Q) Describe how nerve impulse travels along the axon of a myelinated neurone
Main points | |
Key idea: | Explain resting potential (-70mV) of neurone |
· Resting potential -70mV · Axoplasm of neurone more –ve charged than tissue fluid · Reasons : (i) Na/K pumps - actively transport 3 Na+ out, 2 K+ into cell per ATP hydrolyzed (ii) membr more permeable to 2 K+ than Na+ | |
Key idea: | Generation of action potential at Node of Ranvier |
· Action potential (AP) generated only at a node of Ranvier as other areas of neurone are insulated by myelin sheath · AP - rapid change in potential diff a/x axon membr · 3 stages – depolarization, repolarization and hyperpolarization. | |
Key idea: | Na+ channels during depolorization |
· Na+ channels open, influx of Na+, down conc gradient, into axoplasm · Membr potential becomes less -ve until threshold potential (e.g. -55mV) · More Na+ channels open, greater influx of Na+ · Membr potential changes from -70 to +40mV (i.e. depolarization) | |
Key idea: | Influx of Na+ at node of Ranvier depolarizes next node of Ranvier |
· Inside of neurone at point of stimulus is more +ve charged than outside · But, next node of Ranvier still at resting potential · Creates a local electrical circuit, allows Na+ to diffuse to next node causing Na+ channels open, influx of Na+ ; hence deplorization, new AP generated · Another new local electrical circuit is established b/w newly depolarized portion of axon and next node · Hence, AP ‘jumps’ from one node to another (i.e. saltatory conduction) | |
Key idea: | Unidirectional movement of nerve impulse along axon |
· At original point of impulse, Na+ channels close, K+ channels open, efflux of K+, down its conc gradient, out of the neurone · Restores resting potential as membr potential changes fr +40mV back to -70mV · K+ channels slow in closing, membr potential turns more -ve than -70mV (hyperpolarization) before resting potential is restored · Ref to refractory period where no new AP can be generated in response to a new stimulus · Result: unidirectional movement of nerve impulse | |
Comments: (i) Visualize the answer with the aid of a labelled diagram. (ii) Verbalize the answer + write down the main points w/o referring to the answer (use abbreviations) (iii) Read again within the next 24h & once more within 72h (do not spend more than 10min) (iv) The transmission of the action potential can be described as a wave of depolarization transmitted in one direction over infinite distance of the neurone with no change in speed and amplitude | |
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