The Muscular System Quiz

The Muscular System

• The muscular system is responsible for movement in the entire organism and the movement of materials, e.g., blood, food, within the body.
• Three types of muscle tissue exist: smooth, cardiac, and skeletal.
• Muscle cells are also known as muscle fibers.

Smooth Muscle

• Smooth muscle fibers are tapered cylinders with a single nucleus.
• They are arranged in parallel lines, forming sheets.
• Smooth muscle is found in the walls of hollow internal organs and blood vessels, causing those walls to contract.
• The contraction of smooth muscle is involuntary.
• Smooth muscle can sustain prolonged contractions and does not fatigue easily.

Cardiac Muscle

• Cardiac muscle forms the heart wall.
• Cardiac muscle fibers are branched, striated, and tubular, with a single nucleus.
• They interlock at intercalated discs, containing gap junctions that permit contractions to spread throughout the heart wall.
• Cardiac muscle relaxes completely between contractions, preventing fatigue.
• Cardiac muscle contracts rhythmically without nervous stimulation; the contraction is involuntary.

Skeletal Muscle

• Skeletal muscle fibers are tubular, multinucleated, and striated.
• They make up skeletal muscles attached to the skeleton.
• This type of muscle fiber is very long, running the entire length of the muscle.
• Skeletal muscles are voluntarily controlled.
• Skeletal muscles function in groups.

Skeletal Muscle Structure

• Fascicle: A bundle of skeletal muscle fibers.
• Fascia: Connective tissue covering muscles, extending to form tendons.
• Bursae: Small, fluid-filled sacs between tendons and bones, acting as cushions and lubrication.
• Origin: The attachment site of a muscle to a stationary bone.
• Insertion: The attachment site of a muscle to a bone that moves during contraction.
• Groups of Muscles: Muscles function in groups, agonist, synergist, and antagonist. - Agonist: The primary mover doing most of the work. - Synergist: Assists the agonist. - Antagonist: Acts opposite to the prime mover.

Names and Actions of Skeletal Muscles

• Muscle names often show size (e.g., gluteus maximus, gluteus minimus), form (e.g., deltoid, trapezius), or location (e.g., external obliques, internal obliques).
• Other terms describe direction of muscle fibers (e.g., rectus abdominis), attachment sites (e.g., sternocleidomastoid), and numbers of attachments.

Components of Muscle Fibers

• Sarcolemma: Plasma membrane of a muscle fiber; forms T tubules.
• Sarcoplasm: Cytoplasm of a muscle fiber; contains organelles, including myofibrils.
• Myoglobin: Red pigment storing oxygen for muscle contraction.
• T tubules: Extension of sarcolemma; conveys impulses that cause calcium release.
• Sarcoplasmic reticulum: Smooth endoplasmic reticulum of a muscle fiber; stores calcium.
• Myofibril: A bundle of myofilaments that contracts.
• Myofilaments: Actin or myosin filaments.
• Sarcomeres: Contractile units within a myofibril.
• Two Types of Myofilaments: Thick (myosin) and Thin (actin). - Thick (myosin) myofilaments: Shaped like golf clubs, ending in cross-bridges. - Thin (actin) myofilaments: Composed of two intertwined strands of actin.
• Bands within a Sarcomere: I band (thin filaments only), A band (overlapping thin and thick filaments), H band (thick filaments only). Z line/discs is found at the end of every sarcomere and at the edge of each myofilament

The Sliding Filament Theory

• As the muscle fiber contracts, the sarcomeres shorten.
• Myofilaments slide past each other; their length remains the same.
• I band shortens and the Z lines move inward, alongside an almost disappearing H band.
• ATP provides energy for myosin heads to bind to actin, break down the ATP, and pull actin filaments toward the center of the sarcomere.
• Rigor mortis: the muscle stays contracted at death because ATP is needed to break cross-bridges.

Muscle Fiber Contraction.

• Motor neuron: A nerve cell stimulating muscle fibers to contract.
• Nerve: Group of axons.
• Axon: Long extension of a neuron that stimulates a muscle fiber.
• Neuromuscular junction: Where axon terminals form a synapse with the sarcolemma, transmitting signals.
• Synaptic cleft: The space separating the axon terminal and the sarcolemma.
• Acetylcholine (ACh): neurotransmitter released at the neuromuscular junction.

Motor Units and Muscle Twitch

• Motor unit: A motor neuron and all the muscle fibers it innervates.
• Muscle twitch: A single contraction of a muscle fiber lasting a fraction of a second.
• Stages of Muscle Twitch: Latent period, contraction period and relaxation period.
• Summation of Muscle Twitch: Increased muscle contraction due to increased calcium.
• Tetanus: A maximal sustained contraction.
• Fatigue: Muscle relaxes even when stimulation continues, often due to depletion of energy reserves.

Energy for Muscle Contraction

• Creatine phosphate (CP) pathway: Simplest and fastest way to produce ATP during exercise.
• Fermentation: Anaerobic process that produces ATP from glycogen.
• Cellular respiration: Aerobic process in mitochondria, most efficient way to produce ATP.

Fast-twitch and Slow-twitch Fibers

• Fast-twitch fibers (white): Designed for strength and explosions of energy, not endurance; primarily anaerobic.
• Slow-twitch fibers (red): Endurance sports; primarily aerobic.

Common Muscular Conditions

• Spasms: Sudden involuntary muscle contractions.
• Cramps: Strong painful muscle spasms.
• Convulsion: Multiple spasms.
• Strain: Stretching or tearing of a muscle.
• Sprain: Twisting of a joint, typically injuring muscles, ligaments, tendons, and blood vessels.
• Tendinitis: Inflammation of a tendon.

Muscular Diseases

• Myalgia: Achy muscles, often due to overuse.
• Myositis: Inflammation of muscles, commonly caused by infection or immune disorders.
• Fibromyalgia: Chronic condition with widespread pain, tenderness, and stiffness in muscles.
• Sarcomas: Cancers originating in muscle or connective tissues surrounding muscles; can also occur in bone, adipose, and cartilage.
• Muscular Dystrophy: Progressive degeneration and weakening of muscles due to genetic defects.
• Myasthenia gravis: Autoimmune disease weakening muscles of the eyelids, face, neck, and extremities; typically caused by immune system attacking acetylcholine receptors.