Twitch Muscle Fibers Essays

Twitch Muscle Fibers Essays Twitch Muscle Fibers Paper Twitch Muscle Fibers Paper muscle contraction Introduction For a skeletal muscle fiber to contract, a stimulus must be applied to it. The stimulus is delivered by a nerve cell, or neuron. A neuron has a threadlike process called and axon that my run 91 cm or more to a muscle. A bundle of such fibers from man different neurons composes a nerve. A neuron that stimulates muscle tissue is called a motor neuron. The motor neuron branches into terminal structures called telodendria that come into close approximation with a portion of the sarcolemma o a muscle fiber (muscle cell). The site where the neuron and muscle fiber meet is called a neuromuscular junction. At this junction the muscle fiber membrane is specialized to form a motor end plate. The sensitivity of a muscle fiber to electrical stimulation through the skin surface is largely a function of the proximity of a neuromuscular junction, the thickness of the skin, and variations in skin conductivity. An electrical stimulus that causes a muscle twitch at one spot may have no effect on muscle fibers that are only a centimeter away. Specific spots on the skin where minimal electrical stimuli cause muscle twitching are called motor points. The purpose of this exercise is to identify specific spots on the skin where these motor points are located. If the electrical stimulus is close enough to pain receptors (naked nerve endings) in the skin, one may experience a slight pricking sensation as the stimulus is applied. Since the skin has many of these pain receptors, this sensation is often experienced; however, a muscle twitch may or may not be produced simultaneously with the pricking sensation. Our intent will be to concentrate on the identification of motor points. Muscles are made up of functional units called motor units. Each motor unit consists of a neuron which supplies one or more muscle fibers. When a motor unit is stimulated, its muscular component will contract with all of the orce they (it) can generate, or they (it) are not contracting at all / in a given muscle there are many motor units, these units vary in their level of irritability. Tat is some will respond to a weak stimulus, while others may require a much stronger stimulus. Often the smaller units are less irritable than the larger units, therefore we often have better control when the task requires only slight strength, and we have much poorer control when greater strength is needed. The overall strength of a contraction of a muscle is the summation of the pull of the motor units operating at any one time. There are two types of summation, spatial and temporal. An increase in the strength of the overall muscle contraction due to spatial summation is brought about by increasing the strength of a stimulus which increases the number of motor units that are operating. Remember that the muscle fibers that are contracting are doing so with all of their strength. Temporal summation is brought about by increasing the rate of stimulus so that the same muscle fibers are contracting again before they have had a chance to completely relax. Each individual fiber can contract more powerfully when the stimuli arrive close together in this fashion, then it can contract in a single twitch. This may at first seem to violate the all or nothing principle. The explanation for temporal summation involves the elastic properties of muscles. Tension developed is translated through many structures. It is transmitted from the cross bridges through thick and thin filaments, across Z lines, extracellular connective tissue, muscle and bone. Each of these components has a certain amount of elasticity. Only when all of these elastic structures are taut can increasing contraction by the muscle occur. When a second stimulus occurs very close to the first the elastic structures are not yet slack and the result is the contraction is stronger than any simple muscle twitch. Temporal summation will not be studied at this time. Human muscle can be directly stimulated through the skin using a relatively strong stimulus. When stimulating skeletal muscles in this way, certain sensitive spots can be located that elicit a much greater response. These spots are called motor points. Motor points are usually located directly over the area where the nerve supplying the muscle being stimulated enters the muscles, often the belly of the muscle. IN the first part of this exercise, we will study the motor points of the hand using the isolated square wave stimulator. In the second part of this exercise we will use the Physiogrip system to study the details of a single muscle twitch which can be separated into three phases; the latent period, the period of contracting, and the period of relaxation. Refer to Chapter 10 for details of muscle physiology. We will establish the minimal or threshold stimulus required to cause your flexor pollicis brevis or flexor digitorum muscle to contract, and see the consequences of increasing the stimulus strength which increases the force of contraction due solely to spatial summation. The flexor pollicis brevis is located at the base of the thumb, if flexes and adducts the thumb, and it is innervated by the median and ulnar nerve. The flexor digitorum superficialis is located on the inside of the forearm, and while not a deep muscle, it is beneath the palmaris longus, flexor carpi radialis, pronator teres and the brachioradialis. The flexor digitorum superficialis flexes first the middle and then the proximal phalanges. This muscle is involved only in rapid, forceful flexion of the digits in grasping movements; it is not at all involved with gentle, unresisted flexion. The flexor digitorum superficialis is innervated by the muscular branch of the median nerve. The third part of this exercise will involve further analysis of the simple muscle twitch and the knee jerk reflex using the Flexicomp program. Using a transducer attached to your knee the physiology of the knee jerk reflex (an example of a monosynaptic reflex arc) will be studied. Procedure Introduction: Each experiment (motor points of the hand, Physiogrip, and Flexicomp) will be performed by the same groups of 3 students. One member will be the subject, a second will be the stimulator or apparatus operator, and the third member will be the recorder. Subject Preparation: For each procedure there will be a considerable amount of preparation. Take notes while your instructor discusses each procedure. For determining the motor points of the hand, use the abrasive pad, detergent and paper towels (or Kleenex) to thoroughly clean your hand. The superficial layers of the stratum corneum and excessive skin oils must be removed to obtain proper conductivity. Remove rings which will interfere with identification of motor points. After the skin is completely dried position the subject’s arm in a relaxed manner on the table, with palm up. Electrode gel will be applied to the areas to be tested (hand and forearm) to improve conductivity. The Physiogrip procedure involves attaching a flat electrode (with electrode gel) to the back of the hand while probing for the motor point of the flexor pollicis brevis or flexor digitorum superficialis muscles. The Flexicomp procedure involves attaching a knee jerk transducer to the knee and then properly calibrating it using onscreen directions and the help or your instructor. Stimulators: Your instructor will discuss the stimulators. Take notes. The electrical stimuli to be applied to the skin will be minimal, or threshold, must sufficient to elcit a response. The threshold stimulus is governed by duration and voltage. If the duration is too short, more voltage is required. Before starting each experiment make sure the stimulator voltage is zero. As the voltage is increased in 5-volt increments, a point will be reached when a muscle twitch will occur, or the subject will indicate a sensation of minor pain. If a twitch occurs, you have reached the minimum voltage or threshold for muscle stimulation. Stimulation process: For determining the motor points of the hand, outline your hand in the report section and record the minimum voltage required to elicit a twitch for each spot. Continue to increase the voltage and to probe the hand until 2 or more motor points have been identified or until the stimulation process becomes uncomfortable. For the Physiogrip procedure, locate the motor point for the flexor pollicis brevis or other flexor muscles of the hand muscle. Follow the onscreen directions of the computer to determine muscle twitch information. Your instructor will help you select the proper responses. For the Flexicomp a modified patellar hammer will be used to stimulate the knee. When you tap the patellar ligament a simultaneous signal will be sent to the computer. Your instructor will show you how to print out and evaluate the simple muscle twitch obtained using the Flexicomp equipment. Detailed Setup for the Physiogrip Procedure: . Put a small amount of electrode gel on the flat electrode and strap the electrode to the back of your right or left wrist. The wrist area should be cleaned with alcohol prior to attachment of the electrode. Put a small amount of electrode gel on the front of the wrist. This gel will be spread around with the stimulus electrode as you search for the flexor digitorum superficialis mo tor point. 2. Grasp the pistol grip and apply enough pressure on the trigger to lift the screen plot off the bottom of the screen. 3. Your lab instructor will show you how to analyze and print out your data. Staple your Physiogrip and Flexicomp printouts to the Lab Report #14. Label your data with the following information: a. The stimulus voltage b. The latent period and its duration in seconds c. The contraction period and its duration in seconds d. The relaxation period and its duration in seconds 1. Threshold voltages for motor points in the hand can vary from person to person. What could account for these differences in threshold voltages? The threshold voltages for motor points in the hand can vary from person to person due to the thickness of the skin, dirt on the hand, or oily hands. . Outline your hand in the space below and then for each part of the palm and fingers record the minimum voltage required to elicit a muscle twitch. [pic] A = 230 X 0. 1 = 23 mv (lumbricales) B = 350 X 0. 1 = 35 mv (Flexor Pollicis Brevis) 3. ) When a nerve cell is at rest, there is a relatively greater concentration of sodium ions on the outside of its membrane 4. ) When sodium ions are actively tra nsported outward through a nerve cell membrane, they are exchanged for K+ ions which are transported inward. 5. The difference in electrical charge between the inside and the outside of a resting nerve cell (neuron) is called the resting membrane potential. 6. ) If the resting potential becomes less negative (-70 mV toward 0) in response to stimulation, the process is called depolarizing graded potential. 7. ) Any stimulus weaker than a threshold stimulus is termed a subthreshold stimulus. 8. ) If a series of subthreshold stimuli is quickly applied to the neuron, the cumulative effect may be sufficient to initiate an impulse. This phenomenon is called temporal summation. 9. Following depolarization, potassium ions diffuse outward and cause the cell membrane to become more negative. 10. ) An action potential (or nerve impulse) is a rapid sequence of changes involving the inward movement of Na+ ions followed by the outward movement of K+ ions. 11. ) Following the passage of a nerve im pulse there is a period during which a threshold stimulus will not trigger another impulse. This period is called the refractory period. 12. ) A single nerve cell, just like a single muscle fiber, transmits an action potential according to the all or none principle. 3. ) Nodes of Ranvier (neurofibral nodes) occur between adjacent axon segments of the Schwann cell. 14. The type of nerve conduction in which and impulse jumps from node to node is called saltatory conduction. 15. ) The greater the diameter of a nerve fiber, the faster the impulse travels. 16. ) Explain in detail what physiological events are taking place during the latent period. In your discussion explain the difference between the â€Å"latent period† for the Physiogrip experiment and the â€Å"latent period† for the Flexicomp experiment. In the laboratory, a witch can be produced by direct electrical stimulation of a motor neuron or its muscle fibers. The record of a muscle contraction is called a myogram. Twitches of skeletal muscle fibers last anywhere form 20-200 Msec. This is very long compared to the brief 1-2 msec that a muscle action potential lasts. Note that a brief delay occurs between application of the stimulus and the beginning of contraction. The delay, which lasts about 2 msec, is termed the latent period. During the latent period, the muscle action potential sweeps over the sarcolemma and calcium ions are released from the sarcoplasmic reticulum. If two stimuli are applied, one immediately after the other, the muscle will respond to the first stimulus, but not to the second. When a muscle fiber receives enough stimulation to contract, it temporarily loses its excitability and cannot respond for a time. Physiogrip: It also showed that there was a less latent period because it only interacted with the nerves to the fingers. Flexicomp: The impulse generated below the knee has to travel to the spinal cord and then reaction takes place, making the latent period longer.