Download 08_lecture_ppt Muscle Physiology B

Chapter 8
Histology and
Physiology of
Muscles
Skeletal Muscle Fibers
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Muscle Relaxation
• Calcium ions are transported back into
the sarcoplasmic reticulum
• Calcium ions diffuse away from troponin
and tropomyosin moves, preventing
further cross-bridge formation
Muscle Twitch
• contraction of a
muscle as a result of
one or more
muscle fibers
contracting
• lag, contraction, and
relaxation phases
Table 8.2
Motor Unit
Fig. 8.13
Strength of Muscle Contraction
• For a given condition, a muscle fiber or motor
unit contracts with a consistent force in response
to each AP
multiple motor unit summation
• For a whole muscle, stimuli of increasing
strength result in graded contractions of
increased force as more motor units are
recruited
multiple-wave summation
• Stimulus of increasing frequency increase force
of contraction
Strength of Muscle Contraction
• Incomplete tetanus is partial relaxation between
contractions
• Complete tetanus is no relaxation between
contractions
• force of contraction of a whole muscle increases
with increased frequency of stimulation because
of an increasing concentration of Ca2+ around
the myofibrils
Treppe
increase in force of contraction during first few
contractions of a rested muscle
Fig. 8.15
Multiple Motor Unit
Summation
Treppe
Multiple-Wave
Summation
Types of Muscle Contraction
• Isometric contractions cause a change in muscle tension but no
change in muscle length
– Postural muscles
• Isotonic contractions cause a change in muscle length but no
change in muscle tension
– Fingers, upper extremities
• Concentric contractions are isotonic contractions that cause muscles
to shorten
– Flexing elbow
• Eccentric contractions are isotonic contractions that enable muscles
to lengthen
– Extension of elbow
• Muscle tone is the maintenance of a steady tension for long periods
• Asynchronous contractions of motor units produce smooth, steady
muscle contractions
Muscle Length and Tension
• Muscle
contracts with
less than
maximum
force if its
initial length is
shorter or
longer than
optimal
Fig. 8.17
Fatigue
• decreased ability to do work
• Can be caused by
– central nervous system (psychologic fatigue)
– Depletion of ATP in muscles (muscular
fatigue)
• Physiologic contracture (inability of
muscles to contract or relax) and rigor
mortis (stiff muscles after death) result
from inadequate amounts of ATP
Energy Sources
• Creatine phosphate
– ATP is synthesized
when ADP reacts
with creatine
phosphate to form
creatine and ATP
– ATP from this
source provides
energy for a short
time
Fig. 8.18
Energy Sources
• Anaerobic respiration
– ATP synthesized provides
energy for a short time at
beginning of exercise and
during intense exercise
– Produces ATP less
efficiently but more rapidly
than aerobic respiration
– Lactic acid levels
increase because of
anaerobic respiration
Fig. 8.18
Energy Sources
• Aerobic
respiration
– Requires oxygen
– Produces energy
for longer time-muscle
contractions under
resting conditions
or during
endurance exercise
Fig. 8.18
Fig. 8.18
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_energy_sources_for_prolonged_exercise.html
Speed of Contraction
• 3 main types of skeletal muscle fibers
– Slow-twitch oxidative (SO) fibers
– Fast-twitch glycolytic (FG) fibers
– Fast-twitch oxidative glycolytic (FOG) fibers
• SO fibers contract more slowly than FG
and FOG fibers because they have slower
myosin ATPases than FG and FOG fibers
Fatigue Resistance
• SO fibers are fatigue-resistant and rely on
aerobic respiration
– Many mitochondria, a rich blood supply, and
myoglobin
• FG fibers are fatigable
– Rely on anaerobic respiration and have a high
concentration of glycogen
• FOG fibers have fatigue resistance
intermediate between SO and FG fibers
– Rely on aerobic and anaerobic respiration
Functions
• SO fibers maintain posture and are involved with
prolonged exercise
– Long-distance runners have a higher percentage of
SO fibers
• Chicken legs!
• FG fibers produce powerful contractions of short
duration
– Sprinters have a higher percentage of FG fibers
• Chicken wings & breasts!
• FOG fibers support moderate-intensity
endurance exercises
– Aerobic exercise can result in the conversion of FG
fibers to FOG fibers
Tab. 8.3
Muscular Hypertrophy and Atrophy
• Hypertrophy is an increase in size of muscles
– Due to increase in size of muscle fibers resulting from
an increase in number of myofibrils in muscle fibers
• Aerobic exercise
– Increases vascularity of muscle
– Greater hypertrophy of slow-twitch fibers than fast-twitch fibers
• Intense anaerobic exercise
– Greater hypertrophy of fast-twitch fibers than slow-twitch
• Atrophy is a decrease in the size of muscle
– Due to a decrease in size of muscle fibers or loss of
muscle fibers
Effects of Aging on Skeletal Muscle
• By 80 years of age 50% of muscle mass is
gone
– Due to a loss in muscle fibers
– Fast-twitch muscle fibers decrease in number
more rapidly than slow-twitch fibers
• Can be dramatically slowed if people
remain physically
active!!!!!!!!!!!!!!!!!!!!!!!!!!!
SMOOTH &
CARDIAC MUSCLE
Types of Smooth Muscle
• Visceral smooth muscle fibers have many
gap junctions and contract as a single unit
• Multiunit smooth muscle fibers have few
gap junctions and function independently
– Found in walls of hollow visceral organs, such
as stomach, urinary bladder, and respiratory
passages
– Forces food and other substances through
internal body channels
– It is not striated and is involuntary
Regulation of Smooth Muscle
• Contraction is involuntary
– Multiunit smooth muscle contracts when
externally stimulated by nerves, hormones, or
other substances
– Visceral smooth muscle contracts
autorhythmically or when stimulated externally
• Hormones are important in regulating
smooth muscle
Structure of Smooth Muscle Cells
• Spindle-shaped with a single nucleus
• Have actin and myosin myofilaments
– Actin myofilaments are connected to dense bodies
and dense areas
• Not striated
• No T tubule
system and
most have less
SR than skeletal
muscle
• No troponin
Contraction and Relaxation of Smooth Muscle
• Calcium ions enter the cell to initiate
contraction
– Bind to calmodulin
– Activate myosin kinase, which transfers a
phosphate group from ATP to myosin
– When phosphate groups are attached to
myosin, cross-bridges form
• Relaxation results when myosin
phosphatase removes a phosphate group
from the myosin molecule
Fig. 8.19
Functional Properties of Smooth Muscle
• Pacemaker cells are autorhythmic smooth muscle
cells that control contraction of other smooth muscle
cells
• Smooth muscle cells contract more slowly than
skeletal muscle cells
• Smooth muscle tone is ability of smooth muscle to
maintain a steady tension for long periods with little
expenditure of energy
• Smooth muscle in walls of hollow organs maintain a
relatively constant pressure on the contents of organ
despite changes in content volume
• The force of smooth muscle contraction remains
nearly constant despite changes in muscle length
Cardiac Muscle Cells
• Occurs only in the heart
• Is striated like skeletal muscle but is not
voluntary
• Have a single nucleus
• Connected by intercalated disks that allowing
them to function as a single unit
• Capable of autorhythmicity
• Contracts at a fairly steady rate set by the
heart’s pacemaker
• Neural controls allow the heart to respond to
changes in bodily needs
Tab. 8.1
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