Changes in Membrane Potential Flashcards
electrically excitable
- nerve and muscle cells are specialized to use rapid changes in their electrical properties for signaling or mechanical work
- due to presence of gated ion channels
dendrites
- numerous
- receive input
- generate local potentials
- LGIC (can also be mechano-gated)
cell body
- integrates local potentials
- contains cell body and organelles
- LGIC
axon hillock
- site of AP generation
- VG Na+
- VG K+
axon
- send information one way via action potential
- one per neuron
- VG Na+
- VG K+
axon terminal
- release of neurotransmitter
- VG Na+
- VG K+
- VG Ca+2
ligand receptor
- neuron stimulated by chemical
- ion channel opens
- ionic current flow
- local change in membrane potential
depolarizing
toward zero
overshoot
polarity reversed (inside positive, outside negative)
repolarizing
towards resting membrane potential
resting membrane potential
-70 mV
hyperpolarizing
more negative than resting membrane potential
types of potentials
- local (graded) potential
- action potential
local (graded) potential
small change in membrane potential confined to small region of membrane
- small distance signals
- produced by non-voltage gated channels
- primary at dendrites and cell body
local potential use:
- LGIC (nerve / muscle)
- mechano (sensory receptors)
local potential characteristics
- “graded”
- decremental
- depolarizing or hyperpolarizing
- can summate
“graded”
proportional to size of stimulus
- magnitude of potential can vary
- strong stimulus = more channels will open
decremental
decay with distance from stimulus because charge leaks through membrane
- flow of charge decreases as the distance from the site of the potential increases
characteristic: depolarizing
Na+ in
characteristic: hyperpolarizing
Cl- in, inside is more positive or K+ out, inside is more negative
summate
add together
- if threshold is reached (-55mV) neuron will generate action potential
excitatory
depolarizing
inhibitory
hyperpolarizng
action potential
large and rapid change in membrane potential that propagates over long distances
- use voltage gated channels
- only excitable membranes (nerve, muscle cells)
action potential characteristics
- all or none
- not graded by stimulus size
- not decremental (self-propagating)
- cannot summate (refectory period)
action potential step #1
steady state RMP P k > P Na due to leak channels
action potential step #2
threshold reached due to local potentials
action potential step #3
VG Na+ channels open rapidly depolarizing membrane –> Na+ in (positive feedback
- depolarization
action potential step #4
inactivation of Na+ channels and delayed opening of K+ channels stop depolarization
action potential step #5
open VG K+ channels depolarize to negative potential (K+ out)
- repolarization
action potential step #6
slow closing K+ channels hyperpolarize membrane closer to EKj Na+ channels return to closed state
- hyperpolarization
action potential step #7
Na+ / K+ ATPase establishes RMP
local anesthetics (novocaine, lidocaine)
- VG Na+ channel blockers
- pain receptors can’t send signals to brain
- NO action potential
tetrodotoxin (TTX)
VG Na+ channel blocker
saxitoxin
- produced by algae
- VG Na+ channel blocker
scorpion toxin mechanism of action
- prevents Na+ channel in activation
- blocks neural transmission
tetraethylammonium (TEA) mechanism of action
blocks VG K+ channels
absolute refractory period
- no amount of stimulation will produce another action potential
- because Na+ channels already open or inactivated
relative refractory period
- only a very strong stimulus will produce another action potential
- hyperpolarization because K+ channels open drives Vm more negative
refractory period
- contributes to the separation of action potentials so that individual electrical signals pass down axon
- as V, approaches RMP, threshold stimulus strength decreases
propagation
one produces the next
unmyelinated fibers
- local current from the opening of LGIC in dendrites and cell body causes an action potential to be insulated in region 1
- local current depolarizes in region 2
- will not propate backwards because of refractory period
- region 1 is refractory (Na+ channels inactive) –> repolarizing
- ex: digestive system
myelination
protein and lipid (80%) insulation
PNS myelination
Schwann cells
CNS myelination
oligodendrocytes
nodes of ranvier
not a continuous sheet of insulation in the central nervous system (CNS)
myelinated fibers
- VG Na+ channels concentrated at nodes
- current flows thru axon to next node
- action potentials ‘jump’ from node to node
- Na+ ions can’t flow in/out where there is myelin
- ex: pain receptors
conduction velocity increases with:
- axon diameter
- myelination
Cm
membrane capacitance, electrostatic forces acting through bilayer
Rm
membrane resistance
- how many leak channels are open
Ri
current path provided in axoplasm
length
how far current will flow
time
time to change Vm
- T is proportional to Cm
larger axons
- more cross sectional area
- more charge carriers
- leads to lower Ri
- local current decay less over a given distance
- larger local current
- increases conduction velocity
internal regions
where there is myelin
saltatory conduction
larger local currents are available to depolarize axon form node to node
myelin covering
no flow of ions
pathogenesis
- increased B and T cell sin CNS
- increased astrocyte and microglia activity
- inflammation
- demyelination and neurotoxicity in CNS
- current is lost through exposed membrane –> conduction block may occur
- myelin replaced with scar tissue due to increased astrocyte activity = sclerosis
conduction block in MS
disrupts ability of neurons to communicate
symptoms of MS
life expectancy: 5-10 years less than unaffected
