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Transcript
Lecture 1, Chapter 1 Overview: History and the neuron History Galen Nerves were tubes, brain had holes (ventricles) __________ flowed through the tubes, much like ____________ Ramon y Cajal Showed that nerve cells were ____________________ ____________________ and not networks of tubes Noticed __________ between units Noticed ____________________ (cell bodies) How does one part of the body communicate with another part? Loewi (1921) Stimulated the vagus nerve in one heart to __________ it down Removed some of the fluid it was bathed in Added that removed fluid to a different heart The second heart ______________________________ Some ______________________________ must be working to slow the heart What is the nature of these chemicals? Hodgkin and Katz (1949) Working with giant squids can lead to problems “The values for spike height are in good agreement with those obtained by Hodgkin and Huxley (1945), but are considerably smaller than those reported by Curtis and Cole (1942). The average value for the resting potential is slightly smaller than that given by Curtis and Cole… The average action potential was about 20mV smaller than that given by Curtis and Cole. But a more serious discrepancy arises from the fact that we have never observed action potentials greater than 100mV at 18-23 degrees C, whereas Curtis and Cole describe a spike as large as 168mV in a fibre which gave a resting potential of 58mV. The matter is not one that can be lightly dismissed, because the existence of a fibre capable of giving an overshoot of 110mV has far reaching implications. We can no longer be inclined to think that our relatively small action potentials can be attributed to the poor condition of the experimental animals, since a number of the squids employed were extremely lively and in perfect condition. Nor does it seem likely that axons were damaged in the process of isolation… the only explanation that can be offered is that there is a real difference between the properties of L.Peali used at Woods Hold and L forbesi used at Plymouth.” What did they find? Knew that when frog muscles are put into a solution with low levels of sodium and chloride, they were ________________________ So either sodium or chloride in needed for excitation Replaced chloride in sea water solution with sulphate and the nerve was ________________________________________________ Conclusion: ________________________is at the root of the action potential What about increasing sodium levels? “The concentration of sodium chloride was increased… This solution…damaged the axon by osmotic effects in 5-15 min. But before the osmotic effects became apparent the axon gave an increased action potential with characteristics which were the converse of those in sodium-deficient solutions.” The Resting Potential A difference between the voltage of the inside and the outside of the neuron (approx -70mV) Occurs because ions are found in different amounts inside of the cell compared to outside of the cell The players: ________________________ (Na+) ________________________ (K+) ________________________ (Cl-) Other Anions (A-) Two forces at work in a neuron Electrostatic gradient Charge- ___________________________________________ Diffusion gradient ________________________________________________ Why don’t ions just move to where the forces push them? _____________________________________________membranes Contain ion channels which open and close to let certain ions in or out of the cell Two types of channels ________________________________________________channels Opened by a chemical ________________________________________________channels Opened by a change in voltage The sodium potassium pump Forces three sodium ions out of the cell for every two potassium ions brought back in Creates an ________________________between the inside and the outside The outside gets more sodium, which makes it more ________________________ Takes up to 20% of a cell’s energy to keep going In addition Ion channels are “________________________” Small amounts of ions get through even when the channels are “closed” This further contributes to the status of the resting potential ________________________by the sodium potassium pump The action potential Ligand gated channels play a large role Opened by a ________________________ The chemical is usually a neurotransmitter Some chemicals ________________________the amount of positive ions entering the cell Others increase the amount of ________________________ions entering the cell EPSPs and IPSPs If the inside of the cell becomes more positive, this is called an ________________________________________________ (EPSP) If the inside of the cell becomes more negative, this is called an ________________________________________________ (IPSP) Keys to the action potential If enough of an EPSP leads to the surpassing of the threshold (approx -55mV), an ______________________________________________ The increase in voltage inside the cell triggers the opening of more and more ___________________________________sodium channels These allow a LOT of sodium to enter the neuron and increase the positive charge inside the cell Keys to the action potential Around +40 to +50mV, the sodium channels ___________________, leaving just the potassium channels _____________________ Potassium is now repelled to exit the neuron by the positive charge inside (electrostatic force) and by its concentration gradient Too much potassium leaves and the cell becomes ____________________________ Refractory period While the cell is hyperpolarizing, it cannot fire again (sodium channels are unable to open) ________________________refractory period Once the potassium has overshot the resting potential (too much left), the cell can re-fire if it receives a larger than average EPSP ________________________refractory period Getting back to rest Due to the sodium potassium pump, the resting potential is eventually ________________________and the cell is ready to go again “leaky” channels aid in this process as well The axon terminal The change in voltage in the axon terminal leads to the opening of voltage gated ________________________channels ________________________________________________ Calcium leads to ________________________- the neurotransmitter is released into the synapse The synapse The neurotransmitter crosses the synaptic gap and ________________ ___________________________________on the postsynaptic neuron Many different types of receptors exist Action of NTs at synapse Receptors can be either single step or multiple steps ________________________: single step NT binds to channels and opens them Can work quickly: ________________________ ________________________receptors Multiple steps NT binds to receptor- triggers G protein to bind to ion channel and open it The majority of receptors in the brain are metabotropic Slower than ionotropic receptors: ________________________