Concept Check 27.5
Camila Ruales
Block B

1. Explain how muscles work in pairs in moving limbs.

Muscles move our limbs or different body parts by contracting and the relaxing. However, they can only pull bones they are attached to by tendons, but not push them back to where they were originally, therefore they have to work in pairs. Each skeletal muscle contracting has an opposing muscle that is relaxed but then contracts pulling the bone attached to the opposite direction and then the first muscle that contracted is now relaxed. These muscles are called flexors and extensors, and as the flexor contracts, moves or bends the bone, then relaxes once the extensor contracts straightening the bone.

2. Identify the structures that make up a skeletal muscle.

The skeletal muscle is made up of thousands of parallel muscle fibers and nerves and blood vessels. Each muscle fiber is a single muscle cell that has many nuclei. They are the biggest cells in the body and are long and cylindrical. In the interior of these muscle fibers you can find bundles or packages of smaller units known as myofibrils. They have alternating light and dark stripes causing the skeletal muscle to be striated. Each myofibril is made of sarcomeres which are the muscle’s basic unit of contraction and they are made of two types of filaments. The thin filament is composed of a protein called actin and they have a twisted structure. The thick filaments are made of the protein myosin and have bulges called myosin heads.

3. Identify at least three organ systems involved in a handshake. Describe what each system contributes to a handshake.

Three organ systems involved in a handshake are the nervous, muscular, and skeletal system. The process starts with the nervous system, when you identify the other person's hand with your eyes and send signals to your brain which is the control center directing all your movements. Messages are transmitted trough your nerves to your muscular system and several of your muscles start a series of contractions. The muscles involved are located in your back, shoulder, upper arm, forearm and wrist. In addition, 27 bones are also involved in the process, these are moved by your muscles. Through this whole process, nutrients and oxygen reach your muscles in the blood cells carried in the blood vessels. We can see how the three organ sytems work together to enable a handshake.

4. Explain how actin and myosin interact as a muscle cell contracts.

The interaction between the two proteins composing the filaments inside the sarcomeres are the cause of the contractions. First the myosin heads attach to the thin filaments or actin and later bend, in this way they pull the actin toward the center of the sarcomere. In this step, the two types of filaments are parallel to each other and aligned. When the two types of filaments are parallel, the muscle contracts. Then, the myosin heads are released form the actin by ATP and attach to farther places and pull the thin filaments along. When this happens, the filaments are no longer parallel to each other, and the muscle relaxes. As the filaments slide along each other, the sarcomere shortens, and once sarcomeres from multiple muscle fibers shorten together, the muscle contracts. Not all myosin heads are attached to thin filament at the same time, but while some hold the actin in position, others are free to connect to new binding sites.