Changing membrane potentials Flashcards
capacitance
two conducting materials separated by an insulator
stores charges of opposite sign on two opposing surfaces
amount of charge stored is inversely proportoinal to the thickness of the capacitor
difference between capacitive current and ionic current
the initial injection of a current into a capacitor rearranges the distribution of charges within the capacitor- capacitive current
as the ions rearrange, it changes the distribution of ions on the other side of the membrane, changing the voltage- this is the ionic current
this takes time, and is responsible for the lag in the change in membrane potential
time constant
the time required to rise to approximately 64% of the total change in potential
usually b/t 1-10 milliseconds
why is the time constant important?
the rate at which the membrane changes in respone to an individual stimulus and its subsequent relaxation determines how closely spaced sequential stimuli must be befre there is a summation of their effects
cells w/ short time constants sum their potentials more easily
temporal summation
depolarization events add together and through their combined influence depolarize the cell enough to reach threshold for firing an action potential
cable properties
the electrical characteristics involved in the spread of current and voltage changes along an axon
Ri
resistance to current flow through the inside of the axon
related to axon size- big axons = less resistance
the length constant
the distance at which ~37% of the original change in membrane potential still occurs
speed of propagation
is proportional to 1/ (sqrt(membrane resistance * membrane capacitance * internal resistance to current flow))
how is axon diameter related to capcitance and resistance
proportional to capacitance
inversely proportional to membrance membrane resistance
inversely proportional to the square of internal resistance to current flow
action potential
rapid depolarization and then repolarization of the membrane to a membrane depolarization of sufficient magnitude, but not hyperpolarization
resting membrane potential
-70 to 80 mV
describe the events occuring during an action potential
Na channels open in response to depolarization. It is only open for about 1 ms before it is closed also by depolarization. the membrane must be repolarized before they open again. (DEPOLARIZATION PHASE)
delayed rectifier K channels also open in response to depolarization, but w/ a slight delay. these channels do not inactivate until repolarization of the membrane. (REPOLARIZATION PHASE)
afterhypolarization
d/t residual k channel activity, resulting from their lag in response to membrane potential
threshold
the membrane potential at which the inward current through the NA channels that are opening up is finally greater than the outward K current through other channels
(Ina > Ik + Ileak)
refractory period
a period in time in whihc another AP cannot be generated (absolute refractory period) or can only be generated w/ some difficulty (relative refractory period)
result of:
Na channel inactivation
High levels of K channel activation
describe what proportion of ions move during an AP
very low, overall concentrations change very little
how important is the Na/K ATPase to APs?
not at all. important to maintaining the resting potential
general features of voltage gated channels
4 homologous domains/channels
6 transmembrane regions/domain
- pore loops- selectivity filter (region that does not quite become a transmembrane region)-
2 voltage sensors- 4th domain
3 inactivatoin loops
how does the pore loop confer selectivity?
done by size of hyrated molecule and energy required to remove water from ion
properties of the S4 voltage gates
go through all-or-none transitions between discrete conformations
S4 region has positively charged AAs inside the membrane every 3rd electron
depolarizing the membrane repels the positive charges, moving it in a helical twist
ball and chain model
cluster of positively charged amino acids, as the membrane becomes depolarized, the “ball” will plug the channel
applies to K channels
Na channels have n analagous loop of positively charged amino acids that flip up and clog the membrane
t vs l type Ca channels
t- transient response to depolarization- involved w/ transmitter
l- longer response to depolarization
how does the speed of Ca and Na channels compare?
Na is faster to activate and deactivate
delayed rectifier K channels do not inactivate
ok
A current potassium channel
inactivate rapdily
will counteract a depolarization that may lead to an AP, prolong the interval b/t APs
HCN channels
cation selective- K and Na both pass
open in response to hyperpolarization, helping raise the membrane potential reach a new AP, shortening the interspike interval
responsive to cAMP
important in pacemaking cells
tetrodotoxin and saxitoxin
bind w/ high affinity to external site of Na channels and block the pore
TTX- puffer fish
STX- from algae that occasionally proliferate and cause a red tide- cooking does not inactivate
conus toxins
class of toxins specific to voltage gated Ca channels
TEA
a K channel selective blocker
procaine/lidocaine
block the flow of ions through the Na channels, preferentially binding inactivated Na channels
neuroma
elevated Na channel expression occurs in perihperal sensory nerves that have been cut- causes phantom pain
propogation of an AP depends on both active and passive membrane properties
ok
why is AP propogation unidirectional
retrograde channels are in the refractory period
axon hillock (AIS)
elaborate accumulatoin of proteins, lipids and cytoskeletal elements that serve as a barrier to diffusion, effetiely separating the axon component
site of AP start
range of measured axon speeds
1 m/s to 120 m/s
myelins effect on conduction velocity
increases velocity
decreases capcitance and increases resistance
causes an increase in the length constant
myelination allows us to save space by creating quick neurons w/ small diameters
ok
what cells are responsible for myelination?
CNS- oligodendrocytes- myelinate as many as axons as possible
PNS- schwann cells- only one at a time
schwann cells also “encase” PNS axons, not true myelination, SCs interact with many axons
where are the channels located in myelinated axons
in the nodes of ranvier
how many layers of glial cells make up myelin
10-150
difference between control of myelin expression in CNS and PNS
CNS- independent of axon presence
PNS- dependent on axon contact
neuregulins
transmembrane factors expressed in axons, and they interact w/ ErbB receptor tyrosine kinase on glial cells
help communication between the two
how are are the locations of the nodes of Ranvier determined
some initial axon specialization, followed by glial cell contact and further specializatoin
MAG
myelin associated glycoprotein- in both the PNS and CNS- expressed in the earliest stages of myelination and primarily in the first wrap of the glial cell
involved in initial axonal-glial recognition and myelination
Po
a major structural protein of PNS myelin, accounting for over 50% of protein in SC
mediates myelin compaction
PLP
proteolipid protein- Po equivalent in CNS
MBP
myelin basic protein
found in both PNS and CNS. mediates close apposition of inner membrane
saltatory conduction
AP is only regenerated at each node of Ranvier
saves energy b/c it leaves less work for Na/K pump
dysmyelinating disorder
some myelin present, defected
ex. pelizaes-merzbacher disease
demyelinating disease
myelin sheath is absent
guillain-barre syndrome
myelin is deficient in PNS. primarily affects motoneurons associated w/ inflammatory damage to myelin
charcot-marie-tooth
peripheral motor and sensory neuropathy. leg weakness, difficulty running, hand weakness.
decreased nerve conduction
associated w/ gap junctoin subunit (Cx32)
MS
autoimmune disease where T cells cause gaps or lesions in myelin of CNS. characterized by plaques of demylination. results in the slowing or blocking of APs
most people develop symptoms 20-50
cause unknown- environmental and genetic. some evidence that viral infection causes increased permeability in the BBB to t lymphocytes
MS treatment
immunosuppressives
oligodendrocyte transplantation
replacement myelin, but it results in thinner myelin than original
potential MS treatment
