Lecture 21 – Nervous System III

Enough to reach threshold the immediately adjacent

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enough to reach threshold, the immediately adjacent inactive zones will become active  and fire their own AP The more distant zones will not receive enough current to generate an AP In the magnitudes of the outward currents decrease smoothly with increasing distance  from the active zone Myelinated Current flows only through the nodes Nodes have a very high density of Na channels Current does not flow through the intermodal membranes because myelin has a high  resistance
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In the PNS, the Schwann Cells In CNS, Oligodendrocytes No “leakage” of ions out of the membrane, so it isn’t depolarized Spacing is critical because it has t get to the next node and depolarize it As a result, the current flowing down the axon is conserved, and the current density at  the nodes is very high This high current density results in the generation of an action potential at the node Thus, the active zones jump in a salutatory manner from one node to another The intermodal membranes may not even fire an AP Rate of AP Propagation Large diameter propagates action potentials faster Large fibers are less resistant to current flow Lower resistance More ions will flow per unit time which brings adjacent regions to threshold potential  more rapidly Myelination Myelinated axons makes it difficult for current to flow in and out of the cell There is ales leakage of charge and a local current can spread a lot faster Velocity of Conduction Because there is less leakage of charge across the cell membrane, local current can  spread father along the membrane The density of voltage gated Na channels in myelinated regions of the axon is low Action potential only occurs at the nodes of Ranvier where the myelin coat is interrupted  and the concentration of voltage gated channels is high Propagation in myelinated fibers is called salutatory conduction Synapses Activity of synapses can increase or decrease the likelihood of AP generation in the  postsynaptic cell Mechanism:  graded  depolarizing or hyperpolarizing MP AP – electrical Synapse – chemical To electrical AP effect Neuron can be synapsing with up to 100k other cells Complex methods of integration Electrical Synapses
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Simple, but uncommon Gap junctions Rapid communication Ions, second messengers Identified in retina, cortex, brainstem, and hypothalamus Bidirectional Chemical End of axon has swollen end – axon terminal
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