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Motor ReflexJeanne Charoy

I. INTRODUCTION

IN order to survive, living organisms are constantlydetecting changes in their environment and reacting

accordingly. Reflexes, stereotyped rections of the CentralNervous System to sensory stimuli, are a good example ofthis kind of stimulus-response behavior.In this paper, we will be describing experimental observationsof one of the reflex arcs involved in controlling the motorsystem, the monosynaptic arc of the stretch reflex. Indeed,we measured the knee-jerk reflex, also known as the patellarreflex, as well as the ankle-jerk reflex on a relax volunteerusing EMG. We also gave an attempt at recording the H-reflexwhich proved unsuccessful. To discuss the H-reflex we willrefer to recordings giving to us with the lab handout. All ourresults and observations will be discuss below.

II. KNEE JERK REFLEX

The knee-jerk reflex is evoked when the tendon of thequadriceps muscle below the kneecap is stroked.The normalresponse should be for the muscle to contract and the leg togo up very briefly. This relfex is often tested during medicalexamination and most of us have memories of the doctorhitting us with a hammer.

Method

A relaxed volunteer sat with her leg hanging freelyover the edge of the chair. We then placed two electrodes,approximately 5 cm apart, on the quadriceps muscles on thefront thigh and a ground electrode inside of the ankle on thesame leg. The electrodes were plug into a transducer, whichallowed us to observe the EMG activity in the form of graphson the computer.A reflex hammer was also used. Like the electrodes, it wasplugged to the transducer, which allowed us to know preciselyat what time and with what amplitude the shock occurred.This also was also converted and displayed on the computerscreen.As seen in Fig. 1, we then proceeded to lightly hit thevolunteer under her kneecap, in order to elicit the reflex. Werepeated the maneuver several times in order to get 5 goodrecordings and have enough data to make some conclusions.In a second experiment, we proceeded in the same way butfor one detail. The volunteer was this time executing what iscalled the Jendrassik maneuver while getting hit on the knee.The Jendrassik maneuver is a medical maneuver wherein thepatient flexes both sets of fingers into a hook-like form andinterlocks those sets of fingers together (definition from thelab handout). While the subject was doing this, he was hit

under his kneecap, like during the regular knee-jerk reflex test.

Results

As we can see in Fig. 1, the EMG activity recorded showsthat, a short period of time after having been hit, the musclecontracted slightly. The peak EMG response averaged at0,620 mv. This period of time is also called latency or reflextime. From our recordings, the latency averaged at around 28ms (Fig. 2)

Fig. 1. Example of a graph we obtained while testing the knee-jerk reflex.The top graph is generated by the hit of the hammer (the stimulus) while thegraph below is the recorded EMG activity (the response)

Fig. 2. The recorded latency, as well as the time of the peak EMG response,in both normal conditions and Jendrassik maneuver conditions for the Knee-Jerk reflex

In the case of the Jendrassik maneuver, we observed moreor less the same latency (around 30ms). The peak EMG

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response, however, was significantly higher, as it averagedat 1.050 mv, almost twice as much as in normal conditions.(Fig. 3)

Fig. 3. Differences between the latency and the peak EMG activity in normalconditions and Jendrassik maneuver conditions. While the latency doesn’t varymuch, we can see a clear difference in the EMG response.

Discussion

As we expected, a light hit under the kneecap seems tomake the quadriceps muscle of the thigh contract which inturns makes the leg go up, and this all happens involuntarily.When we hit the tendon with the hammer, it causes the muscleto stretch a little. Inside muscles we can find a number ofthinner and shorter muscle fibers encased in a connective tissuecapsule. Those are called the muscle-spindles, a stretch recep-tor. They are innervated with a type of afferent fibers called theGroup 1a fibers. When muscle-spindles are stretched, the firingof the 1a fibers increase, when muscle-spindles are relaxed, thefiring of the 1a fibers decreases and can reach 0. In short, themuscle-spindles signal the length of the muscle.1a fibers enter the spinal cord through the dorsal roots andthere form excitatory synapses in the anterior horn withhomonymous motoneurons, which, when excited, cause thesame extensor muscle to contract. As a result, a brief stretchingof the muscle will produce, after a short latency, a contractionof the muscle. This is a reflex arc (Fig. 3) and it is why,after the muscle stretches due to us hitting the tendon with ahammer, we observe a slight contraction of said muscle.The latency is also called reflex time. It is determined by theconduction time of the action potentials in the 1a fibers (frommuscle spindle to motorneuron) and the motor axons (frommotorneuron to muscle cells) respectively. In adult human, thepathway from quadriceps to the spinal cord and back is about160 cm. The speed of conduction in 1a fibers and motor axonsis of approximately 100m/s, which means it should take thesignal about 16ms to go from sensory receptor to the spinalcord and then go back to the muscle and cause it to contract.What we observe is closer to 30ms. This is the result of aseries of delays :

1) the delay between the stretch and the first action poten-tial discharge by the muscle-spindle

2) the time for transmission in the synapses, also calledsynaptic delay

3) the time for the action potentials to spread along thefibers

4) the time for the contraction to be triggered by the musclefibers’ action potentials

Altogether, the monosynaptic stretch reflex is considered totake form 25 to 30 ms, which is what we obtained empirically.

Fig. 4. Illustration of the reflex arc involved in the knee-jerk reflex

The results obtained during the Jendrassik maneuver exper-iment confirmed the well-recognized accentuating effect thismethod has on tendon-tap jerks. It seems, however, that thereasons why this happens are still unclear. Some suggestedthat the Jendrassik maneuver causes the fusimotor drive tothe muscle-spindles to increase, making them more sensibleto the tendon tap. However, the maneuver also seems to in-crease the response while testing the H-reflex, which bypassesthe spindles completely. Other exlained that the Jendrassikmaneuver directly increases motoneurons excitability, but thiswas then deemed unlikely (Downan and Wolpaw 1988). Asseen in the lab handout, Gregory et al. also showed thatthe fusimotor neither system involved in reinforcement norare direct excitatory or presynaptic disinhibitory effects onmotoneurones.Given the results obtained using the Jendrassik maneuver, wecould conclude that ’simple’ reflexes are quite low in intensity.

III. ANKLE JERK REFLEX

The ankle-jerk reflex, otherwise known as the Achillesrelfex, is another example of myotatic reflex. Like theknee-jerk reflex, it is used to diagnose damage to the spinalcord or the nerves.

Method

This time, the subject was standing up on one leg, whilethe other was resting on a chair, bent at the knee, his foothanging over the edge. Two electrodes were glued inside ofthe calves muscle, about 7cm apart. As before, the groundelectrode was attached inside the ankle of the same leg.Using the reflex hammer again, the subject was hit on theAchilles tendon, behind the ankle, just above the heel. Thestimulus and EMG activity were recorded and displayed

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on the computer in the form of graphs. The maneuver wasrepeated several times in order to obtain satisfying graphs.This experience was also repeated using Jendrassik maneuver.

Results

As with the knee-jerk reflex, tapping the tendon seemsto elicit a slight muscle contraction (see Fig. 5). Indeed, ashort time after hitting the Achilles tendon, we observed aplantar flexion of the foot ( A plantar flexion is a movementthat decreases the angle between the sole of the foot and theback of the leg (Wikipedia) ). This flexion is induced by thecontraction of the calf muscle.

Fig. 5. Example of one of the graphs obtained while recording the ankle-jerk reflex. Though we can see a lot of artifacts, the muscle contraction inresponse to the stimulus is still clearly observable.

Again, we conducted this experiment in both normal andJendrassik maneuver settings. While the latency seemed tostay constant in both conditions, averaging at around 40ms,the peak EMG response showed significant differences, go-ing from 0,078mv average in normal conditions to 0,143mvaverage in Jendrassik ones.

Fig. 6. Latency and EMG peak response recorded during experimentationof the ankle-jerk reflex in both normal and Jendrassik maneuver conditions.

Discussion

We obtained results really similar to the knee-jerk experi-ment, which seems to indicate that the ankle-jerk reflex oper-ates in the same way. Hitting the tendon causes the muscle, andconsequently the muscle-spindles, to stretch. The 1a fibers fireaction potentials, exciting the muscle’s motorneurons locatedin the spinal cord (anterior horn), which in turn fire actionpotentials and cause the same muscle to contract. It’s amonosynaptic reflex arc.The latency observed is a bit superior to the one observed inthe knee-jerk reflex because the calf muscle is farther awayfrom the spine than the thigh muscle and the distance actionpotentials have to travel is thus longer. Of course, the seriesof delay mentioned above, such as synaptic delays or delaysin transmission, also occur here.

Fig. 7. Differences in peak EMG response and latency between normalankle-jerk reflex test and Jendrassik maneuver ankle-jerk reflex test

From our observations, the Jendrassik maneuver seems toalso increase the EMG response in the ankle-jerk reflex. Asexplained in the first part of this paper (Knee-Jerk Reflex), thereason for this potentiation is not well understood.

IV. HOFFMAN REFLEX

H-reflexes are the electrical equivalent of the monosynapticstretch reflex, which we elicited mechanically in the firsttwo experiment. The pathway for the two is identical. H-reflexes are normally elicited in a few muscle, such as thecalf muscles, through the electrical submaximal stimulationof the large afferent 1a sensory fibers which is followed byactivation of motoneurons in the anterior horn of the spinalcord. Neuromuscular spindles are completely by passed here.

Method

The subject was asked to stand up. The electrodes were leftin the same position as for the ankle-jerk reflex experiment,that is two on the calf muscle, inside the leg, about 7 cmapart, and the ground electrode inside the ankle. We thenattempted to stimulated the tibial nerve in the poplitealfossa by mean of a subcutaneaous electrode. Unfortunately,we were enable to locate the nerve well enough to evokea response in the subject. As a result, we were enable toobtain satisfactory electromyographic recording of the reflexresponse.In order to be able to move on to discussion about theH-reflex, we used the recordings provided with the lab

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handout.

Results

We observed three different pattern of EMG activity.1) Below 10mA electrical stimulation, we could see one

wave of EMG activity.2) From 10mA to 14mA electrical stimulations, we then

observed two consecutive waves of approximately thesame amplitude.

3) From 18mA onwards, the first observed wave is largelysuperior in amplitude to the second one, which hasdecreased to a level close to 0.

Fig. 8. EMG response of the calf muscle after electrical stimulation of 8mAand 10mA

Fig. 9. EMG response of the calf muscle after electrical stimulation of 10mA,12mA and 14mA

Fig. 10. EMG response of the calf muscle after electrical stimulation of14mA and 18mA

Discussion

As we explained above, the H-reflex is elicited by electricalstimulation of the sensory 1a fibers. The stimulus then travels

to the spinal cord and is transmitted to the anterior horn cellwhich fires it down along the alpha motor axon to the muscle,causing contraction.A low stimulation will only activate the sensory fibers (1afibers) and yield what is called an H-wave, the expressionof the monosynaptic reflex. As we increase the stimulation,however, direct activation of the efferent fibers (motor axon)is caused, sending action potentials directly form the point ofstimulation to the neuromuscular junction. This arc producesan EMG response called the muscle response or M-wave. Itappears before the H-wave (Fig.9 and Fig.10).The reason why this happens at a higher-intensity stimulus isbecause the threshold of activation of motor axons is higherthan that of Ia sensory neurons, due to the latter’s smallersize. When the depolarization threshold for the motor axonsis reached, action potentials are generated and fired towardsthe muscle, hence causing a muscle contraction. It is not calleda reflex, but simply a motor response,because it did not travelthrough the spinal called. The M-wave appears before the H-wave because the path the aciton potentials have to travel isshorter.What we observed then is that continuous increasing of thestimulus eventually result in the disappearance of the H-wave, whereas the M-wave seems to remain present. Thedisappearance of the H-reflex is due to an effect known as theantidromic collision. An electric activity is called antidromicwhen it travels in the ’wrong direction’ in the motor axons.As the antidromic volley of electric activity travels up themotor axons to the spinal cord, it eventually collides with theorthodromic volley coming from the sensory axon and passingthrough the spinal cord. What happens then is a matter ofsize of the volleys. If the antidromic volley is smaller thanthe orthodromic one, then the latter is reduced but still goeson to the muscle. We then observe two waves. However, ifthe antidromic volley is equal or superior to the orthodromicone, it is suppressed and does not proceed to the muscle. Thisexplains why the H-wave tends to disappear as the electricalstimulation increases. (Fig.11)

V. CONCLUSION

We conducted a series of experiment in order to understandthe underlying principles of reflexes. First we tested theknee-jerk reflex then the ankle-jerk reflex. Both were testedin normal condition, as well as when using what is calledthe Jendrassik maneuver. We then proceeded to observe theH-reflex but were unfortunately unable to elicit an EMGresponse in our volunteer subject.What we observed is that, be it mechanical or electrical,a stimulation of the sensory 1a fiber in a muscle will befollowed by a slight contraction of that same muscle. This isdue to what is called a reflex arc. In the case of the stretchreflex, this arc is monosynaptic, meaning the sensory fibersynapses directly to an homonymous motorneuron, withoutany interneuron connexions. In the case of the H-reflex, aninteresting phenomenon occurs as the electrical stimulationgets increased. Indeed, a motor response is then elicited anda antidromic signal is also produced. If strong enough, this

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Fig. 11. Illustration of the H-reflex and the behaviors of the H and M wavesas the stimulus increases in intensity

signal will annihilate the afferent sensory signal and we willobserve the disappearance of the H-reflex, leaving only themotor response apparent through what is called an M-waveon the recordings.

REFERENCES

[1] J. Gregory, S. Wood, U. Proske. An investigation into mechanisms ofreflex reinforcement by the Jendrassik manoeuvre Experimental BrainResearch, Volume 138, Number 3 / May, 2001

[2] Riann M. Palmieri, Christopher D. Ingersoll, and Mark A. Hoffman. TheHoffmann Reflex: Methodologic Considerations and Applications for Usein Sports Medicine and Athletic Training Research Journal of AthleticTraining 39(3): 268277; Jul-Sep. 2004

[3] M.A. Fisher Encyclopedia of the Neurological Sciences Elsevier Inc., 2ndedition, Pages 598599, 2014

[4] R.F. Schmidt Fundamentals of Neurophysiology Springer-Verlag, 3rdedition, Pages 103121, 1985