skeletal muscle gross muscle plasma membrane neuromuscular junction action potential
Post on 17-Dec-2015
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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)
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
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 voltage- or 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
Na+
channel
K+
Na+
Na+ Na+
Na+
Na+ Na+
Na+
Na+ Na+
Na+
Na+ Na+Na+ Na+Na+
Na+Na+Na+
Na+K+ K+
K+K+
K+ K+K+ K+
K+ K+K+ K+
K+ K+K+
K+K+K+K+
Mem
bra
ne
po
ten
tial
(m
V)
+20
0
-20
-40
-60
-80
Time (ms)
ATP
PiADP
ATPase
intracellular
K+
channelNa+-K+
exchange pump
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
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
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
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