Anatomy Chapter 4.docx

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Biological Sciences
Connie Soros

Chapter 4 Neuronal Physiology • Terms - excitable tissues: capable of producing electrical signals (transient, rapid changes in membrane potential), nerve & muscle - resting membrane potential: the membrane potential that exists when no net changes in potential are occurring - graded potentials: local changes in membrane potential that vary in magnitude (flow of ions) - action potentials: brief, rapid reversals in membrane potential, which can spread throughout the membrane (flow of ions) - voltage-gated channels: membrane channels that open or close in response to changes in potential - polarization: a membrane that has potential is polarized - depolarization: a decrease in membrane potential (inside becomes more positive) - triggering event: event that initiates a depolarization (stimulus like light or touch, chemical messenger) - hyperpolarization: an increase in membrane potential (inside becomes more negative) - repolarization: return to resting potential after a depolarization • Graded Potentials - magnitude of graded potential related to magnitude of triggering event 1. stronger trigger  greater magnitude of change in potential - current flow (movement of charges) 1. when a graded potential occurs, a piece of the membrane (called the active area) has a different potential than the rest of the membrane (which is at resting potential, called the inactive area) a. current flows between active area and adjacent inactive areas (opposite charges attract) b. previously inactive areas become active and more current flow occurs 2. spread of graded potential is decremental a. decreases as it moves along the membrane b. current leaks into ECF c. function as signals over short distances 3. usually not an actual reversal of charges - just a reduction in potential (inside becomes less negative than before, small depolarization) 4. important for nerve and muscle cells (e.g., postsynaptic potentials) • Action Potentials (AP) - can be transmitted over long distances without losing strength - Depolarization 1. triggering event causes depolarization to occur relatively slowly until threshold potential is reached (about -50 to -55 mV) a. once threshold is reached, membrane quickly depolarizes to +30 mV + (1) when triggering event begins depolarization, some of the voltage-gated Na + channels open, Na flows into cell (proteins that make up the channel have charged portions, shape change occurs as those charges interact with charges surrounding the membrane) + (2) this further depolarizes the membrane, causing even more Na channels to open + (3) at threshold all the Na channels are open and there is an explosive increase + + in Na permeability (P Na ) + (4) at peak depolarization, the Na channels close (the channel is constructed so that the same depolarization that opens them also closes them) - Repolarization begins + + + 1. as Na channels close, K channels open (P K increases) due to delayed voltage-gated + response to the depolarization, K flows out of cell 2. this restores internal negativity 3. as repolarization progresses... + a. Na channels resume original conformation (closed but capable of opening) + b. newly opened K channels close (1) hyperpolarization occurs before channels close (membrane even more negative than at resting potential) (2) resting potential restored - AP lasts about 1 millisecond - ion gradient restored + + 1. Na -K pump restores ion gradients a. important for long term maintenance of gradient b. not necessary between APs (1) ion shifts during AP are not so great that they wipe out concentration gradients, so many APs can occur in succession • Neurons (nerve cells) - 3 basic parts 1. cell body a. houses nucleus and organelles b. receives signals from other cells (contains receptors for chemical messengers) 2. dendrites a. projections from cell body b. increase surface area for receiving signals 3. axon (nerve fiber) a. single elongated projection b. conducts APs away from cell body c. often has collaterals (side branches) d. axon hillock (part of cell body and first part of axon) is area where APs generated in most neurons e. ends in branches called axon terminals that release chemical messengers f. may be less than a mm or more than a m • Propagation of an AP - Conduction by local current flow (contiguous conduction) 1. AP at axon hillock a. local current flow between this active area and adjacent inactive area causes new AP b. AP passed section by section along axon - Saltatory conduction 1. occurs in myelinated fibers a. special cells form barrier that is impermeable to ions (1) wrap around fiber (2) mostly lipids (3) formed by oligodendrocytes in central nervous system (CNS), by Schwann cells in peripheral nervous system (PNS) b. nodes of Ranvier (1) bare spaces between myelin + (2) contain Na channels 2. AP "jumps" from node to node a. much f
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