During the following class we will develop all the processes that lead to muscular contraction and relaxation. We will be interested to analyze biochemistry and mechanical processes and also do not forget the catalyzing factor: the nervous signal. Muscle When we make any reference to the muscle we are talking about a tissue, this means a cellular composition that shares a particular function, contraction in this case. These muscular cells known as miocytes are surrounded by intercellular substance and connective tissue.

The connective tissue that surrounds the miocytes is filled with blood vessels that will provide nutrients and collect discarded elements produced by their activity. Muscular tissue has two basic properties that distinguish them from other tissues: It is an excitable tissue that is able to generate an action potential that will be the key to muscular contraction. It is also a tissue that is able to transform the chemical energy provided by TPA’s hydrolysis into mechanical energy (work, contraction) and heat (unutilized energy). Functions As we have mentioned it before the activity developed by the muscular tissue is contraction and relaxation. Because of this fact, the muscle develops several functions in different parts of the body.


Locomotion: muscular activity allows us to get around. Evacuation of visceral contents: muscular contraction allows us to empty organs such as the stomach and the bladder. Mastication, ingestion: muscles make movements, which lead to alimentary bolus and its arrival to the stomach. Arterial pressure regulation: the blood vessels and heart walls are made of miocytes. Contraction Types We will briefly describe two types of muscular contraction, which might take place. Isotonic contraction is an activity characterized by the muscle’s shortening. In order to consider a muscular activity an isotonic one, the charge must be constant.

Isometric contraction is carried through the muscular fiber fixation, the muscle’s shortening is thus impeded, and its length is constantly kept. During isometric activity, muscular tension raises substantially. Miocyte The miocyte or muscular fiber is a contraction highly specialized cell with an important concentration of myofibrils (hundreds or even thousands). Its extreme specialization has allowed it to lose its splitting ability. Miocytes do not carry on mitosis. When analyzing cellular morphology we find two types of muscles in the body: Striated, which characterizes by possessing transversal striations (formations only visible if Optic Microscope is utilized). These striations have their origin in a structure named sarcomerus, which allows a faster contraction. There are two types of striated muscle, the skeletal and the cardiac one.


The skeletal we find it in all muscles which allow our locomotion and our stand up position. Its innervations are voluntary, which means that we regulate their activity consciously. The cardiac muscle is the one that builds the heart walls, and carries the responsibility the pumping of blood throughout the organism. In this case, its regulation is involuntary. Straight or Plain muscle: This type of muscle possesses myofibrils. However they do not organize themselves forming a sarcomerus. Its contraction is slower than in the striated case, but it is also more resistant and prolonged (that is why we find ´plain muscles forming the walls of the stomach or the vessels which have a constant contractile activity), Its nervous regulation is involuntary (autonomic). TPA Before we continue, it is necessary to include a brief description of the energy exchange coin that is used by our organism and also our muscular tissue: the TPA. The Tri-phosphate Adenosine is a molecule that possesses two high-energy links, which are established between two phosphate groups. These link’ rupture (generally carried through hydrolysis) produces the liberation of a great amount of energy used by cells’ metabolism of our whole body. In order to form TPA it is necessary to obtain nutrients through alimentation (mainly fats, carbon hydrates, to a lesser extent proteins) and also oxygen obtained through lung gaseous exchange. Two processes lead to the TPA’s synthesis: The oxidation-phosphorilation, which is carried in the mitochondria (slow but lasting process) and the Glucolisis which takes place within the cytoplasm (faster but less lasting).

THEORETICAL MODEL: ORGANIZATION OF THE SKELETAL MUSCLE, SARCOMERUS

Muscular tissue characteristics In order to comprehend the muscular tissue metabolism it is necessary to retain the following cells characteristics: High specialization level (contraction): Biochemistry structure, which allows this contraction, is the myofibrils. They do not reach mitoses nor do they regenerate themselves. Cell division is genetically inactivated. They grow when the cells volume rises. This is called hypertrophy. It happens due to a rise in the proteins’ synthesis speed (actinia and myosin). In this case, proteins degradation speed maintains itself constant. Muscular fibers have a high mitochondria density (thus favoring the oxidative phosphorilation) and a protein denominated mioglobine, which allows the oxygen fixation. These two characteristics allow a high and efficient TPA synthesis. We should not forget that muscular contraction implies a TPA high level of utilization. In the myocites’ cytoplasm we find a known formation of T tubules. These tubules are an encapsulation of the cell membrane: Nervous terminals reach this encapsulation (axons of the a moto-neuron), which will transmit the necessary electrical impulse for contraction. The structure formed by the nervous terminal and the T tubule is called Motor Plaque. The sarcoplasmic reticule (or endoplasmic reticule) is very developed within the myocites. This microscopic organism becomes the intracellular deposit of Calcium. This ion is indispensable for the propagation of the contraction signal. The fibers (myocites) group themselves forming fascicles. And fascicles do the same in order to form muscles. We can find two varieties of skeletal muscular fibers. In order to make this distinction the use of an electronic microscope is necessary.


Type I: they are small cells. In their cytoplasm we might find numerous mitochondria and abundant mioglobine. These myocites are much irrigated and receive a great amount of oxygen. Due to this factor, and compared to the Type II cells, their TPA hydrolysis speed is relatively low. And their contraction is slow. However, since they possess a high amount of TPA reserves, contraction ends to be persistent (higher resistance to tiredness). Mioglobine high presence gives it a reddish coloration. High percentages of this type of muscular fiber are to be found in posture and anti-gravitating muscles.

Type II: They are larger. Their mitochondria amount is lower and irrigation is less profuse. However, they possess a large sarcoplasmic reticule. These characteristics favor a fast liberation of calcium. This fact allows a very oldstrong contraction. Generally speaking the contractions are short and intense. There exists a great amount of glucolitic enzymes to allow a fast liberation of energy. Mioglobine’s deficit provokes a whitish coloration. Miofibriles We will now analyze the elements, which give a structure to the muscular fibers. Myofibrils are cylindrical structures formed by two contractile proteins: actinia and myosin. These proteins are polymerized and form microfilaments and their particular organization will origin the myofibrils. Secondarily, we also find other two proteins forming these microfilaments: tropomiosine and troponines (C, I and T). Actinia It is the main component of the thin microfilaments. We find it in two shapes: globular and fibrillar. Globular actinia (G) is formed by protein individual units. On the other hand, fibrillar actinia (F) is a polymerization of the actinia G units, which specially organize forming a sort of double helix. This structure maintains a stable constitution because of the presence of two other proteins: Troponine (three types): Troponine I that inhibits the union between actinia and myosin (necessary for contraction). At rest, it is united to actinia. During contraction, it frees it. Troponine C is a calcium linking protein. Once it links calcium, it separates Troponine I from Actinia, thus liberating the last one. Troponine T: its function is structural. It joins C and I Troponines with Tropomiosine. Tropomiosine: This protein covers the joining links between the myosin and actinia transversal bridges. During contraction frees these links allowing the union of actinia and myosin. Myosin Myosin forms thick microfilaments (myosin microfilaments). Myosin molecule is formed by a head (normally covered by a TPA molecule which has the ability to hydrolyze it) and a tail with a structural function. Sarcomerus It is the structural unit of the muscular contraction. It is formed by a regular sequence of dark and clear stripes, which give the myocite a typical striated aspect. We will now describe each of the stripes, which form a sarcomerus. Z Disks: they constitute the sarcomerus’ limits. A protein called actinine where the actinia microfilaments organize themselves forms them. I Band: this band is formed by actinia thin filaments, with no superposition whatsoever with other filaments. It is a clear band or stripe (in a polarized light microscope we can watch it as isotropic). It is located at the sarcomerus’ ends. During contraction it shortens. Its length diminishes since actinia will to a great extent superpose with myosin. A Band: myosin thick filaments form it. And two sections can be clearly be distinguished. One of them shows no superposition. The other one shows superposition of actinia and myosin filaments. It is a dark band (an isotropic) and it occupies the sarcomerus’ center. During contraction its length remains constant. H Band: It is formed by myosin filaments without any superposition (it is included within Band A). It shortens during contraction. M Band: It is found in the H Band middle. It is formed by proteins, which support and organize the thick myosin filaments.

FILAMENTS’ SLIDING THEORY Sarcomerus shortening During relaxation, the sarcomerus’ length is of 2 to 4 mm. In order to reach muscular contraction thin filaments slide over the thick ones towards the sarcomerus center. Facts usually associated to contraction are the following: Formation of Transversal Bridges (reversible chemical union) between the actinia and myosin filaments Association between actinia and myosin heads: This is possible since the myosin head hydrolyzes the TPA, which obstructs it during relaxation. Freed energy is in part utilized to make a swinging movement destined to produce the actinia movement towards the sarcomerus center. During relaxation the myosin head disassociates itself from the actinia and a TPA molecule covers it again. Excito-Contractile mating A nervous signal is necessary in order to unlock the facts, which produce muscular contraction, and it comes from the central nervous system. This signal produces a depolarization of the cell membrane. This depolarization generates a signal, which reaches the Sarcoplasmic reticule through the T tubule. Inside the intracellular deposits, Calcium is thus freed. This fact produces the union of Calcium and C Troponine. I Troponine and actinia union is weakened and actinia-myosin union gets oldstronger. During relaxation calcium is recaptured by a TPA dependent pump, which is found within the sarcoplasmic reticule membrane. Two enzymes participate in this process: calsecuesrine and parbalbumine.



Both are present in the sarcoplasmuc reticule. Muscular contraction and relaxation are two active processes, which require TPA: CONTRACTION IS AN INTEGRATED SEQUENCE OF EVENTS We will now explain the events that are involved during muscular contraction. We will divide into three moments. Moment Zero At this point, the plasmatic membrane is polarized, Calcium is found in the sarcoplasmic reticule, I Troponine is united to actinia and the myosin head is obstructed by a TPA. Sarcomerus is relaxed. Under these circumstances a depolarizing nervous impulse is launched from the central nervous system. Moment One Calcium deposits liberation is a consequence of the depolarizing impulse, which comes from the T tubule to the sarcolplasmic reticule. This ion is transported because of a gradient concentration (passive transportation). Calcium will unite C Troponine provoking I Troponine liberation of actinia. Also, Calcium provokes the TPA hydrolysis found in the myosin head thus freeing the necessary energy, which will allow contraction. Consequently the myosin-free head joins the actinia displacing it towards the sarcomerus center. Muscle is contracted. Moment Two Calcium returns to the sarcoplasmic reticule through a TPA usage since it does so in a counter gradient manner. C Troponine abandons its union to Calcium, and allowing the I Troponine to join the actinia. This TPA molecule returns to cover the myosin head. The muscle is relaxed once again. Chemical energy conversion into heat and mechanical energy Chemical energy transformation into a mechanical one and heat is directly proportional to Contraction’s duration and tension and follows this equation: Chemical energy = mechanical energy plus heat Global mechanical efficiency of muscular contraction reaches 20%. This means that out of the TPA hydrolysis produced energy only a 20% is used in contraction and some 80% turns into heat or it is used in the energy’s recovery (re-captivation of Calcium, re-synthesis of TPA). Maximum Contraction Strength per Muscular Section This parameter is equal in men and women and it reaches a value of 2 or 3 kg/cm2 of each muscular section. However, and due to hormonal differences, men has more muscular mass than women whereas women tend to possess more adipose tissue. This is the reason why men can develop more strength that women. Skeletal Muscle excitation Skeletal muscle fibers are innervated by nervous fibers, which are originated in the moto-neurons within the anterior segment f the Spine Medulla. Each nervous end establishes a neuromuscular union. Nervous fiber branches encapsulate within the muscular fiber interior. This structure is called terminal motor plaque. Contraction is provoked by the following events. A nervous impulse should reach the neuromuscular union in order to produce the liberation of acetilcoline. Acetilcoline unchains the opening of the ionic channels, which are in the cell membrane. The opening of these channels provokes the entering of large quantities of sodium ions (Na+) to the cell interior and they will produce the membrane depolarization. An enzyme known as acetilcolinesterase will later take care of the acetilcoline hydrolyzing thus closing the Na+ channels. The knowledge of these processes is very important to analyze the actions of nicotine or nervous gas, for example. Nicotine shows an acetilcoline analogous action. Fluor-phosphate Disopropile (nervous gas) inactivates the acetilcolinesterase during weeks. Both actions might produce muscular spasms. Nervous gas case is very worrying since it produces asphyxia through larynges spasms. Nicotine overdose might also produce that phenomenon.

ANATOMIC AND PHYSIOLOGICAL INTEGRATION OF THE NERVOUS SYSTEM

According to its topographic distribution we can divide our nervous system into the CNS (central nervous system) formed by the spine medulla and the encephalon (encephalic trunk, cerebellum and brain) and the PNS (peripheral nervous system) integrated by peripheral nerves and glands. According to its function we might distinguish a SNS (somatic nervous system), which is characterized by being conscious and voluntary, and an ANS (autonomic nervous system) which s unconscious, involuntary and automatic. Within the ANS we find two subdivisions: the Sympathetic and the Para-sympathetic. The sympathetic characterizes itself by the utilization of neurotransmitters such as nor adrenalin and adrenaline. This system is activated in stress situations such as fasting, exercising, struggle or fleeing. This response shows through vessel constriction, rise of the five cardiac properties (automatism, excitability, conductibility, contractility and relaxation), bronco dilation, midriasis, glucemia rise, blood flow rise towards the skeletal muscle, and gastrointestinal blood flow diminution. On the other hand, the Para-sympathetic ANS uses acetilcoline (Ach) as neurotransmitter. This system is activated during resting and satiation (postprandium). Para-sympathetic response characterizes itself through vessel dilation, five cardiac properties diminution, vessel constriction, meiosis, glucemia diminution, blood flow diminution towards the skeletal muscle, gastrointestinal blood flow diminution, acid secretion rise, motility, stomach emptying, and motility rise and transit in the thin intestine.

MOTOR SYSTEM The somatic motor system is organized in three hierarchical levels: Movement Diagramming Decision of the type of program to be executed: It requires the will, ability to develop some movement with some defined goal. Motor cortex performs that task. Its damage provokes paralysis. Program selection: It selects the muscles, which have to intervene, and its sequence of activation. The motor of this activity is the pre-motor area. Its damage provokes motor apraxia (without movement). Execution and control of the motor program: base ganglion and cerebellum intervene in the voluntary motor act. Direction, speed and size of the movement are regulated. Movement execution: It is carried by the encephalic trunk (mesenphalus, protuberance and bulb). Muscular contraction direct responsible: This activity is carried on by the spine medulla moto-neurons.