Lab report written for my Higher Cortical Function module at University College Dublin. It's about monosynaptic reflex

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Motor Reflex
Jeanne Charoy
I. INTRODUCTION
IN order to survive, living organisms are constantly
detecting changes in their environment and reacting
accordingly. Reflexes, stereotyped rections of the Central
Nervous System to sensory stimuli, are a good example of
this kind of stimulus-response behavior.
In this paper, we will be describing experimental observations
of one of the reflex arcs involved in controlling the motor
system, the monosynaptic arc of the stretch reflex. Indeed,
we measured the knee-jerk reflex, also known as the patellar
reflex, as well as the ankle-jerk reflex on a relax volunteer
using EMG. We also gave an attempt at recording the H-reflex
which proved unsuccessful. To discuss the H-reflex we will
refer to recordings giving to us with the lab handout. All our
results and observations will be discuss below.
II. KNEE JERK REFLEX
The knee-jerk reflex is evoked when the tendon of the
quadriceps muscle below the kneecap is stroked.The normal
response should be for the muscle to contract and the leg to
go up very briefly. This relfex is often tested during medical
examination and most of us have memories of the doctor
hitting us with a hammer.
Method
A relaxed volunteer sat with her leg hanging freely
over the edge of the chair. We then placed two electrodes,
approximately 5 cm apart, on the quadriceps muscles on the
front thigh and a ground electrode inside of the ankle on the
same leg. The electrodes were plug into a transducer, which
allowed us to observe the EMG activity in the form of graphs
on the computer.
A reflex hammer was also used. Like the electrodes, it was
plugged to the transducer, which allowed us to know precisely
at what time and with what amplitude the shock occurred.
This also was also converted and displayed on the computer
screen.
As seen in Fig. 1, we then proceeded to lightly hit the
volunteer under her kneecap, in order to elicit the reflex. We
repeated the maneuver several times in order to get 5 good
recordings and have enough data to make some conclusions.
In a second experiment, we proceeded in the same way but
for one detail. The volunteer was this time executing what is
called the Jendrassik maneuver while getting hit on the knee.
The Jendrassik maneuver is a medical maneuver wherein the
patient flexes both sets of fingers into a hook-like form and
interlocks those sets of fingers together (definition from the
lab 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 shows
that, a short period of time after having been hit, the muscle
contracted slightly. The peak EMG response averaged at
0,620 mv. This period of time is also called latency or reflex
time. From our recordings, the latency averaged at around 28
ms (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 the
graph 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 more
or less the same latency (around 30ms). The peak EMG

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response, however, was significantly higher, as it averaged
at 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 normal
conditions and Jendrassik maneuver conditions. While the latency doesn’t vary
much, we can see a clear difference in the EMG response.
Discussion
As we expected, a light hit under the kneecap seems to
make the quadriceps muscle of the thigh contract which in
turns makes the leg go up, and this all happens involuntarily.
When we hit the tendon with the hammer, it causes the muscle
to stretch a little. Inside muscles we can find a number of
thinner and shorter muscle fibers encased in a connective tissue
capsule. Those are called the muscle-spindles, a stretch recep-
tor. They are innervated with a type of afferent fibers called the
Group 1a fibers. When muscle-spindles are stretched, the firing
of the 1a fibers increase, when muscle-spindles are relaxed, the
firing of the 1a fibers decreases and can reach 0. In short, the
muscle-spindles signal the length of the muscle.
1a fibers enter the spinal cord through the dorsal roots and
there form excitatory synapses in the anterior horn with
homonymous motoneurons, which, when excited, cause the
same extensor muscle to contract. As a result, a brief stretching
of the muscle will produce, after a short latency, a contraction
of the muscle. This is a reflex arc (Fig. 3) and it is why,
after the muscle stretches due to us hitting the tendon with a
hammer, we observe a slight contraction of said muscle.
The latency is also called reflex time. It is determined by the
conduction time of the action potentials in the 1a fibers (from
muscle spindle to motorneuron) and the motor axons (from
motorneuron to muscle cells) respectively. In adult human, the
pathway from quadriceps to the spinal cord and back is about
160 cm. The speed of conduction in 1a fibers and motor axons
is of approximately 100m/s, which means it should take the
signal about 16ms to go from sensory receptor to the spinal
cord and then go back to the muscle and cause it to contract.
What we observe is closer to 30ms. This is the result of a
series 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 called
synaptic delay
3) the time for the action potentials to spread along the
fibers
4) the time for the contraction to be triggered by the muscle
fibers’ action potentials
Altogether, the monosynaptic stretch reflex is considered to
take 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 this
method has on tendon-tap jerks. It seems, however, that the
reasons why this happens are still unclear. Some suggested
that the Jendrassik maneuver causes the fusimotor drive to
the muscle-spindles to increase, making them more sensible
to the tendon tap. However, the maneuver also seems to in-
crease the response while testing the H-reflex, which bypasses
the spindles completely. Other exlained that the Jendrassik
maneuver directly increases motoneurons excitability, but this
was then deemed unlikely (Downan and Wolpaw 1988). As
seen in the lab handout, Gregory et al. also showed that
the fusimotor neither system involved in reinforcement nor
are direct excitatory or presynaptic disinhibitory effects on
motoneurones.
Given the results obtained using the Jendrassik maneuver, we
could conclude that ’simple’ reflexes are quite low in intensity.
III. ANKLE JERK REFLEX
The ankle-jerk reflex, otherwise known as the Achilles
relfex, is another example of myotatic reflex. Like the
knee-jerk reflex, it is used to diagnose damage to the spinal
cord or the nerves.
Method
This time, the subject was standing up on one leg, while
the other was resting on a chair, bent at the knee, his foot
hanging over the edge. Two electrodes were glued inside of
the calves muscle, about 7cm apart. As before, the ground
electrode was attached inside the ankle of the same leg.
Using the reflex hammer again, the subject was hit on the
Achilles tendon, behind the ankle, just above the heel. The
stimulus and EMG activity were recorded and displayed

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on the computer in the form of graphs. The maneuver was
repeated 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 seems
to elicit a slight muscle contraction (see Fig. 5). Indeed, a
short time after hitting the Achilles tendon, we observed a
plantar flexion of the foot ( A plantar flexion is a movement
that decreases the angle between the sole of the foot and the
back of the leg (Wikipedia) ). This flexion is induced by the
contraction 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 in
response to the stimulus is still clearly observable.
Again, we conducted this experiment in both normal and
Jendrassik maneuver settings. While the latency seemed to
stay 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 0,143mv
average in Jendrassik ones.
Fig. 6. Latency and EMG peak response recorded during experimentation
of 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, and
consequently the muscle-spindles, to stretch. The 1a fibers fire
action potentials, exciting the muscle’s motorneurons located
in the spinal cord (anterior horn), which in turn fire action
potentials and cause the same muscle to contract. It’s a
monosynaptic reflex arc.
The latency observed is a bit superior to the one observed in
the knee-jerk reflex because the calf muscle is farther away
from the spine than the thigh muscle and the distance action
potentials have to travel is thus longer. Of course, the series
of delay mentioned above, such as synaptic delays or delays
in transmission, also occur here.
Fig. 7. Differences in peak EMG response and latency between normal
ankle-jerk reflex test and Jendrassik maneuver ankle-jerk reflex test
From our observations, the Jendrassik maneuver seems to
also increase the EMG response in the ankle-jerk reflex. As
explained in the first part of this paper (Knee-Jerk Reflex), the
reason for this potentiation is not well understood.
IV. HOFFMAN REFLEX
H-reflexes are the electrical equivalent of the monosynaptic
stretch reflex, which we elicited mechanically in the first
two experiment. The pathway for the two is identical. H-
reflexes are normally elicited in a few muscle, such as the
calf muscles, through the electrical submaximal stimulation
of the large afferent 1a sensory fibers which is followed by
activation of motoneurons in the anterior horn of the spinal
cord. Neuromuscular spindles are completely by passed here.
Method
The subject was asked to stand up. The electrodes were left
in the same position as for the ankle-jerk reflex experiment,
that is two on the calf muscle, inside the leg, about 7 cm
apart, and the ground electrode inside the ankle. We then
attempted to stimulated the tibial nerve in the popliteal
fossa by mean of a subcutaneaous electrode. Unfortunately,
we were enable to locate the nerve well enough to evoke
a response in the subject. As a result, we were enable to
obtain satisfactory electromyographic recording of the reflex
response.
In order to be able to move on to discussion about the
H-reflex, we used the recordings provided with the lab

4. UNIVERSITY COLLEGE DUBLIN, 18/02/2015 4
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 the
same amplitude.
3) From 18mA onwards, the first observed wave is largely
superior in amplitude to the second one, which has
decreased to a level close to 0.
Fig. 8. EMG response of the calf muscle after electrical stimulation of 8mA
and 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 of
14mA and 18mA
Discussion
As we explained above, the H-reflex is elicited by electrical
stimulation of the sensory 1a fibers. The stimulus then travels
to the spinal cord and is transmitted to the anterior horn cell
which fires it down along the alpha motor axon to the muscle,
causing contraction.
A low stimulation will only activate the sensory fibers (1a
fibers) and yield what is called an H-wave, the expression
of 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 of
stimulation to the neuromuscular junction. This arc produces
an EMG response called the muscle response or M-wave. It
appears before the H-wave (Fig.9 and Fig.10).
The reason why this happens at a higher-intensity stimulus is
because the threshold of activation of motor axons is higher
than that of Ia sensory neurons, due to the latter’s smaller
size. When the depolarization threshold for the motor axons
is reached, action potentials are generated and fired towards
the muscle, hence causing a muscle contraction. It is not called
a reflex, but simply a motor response,because it did not travel
through the spinal called. The M-wave appears before the H-
wave because the path the aciton potentials have to travel is
shorter.
What we observed then is that continuous increasing of the
stimulus eventually result in the disappearance of the H-
wave, whereas the M-wave seems to remain present. The
disappearance of the H-reflex is due to an effect known as the
antidromic collision. An electric activity is called antidromic
when it travels in the ’wrong direction’ in the motor axons.
As the antidromic volley of electric activity travels up the
motor axons to the spinal cord, it eventually collides with the
orthodromic volley coming from the sensory axon and passing
through the spinal cord. What happens then is a matter of
size of the volleys. If the antidromic volley is smaller than
the orthodromic one, then the latter is reduced but still goes
on to the muscle. We then observe two waves. However, if
the antidromic volley is equal or superior to the orthodromic
one, it is suppressed and does not proceed to the muscle. This
explains why the H-wave tends to disappear as the electrical
stimulation increases. (Fig.11)
V. CONCLUSION
We conducted a series of experiment in order to understand
the underlying principles of reflexes. First we tested the
knee-jerk reflex then the ankle-jerk reflex. Both were tested
in normal condition, as well as when using what is called
the Jendrassik maneuver. We then proceeded to observe the
H-reflex but were unfortunately unable to elicit an EMG
response 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 be
followed by a slight contraction of that same muscle. This is
due to what is called a reflex arc. In the case of the stretch
reflex, this arc is monosynaptic, meaning the sensory fiber
synapses directly to an homonymous motorneuron, without
any interneuron connexions. In the case of the H-reflex, an
interesting phenomenon occurs as the electrical stimulation
gets increased. Indeed, a motor response is then elicited and
a antidromic signal is also produced. If strong enough, this

5. UNIVERSITY COLLEGE DUBLIN, 18/02/2015 5
Fig. 11. Illustration of the H-reflex and the behaviors of the H and M waves
as the stimulus increases in intensity
signal will annihilate the afferent sensory signal and we will
observe the disappearance of the H-reflex, leaving only the
motor response apparent through what is called an M-wave
on the recordings.
REFERENCES
[1] J. Gregory, S. Wood, U. Proske. An investigation into mechanisms of
reflex reinforcement by the Jendrassik manoeuvre Experimental Brain
Research, Volume 138, Number 3 / May, 2001
[2] Riann M. Palmieri, Christopher D. Ingersoll, and Mark A. Hoffman. The
Hoffmann Reflex: Methodologic Considerations and Applications for Use
in Sports Medicine and Athletic Training Research Journal of Athletic
Training 39(3): 268277; Jul-Sep. 2004
[3] M.A. Fisher Encyclopedia of the Neurological Sciences Elsevier Inc., 2nd
edition, Pages 598599, 2014
[4] R.F. Schmidt Fundamentals of Neurophysiology Springer-Verlag, 3rd
edition, Pages 103121, 1985