Muscle is responsible for maintaining posture, internal movement within the body, body movement, and the generation of heat to maintain normal body temperature. Muscle produces force by contracting, meaning muscle can pull but cannot push.

“If you look right over these mountains,” Jamie begun with a smug, raising his arm, pointing to his muscles, “you’ll see these muscles help Jamie get the ladies.”

“You do not get the ladies …” Mandy replied.

Muscle can be divided into:

• Skeletal muscle, discussed
• Cardiac muscle, which is what the heart is mainly composed of. Cardiac muscle is striated, meaning it has sarcomeres (discussed ). Cardiac muscle cells only has one nucleus. Unlike skeletal muscle, cardiac muscle doesn’t attach to bone. Rather, cardiac muscle forms the wall of the heart, which contracts to reduce the size of the heart, pumping blood out of the heart. Cardiac muscle is connected to each other by intercalated discs. Intercalated discs have gap junctions, which as expressed , permit action potential to spread from one cell to another. The interconnection by gap junctions is known as syncytial. Cardiac muscle is involuntary, meaning it is innervated by the autonomic nervous system (as mentioned )
• Smooth muscle, which is also involuntary, and only contains one nucleus. Smooth muscle is not striated, meaning it doesn’t contain sarcomeres. Rather, actin filaments (discussed ) are anchored in dense bodies. The myosin head binds to actin like in skeletal muscle (see ). Upon muscle contraction, dense bodies are pulled together and the cell shortens. Smooth muscle can be divided into:
  • Single-unit smooth muscle, where a myriad of smooth muscle fibers contract as a single unit. Although only one cell is innervated by a neuron, the action potential propagates to neighboring muscle cells through gap junctions. Note therefore, that single-unit smooth muscle is also syncytial (connected by gap junctions, as just defined). Single-unit smooth muscle is also known as visceral smooth muscle, as it is found in most viscera of the body. Viscera are internal organs inside the body
  • Multiunit smooth muscle, where each smooth muscle fiber is innervated by its own neuron, thus capable of independent contraction. For example, the iris of the eye

Skeletal muscle is voluntary, meaning it can be consciously controlled. Skeletal muscle, by its nature of causing skeletal movement, is attached to bone, via tendons. Evidently, tendons connect muscle to bone. In contrast, ligament connects bone to bone. Muscle usually originates from a larger bone [usually closer to the body], and inserts on a smaller bone [usually further from the body]. Synergistic muscles are helper muscles, which help fix certain joints, so others can move more effectively. In the example, the larger bone is stabilized by synergistic muscles, so that the other end can operate more effectively. Noting that muscle can only contract, muscles must be paired to work in opposing directions, known as agonist and antagonist. For example, biceps [brachii] and triceps [brachii] are antagonistic, such that when the arm contracts (distal arm containing radius and ulna is brought up and closer to the more proximal arm known as the humerus), the biceps (known classically as the muscle “bulge”) are at work; in contrast, when the arm straightens, the triceps (located on the back of the arm) are at work. Note that as the agonist contracts, its antagonist relaxes.

The functional unit of skeletal muscle is sarcomere. Muscle with sarcomeres are known as striated. Sarcomere consists of the thick filament [formed from the protein myosin] and thin filament [formed from the protein actin] laid as layers side by side, to form a tube. Sarcomeres are stacked end-to-end to form the long myofibril. Each myofibril is wrapped by a sarcoplasmic reticulum. Note that sarcoplasmic reticulum is an endoplasmic reticulum, and remember from  that endoplasmic reticulum is a membrane, and therefore sarcoplasmic reticulum consists of a double phospholipid bilayer (iterated ). Several myofibrils [with their respective sarcoplasmic reticulum] are wrapped within the sarcolemma, which is the membrane of the muscle cell. Muscle cells are also known as muscle fibers.

[img]muscle-fiber.png[/img]

Muscle cells are multinucleated, meaning each cell [and thus cytoplasm] has more than one nucleus. Like the neuron, skeletal muscle fibers are such specialized cells they are unable to undergo cell division. Thus, growth doesn’t occur by increase in the number of skeletal muscle fibers/cells. Rather, skeletal muscle increases in size known as hypertrophy, where the number of myosin and actin filaments increase, thus increasing the size of myofibrils.

Sarcolemma has invaginations known as T-tubules, which are tunnels that run deep into the cell. Action potentials travel along the membrane, and spreads the action potential to the interior of the cell through the T-tubules, to the sarcoplasmic reticulum.

Sarcomere (tube) is defined as the distance between one Z-line and the next. The region immediately surrounding the Z-line is known as the I-band, which represents the distance in which thin filaments don’t cross over with thick filaments. The region next to the I-band is the A-band, which represents the entire distance of the thick filament (remembering from  that this is composed of myosin). Within the A-band is the H-zone, which represents the distance in which thick filaments don’t cross over with thin filaments. Inside the H-zone [and thus A-band] is the M-line, which is the middle of the sarcomere.

[img]sarcomere.png[/img]

Upon muscle contraction, the H-zone and I-band shortens, which can be memorized with the mnemonic “HI”. The A-band however, does not change length. As stated , the thick filament is formed from a bundle of myosin protein, which has many globular heads oriented in different directions. The myosin head wishes to bind to actin, but is unable to, as the protein tropomyosin is already bound. As stated , sarcoplasmic reticulum is a unique kind of endoplasmic reticulum, that stores and pumps calcium ions. An action potential will cause the sarcoplasmic reticulum to release its calcium ions [out of the sarcoplasmic reticulum lumen] into the cytosol [remembering from , the principle that the lumen of an organelle is separated from the cytosol of the cell]. Calcium ions bind to the troponin, which is a protein attached to tropomyosin, therefore permitting troponin to pull tropomyosin away from the binding site on actin (for the myosin head). The myosin head can now bond to actin, and bend hard. As this occurs at both ends of a sarcomere, the thin filament (actin) are pulled toward each other, causing the sarcomere to shorten. If ATP is present, it binds to the myosin head, causing it to release actin. ATP then hydrolyzes, the energy used to cock the myosin head, to bend back to its high energy configuration [and wait for the active site to be uncovered again]. If ATP is not present, the myosin head is stuck to actin. This occur, for example, during death, known as rigor mortis, lasting around two days before the myosin heads are degraded, finally allowing the body to relax.

Force of a muscle is determined by motor unit, which is the collection of a neuron and the skeletal muscle fibers it innervates. Small motor units innervate less muscle fibers, and larger motor units innervate more muscle fibers. Motor units are recruited sequentially, from small to large, until the desired force is achieved. Muscles however, generally function to have a limited range of forces, and as such, generally have limited sizes. This is because only small motor units can have precision. Therefore, precision and strength are tradeoffs. For example, fingers are precise (can write with) but can provide little strength, and the leg provides greater strength but less precision (cannot write with).

“Reminds me of ‘opportunity cost’ from economics,” Mandy commented, “You can’t have your cake and eat it.”

Skeletal muscle can be divided into various types, including type I fibers (aka slow twitch), which are red in color; and type II fibers (aka fast twitch), which are white in color. Type I fibers are slower to contract, but also slower to fatigue. Type I fibers are also less powerful. The ratio of type I and II fibers depends on genetics, and like size of motor units, location of muscle. For example, a marathon runner is likely born with more type I fibers than others.

Bone is living tissue, and has various functions, including:

• Protect internal organs, such as the skull protecting the brain, and ribs protecting the heart and lungs
• Support softer tissue, providing a frame in which the body is supported
• Permit movement, since skeletal muscle require bones to generate force
• Blood cell production
• Mineral storage, acting as a reserve of minerals important to the body, most notably calcium and phosphorus in the form of hydroxyapatite. Hydroxyapatite are crystals, distributed among the collagen fibers, providing bone greater compressive strength than concrete. Bone can be resorbed to increase calcium and phosphate concentration in blood, or form bone thereby decreasing calcium and phosphate concentration in blood
• Storage of energy in the form of adipose tissue (fat), the bone marrow acting as a storage reserve of fat cells

The difference types of bone include:

• Long bones, which are slightly curved for strength. Examples include leg, arm, fingers and toe bones
• Short bones, which are cube like. Examples include ankle or wrist bones
• Flat bones, which have large surfaces for organ protection and muscle attachment. Examples include skull, sternum, rib cage, and shoulder blade
• Irregular bones, which have an irregular shape not fitting into the categories . Examples include vertebra

The bone types are distinct based on their ratio of compact to spongy bone. Spongy bone has the appearance of porous sponge. Spongy bone contains red (bone) marrow, where platelets, RBC and WBC are developed, as mentioned . Red bone marrow also contains macrophages and fat cells. Yellow (bone) marrow is found in medullary cavity, the innermost cavity of long bone. Yellow bone marrow is mainly composed of fat cells. Compact bone forms the outer shell of bone, and the shaft of long bones. The functional unit of compact bone is the osteon (aka Haversian system). Osteons are formed by the following process:

• Osteoclasts tunnel through compact bone by resorbing bone
• Osteoblasts follow, remodeling the bone, by secreting collagen and other materials required to form bone
• Osteocytes are osteoblasts that are trapped within the bone, in a hollow space called lacunae. Osteocytes therefore cease to secrete matrix material. Lacunae are connected to one another by small tunnels known as canaliculi, which are used for exchange of nutrients. Thin extensions of osteocyte cytoplasm extend through the canaliculi to form gap junctions with the neighboring osteocyte

Osteon contains blood, nerve and lymph vessels. Bone remodeling is a lifelong continuous process, meaning it is a living tissue, which is defined as dynamic tissue whose cells are replaced when die.

Cartilage is flexible and resilient connective tissue. As stated , connective tissue have an extensive matrix. Cartilage is composed of chondroitin sulfate and collagen. Chondroitin sulfate provides elasticity, and collagen provides strength. As defined , elasticity is the property of materials to return to their original shape after deformation. The cells of cartilage, known as chondrocytes, are trapped within bone, in the lacunae. Cartilage does not contain blood vessels or nerves, except in the perichondrium.

Although joints are classically thought of as locations which permit movement, joint is the location where two or more bones make contact. Joints can be classified into:

• Fibrous joint, which permit little or no movement. An example is the lines between the bones of the skull
• Cartilaginous joint, which permit slight movement. An example is the articulation of the sternum with the ribs
• Synovial joint, which are freely movable. Synovial joints are distinct because they have a capsule of synovial fluid. An example of a synovial joint is the elbow and knee