Editing F wave (section)

== Physiology ==
F-waves are evoked by strong electrical stimuli (supramaximal) applied to the skin surface above the distal portion of a nerve.<ref name=":2" /> This impulse travels both in [[orthodromic]] fashion (towards the [[muscle fibers]]) and [[antidromic]] fashion (towards the cell body in the spinal cord) along the [[alpha motor neuron]].<ref name=":3" /><ref name=":5" /><ref name=":8"/><ref>{{Cite journal|last1=Sathya|first1=G. R.|last2=Krishnamurthy|first2=N.|last3=Veliath|first3=Susheela|last4=Arulneyam|first4=Jayanthi|last5=Venkatachalam|first5=J.|date=2017|title=F wave index: A diagnostic tool for peripheral neuropathy|journal=The Indian Journal of Medical Research|volume=145|issue=3|pages=353–357|doi=10.4103/ijmr.IJMR_1087_14|issn=0971-5916|pmc=5555064|pmid=28749398|doi-access=free}}</ref> As the orthodromic impulse reaches innervated muscle fibers, a strong direct motor response (M) is evoked in these muscle fibers, resulting in a primary [[Compound muscle action potential|compound muscle action potential (CMAP)]].<ref name=":2" /><ref name=":5" /> As the antidromic impulse reaches the [[Soma (biology)|cell bodies]] within the [[Anterior horn of spinal cord|anterior horn]] of the motor neuron pool by retrograde transmission, a select portion of these alpha motor neurons, (roughly 5-10% of available motor neurons), 'backfire' or rebound.<ref name=":1" /><ref name=":2" /><ref name=":3" /><ref name=":4" /> This antidromic 'backfiring' elicits an orthodromic impulse that follows back down the alpha motor neuron, towards innervated muscle fibers. Conventionally, axonal segments of motor neurons previously depolarized by preceding antidromic impulses enter a [[Hyperpolarization (biology)|hyperpolarized]] state, disallowing the travel of impulses along them.<ref name=":7">{{Cite book|last=Kimura|first=Jun|title=Clinical Neurophysiology of Motor Neuron Diseases|chapter=Peripheral nerve conduction studies and neuromuscular junction testing|date=2004-01-01|chapter-url=http://www.sciencedirect.com/science/article/pii/S1567423104040122|series=Handbook of Clinical Neurophysiology |volume=4|pages=241–270|editor-last=Eisen|editor-first=Andrew|publisher=Elsevier|language=en|access-date=2020-02-26|doi=10.1016/S1567-4231(04)04012-2|isbn=9780444513595}}</ref> However, these same axonal segments remains excitable or relatively depolarized for a sufficient period of time, allowing for rapid antidromic backfiring, and thus the continuation of the orthodromic impulse towards innervated muscle fibers.<ref name=":7" /><ref name=":8">{{Cite journal|last1=Panayiotopoulos|first1=C. P.|last2=Chroni|first2=E.|date=1996|title=F-waves in clinical neurophysiology: a review, methodological issues and overall value in peripheral neuropathies|journal=Electroencephalography and Clinical Neurophysiology|volume=101|issue=5|pages=365–374|issn=0013-4694|pmid=8913188}}</ref> This successive orthodromic stimulus then evokes a smaller population of muscle fibers, resulting in a smaller CMAP known as an F-wave.<ref name=":2" />

Several physiological factors may possibly influence the presence of F-waves after peripheral nerve stimulation. The shape and size of F-waves, along with the probability of their presence is small, as a high degree of variability exists in motor unit (MU) activation for any given stimulation.<ref name=":3" /> Thus, the generation of CMAP's which elicit F-waves is subject to the variability in activation of motor units in a given pool over successive stimuli.<ref name=":6" /> Moreover, stimulation of peripheral nerve fibers account for both orthodromic impulses (along sensory fibers, towards the dorsal horn), as well as antidromic activity (along alpha motor neurons towards the [[Anterior horn of spinal cord|ventral horn]]).<ref name=":3" /> Antidromic activity along collateral branches of alpha motor neurons may result in the activation of inhibitory [[Renshaw cell]]s or direct inhibitory collaterals between motorneurons.<ref>{{Cite journal|last1=Moore|first1=Niall J.|last2=Bhumbra|first2=Gardave S.|last3=Foster|first3=Joshua D.|last4=Beato|first4=Marco|date=2015-10-07|title=Synaptic Connectivity between Renshaw Cells and Motoneurons in the Recurrent Inhibitory Circuit of the Spinal Cord|journal=The Journal of Neuroscience|volume=35|issue=40|pages=13673–13686|doi=10.1523/JNEUROSCI.2541-15.2015|issn=0270-6474|pmc=4595620|pmid=26446220}}</ref> Inhibition by these means may lower excitability of adjacent motor neurons and decrease the potential for antidromic backfiring and resultant F-waves; although it has been argued Renshaw cells preferentially inhibit smaller alpha motor neurons limited influence on modulation of antidromic backfiring.<ref name=":5" />

Because a different population of anterior horn cells is stimulated with each stimulation, F waves are characterized as ubiquitous, low amplitude, late motor responses, which can vary in amplitude, latency and configuration across a series of stimuli.<ref name=":3" /><ref>{{Cite journal|last1=Fisher|first1=Morris A.|last2=Patil|first2=Vijaya K.|last3=Webber|first3=Charles L.|date=2015|title=Recurrence Quantification Analysis of F-Waves and the Evaluation of Neuropathies|journal=Neurology Research International|volume=2015|pages=183608|doi=10.1155/2015/183608|issn=2090-1852|pmc=4672360|pmid=26688754|doi-access=free}}</ref>