Understanding Muscle Cells - AP Biology

Skeletal muscles are used for voluntary movement of the body. They are striated and have multiple nuclei at the periphery of each cell.

Cardiac muscle is used for involuntary contractions of the heart, allowing for the movement of blood. Cardiac muscles are striated and have a single, central nucleus in each cell.

Sarcomeres represent the basic contractile unit of the muscle and can be used to show the process of contraction. Numerous sarcomeres make up myofibril, which combine to form a muscle fiber (myocyte), which combine to form a fasciculus, which combine to form muscle.

During muscle contraction thick filaments (myosin) remain stationary, while thin filaments (actin) move towards one another. In the process, the H zone and I band shorten, as the Z lines get closer together. The A band, however, never changes in length.

Skeletal muscle is attached to bones by tendons, allowing you to move around. These are the muscles that you can consciously control.

Smooth muscle and cardiac muscle are not controlled consciously. You cannot consciously control your heart beat or your blood vessels constriction.

Cardiac muscle tissue is located only in the heart and contractions in this type of tissue are critical in propelling blood through the circulatory system. Contraction is involuntary, rhythmic, and continues through the entire life of a human.

Swallowing involves both skeletal muscle and smooth muscle. While skeletal muscle is voluntary (somatic), smooth muscle is involuntary (autonomic). Swallowing involves both of these systems. One can voluntarily induce swallowing, but will also swallow involuntarily, such as while sleeping.

Most of the muscles in the esophagus are smooth muscle; these will be the muscles largely responsible for counter-acting gravity in the example. The upper region of the esophagus (the pharynx) and the tongue are composed of skeletal muscle, allowing us to induce the swallowing motion voluntarily.

The three types of muscle in the body are smooth, skeletal, and cardiac muscle. The type of muscle lining artery walls is smooth muscle, which is also present in many organs such as the intestines and the urinary bladder. Unlike skeletal muscle, smooth muscle control is involuntary and, unlike both skeletal and cardiac muscle, it is non-striated. Smooth muscle around the arteries and arterioles is important for regulating blood pressure and directing blood flow in the body.

A muscle consists of millions of tiny subunits called sarcomeres. This is the functional unit of the muscle cell responsible for shortening and causing contractile force. Sarcomeres combine to form myofibrils, which form together to become a single muscle fiber, or muscle cell. Multiple muscle fibers form a muscle belly, the macrostructure of the muscle.

Calcium ions facilitate the translocation of the troponin-tropomyosin complex, which inhibits cross-bridge formation between the myosin head (thick filament) and actin (thin filament). In order to inhibit muscle contraction, the troponin-tropomyosin must slide back into its original position, thereby inhibiting physical and chemical contact between the thick and thin filaments. In order to accomplish this, the calcium ions must be actively pumped out of the cytoplasm and into the sarcoplasmic reticulum (SR). It is important to remember that in a muscle fiber, the SR acts as a reservoir to store calcium ions. In order for them to be stored in a concentration greater than that in the cytoplasm, active transport must be involved to pump the ions against their gradient.

Muscle cells are called myocytes. "Myo" is a prefix for anything related to muscles. Terms such as myoblasts and myogenesis are related to muscle cells. "Cyte" means cell, and therefore "myo" and "cyte" means "muscle cell."

Melanocytes are found in the skin, and are responsible for producing the pigment melanin. Astrocytes are glial cells in the nervous system that create the blood-brain barrier. Acanthocytes are abnormal red blood cells that have become damaged, resulting in a change in their appearance.

Sarcomeres are not cells; they are a structure found in myocytes. Sarcomeres are composed of actin and myosin filaments, and are the fundamental contractile unit of the muscle fiber.

In order for muscle contraction to occur, myosin binding sites must be exposed on actin filaments. This is accomplished by releasing calcium ions from the sarcoplasmic reticulum. Calcium interacts with troponin, which in turn allows the myosin binding site to be exposed via interaction with tropomyosin. Troponin is not degraded during this process, only used to move tropomyosin out of the way. Myosin can then bind actin, hydrolyze ATP, and begin contraction.

GTP is not involved in the process and actin does not have an ATPase function.

Type I skeletal muscle tissue is a red—oxygenated—muscle tissue that carries oxygen and is involved in aerobic activity. As a result, type I tissue is a slow twitch muscle. Type I skeletal muscle tissue contains many mitochondria.

There are two compounds that are absolutely necessary for muscle contraction: ATP and calcium ions. Calcium ions bind to troponin, which removes tropomyosin from the binding site on actin. Only after this change occurs can myosin bind to actin and cause a contraction. ATP is hydrolyzed to alter myosin into a high energy state. This energy is released during the muscle contraction when myosin binds to actin. Even if ATP is present, the contraction cannot occur without calcium, and even if calcium is present it cannot occur without ATP.

During the contraction cycle, ATP binds to myosin in its low-energy state. ATP is converted to ADP, and the resulting energy is stored in the myosin head. The myosin then binds to actin, still carrying the ADP, and uses the energy to transition back to the low-energy state by pulling actin and shortening the sarcomere. This causes the ADP to be released from myosin. The binding of a new ATP to the myosin allows it to release actin, and the cycle begins again.

There are seven steps involved in the signaling cascade from the initiation of contraction to the subsquent relaxation of muscle fibers.

1. Motor neurons release acetylcholine (ACh), which binds to receptors on the muscle fiber's cell membrane.
2. ACh receptor binding and activation creates an action potential that propagates along the muscle fiber membrane and down T tubules.
3. The action potential triggers the release of Ca2+ ions from the sarcoplasmic reticulum.
4. The released Ca2+ ions bind to troponin, which causes the displacement of tropomyosin, revealing the myosin-binding sites on the actin filament.
5. Myosin cross-bridges attach to actin at exposed binding sites and enter a cycle of shifts to crawl along the actin fiber. This causes the sarcomere to shorten. ATP is required for this reaction.
6. Cytosolic Ca2+ ions are removed and brought back into the sarcoplasmic reticulum by active transport (by a Ca2+ ATPase pump).
7. Tropomyosin blockage of myosin-binding sites is restored by Ca2+ ion removal). Contraction ends and the muscle fiber relaxes.

The contraction of muscle cells results from the controlled interaction between actin and myosin proteins.

Actin forms a thin filament coil structure comprised to two strands of actin and some additional regulatory proteins (troponin and tropomyosin). The actin coils are arranged above and below myosin bundles, which are also called thick filaments. One end of myosin forms a globular head shape that, in a relaxed state, is not able to bind to a site on actin. When the muscle is stimulated by a motor neuron, a signal cascade results in a conformational change that allows the myosin head to bind to actin and shift positions, allowing the muscle to shorten and contract.

While ATP is an essential component in the biochemistry of muscle contraction, it is not actually a protein. ATP is, instead, considered an amino acid derivative. Titin is the elastic protein that connects the ends of a sarcomere and allows the muscle to recoil when over-stretched.

Ca2+ plays a critical role in the regulation of muscle cell contraction and relaxation. Motor neuron stimulation and subsequent the action potential signals the release of Ca2+ from the sarcoplasmic reticulum. The ions bind to troponin—a protein found in a complex with tropomyosin and bound to actin filaments—and exposes the myosin-binding sites on the actin filament by shifting the troponin-tropomyosin complex. With the myosin-binding sites revealed, the myocyte will be able to contract.

In the absence of calcium in the cell, the binding sites on the actin filament cannot be made available and myosin will not bind. No contraction will occur.

There are three primary types of muscle: skeletal, smooth, and cardiac.

Skeletal muscle is multinucleated and striated. Smooth muscle is mononucleated and not striated. Cardiac muscle is mononucleated and striated. If we know that the muscle type is not multinucleated, then it must be mononucleated and could be either smooth muscle or cardiac muscle.

Type II skeletal muscle tissues are all fast twitch muscles, but vary in their use of oxygen.