Transcript 5-2 Neuromuscular Junctio

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 crossbridge 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 crossbridge
 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
Actin
has globular
bridges
 Ca+2 ions help
cross bridges
react with actin

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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 crossbridges
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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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Energy Source for Muscle
Contraction
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
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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)
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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
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