Electrical Properties of Cells Flashcards
Exam 1, FOM 1, Lecture 18
What are excitable cells?
cells that are capable of generating action potentials on stimulation
What are the two types of excitable cells?
neurons
muscle cells
What is an action potential?
short lasting event when the electrical membrane potential of a cell rapidly rises and falls
What is membrane potential?
difference in voltage between the interior and exterior of a cell
What is resting membrane potential?
when there is no net flow of the ion
What two components make up the electrochemical gradient?
voltage gradient
concentration gradient
How is the electrochemical equilibrium calculated?
NERNST equation
What is the NERNST equation?
E = RT/zF ln ([ion]o/[ion]i)
What is E in the NERNST equation?
equilibrium potential
What is R in the NERNST equation?
gas constant (2 cal mol-1 K-1)
What is T in the NERNST equation?
absolute temperature (K)
What is z in the NERNST equation?
valence (charge) of the ion
What is F in the NERNST equation?
Faraday’s constant (2.3 x 10^4 cal V-1 mol-1)
What is the equilibrium potential?
membrane potential at which there is no net flow of the ion
What is depolarization?
makes the membrane potential (cell interior) less negative
What is hyperpolarization?
makes the membrane potential (cell interior) more negative
What is inward current?
flow of positive charges into the cell
How does inward current impact the membrane potential?
depolarizes the membrane potential
What is outward current?
flow of positive charges out of the cell
How does outward current impact the membrane potential?
hyperpolarizes the membrane potential
What is an action potential?
property of excitable cells that consists of rapid
depolarization followed by repolarization of the membrane
What is the threshold?
membrane potential at which the action potential
is inevitable
What is the Resting Membrane Potential of a nerve?
~ -70mV
At rest, are the Na+ channels opened or closed?
closed
At rest, what is the level of K+ conductance?
high
What are the three phases of an action potential?
depolarization (upstroke)
repolarization
hyperpolarization (undershoot)
What are the steps in the upstroke/depolarization phase of an action potential?
- stimulation
- inward current depolarizes membrane current to threshold
- depolarization causes opening of Na+ channels
What are the steps in the repolarization phase of an action potential?
- depolarization closes Na+ channel (more slowly than activated)
- Na+ conductance returns toward zero
- depolarization slowly opens K+ channels and increases K+ conductance to higher levels than at rest
- combined effect of closing Na+ channels and opening K+ channels contribute to repolarization
- outward K+ movement
What are the steps in the hyperpolarization phase of an action potential?
- K+ channel conductance remains higher than at rest
- makes it approach the K+ equilibrium potential
The molecular events that initiate repolarization of the neuron can most accurately be described by which of the following:
- Opening of a voltage activated calcium channel
- Closing of the voltage activated Na+ channel and opening of a voltage activated K+ channel
- Prolong refractory period
- Inward movement of K+
Closing of the voltage activated Na+ channel and opening of a voltage activated K+ channel
What is an absolute refractory period?
period in which another action potential cannot be elicited no matter how large the stimulus
What is the relationship between the refractory period and the refractory period?
Coincides with the entire duration of the action potential
What is the explanation for the absolute refractory period?
Activation gates of Na+ channels close when the membrane potential is depolarized, and remain closed until repolarization has occurred and gates have reset.
What is a relative refractory period?
begins at the end of the absolute refractory period and continues until the membrane potential returns to resting levels
In what context can an action potential be elicited during the relative refractory period?
if a stronger stimulus is provided
What is the explanation for the relative refractory period?
K+ conductance is higher than at rest and the membrane potential is closer to the K+ equilibrium and further from the threshold
What is the accommodation refractory period?
when the cell membrane is held at a depolarized level so the threshold potential is passed without firing an action potential
What is a clinical example of the accommodation refractory period?
- Hyperkalemia (high serum K+ levels)
- causes depolarization of skeletal muscle membrane
- although membrane potential is closer to threshold, action potentials do not occur because inactivation gates of Na+ channels are closed by depolarization
- leads to muscle weakness
Which ion’s equilibrium potential is a nerve’s resting potential closest to?
potassium equilibrium potential
Which ion contributes most predominantly to the
upstroke? Does it move in or out of the cell?
sodium, in
What two events contribute to the repolarization phase?
Na+ channels close
K+ channels open
What is propagation of an action potential?
every next part of the membrane being sequentially depolarized
If you measure membrane potential at several different points along an axon after an AP has fired, how will it look at each point?
identical
What is a subthreshold stimulus?
stimulus that is too small in magnitude to produce an action potential in excitable cells
How are subthreshold stimuli carried down an axon?
electrotonically conducted
What is electrotonically conduction?
passive flow of a change in electric potential along a nerve or muscle membrane
If you measure membrane potential at several different points along an axon after an subthreshold stimulus has fired, how will it look at each point?
the amplitude of the stimulus decays with distance as it moves away from the stimulus site
What is conduction velocity?
the rate of the action potential conduction along a neuron limits the flow of information from the nervous system
How does increasing the diameter of an axon impact the internal resistance?
decreases
How does decreasing the diameter of an axon impact the internal resistance?
increases
What is the relationship between the diameter of an axon and the internal resistance?
inverse
What is Saltatory conduction?
action potential jumps from one node to the next
What is the relationship between conduction velocity and axon diameter?
direct
What is the relationship between conduction velocity and myelination?
exponential
What neurotransmitter is present at a neuromuscular junction?
acetylcholine
What are the steps for transmitting a signal at a neuromuscular junction?
- nerve impulse arrives at axon terminal of motor neuron
- triggers release of acetylcholine (ACh)
- ACh diffuses, binds to its receptors
- triggers muscle action potential
- Acetylcholinesterase destroys ACh so more action potentials don’t arise
- AP traveling along t tubule opens Ca2+ (calcium ion) channels in the SR
- calcium enters into sarcoplasm
- Ca2+ binds to troponin
- Contraction
- Ca2+ channels close and calcium transport pumps use ATP to restore low levels of calcium in sarcoplasm
- Troponin complex returns to original position to block myosin binding sites on actin
- Muscle Relaxes
