Muscle Flashcards
Properties of skeletal muscle
Long, cylindrical, multi-nucleated cells composed of a repeating alignment of myofilaments that creates a striated appearance; nuclei are located on the periphery of skeletal muscle cells, except for in the case of damaged cells
Skeletal muscle cells can be fast twitch or slow twitch, largely dependent on the density of mitochondria as well as the myosin ATPase gene expressed
Properties of cardiac muscle cells
Single nucleated cells (myocytes) separated by intercalated discs, which physically link contracting cells together and form gap junctions for rapid electrical communication & synchronous contraction
Properties of smooth muscle cells
Single nucleated, spindle-shaped cells linked by gap junctions
Sarcomere
Basic unit of muscle contraction; defined as extending from one Z line to the next Z line;
~10,000 sarcomeres can make up 1 skeletal muscle cell
Thin Filaments
Comprised of two filamentous actin strands wound in a helical arrangement
Thick Filament
Bundles of 300-400 myosin pairs; myosin is comprised of a pair of heavy chains and two pairs of light chains; heavy chains form globular heads with alpha helical regions and light chains associate with the globular heads; globular heads have both actin binding and ATP binding sites
Myosin cycle - skeletal muscle
In the relaxed state, Ca2+ concentration is low and myosin binding sites on actin are covered by tropomyosin
Rising intracellular Ca2+ binds to troponin, inducing a conformational change that is transferred to tropomyosin to allow uncover myosin binding sites on actin; myosin binds actin and immediately “pulls” the actin 8nm forward
ATP binding to myosin triggers it’s dissociation from actin; hydrolysis of ATP by the myosin head ATPase powers the conformational change that “cocks the head” of myosin, preparing it for the next binding cycle
Myosin cycle in smooth muscle
Increased intracellular Ca2+ binds Calmodulin and Ca-Calmodulin activates CaM Kinase; CaM kinase phosphorylates one of the myosin light chains, which binds actin to generate force
Ca pumps and Na-Ca exchangers in the sarcolemma remove calcium, leading to inactivation of the kinase and dephosphorylation of myosin by a phosphatase
Function of dystrophin
Dystrophin is part of the protein machinery that anchors actin beneath the plasma membrane to extracellular matrix molecules
Function of Titin, Nebulin, and a-actinin
Titin links myosin thick filaments to the Z line, keeping myosin centered in the sarcomere
Nebulin and a-actinin associate with actin filaments, maintaining a regular arrangement of the sarcomere
Both proteins contribute to the passive tension in a muscle
Mechanism of AP-generated contraction in skeletal muscle
A depolarized nerve axon releases ACh at the neuromuscular junction, which binds AChR in the muscle post-synaptic membrane; AChR is an ion channel that opens, causing depolarization and triggering an AP which propagates in both directions from the endplate toward the tendon, and along the t-tubule network
Depolarization triggers opening of the DHPR channel, a voltage-gated Ca2+ channel in the t-tubule; the DHPR is mechanically coupled to the RyR Ca2+ release channel in the SR, which opens to allow Ca2+ efflux from the SR into the cytoplasm where it initiates actin/myosin interaction
Excitation - Contraction Coupling (Skeletal Muscle)
The mechanism by which depolarization in the t-tubule system is translated into Ca2+ release from the SR
Termination of Ca2+ signaling
Ca-ATPase pumps in the SR membrane transport Ca2+ back into the SR and bring cytoplasmic Ca2+ back to a low level
E-C Coupling (Cardiac Muscle)
In cardiac muscle, Ca2+ entry through the voltage-gated Ca2+ channel in the t-tubule is required to bind the RyR in the SR, triggering it to open and release Ca2+ from the SR into the cytoplasm; this is Calcium Induced Calcium Release (CICR)
Motor Unit
Refers to 1 motor neuron and the collection of muscle fibers that it innervates; one motor neuron may innervate multiple fibers but each muscle fiber is only innervated by one motor neuron
Motor unit size varies, from 2-3 fibers (for very fine movements) to 500 muscle fibers (for large muscles)
Grading muscle tension (skeletal) - 2 major mechanisms
- Increase the frequency of action potentials until a maximal (tetanic) contraction is achieved
- Recruit additional motor units until all motor neurons innervating the muscle are stimulated
Satellite Cells
Stem cells that give rise to new myoblasts to repair injured skeletal muscle; damaged cells produce factors such as leukemia inhibitory factor (LIF) to trigger proliferation of satellite cells
Effect of exercise on muscles
Exercise causes hypertrophy of muscles; the cross-sectional area of each cell is increased as myoblasts are recruited and new myofibrils are formed
Muscle fatigue
Increased organic phosphate and hydrogen ions produced as a result of massive ATP consumption by skeletal muscle leads to decreased Ca2+ release from the SR as well as decreased binding of Ca2+ to troponin, causing fewer myosin-binding sites to be exposed and decreasing the overall efficiency and power of contraction
Hypertrophic Cardiomyopathy - Cardiac phenotype
Characterized by thickening & stiffening of the heart wall, especially in the left ventricle; impairs the heart’s ability to fill and pump effectively.
Phenotypic features include:
Cardiomyocyte and cardiac hypertrophy
Myocyte disarray
Interstittial and replacement fibrosis –> arrhythmia
Dysplastic intramyocardial arterioles –> ischemia
Molecular defect of hypertrophic cardiomyopathy
Most common mutation is a missense point mutation Asp –> Tyr that may occur in different proteins of the sarcomere complex
Cardiomyopathies are often autosomal dominant, demonstrating incomplete penetrance and genetic heterogeneity;
Malignant hyperthermia phenotype
Individuals are healthy throughout life until exposed to anesthesia agents (halothane and/or succinylcholine); this exposure triggers hypermetabolism, hyperthermia, skeletal muscle damage, and death if untreated (70%)
Specific signs: Muscle rigidity (masseter), increased CO2 production, rhabdomyolisis, hyperthermia, acidosis, hyperkalemia, tachycardia
Treatment = dantrolene sodium
Molecular defect of malignant hyperthermia
Autosomal dominant mutation in RYR1 gene; leads to prolonged activation of the RyR protein and extended Ca2+ release in the presence of anesthesia agent trigger molecule (halothane and/or succinylcholine)
Duchenne Muscular Dystrophy phenotype
Primarily affects boys with onset of skeletal muscle disease at 3-5 years old
Phenotypic characteristics include:
Abnormal gait (i.e. toe walking) Gower's sign, Calf pseudohypertropy Wheelchair bound by ~11 years old Death in 20s by progressive involvement of respiratory muscles
Becker Muscular Dystrophy is characterized by milder muscular complaints and later-onset cardiomyopathy
Molecular defect of Duchenne Muscular Dystrophy
Caused by big deletions (often frameshift) in the X-linked DMD gene coding for dystrophin protein; disrupts the association between intracellular actin within muscle cells and the muscle cell membrane
Clinical presentation of hypertrophic cardiomyopathy
Often asymptomatic throughout life BUT may present with dyspnea, angina, syncope, cardiac murmur, or arrhythmia
…also, sometimes sudden cardiac death
Myofibril
An association of thick and thin filaments, arranged into sarcomeres which are aligned in series and surrounded by SR
Source of Ca2+ for smooth muscle
Smooth muscle cells are very thin; Ca2+ entering from channels in the plasma membrane quickly diffuses to the center of the cell; smooth muscle cells do not need t-tubules or SR
Myostatin
Normally made and secreted by muscles as a negative feedback for muscle growth; mutation or selective inhibition leads to increase in muscle mass
