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
One muscle may have
many motor units of
different fiber types.
SPINAL CORD
Neuron 1
Neuron 2
Neuron 3
Motor
nerve
KEY
Muscle fibers
Motor unit 1
Motor unit 2
Motor unit 3
PLAY
Interactive Physiology® Animation: Muscular System:
Contraction of Motor Units
Figure 12-18
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
Schematic of the series elastic elements
Elastic
components
Biceps
muscle
1 Muscle
at rest
Contractile
components
1
Muscle length
Triceps
muscle
2
3
Elastic
element
Sarcomeres
2 Isometric contraction:
3 Isotonic contraction:
Sarcomeres shorten
Muscle has not shortened.
more but, because elastic
Sarcomeres shorten, generating
elements are already
force, but elastic elements
stretched, the entire
stretch, allowing muscle length
muscle must shorten.
to remain the same.
Figure 12-20
The Arm is a Lever and Fulcrum System
(a)
Biceps muscle
Lever
Load
Fulcrum
Figure 12-21a
The Arm is a Lever and Fulcrum System
(b)
F1
5 cm
15 cm
F2 = 2 kg
Figure 12-21b
The Arm is a Lever and Fulcrum System
(c)
A 7-kg load is added
to the hand 25 cm
from the elbow.
D1
D2
5 cm
25 cm
Figure 12-21c
The Arm Amplifies Speed of Movement of the Load
Lever
5 cm
Fulcrum
1 cm
Figure 12-22
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
Skeletal
Smooth
Tension
Cardiac
0
1
2
3
Time (sec)
4
5
Figure 12-24
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
Autonomic neuron
varicosity
Small
intestine
Gap
junctions
Neurotransmitter
Receptor
Smooth muscle
cell
(a) Single-unit smooth muscle cells
Figure 12-25a
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