Cell membranes and synaptic activity Flashcards
(35 cards)
What autoimmune disease is associated with demyelination of central axons (CNS)?
[multiple sclerosis]
What does it mean, ‘membrane potential’?
[refers to a separation of charge across the membrane; due to differences in permeability between ions. This separation yields a ‘potential’ measured in volts that represents potential energy in letting this charge separation disappear]
Do all cells have a membrane potential?
[Yes….it’s not just neurons… but neurons are best at exploiting membrane potential for signaling purposes.. also, only neurons and muscle cells produce action potentials - large, local, quick shifts in membrane potential]
Is it true that the ‘resting potential’ is only due to K+ current?
[true for glial cells, not for neurons. K+ conductance is high at rest, but Cl- conductance is about 1/3 of that, Na+ conductance is about 1/25 of K+. Because the Nernst potential for Cl- is nearly the same as the resting potential, it’s influence is minimal. However, because the Nernst potential for Na+ is so far away from the resting potential, it is a factor for the resting potential ]
Which ion is most responsible for the ‘resting potential’?
[K+]
Which ion’s permeability causes the massive depolarization observed in the action potential?
[Na+]
Which ion is necessary for release of neurotransmitter at the synapse?
[Ca++]
What is the duration of an action potential?
[1 msec]
How much does the membrane potential shift during an action potential?
[100 mV]
What constrains the voltage limits on an action potential?
[Maximum depolarization is constrained by the Nernst potential for Na+ (~50 mV), maximum hyperpolarization is constrained by the Nernst potential for K+ (~-90 mV)
What is a good approximation of the ‘resting’ potential?
[-70 mV]
What does the Nernst potential represent?
[For a single ion, it describes the membrane potential at which the inward and outward current would be zero; equal # of that ion flow in and out across the membrane]
If neurons are so permeable to K+, why isn’t the Nernst potential for K+ also the ‘resting potential’?
[the membrane is also permeable to Na+ and Cl- both of which depolarize the resting potential from Keq (especially Na+)]
If the membrane at rest, is so permeable to Cl-, how come no one cares about Cl-?
[we’re not bigots. The fact is the Nernst potential for Cl- is close to the observed membrane potential…so, moving a few Cl- ions across the membrane will not change much in that regard. However, in synaptic transmission, increasing Cl- conductance does have impact because it negates the influence other ions normally have when they cross the membrane. This effect is known as shunting… it effectively short-circuits the influence of other ions]
At the ‘resting’ potential, does opening a K+ channel generate an outward or inward current?
[outward, K+ ions will leave, hyperpolarizing the cell…trying to bring it to the Nernst potential for K+)]
Name the ‘bag’ that holds neurotransmitter for release?
[vesicle]
What is the refractory period?
[minimal time between action potentials, constrained by availability of voltage-gated Na+ channels]
What is the time constant?
[the time a membrane drop to 1/e (37%) of its charged up membrane potential. It is equal to the membrane resistance X membrane capacitance. The “membrane time constant” of a neuron is simply a way of measuring how quickly a neuron’s voltage level decays to its “resting state” after it receives an input signal.]
What constrains axonal conduction velocity?
[the time it takes to charge up and discharge the membrane; in general, want to minimize Rm, Cm and Ri…. The squid’s giant axon is non-myelinated, and speeds up conduction by minimizing Ri. Myelination speeds conduction because although the fat increases Rm, this is more than offset by the drop in Cm]
What is a good guess for the absolute refractory period for a myelinated axon?
[ 0.5 msec]
What is the relative refractory period?
[a 2nd action potential can occur, but requires more stimulation and its amplitude is decreased because it doesn’t have the full complement of voltage-gated Na+ channels available and some VG K+ channels have opened which also decreases size; in myelinated axons, relative refractory period is ~4-5 msec]
What does the Na+/K+ ‘pump’ do?
[maintains the membrane potential over time, but does require energy, ATP]
Name 3 properties of the Na+/K+ pump?
[it requires ATP to operate, it hyperpolarizes the membrane potential (~ 4-6 mV), and this hyperpolarization is due to a net outward current: 3 Na+ ions out for every 2 K+ ions in]
Identify the two major types of neurotransmitter receptors?
[ionotropic, metabotropic]
