chapter 11 physiology of the muscular system. introduction muscular system is responsible for moving...
TRANSCRIPT
Chapter 11Physiology 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
– 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
Overview of the muscle cell
An electron micrograph of a section through a triad junction of a frog tonic fibre, showing a central t-tubular element flanked on either side by the sarcoplasmic reticulum.
• Myofilaments – Each myofibril contains thousands of thick
and thin myofilaments
Myofilaments
Thick Filament
MyofilamentsFourFour different proteins make up myofilaments different proteins make up myofilaments
Thin 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
• 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
Myofilaments
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
• 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
Calcium binds to Calcium binds to troponin troponin
tropomyosin to tropomyosin to shift and expose shift and expose active sites on active sites on actinactin
ACh Calcium troponin tropomyosin
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
Mechanism of contraction
ACh Calcium troponin tropomyosin myosin actin
(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
Mechanism of contraction
SR
Actin
Myosin
• 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)
Energy sources for muscle contraction
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