suzanne d'anna1 neuromuscular junction. suzanne d'anna2 motor unit l one motor neuron l...
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Suzanne D'Anna 1
Neuromuscular Junction
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Motor Unit
one motor neuron all the skeletal muscles it stimulates
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Fine Muscle Control
few muscle fibers stimulated by one motor neuron
single motor neuron may supply very few fibers (eye)
Result:
- finer control of muscle fibers
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Coarse Muscle Control
many muscle fibers stimulated by one motor neuron
single motor neuron may supply many fibers (large muscle)
Result:
- less control of muscle fibers
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Neuromuscular Junction
contact or junction between motor neuron and a skeletal muscle
- thread-like extensions of neuron branch into many axonal terminals
- each branch forms a junction with sarcolemma (one muscle fiber)
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Nerve Endings
individual branches of axon near muscle fiber loose myelin sheath
divide into several bulb-shaped structures (synaptic end bulb)
- bulbs contain neurotransmitter acetylcholine (ACh)
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Nerve Endings
extremely close to muscle but never touch
space is called synaptic cleft
- filled with interstitial fluid
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Motor End Plate
portion of muscle fiber membrane adjacent to synaptic end bulb of motor neuron
contains receptors for acetylcholine
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Polarization
muscle fiber relaxed (resting sarcolemma) outside sarcolemma + charge
(predominant extracellular ion is Na+) inside sarcolemma - charge
(predominant intracellular ion is K+) sarcolemma is relatively impermeable to
both ions
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Depolarization(generation of the action potential)
stimulation of sarcolemma by motor nerve patch of sarcolemma becomes permeable to
sodium ions (sodium gates open) + sodium ions rush into cell inside sarcolemma + charge outside sarcolemma - charge this rush upsets electrical currents causing
action potential
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Propagation of the Action Potential
+ charge inside sarcolemma changes permeability of adjacent patches on sarcolemma
depolarization is repeated
- therefore action potential spreads along entire length of sarcolemma
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Repolarization
events occur in reverse sarcolemma permeability changes Na+ gates close K+ gates open allowing diffusion of K+ ions
out of cell activation of sodium-potassium pump
restores ionic resting state concentrations
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Repolarization (cont.)
occurs in same direction as depolarization
must occur before muscle can be stimulated again
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Sequence of Events of Muscle Stimulation
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Muscle Stimulation
muscle fibers are stimulated by motor neurons
impulse arrives at axon terminal of motor neuron
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Muscle Stimulation (cont.)
impulse depolarizes plasma membrane opening voltage-sensitive calcium channels (Ca+2)
calcium ions diffuse from extracellular fluid into the axon terminal
- triggers release of acetylcholine from synaptic end bulb
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Muscle Stimulation
ACh diffuses across synaptic cleft ACh interacts with receptors in the motor
end plate of the muscle fiber, thus altering its permeability to sodium ions (Na+)
sodium ions diffuse from extracelluar fluid into muscle fiber, producing local depolarization called end-plate potential
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Muscle Stimulation(power stroke)
end-plate potential generates flow of ions or current to bring adjacent sarcolemma to threshold
current spreads in both directions triggering action potentials
action potential initiate wave of contraction by way of transverse tubules
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Muscle Stimulation (power stroke) (cont.)
action potential triggers release of Ca+2 from sarcoplasmic reticulum
Ca+2 ions bind to troponin molecules on the thin filaments
tropomyosin moves, uncovering cross-bridge binding sites on actin
binding of actin and myosin causes ATP to split releasing energy for the power stroke
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Muscle Stimulation (power stroke) (cont.)
rotational movement of a myosin cross-bridge
one power stroke of a cross-bridge results in a small movement of the thin filament
each cross-bridge produces many cycles of movement during a single twitch contraction
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Muscle Stimulation (power stroke) (cont.)
acetylcholine is quickly decomposed by cholinesterase
its decomposition prevents generation of further end-plate potentials
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Sliding Filament Mechanism
during muscle contraction, neither the thick nor the thin filaments decrease in length
the actin (thin) filaments slide like pistons inward among the myosin (thick) filaments
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Sliding Filament Mechanism
(cont.) in the resting state, the ends of the actin
barely overlap the myosin during contraction, these ends overlap
considerably while the two Z membranes approach the ends of the myosin filaments
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Myosin
has globular bridges
Ca+2 ions help cross bridges react with actin
Actin
ADP molecules on surface act as sites for linkages with cross bridges
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Sources of Energy
phosphate system glycogen-lactic acid system aerobic system
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Phosphate System
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Phosphate System
ATP and creatine phosphate together they provide energy for
muscles to contract maximally for approximately 15 seconds
this system is used for short bursts of energy
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Energy Source for Muscle Contraction
immediate source is ATP (adenosine triphosphate)
supplied by mitochondria near myofibrils enzyme ATPase splits a phosphate group
from ATP, forming ADP (adenosine diphosphate) and P (phosphate group)
energy released when P is split from molecule of ATP activates myosin cross-bridges
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Energy Source for Muscle Contraction
very little ATP present in muscle fibers if exercise is to continue for more than a
few seconds, additional ATP must be produced
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primary energy source available to regenerate ATP from ADP and phosphate is creatine phosphate
contains high-energy phosphate bonds cannot directly supply energy to a cell 3-5 times more abundant in muscle fibers
than ATP
Energy Source for Muscle Contraction
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Creatine Phosphate
stores energy released from mitochondria when sufficient ATP is present, creatine
phosphokinase (enzyme) promotes synthesis of creatine phosphate
energy is stored in its phosphate bonds when ATP is being decomposed, energy
from CP is transferred to ADP and then quickly converted back to ATP
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Glycogen-Lactic Acid System
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Glycogen-Lactic Acid System
with continued activity, muscles require energy after the supply of creatine phosphate is depleted
glucose must be catabolized to generate ATP
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Glycogen-Lactic Acid System
glucose passes into contracted muscles via blood (facilitated diffusion)
glucose is also produced by glycolysis (breakdown of glycogen in muscles)
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Glycolysis
series of ten reactions splits glucose into two molecules of
pyruvic acid and two molecules of ATP anerobic process (no oxygen)
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Glycogen-Lactic Acid System
pyruvic acid formed by glycolysis enters mitochondria
- its oxidation produces large quantities of ATP from ADP
some activities do not supply enough O2 to completely break down pyruvic acid
pyruvic acid is then converted to lactic acid
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Glycogen-Lactic Acid System (cont.)
most lactic acid diffuses from skeletal muscles into the blood
heart muscle fibers, kidney cells and liver cells use lactic acid to produce ATP
liver cells can convert lactic acid back to glucose
some lactic acid is accumulated in blood and muscles
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Glycogen-Lactic Acid System (cont.)
can provide energy for about 30-40 seconds of maximal muscle activity, e.g., a 50 meter swimming race
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Aerobic System
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Aerobic System
reactions that require oxygen carried by the blood
oxygen is bonded to molecules of hemoglobin
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Cellular Respiration
when energy is exhausted, muscles become dependant upon cellular respiration of glucose as a source of energy for synthesis of ATP
muscle activity longer than 30 seconds requires an aerobic process
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Aerobic System
conversion of pyruvic acid into CO2, H20, and ATP
yields 36 molecules of ATP from each glucose molecule
provides energy for muscular activity lasting longer than 30 seconds
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Recovery Oxygen Consumption (oxygen debt)
elevated oxygen use after exercise above resting oxygen consumption elevated oxygen necessary to restore
metabolic conditions to resting state
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Recovery Oxygen Consumption
converts lactic acid back into pyruvic acid
reestablishes glycogen stores in muscle and liver cells
resynthesizes creatine phosphate and ATP
replaces O2 removed from myoglobin
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Recovery Oxygen Consumption (cont.)
ATP production for metabolic reactions (increased rate due to increased body temperature)
ATP production for continued elevated activity of cardiac and skeletal muscles
ATP production needed for an increased rate of tissue repair
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Muscle Fatigue(inability of a muscle to contract)
Condition may result from:
- insufficient O2 delivered to muscle cells
- depletion of glycogen stored in muscle cells
- buildup of lactic acid in body fluids
- insufficient acetylcholine