MICROSCOPIC SKELETAL MUSCLE ANATOMY
Human Anatomy
Unit 2
Skeletal Muscle Tissue
• Characteris9cs – Striated – Mul9nucleate – Voluntary (soma9c)
• Anatomy – Muscle • Myofiber • Myofibril • Myofilament
Muscle Microanatomy
• Mul9nucleate – Develop from the fusion of
mul9ple myoblasts – Each myoblast has its own
nuclues • Myofiber
– Muscle fiber – Muscle cell
• Myofibril – Tubes of contrac9le
proteins • Myofilament
– Contrac9le proteins
Muscle Fiber
• Sarcolemma – Membrane of muscle fiber – Electrically excitable – T-‐tubules
• Sarcoplasm – Muscle cytoplasm
• Sarcoplasmic re9culum – Modified smooth ER – Lateral sacs – Stores calcium (Ca2+)
• Func9onal unit of skeletal muscle is the sarcomere
Sarcomere Structure • Z line
– Defines the sarcomere • A band
– Length of thick filament • H band
– Space between thin filaments within a sarcomere
• I band – Space between thick filaments
between sarcomeres • Thick filament • Thin filament • Elas9c filament
Thin Filament
• 2 strands of ac9n molecules – Has binding sites for
myosin
• Tropomyocin – Covers binding sites on
ac9n for myosin – Res9ng fiber
• Troponin – Holds tropomyocin in place
– Binds to calcium (Ca2+)
Thick and Elas9c Filament
• Thick Filament – Myosin – Tail – Myosin head
• Myosin Head – 2 binding sites
• ADP + pi • Ac9n
• Elas9c filament – Ti9n – Achors myosin in place within sarcomere
Muscle Contrac9on
• Sliding filament theory • Myosin heads bind to ac9n
– 6 thin filaments: 1 thick filament
• Heads change conforma9on and pull the ac9n filaments across myosin molecule – Discon9nuous pulling
• Sarcomere shortens – Thick and thin filaments do
not shorten!
• All or none response
Neuromuscular Junc9on
• Soma9c motor neuron innervates muscle fiber – Synap9c bulb – Vessicles – Acetylcholine (ACh)
• Motor end plate – Invagina9on of sarcolemma
– Receptors for ACh • 1:1 ra9o
Motor Unit
• A single motor neuron and all of the muscle fibers it innervates
• A soma9c motor neuron has collateral branches that innervate mul9ple muscle fibers
• When a motor neuron sends a signal to contract – All fibers in the motor unit
will contract
• A muscle will be comprised of mul9ple motor units – Ra9o of motor units:muscle
Skeletal Muscle
Neuromuscular Junc9on
Cardiac Muscle
Smooth Muscle