midterm exam 2

• Three types of vertebrate muscle:

  • Skeletal: for voluntary movement and breathing
  • Cardiac: for the beating of the heart
  • Smooth: for involuntary movement of internal organs
• Skeletal muscle moves bones and the body and is characterized by a hierarchy of smaller and smaller units
• A skeletal muscle consists of a bundle of long fibers, each a single cell, running along the length of the muscle
• Each muscle fiber is itself a bundle of smaller myofibrils arranged longitudinally
• Skeletal muscle cells are called muscle fibers and are extremely large, multinucleated cells formed by the fusion of embryonic myoblasts
• The functional unit of a muscle is called a sarcomere and is bordered by Z lines, where thin filaments attach
• Each muscle fiber has several myofibrils, which are bundles of actin and myosin filaments
• Skeletal muscle is also called striated muscle because the regular arrangement of myofilaments creates a pattern of light and dark bands
• According to the sliding-filament model, thin and thick filaments ratchet past each other horizontally, powered by the myosin molecules
• The regulatory proteins tropomyosin and the troponin complex bind to actin strands on thin filaments, preventing actin and myosin from interacting
• For a muscle fiber to contract, myosin-binding sites must be exposed, which occurs when calcium ions bind to the troponin complex and expose the myosin-binding sites
• The stimulus leading to contraction of a muscle fiber is an action potential in a motor neuron that synapses with the muscle fiber
• Action potentials travel to the interior of the muscle fiber along transverse (T) tubules, causing the sarcoplasmic reticulum to release Ca2+ and initiate muscle fiber contraction
• Amyotrophic lateral sclerosis (ALS) interferes with the excitation of skeletal muscle fibers and is usually fatal
• When motor neuron input stops, the muscle cell relaxes, and transport proteins in the sarcoplasmic reticulum pump Ca2+ out of the cytosol
• Myasthenia gravis is an autoimmune disease that attacks acetylcholine receptors on muscle fibers; treatments exist for this disease
• Nervous control of muscle tension involves graded contractions of a whole muscle that can be voluntarily altered by varying the number of fibers that contract or the rate at which fibers are stimulated

  
• A motor unit consists of a single motor neuron and all the muscle fibers it controls
• Recruitment is the process by which more and more motor neurons are activated, leading to an increase in the force developed by a muscle
• Twitches are discrete, all-or-none contractions resulting from single action potentials in motor neurons; summation of twitches occurs when action potentials are close together in time, leading to increased tension
• Tetanus is a sustained contraction produced when the rate of stimulation is so high that muscle fibers cannot relax between stimuli; the duration of tetanic contraction depends on the ATP supply needed to break myosin-actin bonds and "re-cock" myosin heads
• Muscles have three systems for obtaining ATP: immediate system using preformed ATP and creatine-phosphate, glycolytic system metabolizing carbohydrates to pyruvate and lactic acid, and oxidative system metabolizing carbohydrates and fatty acids to H2O and CO2
• Types of skeletal muscle fibers include oxidative fibers relying mostly on aerobic respiration, glycolytic fibers using glycolysis as their primary ATP source, and fast-twitch and slow-twitch fibers enabling different types of muscle contractions

  
• Fast-twitch fibers can be either glycolytic or oxidative, enabling brief, rapid, powerful contractions, while slow-twitch fibers contract more slowly but sustain longer contractions
• Cardiac muscle, found only in the heart, consists of striated cells electrically connected by intercalated disks and can generate action potentials without neural input
• Smooth muscle, found mainly in the walls of hollow organs, has relatively slow contractions and lacks striations due to the irregular arrangement of actin and myosin; its contraction is regulated by calcium ions entering the cytosol through the plasma membrane
• The circulatory system's purpose is to transport nutrients, respiratory gases, hormones, metabolic products, and wastes throughout the body, as well as for temperature control

  
• The immune system protects an organism from infection by identifying and killing pathogens
• The circulatory system can be open or closed, with structural elements including a pump (heart), conduits (vessels), and a transport medium (blood or hemolymph)
• Pathogens constantly evolve new ways to avoid detection by the immune system, requiring it to be adaptive and absolutely specific to not attack normal cells and tissues
• Double circulation in vertebrates maintains higher blood pressure in the organs than single circulation
• Vertebrate circulatory systems vary: fish have a single circuit, while amphibians have a double circulation system
• The human heart has 4 chambers (2 atria and 2 ventricles) and functions as two pumps, with valves preventing backflow
• All animals have innate immunity, a defense active immediately upon infection, which includes barrier defenses and vertebrates also have adaptive immunity
• Innate immunity includes barrier defenses like skin, mucous membranes, secretions, phagocytic cells, natural killer cells, antimicrobial proteins, and the inflammatory response
• The cardiac cycle has two phases: "diastole" (relaxation phase) and "systole" (contraction phase), with heart sounds made by heart valves opening and closing
• The adaptive immune response in vertebrates is activated after the innate response and involves recognition of traits specific to particular pathogens using a vast array of receptors
• Effective pumping of the heart requires sequential contraction of chambers and coordinated contraction of muscle cells within the chamber, depending on special features of cardiac muscle
• The adaptive immune response relies on two types of lymphocytes, T cells and B cells, with T cells maturing in the thymus and B cells in bone marrow
• Cardiac muscle cells are connected by gap junctions for coordinated contractions, allowing rapid spread of action potential