The Musculoskeletal System

The Muscular System

Skeletal Muscle

• Voluntary movement (innervated by somatic system)
• Appears as striped or striated since the actin and myosin are arranged in repeating units called sarcomeres.
• Multinucleated since individual muscle cells are fused into long rods
• Myoglobin: Oxygen carrier that uses iron in heme group to bind oxygen. Red colour
• Red Fibers or Slow-Twitch Fibers: high myoglobin and get energy aerobically
  • Contain lots of mitochondria to carry out oxidative phosphorylation
• White Fibers or Fast-Twitch Fibers: less myoglobin, so there is a lighter colour.
• Both fibers mixed in a specific muscle, but if muscle is meant to contract slowly then it contains more red fibers. Muscles that contract rapidly and fatigue quickly contain mostly white fibers

Smooth Muscle

• Involuntary action and controlled by the autonomic nervous system.
• Found in respiratory tree, digestive tract, bladder, uterus, blood vessel walls, etc.
• Actin and Myosin fibers are not well organized so no striations are seen.
• Capable of more sustained contraction as compared to skeletal muscles.
• Tonus: is a constant state of low muscle contraction.
• Myogenic Activity: smooth muscle contracts without input from nervous system.
  • Contract directly in response to stretch or stimuli.

Cardiac Muscle

• Both skeletal and smooth muscle characteristics. Has involuntary control, but are striated
• Primarily have one nucleus
• Muscle cells are connected by intercalated discs that contain gap junctions. These are connections between the cytoplasm of adjacent cells and allows for the flow of ions directly between cells.
  • Allow for more rapid and coordinated muscle cell depolarization and efficient contraction
• Are able to define and maintain their own rhythm through myogenic activity.
  • Sinoatrial Node is where depolarization starts, goes to Atrioventricular nodes. Then spreads to bundle of His and then to the Purkinje Fibers.
  • This happens automatically, but nervous and endocrine system can also alter heart rate
• Vagus nerve provides parasympathetic innervation in order to slow down heart rate.
• NE from the sympathetic neuron or epinephrine from the adrenal medulla can bind to the adrenergic receptors of the heart in order to increase its rate.

Microscopic Structure of Skeletal Muscle

The Sarcomere

• Basic contractile unit of skeletal muscle and is made up of thick and thin filaments
  • Thick Filaments: organized bundles of myosin
  • Thin Filaments: made up of actin and troponin/tropomyosin (proteins)
    • These proteins help regulate the interaction between actin and myosin
  • Titin acts as a spring and anchors the myosin and actin filaments together which prevents the excessive stretching of the muscle
• Z-lines: define the boundary of each sarcomere
• M-line: runs down the center of the sarcomere
• I-band: contain exclusively thin filaments
• H-Zone: contains only thick filaments
• A-band: contains the thick filaments and the overlap with the thin filaments
• During contraction, the distance between everything becomes smaller except for the A-Band size, which remains constant.

Gross Structure of Myocytes

• Sarcomeres are attached end-to-end to form myofibrils, which are surrounded by a sarcoplasmic reticulum (SR).
  • SR is a modified endoplasmic reticulum that contains a high concentration of Ca²⁺ 
• Sarcoplasm is a modified cytoplasm located outside the SR.
• Sarcolemma is the cell membrane of the myocyte
  • Capable of propagating an action potential and can distribute the action potential to all sarcomeres in a muscle using a system of transverse tubules
  • T-tubules are oriented perpendicular to the myofibrils
• Each myocyte contains many myofibrils arranged in parallel and can be called a muscle fiber. Myocytes in a parallel form a muscle
• The nuclei are found at the peripheral of the cells

Muscle Contraction

Initiation

• Contraction starts at neuromuscular junction.
  • This is where the nervous system communicates with muscles via motor neurons
• Signal travels down the neuron until it reaches the nerve terminal (synaptic bouton), where ACh is released into the synapse
  • Nerve terminal also called motor end plate
• Ach causes depolarization. Each nerve terminal controls a group of myocytes, and both the nerve terminal and myocytes makes up the motor unit.
• Depolarization triggers an action potential which spreads down the sarcolemma to the T-tubules.
• T-tubules travel into the SR and Ca²⁺ is released.
• Calcium ions bind to a regulatory subunit in troponin which triggers a change in the tropomyosin
• Change in tropomyosin exposes the myosin-binding sites on the actin thin filament.

Shortening of Sarcomere

• Globular heads of the myosin bind to the exposed sites on the actin, which forms an actin-myosin cross bridge.
• Cross bridge allows for myosin to pull on actin. This draws the thin filaments towards the M-line, which ultimately shortens the sarcomere.

Sliding Filament Model

• Myosin carries ADP + inorganic phosphate binds to the myosin binding site. The ADP + P_i are released and this provides energy for power stroke (the sliding of actin over myosin).
• ATP binds to myosin head and it is released from the Actin.
• ATP is then hydrolyzed into ADP + P_i which subsequently recocks the myosin head
• The model is based on the repetitive binding and releasing of myosin heads on actin filaments. Which allows the thin filament to slide along the thick filament.

Relaxation

• Acetylcholinesterase is the enzyme that degrades Acetylcholine. This results in the termination of the signal at the neuromuscular junction and allows the sarcolemma to repolarize
• Calcium release ceases as the signal decays and the SR takes up calcium into sarcoplasm.
• SR is responsible for the control of calcium ions in order to only allow contraction when it is necessary.
• ATP binds to myosin head, which allows Sarcomeres to relax. And tropomyosin covers the myosin-binding sites.

Stimulation, Summation and Muscle Fatigue

• Strength of a response depends on the number of motor units that are recruited to respond. The strength of a response form one muscle cell cannot be changed.

Simple Twitch

• Response of a single muscle fiber to a brief stimulus.
• Latent period: time between reaching threshold and the onset of contraction.
• P spreads along muscle and allows for calcium to be released from the SR.
• Muscle then contracts and relaxes if calcium is present.

Summation and Tetanus

• Frequency summation is the frequent and prolonged stimulation of a muscle fiber.
• Contractions will combine, become stronger and be prolonged since muscle has insufficient time to relax.
• Tetanus: is when the muscle is unable to relax because the contractions are so frequent.

Oxygen Debt and Muscle Fatigue

• Muscles require ATP to function. For aerobic metabolism, large amounts of oxygen are required to generate the large amount of ATP needed.
• Creatine Phosphate: created by transferring phosphate group from creatine during times of rest. Process can then be reversed to quickly generate ATP.
• Myoglobin: reserves are used to keep aerobic metabolism going.
• Fast-twitch muscles rely mainly on glycolysis & fermentation to make ATP
• When muscles overwhelm the body’s ability to deliver oxygen, then even red muscle fibers switch to anaerobic metabolism and produce lactic acid. This is when muscles begin to fatigue.
• Oxygen Debt: difference between amount of oxygen needed by the muscle and the actual amount present.
• Body must metabolize all lactic acid to pyruvate which requires oxygen. Amount of O₂ needed is equal to the oxygen debt.

The Skeletal System

• Exoskeletal encase the whole organism and are usually found in arthropods
• Endoskeleton are internal, but cannot protect the soft tissue structure as well as exoskeletons do.

Skeletal Structure

• Axial Structure is the skull, vertebral column, ribcage and hyoid bone and it provides the basic central framework for the body.
• Appendicular Skeleton: bones of the limb, pectoral girdle and pelvis.

Bone Composition

• Connective tissue derived from embryonic mesoderm

Macroscopic Bone Structure

• Strength comes from compact bone
• Spongy/Cancellous Bone: lattice structure consists of trabeculae (bony spicules)
• Bone Marrow: cavities between trabeculae
  • Red Marrow: filled with hematopoietic stem cells
  • Yellow Marrow: composed mainly of fat and is inactive
• Long Bones: found in the appendicular skeleton, and are characterized by cylindrical shafts called Metaphyses swell at the end of each diaphyses. Bone ends are called epiphyses.
  • Outermost portions are compact bones and internal is spongy.
  • Diaphyses and metaphyses are full of bone marrow.
  • Epiphyses use spongy cores to disperse the force and pressure on joints.
• Epiphyseal (growth) Plate: at the internal edge of the epiphysis and is a cartilaginous structure where longitudinal growth occurs.
  • Prior to puberty, is filled with mitotic cells that contribute to growth. These plates close at the end of puberty and growth is halted.
• Periosteum: surrounds long bond to protect and acts as a site for muscle attachment.
  • Needed for bone growth and repair.
• Tendons attach muscle to bone & Ligaments are bone to bone at joints
• Bone Matrix: includes organic compounds (collagen, glycoproteins, peptides) and inorganic compounds (calcium, phosphate, hydroxide ions form hydroxyapatite).
  • Is where the strength of compact bones comes from
  • Also stores minerals such as sodium, magnesium, potassium
  • Strength comes from uniform distribution of organic and inorganic compounds
  • Osteons or Haversian Systems: structural unit of bone matrix
    • Lamellae: surround a central microscopic channel. These are concentric circles of bony matrix
    • Haversian Canals: longitudinal channels
    • Volkmann’s canals: transverse channels
      • Canals contain blood vessels, nerve fibers, and lymph vessels that maintain the health of the bone.
    • Lacunae: found between the lamellae and are site for mature bone cells (osteocytes).
    • Canaliculi: connects the lacunae which allows for the exchange of nutrients and wastes between the osteocytes and canals.

Bone Remodeling

• Osteoblasts build bone, while osteoclasts (polynucleotide macrophages) reabsorb it.
• Bone turnover/remodeling occurs in response to stress and the bone is remodeled in a way to accommodate the repetitive stresses.
• Parathyroid Hormone: peptide hormone that promotes the resorption of bone when calcium levels in the blood are low.
• Vitamin D: activated by parathyroid hormone, promotes the absorption of bone.
• Calcitonin: peptide hormone is released in response to high calcium levels. This promotes bone formation and decreases calcium levels.

Cartilage

• Softer and more flexible than bone. Not innervated with blood/lymph vessels – avascular
• Consists of firm and elastic matrix called the chondrin which is secreted by chondrocyte cells.
• Fetus skeletons are mainly cartilage since they need more flexibility to grow in confined environment.
• In adults, is only located in areas that need extra flexibility or cushioning (nose, ear, walls of larynx/trachea, intervertebral discs and joints)
• Endochondral Ossification: creation of bones through the hardening of cartilage. Is responsible for most of the long bones of the body
• Intramembranous Ossification: undifferentiated embryonic connective tissue (mesenchymal tissue) is transformed into bone. Occurs in the bones of the skull

Joints and Movement

• Immovable Joints: bones that are fused together to form sutures. Found mainly in the head (anchor bones of the skull together)
• Moveable Joints: Permit bones to shift relative to each other and are strengthened by ligaments (fibrous tissue that connects bones to one another).
  • Synovial Capsule: encloses joint cavity
  • Synovium: soft tissue that secretes synovial fluid.
  • Articular Cartilage: coats the articular surfaces of the bones in order to restrict impact to lubricated join cartilage
• Origin: end of a muscle with a larger attachment to bone (usually proximal).
• Insertion: End of muscle with smaller attachment to bone (usually distal)
• Antagonistic Pairs: How the muscles usually work – one muscle relaxes while the other contracts.
• Synergistic: Muscles working together to accomplish the same function

Types of Movement

• Flexor Muscle: decreases the angle across the joint
• Extensor Muscle: Increases the angle across the joint
• Abductor: Moves body away from the midline
• Adductor: Moves body towards midline
• Medial rotator: rotates axis of limb towards the midline
• Lateral rotator: rotates axis away from midline