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., O 2, CO 2 ) 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 voltage- or receptor-regulated gate

Transport Times

Na + channel K+K+ Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Membrane potential (mV) Time (ms) ATP Pi ADP ATPase intracellular K + channel Na + -K + exchange pump

Resting Membrane Potential channels and pumps outside inside [Na + ] [K + ] 3 Na + 2 K + Motoneuron resting membrane potential -70 mV Muscle resting membrane potential -90 mV Na-K ATPase membrane channel leakiness

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

depolarization – influx of Na + repolarization – efflux of K + hyperpolarization – overshoot of K + efflux Phases of Action Potential

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 Ca 2+ 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