Mel

Mel's part of Proposal: 1) MEPs – what are they, overview of how they are affected by physical interventions (such as exercise, briefly) The term ‘motor evoked potentials’ (MEPs) is used to describe potentials obtained from muscles by the stimulation of motor structures in the central nervous system (Chiappa, 1997). These potentials can be caused by stimulation along any of the motor pathways, but the cerebral cortex and the cervical spinal cord are the most commonly stimulated areas. Direct electrical activation of the motor pathways using the cerebral cortex has been used in clinical experiments for many years. A study in 1980 by P. A. Merton and H. B. Morton used transcranial electrical stimulation (TES) to cause non-invasive stimulation of MEPs. This method was further refined in 1985 by Barker et al., with the introduction of transcranial magnetic stimulation (TMS). Although peripheral muscles and nerves were first investigated with TMS, studies nowadays often focus on stimulation of the cortex. TMS has been found to be a very effective method to measure MEPs. As stated by Dishman (2000) “…TMS allows for the recording of MEPs from virtually any muscle.” When using TMS to measure MEPs, there is electrical activation over the region being stimulated via an anode and a cathode at a distal site measuring the MEP response. When stimulating an area of cortex, different neural components of the precentral gyrus may become stimulated – the dendrites, the cell bodies or the axon hillocks. MEP generation depends on the activation of the axon hillocks. With magnetic stimulation,” an intense current in and external coil induces local depolarising electrical currents that flow through the neuron and axon hillock” (Daube, 2002). It is this depolarisation that initiates descending action potentials in the corticospinal pathways and cause an MEP to be produced. The size of the MEP obtained from muscle stimulation is a measure of the size or effectiveness of the corticospinal output caused by the stimulus, and also the excitability of the neurons in that pathway (MacDonell et al., 1991). An MEP reading consists of an initial D (direct) wave is followed by several I (indirect) waves, which come at periodic intervals (usually about 1 millisecond) or ‘latency period’, being the time between the MEP waves and the stimulation. D-waves represent the “direct excitation of corticospinal tract neurons” (Jasvinder, 2009), while I-waves reflect “indirect depolarization of the same axons via corticocortical connections” (Jasvinder, 2009). Larger MEPs may be produced with increased excitability of cortical and spinal neurons due to not only direct stimulation of these structures, but also stimulation of surrounding neurons (Capaday and Stein 1987; Butler et al, 1993). Motor neurons that contribute directly to the muscle activation may be unresponsive at the time of stimulation, but as found by Darling, et al.,(2006) “…for low level contractions one would only expect only a small percentage would be in this state because force is maintained by asynchronous activation of a number of motor units.” MEPs in muscles may be elicited by TMS with or without voluntary contraction of the muscle. However, as found by Rothwell et al.,(1991), Maertens de Noordhout et al., (1992), and Hauptman and Hummelsheim (1996), “voluntary preactivation is known to increase the magnitude of motor evoked potentials”. Although with increasing exercise levels the amplitude of MEPs decreases. This was found by Joaquinn, et al., (1993), where a “transient decrease in the amplitude of MEPs to TMS following a period of repetitive muscle activation to the point of subjective fatigue in healthy humans”. The findings from this study were supported by Giampietro, et al., (1995), in that decreases in MEP amplitude following repetitive exercises may indicate a level of central nervous system neurophysiological fatigue. The results from both of these studies also support the theory of this postexercise exhaustion to be due to a depletion of the acetylcholine that is immediately available. The major values of obtaining MEPs and carrying out MEP studies have been in measuring nervous system pathways inoperatively, and also clinically in conditions of the nervous system such as multiple sclerosis, where slowing of impulse conductions can be measured and monitored. In the case of this study the measuring of MEPs will help determine the effectiveness of a particular manual therapy technique, and what effects it does have at a neurophysiological level.

REFERENCES USED: http://emedicine.medscape.com/article/1139085-overview Neurology 1991;41:1441–4.
 * 1) Motor evoked potentials. Chawla Jasvinder. Medscape November 25 2009.
 * 1) Evoked Potentials in Clinical Medicine (third edition). Keith H Chiappa. 1997 by Lippincott-Raven Publishers. Philadelphia, PA, USA.
 * 2) Macdonell RA, Shapiro BE, Chiappa KH, et al. Hemispheric threshold differences for motor evoked potentials produced by magnetic coil stimulation.
 * 1) Clinical Neurophysiology (Contemporary Neurology Series, 66) (second edition). Jasper R Daube. 2002 by Oxford University Press, Inc. New York, New York, USA.
 * 2) Abercromby A, Amonette W, Layne C, McFarlin B, Hinman M & PaloskiW(2007). Variation in neuromuscular responses during acute whole-body vibration exercise. //Med Sci Sports Exerc// **39**, 1642–1650.
 * 3) Postexercise depression of motor evoked potentials: a measure of central nervous system fatigue. Joaquinn P. Brasil-Neto, Alvaro Pascual-Leone, Josep Valls-Solé, Angel Cammarota, Leonardo G. Cohen, Mark Hallett. Experimental Brain Research (1993)93: 181-184.
 * 4) Long-lasting depression of motor-evoked potentials to transcranial magnetic stimulation following exercise. Zanette Giampietro, Bonato Claudio, Polo Alberto, Tinazzi Michele, Manganotti Paolo, Flaschi Antonio. Experimental Brain Research (1995) 107:80-86.
 * 5) DISHMAN
 * 6) Darling, Warren G., Wolf, Steven F., Butler, Andrew J. Variability of motor potentials evoked by transcranial magnetic stimulation depends on muscle activation. Experimental Brain Research 2006. 174: 376-385.

2) Aim The aim of this study is to determine whether high velocity, low amplitude (HVLA) techniques can alter the corticospinal excitability of the motorneurons innervating the gastrocnemius muscle using transcranial magnetic stimulation (TMS). Previous studies have investigated the effects of HVLA using TMS in the paraspinal muscles and the gastrocnemuis muscles, finding that HVLA has a transient increase in central motor excitability. This study aims to build on the results from these studies and to further investigate the effects of HVLA techniques at a neurophysiological level.

SUMMARY Spinal manipulation or high velocity, low amplitude (HVLA) techniques are widely used by the manual therapists. This technique has been found to have a great analgesic effect on the body, not only physiologically (stretching joint capsules, resetting neural pathways, etc.) but also can have a psychological placebo effect on patients. Specifically osteopaths believe that the proper application of HVLA to certain areas of the body can greatly aid in the reduction of TART findings (tissue texture change, asymmetry, decreases in ranges of motion and tenderness), reduce pain levels and have effects more distal to the application site as well. Knowing that the nerve roots that innervate the gastrocnemius muscle originate from the lumbosacral region, manipulation of this area may help reduce clinical findings in this area of the lower limb. Previous studies have been conducted into the effects of HVLA at a neurophysiological level. One of the proposed theories of the mechanism of HVLA is facilitation or sensitisation, meaning that by the technique causes an increase in the excitability or responsiveness of the dorsal horn in the spinal cord, resulting in changes in reflex activity and/or central integration. Other studies have found that manipulation can cause mechanical hypoalgesia, resulting in increases in the pain pressure threshold and increased sympathetic outflow. These studies have looked at various muscular outputs including the H-reflex and muscle evoked potentials (MEPs) from HVLA techniques. The H-reflex is the electrically induced version of the spinal stretch reflex (monosynaptic), whilst MEPs are produced by stimulating higher order regions (the cerebral cortex) to produce depolarisation of descending motor pathways and an MEP to be elicited from a certain muscle. In 1985 a study by Barker et al., pioneered the use of transcranial magnetic stimulation (TMS), a non-invasive technique which used stimulation of the cortex to elicit action potentials through the axon hillocks of specific neurons in specific neural pathways. Since then TMS has been found to be a very effective means of stimulating MEPs, clinically aiding in measuring nervous system pathways inoperatively. In the case of this study, TMS will be used to stimulate the descending motor pathways to the lower limb, namely to the gastrocnemuis muscle, and MEPs recorded. An MEP consists of 2 parts, an initial direct wave followed by several indirect waves. MEPs have been found to be elicited by TMS with or without voluntary contraction of that muscle, however voluntary preactivation has been found to increase the magnitude of the waves produced. The aim of this study is to determine whether high velocity, low amplitude (HVLA) techniques can alter the corticospinal excitability on the motorneurons innervating the gastrocnemius muscle using transcranial magnetic stimulation (TMS). This study intends to build on the results from these studies and investigate the effects of HVLA techniques at a neurophysiological level. HYPOTHESIS? METHOD?