Musculoskeletal system

Skeletal System

• Provides body framework, protects organs, contains red bone marrow, and stores calcium.
• Calcium is removed from bones when blood levels are low and shifted into bones when blood levels are high.

Bone Tissue

• Osteocytes are bone cells.
• The matrix contains calcium salts and collagen.
• Types of calcium salts include calcium carbonate and calcium phosphate.
• Calcium constantly shifts between bone and blood.
• Osteocyte cilia detect calcium content in interstitial fluid.
• Cells regulate calcium deposited or removed from the bone matrix.
• Types of bone tissue include compact and spongy bone.

Compact Bone

• Made of osteons or haversian systems.
• Bone matrix exists as microscopic cylinders.
• Osteocytes are in concentric rings around the central canal.
• Cytoplasmic extensions project into microscopic canaliculi in the matrix.
• Blood vessels in haversian canals provide oxygen and nutrients to innermost osteocytes.

Spongy Bone

• Contains osteocytes, matrix, and blood vessels, but lacks the arrangement of haversian systems.
• Contains visible holes or cavities.
• Often contains red bone marrow.
• Produces erythrocytes, leukocytes, and thrombocytes.
• Yellow marrow comprises fat and mesenchymal stem cells.
• These cells can form chondrocytes or osteocytes.

Articular Cartilage

• Hyaline cartilage covering joint surfaces for smooth movement.

Periosteum

• Outer bone covering comprised of fibrous connective tissue membrane.
• Collagen fibers merge with tendons and ligaments to anchor them on bone.
• Blood vessels enter the bone.
• Osteoblasts activate if the bone is damaged.

Bone Classification

• Long bones include the femur, humerus, and phalanges.
• The diaphysis makes up the long bone shaft comprised of compact bone.
• A hollow medullary canal containing yellow bone marrow forms in the diaphysis.
• Epiphyses make up the ends of long bones.
• Epiphyses consist of spongy bone covered in a thin layer of compact bone.
• Short, flat, and irregular bones are made of spongy bone covered with a thin layer of compact bone.
• Red bone marrow can be found within spongy bone.
• Short bones include carpal and tarsal bones.
• Flat bones include ribs and the scapula.
• Irregular bones include vertebrae and facial bones.

Bone Growth and Remodeling

• Osteoblasts produce (build) bone matrix.
• Osteoclasts perform bone resorption.
• Acids and enzymes secreted by osteoclasts dissolve and digest the bone matrix.
• Remaining minerals and amino acids are absorbed in bone resorption.
• Osteoclasts are very active in embryonic long bones.
• Osteoclasts reabsorb bone matrix in the center of the diaphysis to form the marrow canal.
• Bone remodeling occurs throughout life.
• Osteoclasts remove calcium from bone to increase blood calcium levels.
• Osteoblasts calcium salts and deposit bone matrix to lower blood calcium levels.
• Osteoblasts and osteoclasts respond to weight bearing and increased or decreased loading.

Embryonic Bone Growth

• The skeleton begins as fibrous connective tissue (cranial and facial bones) and cartilage (rest of bones).
• Ossification of the fetal skeleton begins in the 3rd month of gestation, producing bone matrix.
• In cranial and facial bones, bone matrix replacement radiates from the center of ossification in each bone.
• In long bones, the center of ossification is initially in the diaphysis, followed by later centers in the epiphyses.
• Growth in long bones occurs at epiphyseal discs/plates (growth plates) located at the junction of the diaphysis with the epiphysis.
• Epiphyseal plates are cartilage.
• Bones lengthen as more cartilage is produced on the epiphyseal side.
• Osteoblasts on the diaphyseal side produce bone matrix to replace cartilage.
• Osteoblasts transition to maintenance cells (osteocytes) when surrounded by bone.
• Epiphyseal plates close between 16 and 25 years of age.
• Cartilage production slows.
• Bone replacement accelerates.
• The cartilage matrix of the epiphyseal plates is replaced by bone matrix.

Factors Affecting Bone Growth

• Heredity determines genetic potential.
• Bones require proper nutrient levels to grow.
• Calcium, phosphorus, and protein become part of the bone matrix itself.
• Vitamin D is needed for efficient calcium and phosphorus absorption by the small intestine.
• Vitamins A and C are needed for ossification or calcification.
• Adipose tissue produces leptin, which stimulates osteoblasts.
• Osteoblasts produce osteocalcin, which decreases fat storage by adipose tissue and increases insulin production.
• Cells in the small intestine produce serotonin, which inhibits osteoblasts.
• Several hormones are important for bone growth and maintenance.
• Increased loading during exercise or physical stress stimulates osteoblasts, while decreased loading stimulates osteoclasts.

Hormones Affecting Bone Growth

• Growth hormone increases the mitosis rate of chondrocytes and osteoblasts and the rate of protein synthesis (collagen, cartilage matrix, and enzymes for cartilage and bone formation)
• Thyroxine increases the rate of protein synthesis and energy production from all food types
• Insulin increases energy production from glucose
• Parathyroid hormone increases calcium release from bones into the blood, boosting calcium blood levels. It also increases calcium absorption by the small intestine and kidneys into the blood.
• Calcitonin decreases calcium release from bones, lowering blood calcium levels.
• Estrogen or testosterone promotes the closure of epiphyseal plates of long bones and helps retain calcium.

Muscular System

• Consists of skeletal muscles and tendons.
• Skeletal muscle, aka striated or voluntary muscle tissue.
• Tendons are made of fibrous connective tissue.
• Fascia around each muscle is also made of fibrous connective tissue.
• Functions include moving the skeleton and producing heat.

Systems Necessary for Movement

• The skeletal system which is moved by muscles.
• The nervous system transmits electrochemical impulses to muscles.
• The respiratory system exchanges O2 and CO2 between the air and the blood.
• The circulatory system transports O2 to muscles and takes CO2 away.

Muscle Structure

• Each muscle consists of thousands of muscle cells (muscle fibers or myocytes).
• Epimysium is connective tissue surrounding an entire muscle.
• Fascicles (bundles) of muscle fibers are contained in the epimysium
• Perimysium is tissue surrounding each fascicle.
• Endomysium is tissue surrounding each muscle fiber.

Brain Role

• Skeletal muscle contraction depends on brain function.
• The frontal lobe controls volitional movement.
• Motor areas of frontal lobes generate electrochemical impulses that travel along motor neurons to muscle fibers.
• A single axon of a motor neuron may branch extensively with one neuron innervating a few to hundreds of muscle fibers.
• A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
• Small motor units are found in muscles performing small, precise movements while large motor units are found in muscles needing more power.
• Effective movement requires some muscles to contract while others relax.
• Coordinated movements are regulated unconsciously by the cerebellum.

Muscle Tone

• Muscle tone is a state of slight contraction present in healthy muscles.
• Muscle tone is regulated by the cerebellum.
• Alternate fibers (different motor units) contract for posture.
• Coordination and heat production occur.
• Resting tone accounts for ~25% of body heat.

Energy Sources For Muscle Contraction

• ATP is the direct energy source
• Creatine phosphate is an energy-transferring molecule that is broken down to creatine, phosphate, and energy.
• The energy converts back to ATP.
• Most creatine formed converts back to creatine phosphate.
• Creatinine, a nitrogenous waste product from the breakdown of creatine phosphate is excreted from the kidneys.
• Glycogen is the most abundant energy source that is stored in muscles.
• Glycogen is converted to glucose for cellular respiration.

Oxygen and Muscle Cells

• Oxygen sources for muscles include hemoglobin in RBCs and myoglobin in muscle cells.
• Both sources contain iron, enabling them to bond to oxygen.
• Oxygen debt follows strenuous exercise.
• Oxygen stored in myoglobin is quickly used up.
• Normal circulation can't deliver more oxygen fast enough to continue aerobic cell respiration.
• Glucose can't be fully broken down to CO2 and H2O.
• Cell respiration becomes anaerobic without adequate oxygen.
• Glucose converts to lactic acid, lowering pH and contributing to muscle fatigue.

Neuromuscular Junction (NMJ)

• The neuromuscular junction is the site where a motor neuron terminates on a muscle fiber.
• The axon terminal is the enlarged tip of the motor neuron that contains the neurotransmitter acetylcholine (ACh).
• The sarcolemma is the membrane of the muscle fiber.
• The sarcolemma contains receptor sites for Ach and cholinesterase.
• T tubules are inward folds of the sarcolemma that carry the action potential to the interior of the muscle cell.
• The synapse (synaptic cleft) is the small space between the axon terminal and sarcolemma.
• Cholinesterase is an enzyme that breaks down acetylcholine.

Sarcomere Structure

• Sarcomeres are the contractile unit of the muscle arranged end-to-end in cylinders called myofibrils.
• Z lines mark the end boundaries of a sarcomere.
• Myosin and actin filaments are contractile proteins.
• Thick filaments contain myosin in the sarcomere center.
• Myosin filaments are anchored to the Z lines by the protein titin.
• Thin filaments containing actin are attached to the Z lines.
• Troponin & tropomyosin are inhibiting proteins of the thin filaments.
• They prevent sliding of actin & myosin when the muscle fiber is relaxed.
• The sarcoplasmic reticulum is an endoplasmic reticulum of muscle cells, which serves as a reservoir for calcium ions.

Muscle Contraction

• Begins when a nerve impulse arrives at the axon terminal, stimulating the release of ACh.
• ACh generates a shift in ions at the sarcolemma, which initiates sliding filament mechanism of muscle contraction.

Sliding Filament Mechanism

• A nerve impulse arrives at the axon terminal.
• Ach is released and diffuses across the synapse to the sarcolemma.
• Ach binds to receptors and makes the sarcolemma more permeable to Na+ ions.
• Sodium rushes into the cell.
• The sarcolemma depolarizes, becoming negative outside and positive inside the cell.
• T tubules bring the reversal of charges to the interior of the muscle cell.
• Depolarization stimulates the release of Ca2+ ions from the sarcoplasmic reticulum.
• Ca2+ bonds to the troponin–tropomyosin complex, shifting it away from the actin filaments.
• Myosin splits ATP to release its energy.
• Bridges on myosin attach to actin filaments to pull them toward the center of the sarcomere, shortening it.
• All sarcomeres in a muscle fiber shorten.
• The sarcolemma repolarizes.
• Potassium ions leave the cell, restoring a more positive charge outside and a negative charge inside.
• Pumps return Na+ ions outside and K+ ions inside.
• Cholinesterase in the sarcolemma inactivates acetylcholine.
• When there are no further impulses, muscle fibers relax.