LECTURE 6 (Skeletal Muscle)

Question 1

What are the three different types of muscle tissue?

Answer 1

• Cardiac muscle
• Skeletal muscle
• Smooth muscle

Question 2

Describe Skeletal muscle

Answer 2

• Contains bundles of very long, multinucleated cells with cross-striations
• Contraction is quick, forceful & voluntary

Question 3

Describe Cardiac muscle

Answer 3

• Cross-striations with elongated, branched cells bound to one another at INTERCALATED DISCS
• Contraction is involuntary, vigorous and rhythmic

Question 4

Describe Smooth muscle

Answer 4

• Collections of fusiform cells which lack striations
• Slow, involuntary contractions

Question 5

What is contraction caused by?

Answer 5

The sliding interaction of thick myosin filaments along thin actin filaments

Question 6

What is Hypertrophy?

Answer 6

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

Question 7

What is Hyperplasia?

Answer 7

Tissue growth by an increase in the number of cells

Question 8

Describe the development of skeletal muscle

Answer 8

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

Question 9

What is the function of Myotubes?

Answer 9

• Synthesis the proteins to make myofilaments
• Gradually show cross-striations by light microscopy

Question 10

Describe the Epimysium

Answer 10

• External sheath of dense irregular connective tissue
• Surrounds entire muscle
• Carry larger nerves, blood vessels and lymphatics of the muscle

Question 11

Describe the Perimysium

Answer 11

• Thin connective tissue layer that immediately surrounds each FASCICLE
• Nerves, blood vessels and lymphatics penetrate the perimysium to supple each fascicle

Question 12

Each ________ of muscle fibers makes up a functional unit in which the fibers work together

Answer 12

Fascicle

Question 13

Describe the Endomysium

Answer 13

• Very thin, delicate layer of reticular fibers and scattered fibroblasts
• Surrounds individual muscle fibers
• Rich network of nerve fibers and capillaries

Question 14

What are the three important layers of skeletal muscle

Answer 14

• Endomysium
• Perimysium
• Epimysium

Question 15

What do all three layers and the dense irregular connective tissue of the deep fascia of skeletal muscle attach to?

Answer 15

Tendons at MYOTENDINOUS JUNCTIONS joining muscle to bone, skin or other muscle

Question 16

What does each sarcomere extend from?

Answer 16

Z disc to Z disc

Question 17

Describe what can be seen in skeletal muscle striations

Answer 17

• 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

Question 18

Where are Mitochondria and Sarcoplasmic reticulum found?

Answer 18

Between the myofibrils which consist of an end-to-end repetitive arrangement of sarcomeres

Question 19

What are the properties of Myosin filaments?

Answer 19

• 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

Question 20

What are the properties of Actin filaments?

Answer 20

• Thin and helical
• Run between the thick filaments
• Each monomer contains a binding site for myosin
• Two tightly associated regulatory proteins: TROPOMYOSIN & TROPONIN

Question 21

What is Tropomyosin?

Answer 21

A 40-nm-long coil of two polypeptide chains located in the groove between the two twisted actin strands

Question 22

What is Troponin?

Answer 22

A complex of three subunits:
- TnT = attaches to tropomyosin
- TnC = binds Ca2+
- TnI = regulates actin-myosin interaction

Question 23

Where do Troponin complexes attach to?

Answer 23

Specific sites regularly spaced along each tropomyosin molecule

Question 24

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

Answer 24

Myosin

Question 25

Describe the Sarcoplasmic Reticulum

Answer 25

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

Question 26

Describe the T Tubules

Answer 26

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

Question 27

What is adjacent to each T-Tubule?

Answer 27

Expanded terminal cisternae of sarcoplasmic reticulum called a TRIAD (T-tubule + Terminal Cisternae = Triad)

Question 28

What is the function of the Triad complex?

Answer 28

- 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

Question 29

Describe the stages of muscle contraction

Answer 29

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

Question 30

What happens in the absence of ATP?

Answer 30

Actin-myosin cross bridges become stable which accounts for RIGOR MORTIS that occurs as mitochondrial activity stops after death

Question 31

What happens in the relaxed state?

Answer 31

- 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

Question 32

What happens during muscle contraction?

Answer 32

- Z discs are drawn closer together - Titin molecules are compressed

Question 33

Describe the motor nerves in innervation

Answer 33

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

Question 34

Individual __________ muscle fibers do not show graded contraction- they contract either all the way or not at all

Answer 34

Striated

Question 35

How do fibers vary the force of contraction?

Answer 35

The fibers within a muscle fascicle do not all contract at the same time

Question 36

What do Striated muscles and myotendinous junctions contain?

Answer 36

Sensory receptors acting as proprioceptors providing the CNS with data from the musculoskeletal system

Question 37

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

Answer 37

Muscle spindles

Question 38

What do tendon organs detect?

Answer 38

Changes in tension within tendons produced by muscle contraction and act to inhibit motor nerve activity if tension becomes excessive

Question 39

Describe the three main types of skeletal muscle fibers

Answer 39

- 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

Question 40

Describe Duchenne Muscular Dystrophy

Answer 40

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