Topic 13: Muscle Cells Flashcards
What are the three types of contractile cells in mammals?
skeletal muscle: bipolar, striated actomyosin arrays
smooth muscle: actomyosin cortical contraction
cardiac muscle: bipolar, striated actomyosin arrays
all rely on actin-myosin based movement for contraction
have different arrangements of actin and myosin and differ in the signals that cause contraction
What controls contraction in cardiac muscle?
involuntary control at pacemaker
What controls contraction in skeletal muscle?
voluntary control at a neuromuscular junction
What controls contraction in smooth muscle?
controlled involuntary
stimulated by both hormonal and neural inputs
What is the tissue structure of skeletal muscle?
muscle consists of bundles of parallel muscle fibers (single cell)
full skeletal muscle is a network of bundled muscle cells
What is the cell structure in skeletal muscle?
muscle cells from by fusion of muscle precursor cells
multinucleate cells each called a muscle fiber
multinucleate cells share a common cytoplasm and are called a syncytium
What is the cytoskeletal arrangement of skeletal muscle?
sarcomere composed of interdigitated thick and thin filaments
referred to as striated muscle
What are the components of the scarcomere?
thick filaments: contain many molecules of Type II myosin, tail-tail dimer allows formation of bipolar filaments
thin filaments: F actin complexes including structural and regulatory factors (troponin and tropomyosin)
structural proteins: e.g. alpha actinin (actin bundling)
myomesin: bundles myosin into thick filaments, stabilizes tail-tail-dimer
titin: tethers actin and myosin to sacromere
How is the activation of skeletal muscle dependent on calcium?
myosin binding sites on actin are normally blocked by tropomyosin therefore in order to have contraction, the tropomyosin must be moved out of the way
troponin C binds calcium and causes a conformation change that is transmitted to the tropomyosin freeing up the myosin binding sites on actin
What is tropomyosin?
wraps around thin filaments and blocks myosin binding
What is troponin?
Ca++ sensing protein
when Ca++ is present, it binds to troponin and causes a conformational change which displaces tropomyosin, allows myosin to bind
What is the regulation of calcium levels in skeletal muscles?
motor neurons initiate contractile cycle at the neuromuscular junction
causes change in intracellular calcium concentration to initiate contraction
a motor neuron synapsed with a muscle is called a neuromuscular junction and releases the neurotransmitter acetylcholine
What are the steps of activation of a skeletal muscle?
- AP opens voltage gated Ca++ channels
- ACh released into synapse
- ACh binding to receptors causes influx of Na+ = depolarization of muscle cell
- AP transmitted down T-tubule into muscle
- AP causes release of Ca++ from sarcoplasmic reticulum (SER)
What is the sliding filament model?
contraction is due to thin filaments sliding past thick filaments, with no change in the length of either type of filament
thin filaments slide inwards, bring Z disk closer together
contraction = shortening of sarcomere
What is the cell structure of smooth muscle?
non-striated
unicellular
What is the cytoskeletal arrangement of smooth muscle?
actin arranged in a crisscross pattern across the cell anchored in dense bodies
actin and myosin form a mesh on intracellular face = cortex
uses non-muscle myosin (no true filaments)
contraction of myosin causes shortening of entire cell
What is the process of activation of smooth muscle?
neural or hormonal impulses cause opening of Ca channels and an influx of calcium into the cell
calmodulin: Ca++ sensory molecule that is activated when bound to Ca++
active calmodulin binds MLCK (myosin light chain kinase)
MLCK phosphorylate rMLC
phosphorylation of rMLC causes activation of myosin (non-muscle myosin Type II)
What is the contractile cycle of smooth muscle?
actin and myosin bundles contract and pull on dense bodies producing a shortening of the smooth muscle cell
dense body: intersection between MF and IF cytoskeletons
What is the cell structure of cardiac muscle?
striated (contain sarcomeres)
unicellular: connected by intercalated disks
intercalated disks are high in desmosomes and gap junctions
impulse for contraction comes from a set of auto-rhythmic cells = pacemaker
Ca++ is the contractile signal
What is the cytoskeletal arrangement of cardiac muscle?
sarcomeres
actin and myosin
cardiac myosin
What causes the activation of cardiac muscle?
activated by the cardiac pacemaker
spread by gap junctions
longer depolarization allows chamber emptying
opening of voltage gated Ca++ channels prolongs depolarization
Ca++ spreads between cells via gap junctions
