Muscle tissue

Question 1

functions of muscular system

Answer 1

• movement of body, organs + substances (smooth muscle + sphincters)
• posture
• thermoregulation
• support soft tissues
• store nutrient reserves

Question 2

4 key characteristics of muscle

Answer 2

• irritability/ excitability (respond to stimuli)
• contractility (shorten actively)
• extensibility (contract over a range of resting lengths)
• elasticity (recoil to original length after contraction)

Question 3

3 layers of skeletal muscle connective tissue

Answer 3

• epimysium: strong connective tissue around the entire muscle
• perimysium: bundles muscle fibres into fascicles (contains blood vessels and nerves)
• endomysium: surrounds individual muscle cells

Question 4

myofibril

Answer 4

• muscle fibres contain cylindrical myofibrils which contain actin and myosin microfilaments

Question 5

sarcoplasm
sarcolemma

Answer 5

• cytoplasm of a muscle fibre
• plasma membrane of a muscle fibre

Question 6

sarcomere
sarcoplasmic reticulum

Answer 6

• smallest contractile subunit which extends from one Z-line to the next
• specialised smooth ER of a muscle fibre - stores calcium

Question 7

features of a skeletal muscle

Answer 7

• voluntary
• striated due to organisation of actin + myosin filaments
• peripheral nuclei
• T-tubules present at A-I junctions (2 per sarcomere)

Question 8

2 major striations in skeletal muscle

Answer 8

• A bands (anisotropic): dark, where actin (thin) and myosin (thick) bands overlap
• I bands (isotropic): light, contain actin (thin) filaments but no myosin

Question 9

H band
M line

Answer 9

• H band: area where there is myosin only in the middle of an A band, shortens during contraction
• M line in the middle of H band

Question 10

T (transverse) tubule

Answer 10

• located @ A-I junction
• invagination of sarcolemma (plasma membrane) with membrane on each side called terminal cisternae (forms triad)
• allows for electrical impulses to pass through deep into muscle cell for rapid contractions
• allows Ca2+ to be spread from NM junction for uniform contraction of the entire muscle cell @ once

Question 11

skeletal muscle contraction

Answer 11

• action potential arrives at NMJ > depolarisation of sarcolemma > travels thru T tubule
• Ca2+ released from sarcoplasmic reticulum into sarcoplasm, diffuses to myofilaments and binds to troponin C
• cross bridges repeatedly activate and inactivate between actin and myosin > shortens sarcomeres (not filaments)
• cessation of action potentials by tropomyosin returns Ca2+ to sarcoplasmic reticulum which terminates sliding of actin myosin and hence contraction

Question 12

relative size of 3 types of muscle cells

Answer 12

• skeletal = largest
• smooth = middle
• cardiac = smallest

Question 13

structure of cardiac muscle cells

Answer 13

• branched cells w/ one nucleus (sometimes two)
• striated (involuntary)
• has sarcomeres between Z lines with A (dark) and I (light) bands
• T tubules and only one associated piece of sarcoplasmic reticulum form dyads (not triads) @ Z lines
• cardial muscle cells joined end-to-end via intercalated discs (gap junctions, fascia (zonula) adherens + desmosomes) - physical and electrical connection for contraction
• many mitochondria

Question 14

structure of smooth muscle cells

Answer 14

• spindle shaped w/ central nucleus
• cells joined by gap junctions
• non-striated and no sarcomeres but still uses actin/myosin for contraction
• no T tubules, instead caveolae + cytoplasmic vesicles
• actin-binding dense plaques (equivalent of Z lines)
• contraction triggered by mechanical, electrical and chemical stimuli and regulated by calmodulin

Question 15

describe the regenerative capacity of muscle cells

Answer 15

• smooth muscle cells have the greatest capacity for regeneration
• skeletal muscles have satellite cells which facilitate regeneration
• cardiac muscle cells have no equivalent satellite cells > not replaced when they die