Lecture 4: Graded and Action Potentials Flashcards
nerve and muscle cells are _____ tissues that can change their resting potentials into _____ signals
EXCITABLE tissues that can change their resting potentials into ELECTRICAL signals
Neural communication is based on ______ changes in membrane permeability to _____
RAPID changes in membrane permeability to ION
why is ion permeability changes used for cell to cell signaling in the brain over a second messager system or hormones?
SPEED, ion permeability change is FASTER
Graded potentials depend on the _______ changes induced by the neurotransmitter in the postsynaptic neuron
PERMEABILITY CHANGES
What determines whether a neuron fires or not?
based on if the net input is inhibitory or excitatory
When does temporal summation occur
when a single synapse receives many EPSPs (excitatory postsynaptic potentials) in a short period of time
when does spatial summation occur
when a single synapse receives many EPSPs from MANY presynaptic cells
what are the two gates in a Voltage-gated Na+ channel?
ACTIVATION and INACTIVATION gate
How many gates does a Voltage-gated K+ channel have?
One and its either open or closed
All voltage-gated channel gates are triggered to respond at ________ ?
threshold
3 conformations of voltage-gated sodium channels
resting
- closed but can open (-70mV)
activated
- open (from threshold to peak potential -50 to +30 mV)
inactivated
- closed and not capable of opening (+30 to -70mV)
2 Conformations of the voltage-gated Potassium channels
Closed
- at resting potential
-delayed opening occurs at threshold
- remains closed from -70 to +30 mV
Open
- from peak potential till after the hyperpolarization phase (+30 to -80mV)
what is the Absolute Refractory Period
interval during which NO stimulus can elicit an action potential
- most Voltage gated Na+ channels are inactivated
what is the Relative refractory period?
interval when a very large graded potential stimulus is required to elicit an action potential
due to elevated gK coupled w/ residual inactivation of Voltage gated Na+ channels
gk = membrane conductance to potassium ions
What is the purpose of a refractory period
- ensures one-way propagation of the action potential
- limits the frequency of action potentials
- energy conservation and prevents seizures
action potentials will always be of the same _______ for a particular axon regardless of how large the stimulus is
will always be the same MAGNITUDE
if a stimulus exceeds the threshold, an action potential will be ______
INITIATED, all or none response
the strength of a stimulus is determined by the _____ of action potential firing
FREQUENCY of action potential firing
A weak stimulus would cause ____ action potentials to be fired compared to a strong stimulus
FEWER
what determines the speed of conduction
DIAMETER of the FIBER
- larger diameter = slower internal resistance = faster conduction
-rapid fibers (large diameter) ex: motorneurons
-slow fibers (small diameter) ex: internal organs gut, glands
DEGREE OF MYELINATION
-lipid insulator of nerve fibers increases conduction velocity
What are the 2 types of conduction
CONTINUOUS CONDUCTION
- conduction in unmyelinated fibers
- action potential spreads along every portion of the membrane
SALTATORY CONDUCTION
** 50x faster
- impulse jumps over sections of fiber that is covered w/ insulating myelin
increasing ECF K+ will cause the RMP to ??
RMP will decrease, causing the inside of the cell to become more + = depolarization
Repolarization is ?
The return to resting potential after depolarization
When treating critically ill patients with intravenous fluids, which two ions are most important to the neuronal membrane potential?
Na+ and K+
very small shifts that can lead to action potentials IF threshold is reached
graded potentials
what happens to the ion channels when threshold is reached
Na+ channels open and the membrane depolarizes (becomes more +)
what is happening during the Depolarization stage of an AP
Na+ channels are open, Na+ is coming into the cell
what is happening at peak AP
- k+ channels open and K+ begins to efflux out of the cell
- Na+ channels close
what happens during hyperpolarization phase of an AP
- K+ ions continue to efflux out of cell
- cell enters into refractory period during which no more APs can fire
What happens during an AP as the membrane returns to resting membrane potential
K+ channels close and RMP is restored
what are the two types of signal conduction down an axon?
- CONTIGUOUS
- conduction occurs in UNmyelinated fibers
- AP spreads along every portion of the membrane
- SALATORY (50X FASTER)
- rapid conduction along myelinated fibers
- impulse jumps over sections of myelinated fibers
in myelinated axons, exchange of ions across the membrane (and thus generation of an AP), can only occur where??
at the nodes of Ranvier where a high density of Voltage-gated Na+ channels are found
Action Potentials are a result of ?
sequential openings of voltage gated ion channels
Unlike a graded potential, APs are always of the same ______ regardless of the size of the stimulus
magnitude
what determines the strength of an action potential stimulus
- the FREQUENCY of action potential firing
—> a weak stimulus would cause fewer APs to be fired compared to a strong stimulus
what happens if myelin sheath gets degraded or damaged?
the rapid conduction in myelinated fibers would be compromised
ex: degenerative myelopathy is a progressive disease of the spinal cord that leads to paralysis
explain what happens when muscle paralysis occurs as a result of Hyperkalemia
clinical signs: episodes of painful spontaneous muscle contractions followed by paralysis
cause: elevated plasma K+ levels
how it works
- hyperkalemia causes a depolarization of skeletal muscles
- at first, spontaneous APs occur since the RMP is closer to threshold
- but as depolarization becomes greater, Voltage-gated Na+ channels inactivate
- Na+ channel inactivation causes cells to be unable to fire APs, resulting in paralysis
