Chapter 12b

Muscles

Summation of Contractions

Figure 12-17a

Summation of Contractions

Figure 12-17b

Summation of Contractions

Figure 12-17c

Summation of Contractions

Figure 12-17d

Motor Units

Figure 12-18

SPINAL CORD

Neuron 1Neuron 2Neuron 3

Motornerve

Muscle fibers

Motor unit 2

Motor unit 1

Motor unit 3

One muscle may havemany motor units ofdifferent fiber types.

KEY

PLAY Interactive Physiology® Animation: Muscular System: Contraction of Motor Units

Mechanics of Body Movement

• Isotonic contractions create force and move load• Concentric action is a shortening action• climbing

• Eccentric action is a lengthening action• Downhill skiing, going down stairs

• Isometric contractions create force without moving a load• Series elastic elements; sarcomeres shorten

while elastic elements stretch resulting in little change in overall length

Isotonic Contraction

Figure 12-19a

Isometric Contraction

Figure 12-19b

Series Elastic Elements in Muscle

Figure 12-20

Contractilecomponents

Elasticcomponents

TricepsmuscleBicepsmuscle

Elasticelement

Sarcomeres

Schematic of the series elastic elements

Muscleat rest

Isotonic contraction:Sarcomeres shortenmore but, because elasticelements are alreadystretched, the entiremuscle must shorten.

Isometric contraction:Muscle has not shortened.Sarcomeres shorten, generatingforce, but elastic elementsstretch, allowing muscle lengthto remain the same.

Mus

cle

leng

th

1 2 3

1 2 3

The Arm is a Lever and Fulcrum System

Figure 12-21a

LeverLoad

Fulcrum

Biceps muscle

(a)

The Arm is a Lever and Fulcrum System

Figure 12-21b

F2 = 2 kg

F1

5 cm

15 cm

(b)

The Arm is a Lever and Fulcrum System

Figure 12-21c

D1

D2

5 cm

25 cm

A 7-kg load is addedto the hand 25 cmfrom the elbow.

(c)

The Arm Amplifies Speed of Movement of the Load

Figure 12-22

Lever

Fulcrum 1 cm

5 cm

Load-Velocity Relationship in Skeletal Muscle

Figure 12-23

Muscle Disorders

• Muscle cramp: sustained painful contraction• Overuse• Disuse/Atrophy• Acquired disorders• Inherited disorders• Duchenne’s muscular dystrophy • Dystrophin

• McArdle’s disease • Myophosphorylase deficiency glycogenosis –

glycogen not converted to glucose 6-phosphate

Duration of Muscle Contraction in the Three Types of Muscle

Figure 12-24

Skeletal

Cardiac Smooth

Time (sec)0 1 2 3 4 5

Tens

ion

Smooth Muscle

• Contraction and relaxation slower• Uses less energy• Maintains force for long periods• Low oxygen consumption

Smooth Muscle

• Smooth muscle is not studied as much as skeletal muscle because• It has more variety• Anatomy makes functional studies difficult• It is controlled by hormones, paracrines, and

neurotransmitters• It has variable electrical properties• Multiple pathways influence contraction and

relaxation

Types of Smooth Muscle

Figure 12-25a

Smallintestine

Autonomic neuronvaricosity

Neuro-transmitter

Receptor

Gapjunctions

Smooth musclecell

(a) Single-unit smooth muscle cells

Smooth Muscle

• Much smaller than skeletal muscle fibers• Has longer actin and myosin filaments• Myosin ATPase activity much slower• Myosin light chain plays regulatory role• Not arranged in sarcomeres• Has less sarcoplasmic reticulum• IP3-receptor channel is the primary calcium

channel• Calcium also enters cell from extracellular fluid

Cardiac Muscle

• Shares features with both skeletal and smooth muscle• Like skeletal: • Striated; sarcomere structure

• Unlike skeletal: • Muscle fibers shorter; may be branched; have

single nucleus• Like smooth: • Electrically linked to one another; some exhibit

pacemaker potentials; under sympathetic and parasympathetic control as well as hormone control

Muscle Summary

Table 12-3

Muscle Summary

• Skeletal muscles• Origin, insertion, flexors, extensors, and

antagonistic muscles• T-tubules, sarcoplasmic reticulum, myofibrils,

thick filament, thin filament, actin, myosin, and crossbridges

• Sarcomere, Z disks, I bands, A band, H zone, and M line

• Muscle tension, load, sliding filament theory, tropomyosin, troponin, Ca2+-ATPase, myosin ATPase, power stroke, rigor state

Summary

• Skeletal muscle• Excitation-contraction coupling, DHP receptors,

and Ca2+ release channels• Twitch, latent period, phosphocreatine, and

muscle fatigue• Muscle fiber types, myoglobin, tetanus, and

motor unit • Mechanics of body movement• Isotonic versus isometric contractions• Series elastic elements, levers, and fulcrums

Summary

• Smooth muscle• Types of smooth muscle, IP3-receptor channel,

calmodulin, myosin light chain kinase, myosin light protein chains, and myosin phosphatase

• Myogenic contraction, slow wave or pacemaker potentials, and pharmacomechanical coupling

• Cardiac muscle• Comparison to other muscle types