Principles of Neural Communication Flashcards
Which cell types have a membrane potential?
- all cells have a membrane potential!
- only nerve cells and muscle cells, however, have developed a way to utilize the potential
What is the resting potential of a cell? What is the threshold potential needed to initiate an action potential? What is the peak potential reached during an action potential?
- resting: -70 mV
- threshold: -55 to -50 mV
- peak: +30 to +40 mV
Leak Channels vs. Gated Channels
- leak: always open, permitting unregulated movement of a specific ion
- gated: opens in response to a particular stimulus (voltage, chemical, mechanical, thermal)
- (don’t forget cells also have Na+-K+-ATPase pumps that pump these ions against their gradients)
Graded Potentials vs. Action Potentials
- graded: small (10-20 mV) short-distance signals that can initiate an action potential; decrease in strength as they spread outward from the initial source; can be of varying magnitude; no refractory periods
- action: large (100 mV) long-distance signals; do not decrease in strength as they spread - they are continuously propagated along; all-or-nothing (always the same magnitude or nothing); refractory periods
Which ion is the greatest contributor to the resting potential? Why?
- K+
- the concentration of K+ is greater inside the cell than outside, while that of Na+ is greater outside; however, at resting potential, the cells is about 100x more permeable to K+ than Na+, so a net movement of K+ OUT of the cell (via K+ leak channels) occurs and generates the negative resting potential
What is the structure of the voltage gated Na+ channels? What are the three conformations of these channels?
- voltage gated Na+ channels have 2 gates: an activation gate in the middle of the channel and an inactivation gate at the cytoplasmic end
- 1) closed active (can be opened): activation gate closed, inactivation gate open
- 2) open: both gates open
- 3) closed inactive (can’t be opened for a while): activation gate open, inactivation gate closed
Explain what happens to ion channels during the resting, depolarization, and repolarization stages.
- (leak channels and ATPase pump are always open/working)
- resting: voltage gated Na+ channels are closed active, voltage gated K+ channels are closed
- initial depolarization: some voltage Na+ channels open, adding to the depolarization
- threshold: explosive inflow of Na+ due to large amounts of voltage Na+ channels opening; these are then rapidly closed (closed inactive)
- repolarization: voltage gated K+ channels open slowly and then close slowly (results in a slight hyperpolarized state)
- resting: voltage gated K+ channels are closed and voltage gated Na+ channels switch to closed active
Very soon after the threshold potential and action potential are generated, voltage gated Na+ channels are rapidly closed - what mechanism drives this closing?
- the voltage event that triggers these channels to open actually also induces them to close, but at a much slower rate
- the event triggers the rapid-opening of the activation gates and the slow-closing of the inactivation gates (when the peak potential is reached, the inactivation gates will finish closing)
What’s the role of the Na+-K+-ATPase pumps?
- these pumps gradually restore the concentration gradients of Na+ and K+ back to normal after an action potential occurs
- they therefore help generate the resting potential (along with leak channels)
- (note that the concentrations do not need to be fully restored to normal in order for another action potential to be generated because there is still way more K+ inside and Na+ outside)
Where are graded potentials and action potentials generated in the neuron? What determines whether a particular part of the cell will generate a graded vs. action potential?
- graded potentials are generated in the dendrites and cell body
- action potentials are generated in the axon
- areas that generate graded potentials simply have fewer voltage gated Na+ channels
Absolute and Relative Refractory Periods
- absolute refractory period: once an area generates an action potential, it can’t generate another one until resting potential is reached
- relative refractory period: a period following the absolute refractory period (so once resting potential is reached) that requires a stronger signal than the initial triggering event in order to generate another action potential
How do action potentials convey the strength of a stimulus?
- stimulus strength is coded by the FREQUENCY of action potentials; action potentials are all-or-nothing, meaning that they don’t come in varying amplitudes
- (graded potentials convey stimulus strength with amplitude magnitude)
Contiguous vs. Saltatory Conduction
- contiguous conduction occurs in unmyelinated axons
- saltatory conduction occurs in myelinated axons (about 50x faster); the lipid sheath acts as an insulator that prevents current/ion leak, “forcing” the current to jump along the nodes of Ranvier = faster conduction
Which parts of the neuron has the largest concentrations of voltage gated Na+ channels?
- the axon hillock (this is why the action potential is generated here)
- in a myelinated axon, the nodes of Ranvier also have very high concentrations of these channels (the actual myelinated parts have very few)
Why do axons with larger diameters have increased conduction speeds?
- larger diameter means less resistance!
