2 11 15 Action Potentials Threshold Potential o Vm required to activate Na channels o Stimulus then generated by Na influx o Weak depolarization subthreshold potentials Action Potential amplitude aka size if occurs o Independent of stimulus o No change in size is not a graded potential Graded potentials do have a range change in size Our action potential does not This solves the problem we had with our graded potential Action potentials are localized in each segment we are talking about Think about Phys Phabs doing the wave on stage Each one represented an action potential We didn t have one person action potential run down the stage Practical Application o Local anesthetics many block Na channels That means that when we are being cut by the doctor under anesthesia we don t feel anything because we don t reach the threshold You do reach the threshold potential and the sodium channels open but they are blocked by the inactivator Propagation down axon o Figure 6 23 o Phys Phabs come up stage They line up Each person is a section of a plasma membrane The raise their hand over their head to represent the graded potential The graded potential is passed on the next person to travel to the end of the cell There is no movement of an individual action potential Basically end up doing the wave on stage What s wrong with the analogy using staff They aren t the same height we need all the action potentials to be equal Dr B makes joke in class about one staff being short vertically challenged o Adjacent sections of Plasma Membrane Solves the distance problem we had with graded potentials No movement of cells One cell starts the next o Increase distance no change in size of action potential o Unidirectional So no signal change Problem solved Necessary for speed Due to 2 11 15 o Myelin insulator Hyperpolarization o K potassium stays open for a little too long Na channels blocked o We can t get an influx of sodium Depends on context though Can be considered bidirectional if the action potential starts in the middle of the axon and we are looking at both sides Figure 6 24 Aids graded potentials and blocks action potentials We still see a decrease but the myelin makes it decrease much more slowly We increase the speed by having the myelinated sections pass graded potentials down the plasma membrane because they travel faster than action potentials o We can t myelinated everything though because we would fall below the threshold before we got to the final cell o Hence why we have nodes of Ranvier Increases conduction velocity o Saltatory Conduction Figure 6 24 Action potentials only at nodes of Ranvier So we don t necessarily need channels to produce a graded Jump to next node via graded potentials under myelin potential The is a difference in charge under the myelin but that charge came from the ions that came through the channels where the action potential was generated the nodes of ranvier Back to Chemical Synapses Postsynaptic neuron o Inputs from many pre can occur Excitatory stimulatory stimulus depolarizing We can have excitatory or inhibitory synapses Inhibitory hyperpolarizing o Temporal Summations The second stimulus builds off the first one Same close together Same stimulus being sent soon after the other Signal is coming from the same synapse o Spatial Summations Stimuli are different but reaches the post synaptic at Different close together Signal is coming from 2 different synapses at the the same time same time 2 11 15 o Only graded potentials can exhibit these summations o These summations can help us reach the threshold Synapses can change info more more complex o When the presynaptic number is increased the action is more complex which is definitive of special summations When it is more complex there are more pathways to get to the threshold The higher the complexity the more opportunities summation forms there are to get to a threshold potential Figure 6 31 Synapse Strength synapses Presynaptic factors Figure 6 33 The effectiveness strength of a synapse is influenced by both post and pre o Availability of neurotransmitter o Calcium availability More neurotransmitters released more ion channels being opened More calcium more vesicles release neurotransmitters Calcium influx more neurotransmitter release exocytosis o Receptor availability Most presynaptics are also a postsynaptic figure 6 33 Activation of presynaptic receptors influences calcium influx into the terminal and thus the number of neurotransmitter vesicles that are released Can decrease or increase the amount of neurotransmitter o Membrane Potential released Level of polarization affects the amount of messages that are able to get through If the cell is hyper polarized it is going to be less likely for a message to get through Depolarized more likely Hyperpolarized membrane is less likely to send a message because it is less likely to reach the threshold potential Postsynaptic factors o Receptor availability o Membrane potential o Other synaptic inputs Anatomy of the Brain Cerebral Cortex aka cerebrum Gray Matter outer shell o Clustered cell bodies nuclei White Matter inner layer o Axons myelin is white Figure 6 39 o Perception voluntary movement language learning memory
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