has a fixed amplitude, and can travel
greater distances along the membrane..
3. a. Describe how there can be an electrical change on either side of a membrane
thereby generating a membrane potential.
The membrane potential is due to the presence of an ionic concentration difference and
electrical gradient between both sides of the membrane.movement of charged
molecules across the cellular membrane leads to the creation of electrical signals.
b. What two characteristics must a membrane possess in order to generate and maintain electrical
signals critical to nervous tissue?
The two traits required for electrical signal generation and maintenance is the presence of a resting
membrane potentials and ionic pumps that maintain ionic concentration and electrical gradients between
the two sides of the membrane.
Membrane Transport Proteins
4. What three membrane proteins are required if a membrane is to carry electrical
impulses?
These proteins are pores, gated channels, and ion (i.e. sodium/potassium) pumps.
5. Describe the activity of sodium and potassium ion channels.
Pores are ion-specific and provide free access to their respective ions across the plasma membrane. Ions
move through these pathways via diffusion – down their concentration gradient.
6. How do the following differentially affect gated channels?
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Chemicals (ligands)
Voltage changes
Mechanical deformation
Gated channels, like the open-access pores, are ion-specific. These gated passages are dependent on
particular membrane-affecting agents to either open or close the gate. These may be distinct chemicals
referred to as ligands. Voltage changes to the potential across the membrane also can alter these gates.
An alteration in the physical structure of the gate that leads to its mechanical deformation can also act on
opening and closing of these gates.
7. Where is each of the following types of gated channels located AND what type
of function do they serve?
Ligand gated – these are found at the cell bodies and dendrites. Hormones and
neurotransmitters modify these channels for normal cell maintenance.
Voltage gated – these are seen on the axon of neurons and are utilized for the
generation and propagation of action potentials.
Mechanically gated – these are discovered on the dendrites of sensory neurons
and are also seen in sensory receptor cells. Their opening and closing are dependent
upon the distension of the cell membrane from vibrations or pressure. They are
actively involved in providing the individual with sensory input of the surrounding
environment.
8. a. Describe the function of the Sodium/Potassium pump.
Its primary purpose is to actively move 3 – Na+ ions extracellularly while transporting
2 – K+ into the cell.
b. How does the sodium/potassium pump action maintain the membrane
potential?
Through its active transport of Na+ ions out and K+ into the cell, these pumps
counteract the ionic flow of both these ions via the pores and gated channels
which degrade the concentration differences of the ions on both sides of the
membrane. In
doing so, the ionic concentration gradient is preserved and the resting
membrane potential is maintained.
Resting Membrane Potentials
9. Describe the positive and negative charges on either side of a membrane at
resting potential.
Membrane surfaces are charged due to the accumulation of positive and negative
ions on both sides of the membrane. At rest, the inner surface of the membrane is
relatively more negative than the outer surface of the membrane resulting in a
potential difference.
10. a. The opposite “poles” of a battery are positively and negatively charged.
Explain why polarized is a good term to describe a membrane with a resting potential.
Polarized is an excellent term to describe the membrane because the overall charge on
the outer surface is more positive relative to the inner surface, which is more
negative.
b. Where, exactly, do the opposing charges exist? What is the charge of the
cytosol and the extracellular fluid?
These charges exist on the inner and outer surfaces of the membrane. In
comparison, the cytosol and extracellular fluids are electrically inert.
c. What is the typical resting potential for a membrane?
The usual value is a negative 70 millivolts.
11.Describe how the following contribute to the unequal distribution of ions and charges thereby
establishing a resting membrane potential.
K+ permeability – Due to higher availability of K+ pores in the membrane, these leads to K+ ‘s increased
permeability through the cell allowing more K+ to leave the cell versus the number of Na+ entering the cell.
This causes the interior of cell to be slightly more negative relative to the outside layer of the cell, thus
producing the resting membrane potential.
Na+/K+ Pump Activity – The activity of this pump is to counteract the natural flow of Na+ entering the cell
and the K+ exiting the cell by removing 3 - Na+ ions out of the cell for every 2 - K+ ions into the cell thereby
preserving the resting membrane potential.
Graded Potentials
12. Define a graded potential.
This is a change in the resting membrane potential value. This event happens when a
gated channel shuts or opens and a corresponding change occurs in the ion flow
across the membrane.
13. How does opening a gated ion channel affect resting membrane potential?
When opening a gated channel, ions flow through the membrane, which in turn,
causes a change in voltage across the membrane. This ultimately affects the
membrane potential.
14. Correlate the name “graded potential” to the affect
of stimulus intensity.
The intensity of a stimulus governs the number of gated channels
that open or close. Therefore, an increase in the strength of the
impulse
will result in changes in the flow of ions across the membrane which
alters the membrane potential. Thus, the intensity of a stimulus will
dictate the degree of change in the membrane potential and the amplitude of a graded potential.
15. Describe two types of graded potential change.
hyperpolarization
This is an increase in polarity due to a downturn of the number of potassium ions
inside the cell. This decrease causes the inside to be more negative relative to the
outside.
depolarization
This is a decrease in polarity that is generated by the addition of sodium inside the
cell. This extra number of sodium ions intracellularly makes the inside less negative relative to the outside
layer.
16. What is the role of graded potentials?
Graded potentials are important because they either bring the membrane
potential closer to threshold where an action potential can be generated, or
further away from threshold thus making the generation of an action potential
more difficult..
Action Potentials
17. What is the function of a neuron’s trigger zone?
The neuron’s trigger zone produces the action potential.
18. a. Why are Na+ and K+ gated channels essential for an action potential?
The Na+ gated channels must be opened for the membrane potential to depolarize to threshold and
generate an action potential. The K+ gated channels is required to be open to return the membrane
potential back to the resting membrane potential.
b. Describe the electrical change during an action potential.
During an action potential, the membrane potential rapidly depolarizes and reverses such that the inner
membrane surface is more positive relative to the outer membrane surface. The membrane potential then
repolarizes back to resting membrane potential.
19. a. What is the condition of the Na+ gated channels at resting potential?
These gates are closed.
b. Explain the importance of stimulus strength. The stimulus strength (i.e.
graded potential) is critical because it will govern if an action potential will
occur or not.
c. What effect does hyperpolarization or depolarization less than -55 mV have on Na+ gated
channels?
The voltage regulated Na+ channels do not open and an action potential is not generated.
d. Explain why an action potential is sometimes referred to as an “all or
nothing” response.
Once depolarization reaches threshold, an action potential is generated. If
depolarization does not reach threshold, no action potentials are generated.
Action potentials either occur or they don’t.
e. Reversal of the membrane charge closes the Na+ gates and opens the
K+ gates. How does this affect the depolarization? Include the role of the Na+/ K+ pump in your
response.
When the sodium gates shut and the potassium gates open, the membrane begins to return to its
resting potential. This process of repolarization is finally completed when the sodium/potassium pump
actively moves 3 - Na+ ions out of the cell for every 2 - K+ ions into the cell.
20. Describe the refractory period.
There are two types of refractory periods, absolute and relative. During the
absolute refractory period, the membrane is responding to a stimulus and is
unresponsive to any additional stimuli. During the relative refractory period,
the membrane is hyperpolarized and a greater than normal stimulus is
required to generate an action potential.