Power Lab- Index Card

POWERLAB EXPERIMENT ON NERVE CONDUCTION VELOCITY OBJECTIVES:  Measure nerve conduction velocity from difference in latencies between responses evoked by nerve stimulation at wrist and elbow. Setup: Position Bar Stimulus Electrode on medial aspect of front of elbow (Antecubital fossa). Se t the current in the stimulator panel to 8mA Once you have found the best position for the bar stimulus electrode, increase the stimulus to 1520mA. Click start. Repeat several times. When you do you stop? - When the muscle starts contracting. It means you have reached the threshold stimulus. Compute for nerve conduction velocity? Velocity = Distance/Latency timeWhere Latency time= Elbow-WristTherefore, Velocity = 0.235m/ (0.008s-0.004s) Velocity= 58.75 m/s Based on the nerve conduction velocity, classify the Median nerve. The Median nerve is fiber type Aβ for sensing touch and pressure as the conduction velocity computed, 58.75m/s falls in the range of 30-70 m/s.

EXPERIMENT II: PHYSIOLOGIC PROPERTIES OF A NERVE Ringer’s solution is a standard isotonic solution used to make sure that the muscle is still alive during the experiment. While tying the nerve with a ligature, mechanical stimulus was applied, eliciting a response through muscle contraction. Why are you observing the muscle when procedures are done on the nerve? Visible response during electrical stimulation of the nerve is demonstrated by the contraction of the muscle. It is the action potential generated by the nerve that travels toward the muscle, which causes the contraction. Define threshold stimulus. Threshold stimulus is the minimum stimulus or voltage required to elicit a response. Is there a difference in the response when stimulation is done at the free end of the nerve or proximal to the ligature? Yes. There was no response during stimulation of the free end of the nerve. Muscle contraction was observed when nerve proximal to ligature was stimulated. What is the effect of the ligature? The ligature disrupts the surface membrane of the nerve, negatively affecting the conductivity of the nerve. Proximal to the ligature, the nerve is highly conductible as the surface is still intact.

At the site of the ligature what phase of action potential if the membrane in? Depolarization. There was no repolarization because the gated channels were destroyed by the ligature. What physiologic property/ies of a nerve was/were exhibited in this experiment and in which procedure/s? Excitability is the ability of the nerve to produce an action potential in response to astimuli. Conductivity is the ability of the nerve to transmit/propagate the action potential. What does the experiment prove regarding conductivity in nerve?As the nerve was electrically stimulated, it was able to generate an action potential (excitability) that has propagated (conductivity) along the surface of the nerve membrane, as the voltage-gated channels are located there. In the experiment, it was also demonstrated that nerve conductivity can be affected by different factors, such as mechanical obstruction of the membrane. Ligature serves as the mechanical stress to the nerve that impeded the transmission of the action potential from the nerve to the muscle, preventing the muscle to contract. What conditions are necessary for conduction of impulses along the nerve? Stimuli reaches or surpasses threshold Membrane is intact and not depolarizedIon channels are intact Necessary ions are available (Ringer’s solution or ECF simulator)

C. EXPERIMENT III: EXPERIMENT ON THE ACTION OF SUCCINYLCHOLINE Compare the levels of the threshold of nerve and muscle. The resting potential of muscles (-90 mV) is lower than that of nerves (-70 mV). The main function of nerves is to be excitable and be able to conduct impulses. Because of this, nerves are more excitable and responsive and have faster conductions than of muscles. Why did we tie the left thigh?The left thigh was tied so that succinylcholine will not circulate in the thigh. Assuming the succinylcholine was prevented from circulating in the leg, the expected result is that the left muscle would still contract after the injection of succinylcholine. Based on your observation, where is the probable site of action of succinylcholine? What is its mechanism of action?The site of action of succinylcholine is the neuromuscular junction. Succinylcholine is a competitive inhibitor of acetylcholine for the nicotinic acetylcholine receptor. Succinylcholine would stay bound to the receptors becauseacetylcholinesterase will be unable to degrade succinylcholine. Repolarization is therefore blocked and will cause paralysis.

Enumerate the sequence of events that occur in the NMJ. i. Action potential in the motor axon is conducted into the presynaptic terminal. ii. Increase in Ca2+ conductance at the prejunctional membrane and an influx of Ca2+ into the nerve terminal. iii. Fusion of presynaptic vesicles with plasma membrane of presynaptic membrane and release of Ach through exocytosis. iv. Diffusion of Ach across synaptic cleft.v. Binding of Ach to specific receptor in the postsynaptic membrane.vi. Activation of ligandgated channels by the binding of the neurotransmittervii. Increase in conductance of postsynaptic membrane to both K+ and Na+.viii. Transient depolarization of the postsynaptic membrane known as end plate potential. Which step in Excitation-Contraction-Coupling is affected?Excitation, because succinylcholine competes with acetylcholine for the nicotinic receptors. Give an example of non-depolarizing muscle relaxant. Curare

EXPERIMENT IV: SOME FACTORS AFFECTING HEIGHT OF MUSCLE CONTRACTION (STIMULUS STRENGTH) What was the smallest voltage required to produce a contraction (The threshold voltage)? What was the smallest voltage required to produce the maximum (largest) contraction?What proportion of the fibers in the muscles do you think were contracting to produce this maximal response? The highest muscle contraction even with increasing stimuli is the maximum contraction. At this voltage the fast twitch muscle fibers innervated by the larger motor units are recruited. Thus at this point, all muscle fibers both the slow twitch and fast twitch are contracting. What does the height of contraction represent? It represents the strength of stimulus. As the stimulus increases, the contraction height also increases. What happened to the number of fibers contracting as the voltage was raised from threshold to that required to produce a maximal contraction? The number of fibers contracting increased as the voltage was raised up to the point where ALL motor units have been recruited and no increase in muscle contraction is seen even if there is increase in stimulation. Give the relationship between the strength of stimulus and height of contraction. Does it nullify the all or none principle? The relationship is directly proportional. The All-or-None principle states that

under given conditions the response of a nerve or muscle fiber to stimulus at any strength above the threshold is the same: the muscle responds completely or none at all. It does not nullify the Allor-None principle. This principle only refers to a muscle fiber and not the whole muscle. Graded response is due to the fact that individual muscle fibers within a muscle differ from one another in their threshold. If a plateau is produced in your tracing, explain its occurrence. What type of stimulus strength was delivered to the muscle? What type of summation is demonstrated? If a plateau is produced, it means that all motor units at specific stimulus (supramaximal) are recruited for a specific action. This recruitment is also known as the spatial summation. After the supramaximal, there is no more additional response despite the increase in stimulus. EXERCISE 2- EFFECT OF STRETCH 1. What effects does stretching the muscle have on contraction strength? Is this effect linear?Stretching the muscle increases the force of muscle contraction until the optimum length is reached. This effect however is not linear as the contraction begins to weaken after the optimum point. 2. What stretch resulted in the highest contraction force? What happens to the muscle at the highest stretch levels?At 10mm stretch the force of contraction is still increasing so it is safe to assume that the highest contraction force is beyond 10mm stretch. At optimum level where contraction is greatest, there is the greatest number of cross-

bridges formed due to the large availability of active sites. Beyond optimum the number of cross-bridges begins to fall, decreasing contraction strength. 3. Differentiate preload from afterload. Preload is the load on the muscle before muscle contraction. It determines the initial length of the muscle. It is the optimum length – resting length Afterload is the load on the muscle after the beginning of muscle contraction. It does not change the initial length of the muscle. Explain the relationship between initial length and height of contraction.It is theoretically a bell-shaped curve. The peak of the curve is the optimal length of the muscle which generates the most force. The force-stretch relationship is directly proportional until optimal stretched is reach. Beyond the optimum, the relationship becomes inverse. What is the optimal length of the sarcomere? The optimal length of the sarcomere is the resting length. 2.0-2.2 micrometers Differentiate passive tension from active tension. Passive tension is the tension due to the elastic tissue in the muscle. EXERCISE 3 - SUMMATION 1.How does varying the frequency effect contraction force? Which interval caused the greatest contraction?As the frequency between stimulation is increased, the two peaks eventually fused to form a contraction larger than each individual peak. This effect is termed summation. The 20ms interval had the greatest force produced.

EXERCISE 4- TETANUS . Define tetanus. At which stimulus interval did you observe tetanus? Explain the mechanism behind this phenomenon. A tetanus/tetanic contraction occurswhen a motor unit has been maximally stimulated by its motor neuron. This occurs when a muscle's motor unit has been stimulated by multiple impulses at a sufficiently high frequency. Each stimulus causes a twitch. If stimuli are delivered at low frequency, the tension in the muscle will relax between successive twitches. If stimuli are delivered at high frequency, then the twitches will run together, resulting in tetanic contraction. When tetanized, the contracting tension in the muscle remains constant in a steady state. Graphically, it has a plateau-like appearance signifying constant delivery of stimulus and contraction of muscle. Because of the continuous and constant contraction through stimulation, the calcium ions are no longer able to return to the sarcoplasmic reticulum disabling the muscle to relax. 2. What do you call the frequency used to induce tetanus? Tetanic Frequency or Fusion frequency. This is the minimum stimulus required to elicit tetanus in a muscle 3. Describe how the isolated muscle behaved as the stimulus interval was further decreased? At lower stimulus interval, the muscle was observed to have frequent cycles of contractions and relaxations At higher stimulus intervals, the muscle was observed to be in a constant state of contraction

EXERCISE 5 – FATIGUE 1. Describe how the isolated muscle behaved with continued high frequency stimulation.The muscle was observed to be in a tetanic state until the contractions began to continually decrease in intensity. 2. Provide the possible mechanism as to why the muscle was unable to maintain a prolonged contraction.There are two types of fatigue, high frequency and low frequency fatigue. The experiment showed a high frequency fatigue, therefore, there was NO accumulation of lactic acid as there was not enough time for such event to occur. However, there was a failure of conduction of the action potential because of the very high frequency. * When a failure of action potential conduction occurs, what could be a possible consequence?The calcium ions will not be released because you need action potential conduction through your T tubule in order to release said ions. However, if you let the muscle rest, it will recover very quickly as compared to low frequency where recovery is very difficult. 3. Would your results have differed if you were measuring from a smooth muscle? Why or why not? Yes. Smooth muscles are mainly composed of Type 1 Motor Units, making smooth muscles slow to fatigue with a lower maximum contraction velocity. They are highly vascularized have more mitochondria than skeletal muscle fibers.

EXERCISE 6- RELATION OF LOAD AND HEIGHT OF CONTRACTION 1. What is the relationship between the load and height of contraction?The more you increase the weight, the greater the height of the contraction is. 2. Multiply the height of contraction against the load to determine the maximal load and optimal load.* How do you compute for work? - Work = Force x Distance* How do you compute for work in this set up?- The height of contraction is your distance.- At Load = 0, work = 0, this, therefore is not optimal. 3. No work is performed by the muscle at which weight?Work is zero, when there is no load or when there is no contraction (thus, no height) because the muscle can no longer carry the weight 4. What other factor affecting height of contraction may produce similar tracing on muscle contraction? How is this factor controlled in the experiment?The factors were muscle fatigue, and muscle freshness. These were controlled by resting the muscle for 35 seconds and by applying Ringer’s solution respectively. 5. What is the rationale for adjusting the after load screw?To factor in muscle stretching thus minimizing false results displayed by the stretch not by the load applied.

EXPERIMENT V: ISOMETRIC AND ISOTONIC CONTRACTION Instrument: Dumbbell, Handrip Instrument: Dumbbell 1. Explain why one can no longer flex elbow to the whole ROM (range of movement) beyond the 1 RM. At 1 RM, all of the muscle fibers are contracted to carry the load. At this point, any increase in resistance (>1 RM) restricts reaching the full range of movement since there are no more muscle fibers to contract. Instrument: Hand Grip1. Differentiate isotonic from isomeric contraction. Enumerate examples of movement that will demonstrate each type of muscle contraction. Isotonic contraction occurs when there is a shortening of the muscle given tension or resistance. Isometric contraction occurs when tension is produced with no change in muscle length Lifting a bag of groceries from the floor demonstrates isotonic contraction, while gripping the bag to prevent it from dropping demonstrates isometric contraction. 2. Were you able to sustain the grip on the handle for 10 minutes? If not, why? No.Prolonged contraction causes fatigue in the muscles. The ATP and oxygen in the muscles are depleted and enters an anaerobic state that produces lactic acid as a metabolite.