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Neurophysiology
Conduction, transmission and integration of neural signals
Chapter 3
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Summary of neural transmission
Resting potential and action potential signal propagation ionic bases of electrical potentials neural integration
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Review of neuron Input zone (dendrites) Integration zone (axon
hillock) Conduction zone (axon) Output zone (axon terminals) Secretory event
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Resting potential Definition of electrical potential
Potential energyElectrical potential
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Resting potential
Recording and reference electrodes
amplifier oscilloscope
Experimental setup
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Resting potential
Across the membrane
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Changing membrane potential (creating unrest!)
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Graded potentials (decremental conduction)
stimulator
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The action potential-subthreshold-threshold of excitation-overshoot-afterpotential
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Propagation of the action potential-non-decremental conduction
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The ionic basis of membrane potentials
Diffusion gradients-substances move from areas of high concentration to areas of low concentration
Electrostatic gradients-substances can carry charge (ions)-substances move towards areas of unlike charge
Dynamic equilibrium-gradients can balance one another
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The ionic basis of membrane potential
A- = proteinK+=potassiumNa+=sodiumCl-=chloride
+cations-anions
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Dynamic equilibrium
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Dynamic equilibria
For some species of ion (such as potassium), the relative concentrations on either side of the membrane are pretty much mathematically predictable (Nernst equation) just based on the balance of electrostatic and concentration gradients.
For others (such as sodium) this is not the case.
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This can only mean….
The membrane is not equally permeable to all ions.
At least….not all the time!
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Nerve cell membranes are filled with interesting proteins
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The disposition of ions in the resting nerve cell
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Refractory periods
Note the difference between closed and inactivated sodium channels
-sodium channels only open briefly and then cannot open for some period of time
(absolute refractory period)
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The pufferfish as a neuroscientific weapon
About three o’clock in the morning we found ourselves seized with an extraordinary weakness and numbness all over our limbs. I had almost lost the sense of feeling; nor could I distinguish between light and heavy bodies of such as I had strength to move, a quart pot full of water and a feather being the same in my hand…. --Captain James Cook--
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Conduction of action potentials in unmyelinated axons
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Conduction of action potentials in myelinated neurons-faster, cheaper
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Consequences of demyelination -Multiple sclerosis
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Synopsis of synaptic action
•Transmitter release•receptor action (metabotropic vs. ionotropic receptors)•postsynaptic potentials (EPSP’s and IPSP’s)
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The synapse
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Electron micrograph of a synapse
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Synaptic vesicles fuse with presynaptic membrane (movement of vesicles is caused by entry of calcium into presynaptic terminal
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Freeze-fracture view of transmitter release
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Receptors - a special kind of membrane channel
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Ionotropic receptors
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Metabotropic receptors-G-protein coupled
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Varieties of postsynaptic potentials (PSP’s)
Excitatory (EPSP) usually when receptor channels
admit sodium Inhibitory (IPSP)
usually when receptor channels admit chloride
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Termination of synaptic action
1. Deactivation
2. Re-uptake by transporters
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Neural integration Spatial integration-ripples can collide and combine
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Neural integrationSpatial integration-ripples can go up (excitatory) or down (inhibitory)
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Neural integrationSpatial integration-ripples can combine
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Another look at spatial summation
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Neural integrationTemporal integration-ripples can combine to make bigger ripples