nerve/synapse lectures 3-4 Flashcards
what are sodium and potassium gradients maintained by
sodium potassium pump
describe sodium potassium pump
uses energy produced by ATP hydrolysis to pump sodium out and potassium in (against concentration gradients)
what happens if sodium potassium pump doesnt work
resting membrane potential goes down to zero
describe how pumps create the gradients
concentration gradient created by ATP energy
- used to drive sodium potassium pump
concentration gradient creates potential energy gradient
where is sodium potassium pump found
neuron membrane
describe how the sodium potassium pump works - gen
uses ATP energy = binds and hydrolyzes ATP and uses energy
grabs sodium and pump out of cell against concentration gradient
grabs potassium from outside and pumps inside
maintains concentration gradients for sodium and potassium
what do axons do
propagate info from one region of the nervous system to another by electrical impulses = action potentials
where do action potentials start and go towards
start at initial segment of axon and propagate down length of axon to presynaptic terminals
describe action potential - informally
signal
electrical pulse = very rapid change in membrane potential of neuron
propagates like wave
self regenerating = will propagate through whole length
definition of action potential
transient depolarizing spike that moves down axon
describe membrane potential at the action potential peak
membrane potential approaches Ena
describe beginning of AP
membrane potential spikes for 1/1000 of sec to positive value +30mv then drops back to -70mv
how to simulate action potential
use stimulating electrode to change membrane potential by injecting current
will make initial segment more positive = depolarization
what is hyperpolarization
make more negative
inject negative current using simulator if reverse polarity (-70 to -80 mv)
when is action potential initiated
when membrane potential depolarizes to a threshold level
what is threshold determine by
properties of ion channels in the axon membrane (especially the voltage gated sodium channels)
what is threshold potential level
-50mv
what happens if membrane depolarizes from -70 to -65mv
will just go back to resting potential
nothing will happen
what happens if membrane depolarizes from -70 to -50mv
action potential
will not just go back to resting
action potential will start and no way to stop it
what is the rising (depolarizing) phase of action potential caused by
sodium ions flowing into cell through voltage gated sodium channels
name the 3 critical properties of sodium channels
1 - closed at resting membrane potential, open when membrane depolarizes
2 - selective for Na+
3 - open channel rapidly inactivates and stops flow of sodium ions
why does resting membrane have little permeability to sodium
since voltage gated sodium channels are closed
when voltage gated sodium channel is open
allows sodium to flow into cell through pore of voltage gated channels
where are voltage gated sodium channels concentrated
in axon
allows it to propagate
if sodium channel is inactivated can it open again
not directly
must be converted back to closed state before they can open again
the rising phase of the AP is…
a regenerative process
what does depolarization of the membrane to threshold potential activate
small amount of sodium channels
further depolarizes membrane and activates more sodium channels
what kind of mechanism is AP
positive feedback
describe positive feedback effect of ap
maximal activation of sodium channels
a large sodium influx
depolarization of membrane from resting level to near Ena
what does inactivation do
terminates sodium influx
causes membrane to relax back to its original resting level
potassium flows out and brings back to -70mv
describe what happens when 100% of sodium channels are open
very permeable to sodium
so membrane potential will go up near equilibrium potential of sodium
after 1 milisec sodium channels inactivate and drops back to potassium equilibrium potential
why at the peak of ap the Na+ permeability swaps for the resting permeable of K+
density of voltage gated sodium channels in axon membrane is much higher than density of the leak potassium channels
what contributes to falling phase of action potential
sodium channel inactivation
delayed activation of voltage gated potassium channels
describe voltage gated potassium channels
open when membrane is depolarized
much slower than sodium
channel opens after a delay (1 milisec after sodium channels open)
open when sodium channels inactivate
during falling part of ap
what happens when voltage gated potassium channels are open
potassium leaks out
helps bring membrane down to resting potential faster
shut when membrane repolarizes
what 2 factors help make action potentials short
sodium channels inactivate fast
potassium channels open and allow potassium to flow and repolarize membrane
why do we want action potentials to be short
since then you can send more
neuron can send many aps per second or in a short amount of time
what happens without voltage gated potassium
ap would still happen but just repolarizes slower
what happens to sodium and potassium gradients when many aps
they run down faster
neuronal activity must be maintained by pumps (sodium potassium pump)
what happens if sodium potassium pump didnt work
gradients will just run down over time since neurons will still fire aps
name something that can block sodium potassium pump
puffer fish make tetrodotoxin, an extremely potent inhibitor of sodium channels
describe role of sodium potassium pump in maintaining gradients when many aps
pump constantly and move sodium out and potassium in to maintain concentration gradients, so they are not run down by the aps
what is action potential propagation caused by
spread of electronic currents from site of ap - which excites adjacent regions of axon
due to properties of voltage gated sodium ion channels
describe ap propagation
membrane depolarizes and has more + charges, will attract - charges from part of membrane not depolarized (right next to it)
+ flows towards - and depolarizes next part of membrane and initiates ap
will the action potential ever go backwards, why?
NOOOOO
due to inactivation of sodium channels
they are refractory - will not open again
must convert back to being closed, only happens when membrane goes back to resting potential
segment of axon behind ap is unexcitable
once a domino falls, it can’t stand up again right away.
what does refractory mean
Unresponsive
what is absolute refractory period
few milisecs after ap
sodium channels are inactivated and membrane is completely unexcitable
what is relative refractory period
longer period
voltage gated potassium channels open
membrane potential overshoots its resting level - (membrane becomes closer to potassium equilibrium level)
axon is less excitable and unlikely to fire ap (HYPERPOLARIZATION)
what determines how fast axon fires aps
duration of absolute and especially relative refractory period
each ap is an…
all or none event
how do neurons send info
time and frequency and pattern of aps
each ap is a binary event
encode info by how fast aps come and by patterns and timing
what does intensity of stimulus correspond to (Transduction of pressure on skin surface into neuronal activity)
frequency of aps
what are the molecular targets for naturally occurring neurotoxins
sodium channels
all different kinds of ion channels in neurons and other excitable cells like muscle are also targets
describe puffer fish
tetrodotoxin = extremely potent inhibitor of sodium channels
binds with high specificity and affinity
10^-8 M is enough to block all sodium channels
describe phyllobates frogs (south american tree frogs)
secrete from skin batrachotoxin = powerful sodium channel activator
binds to sodium channels and irreversibly opens them, so neurons will fire aps like crazy, super dangerous
animals who come in contact have a bunch of seizures
what are sodium channels also modulated by
pyrethroid insecticides
scorpion and anemonae toxins
what important therapeutic drugs block sodium channels
local anesthetics
antiepileptic agents
describe local anesthetics
lidocaine
benzocaine
tetracaine
cocaine
block voltage gated sodium channels so aps will not get past the area (will not feel stimulus)
describe antiepileptics
phenytoin (dilantin)
carbamazepine (tegretol)
epileptic seizure = electrical storm in brain, drugs block voltage gated sodium channels just enough to suppress seizures
what is rapid propagations of aps important for
survival
rapid reflexive responses
describe how squids send fast moving signals from one end of body to other
giant axons
1000 times fatter than human axons
what is propagation rate of ap proportional to
axon diameter
what happens if put hand on stove
put hand on stove
5-10 sec later = pain
slow burn conveyed from hand to CNS by axons called C fibers (slow, propagate at 0.2m/s)
what are fastest and slowest aps
fastest = 100m/s
slowest = 0.2m/s
there will be evolutionary selection pressure that will want…
to select axons that can propagate aps as fast as possible
why is a fatter axon faster
imagine a hose, wider = more water than a thinner hose
propagation rate of AP is proportional to axon diameter
why are squid axons giant
evolution can select for axons to get fatter - common in invertebrates
giant squid axon runs down middle of squid - good since squids escape mechanism is to run away, signal must be sent fast
are fat axons good for humans
noo
100 billion axons and if each were big - our heads would be be huge
solution is to insulate axons instead (caused by evolutionary selection pressure for complex nervous systems like mammals and humans, higher vertebrates)
what are axons wrapped in (in vertebrate neurons)
insulator called myelin
what is myelin formed by
schwann cells in PNS
oligodendrocytes in CNS
cells sit next to neuron and send out these extensions and wrap around axon to create insulation
is myelin wrapped around whole neuron
no
not continuous
has gaps
what does myelin do
acts as electrical insulator
enables charge to travel farther and faster down axon
what is myelin interrupted by
periodic gaps called node of ranvier
what do node of ranvier contain
very high concentrations of voltage gated sodium channels
Enables signal to be regenerated at periodic intervals
do all axons have myelin
no - shorter ones do not
long ones tend to be myelinated, like for sensory and memory
what happens if no myelin on longer axons
depolarization will spread but since myelin regions have no/little sodium channels = depolarization will just die and ap will stop
describe saltatory conduction - generally
ap starts at initial segment
starts to fizzle out in areas where there is myelin
then hits node just in time and influx of sodium since many channels are open at nodes
faster than just having ap propagate down length of axon
what is multiple sclerosis caused by
loss of myelin
what is multiple sclerosis
autoimmune disorder
episodes in which immune system attacks and degrades myelin and leads to deficits associated with an episode of MS (sensory, movement and cognitive dysfunctions)
immune system then settles down and myelin grows back but then another episode
each time urs harder for myelin to come back
what is white matter
regions of brain and spinal cord that contain mostly myelinated axons
myelin makes things white
white matter = wires & internet cables that send information super fast
what is grey matter
cell bodies
dendrites
synapses
gray matter as a computer’s processor (where thinking happens)
what is synapse
place where one axon makes contact with another neuron
what is upstream and downstream synapse
upstream = presynaptic
downstream = postsynaptic
name the 3 types of synapses
axodendritic
axosomatic
axoaxonic
name and describe axodendritic synapses
spine synapse
shaft synapse
on dendrites (most synapses are on dendrites)
describe spine synapse
between presynaptic terminal and dendritic spine (sticks out on dendrite)
like neurons in cerebral cortex
mainly excitatory
describe shaft synapse
synapse on shaft of dendrite directly
mainly inhibitory
describe axosomatic synapse
Synapses on cell body
describe axoaxonic synapses
presynaptic terminal of neuron to presynaptic terminal of another neuron
how many synapses can one neuron make
make synapses with many other neuron due to branching axon
each neuron has one axon bur can branch and make many synapses
only one ap tho, sent equally in all directions of branches
