1 Somatic nervous system • Signals from CNS are sent to skeletal muscles. Final result is a muscle contraction. • Motor neuron starts in CNS and its axon ends at a muscle cell. Alpha motor neuron Alpha motor neurons branch into several terminals (can be over 1000), each contacting a separate muscle cell. Axon degenerates downstream from injury Repair can happen in PNS, not often in CNS due to astrocyte scarring 1-4 mm/day Nerve regeneration Nerve meets muscle Axon of motor neuron acetylcholine Motor end plate Voltage-gated calcium channels Action potential of motor neuron Acetycholinesterase Voltage-gated Na + channel Act. pot’l propagation in muscle cell Voltage-gated Na + channel
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Somatic nervous system
• Signals from CNS are sent to skeletal muscles. Final result is a muscle contraction.
• Motor neuron starts in CNS and its axon ends at a muscle cell.
Alphamotorneuron
Alpha motor neurons branch into several terminals (can be over 1000), each contacting a separate muscle cell.
Axon degenerates downstream from injury
Repair can happen in PNS, not often in CNS due to astrocyte scarring
1-4 mm/day
Nerve regeneration Nerve meets muscleAxon of
motor neuron
acetylcholine
Motor end plate
Voltage-gated
calcium channels
Action potential
of motor neuron
Acetycholinesterase
Voltage-gated
Na+ channel
Act. pot’l
propagation
in muscle cell
Voltage-gated
Na+ channel
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Organization of cells
Sarcomere
Myofibril
Muscle cell
Z band Z band Z band
sarcomere sarcomere
A band
Myofibril
Sarcomere – the unit of contraction, made of thin (actin) and thick (myosin) filaments
Contraction of filaments
Myosin Actin
Before Contraction After Contraction
Z Z ZZ
Length of sarcomere shortens with contractionbut filament length is unchanged
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Myosin Actin
troponin tropomyosin
actin
myosin
myosin
myosin
binding site
blocked
actin
• Tropomyosin normally covers the myosin binding site on actin
• When calcium binds with troponin, it pulls tropomyosin away from the binding sites
cross-sectional view
Calcium
Myosin Actin
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Muscle Contraction
• Signal from motor neuron causes action
potential in muscle cell
• Calcium ions released (from sarcoplasmic
reticulum)
• Actin and myosin filaments slide relative to
each other
Myosin cross bridge
BINDING Myosin cross bridge binds to actin
POWER STROKE Crossbridge bends, pulling thinfilament.
DETACHMENT Cross bridgedetaches and returns to original shape- *ATP required*
BINDING to next actin molecule;repeat
Rigor complex
Bending (power stroke)
Detachment
Energized Resting
Binding
Myosin has a binding site for ATPase
Myosin needs ATP to
change shape
motorneuron T tubule
Signal coming to muscle
Sarcoplasmic reticulum(Ca+2 storage)
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From action potential to contractionCalcium is the link
Acetylcholine released at the neuromuscular junction - action potential on muscle fiber
Action potential down “T tubule” to sarcoplasmic reticulum at muscle fibers
Calcium released from the SR to muscle fibers
Terminal button
Acetylcholine-gated cationchannel (Na+ in)
Acetylcholine
T tubule
Cross-bridge binding
Myosin powerstroke
Pathway reviewAction potential
Myosin detachment using ATP
A Ca+ pumpin SR takes up Ca+, allows muscle to relax
Muscles contain groups of motor units
Motor unit = motor neuron +muscle fibers it innervates
Units are recruited during motor activityMuscle force depends on # muscle fibers contracting
The number of muscle fibers varies among different motor units.
–muscles can have many small units or a few large units
–Asynchronous recruitment of motor units delays or prevents muscle fatigue.
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Tension and frequency of stimulation
tetanus - twitch summationfrom sustained Ca+2
twitch - brief contractionresulting from 1 action pot’l
Muscle length and force
Differences in time when maximum tension is reached
Fast and slow twitch muscle cells
Slow twitch (Type I) - have myoglobin, many mitochondria, oxidativeFast twitch (Type IIa) - myoglobin, mitochondria, oxidative & glycol.“Very” Fast twitch (Type IIb) - use glycolysis, split ATP quickly
Fast and slow twitch muscle cells
Oxidative - resistant to fatigue, high rate of O2 transfer from blood, recruited 1st
Glycolytic - more prone to fatigue b/c less ATP produced, harder to recruit
Endurance vs. Bursts of power
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People are born with certain ratio of slow vs. fast twitch fibers
usually an even mix in most skeletal muscles
Spindle muscle fibers (deep within muscle) sense stretch, and Golgi tendon organs (in tendons) sense tension.
Sensation at muscle
Intrafusal (spindle) muscle fibers
Golgi tendon organ
Knee spinal reflex
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Isometric contraction muscle tension is not enough to move load. Muscle doesn’t shorten.
Isotonic contraction
Concentric – muscle shortens to lift a load.
Eccentric - shortened muscle has controlled lengthening.
Primary types of contraction
slowly lowering the weight
Exercising your muscles
Endurance training
type IIx type IIa
more mitochondria, glycogen, vascularization
Exercising your muscles
Strength training hypertrophy of type II fibers
Hypertrophy: how muscles get biggerMuscle cells have satellite cells nearby that respond to muscle injury and wear
Why are muscles sore after lifting?
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Satellite cells:
– activated at microtears
– add nucleus to muscle cell
– more myofibrils made
– cell wider
Hypertrophy: how muscles get bigger
Muscle hypertrophy vs. hyperplasia
Hypertrophy Hyperplasia
Overall, your CNS alters recruitment to increase efficiency of movements you train
Better inhibition of antagonistic muscles
Improved pattern of recruitment of muscles over a movement to gain power
Are there changes in muscle recruitment w/different exercise?
Movements generally recruit ‘small’ motor units first (slow twitch)
Thus to weight train, sufficient wt. needed to recruit more units, and thus engage type II units
Endurance runners/bikers should have periods of sprinting, to engage type II
Are there changes in muscle recruitment w/different exercise?
Tend to be fast twitch units
Tend to be slow twitch units
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ATP sources at muscles
1
3b
23a
What does creatine do?
When muscle fibers contract without our control it is a muscle spasm or cramp
Due to motor neurons being hyperexcited, often b/c of a shift in body fluids or ion levels (dehydration, low Ca, Mg, K) or vigorous activity
What are muscle cramps?
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Smooth muscle contraction
• Multiunit - similar to skeletal motor units
• Single unit - gap jxnsb/w muscle cells. Many cells contract as a unit. (uterus, intestine, bladder)
Smooth muscle
Pacemaker cell
Spontaneous action potential
Action potential spreadto other cells
Gap junctions
Cardiac musclePacemaker muscle cells - action potential gradually depolarizes, then repolarizesContraction spreads from pacemaker through gap jxns