refractory period • 1) absolute refractory period – as long as Na + gates remain open an AP is being generated, no stimulus of any strength will trigger another AP – lasts from the current AP to the next RMP
The Refractory Period • 2) relative refractory period – only an especially strong stimulus will trigger new AP – K + gates are still open and any affect of incoming Na + is opposed by the outgoing K + • refractory period only occurs at a small patch of the neuron’s membrane • other parts of the neuron/membrane can still be stimulated while the small patch is in the refractory period
Signal Conduction in Unmyelinated Fibers • for neural communication to occur, a nerve signal must travel to the end of an axon • unmyelinated fibers have voltage-regulated ion gates along their entire length • an AP from the trigger zone causes Na + to enter the axon and diffuse into adjacent regions just beneath the membrane • the depolarization excites voltage-regulated gates immediately distal to the AP • Na + and K + gates open and close producing new APs progressively distal down the axon • by repetition, the membrane distal to that is excited • this chain reaction continues all the way to the end of the axon The Nerve Impulse Video
Nerve Signal Conduction of Unmyelinated Fibers
Saltatory Conduction in Myelinated Fibers • voltage-gated channels are needed for an AP to occur – there are fewer than 25 voltage-gated channels per m 2 in the myelin-covered internodes – BUT… there are up to 12,000 per m 2 in the nodes of Ranvier • Na + diffuses very quickly on internodes – even though the signal weakens under the myelin sheath, it is still just strong enough to produce an AP at the next node of Ranvier • saltatory conduction – the nerve signal seems to jump/hop from node-to-node on its way down a nerve fiber/axon
Saltatory Conduction • saltatory conduction is much faster than conduction in unmyelinated fibers
Synapses • an AP can go no further once it reaches the end of the axon – So, what does it do? – the AP triggers the release of a chemical neurotransmitter (NT) from the synaptic knobs of the axon – NT diffuses across the synapse and produces a new AP in the next cell across the synapse • in a synapse between two neurons – the first neuron in the signal path is the presynaptic neuron that releases the chemical NT – the second neuron is the postsynaptic neuron that binds and responds to the NT • presynaptic neuron can synapse with the dendrite, soma, or axon of the postsynaptic neuron • a neuron can also have an enormous number of synapses – a spinal motor neuron is usually covered by about 10,000 synaptic knobs from other neurons • in the cerebellum of the brain, one neuron can have as many as 100,000 synapses
Structure of a Chemical Synapse • the synaptic knobs of a presynaptic neuron contain synaptic vesicles • the synaptic vesicles contain chemical neurotransmitters (NT) –
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- Spring '13