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Introductory Neurophysiology

Introduction to Gastrocnemius Muscle

The Gastrocnemius muscle (Calf muscle) is generally present on posterior surface of lower limb. The muscle is innervated by Sciatic nerve. All the muscle fiber innervated by single muscle fiber constitutes the Motor unit. Stimulus produced by the nerve results in contraction of muscle. A single action potential causes a single twitch in muscle. The twitch response increases on continuous nerve stimulation and produce increased muscle contraction. Stimulus intensity can also be increased without changing the magnitude by mere changing the frequency of stimulation. The tension sums up from second stimulus before relaxation of tension from first stimulus. This causes an increase in tension of muscle on subsequent frequencies resulting into Wave summation. Tetanus can occur when this tension reaches at its peak level. Stimulus intensity is further increases on increasing voltage of stimulation resulting into activation of more nerve fibers.

In this background, this experiment is performed to investigate and observe the impact of changes in stimulus intensity and stimulus frequency on contraction of Gastrocnemius muscle through twitch and titanic response.

Method of Introductory Neurophysiology

The procedure was performed in two parts, Muscle activity upon titanic contraction of muscle and nerve conduction in toad preparation. The apparatus constitute of a specimen of Gastronemius musce from back of toad, along with intra-section of sciatic nerve which goes to spinal cord.

There is specimen Tranduca which connect via string to Gastronemius muscle. The muscle contract when stimulated and contraction will be registered by the actual displacement of tranduca to offer voltage. Transduca is attached with below by nerve. Nerve is further connected with electrodes which produce voltage on stimulation. This results in contraction of muscle.

The experiment starts with a minimum threshold stimulation which can cause twitch to muscle. It initially stimulates minimum number of fibers in nerve bundle to induce muscle contraction. On subsequently increasing voltage, amplitude of contraction starts increasing slowly, until it reaches to maximum twitch. This can help in measuring light intensity of muscle twitch, amplitude and drop of light from the graph. Here, a single pulse has caused a single twitch in muscle.

In titanic contraction, multiple pulses of activation are processed from nerve and muscle did not respond. Pulses add on, to each other. A second twitch is generated, before muscle from first twitch gets relaxed and stimulus reckon up. Here, strength of contraction was higher than a singles twitch. On increasing frequency to 50/sec, muscle did not get chance to relax and there was a single line of contraction. There was absence of any relaxation drops in graph, leading to full titanic contraction.

Now, the Gastronemius muscle and nerve gets remove from Transduca. They were transported to rectangular bath, full of electrodes. The electrodes are set at equally spaced interval. Electrodes help to record a difference in action potential. This Action potential shows a bipolar response on different stimulus. The difference between the peaks of two bipolar responses projects, the difference in time. The difference between electrodes gives the distance. Conduction velocity of nerve can be calculated with the help of measured distance and time.

Result of Introductory Neurophysiology

Twitch response- The stimulus intensity was started with 0.13V which showed a flat line with no twitch response. The intensity was increased simultaneously to 0.14V. It initiated a response on lab chart and thus, the threshold stimulation was 0.14V. The response continued to amplify further, increasing stimuli to 0.2 V, 0.3V, 0.4V, 0.5V and 0.6V. The response at 0.4V was little faint and smaller. At this stage, muscle mass may get fatigue on continue stimulation. The stimulation reached to its threshold at 0.6V as there was no further response on lab chart. [Table1]

Titanic muscle response- Here, the stimulus was kept a little higher than threshold value which was received during the experiment of twitch response i.e. 0.2V. Instead of increasing voltage, frequency (Hertz) was increased subsequently. The experiment was initiated with 1 Hz at 0.2V which increased to 10Hz, 20 Hz, 30 Hz, 40 Hz and 50 Hz. Summation was observed from the response of pulses with increasing frequency. The graphical line becomes flat on lab chart at 50 Hz. It depicts that nerve muscle has reached the value of tetanus. [Table2]

Nerve conduction Velocity was calculated at 0.2V stimulus and 0.1 cm displacement. The red and black electrodes were positioned at end of muscle and green electrode in middle. The distance between electrodes was 2cm on the side of red electrode and 4cm on black electrodes.

Discussion on Introductory Neurophysiology

This procedure is performed on Gastrocnemius muscle to check the impact of changes in stimulus intensity and stimulus frequency of sciatic nerve through twitch and titanic response. The intensity started increasing from 0.14V and reaches to its threshold on 0.6 V where no further response was observed on lab chart.

In this experiment, the stimulus was set at 0.2V and frequency was increased. The stimulus depicted a titanic response at 50 Hz. Similar response was obtained from an experiment done by Celichowski J et al. where titanic depression was performed on Gastrocnemius muscle of cat. The depression was observed at 30 and 40 Hz (1). Proske U also demonstrated similar twitch and titanic response on lizard in their study (2). Krysciak K investigated the electrical properties of muscle and effect of muscle overload on twenty rats. Similar titanic response was observed at higher frequency as compared to control. The frequency was increased from 20Hz to 100 Hz, initiating wave summation at 20 Hz and causing tetanic response at 100 Hz (3). Allen DG has covered the concept of muscle fatigue in there article where tetany occurs on continuous increase in frequencies. Though, muscle temperature, time of stimulation, extracellular and intracellular components also plays a major role and thus, more studies are required to assess their importance (4). Grottel K also found a correlation between twitch forces, diameters and amplitudes of action potentials in motor unit (5). Such experiments can help in analyzing overload of motor unit and optimizing training loads. Trainers, patients and athletes can schedule their training in light of proper guidance through these studies. However, the experiment was performed in lab. Hence, more experiments are needed to be encouraged among animals and humans to support such evidence at large scale. There is need to measure threshold value of tetany in other muscles like myocardium in near future.

References for Introductory Neurophysiology

Celichowski J, Krutki P, ?ochy?ski D, Grottel K, Mróczy?ski W. Tetanic depression in fast motor units of the cat gastrocnemius muscle. J Physiol Pharmacol. 2004; 55 (2):291?303.

Proske U. The muscle spindles in slow and twitch skeletal muscle of the lizard. J Physiol. 1973; 230(2):429?448. doi:10.1113/jphysiol.1973.sp010196

Kry?ciak K, Celichowski J, Drzyma?a-Celichowska H, Gardiner PF, Krutki P. Force regulation and electrical properties of motor units in overloaded muscle. Muscle Nerve. 2016; 53(1):96?106. doi:10.1002/mus.24690

Allen DG, Lamb GD, Westerblad H. Skeletal muscle fatigue: cellular mechanisms. Physiol Rev. 2008; 88(1):287?332. doi:10.1152/physrev.00015.2007

Grottel K, Celichowski J, Kowalski K. Twitch force and action potentials of single motor units in medial gastrocnemius muscle of the rat. Acta Neurobiol Exp (Wars). 1988; 48(2-3):71?81.

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