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Nervous Tissue: Types of Synapses and Neural Integration
Nervous Tissue: Types of Synapses and Neural Integration

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Northeastern University
BIOL 1117
Christopher Richardson

L24-Nervous Tissue 11/6/13 • The Refractory Period o during an action potential and for a few milliseconds, region of neuron is difficult to stimulate to fire again. o refractory period: the period of resistance to stimulation o two phases of the refractory period  absolute refractory period • no stimulus of any strength will triggerAP • from action potential to RMP • This is as long as the Na+ gates are open or inactivated. • Need to remove the inactivating Na+ gate mechanism and then close Na+ gates before can reopen the Na+ gates or channels in response to another stimulus  relative refractory period • only especially strong stimulus will trigger new AP • This period lasts until hyperpolarization ends; lasts as long as K+ gates open o Requires a strong voltage stimulus to open enough Na gates to allow in enough Na+ to shift the inside of the cell back towards positive because K+ is still leaving as Na+ enters  refractory period is occurring only in a small patch of the neuron’s membrane at one time  Refractory periods limit the total number of action potentials in a given time period • Signal Conduction in Unmyelinated Fibers o for communication to occur, the nerve signal must travel to the end of the axon o unmyelinated fiber has voltage-gated ion channels along its entire length + o action potential at the trigger zone causes Na to enter the axon and diffuse into adjacent regions beneath the membrane o the depolarization immediately excites voltage-gated channels next to the action potential. o Na and K gates open and close producing a new action potential; by repetition the adjacent membrane next over is excited o An action potential does not travel along an axon; it stimulates the production of a new action potential just ahead of it. o No one action potential travels to the end of an axon. o Anerve signal is a chain reaction of action potentials.An action potential is therefore different from a nerve signal.A nerve signal is a traveling wave of excitation produced by the self-propagation of action potentials. o Action potentials do not travel backward because the membrane behind the nerve signal is still in its refractory period. o Anerve signal moves much more slowly than current in a wire o the last action potential has the same voltage as the first one because at each point along the nerve fiber, the RMP and ion gradient is the same. • Saltatory Conduction Myelinated Fibers o voltage-gated ion channels are scarce in the myelin covered internodes + o fast Na diffusion occurs between nodes  signal weakens under myelin sheath, but still strong enough to stimulate an action potential at next node  Decremental as it moves under the axolemma; it cannot travel down the fiber too far before it dissipates; the nodes occur ever few mm; nodes have many volt-gated ion channels o When the diffusing ions reach a node, these gates are opened and a new action potential is generated o saltatory conduction: the nerve signal seems to jump from node to node • Synapses o synapse is an anatomically specialized junction between two neurons  1st neuron in the signal path is the presynaptic neuron • releases n
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