A discussion of microscopic muscle contraction should include all activities, ch

Question

A discussion of microscopic muscle contraction should include all activities, chemicals and structures involved in muscle contraction. So a discussion of the depolarization of the sarcolemma, the role of the ACH receptor, sodium and potassium in generating an action potential as well as the role of calcium, T tubules, the sarcoplasmic reticulum and tropomycin is expected. You should describe the myofilaments and the process of the sliding filaments.

A discussion of the contraction of a whole muscle should include the macroscopic structures of a muscle and the motor end plates, connective tissue covering and the relationships between these structures.

Explanation / Answer

Muscle is a term usually used for the tissues, which have contraction properties. In response to nerve impulse muscle changes its shape and become shorter and thick and on relaxation it becomes normal in shape. Macroscopical and microscopical structure play important role in muscle contraction.

Microscopic muscle contraction:

Muscle cells are elongated fiber like structure arranged like cables metal wire wrapped around each other, these are called muscle fibers surrounded by perimysium. These are called fascicles. Sarcomeres are functional unit of muscle fiber. The sarcomere has the actin and myosin filaments arranged in repeating units separated from each other by bands called Z lines, which give the striated appearance to skeletal muscle. Muscle fiber contain myofibril. Myofibrils are small cylindrical structures rich in mitochondria, arranged parallel to the length of muscle fiber. Stratiation in muscles are due to these myofibrils. There are protein filaments inside the myofibril known as myofilaments. Myofilaments are composed of thick and thin filaments. Thick filaments are made up of myosin while thin filaments are made up of actin proteins. The sarcomere’s actin and myosin filaments arranged in repeating units separated from each other by bands called Z lines, give the striated appearance to skeletal muscle. Sarcomere contains some specialized structure such as sarcolemma,which is a specialized plasma membrane which helps in distribution of charges due to its selectivity; the sarcoplasmic reticulum (SR), which is modified endoplasmic reticulum that stores calcium and transverse tubule of T-tubules, which help in creating excitation inside the cell.

Chemicals involved in muscle contraction are neurotransmitter acetylcholine, ATP, sodium, calcium. Muscle uses glucose and glycogen as fuel.

Structures involved

When neurotransmitter acetylcholine is released from nerve cells it binds to receptors present on the plasma membrane of sarcomere i.e.sarcolemma.

Binding of acetylcholine to sarcolemma results in depolarization of membrane which opens Na+ channels.

Sodium starts entering the cell and results in membrane excitement, due to this calcium is released from sarcoplasmic reticulum into the cytoplasm of muscle fiber.

Ca+2 bound by troponin start to interact with tropomyosin. This result in shifting of its position along the actin filament.

Calcium helps in formation of cross brides in myosin, and therefore, myosin rotates and pull actin towards the center of sarcomere.

This process of making cross bridge in myosin is repeated and actin filaments become shorter enough to overlap. Energy for the whole process is provided by ATP.

When there is no enough stimuli acetylcholine is broken down by enzyme cholinesterase. and therefore in the absence of any stimuli cross bridges are broken and calcium flow back to the sarcoplamic reticulum and muscle relaxes.

Myofilament and sliding filaments: This model of muscle excitement and relaxation, where bridges are formed and muscle become shorter and relaxes and revert to its original form is called sliding filaments.

Macroscopic structure of muscle

Muscles are composed of muscle fibers. Sarcomere is the basic functional unit of muscle fiber. Myoneural junMotor end plate is the structure present at the myoneural junction.

Each Muscle fiber is surrounded by connective tissue called as endomysium and group of muscle fiber is surrounded by muscle fiber called as perimysium form facilcles and each fascicles is surrounded by epimysium to form a complete muscle tissue.

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