General description of muscles:
1- The function of muscles is contraction to produce movement in response to neuronal impulses.
2- Muscle is connected to bones by Tendons
3- Muscles composed of cells that run along the whole distance of muscle (from one Tendon to The other) and all muscle cells are collected parallel to each other, and between them is connective tissue that hold and supports them. The muscle cell is called (muscle fiber).
4- Each muscle fiber receives one branch of axon terminals.
5- Each fiber contains a lot of contractile proteins arranged in filaments called Myofibrils
6- Myofibrils consist of Thick and Thin filaments [myosin filaments (thick), and thin filaments are Actin filaments]
7- These Tow filaments arranged so that they are interdigitating with each other to some degree
8- When action potential reach cell membrane of muscle fiber it will spread all the membrane, in addition, it will spread to deepest parts of muscle fiber [(by some network as we will see later]
9- Action potential in muscle fiber will cause increase in intracellular Ca+2 (released from storage sacs inside the fiber) This Ca+2 will elicit attraction between interdigitating filaments so That one of them (Actin) will continue sliding on other and that is (shortening or—–contraction).
Anatomy of muscle fiber:
1- Myofibrils are surrounded by a network of Ca+2 stores call Sarcoplasmic Reticulum.
2- plasma membrane has invaginations toward inside and deeply so that cross from one side to other, this structure is called Transverse tubules or T- tubules, They are extension of cell membrane and contain extra cellular fluid and called (Transverse) because They are transverse to myofibrils.
3- T- tubules and Sarcoplasmic Reticulum (sacs contain Ca+2) are both associated together, so that when action potential come (Through T- tubules), it will spread directly to Sarcoplasmic Reticulum, to release Ca+2.
*Function of T- tubule is like wires That extend electricity to the deep parts of the fiber .
Arrangement of Myofibril filaments
As we said That myofibril has thick and thin filaments (myosin & Actin) and they are interdigitating or overlapping , area of Actin only, is seen light in microscope and called ( I- band ) , area of overlap between myosin & Actin is seen dark in microscope and called ( A- band ) and this arrangement (light and dark) is found in whole muscle fiber , giving the appearance of striation that’s why we call skeletal muscles as…….(striated muscles) .
* In the middle of A- band there is small area, which is less dark, (because it consist of myosin only without overlap with Actin) this area called H-zone.
* In the middle of I-band, the Actin filaments attach to a skeleton which is some type of proteins that forms a line in middle of I-band called Z-line
* In the middle of H-zone there is a line to which the myosin filaments attach and it is also protein, this line called M-line.
* Functional unit of the muscle, (i.e. smallest part in muscle that can perform contraction), is called Sarcomere and it extends from Z-line to next Z-line
* Length of Sarcomere is 2 micrometer.
* We can notice in the figure that myosin filaments have projections in all directions toward neighboring Actin, these projection called cross bridges , and we will see how these (cross bridges) have main role in contraction.(look to figure 1, and following figure)
* Z- Line (a protein) not only cross one myofibril in I- band. But cross all myofibrils in the muscle fiber, so that it connect all myofibrils
* What hold myosin and Actin in position and this arrangement? It is special large protein between them work as frame work
* Intra cellular fluid of muscle fiber (which fill spaces between myofibrils) is called Sarcoplasm, it contains large number of mitochondria (to provide energy ATP for cross bridging process)
Molecular Features of filaments:
A. Myosin Molecular Features:
* Myosin filaments composed of myosin molecules; myosin molecules have large molecular weight (larger than Actin).
* Myosin is composed of tow strands (chains) wrapped or twisted on each other to form double helix and this helix is called (Tail), in the one end of the tail each strand folded to form globular structure called (Head), so we have to heads for one tail, each head is tilted from the tail in some angle so that the whole structure (Tail and head) appear as (golf club) .
* Between the tail and each head there is little (arm), the head and arm is called (cross bridge) which has the main role in contraction mechanism.
* Each head has tow important sites:
1) Active site (for binding with Actin) and 2) ATPase site (which has enzyme like function to hydrolyze ATP and produce energy ) .
* When myosin molecules collected in myosin filament they are arranged so that the pairs of heads (cross bridges) are projected in all directions surrounding filament and the angle between Tow neighboring pairs is about 120 degree.
* Between the tail and the arm there is what is called (Hinge) which enable arm to move forword and backward, also between arm and head there is another hinge for same function (movement of head forward & back word)
B. Actin Filaments:
* Represent thin filaments and consist of Actin mainly (as a backbone) and Tropomyosin and Troponin.
* Actin is also double strands that are wrapped or twisted on each other to form double helix structure
* Tropomyosin is wrapped on the Actin double helix (in the groove that is made from Actin helix)
* Actin has active sites (That bind with active sites of myosin), these active sites of Actin are covered by Tropomyosin (when it run in the helix) and because of this (covering), the active sites of Actin is hidden from that of myosin, and this is in resting state ( i.e. No contraction present )
* Actin structure in each strand is like multiple spheres attached together and when each strand twist on other
* Troponin attaches both Actin and Tropomyosin together, its role is very important —–how? When Ca+2 enter cell and bind to (Troponin), Troponin will tilt or pull the Tropomyosin away from active sites of Actin (and by this Active sites of both Actin & myosin are exposed to each other and then they will attract each other and contraction happen)
*Troponin has 3 subunits all together form globular structure they are:
[Look at figure 6 and 7]
1- Troponin I: it inhibits interaction between Actin & myosin
2- Troponin T: it is attached to Tropomyosin
3- Troponin C: this subunit bind with calcium (ca+2)
1– Tropomyosin (which cover active sites of Actin) and Troponin (will pull Tropomyosin away from active sites) is both called (regulatory proteins) because of their role in covering & exposing active sites.
2– Active site of Actin is also called (myosin binding sites)
Arrangement of Actin & Myosin Filaments:
* In across section through myofibril (where there are Actin & myosin filaments) we will see special arrangement between Actin & Myosin that has wisdom in contraction process.
A– Myosin is arranged in triangles (look at figure 8)
B– Actin is arranged in hexagonal shape (look at figure 9)
* And the result of both arrangements in combination will make following features:
1- Each Actin filament is surrounded by (3) myosin filaments
2- Each myosin filament is surrounded by (6) Actin filaments
3-Ratio between Actin filament number and myosin filament number is (2: 1) in each myofibril (i.e. Actin is 2 and myosin is 1)
4-From above we conclude that each Actin will react with (3) myosin, and each myosin will react with (6) Actin in each myofibril.
* Sarcoplasmic reticulum has enlarged portion called (lateral sac) or (Terminal cisternae) and they are present in association with T- tubule.
Single neuron will innervate multiple muscle fibers (because as we know the axon will branch to nerve terminals and each Terminal will supply one muscle fiber)
* This structure (neuron and all muscle fibers supplied by it is called (motor unit)
* This neuron is called (motor neuron) and it is present in anterior horn of spinal cord
* Strength of muscle contraction
Depends on factors and one of them is the number of muscle cells contracted as to say, when muscle contract it is not necessary all fibers are contracted —-for example if we want to lift one kg, then the brain will give order (by nerve axons) to half of muscle fibers (for example) to contract, and when we want to lift 5 kg, There might be all fibers contracted, so we increased the strength of contraction by increasing number of muscle fibers that are contracted.