Biology 101 - Muscle and Skeletal System Part 3

Each calcium ion binds to troponin, causing tropomyosin to change its shape. This frees the myosin binding site on the relevant actin molecule.

When calcium is not present, tropomyosin is bound to actin in a position that blocks the myosin binding site. For contraction to occur, this site must be open for the head of a myosin molecule to attach.

These are deep folds in the sarcolemma of muscle cells. Since they bring the cell membrane closer to the central regions of the cell, they allow action potentials to quickly propagate to the sarcomeres.

sliding filament model

According to this theory, myosin heads bind to actin molecules, then undergo a quick stroke to shorten the sarcomere. The filaments themselves do not change in length during this process; they simply overlap.

The length of the A band remains unchanged. In a sarcomere, the A band refers to the length of an entire myosin fiber. Since actin and myosin do not actually shorten during contraction, the A band's length is always constant.

Both the I band and the H zone change in length during contraction. In a sarcomere, the I band refers to the unoverlapped actin region, while the H zone refers to the region that contains myosin alone. Contraction involves the overlapping of filaments, so both of these regions shorten as the actin and myosin pull together.

• Tropomyosin directly attaches to actin. In the absence of calcium, it physically blocks the myosin binding site.

• Troponin binds to tropomyosin. It binds to calcium (when the ion is present) and triggers a conformational change in the tropomyosin molecule.

1. Ca2+ binds to troponin on actin molecules; on each one, tropomyosin is moved away from the myosin binding site.

2. Myosin heads have already bound ATP and are situated in a high-energy position. Now, they can freely bind actin, forming cross-bridges.

3. ATP is hydrolyzed; the myosin pulls back in a "power stroke," shortening the sarcomere.

4. A new ATP molecule is required to remove myosin from its bound position and reset the cycle.

Skeletal muscle becomes permanently contracted in a condition known as rigor mortis.

A common misconception is that ATP is necessary for the initiation of muscle contraction. Technically, ATP is required for the termination of contraction, where myosin dissociates from actin. Deceased individuals would be unable to perform this process, so their muscles would remain contracted.

1. protection of internal organs

2. structure and support

3. calcium storage

4. immune cell and erythrocyte production

1. compact bone

2. spongy bone

Compact bone is also called cortical bone; spongy bone is also called cancellous bone.

It is hard and dense, mainly composed of the minerals, salts, and collagen that form the bony matrix.

Compact bone is comprised of cylindrical subunits called osteons.

It is softer and less dense than compact bone. It contains many blood vessels, as well as gaps in its structure that are filled with bone marrow.

Spongy bone is comprised of thin, sharp subunits called trabeculae.

A strip of tough connective tissue that connects a muscle to a bone. Tendons are composed of collagen.

Tendons generally function to facilitate movement.

A strip of tough connective tissue that connects a bone to another bone. Ligaments are composed of collagen.

Ligaments generally function to stabilize joints. For example, the anterior cruciate ligament, or ACL, stabilizes the knee.

A form of connective tissue that is less rigid than bone. It includes elastin protein fibers, proteoglycan, and collagen.

The collagen in cartilage is produced by specialized cells called chondrocytes.

During development, cartilage solidifies into bone in a process known as ossification. It also provides soft, flexible protection for certain parts of the anatomy.

For example, the trachea, nose, and ears contain cartilage.

endoskeletons

An endoskeleton is an internal rigid structure. It can provide support to the organism, but does not encase the entire body.

exoskeletons

An exoskeleton is an external rigid encasement. It protects the entirety of the body, but must be replaced as the organism grows.

1. axial skeleton

2. appendicular skeleton

The axial skeleton is relatively central; it contains the vertebrae, ribcage, and skull. The appendicular skeleton is more peripheral and includes the bones of the appendages and the pelvis.

calcium

Plasma Ca2+ concentration is controlled by two hormones: calcitonin and parathyroid hormone (PTH). Calcitonin lowers plasma calcium by promoting its storage in bone, while PTH has the opposite effect.

• Osteoclasts break down bone and release its calcium content into the plasma. As a result, osteoclast activity increases blood calcium levels.

• Osteoblasts promote the synthesis of bone, removing calcium from the plasma in the process. As a result, osteoblast activity decreases blood calcium levels.

decrease

Calcitonin is released from the thyroid when plasma calcium levels are high. It stimulates osteoblasts to increase bone formation, taking calcium out of the blood in the process. In contrast, parathyroid hormone stimulates osteoclasts.

parathyroid hormone

| PTH

Abnormally high osteoblast activity means that excess calcium is being taken out of the plasma and stored as bone. This can result from high levels of calcitonin or low levels of PTH.

hydroxyapatite (Ca5(PO4)3OH).

It is important to know that bone contains calcium, phosphate, and hydroxide.

synovial joints

Synovial joints allow for flexibility at the junctions between bones. The rigid bones are separated by synovial fluid. Ball-and-socket joints are an example of this type of structure.