Muscle Tissue

Muscle tissue is highly specialized to convert chemical energy into kinetic energy Your movements occur through the coordinated contracting and relaxing of many muscles When muscles contract (shorten) it causes a part of the body to move

There are 3 types of muscle cells Skeletal Smooth Cardiac Food moves through the intestines due to peristalsis of the smooth muscles Your heart beats because of cardiac muscles And your body moves because skeletal muscles pull on your bones

3 Basic Muscle Types

The cells of smooth muscle tissue are long and tapered at each end and have one nucleus They are arranged in parallel lines, forming sheets Are found in the walls of blood vessels, hollow internal organs, and in the iris of the eye Contraction of these muscles is involuntary (without your control)

The smooth muscles contract slower but can remained contracted for an extended amount of time because they don’t fatigue easily

The cardiac muscle is unique to the heart and forms the walls These cells are tubular and have bands of light and dark (striated), and have one nucleus The muscle cells are branched, creating a netlike structure Like smooth muscles, cardiac muscles contract involuntary

Skeletal muscles are also tubular and striated the “meat” of our bodies are composed of these muscles The contraction of these muscles is voluntary and is controlled by the nervous system The bodies movements is due to over 600 skeletal muscles

The skeletal muscles have the following functions: Supports the body by opposing the force of gravity allowing us to stand upright Makes our body move Helps maintain a constant body temperature by releasing heat when using energy Protects the internal organs and stabilizes the joints

Unlike smooth and cardiac muscle cells, skeletal muscles are very long and have many nuclei which maintain the normal functions of these cells They are usually referred to as fibers rather than cells because of their structure

Cooperation When muscles contract, they shorten This means that muscles can only pull The work of a muscle is done when it contracts and when it relaxes it is in a passive state Muscles that allow motion come in pairs For the action of each muscle, there is another muscle that does the opposite action

Bundles of Fibers Each muscle in the body lies along the length of a bone A tough, heavy band of tissue, called a tendon, attaches each end of a muscle to a different bone The long skeletal muscle fibers can be up to 20 cm in length Muscle fibers are organized into many larger bundles

A muscle then consists of clusters of these bundles of muscle fibers A layer of connective tissue wraps around each fiber Another layer wraps around each bundle and another around the whole muscle itself Blood vessels and nerves run between the bundles of muscle fibers

The blood provides the fibers with nutrients and oxygen required to power the contractions as well as removes cellular wastes The nerves trigger and control muscle contractions Most of the volume of a muscle fiber consists of many subunits called myofibrils And each myofibril is made of even finer myofilaments

Muscle Contractions

Muscle contractions involve the coordinated action of the two types of myofilaments, actin and myosin A thin actin myofilament consists of two strands of protein molecules that are wrapped around each other

A thick myosin myofilament also consists of two strands of protein molecules wound around each other but is about 10 times longer

When a myofilament contracts, the heads of the myosin move first Like flexing your hand at the wrist, the heads bend backward and inward This moves them closer to their rod-like “backbone” and a bit in the direction of the flex Because the heads are attached at this time to an actin myofilament, the actin is pulled along with the myosin heads

As each myosin head flexes one after the other, the myosin walks along the actin Each step requires ATP to provide energy that repositions the myosin head before each flex

The sliding of actin past the myosin is part of the sliding filament model of the muscle contraction The actin is anchored at one end of each myofilament at a position in the muscle tissue called the Z line Because the actin is anchored like this, its movement pulls the Z line along with it

As the Z lines are pulled closer together, the plasma membranes to which they are attached to cause the entire muscle fiber to contract

Aerobic Cellular Respiration Aerobic cellular respiration provides most of a muscle’s ATP The muscle fibers use glucose from fats and oxygen to produce ATP The other products are carbon dioxide, water and heat CO2 leaves the body at the lungs, water is absorbed, and the heat warms the body

About 2/3 of the heat that maintains a constant body temperature comes from the aerobic cellular respiration of the skeletal muscles throughout the body

Role of Calcium When a muscle is relaxed, its myosin heads are raised and ready but not attached to actin due to another protein called tropomyosin blocking the attachment site For a muscle to contract the tropomyosin must be moved out of the way

Another protein called troponin binds to the tropomyosin to form a complex regulated by calcium concentration When calcium concentration is low, tropomyosin prevents myosin from binding to actin When calcium concentration is raised, the complex forms and is shifted away, allowing the binding of myosin and actin

Muscles and Health

During active use, some muscles are contracting and others relaxing Even when muscles appear to be at rest, their fibers are always contracting This continuous low-level activity results in muscle tone Muscle tone is important for maintaining posture If all our muscles were to completely relax, our body would collapse

In general, the skeletal muscles are subject to fewer disorders compared to other organ systems However they are vulnerable to injuries that result from sudden and intense stress placed on them and on tendons Complete the description of the listed muscular system disorders

Muscles may also be impaired simply from lack of use, this is referred to as atrophy Atrophy is a reduction in size, tone, and muscle power If extreme atrophy occurs, the loss of muscle function is permanent Exercise is important to strengthen muscles and allow them to use energy more efficiently

Create your own set of notes for slow-twitch fibers and fast-twitch fibers