GENERAL MUSCLE CHARASTARISTIC AND FIBER TYPES

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2 GENERAL MUSCLE CHARASTARISTIC AND FIBER TYPES UNITARY CONTRACTION OF SMOOTH MUSCLE Smooth muscles are present in hollow/visceral organs, like the Gastrointestinal tract (GIT), Urinary Bladder, and Blood Vessels. The innervation of smooth muscles is fused, which means a branch of a nerve ending with an enlargement (nerve bulb) containing the neurotransmitters that will be released in a diffuse pattern on the surface. In our body, we have two structural types of smooth muscles; the first type, we have a smooth muscle that their cells (smooth muscle cells) are connected with each other by hundreds to thousands of low resistance gaps called gap junctions. So, once we have a release of neurotransmitters at the area of neuromuscular junction of the nerve end in the smooth muscle, the electrical signals can be transmitted between the smooth muscle cells, like the electrical synapse. This type of smooth muscle is found in GIT, in which the smooth muscle will contract as a single unit, and we called it Single Unit Arrangement of Smooth Muscles (SUASM). The second type, is found in some types of smooth muscles, in which the signal transmission is closely related to skeletal muscles. We have a lot of signal transmission units (autonomic neurons, or synapses), but less (or no) amount of gap junctions. It doesn't have a 1:1 relationship with skeletal muscle mechanism, but there is a close association. This type of smooth muscles is found in the Blood Vessels, in which we want just to contract a specific part of the circulatory system. So, many units of smooth muscles contract at different time, according to the needed part to perform contraction, by the action of many neuronal activation units, and we called them Multi-Unit Arrangement Of Smooth Muscles (MUASM). So, a single smooth muscle is innervated by just ONE synapse. Structural Type of smooth muscle The contraction pattern Amount of Gap Junctions Anatomical example SUASM (Single-Unit) One signal unit will contract the whole SM as a one piece High GIT MUASM (Multi-Unit) Many signal units are present to perform the function of contraction Low Blood Vessels 1

3 For example (on MUASM), if we have a cut/injury in the skin, the body will undergo a vasoconstriction to the blood vessels of the injured area of the skin not to the whole Vascular System. SMOOTH MUSCLE TONE/TENSION Sphincters of the GIT are activated in a tonic way (i.e., there is tension all the time). At the neuromuscular junction in smooth muscles we have different types of neurotransmitters depending on the state/function needed to be performed by the muscle. For example, in a (1): normal fully contracted smooth muscle of sphincter, we have a neurotransmitter to inhibit that contraction (i.e., inhibitory neurotransmitters) when the muscle wants to relax, to expel the GI content the other compartment, this what is called Bi-phasic pattern of contraction-relaxation to propel the GIT content. In a (2): normal relaxed smooth muscle of the Urinary Bladder, only the stimulus (when the Bladder is full) will contract the smooth muscle, by the action of activator neurotransmitters. CHARACTERISTICS OF SKELETAL MUSCLE CONTRACTION A single skeletal muscle contraction, will give us a muscle twitch (contraction phase + relaxation phase). [See FIGURE.1] The magnitude (force) of contraction could be recruited by (1): activating/recruiting more motor units * (i.e., the stronger the stimulus, the more motor units will be recruited, the more force generated by muscle contraction). The smaller motor units will be activated at the first, followed by the larger in size, and so on, and this match the force needed with the task. So, it is a motor-size increment (by size) according to the force needed to perform a task. Also, (2): we can increase the force, by increasing the frequency of stimulation by increasing the duration of the load-lifting ( duration of lifting a heavy object, the frequency of stimulation, the force produced by contraction). So, the maximum force is produced by carrying the heavy object for a duration enough to recruit all the motor units, which is the Tetanus Frequency (high frequency in which the force shows no waviness). So, duration of lifting a heavy object, the frequency of stimulation, the duration of the releasing of myoplasmic Calcium (Ca 2+ ), so, more time for myosin and actin to form cross-bridges, higher force to get.] 2

4 NOTES: Single action potential (stimulation) causes single muscle contraction (Twitch). Twitch three phases : Latent (L), contraction (C) and relaxation (R). Don't confuse the action potential (AP) with the muscle twitch. NOTE: [See FIGURE.2] The muscle twitch lasts much longer than the action potential. A very short stimulus causes a single muscle contraction (twitch). Force rises then falls, the falling time is longer than the rise time. So, the contraction force could be increased by: 1- Recruitment of more motor units: Recruitment mechanism: Gradual increase in stimulus strength produces stronger twitch, as progressively increasing stimulus activates more motor neurons, which activates more motor units which leads to more force. [See FIGURE.3] 2- Increasing the frequency of stimulation: Increasing the duration of stimulation. 3

5 TYPES OF SKELETAL MUSCLE CONTRACTIONS Contraction IS NOT the shortening of the muscle, it is the activation of the muscle. We have three types of skeletal muscle contractions: (1): Isometric contraction, (2): Isotonic contraction, and (3): Eccentric (lengthening) contraction. (1): Isometric Contraction (IMC) If you take a single muscle, and you attach it to a fixed area (point), and the other part of the muscle to a force transducer (we are keeping the length of the muscle, constant length), then we stimulate the muscle by an external stimulus (electrical stimulation), we will record (on a strip-chart recorder) a muscle twitch, without shortening of the muscle. This is an Isometric contraction. An example on (IMC) is carrying a heavy subject against the gravity (keyword to differentiate between IMC and ITC), or pushing on the wall; by bushing on the wall, the muscles of your hands will develop a tension without any shortening. [See FIGURE.4 (a, b)] 4

6 (2): Isotonic Contraction (ITC) The modification of the setup (the process); is to add an after load linked to a movable lever to the muscle. By changing the magnitude of the after load, we can change the length of the muscle. Then, we will stimulate that single muscle, by an electrical stimulator, we will record tension with different muscle twitch graph shape (splat area). When we stimulate that muscle, at first, the muscle will not be able to carry the after load (object) until the development of a force equal weight of that object. This (i.e., the start) is an Isometric Contraction (i.e., there is no change in length). After the development of a force equal to the after load, the muscle will start shorten, with the same, constant force (the force will NOT change beyond the after load). An example, when you put a load on your palm, just before you want to carry it upward by rising the palm (by the action of the Biceps muscle), the muscle will generate a force equal to load on your palm without any change of the length of the muscle this is the IMC. Then, you can lift this object (or load), this is the ITC. [See FIGURE.5] - So, all in all, the ITC has 2 phases: a) IMC phase: the muscle will generate a force equal to the load, with no change in length. b) ITC phase: the muscle will not generate any additional force, and there will be a shorten in the muscle. (3): Eccentric (lengthening) Contraction Lengthening contraction happens when you carry a load (object), which is really very heavy, and this will extend (stretch) your Biceps, for example. It is a very strong contraction. It is much higher than the ITC (shortening contraction). Lengthening contraction might lead to an injury to the extended muscle. [See FIGURE.5 (c)] 5

7 TOTAL TENSION, PASSIVE TENSION, AND ACTIVE TENSION We have a relationship between the force developed and the length of the muscle. We have a tension called passive tension, which is the tension that will be formed if we stretch the muscle without stimulation. (increasing the tension without activation of the muscle). This passive tension is produced due to the elasticity of the muscle. The relationship is NOT linear. We have another tension called active tension, which is the tension produced by the activation of muscles. FORCE-LENGTH RELATIONSHIP The sliding theory (the over-lapping b/w the thin and thick filaments) explains this length-tension relationship. The length of the myosin is 1.65 µm, and we have 1 µm of thin (actin) filament from each Z-line. So, we will have 3.65 µm of a sarcomere (fully extended sarcomere) if we don't have any overlapping b/w thin and thick filaments (over-stretching the muscle = passive tension, but very little in amount), the TT = 0, there is no overlapping (no cross-bridges formation, no force, no tension) in an activated over-stretched muscle. If we decrease the width of the sarcomere (less than 3.65 µm) by forming more cross-bridges, until we reach the maximum overlapping, which is 2.2 µm. So, maximum overlapping means maximum tension. The further overlapping will not produce over tension, until the length 1.95 µm, because there is not no further myosin to bind with actin at the center of the sarcomere. When the thin filaments are doubled, at this time the force will start to decrease, there will be distortion of the thin filaments that will approach each other more than the needed length. So, less cross-bridges are formed (at shorter length the thin filaments begin to run into each other and the number of cross bridges decrease). After the thin filaments reach the length of the thick filaments, they will butt up against the Z-disk the force falls precipitously, results in distortion of the structure sarcomere. [See FIGURE.6] 6

8 The active tension (AT) = the total tension (TT) the passive tension (PT). Cross bridges can be decreased by: a) Extending the muscle (so no overlap, the PT > AT.) b) Shortening the muscle (so the overlap will decrease, the PT = 0, AT is very low). - At the AT, we will get the maximum contraction, and it is the resting length of the muscle. [See FIGURE.7] By lengthening (stretching) your muscle, this will give you a higher force. The length of the muscle is limited by the origin and the insertion of that muscle. (The velocity of muscle contraction developed is inversely related to the rate of muscle contraction, and with the afterload). 7

9 Explanation: If you have a very heavy weight, your muscle (for example, Biceps muscle) will be shorten very slowly (this is if you want to carry this heavy object upward), on the other hand, if you have a very light weight, your muscle will be shorten very quickly. So, at Zero velocity we will have maximum force (at this point we will have an IMC), and at Zero force we will have maximum velocity. All the previous cases are at zero and positive velocities. Negative velocities means lengthening of the muscle. [See FIGURE.8 (a)] NOTE: The gluteus muscle, at the standing position has a passive tension, due to the extending of its fibers, when you carry a heavy object, this will contract the muscle and produce an active tension, so, we are having a total tension (TT). MUSCLE POWER: (WORK OUTPUT) The muscle power (or work) is one of the most important feature of the muscles. Power is the force times the velocity. The peak of the muscle power is not at the highest velocity or the highest tension. The peak of the muscle power is at about onethird of maximal force, and one-third of the maximal velocity. In order to increase the muscle work output you have to change both parameters (decreasing the velocity (V) and increasing the force (F) /tension). Now, increasing both the F and V needs training and time (like in the boxing sport). [See FIGURE.8 (b)] SKELETAL MUSCLE TONE Without any stimulation we have a specific tension (basal motor activity) in our muscles. Even when muscles are at rest, a certain amount of tautness usually 8

10 remains. This is the muscle tone that results from the basal motor-neuron activity of your muscles. Because normal skeletal muscle fibers do not contract without an action potential to stimulate the fibers, skeletal muscle tone results entirely from a low rate of nerve impulses coming from the spinal cord. FIBER TYPES AND MUSCLE ENERGETICS If we have different types of muscles, like Extra-ocular muscle, Gastrocnemius, and Soleus. And you put them on a twitches maker-recorder (a device). You will have a different muscle twitch time, with different tensions. So, muscle types can be distinguished by their contractile appearance. So, we have fast muscle fibers (Extraocular muscle), intermediate muscle fibers (Gastrocnemius), and slow muscle fibers (Soleus). This variation is essential to allow the muscle you perform its main function. For example, if you need a continuous contraction (sustain a long-duration tension), with low velocity, this an example of the Soleus. And for the Extra-ocular muscle (eye or eyelids movement) you need the opposite. This various contractile appearance, is due to different ATPase activity of myosin heads (or the turn-over rate). So, a high cycling rate, results in a high contraction velocity. So, high turn-over, means high velocity, and vice-versa. No single muscle has one type of the muscle fibers. They contains the three types of muscle fibers, but in different percentages. For example, most of the Soleus muscle fibers are slow fibers, and in the Extra-ocular muscle most of the fibers are fast. And this is determined by the genetic code (gene isoforms of myosin molecules). So, Slow, intermediate, and fast ATPase myosin heads activity are determined by different genes (each type has a unique gene). By using myosin stain, different fibers are stained according their type (i.e., fast, intermediate, and slow). And the myosin staining process will give us two results: (1): Type I fibers (slow, red muscles), (2): Type II fibers (fast, white muscles); which is subdivided into two subtypes: Type IIa (fast, oxidative), Type IIb (fastest, glycolytic). [See FIGURE.10] 9

11 NOTE: There is also another type of Type II, which is Type IIx (not required). Now, muscles can be classified (Peter and coworker classification) based on their metabolic properties (the source of the energy): 1.Slow oxidative (SO): contracts slowly, for long duration, and no need for a fast supply of ATP by the glycolysis pathway. It uses the oxidation pathway (oxidative phosphorylation of glucose) to get ATP, by Electron Transport Chain (ETC) in the mitochondria.so contains a lot of mitochondria and myoglobin, large oxidative capacity, slower, and fatigue resistant. (SO is Type I) 2.Fast glycolytic (FG): depends on glycolysis, which an anaerobic production of ATP, before the step of the mitochondrial oxidation. (FG is Type IIb) 3.Fast oxidative-glycolytic (FOG): utilize both oxidative and glycolytic pathways (intermediate muscle fibers type).(fog is type IIa) ***********UNTIL HERE IS THE NEEDED MATERIAL**************** - Burke classified muscle fiber based of their mechanical properties into: 1.Slow (S) 2. Fast fatigue resistant (FR) 3. Fast intermediate (FI) 4. Fast fatigable (FF) - Also, Muscle fiber types differ also in the isoforms of many different proteins for example: Fast twitch fiber contains SERCA1a & TnC2, while slow twitch fiber contains SERCA2a & TnC1. ** TnC: Troponin C. - Also, Muscle fiber types also differ in the relative amount of organelles: 1. Mitochondria (more in slow oxidative or Type I) 2. SR volume 3. SR calcium pump 10

12 4. Myoglobin (more in slow oxidative or Type I) - [IMPORTANT]: ATP hydrolysis is the source of energy for mechanical work (Cross-bridges formation) by the action of Myosin ATPase. [ATP (H2O) ADP + Pi + Energy 57 KJ] (Part used in the process of contraction + heat) SOURCES OF ATP 1.Cytoplasmic ATP (5 mµ) can support full contraction for about 1-2 second at most. 2.Creatine phosphate (CP) regenerates ATP fastest to its normal cytoplasmic concentration: CP + ADP = ATP + Creatine. This source of energy supports maximal muscle contraction for another 5 to 8 seconds. 3. Myokinase enzyme, generate ATP from ADP. 4.Glycolysis: rapid, but low capacity supply of ATP for fast twitch fibers (Glycogen or blood). Each 1 glucose gives a net of 2 ATP. 5. Oxidative phosphorylation: slower, but high capacity source of ATP for slow twitch fibers: Electron transport chain 1 glucose gives a net of 30 ATP. - All muscle contractions use the sources Our body utilizes 3 fatty acid to produce 1 ATP (at rest), if we increase our physical activity our fuel source will be shifted to Glucose, if we reach our physical activity to a sever level, we will use glycogen and glycolysis. Moderate, Long period of exercise will burn fatty acid rather than glucose (a good way to lose excess fat). MUSCLE FATIGUE Muscle fatigue is a reduction in developed force resulting from previous muscle activity. So, the maximal force that can be generated from resting muscle, any decrease of this maximal force is called fatigue. Maximal force can be sustained only very short time (only once). Now, Metabolic fatigue is a reduction in submaximal force after prolonged repetitive stimulation, usually exercise is done at submaximal force from many repetition, then we become tired and be unable to do this submaximal force. This is because of the increase of the level of lactic acid in our blood. Because of the usage of fast fibers type IIb. This is because of the conversion of pyruvate to lactate to supply the oxidizing agents. Pyruvate will affects the inorganic phosphate on the myosin head, so this will appears like fatigue. We said that the force of your muscle can be increased by: recruitment, and increasing the frequency of stimulation. Also the muscle mass (cross-sectional area) is a factor. **Signals that control muscle mass (increase the amount of the contractile proteins/ hypertrophy "mainly" more satellite cells, hyperplasia): 11

13 1.Stretch (positive regulator) 2.Hypoxia (positive regulator) 3.Androgens (positive regulator) 4.glucocorticoids (positive regulator) 5.Ca 2+ (positive regulator) 6.Myostatin (negative regulator): it is a gene, that will be expressed to proteins to suppress the muscle overgrowth. In animals, we knock-out that gene, the animal will become muscular, and lean (because it will also affects the adipo-genesis). Your colleague: Qaisar Maaya'h 12