Question

Explain the entire process of contraction in a skeletal muscle, starting with how the signal enters the muscle cell at the neuromuscular junction. Include all relevant ions, molecules, structures, and processes in your explanation. How does the muscle carry out a sustained contraction? How does a muscle relax? How does skeletal muscle differ from the other two muscle types?

Answer 1

Process of contraction in skeletal muscles:

The process of contraction take splace by various roles steps involning,

1. a nerve impulse which travels to the neromuscular junction on a muscle, is the point where axons of the nerve cells meet the muscle cell.

2. The axon relaeses ACh to the rreceptors located on the sarcolemma. This act causes depolarization, of the sarcolemma by openeing the ion channels and allows the Na+ ions into the muscle cells.

3. These Na+ ions diffuses into the muscle fiber and depolarization continues as it creates an action potential from motor neuron across the sarcolemma, which triggers the release of acetylcholine into the motor end plate.

4. The action potential travels across the sarcolemma and down the T-tubules and triggers the sarcoplasmic reticulum to release Ca+ ions. These calcium ions plays a major role in intiating muscular contractions.

5. As the Ca+ ions level increases, Ca+ ions binds with troponin, which removes the blocking action of tropomyosin, from actin binding sites.

6. Myosin readily binds with actin, and form cross bridges which begins the contraction proecess. ATP molecules binds with myosin in order to contract.

6. ATP hydrolysis cause the myosin heads to change and moves them towards the next binding site and returns to its original conformation. This kind of movement drags the actin along myosin in a sliding mechanism.

7. As the process continues, ATP molecules are hydrolysed (broken down) to ADP and Pi, which gives myosin energy to high energy position.

8. The repeated process of myosin heads drag actin filaments along the length of myosin, these actin filaments are anchored to Z lines, which pulls the Z lines closer thus shortening the sarcomere.

9. As the whole process is repeated, the individual sarcomeres becomes shorter in length and thus mucle fibres as a whole contracts. The whole process continues as long as ca+ ions and ATP molecules are available.

Muscle Relaxation:

Muscle relaxation takes place once the ACh released from axons are broken down by AChE, an ending action potential generated from sarcolemma. The sarcoplasmic reticulum, reabsorbs calcium (Ca+) ions, hence the Ca+ ions tends to decline in the sacroplasm. As the calcium ion concentration reaches the normal resting levels, troponin-tropomyosin complex also returns to its normal level, thus preventing further cross-bridging interaction. Without cross-bridge interactions, further sliding do not takes place and the contraction of muscles returns to the resting level.

skeletal muscle differ from the other two muscle types

Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Found attached to bones and skin	Found only in heart	It line walls of the internal organs.
Striated with orderly arranged myofibrils	Striated with many myofibrils in orderly arrangements	Not striated and some myofibrils are of varying lengths
Perform voluntary muscular movements	Perform involuntary muscular movements	Perform involuntary muscular movements
Non Self-stimulating	Self-stimulating	Self-stimulating
Power the joints, aids physical movements	Responsible for pumping blood throughout the body	Movement of internal organs to facilitate their functions
Do not exhibit rhythmic contractions	Exhibit rhythmic contractions	Exhibit rhythmic contractions
Have high energy requirement	Have intermediate energy requirement	Have low energy requirement
Easily fatigue	Do not fatigue	Do not fatigue
Have high speed of contraction	Have intermediate speed of contraction	Have low speed of contraction
Strength increases with stretching	Strength increases with stretching	Exhibit stress-release response
Under the regulation of nervous system	Under the regulation of nervous system, endocrine system and various chemicals	Under the regulation of nervous system, system, endocrine system, various chemicals and stretching.
Comprises of very long, cylindrical, multinucleated cells	Comprises of branching of cells, connected by intercalated discs with a single nucleus	Comprises of single tapering, single nucleated cells