Advice TC

Bones cannot move without muscles. Cell contraction is the changing of the

Shape of the cell. If a cell contracts in one direction, it lengthens in another direction. Some cells are designed to contract, these are muscle cells, which are found in most animals. These types of movements rely on biochemical systes involving tubulin and microtubules. Another basic cell movement depends on the protein actin found in the eukaryotic system. Actin fibers cause movement in the cytoskeleton and cell mambrane, using energy which comes from ATP.

A. Muscle groups

There are three types of muscles

1. Smooth muscles

Smooth, glistening in appearance, also called involuntary muscle. Found in various internal organs such as in the walls of blood vessels and the digestive tract.

2. Cardiac Muscle or Heart Muscle

Has control centers of its own

3. Skeletal Muscle

Striated or voluntary muscle. These muscles are controlled by the somatic nervous system. They can contract much more rapidly than smooth muscle or cardiac muscle, but they cant stay contracted for long periods. Skeletal muscles are multinucleated; the nuclei lay just beneath the cell surface.

B. Muscle Anatomy

Each muscle fiber contains a precise arrangement of protein. A whole bunch of these fibers is called a fasciculi and gives meat its stringy appearance. Blood vessels run throughout the fasciculi supplying oxygen and nutrients while getting rid of wastes and carbon dioxide. Nerves also penetrate the bundles and divide so that each fiber has a nerve. The whole mass of aligned fasciculi is the belly of the muscle. The belly is enclosed by fascia. At the end of the muscles the fascia unites to from a collagenous tissue called a tendon which fastens the muscle to the bones. When muscles contract, one end moves and one does not move much. Bones are returned to their original position by opposing actions.

In a hydrostatic skeleton, the body returns to its original position with incoming fluids.

C. Muscle Anatomy

1.Antagonistic Muscles

Two opposing muscle groups are called antagonistic muscles.

a. Flexor: Biceps brachii

Originates at two points, inserts on the radius

b. Antagonist

Lies on the back of the arm. It originates at three places, two on the humerus and one on the scapula

D. Muscle Fiber

1.Each muscle fiber is surrounded by a cell membrane, sarcolemma

1. the sarcolemma receives the endings of the motor neurons at the neuromuscular junction.
2. Motor neurons send impulses from the central nervous system to muscle which causes the muscles to contract
3. Just below the sarcolemma there are a number of mitochondria, nuclei, and glycogen granules, a sarcoplasmic reticulum and T-tubules which spread the action potential ot the sarcoplasmic reticulum
4. Below these structures are rod shaped myofibrils.

Lets look at one myofibril: we see banding which is called a Z line. Between the Z lines there is the contractile unit called the sarcomere.

Toward the center is a broad region called the A band with a smaller lighter region called the H zone. The A band is made up of overlapping myofilaments. Myofilaments are filamentous protein structures of actin and myosin. The dark lines extending across the A band and running through the H zone are myosin filaments. The myosin filaments are overlapped by actin filaments which begin at the Z line and run part way through the A band. The I bands consist of actin filaments alone.

E. Muscle Contraction

When the muscle fiber is stimulated, the Z lines move together and the sarcomere is shortened. The banding changes producing a dark line where the H zone was. Actin myofilaments slide inward through the myosin. The I bands become reduced

1. How will this happen?

Myosin consists of a fibrous “tail” with a globular head. The tail s where the individual myosin molecules join to form a thicker filament. The myosin head binds to an ATP molecule which can me hydrolyzed to ADP and P. the energy released by the cleaving is transferred to the myosin and changes the shape of myosin to a high energy configuration. The high energy myosin binds to a specific site of actin and forms a cross-bridge. Once attached to actin, the stored energy is released, and the myosin head relaxes to its lower energy configuration. When it relaxes, it changes the angle of the attachment of the myosin head to the fibrous myosin tail. The myosin bends inward on itself and pulls the thin filament toward the center of the sarcomere. The bond between the lower energy myosin and actin is broken when a new molecule of ATP binds to the head. The process repeats itself.

Each of the approximately 350 heads of the myosin filament join and rejoin about 5 cross bridges per second.

A muscle stores enough ATP for a few contractions. Although muscles store glycogen between the myofibrils, most of the energy needed for repetitive muscle contraction is stored in phosphagens. These substances can supply a phosphate group to make ATP from ADP. When at rest, the myosin binding sites on the actin molecules are blocked by topomyosin, a regulatory protein. The troponin complex, another set of regulatory proteins, positions the tropomyosin on the actin filaments.

For a muscle cell to contract, the myosin-binding sites on the actin must be exposed. Calcium ions bind to the troponin complex which changes the shape of the complex. This changes the interaction between the troponin complex and tropomyosin. This change exposes the myosin binding sites on the actin. The membrane of the sarcoplasmic reticulum actively transports calcium from the cytoplasm into the interior of the reticulum, which is and intracellular storehouse for calcium.

1. Steps for muscle contraction

a. An impulse moves down the nerve cell and through the motor neuron.

1. The motor neuron stimulates the sarcolemma. The action potential spreads deep into the interior of the muscle cell along the infoldings of the tubules.
2. The sarcolemma stimulates the sarcoplasmic reticulum which then releases calcium.
3. The calcium floods the sarcomere and binds to the troponin complex. The interaction between the troponin and tropomyosin is affected. The myosin binding sites on the actin are now exposed.
4. ATP on the myosin head hydrolyzes. The energy is transferred to the myosin and changes the shape of the molecule. The energized myosin binds to a specific site on the actin and forms a cross bridge
5. The energy stored in myosin is released. Myosin relaxes to the lower energy shape. As it relaxes, the myosin bends on itself, pulling the actin filament towards the center of the sarcomere. The cross-bridge between the actin and myosin is now broken.
6. A new ATP molecule binds to the myosin head.
7. This continues until the muscle has contracted
8. The muscle returns back to its original position, when the antagonistic muscle contracts.

Rigor Mortis occurs when there is no more ATP available, and the calcium is not removed from the sarcomere. The muscle stays contracted. Rigor Mortis ends because bacteria have begun to break down the muscle filaments.

1. Fast and slow muscles

Fast and slow muscles differ in the duration of twitches.

Slow fibers have less sarcoplasmic reticula than fast fibers. Calium remains in the cytoplasm longer. The twictch lasts up to 5 times longer than the fast fiber.

Slow twitch fibers have many mitochondria, a rich blood supply, and myoglobin( an oxygen storing protein). Myoglobin is a brownish red pigment which binds oxygen more tightly than hemoglobin. Myoglobin is found in the dark meat of poultry and fish.

1. Osteoporosis and muscle contraction

Postmenopausal women do not easily absorb dietary calcium, but they need calcium for muscle contractions. The calcium is removed from the bones. Bones become brittle over time and break easily. Females need to increase calcium intake when young to prevent osteoporosis.