! Depolarization continued. AP Biology. " The final phase of a local action

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1 ! Resting State Resting potential is maintained mainly by non-gated K channels which allow K to diffuse out! Voltage-gated ion K and channels along axon are closed! Depolarization A stimulus causes channels to open! influx (into cell) depolarizes the membrane # Depolarization = reduction in the magnitude of membrane potential (cell becomes less negative, or The 1st domino goes down! more positive)! Depolarization opens voltage-gated channels! Depolarization Continued Opening of voltage-gated channels causes more influx! Cell further depolarized # Triggers still more voltage-gated channels to open $ Net result: Large flow of current inward If membrane voltage is increased to a particular value called the threshold (-55 mv in mammals), triggered!! Action potentials = massive change in membrane voltage Look! POSITIVE FEEDBACK! # Function as nerve signals, a.k.a nerve impulses $ All-or-none response to stimuli Gate closed open! Depolarization continued Initiating the! channels open rapidly, causing rapid depolarization # Resulting in the Rising phase of an! channels then self-inactivate blocking more flow K channels also opened in response to more positive membrane potential caused by a stimulus but! K channels open more slowly and remain open throughout the whole # After channels close, K continues to flow out and membrane potential drops again $ Results in the falling phase of How is a nerve impulse generated?! Depolarization continued The final phase of a local action potential is called the undershoot! Membrane permeability to K is higher than a neuron at rest! More K leaves and the cell is temporarily hyperpolarized # Hyperpolarized = increase in the negative magnitude of the membrane potential

2 Gate Voltage-gated channels remain inactivated during the falling phase and early undershoot phase of the action potential The nerve impulse travels down the neuron s axon! An in one region triggers an in a neighboring region etc # ions entering one region of an axon diffuse through the axon s cytoplasm The rest # This electrical current of the depolarizes neighboring dominoes fall! regions in the axon s membrane $ If neighboring membrane reaches threshold, and K voltage-gated membrane channels open and an action potential is initiated Membrane ion permeability! A second depolarizing stimulus during this period is unable to trigger another # Downtime following an is called the refractory period How does a nerve impulse travel?! Just like with dominos! The Refractory Period closed K open K K

3 How does a nerve impulse travel? How does a nerve impulse travel?! Signal always moves in one direction!!!!! Like dominoes, one triggers a neighboring one which triggers a neighboring one etc Action potentials or nerve impulses are propagated down the axon starting at the axon hillock! Each one is of the same magnitude, speed, and duration Signal ALWAYS moves in one direction!!!! How does a nerve impulse travel? Behind the traveling zone of depolarization due to inflow is a zone of repolarization due to K outflow! In this zone, channels are inactivated (refractory period) # Inward current of can only depolarize the axons Structure membrane AHEAD of the last of neurons not behind it fits their $ Action potentials function move in only one direction towards synaptic terminals $ voltage gated channels are unblocked and ready to be activated again when the membrane is fully repolarized to resting potential How does the nerve re-set itself?! After firing a neuron has to re-set itself needs to move back out K needs to move back in! They need to travel against concentration gradients # need a pump!! A lot of work to do here! K K K K K K K K K K K K K

4 How does the nerve re-set itself?! Sodium-Potassium pump active transport protein in membrane! requires ATP 3 pumped out ATP 2 K pumped in # While K voltage gated channels are closed again, some permanently open K channels will allow som eof this K to leak out again! re-sets charge across the plasma membrane That s a lot of ATP! Feed me some sugar quick! Neuron is ready to fire again K aaaa - K aa - K K K K K K aa - aa - aa - K Ready for next time! resting potential Action potential graph 1. Resting potential (-70mV) 2. Stimulus reaches threshold potential (-55mV) 3. Depolarization channels open rapidly! flows in K channels open slowly 4. channels close and inactivate; K channels all open (35mV) 5. Repolarization K flows out & resets charge gradient 6. Undershoot K channels close slowly (-75mV)! K levels return to normal Membrane ready to be depolarized again if new signal arrives Membrane potential 40 mv 30 mv 20 mv 10 mv Depolarization flows in 0 mv 10 mv 20 mv 30 mv 40 mv 50 mv 60 mv 70 mv 80 mv Threshold Resting potential Repolarization K flows out 5 Hyperpolarization (undershoot) 6 Resting Frequency of s vary in response to input! Stronger stimuli result in more frequent s Louder noise = more action potentials moving down an axon

5 Myelin sheath Conduction Speed! Action potential are propagated! Produced by special glial cells down axons at incredible speeds The wider the axon the faster the s are conducted! Wrap axons in many layers of Invertebrates like mollusks and insects have giant axons which function in rapid behavior response 30m/sec plasma membrane # Mostly lipid based = poor conductor of electricity $ Vertebrates have narrowing axons In the blink of an eye! Yet, brain! finger tips in milliseconds! Provides electrical insulation When axon depolarizes, the current generated by influx spreads farther along the interior of the axon! More distant regions of the How? Oligodendrocytes in CNS Schwann cells in PNS axon are brought to threshold sooner Thanks to a layer of electrical insulation surrounding vertebrate axons known as a myelin sheath $ Saltatory Conduction Action potential reaches terminal sooner Enhanced conduction speed due to myelin! Voltage-gated sodium channels are restricted to gaps in between myelin sheaths = Nodes of Ranvier Action potentials are NOT generated in the regions between nodes! Current produced at one node travels all the way to next node and depolarizes the membrane there # Action potentials skip stretches of axon & move from node to node! Saltatory conduction saltatory conduction myelin axon 150 m/sec vs. 5 m/sec (330 mph vs. 11 mph)! Can conduct faster than giant squid unmyelenated axon despite being 40x narrower! Space efficient

6 Multiple Sclerosis (MS) Autoimmune disease! immune system (T cells) attack myelin sheath! loss of proper signal conduction