Action Potential and Synapse

Question 1

what are the steps of an action potential?

Answer 1

1. when threshold potential is met voltage-gated Na+ channels open and Na+ enters the cell
2. as the cell depolarizes voltage-gated K+ channels open and K+ leaves the cell, slowing the depolarization
3. at peak voltage-gated Na+ channels close and the cell starts to repolarize
4. at bottom voltage-gated Na+ channels return to their normal state (closed but can open) and voltage-gated K+ channels close
5. once cell is hyperpolarized voltage-gated Na+ and K+ channels return to their resting states

Question 2

absolute vs relative refractory period

Answer 2

absolute: cell physically cannot depolarize again (Na+ channels inactivated, K+ channels are open until MP reaches resting)
relative: Na+ is able to open but the cell is so negative that a very strong signal is needed to start another action potential (channels are at resting states but MP is below -70)

Question 3

what does it mean that action potentials are unidirectional?

Answer 3

ions can flow in both directions but action potential can only move towards the axon terminal

Question 4

what are the two theoretical directions of the action potential?

Answer 4

orthodromic: action potential moves down towards axon terminal
antidromic (backpropagating): action potential moves up the axon , from the hillock (MPt: -65 mV) to the soma (MPt: -35 mV)
EPSPs/IPSPs form axoaxonic synapses and use backwards AP to remove the Mg+ so that glutamate can move through NMDAR

Question 5

what are the factors determining action potential conduction velocity in the axon?

Answer 5

membrane capacitance, membrane conductance, and intracellular resistance

Question 6

how does membrane capacitance determine action potential velocity in the axon?

Answer 6

ability to retain charge/separate ions, myelination decreases leak channels and increases capacitance, the more ions traveling the better

Question 7

how does membrane conductance determine action potential velocity in the axon?

Answer 7

efficiency of conducting current based on how long channels stay open, how fast they open, etc. voltage-gated channels and increased MP increase conductance

Question 8

how does intracellular resistance determine action potential velocity in the axon?

Answer 8

factors that reduce internal current by blocking flow of ions and properties of the axoplasm, axon length and diameter, blocking flow of ions stops the next channel on the axon from raising the MP and moving the AP

Question 9

what is the all-or-none law?

Answer 9

that idea that the amplitude of the AP is independent of the size of the eliciting stimulus and is constant for a particular neuron type

Question 10

what factors code stimulus intensity of an action potential?

Answer 10

AP frequency (rate law) of a neuron and the number of neurons firing in the brain region determine intensity, volley principle: firing AP out of sequence between each other encodes a more intense stimulus

Question 11

what is continuous conduction on an unmyelinated axon?

Answer 11

Na+ ions diffuse and depolarize adjacent axon membrane, thus triggering an action potential in an adjacent area of the axon (0.5-2 meter/sec)

Question 12

what is saltatory conduction on a myelinated axon?

Answer 12

triggered in the nonmyelinated initial segment of the axon, AP occur only at the nodes of ranvier, elsewhere neural impulses diffuse using the cable properties of myelin, requires less energy (120 meters/sec)

Question 13

what does the node on myelinated axons do?

Answer 13

contains voltage-gated Na+ channels which allows Na+ into the cell, Kv7 K+ channels are activated at subthreshold, are very sensitive and fast-opening/non-inactivating which allows the action potential to progress down the axon

Question 14

what does the paranode on myelinated axons do?

Answer 14

glial loops are linked with gap junctions and are anchored to the edge of the myelin sheet with cell adhesion molecules

Question 15

what does the juxtaparanode on myelinated axons do?

Answer 15

made up of anchored voltage-gated inward rectifier K+ channels, regulates K+ flow into the cell at membrane potentials above -70 which helps restore membrane potential after action potentials

Question 16

what does the internode on myelinated axons do?

Answer 16

contains active transporters and leak channels to help restore the membrane potential after action potentials

Question 17

what are the different types of synpases?

Answer 17

axodendritic: the axon synapses with dendrites of another neuron
axosomatic: the axon synapses with the soma of another neuron
dendrodendritic: dendrites of two neurons form a synapse
axoaxonic: the axons of two neurons form a synapse
neuromuscular junction: the axon synapses with a muscle

Question 18

what are some features of electrical synapses?

Answer 18

contain gap junctions made of connexons, voltage-gated with a high threshold or signal gated (H+, Ca2+), nonselective (ions, cAMP, and ATP can pass), bidirectional but it can be rectifying, EPSPS or IPSPs can pass, assist with rapid, synchronized/oscillations between cells, glia prefer electrical synapses (Ca2+ wave)

Question 19

what are anaxonic neurons?

Answer 19

a neuron without an axon, forming dendrodendritic synapses, form electrical synapses

Question 20

what are some features of chemical synapses?

Answer 20

presynaptic terminal: synaptic vesicles (small clear core vesicles) are small and light and release neurotransmitter, dense core vesicles are large and dark and release neuropeptides, contains exosomes
postsynaptic terminal: has receptors and effector systems

Question 21

what are some features of exosomes?

Answer 21

exosomes are vesicles containing enzymes, miRNA, and lipids that alter biochemistry, cross BBB/cell membranes via endocytosis, have neuro-, glial-, glia-neuron, gut-brain communication, long-range communication by series of neurons, neuroplasticity, disease/drug delivery