Nerve Conduction Flashcards
What is the function of a neuron?
to receive stimuli and conduct electrical impulses
How does the structure of a neuron assist its function?
the membrane: cytoplasm ratio is very high
ion channels in the membrane conduct electrical impulses
lots of membrane = lots of signals conducted
what will the signal always be regardless of what type of signal a neuron is sending?
the signal will always be changes in electrical potential across the neuron plasma membrane
What are the 4 parts of a neuron structure?
cell body
dendrites
axon
terminal branches
Describe the structure and function of dendrites
Extensive network of ? that receive stimuli
Describe the structure of the myelin sheath
layers of the plasma membranes of Schwann cells (glial cells) wrapping around an axon
What are the kind of cells that make up the myelin sheath with their membranes?
Schwann cells
What are the exposed intervals between Schwann cells called?
Nodes of Ranvier
Describe the synapse
the gap between the synaptic terminals (terminal branches) of one neuron and the dendrites of the next neuron
How is an electrical signal passed between neurons?
neurotransmitters are released by the presynaptic cell and cross the synaptic gap/cleft to trigger an electrical signal in the postsynaptic cell
What is required for an electrical signal to be transmitted between neurons?
the chemical signal of a neurotransmitter
How are the membrane potentials in axons measured?
Using glass microelectrodes
Describe the technique of measuring axon membrane potential
- penetrate axon (axoplasm) with one electrode
- Put second electrode outside the axon in the extracellular fluid
- connect the electrodes to a voltmeter and measure difference in charges
What organism was the technique of measuring axon potential measured in? Why?
Squids - they have giant axons (~1.5 mm in diameter)
Why do squids have giant axons?
to quickly propel themselves through water
How large are the axons of squids?
Up to 1.5 mm
Over 1000x wider than human axons
What are the conditions of a resting cell?
Na+/K+ pump functioning
K+ and Na+ leak channels open
No net flux of ions
Most other channels closed
Resting potential of axon = -70 mV
What is the resting potential of an axon?
-70 mV
What are the 5 basic steps of action potential?
- stimulus must reach threshold
- depolarization
- repolarization
- hyperpolarization
- reestablishment
What is the threshold voltage required to initiate action potential?
-55 mV
What causes the depolarization of the membrane to reach the threshold?
A stimulus (ex. neurotransmitter) will reach the threshold of -55 mV when Na+ flows in
What happens if the membrane depolarization does not reach the threshold?
Action potential will not occur and the resting membrane potential is reestablished
At what voltage will action potential occur?
+40 mV
Where are most action potentials initiated? What else is located here?
Initiated at the axon hillock
Many sodium channels located here
What happens when the threshold is reached?
All the voltage gated Na+ channels open and Na+ rushes into the cell passively
Is the movement of Na+ into the cell passive or active?
Passive - moving inwards is movement along its electrochemical gradient
What is an example of a stimulus that can trigger action potential?
the neurotransmitter acetylcholine released by the presynaptic neuron will cross the synaptic cleft and bind the Ach-Na+ channels (ligand-gated channel), opening the channel and allowing Na+ to rush into the post synaptic neuron
this influx of Na+ can cause a depolarization of the membrane to the threshold (-55 mV) and trigger AP
What role does the inactivation region of the channel protein play in the action potential process?
the inactivation region will block the pore of the channel when it has been open for too long to stop the influx of Na+
at this point, the membrane potential is around +40 mV
At what voltage will the membrane potential be when inactivation of the sodium channels occurs?
+40 mV
What does reaching +40 mV membrane potential trigger?
the voltage gated K+ channels to open and K+ to rush out of the cell
What is the result of K+ rushing out of the cell from the membrane potential reaching +40 mV?
repolarization
the membrane potential will dramatically decrease as K+ ions leave the cell
What 3 things cause the repolarization of the membrane potential?
- inactivation of sodium channels
- Na+ pump moving Na+ out of cell
- +40 mV membrane potential causing voltage gated K+ channels to open and K+ leave cell
Describe the hyperpolarization of the membrane potential
the membrane potential will overshoot -70 mV and become even more negative
What happens when the membrane potential is hyperpolarized?
the membrane potential is lower than -70 mV and the voltage gated K+ channels close
How does the membrane potential reestablish resting potential?
with the gated channels closed, the Na+/K+ pump and leak channels return the cell to resting potential of -70 mV
What has to happen before another AP can occur?
All of the Na+ channels that were inactivated need to first convert back to closed
What is the period called where no action potential can occur?
Refractory period
What are two things that contribute to the refractory period?
- inactivation of sodium channels
2. threshold is still -55 mV - it is much harder to reach threshold when in hyperpolarization state
How long does it take for the entire action potential process to happen in a myelinated mammalian neuron? in a squid axon?
mammalian: > 1 millisecond
squid: ~5 milliseconds
How do nerve impulses move along an axon?
they are propagated
How are nerve impulses propagated?
An action potential at one site has effect on the adjacent side
Depolarization of one site triggers the region just ahead of the action potential to reach the threshold of -55 mV
T or F: A succession of action potentials passes down an axon and loses some intensity along the way
FALSE, it loses no intensity along the way
How many directions can action potentials move? why?
only forward because the Na+ channels behind are inactivated and the membrane is hyperpolarized
the REFRACTORY PERIOD, it can’t move backwards
Describe saltatory conduction
the action potential at one node of Ranvier triggers the depolarization at the next node of Ranvier and nerve impulses hop from node to node down axon
What kind of neurons will saltatory conduction occur on?
ones with the myelin sheath
What is the purpose of saltatory conduction?
Neurons with myelin sheaths have no significant access to extracellular fluid and there’s no channels
so saltatory conduction allows the signal to propagate more than 20x faster down the axon
is the [Ca2+] high or low in the cytoplasm?
normally very low
What happens at the synapse when the action potential depolarization arrives?
Ca2+ voltage gated channels open
Which direction does Ca2+ flow when the voltage gated channels open?
INTO the cell
What does the rush of Ca2+ into the cell trigger?
vesicles with acetylcholine move to the cell surface
what happens when the acetylcholine vesicles move to the cell surface?
Ach is released into the synapse and travels across the synaptic cleft
What happens when Ach reaches the post synaptic nerve cell?
Ach binds to ligand-gated Na+ channels to and the channel opens
What happens when the ligand-gated Na+ channel opens?
Na+ rushes into the cell which can cause the membrane potential to reach the threshold which will cause the action potential on the post synaptic nerve cell which can propagate
T or F: neurotransmitters stay bound to ligand gated channels on the post synaptic membrane forever. why/why not?
FALSE, they must dissociate otherwise there would be a very long signal
How are neurotransmitters removed from the ligand gated channels on the post synaptic membrane?
- they can be broken down by enzymes either facing or released into the synaptic cleft
- they can be returned to the presynaptic neurons by receptor-mediated endocytosis
