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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