Module 3: Lecture 4 Flashcards
what is the refractory period?
- concept in action potential
- refers to the fact that you are unable to generate a second action potential until the first one has completed
- there is two general phases
what are the two general phases of the refractory period?
- absolute refractory period: you cannot generate a second action potential
- relative refractory period: you can generate a second action potential because sodium channels are closed and capable to opening- they have been reset
why can a second action potential not happen until the first one is complete?
because the first action potential is already using all the voltage gated sodium ion channels so if you receive that input during the upward depolarization stage, there is nothing more you can do
- they are all fully open or just recently inactivated so they cannot open more or no longer allow sodium to enter the cell because the inactivation gate is blocking the pore and it will not open again
why is it known as the relative refractory period?
because you can generate an action potential but requires a strong input stimulus
- occurs during hyperpolarization
why does the stimulus need to be stronger in order to generate an action potential in the relative refractory period?
because relative reflection period generates an action potential in the hyperpolarization phase. the hyperpolarization phase is at about -80mV of membrane potential and a stimulus generally causes a change of -15mV which will only bring the mV to -65mV when the threshold potential is -55mV
what is “one-way propagation”?
prevents the signals from going backwards
what is critical to ensure one way propogation?
the structure and absolute refractory period of action potential
what directions does nerves communicate?
unilateral directions
- this is because of the absolute refractory period
when the action potential is completed, the resting membrane potential is restored through _________ _________.
leak channels
- they establish your resting membrane potential by allowing sodium and potassium ions to re equilibrate
how fast is the restoration of concentration gradients after an action potential?
rapid
- almost the same amount of time as the depolarization period of the action potential
how do we restore the original concentrations of Na+ and K+ after they are altered across the membrane from action potentials?
through Na+/K+ ATPase pump
what counterbalances the rate of passive leakage from the channels?
the sodium-potassium pump
how many sodium ions are pumped out of a cell for every 2 potassium ions pumped into the cell from the sodium potassium pump?
3 sodium ions
what type of transport does the sodium potassium pump use?
- primary active transport
- requires energy (ATP) for providing the energy required for the conformational change of the transporter (‘ATPase’ pump)
why is the transport from the sodium potassium pump NOT electrically neutral?
there is a net transfer of 1 positive charge OUT of the cell for each molecule of ATP hydrolyzed (makes the inside of the cell more negative)
- pump is therefore critical for restoring the normal chemical concentration
how do the sodium potassium pump work?
- transmembrane proteins starts by facing its active site which is intracellularly
- have 3 binding sits for sodium(high-affinity) and two binding sites for potassium ions(low affinity)
- phosphorylation of the protein pump is causes a change to face it extracellularly and decreases its affinity for sodium and increases its affinity for potassium then reverts back to its original form when phosphate it removed
what is the mechanism for the long distance cellular communication?
action potential
how should a signal be transmitted?
from one cell to another, along a specific pathway
what are the four zones of a nerve cell?
- input zone
- trigger zone
- conducting zone
- output zone
what is the input zone of our nerve cells?
where the cell receives all of its communication from other nerves
- includes dendrites (mostly) AND cell body
where are all of your other nerves going to contact with that specific nerve and communicate with it?
the input zone
what is the trigger zone of our nerve cells?
where action potentials are initiated because they have a high concentration of voltage gated sodium and potassium channels
- includes the axon hillock
where are the graded potentials occuring?
in the input zone
in the nerve cell, where does the action potential get initiated?
the axon hillock in the trigger zone
where does the action potential go in the nerve if it is generated?
it will travel down the conducting zone which is essential the cell or the nerves axon
what is the conducting zone in the nerve?
part that conducts action potential often over long distances
- includes the axon
where does the action potential terminate?
the output zone
what is the output zone of a cell?
part that releases a neurotransmitter that influences other cells
- includes the axon terminals
once generated at the axon hillock, will the action potential be conducted with further stimulation?
no. no more further input and it will be able to be at the same magnitude throughout
- NOT decremental unlike graded potentials
what are the two methods that action potentials travel down the axon?
- contiguous conduction
- saltatory conduction
what is contiguous conduction?
propagation of the action potential along every patch of membrane down the length of the axon
how does an action potential travel across an axon through contiguous conduction?
the voltage gated channels are letting tons of sodium into the cell into an area and causes it to disperse across the cell. once enough sodium has dispersed into a new part of the axon and has reached threshold potential, it causes the voltage sodium ion gated channels to open in this new area as well and cause action potential. this continues the whole axon down. the previous active area now returns to resting potential
once an action potential is initiated in one part of a nerve cell membrane, a ________________________ is initiated so that the action potential is propagated along the rest of the fibre __________.
- a self-perpetuating cycle is initiated
- automatically
does an action potential travel along the membrane?
no. instead it triggers an identical new action potential in the adjacent area of the membrane
what prevents action potential from being reinitiated in the previously inactive area since sodium travels in both directions in contiguous conduction?
the inactivation gate of sodium channels not being able to respond again due to the absolute refractory period so that there is only “one-way propagation” and sodium travels one way
what is preventing backwards propagation?
the absolute refractory period
what is saltatory conduction relevant to?
myelinated fibres
what is a myelinated axon?
it has an insulating sheath running down the entire length of the cell axon
how does saltatory conduction travel across an axon?
jumps from node of ranvier to node of ranvier
what is a node of ranvier?
a gap in the insulating sheath of an axon that allow the cell membrane to rather than have to excite the entire axon, it only initiated the action potential at very key areas at these regular intervals along this myelin sheath
is myelin sheath hydrophillic?
no, hydrophobic
- has a lot of phospholipids so it is a really strong insulator
what are the components of a myelin sheath?
- 80% phospholipids
- 20% proteins
what does myelin sheath help with?
increasing the conduction velocity of signals in a more efficient manner
in the peripheral nervous system, myelination occurs due to?
Schwann cells in which the individual cells wrap around the axon creating that insulating sheath, and it relieves these very regular intervals known as the nodes of ranvier
what is the length of the nodes of ranvier?
2 microns and occur about every 1mm
at what location is the axon exposed to the extracellular fluid?
nodes of ranvier
- this is where all the voltage gated channels are going to concentrate
