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Transcript
Skeletal Muscle Gross muscle Plasma membrane Neuromuscular junction Action potential Muscle Connective Tissue • provides structure & form to muscle • allows force to be transmitted to tendons/bones • three layers of connective tissue-composed primarily of collagen fibers – epimysium (outer layer) – perimysium (groups fibers into bundles (fascicles)) – endomysium (surrounds each fiber) Muscle Connective Tissue Skeletal Muscle Endomysial connective tissue within skeletal muscle Connective Tissue Functions • provides “scaffolding” upon which fibers can form • holds fibers together • perimysium provides conduit for arterioles/venules and intramuscular nerves • distributes strain/force over entire muscle • endomysium conveys part of contractile force to tendon • fibers taper near tendon attachment; folding of plasma membrane Myon Myonuclei of skeletal fiber Sarcolemma • surrounds each fiber and composed of: – basement membrane (outer side) – plasma membrane • basement membrane contains: – acetylcholinesterase – collagen • functions of basement membrane – termination of synaptic transmission – attachment of fiber to endomysium – scaffolding for muscle fiber regeneration Plasma Membrane plasma membrane composed of lipid bilayer – has fluid properties – regulates fiber ion concentrations with membrane protein pumps and channels Plasma Membrane Proteins • myonuclei and satellite cells – bound to inter surface of plasma membrane • peripheral proteins (plasma membrane receptors) – associated with surface of bilayer – e.g., adenylate cyclase, kinases, hormone receptors – integrins • class of connective proteins • link basement membrane to plasma membrane and cytoskeletal structures • integral proteins function as “gatekeepers” – embedded in phospholipid bilayer – selectively let ions pass Methods of transport • • • • osmosis (i.e., water) simple diffusion (e.g., O2, CO2) facilitated diffusion (e.g., glucose, lactate) active transport (e.g., Na+, K+) • several thousand ` amino acids arranged in 1 or more subunits • hundreds of sugar residues linked • controlled by voltageor receptor-regulated gate Transport Times movement of side chain on protein 10-10 s movement of Na+ through a pore 10-8 s fastest enzyme turnover 10-6 s activation of a channel (rate-limiting step) 10-4 s actin-myosin turnover 10-2 s Membrane potential (mV) +20 0 -20 -40 -60 -80 Time (ms) Na+ K+ Na+ Na+ Na+ Na+ K+ Na+ Na+ Na+ Na+ channel K+ K+ K+ intracellular K+ K+ K+ Na+ Na+ Na+ Na+-K+ exchange pump K+ Na+ Na+ K+ K+ Na+ ATPase K+ ADP Pi K+ Na+ Na+ K+ channel K+ K+ Na+ ATP Na+ K+ K+ K+ K+ Na+ K+ Distribution of Na+-K+ pumps in skeletal muscle and muscle-nerve bundles (N). Pumps are lit from exposure to a labeled antibody Action Potential results from disturbance (e.g. electrical) to membrane affects membrane permeability to Na+ and K+ follows “all-or-nothing” principle Phases of Action Potential depolarization – influx of Na+ repolarization – efflux of K+ hyperpolarization – overshoot of K+ efflux Action Potential Motor Unit Motoneuron • inputs to motoneuron are both excitatory and inhibitory • continuous nerve from spinal cord to neuromuscular junction • are all myelinated – – – – wrapped with myelin (Schwann cells) nodes of Ranvier AP conducted by saltatory conduction greatly conduction velocity • extrafusal motor units innervated by motoneurons Motor End Plate Neuromuscular Junction AP at motor end plate (active zones) causes Ca2+ influx stimulates vesicles to migrate/fuse to membrane and release acetylcholine (ACh) ACh diffuses across synapse and binds with postsynaptic ACh receptors most ACh metabolized by cholinesterase postsynaptic ACh binding causes Na+ influx and K+ efflux depolarization causes development of APs Curare blocks ACh receptors Anticholinesterase drugs (e.g., mustard gas, sarin) prevent hydrolysis of ACh Botulism (bacterium) blocks release of ACh