Actin and Myosin

Actin and myosin are both proteins that are found in every type of muscle tissue.

Thick myosin filaments and thin actin filaments work together to generate muscle contractions and movement. 

Myosin converts chemical energy released from ATP into mechanical energy, which  used to pull the actin filaments along, causing muscle fibers to contract and, thus, generates muscle movement.

The three types of muscle tissue found in the human body are:

Smooth muscle – Smooth muscle contracts involuntarily and is found in the internal organs (except the heart) and blood vessels.

Cardiac muscle – Cardiac muscle is found only in the heart and also contracts involuntarily.

Skeletal muscle – Skeletal muscle is attached to the bones, is the most abundant type of muscle tissue in the human body, and the only type of muscle that can be moved voluntarily. 

Skeletal muscle contractions pull on the bones of the skeleton and allow those bones to move.

All three muscle types contain actin and myosin filaments, which work together to produce muscle contractions.

Actin and myosin filaments work together to generate force that produces the muscle cell contractions that facilitate the movement of the muscles and, therefore, of body structures.

Muscle tissue is made up of bundles of muscle fibers. 

Muscle fibers are long, skinny cells that can be up to several inches long and, in the case of skeletal muscle, may contain several nuclei. 

The cytoplasm of muscle fibers contains long, thread-like structures called myofibrils, which are made up of bundles of thick, myosin filaments and thin  actin filaments. 

Surrounding the actin and myosin filaments is a structure called the sarcoplasmic reticulum (SR); which is a network of tubules that store calcium ions. 

The SR also plays an important role in transmitting electrical signals.

These electrical signals are delivered to the muscle cells by neurons.

Muscle tissue is made of bundles of muscle fibers.

Each myofibril is made of a chain of repeating contractile units called sarcomeres. 

At the end of each sarcomere is a Z disc.

Sarcomeres contain two distinct types of bands: 

dark-colored A bands, which contain thick, myosin filaments

I bands, which have a lighter color and contain only thin, actin filaments. 

The actin filaments are attached to the Z disc, whereas the myosin filaments are anchored to a region in the middle of the sarcomere called the M line.

Sarcomeres are the contractile units that make up myofibrils

Actin and myosin work together to produce muscle contractions and, movement. 

First, a motor neuron delivers an electrical signal to the muscle cell from the brain/spinal cord.

This triggers the release of acetylcholine. 

Acetylcholine causes calcium ions to be released from the sarcoplasmic reticulum. 

The calcium ions bind to troponin protein.

Troponin is attached to another protein, called tropomyosin, and is found between the actin filaments in muscle tissue.

When calcium ions bind to troponin, it’s shape changes, moving  tropomyosin from the myosin-binding sites on the actin filament and making it possible for the myosin heads to bind to the actin filament.

When the myosin heads bind to the exposed binding sites on the actin filaments, the actin-myosin cross-bridges allow muscle contractions to begin.

The myosin works like a motor to convert this chemical energy into mechanical energy.

The mechanical energy of the myosin move its head groups towards the middle of the sarcomere, and pulls the actin filaments towards the center of the sarcomere, causing the sarcomere to shorten and contract. 

The contraction of the sarcomere causes the muscle fiber to contract and generates muscle movement.

Leave a Reply

Your email address will not be published. Required fields are marked *