Excitable Cells Flashcards
What are three mechanisms for cell-to-cell (“intercellular”) communication?
- very rapid
- allow precise timing of signals between cells
- all complex patterns of communication between cells
How do electrical events provide a mechanism for sensing environmental changes (light, sound, temperature, etc.)?
stimuli from environment lead to electrical events in receptor cells
How do electrical events provide a mechanism for triggering intracellular events?
electrical events in cells leads to intracellular changes that are either rapid or can accumulate over time.
What are the three components of cell excitation?
- extracellular fluid (ECF)
- intracellular fluid
- membrane
Extracellular fluid is also known as ___ fluid. It is composed of primarily ___, ___, and other molecules. It is similar to ___ with respect to ionic composition.
interstitial; ions; proteins; plasma
What is the ionic composition of the extracellular fluid?
high [NaCl] (or high [Na+] and [Cl-])
Intracellular fluid is called ___. It is composed of ions, much higher concentrations of ___ and other molecules.
cytosol; proteins
What is the predominant salt of the cytosol?
KCl (or K+ and Cl-)
What contains a phospholipid bilayer and proteins?
membranes
T/F. There are equal amounts of protein and lipid and a small amount of carbohydrate in membranes.
True.
What is defined as an index of the ability of the ion to cross the membrane?
permeability
What type of permeability changes are essential to excitation in cells?
Changes in permeability need to be ion-specific and exquistely timed
What is defined as the random diffusion of organic molecules or ions down an electrical or concentration gradient?
simple diffusion
___-___ organic molecules diffuse rapidly through the membrane which is ___% lipid - solubility in lipid is ___. No ___ requirement.
Non-polar; 50; high; energy
___ cross membranes through channels which are proteins, often several ___ and generally ___-specific.
Ions; subunits; ion
List four examples of non-polar substances.
- O2
- CO2
- Fatty acids
- steroid hormones
The amount of substance crossing a surface per unit of time is known as what?
flux
T/F. Diffusion between two compartments is ALWAYS bidirectional.
True.
___ flux is the difference between the two unidirectional fluxes. When it is = 0, the system is referred to as being in a state of “___ ___”.
Net; diffusion equilibrium
What are the two types of mediated transport?
facilitated diffusion and active transport
In facilitated diffusion there is ___ energy requirement. A membrane ___ serves as a carrier to translocate a molecule across the membrane. All cells have ___ transporters.
no; protein; glucose
Active transport requires ___. The molecule or ion is bound to a transporter in the membrane and moves ___ its concentration gradient and/or electrical gradient. These transporters are referred to as ___.
energy; up; pumps
Energy can affect the ___ of the transporter for the ligand on one side of the membrane more than the other side AND the ___ of transporter conformational change.
affinity; rate
What is the energy source of primary active transport?
hydrolysis of ATP. the transporter becomes phosphorylated and changes the affinity for the solute (ligand) which leads to increases in transport rate.
What does secondary active transport model (coupled) use energy from? How many binding sites does its transporter have?
uses energy in ion concentration gradient across membrane
transporter has two binding sites
What are the three steps in mediated transport?
- ligand binds to transporter
- transporter undergoes conformational change
- ligand is released on other side of membrane
What three factors determine the rate of flux?
- number of transporters in membrane
- extent of transporter saturation (which is affected by i. transporter affinity and ii. ligand concentration)
- rate of transporter conformational change
What are the three type of channels?
- ligand-sensitive
- voltage-sensitive
- mechanosensitive
Define osmosis.
bulk flow of water across a membrane
water is ___ and therefore, diffuses through channels called ___. Water diffuses ___ its concentration gradient.
polar; aquaporins; down
What is the total solute concentration in a solution?
Osmolarity
If 1.0 osmol = 1.0 mole of solute, what does 1.0 mole of NaCl equal?
2.0 osmoles because NaCl dissociates and each contributes to osmolarity
What is the osmolarity of extracellular fluid?
300 mOsm
Isotonic = ___ mOsm of non-penetrating solutes with ___ change is cell volume.
300; no
Hypotonic =
swell because water travels in
Hypertonic =>300 mOsm of non-penetrating solutes causes the cell to do what? why?
shrink because water travels out
Why would a cell be termed iso-OSMOTIC or hypo-OSMOTIC or hyper-OSMOTIC?
nonpenetrating and penetrating solutes present
Explain how a solution can be hyperosmotic and isotonic?
When a cell is placed in this solution, it will initially shrink because water will leave the cell down is concentration gradient. That is the external solution is hyperosmotic.
The penetrating solute will enter the cell down its concentration gradient creating the osmolarity of the cell to be higher than the environment. Therefore, water will enter the cell and the original volume is restored.
If a cell has 150 mosmol Na, 150 mosmol Cl, and 100 mosmol urea, what is the total osmolarity of the solution? What is the tonicity?
total osmolarity is 400 mosmol. The tonicity is 300 because Na and Cl are considered non-penetrating but urea is penetrating.
The electrical charge difference between the inside and outside of a cell is a ___. Therefore, the plasma membrane must have a ___, R. When R changes, ions cross the membrane and this ion movement is a ___, I.
voltage; resistance; current
Define Ohm’s law.
V=IR
T/F. Conductance = current.
False, conductance is a membrane property. It is the reciprocal of resistance
g=1/R
Why would the movement of an ion [X+] stop before [X+]1=[X+]2?
charge difference develops between 1 and 2.
What is the Nernst equation?
Ex= (58/z) log10 ([X1]/[X2])
What does [X1] and [X2] equal?
[X1] = outside [X2] = inside
The Nernst equation yields the equilibrium potential of a ___ ionic species. The equilibrium potential is the voltage across a cell membrane that exactly balances the force in the concentration gradient of a ___ ion.
single; permeable
log10 0.1 = ?
log10 1 = ?
log 10 10 = ?
log10 100 = ?
-1
0
1
2
If the diffusion potential is transient, the membrane becomes ___ permeable to both Na and Cl, but Cl ions have a greater mobility. Therefore, one compartment will have an excess “+” charge, which is a ___ ___. Eventually, the Na and Cl concentrations will be the same in both compartments and the diffusion potential will disappear.
equally; diffusion potential
In steady diffusion, only NaCl in 1 and KCl in 2. The membrane is permeable to only K+ rendering 1 with a net “+” charge and 2 with a net “-“ charge. When will K+ stop moving from 2 to 1?
when the concentration force (favoring the movement of K+ from 2 to 1) is balanced by the electrical force (opposing movement of K+ from 2 to 1). At this point the diffusion potential stops changing - it becomes steady.
When the diffusion potential becomes steady, the system is now at equilibrium and the charge difference is referred to as an “___ ___”. The [K+] in 2 is ___ than [K+] in 1.
equilibrium potential; greater
There is a higher concentration of what inside the cell? outside?
K+ inside and Na+ outside
What is the membrane of the resting cell most permeable to?
K+
What is the permeability ratio (Pk/Pna) equal to?
- K+ ions diffuse across a typical cell membrane ~60 times more readily than do Na+ ions when a cell is at rest
In resting membrane potential, equilibrium is reached when the force in the ___ gradient for K+ ions to move out the cell is balanced by opposing force in ___ gradient which favors keeping K+ ions inside cell.
concentration; electrical
When equilibrium is reached in a resting cell, what is the membrane potential?
~ -70-90mV, which is close to the equilibrium potential for K+ (~ -100mV)
What does it tell you if the resting membrane potential (-70-90mV) does NOT equal the Ek (-100mV)?
Other ion(s) is (are) contributing to the resting potential.
T/F. The resting membrane potential is only permeable to K+ ions?
False, it is slightly permeable to Na+ ions also.
What keeps the resting membrane potential slightly lower than Ek?
Na+ diffuses into cell down both its concentration and electrical gradient. Therefore, K+ ions diffuse out and Na+ ions diffuse into the cell at rest.
T/F. The resting membrane potential is transient and not a steady diffusion potential.
False, it IS steady. Therefore, the resting potential is NOT due to instantaneous movement of ions. It is due to the fact that ions have moved across the membrane specifically K+ ions.
Define depolarization, repolarization and hyperpolarization.
toward 0mV (“hypopolarization”)
toward resting potential
increase in membrane potential (more negative)
T/F. The Na/K pump prevents the resting membrane potential from slowly dissipating to 0mV by counteracting the passive diffusion of ions.
True.
Where does an action potential occur?
at the initial segment of the axon because it contains a high [ ] of voltage channels
T/F. The resting membrane potential is steady but an action potential is transient.
True.
Why does the peak of AP stop approximately + 40mV?
Because the peak of AP (-40mV) approaches Ena (~58mV) AND
The end of the repolarization phase (ie the resting membrane potential) is close to Ek.
Which type of stimulus is insufficient to take a membrane to threshold or elicit an AP?
subthreshold stimuli (temporal and spatial summation)
What type of stimuli lead to APs?
threshold and suprathreshold
Define the absolute refractory period.
brief period when a second threshold stimulus or even a suprathreshold stimulus cannot elicit a second AP.
Under what condition can a suprathreshold but not a threshold stimulus elicit another AP if some time has passed?
relative refractory period.
what does TTX bind and block?
it binds to sodium channels and blocks sodium influx
If you apply a 10mV square-wave stimulus to a cell you elicit an AP but if you apply a 10mV ramp stimulus to a cell you dont. why?
too slow the brain wont respond
This is a property of nerve cells called ADAPTATION. and is due to a property of channels, referred to as ACCOMMODATION.
Cathode causes ___ to move and Anode attracts ___.
cations; anions
what helps AP propagate in one direction?
na inactivation
AP skip along the axon in a process called what?
saltatory conduction
Axons with larger diameters have ___ velocities of conduction.
higher
