11.14.11+-+Muscle


 * I. Skeletal Muscle
 * **A. Describe the organization of myofibers and connective tissue in skeletal muscle. Define fascicle, endomysium, perimysium, and epimysium.**
 * Myofibers are syncytial (multi-nucleate) bundles of myofibrils arranged in parallel and wrapped in endomysium. Multiple muscle fibers are in turn wrapped in perimysium, a more substantial connective tissue bundle. These groupings are called fascicles. Fascicles can move independently of each other. Fascicles are grouped together by a dense sheath of connective tissue called the epimysium.
 * **B. Describe the structure and location of the following components of skeletal muscle fibers: sarcoplasm, sarcolemma, sarcomere, sarcoplasmic reticulum, T (transverse) tubules, terminal cisternae, triads, myofibrils, thick and thin myofilaments, (neuro)motor unit, tropomyosin, troponin, motor end plate, synaptic cleft, subneural cleft, A, I, and H bands, and Z line (disk).**
 * Sarcoplasm: the muscle version of the cytoplasm; the cytosolic fluid within the myofiber.
 * Sarcolemma: the muscle version of the cell membrane
 * Sarcomere: the functional contractile unit of the muscle; bounded laterally by z-bands
 * Sarcoplasmic Reticulum: the muscle version of the endoplasmic reticulum; surrounds the t-tubule as the terminal cisternae to form the triad
 * T (transverse) tubule: invagination of the sarcolemma at the A/I band junction; allows equal dispersion of AP and Ca2+ influx
 * Terminal Cisternae: expansion of the sarcoplasmic reticulum around t-tubules
 * Triads: Terminal Cisternae - T-Tubule - Terminal Cisternae
 * Myofibrils: a series of sarcomeres; bundled together to form myofiber
 * Thick and thin myofilaments: made of myosin and actin respectively; interact during muscle contraction
 * (Neuro)motor unit: the set of muscle fibers innervated by a single nerve; can be 1 fiber (iris) or >100 (limbs)
 * Tropomyosin: the protein covering the actin-myosin binding site at rest
 * Troponin: the lever arm that retracts tropomyosin upon Ca2+ binding to its binding site
 * Motor end plate: the location of synapse between myofiber motor neuron axon; splits the basal lamina of the myofiber
 * Synaptic cleft: the trough in which lies the motor end plate
 * Subneural cleft: the fold of the sarcolemma in which lies the motor neuron axon
 * A Zone: the region of the thick fibers (dArk)
 * I Zone: the region of the thin fibers (lIght)
 * H Band: the region of ONLY thick fibers
 * Z Line: a protein disc to which attaches the plus ends of the thin (actin) filaments
 * **C. Describe the role of each component in the preceding list as you track the events that occur in skeletal muscle from the arrival of a nerve impulse at the neuromuscular junction to the contraction and subsequent relaxation of the muscle via the sliding filament mechanism.**
 * Already doneish
 * **D. Compare and contrast the morphological, biochemical and functional characteristics of slow twitch (red), intermediate, and fast twitch (white) muscle fibers.**
 * It's all pretty intuitive. Fast twitch responds the most quickly, has the fewest mitochondria and myoglobin molecules, has the largest fiber diameter, and tires quickly. The only thing to note is that the fast twitch fibers are also the most difficult to stimulate; this makes sense to me because they are working on limited resources and can't go all day, so you want to make sure you need them when you do contract them.
 * **E. Briefly explain how muscle tension is regulated by recruitment of motor units and frequency of firing (summation).**
 * Motor units can fire individually. Motor units recruit other units based on power required to flex. The entire muscle does not fire each time unless it is necessary.
 * **F. Explain the role of afferent nerves in the regulation of muscle tension. Describe the structure and function of Golgi Tendon organs and neuromuscular spindles.**
 * There are two organs that send information from the muscles to the CNS. They are the golgi tendon organs and the muscle spindles. The golgi tendon bodies are placed within the tendon-muscle junction and they signal changes in tension (tendon-->tension). The muscles spindles are modified muscle fibers that measure amount and rate of stretch.
 * **G. Describe the mechanism and limitations of skeletal muscle regeneration by satellite cells.**
 * Satellite cells are able to regenerate skeletal muscle (produce more myofibrils) if the basal lamina is still intact, but if the basal lamina is disrupted, fibroblasts invade and repair the injury with scar tissue.
 * II. Cardiac Muscle
 * **A. Compare ventricular cardiac muscle and skeletal muscle with respect to:**
 * **1. The cellular structure of myofibers:** cardiac muscle fibers have one boxy nucleus that is centrally placed within the fiber; mitochondria are ~40% of fiber volume in cardiac myofibers, but only 2% in skeletal myofibers; cardiac myofibers branch and form 3d network, while skeletal muscles are functionally 1d.
 * **2. Associated connective tissue:** no endo/peri/epimysium, but there is strong attachment between cardiac myofibers.
 * **3. Subcellular structures including sarcoplasmic reticulum, mitochondria, T-tubules, nuclei, and myofibrils.**
 * Sarcoplasmic reticulum: far less developed with small terminal cisternae that generally don't encircle t-tubule to form triad; instead one of them forms diad.
 * Mitochondria: lots! 40% of volume.
 * T-tubules: At the Z-line (as opposed to skeletal muscle at A/I junction)
 * Nuclei: boxy, central, 1/myofiber
 * Myofibrils: branchy, 3D, striated
 * **4. Regeneration after injury:** cannot do it.
 * **5. Energy source(s):** very little glycogen present, instead reliant upon fatty acids.
 * **B. Illustrate the structural and communicative components of intercalated disks:** Move laterally at Z-lines and longitudinally between myofibrils. Lateral portions (participating z-lines) are sites of desmosomes as well as the usual fascia adherents (actin filament binding). The longitudinal portions of the intercalated discs are sits of many gap junctions which function to disperse contraction AP three dimensionally.
 * III. Smooth Muscle
 * **A. Compare smooth muscle with skeletal and cardiac muscle in terms of tissue morphology and organization of myofilaments.**
 * Myofilaments are massed together in a matrix rather than striated. The cell shape is a fusiform uninucleate cell, as opposed to the striated cells of the cardiac and skeletal myofibers. The dense body in the center of the cell serves as the anchor for the thin filaments (like the z-line) during contraction. THe actin and myosin form a network dispersed throughout the cytoplasm.
 * **B. Define single-unit and multiunit smooth muscle and explain their regulation of contraction by nerves, hormones, gap junctions, and physical stretching.**
 * Nerves and Gap Junctions: Single-unit smooth muscle is present in the viscera and in smaller blood vessels. It is less responsive to innervation because it relies on gap junctions to spread the AP. In contrast, multiunit fibers (present in the iris, larger arteries, and the vas deferens) are more responsive to innervation because each cell has a nerve terminal.
 * Hormones: Smooth muscles respond to both epinephrine and oxytocin. Epinephrine causes the smooth muscle of the digestive tract to relax and the smooth muscle of arterial walls to contract. Oxytocin causes the smooth muscle of the uterus to contract.
 * Physical Stretching: Stretch initiates waves of contraction in GI tract and urinary bladder.
 * **C. Explain how smooth and skeletal muscles differ in regulation, speed, duration, degree, and energy expense of muscle contraction.**
 * Regulation: Smooth is involuntary: Skeletal voluntary
 * Speed: Smooth slower than Skeletal
 * Duration: Smooth can maintain contraction at 1/5th to 1/10th the energy of cardiac muscle
 * Degree: Smooth muscle cells can contract more proportionally than skeletal myofibers
 * **D. Identify the source of elastin and other extracellular matrix components in smooth muscle bundles and sheets.**
 * Smooth muscle cells secrete their own ECM! It's amazing! Collagen (III)! Reticular fibers! Elastin! Proteoglycans!
 * IV. Identify the following microanatomical features in photomicrographs of histological sections:
 * 1) skeletal muscle
 * 2) smooth muscle
 * 3) cardiac muscle
 * 4) endomysium
 * 5) perimysium
 * 6) intercalated disk
 * 7) A band
 * 8) I band
 * 9) Z line (disk)
 * 10) sarcomere
 * 1) sarcomere