What are the Steps in the Skeletal Muscle Contraction Cycle?

The skeletal muscle contraction cycle is activated by calcium ions from the sarcoplasmic reticulum. The calcium ions bind to the troponin, which reveals the active site on actin by removing tropomyosin. It helps the myosin head to bind the actin. The adenosine triphosphate (ATP) binds to myosin causing the head of the myosin to release from the active site of the actin molecule of the thin filament.

How do the Muscles Contract?   

Before the muscle contraction begins, the brain initiates an electrical signal in the form of an action potential. The action potential is sent to the somatic nervous system. The somatic nervous system contains motor neurons which receive the signal sent by the brain and carries it in the form of an action potential. The action potential reaches the end of the axon or the axon terminal and results in the fusion of synaptic vesicles (containing neurotransmitters) to the presynaptic membrane. The neurotransmitters bind to the receptor proteins present on the postsynaptic membrane of another neuron or target cell such as muscle fiber. This eventually activates the cell membrane of the muscle fiber. 

The muscle fiber is covered by a membrane called sarcolemma consisting of certain invagination. Binding of neurotransmitter to the receptor on the sarcolemma and results in membrane depolarization. The action potential will travel into the deep invagination of the sarcolemma and lets the potential spread evenly through the muscle. 

After this, the activation of the calcium channels present on the sarcoplasmic reticulum will open and allows the high concentrated calcium ions present inside to get into the sarcoplasm (cytoplasm of the striated muscles). During the relaxed state, the concentration of calcium ion is low in the cytosol. During the contraction cycle, due to membrane depolarization, its concentration increases in the cell. Inside the muscle cell, there are numerous myofibrils that have the sarcomere (the smallest functional unit of the skeletal muscle).

The shortening of the sarcomere is termed muscle contraction. The force that is specifically generated by muscle contraction is termed muscle tension. The process utilized by muscles for contraction is termed the sliding filament model. This model is a cycle of repetitive events that contracts the sarcomere. This results in the generation of muscle tension.

Note: Sarcoplasmic reticulum is the specialized type of endoplasmic reticulum that is found inside our muscle cell.

How does a Sarcomere Appear and What are the Regions it Contains?

In the sarcomere, two bands are present and named as I-band which are lighter and A-bands that are darker. These are responsible for the striated appearance of skeletal muscle. In the A-band, there is a lighter region called the H zone and it is split by an M line made up of protein myosin.  The I-bands are split by a region called the Z disc.

Note: The functional unit of a skeletal muscle is the individual sarcomeres; those are identified by the section of the Z disc. The thick filaments contain myosin and extend across the A-band, connected at the M line.

The thin filament containing actin that extends across the I-band, connected at the A-band. The core of the thick filament is titin that is made of elastic filaments.

Myosin : It is a protein with a globular head that points outward.It consists of a long tail. The head is the site of activity as it contains the ATP binding sites and actin-binding sites. 

Actin : Two actin filaments will twist to form the backbone of the thin filament. Each actin has an active site to which the myosin head binds.

Troponin : It is a globular complex with three polypeptides. Each of them binds with actin, tropomyosin, and calcium respectively.

Note: Troponin is a protein found in cardiac muscles and skeletal muscles. They are not found in blood, rather sent to the bloodstream when damaged heart muscles are encountered.

The Interface Between the Nervous System and the Skeletal Muscle

The connecting link between both the system, that is, the nervous system and skeletal muscle is called the neuromuscular junction.  Each muscle fiber has a neuromuscular junction also referred to as the synaptic cleft. This is the region where the muscle produces junctional folds within the postsynaptic membrane. The axon terminal releases acetylcholine into the synaptic cleft, the junctional folds contain the receptors which bind the incoming acetylcholine. The binding of acetylcholine opens the ligand-gated ion channels. The sodium ions enter the sarcoplasm and result in depolarization.  

Interaction of Calcium with the Thin and Thick Filament

In the sarcomere, a group of thin filaments is present which are composed of globular proteins referred to as actin. The thick filament consists of myosin. The interaction can be understood better by the following steps:

• The release of calcium ions from the sarcolemma occurs in the cytosol.
• Tropomyosin is a contractile protein that is coiled and helps in the interaction of actin and myosin. 
• The binding of the calcium to the protein called troponin takes place on the actin filament.
• Before binding to the actin, the myosin heads have to hydrolyze an ATP molecule into adenosine diphosphate (ADP) and a phosphate group by the process called hydrolysis. These products stay on the myosin heads and are positioned at a 90’ angle. 
• After the binding of calcium to the troponin, it changes its shape and causes the tropomyosin to shift. During this time, the Tropomyosin exposes the myosin-binding sites on the actin filament.
• The binding also causes the release of the ADP and phosphate group from the myosin.
• The Z lines on the thick filament are pulled towards or inward to each other.
• After the contraction, the myosin head has to be detached, which can be done when the ATP molecule binds onto the myosin head. This releases the thin filament and calcium. And the process is repeated as the muscles move back and forth.

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Zoology
• Bachelors in Biochemistry
• Masters in Biochemistry
• Masters in Anatomy and Physiology

 Related Concepts

• Neuromuscular junctions
• Nervous system
• Muscular system
• The electrical signal in our body

Practice Problems 

1) What are skeletal muscles made of?

Skeletal muscles are made up of fascicles and this fascicle is made of muscle fiber. The muscle fiber is a multinucleated muscle cell.

2) Which is the site of contraction?

The myofibril contains compressed myofilaments.These are arranged into sarcomeres, where the contraction takes place. So, the site of contraction is a sarcomere. 

3) What happens when a muscle fiber is relaxed?

When the muscle fiber is relaxed, the active sites are blocked by the spiralling strand of tropomyosin. 

4) Where does the T-tubule present?

The T-tubule sits at each of the A-band- I-band junction that encircles the sarcomere and helps the signals to reach every region of the muscle cell.