If axons become hyperpolarized for example, after conducting trains of impulses during a sustained muscle contraction , they are less excitable and more current is required. This technique of threshold tracking provides measures of excitability which are determined by the electrical properties of the axonal membrane at the point of stimulation. The procedure differs from nerve conduction studies in which a supramaximal stimulus is used to ensure that all axons within the nerve are excited.
The field of nerve excitability owes a debt to Joseph Bergmans who, in , documented the excitability properties of single human motor axons. He found that single motor axons could be selectively activated over a large range of stimulation strengths and that the same motor units could be activated by threshold stimuli on repeated occasions when the stimulating electrodes were placed in the same location.
He defined threshold as the lowest stimulation strength that would elicit three to five consecutive motor unit potentials a motor unit potential is the potential generated by the muscle fibres innervated by a single motor axon and then painstakingly adjusted stimulus voltage by hand to track changes in the threshold of the axon. He made meticulous and methodical recordings from single motor units in order to study recovery from single and repetitive activation and the influence of ischaemia, electrical polarization and temperature on these processes.
From these non-invasive, in vivo studies, Bergmans was able to describe the mechanisms responsible for the different phases of post-tetanic hyperpolarization. The development of an automated technique for tracking the threshold of a compound sensory or muscle action potential Bostock et al.
In a matter of minutes, the stimulus—response relationship, strength—duration properties, accommodation to subthreshold changes in membrane potential and recovery after activation can be measured, providing information about both nodal and internodal conductances.
The measurements are made non-invasively and in real time, allowing the time course of interventions such as drug administration and renal dialysis to be followed. Many studies have been conducted using this tool and there is now a considerable body of literature describing findings in normal nerves and in numerous pathophysiological conditions ranging from nerve disorders, such as channelopathies and demyelinating conditions, to metabolic disorders, such as renal failure and diabetes.
Further, differences between the excitability properties of axons in the upper and lower limbs and also between the proximal and distal segments within the same nerves have been found. The technique has also been successfully applied to the study of animal nerves and single axons , giving rise to a body of literature on the changes in axonal excitability with maturation.
Automated threshold tracking has recently been applied to investigate the excitability of selectively activated single motor units a motor unit being the motoneurone, its motor axon and the muscle fibres it innervates to determine whether the reason that they have a low threshold to electrical recruitment is due to intrinsic properties Trevillion et al. Threshold tracking enabled comparisons to be made between the excitability properties of the single motor axons with those of higher threshold motor axons recruited in compound potentials recorded in the same experimental session.
However, this proved not to be the case. The key point of differentiation between the axons was found to be in the accommodation to hyperpolarizing currents Fig. Low-threshold axons were found to have greater accommodation to hyperpolarizing changes in membrane potential, presumably due to a greater hyperpolarization-activated inwardly rectifying conductance, Ih.
This conductance has been shown to have a pacemaker role in spontaneously firing neurones and the sinoatrial node. HCN channels mediate the inwardly rectifying conductance and are known to exist in four isoforms. All have been found in the mammalian nervous system, although channel expression in the human peripheral nervous system is yet to be characterised. Also at about this time, sodium channels start to close. This causes the action potential to go back toward mV a repolarization.
The action potential actually goes past mV a hyperpolarization because the potassium channels stay open a bit too long.
Gradually, the ion concentrations go back to resting levels and the cell returns to mV. Lights, Camera, Action Potential This page describes how neurons work.
Resting Membrane Potential When a neuron is not sending a signal, it is "at rest. Action Potential The resting potential tells about what happens when a neuron is at rest. And there you have it Do you like interactive word search puzzles? Read about the physical factors behind the action potential.
Nerve Signaling - from NobelPrize. The giant axon of the squid can be to times larger than a mammalian axon. Not earth-shattering but has some interesting implications for previous studies that base their learning results upon DG plasticity.
Some facts on neuronal excitability: Excitable: the ability to fire action potentials. More excitable: fires action potentials, but more. More LTP: not the same as more excitable. Less inhibition: also not the same as more excitable, though the two may go hand in hand. The Scholarpedia page on neuronal excitability , which was last modified on 13 August , has been accessed 49, times, and contains no information peer review is slow.
Young neurons are spikier. Access to the complete content on Oxford Handbooks Online requires a subscription or purchase. Public users are able to search the site and view the abstracts and keywords for each book and chapter without a subscription.
Please subscribe or login to access full text content. If you have purchased a print title that contains an access token, please see the token for information about how to register your code. For questions on access or troubleshooting, please check our FAQs , and if you can''t find the answer there, please contact us. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use for details see Privacy Policy and Legal Notice.
Oxford Handbooks Online. Publications Pages Publications Pages.
0コメント