книги студ / color atlas of physiology 5th ed[1]. (a. despopoulos et al, thieme 2003)

Active motility (ability to move) is due to

either the interaction of energy-consuming

motor proteins (fueled by ATPase) such as my-

osin, kinesin and dynein with other proteins

such as actin or the polymerization and

depolymerization of actin and tubulin. Cell di-

Work

vision (cytokinesis), cell migration (!p. 30),

intracellular vesicular transport and cytosis

Physical

(!p. 12f.), sperm motility (!p. 306f.), axonal

examples of cell and organelle motility.

transport (!p. 42), electromotility of hair cells

(!p. 366), and ciliary motility (!p. 110) are

Muscle,

The muscles consist of cells (fibers) that

contract when stimulated. Skeletal muscle is

responsible for locomotion, positional change,

and

and the convection of respiratory gases. Car-

diac muscle (!p. 190 ff.) is

responsible for

Nerve

pumping the blood, and

smooth muscle

(!p. 70) serves as the motor of internal organs

2

and blood vessels. The different muscle types

Motility and Muscle Types

Two types of skeletal muscle fibers can be

distinguished: S – slow-twitch fibers (type 1)

and F – fast-twitch fibers (type 2), including

two subtypes, FR (2 A) and FF (2 B). Since each

58

motor unit contains only one type of fiber, this

classification also applies to the motor unit.

Usage .reserved rights All

Atlas Color Despopoulos,

terms to subject

© Physiology of

conditions and

Thieme 2003

.license of

A. Structure and function of heart, skeletal and smooth muscle

Structure and

Smooth muscle

Cardiac muscle (striated)

Skeletal muscle (striated)

function

Motor

end-plates

None

None

Yes

Fibers

Fusiform, short (< 0.2 mm)

Branched

Cylindrical, long (< 15cm)

–

–

Mitochondria

Few

Many

Few (depending on muscle type)

Nucleus per fiber

1

1

Multiple

Sarcomeres

None

Yes, length < 2.6 m

Yes, length <

3.65 m

–

–

Electr. coupling

Some (single-unit type)

Yes (functional syncytium)

No

Sarcoplasmic

Little developed

Moderately developed

Highly developed

reticulum

Ca2+ “switch”

Calmodulin/caldesmon

Troponin

Troponin

Pacemaker

Some spontaneous rhythmic activity

Yes (sinus nodes ca.1s–1)

No (requires nerve stimulus)

(1s–1 –1h–1)

Response to stimulus

Change in tone or rhythm frequency

All or none

Graded

Tetanizable

Yes

No

Yes

Work range

Length-force curve

In rising

At peak of

is variable

length-force curve

(see 2.15E)

length-force curve

(see 2.15E)

+ 20

Absolutely

Relatively

Absolutely

Response

“Spike”

+ 20

refractory

refractory

+ 50

refractory

mV

0

mV

0

to stimulus

Spontaneous

mV 0

– 20

– 20

fluctuation

– 50

– 40

– 60

Potential

– 60

– 100

–100

Muscle

0

200

400

600

0

100

200

300

400

0

10

20

30

tension

ms

ms

ms

Contraction of Striated Muscle

ATP (!p. 72) is essential for filament sliding

and, hence, for muscle contraction. Due to

Stimulation of muscle fibers. The release of

their ATPase

activity,

the myosin

heads

acetylcholine at the motor end-plate of skeletal

(!p. 60) act as the motors (motor proteins) of

muscle leads to an end-plate current that

this process. The myosin-II and actin filaments

spreads electrotonically and activates voltage-

of a sarcomere (!p. 60) are arranged in such a

gated Na+

channels

in the

sarcolemma

way that they can slide past each other. The

(!p. 56). This leads to the firing of action

myosin heads connect with the actin filaments

potentials (AP) that travel at a rate of 2 m/s

at a particular angle, forming so-called cross-

along the sarcolemma of the entire muscle

bridges (!C1). Due to a conformational change

fiber, and penetrate rapidly into the depths of

in the region of the nucleotide binding site of

the fiber along the T system (!A).

myosin-II (!p. 61 C),

the spatial extent of

The conversion of this excitation into a con-

which is increased by concerted movement of

traction is called electromechanical coupling

the neck region, the myosin head tilts down,

(!B). In the skeletal muscle, this process

drawing the thin filament a length of roughly

begins with the action potential exciting volt-

4 nm (!C2). The second myosin head may also

age-sensitive

dihydropyridine

receptors

move an adjacent actin filament. The head

(DHPR) of the sarcolemma in the region of the

then detaches and “tenses” in preparation for

triads. The DHPR are arranged in rows, and

the next “oarstroke” when it binds to actin

directly opposite them in the adjacent mem-

anew (!C3).

brane of the sarcoplasmic reticulum (SR) are

Kinesin, another motor protein (!pp. 42 u.

rows of Ca2+ channels

called ryanodine recep-

58), independently advances on the micro-

tors (type 1 in skeletal muscle: RYR1). Every

tubule by incremental movement of its two

other RYR1 is associated with a DHPR (!B2).

heads (8 nm increments), as in tug-of-war. In

RYR1 open when they directly “sense” by me-

this case, fifty percent of the cycle time is

chanical means an AP-related conformational

“work time” (duty ratio = 0.5). Between two

change in the DHPR. In the myocardium, on the

consecutive interactions with actin in skeletal

other hand, each DHPR is part of a voltage-

muscle, on the other hand, myosin-II “jumps”

gated Ca2+ channel of the sarcolemma that

36 nm (or multiples of 36, e.g. 396 nm or more

opens in response to an action potential. Small

in rapid contractions) to reach the next (or the

quantities of extracellular Ca2+ enter the cell

11th) suitably located actin binding site (!C3,

through this channel, leading to the opening of

jump from a to b). Meanwhile, the other my-

myocardial RYR2 (so-called trigger effect of

osin heads working on this particular actin

Ca2+ or Ca2+ spark; !B3). Ca2+ ions stored in

filament must make at least another 10 to 100

the SR now flow through the opened RYR1 or

oarstrokes of around 4 nm each. The duty ratio

RYR2 into the cytosol, increasing the cytosolic

of a myosin-II head is therefore 0.1 to 0.01. This

Ca2+ concentration [Ca2+]i from a resting value

division of labor by the myosin heads ensures

of ca. 0.01 µmol/L to over 1 µmol/L (!B1). In

that a certain percentage of the heads will al-

skeletal muscle, DHPR stimulation at a single

ways be ready to generate rapid contractions.

site is enough to trigger the coordinated open-

When filament sliding occurs, the Z plates

ing of an entire group of RYR1, thereby increas-

approach each other and the overlap region of

ing the reliability of impulse transmission. The

thick and thin filaments becomes larger, but

increased cytosolic Ca2+ concentration satu-

the length of the filaments remains un-

rates the Ca2+ binding sites on troponin-C,

changed. This

results

in shortening

of the

thereby canceling the troponin-mediated in-

I band and H zone (!p. 60). When the ends of

hibitory effect of tropomyosin on filament

the thick filaments ultimately bump against

sliding (!D). It is still unclear whether this

the Z plate, maximum muscle shortening oc-

type of disinhibition involves actin–myosin

curs, and the ends of the thin filaments overlap

binding or the detachment of ADP and Pi, as

(!p. 67 C). Shortening of the sarcomere there-

described below.

fore occurs at both ends of the myosin bundle,

but in opposite directions.

Actin-myosinII binding

Strong

Weak

Strong

ATP

Pi

ADP

ATP

a

a

b

4nm

36nm or multiple

63

2 Work phase

3 Resting phase (ca. 90% of time; other

(ca.10% of time)

myosin heads are meanwhile active)

DHPR with (heart) and with-

Action

out (muscle) Ca2+channel

potential

RYR

T system

ATP

Myosin

Longitudinal

Tropomyosin

Ca2+

tubule

Actin

Troponin

Resting position

[Ca2+]i

ATP-

Pi

[Ca2+]i

ase

ADP

1 mol/l

mol/l

= 1–10

45° 90°

Ca2+

ATP

1 Actin-myosin binding,

ATP cleavage

50°

90°

4 Loosening of actin–myosin

bond (“softening” effect

of ATP), myosin heads erect

P

i

ATP

2a Myosin heads tilt

due to Pi release

45° 50°

3 ATP binding

With

ATP

Without

ADP

ATP

Stable “rigor complex”

persists

2b

Final head position

(rigor mortis)

after ADP release

Mechanical Features of Skeletal

with the

magnitude of depolarization. The

magnitude of contraction of tonus fibers is

Muscle

regulated by variation of the cytosolic Ca2+ con-

Action potentials generated in muscle fibers

centration (not by action potentials!)

increase the cytosolic Ca2+

concentration

In contrast, the general muscle tone (reflex

[Ca2+]i, thereby triggering a contraction

tone), or the tension of skeletal muscle at rest,

(skeletal

muscle;

!p. 36 B;

myocardium;

is attributable to the arrival of normal action

!p. 194). In skeletal muscles,

gradation of

potentials at the individual motor units. The

contraction force is achieved by variable re-

individual contractions cannot be detected be-

cruitment of motor

units (!p. 58) and by

cause the motor units are alternately (asyn-

changing the action potential frequency. A

chronously) stimulated. When apparently at

single stimulus always leads to maximum Ca2+

rest, muscles such as the postural muscles are

release and, thus, to a maximum single twitch

in this involuntary state of tension. Resting

of skeletal muscle fiber if above threshold (all-

muscle

tone is regulated by reflexes

or-none response). Nonetheless, a single

(!p. 318ff.) and increases as the state of atten-

stimulus does not induce maximum shorten-

tiveness increases.

ing of muscle fiber because it is too brief to

Types of contractions (!B). There are

keep the sliding filaments in motion long

different types of muscle contractions. In

enough for the end position to be reached.

isometric contractions, muscle force (“ten-

Muscle shortening continues only if a second

sion”) varies while the length of the muscle re-

stimulus arrives before the muscle has

mains constant. (In cardiac muscle, this also

completely relaxed after the first stimulus.

represents isovolumetric contraction, because

This type of stimulus repetition leads to in-

the muscle length determines the atrial or

cremental mechanical summation or super-

ventricular volume.) In isotonic contractions,

position of the individual contractions (!A).

the length of the muscle changes while muscle

Should the frequency of stimulation become

force remains constant. (In cardiac muscle, this

so high that the muscle can no longer relax at

also represents isobaric contraction, because

all between stimuli, sustained maximum con-

the muscle force determines the atrial or

traction of the motor units or tetanus will

ventricular pressure.) In auxotonic contrac-

occur (!A). This occurs, for example, at 20 Hz

tions, muscle length and force both vary simul-

in slow-twitch muscles and at 60–100 Hz in

taneously. An isotonic or auxotonic contrac-

fast-twitch muscles

(!p. 58).

The muscle

tion that builds on an isometric one is called an

force during tetanus can be as much as four

afterloaded contraction.

times larger than that of single twitches. The

Muscle extensibility. A resting muscle con-

Ca2+ concentration, which decreases to some

taining ATP can be stretched like a rubber

extent between superpositioned stimuli, re-

band. The force required to start the stretching

mains high in tetanus.

action (!D, E; extension force at rest) is very

Rigor

(!p. 2.13)

as well as contracture,

small, but increases exponentially when the

another state characterized by persistent

muscle is under high elastic strain (see resting

muscle shortening, must be distinguished

tension curve, !D). A muscle’s resistance to

from tetanus. Contracture is not caused by ac-

stretch, which keeps the sliding filaments in

tion potentials, but by persistent local depolari-

the sarcomeres from separating, is influenced

zation due, for example, to increased extra-

to a small extent by the fascia (fibrous tissue).

cellular K+ concentrations (K+ contracture) or

The main factor, however, is the giant filamen-

drug-induced intracellular Ca2+ release, e.g., in

tous elastic molecule called titin (or connectin;

response to caffeine. The contraction of so-

1000 nm long, Mr = 3 to 3.7 MDa) which is in-

called tonus fibers (specific fibers in the exter-

corporated in the sarcomere (6 titin molecules

nal eye

muscles and in muscle spindles;

per myosin filament). In the A band region of

!p. 318) is also a form of contracture. Tonus

the sarcomere (!p. 61 B), titin lies adjacent to

fibers do not respond to stimuli according to

a myosin filament and helps to keep it in the

the all-or-none law, but contract in proportion

center of the sarcomere. Titin molecules in the

Stimulus

Isometric muscleforce

0

2

4

Range of summation

8

10 Time (s)

6

Single contractions

Tetanus

Force

Isometric

Isotonic

Resting

tension

00

curve

Length

Isotonic, then

Auxotonic

After-

isometric

loaded

Rest

Isometric

Rest

Isotonic

contraction

contraction

100

force

80

Skeletal muscle

muscleIsometric maximum)of(%

forcemax.ofRange

60

Cardiac

40

muscle

20

0

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

1.2

Lmax

2.05

2.20

1.90

Actin

Myosin

Sarcomere

m)

3.65

1.50

Sarcomere length (

(skeletal muscle)