Neurophysiology NROSCI 0081
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This 8 page Class Notes was uploaded by Arielle Reiner on Monday September 14, 2015. The Class Notes belongs to NROSCI 0081 at University of Pittsburgh taught by Fanselow,Erika in Summer 2015. Since its upload, it has received 30 views. For similar materials see DRUGS AND BEHAVIOR in Neuroscience at University of Pittsburgh.
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Date Created: 09/14/15
Neurophysiology 09112015 What does neurophysiology mean 0 Physis greek for quotnatureoriginquot Ology greek for quotthe study ofquot quotPhysiology is the branch of biology that deals with the functions and processes of living organisms both animals and plants It s biology in motionquot 0 For our purposes neurophysiology will deal with the biochemical basis for how neurons send signals In short how do neurons work Helps to understand how drugs affect neurons and thus behavior What is the job of a single neuron A stereotypical neuron is designated to 0 Receive multiple messages at the dendrites Either from another neuron or from the environment Eg When light hits the cell in the retina or something touches your skin 0 Integrate those messages in the soma e Access them all together 0 Send a message down the axon But only if the integration at the soma reaches a certain threshold 0 Send the message From the axon to other neurons usually by releasing neurotransmitter molecules at the synaptic terminals terminal boutonsaxon terminal Neurons need to tell other neurons or muscles something and what they say is based in part on what they were told by other neurons Cell membrane 0 Cell membranes form the outer layer of a cell such as a neuron OOOOO Molecules in the membrane form a lipid bilayer Permeable to fatty substances Charged region hydrophilic Uncharged region hydrophobic Impermeable to ions When salt is put in water it separates NaCl The sodium and chloride ght over the extra electron and the chloride gets it Sodium has a positive charge They can t survive outside of uid 3 relevant ions n Chloride Cl n Potassium K n Sodium Na There are ions inside and outside of neurons but not in equal amounts Concentration of potassium is high inside neurons and low outside Concentration of sodium is low inside neurons and high outside Concentration of chloride is low inside neurons and high outside Ions are charged particles so they are hydrophilic n Thus they can be near the outer edges of a cell membrane but cannot cross through the hydrophobic center of the cell membrane on their own Neurotransmitter receptors are embedded in cell membranes 0 These receptors contain channelstubes through which ions can cross the cell membrane o This is the site where many drugs act Receptor selectivity and gating o Selectivity many receptors are highly selective about which ions they will allow through and so there are sodium channels potassium channels etc Gating many receptors are not open at all times and only open when triggered to do so while others are open at all times 0 Gate prevents ions from going through the channel 0 Ion channel gating o A potassium permeable receptor has a channel that is always open so K can move through it at any time o In contrast a sodium receptor has a channel that is only opengated when triggered to be so which means it only allows Na ions to go through at speci c and brief times Receptors can be gated by multiple things 0 Ligand gated when a ligand eg drug or neurotransmitter binds to this type of receptor the binding causes a confrontational change in the receptor molecule which either opens or closes the receptor channel 0 This opening or closing of a receptor channel changes whether ions can cross the membrane while the ligand is bound 0 Voltage gated when there is a change in the relative charge across the cell membrane ie change in voltage and this causes a conformational change in the receptor molecule which either opens or closes the receptor channel So far we have learned Neurons need to generate a message or signal at the soma and send it to the end of the axon Neuron cell membranes are impermeable to ions on the ir own 0 Charged ions are found both inside neurons and in the CSF surrounding them but in unequal amounts Neurotransmitter receptors are located in cell membranes ad they can let ions cross into or out of a neuron Some receptors need to be triggered to open or close 0 When speci c charged ions cross the membrane in a speci c sequence this can cause a change in the number of charged particles inside and outside of a cell This causes a voltage change which can initiate a signal and allow it to travel down the axon o This is a really fast signaling process What is voltage o A voltage is a separation of charge 0 AKA a difference in the number of charges of a given polarity 0 Le positive or negative 0 The charge could involve charged atomic particles such as electrons in a copper wire 0 Or the charge could involve charged ions such as are found inside and outside of neurons 0 Electricity is carried on electrons negatively charged How is voltage related to neurons 0 There can be different amounts of charged ions on different sides of the cell membrane 0 This difference creates a voltage across the cell membrane How would you know a neuron has a voltage 0 Insert electrode that can measure how many charged ions are present 0 Device that can measure the voltage difference in number of charged ions between the electrode and 0 point Cells not just neurons have a membrane potential 0 The presence of a voltage a separation or difference in charge means a cell has a membrane voltage 0 Note that neurons are not the only cells that have a membrane vo age 0 Most cells do 0 Another term for voltage is potential 0 The more common term for a membrane voltage is a membrane potential 5 important questions about neuron signaling 0 Why do ions move through ion channels 0 When ions are in water they diffuse through the water moving from being highly concentrated in 1 region to being equally distributed This is known as the diffusion force 0 If the concentration of an ion is high in 1 region and low in another there is said to be a concentration gradient for that ion 0 Thus if there is a concentration gradient across a neuronal membrane some ions will move to the other side so the numbers of ions are equal This will only happen if the ions are given the opportunity eg via open ion channels 0 Note ions of different types will reach equal concentrations essentially independently of other types of ions that may be present 0 lons all on 1 side channels open and ions begin to move to the other side each type of ion is distributed equally across the membrane 0 2 main things maintain the neuronal ion concentration gradients The NaK pump moves 3 Na ions out of the neuron for every 2 K it brings in contributing to the net negative charge inside the neuron This pump uses energy in the form of ATP a molecule cells in the body use as energy for many cellular funcUons This means that maintaining these ion gradients comes at a cost to the cell The net negative charge inside a neuron will tend to pull positive ions in and push negative ones out n This is known as electrostatic force Where does each type of ion go when ion channels are open 0 K goes outside of the neuron o Na goes inside of the neuron 0 Cl goes inside of the neuron How would ions crossing the membrane affect the membrane potential 0 Milli thousandth Eg 1 milliliter 11000 liters So 1 mV 11000 bolts 0 Resting membrane potential when a neuron is neither receiving nor sending a signal This is usually about 70 mV in a typical neuron Polarization a separation of charge such that there are positive and negative regions Hyperpolarization when the membrane potential becomes more negative more polarized Depolarization when the membrane potential becomes more positive less polarized If Na channels open Na ow will push the membrane potential less negative depolarized If K channels open K ow will push the membrane potential more negative hyperpolarized If Cl channels open Cl ow will push the membrane potential more negative hyperpolarized Different ion channels can open and close moving the membrane potential away from the resting membrane potential in either direction back and forth 0 What can cause ion channels to open 0 0 Ion channels can be ligand gated lon channels can be voltage gated When these are triggered by membrane depolarization they open letting Na into the cell First ligand gated ion channels open depolarizing the membrane a As the membrane depolarizes voltage gated Na channels open letting Na into the neuron which further depolarizes the membrane 0 Why does it matter to the neuron what its membrane potential is O Depolarization open Na channels Na ows into neuron depolarization This is called the positive feedback loop Action potential threshold unconstrained Na channel opening causes the membrane potential to depolarize very rapidly Voltage gated K channels now open and K goes out of neuron which beings to hyperpolarize the neuron Na channels inactivate close so Na ow stops Membrane repolarizes This is the negative feedback loop 0 Na and K ow during an action potential Timing is critical Voltage gated Na channels open rst and rapidly inactivate close Voltage gated K channels open later and stay open longer before inactivating 0 An action potential undershoot Action potential undershoot this occurs because K channels inactivate slowly This allows K to continue to exit the neuron and this pulls the membrane potential below the resting potential Relative refractory period the still open K channels make it more difficult to start a second action potential too close in time to the rst Absolute refractory period inactivated Na channels prevent another action potential from starting How does a toilet ushing resemble an action potential 0 Resting potential full tank of water 0 With insufficient push of the handle you will get a transient ow of water but no ush 0 With sufficient push of the handle get an all of nothing ush o Refractory period after the ush The excitable membrane allows the action potential to regenerate and propagate along the axon 0 Action potential depolarizes the axon o Depolarization opens nearby Na channels 0 Na enters cell 0 Depolarization opens nearby Na channels Passive cable axon as a quotleaky hosequot o Leaking of ions would prevent the action potential from continuing down the axon Myelin insulates neuron so action potential can travel faster 0 Oligodendrocytes CNS or Schwann cells PNS wrap layers of myelin around axons o This reduces the amount of ions that leak out of the axons as the action potential moves along it Saltatory conduction 0 Action potential generation in the nodes of Ranvier active region Nodes of Ranvier speed up action potentials o No action potentials in the myelin passive conduction internode region Importance of Myelin Multiple Sclerosis O O O O Characterized by a loss of myelin Autoimmune disease in which the body s immune system attacks the CNS leading to demyelination Prevents neurons from effectively conducting signals Can lead to any neurological symptom Most commonly associated with sensory loss fatigue depression
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