Download Chapter 7 Body Systems

Chapter 11
Physiology of the Muscular
System
Introduction
• Muscular system is responsible for moving
the framework of the body
• In addition to movement, muscle tissue
performs various other functions
General Functions
• Movement of the body as a whole or of
its parts
• Heat production
• Posture
Function of Skeletal Muscle
Tissue
• Characteristics of skeletal muscle cells
– Excitability (irritability)— ability to be
stimulated
– Contractility— ability to contract, or shorten,
and produce body movement
– Extensibility— ability to extend, or stretch,
allowing muscles to return to their resting
length
Overview of the muscle cell
• Muscle cells are called fibers because of
their threadlike shape
– Sarcolemma—plasma membrane of muscle
fibers
– Sarcoplasmic reticulum (SR)
• Network of tubules and sacs found within muscle
fibers
• Membrane of the sarcoplasmic reticulum continually
pumps calcium ions from the sarcoplasm and stores
the ions within its sacs for later release
Overview of the muscle cell
– Muscle fibers contain many mitochondria
and several nuclei
– Myofibrils— numerous fine fibers packed
close together in sarcoplasm
– Sarcomere
• Segment of myofibril between two
successive Z lines
• Each myofibril consists of many
sarcomeres
• Contractile unit of muscle fibers
Overview of the muscle cell
• Striated muscle
• Dark stripes called A bands
– light H zone runs across midsection of each
dark A band
• Light stripes called I bands
– dark Z line extends across center of each light I
band
Overview of the muscle cell
Tranverse tubules (T Tubules)
• T tubules extend across
sarcoplasm at right angles to long
axis of muscle fiber
• Formed by inward folds of
sarcolemma
• Membrane has ion pumps that
continually transport Ca++ ions
inward from sarcoplasm
• Allow electrical impulses
traveling along sarcolemma to
move deeper into cell
Overview of the muscle cell
Triad
• Triplet of tubes
• a T tubule sandwiched between
two sacs of SR
• allows an electrical impulse
traveling along a T tubule to
stimulate the adjacent SR
An electron micrograph of
a section through a triad
junction of a frog tonic
fibre, showing a central ttubular element flanked
on either side by the
sarcoplasmic reticulum.
Myofilaments
• Myofilaments
– Each myofibril contains thousands of thick
and thin myofilaments
Myofilaments
Four different proteins make up myofilaments
Thin Filament
Thick Filament
Myofilaments
• Myosin
– Makes up almost all the thick filament
– Myosin “heads” are attracted to actin
– Myosin “heads” attach to actin and form cross
bridges
Thick Filament
Myofilaments
• Actin—protein that forms two strands that twist
around each other to form the thin filament
– Myosin attaches to actin on the active sites
– Contains tropomyosin and troponin
• Tropomyosin—protein that blocks the active sites
on actin
• Troponin—protein that holds tropomyosin in place
Myofilaments
– Thin filaments attach to both Z lines (Z disks) of a
sarcomere and extend partway toward the center
– Thick myosin filaments do not attach to the Z lines
Mechanism of contraction
• Excitation and contraction
• A skeletal muscle fiber remains at rest until stimulated by
a motor neuron
• Neuromuscular junction (NMJ)— motor neurons connect
to sarcolemma
• Neuromuscular junction is a synapse where
neurotransmitter
transmit signals
Mechanism of contraction
• Acetylcholine (ACh)— neurotransmitter released into synaptic cleft that
diffuses across gap, stimulates receptors, and initiates impulse in
sarcolemma
• Nerve impulse travels over sarcolemma and inward along T tubules
triggers release of calcium ions
Mechanism of contraction
ACh  Calcium  troponin  tropomyosin
Calcium binds to troponin
tropomyosin to shift
and expose active
sites on actin
Mechanism of contraction
–
Excitation and contraction (cont.)
• Sliding filament model
1. When active sites on actin are exposed, myosin
heads bind to them
2. Myosin heads bend, pulling the thin filaments past
them
3. Each head releases, binds to next active site, and
pulls again
4. Entire myofibril shortens
ACh  Calcium  troponin  tropomyosin  myosin  actin
(contraction)
Mechanism of contraction
Relaxation
• Immediately after Ca++ ions are released, SR begins
actively pumping them back into sacs
• Ca++ ions are removed from troponin molecules,
shutting down contraction
SR
Actin
Myosin
Energy sources for muscle
contraction
• Hydrolysis of ATP yields energy required for muscular
contraction
• Adenosine triphosphate (ATP) binds to myosin head
 transfers its energy to myosin head to perform work of
pulling thin filament during contraction
Energy sources for muscle
contraction
• Catabolism by muscle fibers requires glucose and
oxygen
• Aerobic respiration: occurs when adequate O2 is
available
• Anaerobic respiration: occurs when low levels of O2
results in formation of lactic acid (muscle soreness)
• Glucose and oxygen supplied to muscle fibers by
blood capillaries
• Skeletal muscle contraction produces waste heat that
can be used to help maintain body temperature
(HOMEOSTASIS)
Function of Skeletal Muscle
Organs
• Muscles are composed of bundles of muscle fibers that are
held together by fibrous connective tissue
• Motor unit: motor neuron plus the muscle fibers to which it
attaches
– Some motor units consist of only a few muscle fibers,
whereas others consist of numerous fibers
– Generally, the smaller the number of fibers in a motor unit,
the more precise the movement
available; the larger the number
of fibers in a motor unit, the
more powerful the contraction
available