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.

Alphamotorneuron

Alpha motor neurons branch into several terminals (can be over 1000), each contacting a separate muscle cell.

Nerve meets muscleAxon of motor neuron

Terminal button

acetylcholine

Motor end plate

Voltage-gatedcalcium channels

Action potentialof motor neuron

Acetycholinesterase

Voltage-gatedNa+ channel

Action potentialpropagationin muscle fiber

Organization of cells

2

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

Myosin

3

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

Figure 8.2 (3)Page 259

Sarcomere

Myosin Actin

MyosinActin

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

4

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

Figure 8.13 Page 267

Rigor complex

Bending (power stroke)

Detachment

Energized Resting

Binding

Myosin has a binding site for ATPase

Myosin needs ATP tochange shape

motorneuron T tubule

Signal coming to muscle

Sarcoplasmic reticulum(Ca+2 storage)

From action potential to contraction

• Calcium 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

5

Terminal button

Acetylcholine-gated cationchannel (Na+moves in)

Acetylcholine

T tubule

TropomyosinTroponin

Cross-bridge binding

Myosin cross bridge

Actin

Pathway review

A calcium pumpin SR allowsmuscle to relax

Action potential

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.

Tension and frequency of stimulation

tetanus - twitch summationfrom sustained Ca+2

twitch - brief contractionresulting from 1 action pot’l

6

Muscle length and forceDifferences 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

• People are born with certain ratio of slow vs. fast twitch fibers– usually an even mix in most skeletal muscles

7

Sensation at muscleSpindle muscle fibers (deep within muscle) sense stretch, and Golgi tendon organs (in tendons) sense tension.

Intrafusal (spindle) muscle fibers

Patellar tendon

Extensor muscle

Musclespindle

motorneuron

Knee spinal reflex Primary types of contraction

• 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.

slowly lowering the weight

8

Exercising your muscles

Endurance training

type IIb 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?

Satellite cells:

– activated at microtears

– add nucleus to muscle cell

– more myofibrils made

– cell wider

Hypertrophy: how muscles get bigger

9

Muscle hypertrophy vs. hyperplasia

Hypertrophy Hyperplasia

• The CNS can become trained to provide more force (apparent in early training)

– Better inhibition of antagonistic muscles

– Improved recruitment of different muscles over a movement to gain power

How can muscle recruitment change with exercise?

ATP sources at muscles

1

3b

23a

What does creatine do?

10

• 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?

• Smooth muscle cells are small and unstriated

–No sarcomeres

–Smooth muscle cells contract when Ca+2

enters

–Myosin cross bridges are phosphorylated and bind to actin

Smooth muscle• Is there a point where being muscular negatively

affects our bodies?

• What is largest muscle in the body and why

• Why do our eyes twitch sometimes, seemingly for no reason.

11

• Multiunit - similar to skeletal motor units

• Single unit - gap jxns b/w muscle cells. Many cells contract as a unit. (uterus, intestine, bladder)

Smooth muscle

Smooth muscle

Striated 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