Lecture 9- Action potentials Flashcards
what are graded potentials?
signals often communicated from one neuron to the next. also called postsynaptic potentials
normally a stimulus
how are graded potentials caused?
by the flow of ions through a few ion channels
are graded potentials depolarizing or hyperolarizing?
both!
what is depolarization and hyperpolarizing?
depolarizing: (+)
hyperpolarizing: (-)
what does it mean that graded potentials are passive?
they do not regenerate, they will gradually dissipate as they travel through a cell
unique features of graded potential? (4)
- there are small before threshold changes in membrane potential
- graded potentials are proportional to the size of the stimulus (lg stimulus = lg potential)
- can be added together to form a larger potential
- can be long lasting (mins to hrs)
how is a graded potential like a ripple on a pond?
when you throw a rock there are ripples that come from that rock that fade away
with a graded potential it moves outward from the source and degrades as it moves farther away, eventually degrades into nothing
why does a graded potential take so long?
due to the rippling effect it is more time consuming for the graded potential to get from one synapse to the axon hillock
why does the graded potential signal degrade?
cytoplasm has electrical resistance so allows potential to degrade over time
cell membrane is leaky to ions
what is an action potential?
wave of depolarization that actively propagates across neuronal membrane
is an action potential passive?
NO! its regenerative!
what does it mean that an action potential is regenerative and has a refractory period?
when the potential is done it can ignite again
what are two things an action potential requires?
membrane to be depolarized (+) past a threshold
voltage gated ion channels
can an action potential be summed?
NO
where is the site of action potential generation in neurons?
axon hillock
what are unique things about action potentials?
all or none (no semi- potentials)
large amplitude (100mV)
does an action potential depolarize or hyperpolarize?
ALWAYS depolarizing
what is the ionic basis of the action potential? 9 steps of an action potential
- resting membrane potential
- cell is depolarized by graded potential
- membrane depolarized to threshold (voltage gated Na+ channels open quickly, Na enters cell. voltage gated K+ channels begin to open, but slowly)
- rapid Na+ entry depolarizes cell
- Na+ channels inactivate and slower K+ channels fully open
- K+ leaves cell, membrane potential repolarizing
- K+ channels remain open and additional K+ leaves cell, hyperpolarizing it (afterhyperpolarization)
- voltage gated K+ channels close, less K+ leakes out of the cell. Na+ channels begin to recover
- cell returns to resting ion permeability and resting membrane potential. Na+ channels mostly recovered
what is absolute refractory?
new action potential CANNOT happen
what is relative refractory?
you can get a new action potential now but you have to stimulate the cell even more than you did the first time
what are the 3 patterns of action potentials?
single action potential
tonic action potential
bursting action potential
what is tonic action potential?
fire one after the other after the other
what is bursting action potential?
fires a couple AP, rests for a bit, fires a couple AP
is there just one version of Na+ channels?
no no no, theres like 80… each pattern of action potentials has a different Na+ channel
does the concentration of Na+ and K+ change after a single action potential?
no
theres almost no change after an AP for Na+ and K+ conc. because its an electrical event, not based on concentration
is Na+/K+/ATPase responsible for repolarizing the membrane after an action potential?
no, hyperpolarization does that
does the refractory period rely on Na+/ K+/ ATPase to reset Na+ and K+
no
how does K+ influence action potentials?
an action potential is pretty dependent on K+, if theres lots of K+ concentration (hyperkalemia) then it will be easier to create an action potential
if theres not much K+ concentration (hypokalemia) then you will need a larger stimulus to create an action potential.
what is the direction of an action potential?
soma (cell body) to the terminals
where is the action potential?
axon hillock
what is local current flow?
when the stimulated part of the axon moves into adjacent sections of the axon to continue moving the action potential
when does local current flow occur?
when a section of axon depolarizes, positive charges move by local current flow, on the extracellular, positive charges flow toward the depolarized region
what is the process of an action potential through an axon?
- a graded potential above threshold reaches the axon hillock (trigger zone)
- voltage- gated Na+ channels open and Na+ enters the axon causing depolarization (+)
- positive charge flows into adjacent sections of the axon body by local current flow (now refractory)
- local current flow from the active region causes new sections of the membrane to depolarize, K+ leaves the axon behind the AP causing repolarization
what is myelin formed from?
concentric layers of glial membranes compacted together (excellent insulators)
how many layers are there in myelin sheath?
up to 200
are there ion channels in myelin sheath?
very few ion channels on myelin sheath and under myelin sheath (axonal membrane)
how does myelin increase electrical efficiency?
because theres very few leaks
what happens if there is a leak in myelin?
becomes leaky and local current flow becomes dissipated
if partially demyelinated then local current flow can get through but will be slow
what channels are found in high density at nodes of ranvier?
Na+ channels
K+ channels are found nearby
what is saltatory conduction?
the action jumps from node to node
what are the two ways to speed up velocity of action potentials along an axon?
myelination
increase axon diameter
how does myelination speed up velocity of action potentials along an axon?
increases velocity because insulated areas mean less leakage of Na+ and K+, also means less ATP used
myelination allows axons to be smaller (so you can fit more into a space)
how does increase in axon diameter speed up velocity of action potentials along an axon?
increases velocity because as the axon radius becomes larger, internal resistance decreases
what is a big difference between large diameter axons and myelination axons, in respect to speed?
large diameter do speed up AP but myelination allows EVEN FASTER conduction in LESS SPACE
how fast is the largest diameter myelinated axon?
432 km/h
what are two demyelinating diseases?
multiple sclerosis
guillain barre syndrome
what is multiple sclerosis? type, cause, what it is
autoimmune disease
unknown cause
demyelination of CNS axons
what are the symptoms of multiple sclerosis?
loss of balance
loss of speech
loss of vision
abnormal pupil reflexes
numbness
pain
how do you know if you have multiple sclerosis?
have the symptoms, after time they go away, then they come back again but worse and for longer and so on
what are multiple sclerosis treatments?
immunosuppressants
what is guillain barre syndrome? type, what it is
autoimmune
demyelination of sensory motor and autonomic axons (PNS)
slowing and loss of AP conduction
what is guillain barre syndrome also associated with?
chronic illness (lupus, HIV)
what did the 1976 flu vaccine do for guillain barre syndrome?
wasnt tested well and had 1 additional case per 100, 000
what are symptoms of guillain barre syndrome?
tingling
weakness
pain in hands and feet
can progress to inability to speak, paralysis, respiratory distress
what is the treatment of guillain barre syndrome?
plasmapheresis (remove antibodies from blood)
immunoglobulin G to inactivate circulating antibodies
what does fugu poison/ tetrodotoxin (TTX) come from?
pufferfish (and other animals
what does fugu do?
prevents entry of Na+ into cells
prevents action potentials in neurons and muscle
do you need to eat a lot of pufferfish to die from it? why?
no because of the high potency and high specificity
what is the first symptom of intoxication of tetrodotoxin?
slight numbness of the lips and tongue
what is the second symptom of intoxication of tetrodotoxin? what is it followed by?
increasing paraesthesia in the face and extremities, followed by sensations of lightness or floating
what is the second stage of the intoxication of tetrotoxin?
increasing paralysis and convulsions
increasing respiratory distress, speech is affected
cardiac arrhythmia
when does death occur with intoxication of tetrotoxin?
within 4 to 6 hours
known range of 20 minutes to 8 hours
