Voltage gated ion channels are scarce in myelinated

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Voltage-gated ion channels are scarce in myelinated internodesa.No action potentials can occur in the internodes 2.When Na+ enters the axon at a node of Ranvier, it diffuses for a short distance along the inner face ofthe axolemmaa.Each sodium ion had an electrical field around itb.When one Na+ moves toward another, its field repels the second ion, which moves slightly andrepels another c.This energy transfer travels down the axon much faster and farther i.The signal grows weaker with distance, partly because the axoplasm resists themovement of the ions and partly because Na+ leaks back out of the axon along the wayii.Therefore, there is a lower concentration of Na+ to relay the charge iii.With a surplus of positive charges on the inner face of the axolemma and a surplus ofnegative charge on the outer, these cations and anions are attracted to each other throughthe membrane1.This results in a “storage” of unmoving charges on the membrane3.Myelin speeds up signal conduction in 2 waysa.By wrapping tightly around the axon, it seals the nerve fiber and greatly increases its resistanceto the leakage of Na+ out of the axoni.Sodium ions maintain a higher density on the inner face of the membrane and transferenergy from one to another more rapidlyb.Myelin create a greater separation between the ICF and ECFi.Cations and anions of the ICF and ECF are therefore less attracted to each other1.Na+ ions can therefore move more freely along the axon4.A signal propagated in the foregoing manner cannot travel much farther than 1 nm before it becomes tooweak to open any voltage-gated Na+ channelsa.There is another node of Ranvier every millimeter along the axon, where the axolemma isexposed to the ECF and there is an abundance of voltage-gated channelsb.When the intermodal signal reaches this point, it is just strong enough to open these channels,admit more Na+ and create a new action potentiali.This action potential has the same strength as the one at the previous node1.Each node of Ranvier boosts the signal back to its original strength (+35 mV) 2.The generation of action potentials is a relatively time-consuming process thatslows down the nerve signals at the nodes5.Since action potentials occur ONLY at the nodes, this mode of conduction creates a false impression thatthe nerve signal jumps from node to nodea.Saltatory conduction – conduction in myelinated fibers 12.5 Synapses 1.A nerve signal soon reaches the end of an axon and in most cases, it triggers the release of aneurotransmitter that stimulates a new wave of electrical activity in the next cell across the synapse2.Two types of neurons a.Presynaptic neuron – neuron that releases a neurotransmitter
i.May synapse with a dendrite, the soma or the axon of a postsynaptic neuron, forming anaxodendritic, axosomatic or axoaxonic synapseb.Postysynaptic neuron – neuron that responds to the neurotransmitterThe Discovery of Neurotransmitters1.Synaptic cleft – gap between neuronsa.

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