ALS 2304, Week 5: Neurophysiology (cont.)
ALS 2304, Week 5: Neurophysiology (cont.) ALS 2304
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This 5 page Class Notes was uploaded by Mara DePena on Sunday February 21, 2016. The Class Notes belongs to ALS 2304 at Virginia Polytechnic Institute and State University taught by Dr. Cline in Spring 2016. Since its upload, it has received 24 views. For similar materials see Animal Physiology and Anatomy in Agricultural & Resource Econ at Virginia Polytechnic Institute and State University.
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Date Created: 02/21/16
ALS 2304 WEEK 5: NEUROPHYSIOLOGY TRANSPORT Neurotransmitters are released out of the terminal. They are made by transcription/translation in the soma and moved down to the terminal. Anterograde transport- Moving something (neurotransmitters) from the soma down to the terminal. Molecular motors are responsible for transport. Carry neurotransmitters to axon terminal. Retrograde transport- Moving something (waste) from the axon terminal to the soma. Myelin sheath- Made up of schwann or oligodendrocyte. Whitish, fatty, segmented sheath around most long axons. o Unmyelinated axon- Schwann cells surround nerve fibers but coiling does not take place. Schwann cells partially enclose 15 or more axons each. ELECTRICITY Voltage (V)- A measure of potential energy. Potential energy generated by separated charge. o Membrane potential is measured in mV. o When ions move across, that is work. Potential difference- Voltage between two points. Current (I)- Flow of electrical charge. o Established by opening a channel. Ions moving= current. Resistance (R)- Hindrance to charge flow. o Membrane itself is the resistance to the ions. RESTING MEMBRANE POTENTIAL Established by the sodium-potassium pump. -70mV. ELECTROCHEMICAL GRADIENT Drive ions into neuronal membrane when channels open. Ions flow along their chemical (concentration) gradient. o Move from an area of high to low. Ions flow along their electrical gradient when they move toward an area of opposite charge. o Negative charge inside the membrane. Electrochemical gradient- The electrical and chemical gradients taken together. When Cl attempts to flow into the neuron, chemical gradient pulls it in and electrical gradient tries to push it out. Chemical gradient is stronger. ION CHANNELS (ON NEURONAL MEMBRANE) Passive/leaky- Always open. Never closes. (Ex: Potassium channel) Chemically gated- Open with binding of specific neurotransmitter. Voltage-gated- Open and close in response to membrane potential. No place for neurotransmitter to bind. Mechanically gated- Open and close in response to physical deformation of receptors. Tension is put on dendrite, which pulls apart. Ion can then flow down its electrochemical gradient. GRADED POTENTIALS Short-lived, local changes in membrane potential. Decrease in intensity with distance. Magnitude varies directly with the strength of stimulus. Sufficiently strong ones can initiate action potentials. o EPSP- Resting membrane potential of -70mV. Sodium channels open, raising it to -55mV (or higher) which fires an action potential down the axon. Subthreshold- When the response is below -70mW. o IPSP- Chlorine channels open. Travel down electrochemical gradient and lower resting membrane potential (more negative). SUMMATION First image: Neuron does not fire, big time separation. Second image: Less time separation. Neuron fires. Second EPSP adds to first and crosses threshold. Temporal summation- One pathway fires repeatedly, rapidly. Fires action potential. Third image: Spatial summation- Two EPSPs occur simultaneously, which means there are two terminals. Fourth image: EPSP and IPSP fired simultaneously. Greatly reduced EPSP. Action potential does not fire. ACTION POTENTIAL (NERVE IMPULSE) On the exam, there should be four individual boxes depicting each stage of the action potential. This all occurs on the axon hillock. Action potential is either all or none. No magnitude difference on same neuron. Same neuron will always fire at exactly the same magnitude and will always propagate at the same speed. When it is propagated down the axon, there is first the exchange of sodium down the neuronal membrane. Sodium travels down the membrane, and Potassium flows out. o Resting sta+e + Na and K channels are closed. Leakage, small movements of both. Each Na channel has 2 voltage regulated gates. Activation gate (at top) close in resting state Inactivation gate (at bottom) open in resting state Potassium channel Has only one gate, which is closed in the resting state Channels open at -55mV with help of EPSP. o Depolarization state Inside is becoming more positive than the outside. EPSP drives potential over -55mV and causes the sodium gates to open and potassium gates to close. Timing mechanism on inactivation gate, which will close after a certain amount of time. Timing mechanism on potassium channel, which will open after a certain amount of time. o Repolarization state Inactivation gate closes, stops sodium current. Potassium gates open, potassium exits and internal negativity of resting neuron is restored. Becoming more negative, towards -70mV (resting membrane potential) o Hyperpolarization state Even more potassium flows out than would have to to return to resting membrane potential. Potassium gates shut close with help of timing mechanism triggered when EPSP got there. Neuron insensitive to stimulus and depolarization. Sodium-potassium pump will later restore the balance. Dip under it is known as the undershoot. Part of action channel is an EPSP, which will go on to open the rest of the channels in a circuit. REFRACTORY PERIODS Absolute o Time from opening of Na activation gates until closing of inactivation gate. Prevents generation of action potential Ensures each action potential is separate Impossible from action potential to occur Enforces one-way transmission of nerve impulses Relative o Interval associated with the undershoot/hyperpolarization. o Interval following absolute refractory period when Sodium gates are closed Potassium gates are open Repolarization is occurring Membrane potential is going to be more negative so will need a higher magnitude EPSP to fire an action potential Normally does not fire Causes pain if so o Very frequent action potentials reaching central system o Threshold level is elevated, allowing strong stimuli to increase frequency of AP events. CODING FOR STIMULUS INTENSITY Remember, action potential is all or none. If brain receives all or none, how can the brain tell which pressure/pain is greater? o Frequency of the action potentials Really close together implies pain AXON CONDUCTION VELOCITIES Vary widely among neurons If a circuit is myelinated, the velocity is much faster than one that is unmyelinated o Pathway from hand to brain is myelinated, hence why we quickly retract our hand when we place it on a burner. o Pathway from balls to brain is unmyelinated, hence why it takes a while to realize you’ve been hit in the crotch. SALTATORY CONDUCTION Myelinated axon o Current only passes at nodes of Ranvier (bare axon, gap between myelinated cells). Voltage-gated sodium channels concentrated at these nodes Action potential triggered only at the nodes and jump from node to node. Faster than conduction along unmyelinated axons. Why myelinated pathway is faster. All pathways to organs are unmyelinated, while all the ones associated with touch are myelinated. This is so you can respond rapidly to external stimuli. SYNAPSES Synapse- Where two neurons meet (space in between). Presynaptic neuron- Conducts impulses toward synapse. Postsynaptic neuron- Transmits impulses away from synapse. Synaptic cleft/space- Gap between two neurons. Extracellular fluid is in this space. o Synaptic vesicles- Store neurotransmitters. Action potential opens calcium channel, which floods exon terminal and leads to exocytosis of neurotransmitter. Forms calcium-calmodulin complex, activates kinase which phosphorylates synapsin, which drags the synaptic vesicle down in proximity to membrane (does not cause exocytosis, just shoves vesicles against wall) SNARE proteins- Synaptobrevin attached to vesicle Syntaxin and SNAP 25 attached to membrane. These all entangle, pulling synaptic vesicle very close to plasma membrane. The membranes fuse and exocytosis of neurotransmitter into synaptic space occurs. Diffusion carries these neurotransmitters to the chemically gated channel. This is the rate limiting step in neuronal communication.
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