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MEDSCI 206 – Lab 5
Neurodegenerative Study Date: 12/10/15 Experiment: Neurodegenerative Study Yu and Gagan Joshi Group: William Lin, Kelly Cudmore, Sam Yu
William Lin ID:6737564
Aims
1) Using the clinical examination of Mr. S as an example to provide insight on the features of an UMN lesion with understanding of the physiology and pathologies relating to these clinical features. Relate the symptoms observed in Mr. S to the experiments conducted below. below. 2) Understand the reflex circuitry mediating the tendon-jerk response demonstrated at the level of the Achilles tendon. Understand and explain the mechanisms of the Jendrassik maneuver and how it relates to latency and amplitudes on EMG recording. Compare reflex response to voluntary contraction and relate your observations to physiological mechanisms. 3) Understand the mechanical properties of the skeletal muscles (Abductor pollicus brevis) in a healthy individual. Explain the theory behind temporal summation and fused tetanus as influenced by stimulus frequency. Relate the findings of this experimental section to that of the clinical examinations of Mr. S.
Introduction Introduction
Upper motor neuron lesions are lesions which occur above the anterior horn of the spinal cord and consists of damage to parts of the descending corticospinal tract pathways (Fix, 2008). Most common causes of UMN lesions are that seen in patients whom have suffered from conditions such as a stroke, multiple sclerosis, traumatic brain injury and cerebral palsy (Fix, 2008). Damage at the region of the Upper motor neurons is often characterized by affected motor performance on the contralateral side of the body; this is consistent with the decussating pattern of the pyramidal neurons which occur at the region of the medulla oblongata (Purves, 2008). The symptoms associated with an UMN lesion are often opposite to LMN lesion symptoms; symptoms of UMN lesion consist of the following: Lateralized weakness, Spasticity, Hypertonia, Hyperreflexia, Positive Babinski’s sign and loss of fine voluntary movements (Purves, ( Purves, 2008). With this being said, clinicians typically target these motor parameters in the assessment and localization of the lesion in aid of diagnosis. The ankle jerk response is a common reflex response used by clinicians to test the activity of spinal reflex arches. This reflex can be induced by tapping the Achilles tendon of the patient’s food; this action causes firing of the Ia afferent fibres which are present in i n the centre of the intrafusal fibres of the muscle spindles and project through the dorsal root of the spinal cord and synapses with alpha motor neurons neurons (Purves, 2008). The firing of Ia afferents will result res ult in the release of an excitatory excita tory neurotransmitter, Glutamate, which will increase the firing of the alpha motor neurons, causing contraction of the homonymous muscle and hence the observed plantarflexion of the foot (Purves, 2008). This reflex response res ponse is present to maintain postural balance in the case of unexpected events such as tripping etc (Purves, 2008). Muscular tone describes the resting, baseline tension which is present in a muscle. The maintenance of an appropriate level of tone in the muscle ensures that the muscle fibres exist are at an optimal length to respond
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to successive contraction (Purves, 2008). Tonic activity of musculature is characterized by the “background” activity of the alpha motor neurons which provides the baseline activity of the muscle groups. The activity of the alpha motor neurons in muscle tone is mediated by the gamma motor neurons present with the muscle spindles which maintenance the sensitivity in changes in length of the muscle fibres and thereby allows for Group II afferents (secondary ending afferents) to modulate the tonic activity of alpha motor neurons. Unlike Group Ia fibres, Group II fibres respond to tonic, sustained changes in length and its effect on muscle tone allows for the stretch reflex to be appropriately induced (Purves, 2008). Initiation of voluntary movement begins at the region of the pre-motor cortex which modulates the firing the pyramidal cells in the primary motor (pre-central gyrus) cortex (Purves, 2008). The major pathway of descending motor neurons is that of the corticospinal tract, this tract of axons travels from the cortical regions and pass through the posterior internal capsules to project down to the brainstem and ultimately synapses at the ventral horn and intermediate grey matter of the spinal cord. It has been suggested that approximately 90% of pyramidal neurons decussate at the medulla to form the lateral pyramidal (corticospinal); the 10% of pyramidal fibres that do not decussate at this level of the brain stem constitute that of the anterior corticospinal tract terminate ipsilaterally or bilaterally near the midline of the ventral white commissure of the spinal cord (Purves, 2008). A restricted population of the pyramidal neurons of the lateral corticospinal tract form direct synapses with alpha motor neurons; these alpha motor neurons which received the privileged direct synaptic connections with UMN innervate muscles of the forearm and the hand. The remainder of UMN from the lateral corticospinal tract innervate local neuronal circuitry pools that act to coordinate LMN activity. Lower motor neurons exist as 3 different classes: Alpha, beta and gamma motor neurons; the alpha motor neurons are the most abundant of all and is primarily re sponsible in the initiation of a voluntary contraction. When the alpha motor neurons fire, the neurotransmitter Acetylcholine is re leased at the synaptic terminals of the motor end plate; one alpha motor neuron is known to innervate numerous muscle fibres and is collectively referred to as one motor unit (Purves, 2008). Motor units exist in 3 types: S (Slow) motor units which are resistant to fatigue, FF (Fast-fatigable) and FR (Fast-fatigable resistant) which is an intermediate motor unit between S and FF motor units (Purves, 2008). Event from their name, each motor unit is characterized to a particular property e.g. the S motor units innerve slow-twitch fibres which are limited in force generated by its contraction can be exerted over a longer period of time compared to the other two motor units and the fibres which they innervate (Purves, 2008). There are also biochemical differences between each type of motor unit such that there is a progressive increase in activation threshold from S, FR to FF fibres. As a result, motor unit recruitment has been explained by a theory known as the Size principle which describes the recruitment of motor units from an order of those with lowest activation threshold i.e. S up to a saturated recruitment of all the three motor units. Increase recruitment of motor units can be demonstrated by an increase in stimulus size i.e. current until a supramaximal response is exhibited i.e. all motor units have been recruited (Purves, 2008). Methods Part 1
A video consisting of the clinical examination of Mr. S is played in class. Students are asked to take notes of any observations from the range of examinations performed by the physician and describe them in the discussion section below. Part 2 A) LabChart settings are set as per instruction in page 69-70 of the laboratory manual. The subject is asked to roll their pants up such that the soleus muscles are exposed for electrode attachment. The soleus muscle is identified by asking the subject to roll their fingers over the lateral borders of the leg and locating the boundary which exists between the gastrocnemius and soleus muscle (ref. figure 1 of laboratory guide). Electrode placements are made such that the negative electrode is placed over the muscle belly of the soleus muscle and the positive electrode is placed 3 cm below the position of the first electrode; the ground electrode is placed
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over the ankle bone region of the foot. Ensuring that electrode placement and LabChart settings are set up correctly, tap the Achilles tendon firmly with the tendon hammer provided. Repeat the stimulation at the Achilles tendon by the tendon hammer 5 times; save and labels traces such that latency and amplitude measurements can be taken.
B) Repeat the same experimental set up as that described in Part 2A i.e. LabChart settings and electrode placements remain the same. The only difference between this trial and the previous experiment is that the subject is asked to firmly grip and clench their fingers together prior to the stimulation by the tendon hammer. Once the Jendrassik maneuver has been performed, tap the Achilles Tendon (with the tendon hammer), record 5 repeated trials and save and label traces for amplitude and latency measurement. C) Change the LabChart settings as per instructions on Page 71 (Part 2C). The Achilles tendon is struck once to record the response on a longer time base. After this, gently tap the side of the ankle with the tendon hammer and ask the subject to perform voluntary plantarflexion of the foot when they feel the tapping of the tendon hammer. Repeat this procedure 5 times, save and label traces in order to make latency measurements. Part C
- Assemble the equipments as per instruction on page 72. Ensure that settings for Grass SD9 and LabChart are set-up correctly as per page 73 of the instruction manual. - Position the subjects hand within the apparatus which in a manner which matches the outlines dra wn. Using the straps provided, tighten the subjects hand such that it is held firmly in place - Position the thumb in an extended position and wrap one end of the thread across the thumb and the other end is threaded across and connected to the force transducer. Make sure that there is a degree of t ension in the thread such that successive contractions can be measured by the force transducer. Note: Ensure that the subject is in a relaxed position during the course of the experimental protocol A) Place the cathode (black) directly over the median nerve (ref. figure 2) close to the wrist and place the anode proximally i.e. upstream to the cathode placement. Set the current setting at 5.0 mA and the pulse to be at 200 !". Switch the mode setting of the Grass D9 stimulator to that of single pulse in order to stimulate the medium nerve, a tingling sensation should be reported by the subject. Increase current gratings until a supramaximal response is evident; make sure that LabChart is recording prior to each stimulus. Ensure that a supramaximal response is achieved by further increasing the current stimulus and make sure that the amplitude does not increase anymore. Record traces and overlay pages to be included in discussion. B) Switch the Grass D9 stimulator to Twin pulses setting and set the delay between each successive pulses to be at 200 ms. Start recording on LabChart and similar to that described in Part 3A, stimulate the median nerve by hitting the mode switch to single. Decrease the time interval between pulses as that described in page 75 of the laboratory manual (200,160,100,80,40,20,10,2,1 ms) and take note of the changes in amplitude size and twitch counts between each different time interval settings; also comment on the force developed. Overlay the traces. (Ensure that stimulus current is at supramaximal) C) Adjust LabChart settings as per page 76 of the laboratory manual. Ensure that the stimulus current is at supramaximal and beginning with a low frequency (2 Hz), record by turning stimulator to repeat mode for approximately 2 seconds; wait until the trace on LabChart disappears and then turn of the stimulator. Allow the subject to rest for 15 seconds between each stimulus. Increase the frequency of stimulation in increments of 2 Hz (all measured over 2 s as per the above procedure); repeat until a fused tetanus i.e. no rela xation phase
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is observed. Overlay traces and save the data before proceeding to the next step. Re-set the LabChart settings by changing the sampling time to a fixed duration of 10s; press start to record and ask the subject to perform maximal voluntary contraction of the adductor pollicus muscle. Compare this result to that of the fused tetanus and provide an overlay of this trace in your report.
Results
Figure 1. Overlaid EMG recording of the soleus muscle of the right leg, stimulated by the tapping of the tendon
hammer at the Achilles tendon. The green trace (G) corresponds to the EMG from the tendon reflex and th e pink trace (P) corresponds to the EMG from performing the Jendrassik Maneuver in addition to the tendon reflex. Overlaid traces are acquired from sample 2 of each respective trial and demonstrates an increase in amplitude of 2.709 mV in the Jendrassik maneuver (3.837 mV) compared to that in the tendon reflex (1.128 mV); Latency values for the two traces remain very similar to each other (G: 0.0262 s, P: 0.0269 s). Ref. Table 1 for latency and amplitude values from other samples
Figure 2. Overlaid traces of EMG recordings of the soleus muscle during a reflex response from stimulation at
the Achilles tendon v ia a tendon h ammer (Green trace) and voluntary plantarflexion of the foot (Pink trace), both recorded fro m the r ight leg. Th e EMG r ecording from reflex response d emonstrated a latency of 0.036 s whereas the traces from the voluntary plantarflexion exh ibited a much longer latency of 0.185 s. Traces from the reflex response generated a single spike compared to traces from the voluntary plantarflexion which exhibited multiple spikes occurring in an irregular manner with varying amplitudes. Ref. Table 1 for latency and amplitude values from other samples.
Conditions
Samples 1
2
3
4
5
Mean
Latency (ms)
26.6
26.6
26.2
26.8
26.8
26.6
Amplitude (mV)
1.581
1.128
1.127
1.395
0.814
1.209
Latency (ms)
26.6
26.7
26.9
27.3
26.8
26.9
Jendrassik Maneuver
Amplitude (mV)
2.712
3.837
2.262
1.971
2.333
2.623
Voluntary movement
Latency (ms)
261.5
179.6
349.3
332.0
175.1
259.5
Stretch reflex
Stretch
reflex
+
Table 1. Records all the measured amplitudes and latencies from the three experimental conditions described in the table.
As demonstrated in figure 1, the latencies between the EMG from the stretch as well as the EMG with the Jendrassik maneuver have been shown to be very close to each other; the mean values as per the table deviates by 0.3 of a millisecond. Results for latency for these two experimental trials also remained very consistent between samples (n=5). The mean amplitude from the EMG with the Jendrassik maneuver exhibited mean value that is 2.17 times higher than that with just the stretch reflex. During the voluntary plantarflexion of the foot, latency values have been shown to be almost 10 times greater i.e. slower than that of the stretch reflex response.
Part 3
Figure 3. EMG recordings of the compound action potential (CAP) produced at the level of the abductor
pollicus brevius with stimulation at different current settings (5-19 mA). Increments in amplitude is correspondent to an increase in current up to a value of 19 mA which demonstrates a supramaximal threshold for stimulation. Point of stimulus is taken at time 0.0 ms.
Figure 5a. Force generated from thumb abduction upon stimulation of the median nerve at different time
intervals between twin pulses ( at supramaximal stimulus - 19 mA); Time intervals used: 200, 160, 100, 80 ms as shown in figure.
Figure 4b Force generated from thumb abduction upon stimulation of the median nerve at different time
intervals between twin pulses (at supramaximal stimulus - 19 mA); Time intervals used: 40, 20, 10, 2 and 1 ms as shown in the figure. It is evident from figure 4a that twin pulses fired between a time of 200160 ms delay produced two pronounced crests which are separated by a s ingle trough; furthermore, the 2nd crest also exhibits greater amplitudes than its previous crest in these interval ranges. At 100 ms time delay, there is still two crests evident but a trough is no longer present; an increase in amplitude from this point of time delay can also be observed. Between time interval ranges of 40 ms and 1 ms, the peak has become one single crest and is characterized by increasing amplitudes in relation to decrease time delay up to a point of 10 ms, which exhibits the greatest force generated of 2.1 N. From time intervals of 10 ms down to 1 ms, a singular peak still remains, but its amplitudes have decreased resultantly.
Interval between stimuli (ms) 200
160
100
80
40
20
10
2
1
Number of twitches (1 or 2)
2
2
2
2
1
1
1
1
1
Twitch amplitude ( ! "# % "# &' compared to
-
%
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%
%
previous interval Table 2. Twitch and amplitude changes of the EMG recording measured at the level of median nerve upon twin
pulse stimulation across varying time delays (200-1 ms, n=9). Twitch numbers remained a t 2 across time delay intervals of 200 to 80 ms. Twitch amplitude suggested at an initial decline at time interval 160 ms, but from there, began to increase up to a time interval of 10 ms where the largest force generated was observed (ref. figure 4b). Twitch amplitude at 2ms and 1 ms followed a progressive path of decline, respectively.
Figure 7a. Force produced from contraction of the right abductor pollicus brevis muscle across different
frequencies of stimulation (2 – 18 Hz) at supramaximal stimulus. Note that fused tetan us was but not ach ieved in this particular trial but an unfused tetanus is evident at a stimulation frequency of 18 Hz . It is evident from the figure that increments of stimulus frequency correspond to a decrease in time interval between each peak i.e. each successive peak occurs more immediately. Between frequencies 2-10 Hz, a progressive increase in peak amplitude is observed with increase frequency of stimulation; from 14 Hz, each successive peak is characterized with increments in force generated. At 18 Hz (Unfused tetanus), peaks have conformed to an irregular manner, with individual twitches becoming less apparent. The maximum force generated at 18 Hz (~9.5 N) is much greater than that evident at 2 Hz (~0.5 N).
Figure 6b. Force generated by the right abductor pollicus brevis upon voluntary contraction. When comp aring
the maximum force generated at 18 Hz (ref. figure 6a) and that from the voluntary contraction, it is evident that max. force values in voluntary contract (10.2 N) is somewhat greater than force generated when stimulated at 18 Hz (9.5 N). It is also apparent that the rate of increase of force generate in voluntary contraction is characterized by a almost vertical peak co mpared to that at 18 Hz, where an inclined slope is evid ent.
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Part 1
During Mr. S’s clinical examination, the following symptoms of: Hyperreflexia. Hypertonia, positive Babinski’s test in the left foot, pyramidal distribution of flexor/extensor strength ratio and lateralized weakness on the left side of the body were all evident. Assuming that the above pathologies are related to an Upper motor neuron (UMN) lesion, the Hyperreflexia i.e. increase in reflex response can be attributed to the removal of supraspinal inhibition of the spinal reflex circuitry at the region of the interneurons (Sheean, 2002); thereby causing hyperactivity of the induced tendon jerk responses observed during the examination of the patient. Hyperreflexia is characteristic feature of Upper motor neuron lesions, in comparison, the presence of Hyporeflexia is not sufficient for diagnosis because patients undergoing spinal shock due to UMN lesions will often exhibit Hyporeflexia periods which are then followed by recovery phases and Hyperreflexia (Purves, 2008). When the clinician examined the tonic activity of Mr. S’s left leg, it was evident that a force was exerted against the passive movement of raising the leg. This observation describes the feature of muscle spasticity, which is a form of hypertonia i.e. increase tone in limbs, this is potentially due to the removal of suppression of the postural centres of the brain and the vestibular nuclei and reticular formation of the cortical region (Purves, 2008). In this scenario, the patient exhibited a condition known as clasp knife phenomenon whereby the passive flexion of the knee is counteracted by initial phases of muscle stiffness and then abrupt collapse (Sheean, 2002). This is also due to the disinhibition of the reflex circuitry mediating the stretch reflex; increasing the sensitivity of alpha motor neurons resulting in hyperactivity of the stretch response. Because the Ia fibres senses dynamic changes in length of the muscle spindles, it is characterized by a velocity-dependent response. When the patient’s legs are passively bent, it is first antagonized by the stretch reflex, however, overtime, the stretch response becomes less sensitive at longer lengths and the velocity of stretch is also counteracted by the resistances of the innate stretch reflex; this results in the ultimate collapse in the patient’s leg as observed during the examination. Furthermore, another feature which is characteristic of an UMN lesion is the positive testing of the Babinski’s sign on the left foot (Sheean, 2002) (Purves, 2008). When the patient’s left food was stroked at the level of the sole, the fanning of his toe was exhibited; this observation is only normal in human infants when the maturation of the corticospinal tract is not complete, therefore the Babinski’s test is a useful diagnosis for UMN lesion. With all the above diagnosis pointing towards that of an UMN lesion, the lateralized weakness on the left side of the patient’s body suggests that it is most likely that of an UMN lesion in the right hemisphere of the cortex. 90% of the pyramidal neurons decussate at the level of the medulla oblongata to form the lateral corticospinal tract; with this being said, the region which the lesion has occurred will most likely be contralateral to the affected side of the body (Purves, 2008). UMN projects down from cortex and synapses with lower motor neurons (LMN) to initiate voluntary contraction of muscle groups; when this descending pathway is affected, it will reduce the number of LMN activated and therefore a reduction in muscle recruitment and force generated. This notion of a muscle weakness can be compared to a current stimulus of 5 mA, whereby a sub-maximal response is observed; this lack of recruitment due to UMN lesion will result in only the low-threshold fibres being recruited and thereby producing minimal force that is perceived by the patient as physical weakness).
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Part 2
The EMG recordings acquired in Part 2A of the experiment demonstrates a reflex response which is primarily modulated by the muscle spindles of the soleus muscle (right leg). Stimulation of the Achilles tendon mediated by the tendon hammer causes activation of Ia afferent fibres which are present at the central region of the intrafusal fibres (Purves, 2008). Under normal conditions, the Ia afferents will be activated upon stretch of its synergist muscle group; however, during the experimental protocol, a similar effect is achieved by stimulating the Achilles tendon. Ia afferent fibres enters through the dorsal root of the spinal cord and form direct synaptic connections with descending alpha motor neurons at the region of the ventral horn; hence the reflex pathway is often described as that of a “monosynaptic pathway” (Purves, 2008). Firing of the Ia afferents will result in the release of the excitatory neurotransmitter glutamate, and this will cause the resultant activation of alpha motor neurons which projects down and innervates a motor unit, thereby causing contraction of the homonymous muscle group (Purves, 2008). The mean latency value (26.6 ms) obtained from Part 2A (ref. table 1) characterizes nerve conduction velocities restricted to a reflex circuitry that is devoid of any significant central input. Furthermore, because the stimulation by the tendon hammer occurred as a single strike of moderate force, it may be assumed that the EMG recordings from Parts 2A and B are confined to the soleus muscle; this assumption can also be validated by understanding the different levels of spindle feedback between the soleus and the gastrocnemius muscle in which the soleus muscles have been reported to have greater spindle feedback at rest as well as during voluntary contraction (TUCKER & TüRKER, 2004). When the experimental protocol was repeated whilst performing the Jendrassik maneuver (JM) (Part 2B), an apparent increase in a mean amplitude of 2.709 mV was evident in comparison to the mean amplitude of the EMG recording consisting solely of the stretch reflex (1.209 mV). With this being said, the mean latency values from Part 2A and 2B remained almost identical to each other, with values of 26.6 ms and 26.9 ms respectively (+/- 0.3 ms). The Jendrassik maneuver has been traditionally used in clinical examinations under circumstances where the tendon-jerk response is of a “sluggish” nature. It has been speculated that the Jendrassik maneuver allows the facilitation of the stretch reflex due to 3 possible mechanisms: Diminished interneuron inhibition activity thereby allowing heightened reflex responses, increase gamma motor neuron activation which results in an increased sensitivity to stretch reflex and contribution of a psychological distraction which factors in to play in reducing inhibition of reflex responses (Nardone & Schieppati, 2008). First of all, the hypothesis of an increase sensitivity due to gamma motor neuron activation has received some controversial responses as it was shown that persistence in Jendrassik maneuver facilitation occurred even after ischemic conditions were induced to the leg. Furthermore, facilitation during H reflex whereby the gamma motor neurons are excluded from the reflex circuitry also comes to suggest the fact that the Jendrassik maneuver does not primarily act at the level of the gamma motor neurons (Gregory, Wood & Proske, 2001); this finding effectively rules out gamma motor neuron activation as a potential mechanism for facilitation. Studies have suggested that not enough evidence exists to prove the diminished inhibition at the level of the pre-synaptic terminal; however, with this being said, a recent study has come to suggest an alternative theory whereby it has been shown that JM exerts its facilitation by inhibiting interneurons at a sub-cortical region as well as facilitation at a segmental level; exerting its facilitating actions at a premotorneuronal region (Nardone & Schieppati, 2008). Although there has been limited literature published in elucidating a direction relationship between the Jendrassik maneuver and the presence of a psychological distraction factoring in the facilitation of a reflex response, studies regarding the conscious inhibition of tendon reflex have come to suggest how cortical projections allow for the inhibition of antagonist Ia fibres and thereby reducing the response of a tendon reflex (Iles & Pisini, 1992). It can be implied from this study that the voluntary cross-linking of finger i.e. Jendrassik maneuver diminishes this
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degree of cortical inhibition on the reflex circuitry and has been used by medical clinicians in the examining of tendon-jerks as well as other modalities such as Romberg’s sign (Vereeck, Truijen, Wuyts & Van de Heyning, 2007). Latencies of the stretch reflex with and without the facilitation of the Jendrassik maneuver remained very similar to each other because it is still mediated by the reflex circuitry. During Part 2C where the subject was asked to perform voluntary plantarflexion of their [right] foot, an average latency of 259.5 ms was observed, accompanied by multiple EMG peaks which were of an irregular manner (ref. Figure 2). The latency during voluntary plantarflexion is significantly longer (approx. 10 times) than that observed in both stretch reflex (with and without JM) because unlike that in the stretch reflex which is mediated at a spinal cord level, voluntary movements are initiated at the region of the cortical region i.e. premotor cortex and is required to travel down via upper motor neurons and synapse with the alpha motor neurons at the ventral horn of the spinal cord (Purves, 2008). The latency values from voluntary plantarflexion characterize both the increased distance for nerve conduction of voluntary motor output as well as the conduction velocities of respective fibres, with Ia afferents pertaining to the fastest conduction velocity of 80-120 ms -1 (Siegel, Sapru & Siegel, 2006). The reason behind the multiple irregular peaks pertaining to the EMG recording of the voluntary plantarflexion of the foot can be explained through the notion that voluntary contraction will recruit other muscle groups e.g. Gastrocnemius thereby causing a background distortion in the EMG readings. Part 3
The results obtained from Part 3A demonstrates the theory known as a “Size principle” which describes the ordered recruitment of muscle fibres within a motor pool in response to increments in synaptic input (Purves, 2008). During the experiment, normal physiological synaptic input was replaced by stimulating electrodes placed at the median nerve of the subject’s right arm. In reference to figure 3, it is evident that the increase in current setting resulted in the progressive increase in amplitude (mV) up to a point of 19 mA which marks the supramaximal threshold of the subject. At lower stimulus input, the only fibres recruited are those pertaining to a low stimulus threshold i.e. S motor units; the increase in stimulus input will result in the respective thresholds of both FR and FF motor units being exceeded, up to a point of supramaximal stimulus which suggests the recruitment of all the motor units within a specified motor pool (Purves, 2008). In Part 3B, we examined the effects of temporal summation on the force generated by the abductor pollicus brevis muscle as measured by the force transducer. At longer time intervals between the twin pulses, the recordings are characterized by two independent peaks which is separated by a baseline force level i.e. trough. This is because the time between each successive pulse allows for the muscle fibres to relax and thus meaning that the next successive peak for force generated will remain independent to its previous contraction (Purves, 2008). However, with this being said, it is still evident that during intervals of 200 and 160 ms, the second peak is of a greater amplitude; this can be related to the notion that time for total calcium removal from the Sarcoplasm may be longer than that of 200 ms and this residual calcium will result in slight increments of force generated in the successive pulse (Melzer, Rios & Schneider, 1984). When time interval was further decreased (from 80 ms), a single twitch became apparent, accompanied by an increase in amplitude. These results can be validated by the theory of temporal summation which describes how when muscle fibres are not given enough time relax, the force generated by the successive pulse will be summed to that of its previous twitch i.e. temporal overlapping of force generated (Purves, 2008). This pattern of temporal summation and an increase in amplitude of force generated is only consistent between time intervals of 40 ms to 10 ms. At interval times of 2 and 1 ms, twitch count still remains at 1 but amplitude begins to decrease henceforth. It can be suggested that at 2 (as well as 1) ms, the muscle is still within its absolute/relative period, as a result, this causes the fibres to become insensitive to stimulation and
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will no respond by contracting (Purves, 2008). It can be assumed that at 2 ms interval, the muscle is potentially approaching the end of its refractory period i.e. in its relatively refractory period because the force generated at this time interval (1.3N) is still greater than that at 1 ms (absolute refractory) where the force generated is almost identical to that of the two independent twitches at 200 ms. ( ref. figures 4a&b and table 2 for twitch and amplitude changes). Results from Part 3C exhibited the progression towards a tetanus contraction by the subject when stimulated at increasing frequencies. Although a fused tetanus could not be achieved as a due to factors such as pain tolerance etc., it is still evident from figure 6a that increasing stimulus frequency is accompanied by increase in force generated and the disappearance of regular peaks in the recordings. Tetanus occurs as a result of insufficient time for Ca2+ to be removed from the sarcoplasm (Boron & Boulpaep, 2009). Ca 2+, is extremely important in the formation of cross-bridge and its concentration within the sarcoplasm has been shown to directly affect the force generated by a specific muscle group. Ca2+ removal occurs via numerous pathways mediated by SERCA as well as PMCA (Boron & Boulpaep, 2009); other transport mechanism for Ca 2+ also exists but will not be discussed. When Ca2+ is present in the cytosol, it binds with Troponin C present on the actin filaments and causes an array of conformational changes that lead to the cross-link which is characterized by the “power stroke”, ultimately leading to the shortening of sarcomere length and the production of force. (Boron & Boulpaep, 2009) When the subject was asked to perform voluntary contraction of the thumb, it is evident that the force generated is slightly greater than that of the tetanic contraction. This observation is due to the fact that, in our experiment, a fused tetanus was not achieved and this will therefore result in a slight decrement in total force produced compared to that of the voluntary contraction. However, voluntary contraction and that of the fused tetanus should theoretically have similar values of force generated; this is because the supramaximal stimulus recruits the maximal amount of motor units within a motor pool in accordance to the size principle and therefore simulates the recruitment of muscle fibres during a maximal voluntary contraction (ref. figure 6b) (Purves, 2008). However, with this being said, the above hypothesis will only remain valid providing that the voluntary contraction of the thumb is isolated to that of the right abductor pollicus brevis and does not recruit other muscle group responsible in thumb flexion.
Conclusion
The experiments performed during the lab allowed for us to understand the clinical features pertaining to an upper motor neuron lesion. Associating these clinical features of an UMN lesion to physiological and pathological mechanisms allowed for us to distinguish the differences between LMN and UMN lesion; furthermore, comparison between Mr. S’s clinical examination as well as results obtained from Parts 2 and 3 of the experiment allowed for better insight on the motor parameters which are affected by UMN lesion e.g. Force generated due to muscle weakness. Part 2 of the experiments demonstrated the effects of the Jendrassik maneuver in facilitating in increases Achilles tendon jerk response. Examination of its latency and amplitude changes compared to that of a simple Achilles tendon reflex allowed for us to understand the mechanisms by which facilitation occurs and parameters that are affected by the maneuver. Part 3 of the experiment demonstrated both elicitations of a temporal summation regarding time interval between pulses in relation to force generated and the theory of a tetanus contraction in response to increments in frequency of stimulation.
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References
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