Action Potentials Lecture Notes PDF

6.1 Action Potential Introduction Action Potential (AP) -​ An AP is large change of membrane potential that travels down an axon -​ Conveys information from the cell body to the synaptic terminal -​ When this very large depolarization reaches the synaptic terminal, it will stimulate...

6.1 Action Potential Introduction Action Potential (AP) -​ An AP is large change of membrane potential that travels down an axon -​ Conveys information from the cell body to the synaptic terminal -​ When this very large depolarization reaches the synaptic terminal, it will stimulate neurotransmitter release -​ Causing the neuron to communicate with another cell AP Behavior - Binary -​ APs are binary processes -​ “All-or-nothing” -​ Deciding factor is if the membrane potential reaches the AP threshold AP Behavior - Amplitude -​ APs for a given neuron will always be the same amplitude -​ Amp = Amount of voltage change -​ Different from graded potentials -​ EPSPs and IPSPs -​ Vary in amplitude -​ AP amplitudes may be different across neurons -​ Because the amplitude of all APs for a cell are the same, the way neurons communicate is by changing the number of APs that are fired -​ Firing rate is the number of APs fired per second -​ Most cells have a baseline spontaneous firing rate -​ Spontaneous firing is random and does not carry any information  -​ Can increase or decrease the rate of AP firing to carry information AP Threshold -​ AP firing is dependent on if the membrane potential reaches the AP threshold -​ -55 mV -​ +15 mV from Em -​ AP will fire when a neuron depolarizes to -55 mV -​ AP will NOT fire if a neuron is more negative than -55 mV EPSP & IPSP -​ EPSP: increases the likelihood of firing an AP -​ Gets closer to AP threshold -​ Typically depolarization -​ IPSP: decreases the likelihood of firing an AP -​ Prevents from reaching AP threshold -​ Typically hyperpolarization 6.3 AP Ion Channels AP Ion Channels -​ APs are primarily dependent on two ion channels -​ Voltage-Gated Na+ Channels -​ Voltage-Gated K+ Channels -​ Both have many similarities and some crucial differences AP Ion Channels - Similarities -​ Both are ion channels that use chemical gradients to move ions across the membrane -​ Both are voltage-gated and open when the neuron depolarizes enough -​ Ion specific -​ Essential for APs to happen Na+ Channels Vs K+ Channels -​ Differences -​ Na+ Channels -​ Ion: Na+ -​ Opening Voltage: -55 mV -​ Effect on Membrane Potential: Depolarize (+) -​ Opening/Closing Speed: Faster -​ Inactivation Mechanism: Yes -​ K+ Channels -​ Ion: K+ -​ Opening Voltage: -45 mV -​ Effect on Membrane Potential: Hyperpolarize (-) -​ Opening/Closing Speed: Slower -​ Inactivation Mechanism: No Channel Open/Closing Speeds -​ Na+ channels open VERY quickly -​ Allows ions to rush into the cell almost instantly -​ Membrane potential changes very quickly -​ K+ channels open and close much slower -​ Will take longer for ions to start rushing out of the cell -​ Will take longer to affect the membrane potential Na+ Inactivation -​ Typical voltage-gated channels only have two states -​ Closed when membrane potential is below its threshold -​ Open when membrane potential is greater than its threshold -​ Na+ channels are very unique because they have an inactivation mechanism -​ Na+ voltage-gated channels will become inactivated after being open for a short period of time -​ The protein has a specialized “ball and chain” segment on the intracellular side -​ Ball and chain will move into channel pore, clogging the pore -​ When the ball and chain is in the pore, the channel is inactivated -​ No ions can flow through the channel, regardless of membrane potential -​ Ball and chain will remove itself from the pore over time -​ Once the ball and chain is removed the channel will be able to open and close Additional Support for AP -​ Working in the background of these channels are many hard-working leak channels and transport proteins working to maintain Em and ion concentration gradients 6.4 AP Morphology * Study from slides 6.5 Refractory Period Refractory Period -​ Period of time that the neuron is unable (or is very unlikely) to fire another AP -​ Has two phases -​ Absolute -​ Relative Absolute Refractory Period -​ The neuron is physically unable to fire another AP -​ When the Na+ channels are open or inactive -​ Cannot fire another AP when an AP is already underway or right after -​ Na+ ball and chain is still clogging the pore, preventing ions from moving -​ Na+ channels will transition from inactivated to closed shortly following the falling phase Relative Refractory Period -​ Following the absolute refractory period -​ The neuron is able to fire another AP, but is very unlikely -​ Cell is hyperpolarized and farther from AP threshold -​ Needs very strong stimuli to fire another AP 6.6 AP Propagation AP Propagation -​ APs do not just get fired at the axon hillock then passively travel down axon -​ APs are an active processes which propagate from the axon hillock to the synaptic terminal Axon Morphology -​ The axon is filled with Na+ and K+ voltage-gated channels -​ Allows APs to be actively propagated along the whole axon -​ Able to fire APs at any point of the axon AP Propagation -​ APs start at the axon hillock, then the depolarization will cause the adjacent region of the axon to hit the AP threshold -​ This will cause that adjacent region to fire an AP -​ Refiring APs along the whole axon -​ Effective for very short axons -​ Not effective for long axons -​ Long axons depend on myelin to allow the APs to travel farther and faster Myelin Segment -​ Myelin wraps axons in segments -​ These segments have no Na+ or K+ voltage-gated channels -​ Thick insulations of fat that reduces the ions from leaking out of the axon Nodes of Ranvir -​ Sections between myelin segments are exposed sections of membrane that are FILLED with Na+ and K+ voltage-gated channels -​ APs can be propagated at the Nodes of Ranvir AP Propagation with Myelin -​ The AP will fire at the axon hillock -​ Depolarization will passively travel through the myelin insulated axon -​ Depolarization is very slow to decay because of insulation -​ Depolarization wave will arrive at the Node of Ranvir -​ Another AP will be fired at the node -​ Causing the depolarization to passively travel long the next myelin insulated segment -​ APs traveling down the axon and being refired at each node is called saltatory conduction -​ Latin: leap/jump -​ AP will jump from node to node Synaptic Terminal -​ The AP will travel from the axon hillock to the synaptic terminal -​ Depolarization will reach the synaptic terminal and facilitate the release of neurotransmitters