Transcript muscle fibers

Chapter 7
Lecture Slides
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Introduction
• Muscle tissue is specialized for
contraction
• Contraction moves the body and body
parts
• Body has three types of muscle,
differing in structure and function
– Skeletal muscle
– Smooth muscle
– Cardiac muscle
7.1 Structure of Skeletal Muscle
• Skeletal muscles is composed of mainly
muscle fibers
• Muscle fibers extend length of muscle
• Muscle fibers are arranged in bundles
called fascicles
• Dense connective tissue surrounds
each fiber, fascicle, and muscle
– Establishes tendons to attach muscle
to bone
– Establishes aponeuroses to attach
muscle to other muscles and
connective tissues
• Skeletal Muscle Fibers
– Are multinucleated, long, thin cylinders with
rounded ends that extend the length of the
muscle
– Sarcolemma is the plasma membrane
– Sarcoplasm is the cytoplasm
– Contain myofibrils, the contractile elements
• Contain thin actin filaments and thick
myosin filaments
– Each myofibril consists of repeating
contractile units called sarcomeres
• The sacroplasmic reticulum is the name
given to the smooth endoplasmic
reticulum in a muscle cell.
– Stores calcium (Ca2+) ions.
• The transverse (T) tubule system
– Invaginations of the sarcolemma that
penetrate into the fiber so that they lie
alongside and contact the sarcoplasmic
reticulum.
• The sacroplasmic reticulum is the
name given to the smooth endoplasmic
reticulum in a muscle cell.
– Stores calcium (Ca2+) ions.
• The transverse (T) tubule system
– Extensions of the sarcolemma that
penetrate into the fiber.
– Carries impulses that causes the
sarcoplasmic reticulum to release
Ca2+.
• Neuromuscular Interaction
– A motor neuron sends impulses to a
muscle fiber, which produces an
action
– Each muscle fiber is innervated and
controlled by a motor neuron
– Without nervous stimulation, a muscle
fiber cannot contract
• Motor Units
– A motor neuron and all the muscle
fibers it contacts
– Precise control: motor unit with very
few muscle fibers
– No precise control: motor units contain
100s of muscle fibers
– Motor neuron activation causes
contraction of all associated muscle
fibers
• Neuromuscular Junction
– Connection between axon of motor neuron
and sarcolemma of muscle fiber
– Space between axon and sarcolemma is
synaptic cleft
– Axon terminal has vesicles with
neurotransmitter acetylcholine (ACh)
– Nerve impulse reaches the axon terminal and
ACh is released into the synaptic cleft
– ACh attaches to receptors on the sarcolemma
at the motor end plate
– Begins series of reactions leading to
contraction
7.2 Physiology of Muscle
Contraction
• Contraction involves a number of rapid
structural and chemical changes within a
muscle fiber
• Explained by the Sliding Filament Model
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• Mechanism of Contraction
– ACh binds to receptors on the
sarcolemma
– Stimulates the release of Ca2+ from
sarcoplasmic reticulum
– Ca2+ binds to actin filaments,
exposing active sites on actin
filaments (2)
– Myosin cross-bridges attach to actin
active sites (3)
– Using ATP energy, cross-bridges bend
and pull actin filaments towards center of
sarcomere (4)
– Cross-bridges detach and reattach to
active sites (5-6)
– Cycle repeats as long as Ca2+ and ATP
are present
– Acetylcholinesterase from sarcolemma
decomposes Ach in the synaptic cleft
• Prevent continual stimulation of muscle fiber
• Prepares fiber for next stimulus
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operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
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All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
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Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
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• Energy for Contraction
– Energy comes from ATP
– Due to small amount of ATP in cells,
more ATP must be formed to support
contractions
– Two possible pathways for making
energy
• Creatine phosphate
• Cellular respiration
– Creatine
phosphate
• Storage form of
readily available
energy
– Stores energy from
excess ATP
• Energy is
transferred back to
ADP when ATP
levels decrease
• Depleted quickly in
rapidly contracting
muscle
• Oxygen and Cellular Respiration
– Cellular respiration is a 2 step
process
• Anaerobic phase
• Aerobic phase
– Requires oxygen to operate and produce
ATP
– Most ATP is produced during this phase
– Oxygen for aerobic phase comes
from
• Hemoglobin in red blood cells
• Myoglobin in muscle cells
– Glucose is the main energy source
– Sufficient oxygen levels allow for
aerobic respiration to occur
• Pyruvic acid from anaerobic phase to CO2
and H2O
– Insufficient oxygen with strenuous
exercise prevents aerobic respiration
• Pyruvic acid from anaerobic phase
converts to lactic acid
– Causes discomfort and rapid, deep breathing
– To remove lactic acid, it must be
• Broken down by aerobic respiration
• Converted back into glucose
• Both require oxygen to occur
– Oxygen debt: amount of oxygen
required to metabolize lactic acid
• Also restore normal ATP and creatine
phosphate levels
• Deep breathing occurs until debt is paid
– Endurance training increases efficiency
of aerobic cellular respiration by
increasing
• Number of mitochondria
• Efficiency of obtaining oxygen from blood
• Concentration of myoglobin
• Fatigue: the reduced ability to do
work
– Occurs with continued nervous stimulation
– Gradual decrease in contraction ending
with an inability to contract with stimulation
– Causes
• Accumulated lactic acid and carbon
dioxide
• Depletion of ATP
• Heat Production
– Heat from muscle contraction is used
to maintain normal body temperature
• Decrease in body temperature results in
shivering
– Heat production in muscle is caused
by
• Cellular respiration
• Other chemical reactions within the cell
Types of Skeletal Muscle Fibers
• Muscle fibers contract at different speeds, and
vary in how quickly they fatigue
• Muscle fibers are classified into three main types
– Slow oxidative fibers (Type I)
– Fast oxidative-glycolytic fibers (Type IIA)
– Fast glycolytic fibers (Type IIB)
Slow Oxidative Fibers (SO fibers)
– Least powerful type of muscle fibers
– Appear dark red (more myoglobin)
– Generate ATP mainly by aerobic cellular
respiration
• Rich blood supply and many mitochondria
– Have a slow speed of contraction
– Very resistant to fatigue
– Capable of prolonged, sustained contractions
for many hours
– Adapted for maintaining posture and for
aerobic, endurance-type activities such as
running a marathon
Fast Oxidative–Glycolytic Fibers
(FOG fibers)
– Intermediate fibers
– Generate considerable ATP by aerobic
cellular respiration (still has decent blood
supply, large amount of myglobin, and many
mitochondria)
– Resistance to fatigue is intermediate
– Generate some ATP by anaerobic glycolysis
– Speed of contraction faster
– Contribute to activities such as walking
Fast Glycolytic Fibers (FG fibers)
– Generate the most powerful contractions
– Have low myoglobin content
– Relatively few blood capillaries
– Few mitochondria
– Appear white in color
– Generate ATP mainly by glycolysis
– Fibers contract strongly and quickly
– Fatigue quickly
– Adapted for intense anaerobic movements of
short duration like weight lifting or sprinting
Types of Skeletal Muscle Fibers
• Distribution and Recruitment of Different
Types of Fibers
– Most muscles are a mixture of all three types
of muscle fibers
– Proportions vary, depending on the action of
the muscle, the person’s training regimen,
and genetic factors
• Postural muscles of the neck, back, and legs have
a high proportion of SO fibers
• Muscles of the shoulders and arms have a high
proportion of FG fibers
• Leg muscles have large numbers of both SO and
FOG fibers
• Contraction Characteristics
– Contraction of a Single Muscle Fiber
• Threshold stimulus is the minimal
stimulus that will cause a muscle fiber to
contract
• At threshold, contraction of a muscle
fiber always follows the all-or-none
response
– Always contracts completely or not at all
– Contraction is not proportional to stimulus
strength
– Contraction of
Whole Muscles
• Muscle contractions
are recorded in
myograms
• A single contraction
due to a threshold
stimulus has three
intervals
– Latent period
– Period of
contraction
– Period of
relaxation
• Graded Responses
– Varying degrees of contraction in whole
muscles
• Due to presence of different motor units
responding to different thresholds of stimulation
• Recruitment
– increasing the number of motor units to
work
– Maximal stimulus activates all motor units
and produces maximal contraction
• Further increases in stimulus strength does not
produce stronger contractions
• Muscle Tone
– State of partial contraction in relaxed
muscle
– Keeps a muscle ready to respond
– Due to alternating activation of different
motor units
– Loss of nervous innervation results in
loss of muscle tone and atrophy
(decrease in muscle size)
7.3 Actions of Skeletal Muscles
• Origin and insertion
– Insertion is the movable muscle attachment
– Origin is the immovable muscle attachment
– Isotonic contractions cause movement of a
joint
– Isometric contractions increase tension but
do not cause movement
– Effects of exercise on skeletal muscles
• Strength training causes hypertrophy
– Increase in myofibril number in muscle fibers
• Endurance training improves the efficiency
of muscle action but not hypertrophy
– Increases number of mitochondria and blood
vessels
– Increases oxygen, nutrient, and ATP supply
• Muscle Interactions
– Muscles function in groups
– Groups arranged to provide opposing
movements
• Agonists produce an action
• Antagonists produce the opposite action
– Agonists and antagonists contract
alternately
Muscle Disorders
• Cramps
– Involuntary, painful, sustained tetanic
contractions
– Possible causes
• Chemical changes in the muscle
• Physical blow to the muscle
• Strains or “pulled muscles”
– Due to excessive muscle stretching
– Mild strains damage only a few muscle
fibers
– Severe strains tear both connective and
muscle tissues
• Severe impairment of muscle function
• Spasms
– Sudden, involuntary contractions of a
muscle or group of muscles
– Causes
• Irritation of motor neurons
• Emotional stress
• Neurological disorders