Copyright © 2010 Pearson Education, Inc. Table 9.3.

Copyright © 2010 Pearson Education, Inc.

Special Characteristics of Muscle Tissue

• Excitability (responsiveness or irritability): ability to receive and respond to stimuli

• Contractility: ability to shorten when stimulated

• Extensibility: ability to be stretched

• Elasticity: ability to recoil to resting length


Muscle Functions

1. Movement of bones or fluids (e.g., blood)

2. Maintaining posture and body position

3. Stabilizing joints

4. Heat generation (especially skeletal muscle)


Skeletal Muscle

• Each muscle is served by one artery, one nerve, and one or more veins

Copyright © 2010 Pearson Education, Inc. Figure 9.1

Bone

Perimysium

Endomysium(between individualmuscle fibers)

Muscle fiber

Fascicle(wrapped by perimysium)

Epimysium

Tendon

Epimysium

Muscle fiberin middle ofa fascicle

Blood vessel

Perimysium

Endomysium

Fascicle(a)

(b)


Microscopic Anatomy of a Skeletal Muscle Fiber

• Cylindrical cell 10 to 100 m in diameter, up to 30 cm long

• Multiple nuclei

• Many mitochondria

• Glycogen storage, myoglobin for O2 storage


NucleusLight I bandDark A band

Sarcolemma

Mitochondrion

(b) Diagram of part of a muscle fiber showing the myofibrils. Onemyofibril is extended afrom the cut end of the fiber.

Myofibril

Copyright © 2010 Pearson Education, Inc. Figure 9.2c, d

I band I bandA bandSarcomere

H zoneThin (actin)filament

Thick (myosin)filament

Z disc Z disc

M line

(c) Small part of one myofibril enlarged to show the myofilamentsresponsible for the banding pattern. Each sarcomere extends fromone Z disc to the next.

Z disc Z discM line

Sarcomere

Thin (actin)filament

Thick(myosin)filament

Elastic (titin)filaments

(d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments.


Flexible hinge region

Tail

Tropomyosin Troponin Actin

Myosin head

ATP-bindingsite

Heads Active sitesfor myosinattachment

Actinsubunits

Actin-binding sites

Thick filamentEach thick filament consists of manymyosin molecules whose heads protrude at opposite ends of the filament.

Thin filamentA thin filament consists of two strandsof actin subunits twisted into a helix plus two types of regulatory proteins(troponin and tropomyosin).

Thin filamentThick filament

In the center of the sarcomere, the thickfilaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap.

Longitudinal section of filamentswithin one sarcomere of a myofibril

Portion of a thick filamentPortion of a thin filament

Myosin molecule Actin subunits


Myofibril

Myofibrils

Triad:

Tubules ofthe SR

Sarcolemma

Sarcolemma

Mitochondria

I band I bandA band

H zone Z discZ disc

Part of a skeletalmuscle fiber (cell)

• T tubule• Terminal

cisternaeof the SR (2)

M line


I

Fully relaxed sarcomere of a muscle fiber

Fully contracted sarcomere of a muscle fiber

IA

Z ZH

I IA

Z Z

1

2


Nucleus

Actionpotential (AP)

Myelinated axonof motor neuron

Axon terminal ofneuromuscular junction

Sarcolemma ofthe muscle fiber

Ca2+Ca2+

Axon terminalof motor neuron

Synaptic vesiclecontaining ACh

MitochondrionSynapticcleft

Fusing synaptic vesicles

1 Action potential arrives ataxon terminal of motor neuron.

2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.

Figure 9.8


Nucleus

Actionpotential (AP)

Myelinated axonof motor neuron

Axon terminal ofneuromuscular junction

Sarcolemma ofthe muscle fiber

Ca2+Ca2+


Synaptic vesiclecontaining AChMitochondrionSynapticcleft

Junctionalfolds ofsarcolemma

Fusing synaptic vesicles

ACh

Sarcoplasm ofmuscle fiber

Postsynaptic membraneion channel opens;ions pass.

Na+ K+

Ach–

Na+

K+

Degraded ACh

Acetyl-cholinesterase

Postsynaptic membraneion channel closed;ions cannot pass.

1 Action potential arrives ataxon terminal of motor neuron.

2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.

3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.

4 Acetylcholine, aneurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.

5 ACh binding opens ionchannels that allow simultaneous passage of Na+ into the musclefiber and K+ out of the muscle fiber.

6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.

Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 1


Muscle fiberTriad

One sarcomere

Synaptic cleft

Setting the stage

Sarcolemma

Action potentialis generated

Terminal cisterna of SR ACh

Ca2+

Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 2

Action potential is propagated alongthe sarcolemma and down the T tubules.

Steps in E-C Coupling:

Troponin Tropomyosinblocking active sites

Myosin

Actin

Active sites exposed and ready for myosin binding

Ca2+

Terminal cisterna of SR

Voltage-sensitivetubule protein

T tubule

Ca2+

releasechannel

Myosincross bridge

Ca2+

Sarcolemma

Calcium ions are released.

Calcium binds to troponin andremoves the blocking action oftropomyosin.

Contraction begins

The aftermath

1

2

3

4


1

Actin

Cross bridge formation.

Cocking of myosin head. The power (working) stroke.

Cross bridge detachment.

Ca2+

Myosincross bridge

Thick filament

Thin filament

ADP

Myosin

Pi

ATPhydrolysis

ATP

ATP

24

3

ADP

Pi

ADPPi


Motor Unit: The Nerve-Muscle Functional Unit

• Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies

Copyright © 2010 Pearson Education, Inc. Figure 9.13a

Spinal cord

Motor neuroncell body

Muscle

Nerve

Motorunit 1

Motorunit 2

Musclefibers

Motorneuronaxon

Axon terminals atneuromuscular junctions

Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle.


Motor Unit

• Small motor units in muscles that control fine movements (fingers, eyes)

• Large motor units in large weight-bearing muscles (thighs, hips)


Motor Unit

• Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle

• Motor units in a muscle usually contract asynchronously; helps prevent fatigue


Motorunit 1Recruited(smallfibers)

Motorunit 2recruited(mediumfibers)

Motorunit 3recruited(largefibers)


Muscle Tone

• Constant, slightly contracted state of all muscles

• Due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in muscles

• Keeps muscles firm, healthy, and ready to respond


Muscle Metabolism: Energy for Contraction

• ATP is the only source used directly for contractile activities

• Available stores of ATP are depleted in 4–6 seconds


Muscle Metabolism: Energy for Contraction

• ATP is regenerated by:

• Direct phosphorylation of ADP by creatine phosphate

• Anaerobic pathway

• Aerobic respiration


Anaerobic Pathway

• At 70% of max contractile activity:

• Bulging muscles compress blood vessels

• Oxygen delivery is impaired

• Build up lactic acid

Copyright © 2010 Pearson Education, Inc. Figure 9.19b

Energy source: glucose

Glycolysis and lactic acid formation

(b) Anaerobic pathway

Oxygen use: NoneProducts: 2 ATP per glucose, lactic acidDuration of energy provision:60 seconds, or slightly more

Glucose (fromglycogen breakdown ordelivered from blood)

Glycolysisin cytosol

Pyruvic acid

Releasedto blood

net gain

2

Lactic acid

O2

O2ATP


Effects of Exercise

Aerobic (endurance) exercise:

• Leads to increased:

• Muscle capillaries

• Number of mitochondria

• Myoglobin synthesis

• Results in greater endurance, strength, and resistance to fatigue

• May convert fast glycolytic fibers into fast oxidative fibers


Effects of Resistance Exercise

• Resistance exercise (typically anaerobic) results in:

• Muscle hypertrophy (due to increase in fiber size)

• Increased mitochondria, myofilaments, glycogen stores, and connective tissue


The Overload Principle

• Forcing a muscle to work hard promotes increased muscle strength and endurance

• Muscles adapt to increased demands

• Muscles must be overloaded to produce further gains


Smooth Muscle

• Found in walls of most hollow organs(except heart)

• Usually in two layers (longitudinal and circular)


Smallintestine

(a) (b) Cross section of theintestine showing thesmooth muscle layers(one circular and theother longitudinal)running at rightangles to each other.

Mucosa

Longitudinal layerof smooth muscle (shows smooth muscle fibers in cross section)

Circular layer ofsmooth muscle (shows longitudinalviews of smooth muscle fibers)


Peristalsis

• Alternating contractions and relaxations of smooth muscle layers that mix and squeeze substances through the lumen of hollow organs

• Longitudinal layer contracts; organ dilates and shortens

• Circular layer contracts; organ constricts and elongates


Contraction of Smooth Muscle

• Very energy efficient (slow ATPases)

• Myofilaments may maintain for prolonged contractions


Special Features of Smooth Muscle Contraction

Hyperplasia:

• Smooth muscle cells can divide and increase their numbers

• Example:

• estrogen effects on uterus at puberty and during pregnancy


Developmental Aspects

•With age, connective tissue increases and muscle fibers decrease

• By age 30, loss of muscle mass (sarcopenia) begins

• Regular exercise reverses sarcopenia

• Atherosclerosis may block distal arteries, leading to intermittent claudication and severe pain in leg muscles

Copyright © 2010 Pearson Education, Inc. Table 9.3.

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