LECTURE 6 (Skeletal Muscle) Flashcards
What are the three different types of muscle tissue?
- Cardiac muscle
- Skeletal muscle
- Smooth muscle
Describe Skeletal muscle
- Contains bundles of very long, multinucleated cells with cross-striations
- Contraction is quick, forceful & voluntary
Describe Cardiac muscle
- Cross-striations with elongated, branched cells bound to one another at INTERCALATED DISCS
- Contraction is involuntary, vigorous and rhythmic
Describe Smooth muscle
- Collections of fusiform cells which lack striations
- Slow, involuntary contractions
What is contraction caused by?
The sliding interaction of thick myosin filaments along thin actin filaments
What is Hypertrophy?
When activities (e.g exercise) enlarges the skeletal musculature by stimulating formation of new myofibrils and growth in the diameter of individual muscle fibers -> increases cell volume
What is Hyperplasia?
Tissue growth by an increase in the number of cells
Describe the development of skeletal muscle
1) Mesenchymal cells (MYOBLASTS) align and fuse together to make longer, multinucleated tubes called MYOTUBES
2) Myotubes continue differentiating to form functional MYOFILAMENTS and the ELONGATED NUCLEI are displaced against the SARCOLEMMA
3) Some of the mesenchymal cells do not fuse to form MUSCLE SATELLITE CELLS located on the external surface of muscle fibers
4) Satellite cells proliferate and produce new muscle fibers following muscle injury
What is the function of Myotubes?
- Synthesis the proteins to make myofilaments
- Gradually show cross-striations by light microscopy
Describe the Epimysium
- External sheath of dense irregular connective tissue
- Surrounds entire muscle
- Carry larger nerves, blood vessels and lymphatics of the muscle
Describe the Perimysium
- Thin connective tissue layer that immediately surrounds each FASCICLE
- Nerves, blood vessels and lymphatics penetrate the perimysium to supple each fascicle
Each ________ of muscle fibers makes up a functional unit in which the fibers work together
Fascicle
Describe the Endomysium
- Very thin, delicate layer of reticular fibers and scattered fibroblasts
- Surrounds individual muscle fibers
- Rich network of nerve fibers and capillaries
What are the three important layers of skeletal muscle
- Endomysium
- Perimysium
- Epimysium
What do all three layers and the dense irregular connective tissue of the deep fascia of skeletal muscle attach to?
Tendons at MYOTENDINOUS JUNCTIONS joining muscle to bone, skin or other muscle
What does each sarcomere extend from?
Z disc to Z disc
Describe what can be seen in skeletal muscle striations
- Alternating light and dark bands
- A bands = dark bands
- I bands = light bands
- Each I band is bisected by a dark transverse line, the Z disc
Where are Mitochondria and Sarcoplasmic reticulum found?
Between the myofibrils which consist of an end-to-end repetitive arrangement of sarcomeres
What are the properties of Myosin filaments?
- Occupy the A band at the middle region of the sarcomere
- Two identical heavy chains and two pairs of light chains
- Myosin heavy chains are thin, rodlike motor proteins twisted together as myosin tails
- Myosin heads bind both actin (forming transient cross bridges between thick and thin filaments) and ATP which catalyses energy release
What are the properties of Actin filaments?
- Thin and helical
- Run between the thick filaments
- Each monomer contains a binding site for myosin
- Two tightly associated regulatory proteins: TROPOMYOSIN & TROPONIN
What is Tropomyosin?
A 40-nm-long coil of two polypeptide chains located in the groove between the two twisted actin strands
What is Troponin?
A complex of three subunits:
- TnT = attaches to tropomyosin
- TnC = binds Ca2+
- TnI = regulates actin-myosin interaction
Where do Troponin complexes attach to?
Specific sites regularly spaced along each tropomyosin molecule
Thick filaments are bundles of _________ which span the entire A band and are bound to proteins of the M Line and to the Z disc across the I bands by a protein called TITIN
Myosin
Describe the Sarcoplasmic Reticulum
SARCOPLASMIC RETICULUM contains pumps for Ca2+ sequestration and surrounds the myofibrils -> Ca2+ release from cisternae through VOLTAGE GATED Ca2+ channels is triggered by membrane depolarisation by a motor nerve
Describe the T Tubules
Long, fingerlike invaginations of the cell membrane penetrate deeply into the sarcoplasm and encircle each myofibril -> Trigger Ca2+ release from sarcoplasmic reticulum throughout muscle fibre SIMULTANEOUSLY and produce uniform contraction of all myofibrils
What is adjacent to each T-Tubule?
Expanded terminal cisternae of sarcoplasmic reticulum called a TRIAD (T-tubule + Terminal Cisternae = Triad)
What is the function of the Triad complex?
- Allow depolarisation of the sarcolemma in a T-tubule to affect the sarcoplasmic reticulum
- Triggers release of Ca2+ ions into cytoplasm around thick and thin filaments which initiates contraction of sarcomeres
Describe the stages of muscle contraction
1) A nerve impulse triggers release of ACETYLCHOLINE from the synaptic knob into the synaptic cleft. ACh binds to ACh receptors in the motor end plate of the NEUROMUSCULAR JUNCTION initiating a muscle impulse in the sarcolemma of the muscle fiber
2) Impulse from sarcolemma to T-Tubules -> Ca2+ ions released from TERMINAL CISTERNAE into SARCOPLASM
3) CALCIUM IONS bind to TROPONIN. Troponin changes shape, moving tropomyosin on the actin to expose active sites on actin molecules of thin filaments. Myosin heads of thick filaments attach to EXPOSED ACTIVE SITES to form CROSSBRIDGES
4) Myosin heads pivot, moving thin filaments toward the SARCOMERE CENTRE. ATP binds myosin heads and is broken down into ADP and Pi. Myosin heads detach and return to original position. The cycle repeats which slides thick and thin filaments past each other -> Sarcomere shortens and muscle contracts -> Cycle continues as long as calcium ions remain bound to troponin to keep active sites exposed
5) When impulse stops, Ca2+ ions are ACTIVELY TRANSPORTED into SARCOPLASMIC RETICULUM, TROPOMYOSIN re-covers active sites and filaments passively slide back to their relaxed state
What happens in the absence of ATP?
Actin-myosin cross bridges become stable which accounts for RIGOR MORTIS that occurs as mitochondrial activity stops after death
What happens in the relaxed state?
- The sarcomere, I band and H zone are at their expanded length
- Springlike action of titin molecules help pull thin and thick filaments past one another
What happens during muscle contraction?
- Z discs are drawn closer together
- Titin molecules are compressed
Describe the motor nerves in innervation
Myelinated motor nerves branch out within the perimysium where each nerve gives rise to several UNMYELINATED TERMINAL TWIGS that pass through ENDOMYSIUM and form synapses with individual muscle fibers
Individual __________ muscle fibers do not show graded contraction- they contract either all the way or not at all
Striated
How do fibers vary the force of contraction?
The fibers within a muscle fascicle do not all contract at the same time
What do Striated muscles and myotendinous junctions contain?
Sensory receptors acting as proprioceptors providing the CNS with data from the musculoskeletal system
Changes in length of the surrounding muscle fibers caused by body movements are detected by _______________________ and the sensory nerves relay this information to the spinal cord
Muscle spindles
What do tendon organs detect?
Changes in tension within tendons produced by muscle contraction and act to inhibit motor nerve activity if tension becomes excessive
Describe the three main types of skeletal muscle fibers
- SLOW OXIDATIVE (type I) = high levels of acidic ATPase activity and stain the darkest
- FAST OXIDATIVE-GLYCOLYTIC (type IIa) = Intermediate
- FAST GLYCOLYTIC (type IIb) = stain the lightest
Describe Duchenne Muscular Dystrophy
Dystrophin is a large actin-binding protein located just inside the sarcolemma of skeletal muscle fibers
Mutations in DMD gene lead to defective linkages between the cytoskeleton and ECM -> Muscle contractions disrupt the weak linkages causing atrophy of muscle fibers
