I- Introduction :
The muscle tissue is almost half of our body weight.
The main characteristic of muscle tissue, the functional point of view, is its ability to convert chemical energy (form of ATP) energy mécanique.Grace this property, muscles are able to exert a force.
muscles can be seen as the body's engine.
The mobility of the body as a whole results from the activity of skeletal muscles which are distinguished from internal organs muscles most of which circulate liquid in the channels of our organism.
II- Main Features :
A- Types of muscles :
There are three types of muscles : skeletal, smooth and cardiac.
These three types different from : the structure of their cells, their location in the body, their function and trigger mode of contraction.
B- Functions of the muscles :
- Production moves
- Maintaining posture
- Stabilization of the joints
- heat release
C- Functional characteristics of muscles :
- excitability : is the ability to perceive stimuli and respond. The stimulus may be a neurotransmitter released by a nerve cell, the response is the production along the sarcolemma, an electrical signal which is at the origin of muscle contraction.
- contractility : is the ability to contract forcefully in the presence of the appropriate stimulation.
- scalability : is the stretch ability ; when contract, muscle fibers become shorter, but when they are relaxed, can stretch beyond their resting length.
- elasticity : is the ability of the muscle fibers to return to their rest length when released.
III- Skeletal muscle :
A- Macroscopic Anatomy :
Skeletal muscle is a well-defined body, it contains blood vessels qes, of nerve fibers and a large amount of connective tissue
1- connective tissue envelopes :
- Each muscle fiber is found at the’interior b’a thin connective tissue sheath called an endomyslum. Several fibers and endomysium are placed at odds ribs and form a named bundle.
- Each beam is in turn delimited by a thicker sheath of connective tissue called perimysium. The bundles are grouped together in a coarser covering composed of denser connective tissue which envelops the’whole Pu muscle called epimysium.
2- Innervation and vasculature :
- Each muscle fiber is endowed with’a nerve ending that governs its activity
- Each muscle is served by an artery and one or more veins The arteries carry nutrients and l’oxygen, by against the veins evacuate metabolic waste
B- microscopic anatomy of a skeletal muscle fiber :
1- Muscular Fiber :
– Each muscle fiber is a long cylindrical cell containing many nuclei. It is surrounded by a membrane : sarcolemma
– The sarcoplasm of’a muscular fiber is com pared with the cytoplasm of other cells, but it contains important reserves of glycogen as well as m yogi o b ne, a protein that binds to the’oxygen and n’exists in no other cell type.
2- Myofibrilles :
Each muscle title has a large number of parallel myofibrils that run the entire length of the cell.
3- Myofilaments :
Along the length of each mmyofibrille, we see a dark bands alternating clear and called striae.
– The dark bands are called s s bands A (stries A), I called the light bands (stries I).
Each band A has in its middle a zone due dajre c’is the clear area or striae H (zone H).
Each clear zone estdivsée in two by a line or be som M line.
– In the mid tendes I, there is also a dark area due to the’is nomme : Z line
– The region of’a myofibril formed between two successive Z lines is called Sarcomere, measuring 2 microns and represents I Functional unit muscle.
At the molecular level each myofibril is formed very uniformly arranged filaments : thick and thin filaments. The thick filaments are made of l’assembly of molecules d’a protein : la myosine (present in the bands A to sarcomeric center), while the major component of thin filaments is another protein : gaps ; present in the bands I)
• The thick filaments : myosin molecule has a very specific structure, made of two identical subunits having the form of’a golf club with coiled tails l’one on the’other and whose two spherical heads protrude from the’one end
The myosin heads contain the binding sites of the’actin, of the’ATP as well as ATPaæs enzymes which dissociate’ATP to produce l’energy required for muscle contraction. The heads are also involved in the formation of cross bridges.
• The fine or thin filaments : Are made of three proteins : F-actin (filamentous), troponin and tropomyosin.
- Two chains of filamentous actin made of a succession of globular actin molecules which are wound one around the other.
- Actin door binding sites on which myosin heads (bridges unions) attach during contraction.
- The troponin molecules which consist of three spherical form units (TnI inhibitory =, binds to actin,TnT = binds to tropomyosin,TNC = if lie aux ions calciums).
- The tropomyosin filaments form a ribbon resting on the groove of the helix of actin and block (mask) actin active sites so that the myosin heads can not bind with thin filaments.
4- Sarcoplasmic reticulum and transverse tubules T :
– Tubules transverses T : at the junction of ridges A and I, sarcolemma present invaginations which penetrate inside the muscular fiber and form tubules Transverses (tubule T).
– The sarcoplasmic reticulum (RS) : is an endoplasmic reticulum particular, forming a network of fine tubules surrounding each myofibril its meshes over its entire length. It Includes terminal tanks that establish intimate contacts with the T tubules. The T tubule and the wing tanks located on each side form : The triad
– The sarcoplasmic reticulum plays a fundamental role in :
- Calcium storage : within the RS, calcium is bound to protein : the calséquestrine
- The release of calcium from the sarcoplasmic
- La recapture du calcium : the intracellular medium inwardly of the RS using the calcium pumps ATP- dependent.
– Triad role in the transmission of information :
The transmission of information (influx nerveuxjdu tubular system sarcoplasmic reticulum (which led to the release of intracellular calcium)involves a specific mechanism. This involved :
Voltages dependent channels located within the tubular membrane and blocked by the dihydropyridine (Dःf)hence the channel name or DHP receptor dihydropyridine (Dःfri).
Each receiver dihydropyridine (Dःfः) is in contiguity with a calcium channel of the sarcoplasmic reticulum sensitive Ryanodine hence their name channels or Ryanodine ryanodine receptor (RyR)
Under the effect of membrane depolarization, the DHPR acts as an intensity detector electric current or voltage and undergoes conformational changes resulting in a molecular interaction with the RyR.
This promotes openness and calcium release from the sarcoplasmic reticulum stocks
C- Mechanisms of contraction :
1- filaments slip model :
The theory of contraction sliding filament; developed by Hugh Huxley 1954, offers’following explanation : "During contraction, thin filaments (actin) slide along the thick filaments (myosine), so that the filaments d’actin and myosin overlap more "
Break the thick and thin filaments overlap only on a small part of their lengths, but when muscle cells are stimulated, myosin heads’cling to the binding sites of the’actin and slip s’primer The myosin heads pull the thin filaments towards the center of the sarcomere : C’is the shortening of the sarcomere The length of the A bands does not change during the shortening but that of the I and H bands decreases.
2- coupling excitation- contraction :
C’is the succession of’events by which the potential d’action transmitted along the sarcolemma causes the myofilaments to slip.
The excitation coupling – contraction involves the following steps :
1- The potential of’action spreads along the sarcolemma and transverse tubules.
2- When the potential d’action reaches the triads, DHPR acts as a detector of’intensity of electric current or voltage and undergoes conformational changes leading to molecular interaction with the RyR This promotes its opening and release of callus from the stores of the sarcoplasmic reticulum
3- Once intracellularly (sarcoplasme), calcium binds to troponin C (T11C), four calcium molecules bind to a molecule deTnC.
4- Troponin then changes its three-dimensional structure, causing the lateral displacement of tropomyosin and therefore release (dêmasquage) the actin binding sites.
5- As soon as the actin binding sites are exposed, myosinesse the heads immediately bind to actin forming a complex : l’actomyosine
6- At the same time the binding of calcium on TnC allows the removal of the inhibition exercised by troponin I on the ATPase activity of the myosin head, This ATPase activity enables hydrolysis of ATP into ADP and Pi (this reaction is dependent Mg +).
7- The detachment of Pi and ADP heads myosins, allows bending heads myosins (angle changes consisting of myosin heads attached to actin) = Sliding of actin filaments on the filaments of myosin)
8- The binding of actin with myosin remains stable and only the presence of a new molecule of ATP(which binds to the myosin head), allows the breaking of actin and myosin liaisonenfre.
9- The contraction continues as long as the calcium signal and ATP are present.
10- In the absence of action potential, the sarcoplasmic reticulum calcium recovers sarcoplasme, the troponin again changes its shape and tropomyosinemasque binding sites of myosin heads to actin so the contraction ends and the filaments resume their initial position = Muscle Relaxation
NB : Cadaveric rigidity is a good illustration of the fact that c’is’ATP which allows the detachment of the myosin heads from the’actin. Indeed after death the synthesis of I’ATP ends and detachment of myosin heads becomes impossible. L’actin and myosin are then bound irreversibly, causing rigor mortis, which disappears when the muscle proteins are degraded within hours after death.
D- Metabolism of Skeletal Muscle : ATP regeneration
- During muscle contraction energy for contractile activity (flexion, detachment destêtesde myosin and operation of the calcium pump) is provided by ATP.
- Since ATP is the only energy source that can feed directly contraction, and the ATP immediately available stocks are low in the muscle and allowing contraction 4 at 6 seconds, it must be regenerated continuously so that the contraction may continue. Fortunately regeneration is done in a fraction of a second following three routes :
a- Interaction of ADP with T creat ine phosphate(CP) :
– At the beginning of the contraction, once low ATP reserves were consumed,an additional ATP is rapidly reconstituted partird'une high-energy molecule : Creatine phosphate (CP) :
PhosphoCréatine + ADP Creatine -► + ATP
- This reaction is catalyzed by creatine kinase.
- Maximum muscle strength can be maintained pendantIO 15s
b- glycolysis anaerobic :
muscle glycogen reserves are transformed into lactic acid with production of two (02) ATP molecules. Ensembles, the reserves of ATP and creatine phosphate and aerobic glycolysis can maintain muscle activity for one minute.
c- aerobic cellular respiration : phosphorylation oxydative
In a slight but prolonged muscular activity, ATP used by the muscles is provided by the aerobic cellular respiration which takes place in the mitochondria and require the presence of oxygen and involves a series of chemical reaction (cycle de krebs, and respiratory chain electron transport). During aerobic respiration glucose is completely degraded ; complete the oxidation of one molecule of glucose provides 36 ATP molecules.
E- Types of muscle fibers :
Fibers oxidative slow twitch (de type I) :
- Red color, – Plenty of myoglobin.
- Low glycogen stores, – Large fatigue strength.
- The aerobic pathway is the main route of the synthesis of ATP
Oxidative fibers fast-twitch (de type IIa) :
- Red color – Important glycogen.
- Abundances of myoglobin – Moderate resistance to fatigue.
Fibers glycolytic fast twitch (de type IIb) :
- Important glycogen. – Low content of myoglobin
- White color – fatiguable
ll- Smooth Muscle Tissue :
A- smooth muscle structure :
- Present in the wall of most of the body's hollow organs : Airway, vessels, Digestive, and genitourinary.
- Each smooth muscular fiber is a fusiform cell that contains a single ring, the diameter of these small fibers is between 2 and 4 one.
- Smooth muscle fibers do not have nerve endings sophisticated as that found in skeletal muscle, by cons they are connected to nerve fibers of the autonomic nervous system.
- The sarcoplasmic reticulum of smooth muscle fibers is less developed than that of skeletal muscle fibers and there are no transverse tubules T.
- smooth muscles do not exhibit transverse grooves, Although they contain thick and thin filaments but these filaments are different from those found in skeletal muscles :
– The filaments thick and thin are not arranged in sarcomeres
- The thick filaments smooth muscle myosin heads wear throughout their length, a feature that allows these muscles to be as powerful.
- Tropomyosin is associated with thin filaments but no troponin.
B- Excitation-contraction coupling in smooth muscle :
The smooth muscle contraction mechanism is similar to that of skeletal muscles, and Surles following plans :
- The sliding myofilaments is due to the interaction of actin and myosin
- The contraction is triggered by the increase in intracellular calcium ion concentration
- The sliding of myofilaments requires ATP
• The stages of muscle contraction :
- During the excitement coupling- contraction, calcium is released from the sarcoplasmic reticulum, but it also penetrates from the interstitial fluid
- To activate myosin, calcium interacts with regulatory proteins : calmodulin located on myosin filaments and a known kinase Kinase the light chains of myosin (MLCK),
- And as the thin filaments lack troponin to mask the binding site for myosin heads and are always willing to contract
- Calcium binds to calmodulin and calcium-calmodulin complex binds and activates the kinase of the light chains of myosin (MLCK).
- The activated kinase phosphorylates hydrolysis of ATP and myosin which allows the- latter to interact with actin : shortening occurs.
- Like skeletal muscles, smooth muscles relax when intracellular calcium decreases.
- Elaine N. Marieb : Anatomy and Human Physiology
- H.Guenard : Human physiology
- P. Rigoard,K. Buffenoir : molecular architecture of the sarcoplasmic reticulum and its role in the excitation coupling – contraction
Dr M's course. MARTANI – Faculty of Constantine