Week 1, Lecture 2 Notes
Week 1, Lecture 2 Notes Biol-K416
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Popular in Neuroscience
This 5 page Class Notes was uploaded by Malissa Notetaker on Monday September 5, 2016. The Class Notes belongs to Biol-K416 at Indiana University Purdue University - Indianapolis taught by Dr. Jason Meyer in Fall 2016. Since its upload, it has received 7 views. For similar materials see Cell & Molecular Neuroscience in Neuroscience at Indiana University Purdue University - Indianapolis.
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Date Created: 09/05/16
Cellular and Molecular Neuroscience Lecture 2 8/24/16 Topic: How do neurons convey signals throughout the nervous system? ● Neurons inherently have an internal negative charge elative to the outside ○ NOT an absolute negative charge ○ Have a means of generating constant voltage across their membranes when at rest ■ Known as resting membrane potential ■ Usually around 70mV, but can be between 40 mV and 90 mV, but depends on type of neuron ■ Potential Test question: “Which of the following would most likely be the charge of a neuron at rest?” ● 100mV ● 70mV ● 20mV ● +50mV ● Electrical signals produced by neurons are caused by responses to stimuli ○ Stimuli act to change resting membrane potential, either making it more positive or more negative ○ Three types of stimuli ■ Receptor potentials ■ Synaptic potentials ■ Action potentials ● Receptor Potentials ○ Created due to activation of sensory neurons by external stimuli ■ Light photoreceptors in eye ■ Heat receptors in skin ■ pressure/touch ■ Sound ■ Smells ■ Tastes ○ Converted to nervous system impulse ○ Pacinian corpuscles receptor neurons that sense mechanical disturbances in skin ■ Respond to touch w/ a receptor potential that changes the resting membrane potential for a fraction of a second ■ Respond by making some kind of receptor potential change in charge across membrane ○ After change in resting membrane potential, sensitization/ habituation will occur ■ Allow response to stimuli to change ■ Ex put on clothes in the morning, but you aren’t aware of the feeling of clothes on your body all day ● Activity of cortical neurons: ○ Center of the receptive field (where the touch takes place) increased cell firing ○ Surrounding of receptive field decreased firing ○ Outside receptive field no effect on firing ○ Helps show us exactly where the disturbance is ● Synaptic Potentials ○ Allow transfer of info from one neuron to another neuron across synapse ○ Causes a brief change in resting potential of postsynaptic (more downstream) neuron ○ Serves as a means of info exchange in complex neural circuits ○ Magnitude for synaptic & receptor potentials are around the same ● Action Potentials ○ Neurons that create this signal that travels along axon ○ Responsible for longrange transmission of info in nervous system ○ Membrane potential change is >>> than in synaptic or receptor ■ Repolarization & depolarization occurs faster ■ More consistent ○ Has hyperpolarization ○ Current delivered to neuron that makes membrane potential more negative is hyperpolarization (or repolarization) ■ Nothing really happens, referred to as passive electrical response ○ Current delivered to neuron makes the membrane potential more +, called depolarization ■ If membrane potential becomes + enough & reaches threshold potential, will result in an action potential ○ Synaptic is kind of a precursor to action ● Characteristics of Action Potentials ○ Usually brief (~1 ms) change from to + charge of membrane potential ○ Amplitude of action potential is NOT necessarily due to magnitude of current used to evoke it ○ Action potentials are all or nothing if an action potential is evoked at all, it will be of the same amplitude regardless of strength of stimulus ■ But magnitude of stimulus CAN affect the FREQUENCY of the action potential firing ○ Depolarization that doesn’t hit the threshold line is probably synaptic, not an action potential until it reaches threshold ○ Action potentials are all capping out @ the same level “all or none” ■ But increase in frequency of action potential means increase in the current ● LongDistance Transmission of Electrical Signals ○ Axons can be very long, can even extend over a meter ○ Are VERY poor conductors of electricity ○ If depolarization event is too small to elicit action potential, signal that is traveling will rapidly degrade, usually within a few millimeters ■ Occurs because the current “leaks” out across axon’s membrane ○ But signal will NOT decay down the axon if the stimulus is large enough to generate an action potential ■ As it goes down the axon, same strength but delayed ■ “All or none” action potential ● Ion Movements Produce Electrical Signals ○ Electrical potentials are generated across membranes due to 2 mechanisms: ■ Differences in concentrations of specific ions across nerve cell membranes ■ Membranes are selectively permeable to some of the ions ● Squid Giant Axon is 400x larger than mammalian axon, making it a better to study with ● Know which ions are > inside or outside the neuron ○ Potassium is > inside ○ Sodium is > outside ○ Chloride is > outside ○ Calcium is > outside ● Ion Movements Produce Electrical Signals (continued) ○ Active Transporters actively move ions in or out of cells against concentration gradient ■ Create ion concentration gradients ■ Can make the outside more + than inside ○ Ion channels allow only certain kinds of ions to cross the membrane in the direction of their gradients ■ Are selectively permeable to certain ions ● How An Electrochemical Equilibrium is Established ○ Can be demonstrated through the use of an artificial membrane that separates 2 compartments containing solutions of ions ■ Left side is “inside” of cell and right side is “outside” ■ If concentration of ions is =, there will be no electrical potential measured across it ● No net flux of ions ■ If concentration of K₊ ions is NOT the same on both sides, electrical potential will be created ■ If higher inside, electrical potential inside will become more negative relative to outside due to K₊ ions flowing down concentration gradient ○ Neuronal membranes CAN pump K₊ into cell ■ Have K₊ permeable channels in membrane to allow K₊ flow out of cell ₊ ● Continual resting efflux of K is responsible for resting membrane potential ○ Remember chemical gradient vs. chemical gradient ■ Chemical gradient sheer amount of ions (one side vs. the other) ■ Electrical gradient charge of ions ● Electrochemical Equilibrium ○ A point where there is no net flow of ions (in the case K₊) out of the cell ■ Generally in or out of the cell ○ Due to 2 opposing forces ■ There is always a higher concentration of K₊ ions inside the cell ■ As membrane potential changes, electrical gradient works to stop K₊ from moving across the membrane ○ Thus establishing resting membrane potential ● To move ions, NEED ion channels, then can pass through ○ Ions can only go through their specific channels ● Movement of ions depends on BOTH electrical and chemical gradient ● Tetrodotoxin, TTX, poison from pufferfish ○ Will selectively bind & block sodium ion channels ● Tetraethylammonium, TEA, will selectively block certain K₊ channels ● What ions are involved in the action potential? ○ Beginning of resting state Pk>>> Pna ○ Depolarization Pna increases ■ Due to opening of Na channels, rushes in ○ Peak of action potential Pna>>>Pk ○ Repolarization decrease of Pna ■ Increased permeability to K₊ ○ Back to resting state ■ Pk>>>Pna ● Components of an Action Potential ○ Rising and overshoot phase due to increased membrane permeability to Na+ ■ Depolarization ○ Falling phase due to activation of voltagegated K₊ channels ■ Repolarization ○ Undershoot phase due to K₊ ion channels ■ Hyperpolarization
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