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Course : PG Pathshala-Biophysics
Paper 13 : Physiological Biophysics
Module 03: Molecular Structure Of Muscle And Physiology Of Contraction
Principal Investigator: Dr. Moganty R. Rajeswari, Professor AIIMS, New Delhi
Co-Principal Investigator: Dr. T. P. Singh, Prof. , AIIMS, New Delhi
Paper Coordinator: Dr. K. K. Deepak, Prof. & HOD, Physiology AIIMS, New Delhi
Content Writer: Dr. Atanu Roy, Sr. Resident, AIIMS, New Delhi
Content Reviewer: Dr. Renuka Sharma, Professor VMMC & SJH, New Delhi
3.1 Objectives
- Introduction
- Types of muscle
- Understanding the muscle structure
- Sliding filament theory
- Summary
3.2 Introduction: The term “muscle” is derived from Latin word “Musculus”. “Mus” meaning mouse. The belly resembles body of the mouse and their tendons resemble mouse’s tail, that’s why it’s named muscle. Muscle and skeletal system provides the basic framework for the body. The skeletal muscle lies in between two bones and are always in a state of partial contraction. The main property of a muscle is its contractility and the responsible factors are the contractile proteins.Their arrangement is unique. In this module our main focus will be on skeletal muscle and its contraction.
3.3 Types of muscles:
Skeletal Muscles:
Most abundant and are attached to the skeleton. That’s why they are called skeletal muscles.
They are also known as striped, striated, somatic and voluntary muscles.
They are innervated by somatic nervous system and so they are under voluntary control. So we are able to move the muscles at our will.
They are quick to respond to stimuli and are capable of rapid contraction and get fatigued easily because of their rapidity.
Each muscle fiber is cylindrical, multinucleated cell containing groups of myofibrils. The myofibrils are in turn made up of myofilaments that is of three types i) actin, ii) myosin, and iii) tropomyosin. Examples of skeletal muscles include all muscles of body wall.
Smooth muscles:
They are known as involuntary, plain, unstriped and visceral muscle.
They do not exhibit cross striations under the microscope and so they got the name “smooth”.
They are supplied by autonomic nervous system and are therefore involuntary in nature.
They are slow to respond to stimuli and can undergo long term sustained contractions.
Each smooth muscle fiber is an elongated spindle shaped cell. Single nucleus placed at the center. They also have actin and myosin filaments but the arrangement of these filaments are different as compared to the skeletal muscles.
Examples of smooth muscles include muscles of the gut,blood vessels etc.
Cardiac muscles:
They are the main architect of the myocardium of human heart.
Structurally they are intermediate between the skeletal and smooth muscles. They are striated like skeletal muscle, they are involuntary and have uninuclear cells like smooth muscles.
Each muscle fiber has a single centrally placed nucleus. The fibers branch and anastomoses with each other forming a syncitium. Surrounding cells are joined by intercalated discs which provide conductive pathways from one cell to another.
3.4 Understanding muscle structure: The muscle bundles consists of large no of muscle fibers. Each muscle fiber is equivalent to a cell. Each fiber is made of myofibrils, that in turn consists of densely packed contractile components, arranged in a regular fashion. It gives a striped appearance under the light microscope.Each myofibril consists of a thick filament ie myosin and a thin filament ie actin. It’s the interaction between these filaments which brings out the contraction of the muscle in a coordinated fashion.
Figure 2
Animation present in the power point
3.5 Microscopic observation : Under light microscope there are darkly stained areas called as the A band and lightly stained areas called as I band. The main composition of A band is mainly the myosin and I band actin. The myofibril consists of repeated patterns of lines and bands. Between two Z lines the, portion of myofibril is called sarcomere. It’s the contractile unit of the muscle. I band lies immediately adjacent to the Z line. The A band lies in the center of the sarcomere and H band is situated in the middle the A band.
Tropomyosin: It also constitutes the thin filament. Structurally it’s a double helical structure wrapping spirally around the actin filament. Tropomyosin lies in the grooves between two F- actin coils which forms the actin filament. During contraction they sink deep inside the grooves, thereby exposing the active sites.
Troponin: It’s a globular protein associated with thin filaments. It is made of three subunits i) Troponin I ( it has a very strong affinity with actin molecules, which constitutes the I band)
ii) troponin T- It has strong affinity for tropomyosin
iii) Troponin C- It has a strong affinity with calcium.
Some other additional structural proteins :
i) Actinin- it binds actin with Z lines.
ii) Titin- It connects the Z lines to the M lines and it’s also the largest protein known.
iii) Desmin- It binds Z lines to the plasma membrane.
3.6 The sarcotubular system: The endoplasmic reticulum is modified to form sarcoplasmic reticulum. It in turn forms a dense network around the myofibrils. It helps to distinguish myofibrils from the myofilaments. The cell membrane around the muscle fiber invaginates
transversely deep inside , helping in spread of wave of excitation throughout the cell. This is called the T tubules. At the A-I junctions, the sarcoplasmic reticulum comes very close to the T tubules. These are the areas where tubules of sarcoplasmic reticulum are enlarged, forming cisternae ( a sac like structure ). When two cisternae comes close to a T tubule, it forms a triad.
3.7 Concept of excitation contraction coupling : When an action potential is generated, it traverses along the sarcolemma. And as we have studied earlier, that the sarcolemma invaginates inside the muscle fiber in the form of T- tubules. So, when the action potential is generated , it dips deep down , via the T tubules. It in turn helps to spread the wave of excitation deeper inside the muscle fiber. The sarcoplasmic reticulum comes closer to the T tubules at the A- I junction, thereby forming a triad. When the action potential reaches the T tubules , it opens the Dihydropyridine Receptors (DHPR). DHPR is voltage sensitive. They undergo a conformational change which in turn leads to release of calcium ions from the Ryanodine Receptors(located on the cisternae) by physical interaction. Ryanodine receptors are calcium channels, that help in release of calcium from the cisternae. The DHPR and RyR interaction release calcium from the cisternae, thereby increasing the concentration of calcium inside the sarcoplasm of muscle cell. Which in turn help in contraction of muscle. The relaxation depends on the active transport of calcium back into sarcoplasmic reticulum.
3.8 Sliding filament theory: This theory proposes the mechanism of muscle contraction. Its proposed that the thin filaments slide over the thick filaments with the interaction between actin, myosin, troponin and tropomyosin. Lets see in detail how it happens.
To begin with let’s suppose the muscle is in a resting state. The myosin has two heads, one free and another attached with actin. The troponin I is bound to actin and tropomyosin, covering the sites, where myosin head attaches with actin. Myosin head has tightly bound adenosine phosphate (ADP) or “cocked state” which is released during crossbridge formation. Now let’s suppose that an action potential has arrived via the sarcolemma and with the help of T- tubules, they have spread inside the cell. This depolarizing current caused a conformational change in the DHPR-RyR complex, leading to the release of calcium in the cytoplasm. This free calcium will now go and bind with the Trop-C. The binding causes weakening of interaction between Actin and troponin I, resulting in exposing the sites for the actin binding on myosin. This leads to cross-bridge formation. Upon formation of cross-bridge, the ADP is released and that causes a conformation change in the myosin head, the head bends at the neck and helps it slide past the actin filament, comprising the “power stroke”. Immediately after the power stroke, ATP quickly binds with the myosin’s free site leading to the detachment of myosin from the actin. Hydrolysis of ATP releases inorganic pi which causes “recocking” of myosin head thereby completing the cycle. The sarcomere contract contract by 10nm during each contraction.
